ISS Utilization: AMS-02 (Alpha Magnetic Spectrometer)
AMS is a NASA-selected payload, a fundamental physics experiment and the first sensitive spaceborne magnetic spectrometer, with a broad international cooperation basis. The project is in fact the culmination of 30 years of experience and research of a group of high-energy physicists led by 1976 Nobel Laureate Samuel C. C. Ting and by Thomas Dudley Cabot of the Laboratory for Nuclear Science at the Massachusetts Institute of Technology (MIT).
Construction, integration, testing and operation of the Alpha Magnetic Spectrometer (AMS-02 Experiment) is carried out by an international team, referred to as the AMS Collaboration, composed of about 600 physicists and 56 institutes and organizations from 16 countries: North America (USA, Mexico), Asia (China, Korea, Taiwan), Europe, including nine ESA Member States (Denmark, Finland, France, Germany, Italy, The Netherlands, Portugal, Spain, Switzerland) and Russia, and organized under an agreement between NASA and the U.S. Department of Energy (DOE). AMS-02 will not only be the largest scientific instrument to be installed on the ISS, but it could also be considered the result of the largest international collaboration for a single experiment in space. 1) 2) 3)
The AMS-02 experiment intends to advance knowledge of the universe and the likelihood of a clearer understanding of the universe's origin. The key science objective is to search for antimatter and of the missing matter in the universe (broadly stated, the AMS goal is to provide first detailed observations of charged particles outside the Earth's atmosphere - in the cosmic void). The search is basically for anti-helium and anti-carbon with a detector sensitivity 104 to 105 times better than current limits. The AMS will help scientists better understand the fundamental issues on the origin and structure of the Universe by observing dark matter, missing matter and antimatter. 4) 5) 6)
• Search for antimatter (antinuclei)
• Search for dark matter (90% of the missing matter in the universe)
• Search for other exotic matter
• Cosmic ray studies (specifically, cosmic ray propagation and confinement time in the Galaxy).
At its heart, the AMS-02 is a cosmic ray detector. Cosmic rays will pass through the AMS-02 and be bent by its magnetic field, generated by a permanent magnet 4,000 times stronger than the Earth's own magnetic field. Instruments mounted in the AMS-02 will measure the mass, charge, and energy of the particles that pass through it, including the ability to discern matter from antimatter. Located above the Earth's atmosphere, the AMS will be able to detect high-energy particles that would not make it to the surface.
The design and construction of the AMS-02 Experiment, a state-of-the-art particle physics detector containing a large, cryogenic superfluid helium superconducting magnet (Cryomag), was completed mostly in Europe and Asia. AMS-02 is a DOE organized payload for which NASA is providing integration, delivery, and ISS accommodations. The DOE sponsorship is defined by a signed MOU (Memorandum Of Understanding) of September 20, 1995 that states DOE is responsible for submission of data required for "Interface, integration and safety data" only. NASA is not responsible for mission success and provides minimal avionics support systems.
For the AMS program, NASA officially interfaces with DOE, which is the sponsor for the AMS-02 experiment. DOE in turn sponsors the United States portion of the international AMS Collaboration. NASA/HQ is responsible for the overall NASA management of the AMS program interface activity between NASA and DOE and for overall program management of the NASA AMS program objectives. The Engineering Directorate at NASA/JSC has been delegated responsibility for implementing the AMS project for NASA/HQ. 7) 8)
The flight hardware is referred to as the "AMS Payload" and is comprised of two parts: the "AMS Experiment" provided by the International AMS Collaboration led by the MIT (Massachusetts Institute of Technology), and the "AMS Payload Integration Hardware (PIH)" provided by NASA/JSC (Johnson Space Center), Houston, TX, with the support of Lockheed Martin Space Operations (LMSO).
A precursor AMS experiment, AMS-01, was initially flown on Shuttle flight STS-91 (Discovery, June 2-12, 1998, the final shuttle-MIR docking). AMS-1 was an AMS prototype (a simplified version of the spectrometer) and was used to test particle physics technology in LEO. The early experiment was particularly helpful in calibrating the instrument. AMS-1 observed millions of helium nuclei, but no antihelium. The success of the 10-day mission allowed to obtain relevant results concerning particle fluxes in near Earth orbit and improved limits on the presence of primary antimatter. 9) 10) 11) 12)
For AMS-01, ten permanent magnets were made:
• Seven magnets to understand the field calculation, leakage and dipole moment.
• Three full-size magnets for 1) space qualification, 2) destructive testing and 3) flight.
In spite of the successful 10-day operation of the AMS-01 prototype, the 3-5 year operation of AMS-02 long-duration mission on the ISS required a major redesign of the spectrometer to guarantee that the mission objectives could be reliably accomplished.
AMS Collaboration: The AMS Collaboration is a partnership of physicists and engineers belonging to Institutes, supported by Funding Agencies whose objective is designing, constructing, testing and operation of the AMS-02 Experiment. The different partners have secured the support of their National/International Funding Agencies to enable them to participate in the AMS Collaboration. They approved the experiment, which includes a precursor flight in June 1998 on the shuttle Discovery (AMS-01) and a flight on the International Space Station (ISS).
In 1997 the European Organization for Nuclear Research (CERN, Geneva, Switzerland) accepted the AMS experiment as a "Recognized Experiment" (CERN/DG/RB 97-261). In December 2003, CERN and the AMS Collaboration have signed a new MOU with a duration of five years renewable. It foresees the establishment at CERN of the POCC (Payload Operations and Control Center), the SOC (Science Operations Center), assembly and testing areas, office space and secretarial support. 13) 14)
Figure 1: Overview of AMS-02: a U.S. DOE led International Collaboration of 15 Countries, 44 Institutes and 600 Physicists (image credit: AMS collaboration, Ref. 34)
The main objective of the mission with AMS-02 is to contribute to solving the mysteries of dark matter and antimatter. Our present-day knowledge of physics can only explain four percent of the composition of our universe; the remaining 96%, which has been called 'dark matter' and 'dark energy', we know virtually nothing about. Currently, science assumes that dark matter consists of new elementary particles that ensure, for example, that our Sun orbits the center of the Milky Way on a stable path. "We can only determine the validity of this theory by finding evidence for the existence of dark matter." The AMS will also search for antimatter in space with an unprecedented sensitivity. According to Schael, this is one of the most important issues in physics at present. The current hypothesis in astrophysics states that, after the Big Bang, identical amounts of matter and antimatter were created. But no antimatter has been discovered in space yet. "If the AMS were to detect an anti-carbon nucleus, for example, it would indicate that the universe is in fact symmetrical, and that a spatial separation between matter and antimatter took place after the Big Bang."
The AMS detectors will 'see' 2000 particles per second as they fly through the experiment outside the ISS. The experiment will be able to determine, not just the energy, but also the mass and electrical charge of these particles, which are, for example, evidence of the remnants of massive supernovae. "With the AMS, we are creating what is essentially a kind of photograph of this particle flux using all the detectors" (Stefan Schael).
AMS-02 (Alpha Magnetic Spectrometer-02) Experiment
AMS-02 is the Alpha Magnetic Spectrometer designed to be mounted and operated on the ISS. The AMS-02 instrument assembly is a cube-shaped structure with a total mass of ~7,000 kg. The spectrometer consists of a huge superconducting magnet and six specialized detectors, and requires 2.4 kW of power @ 28 V.
The experiment has a 10 Gbit/s internal data pipeline and will have a dedicated 2 MB/s connection to ground stations. AMS-02 will gather approximately 200 TB of scientific data during its mission. Four 750 MHz PowerPC computers running Linux will provide the computing power.
An act of Congress in 2008 added another flight to the schedule near the end of the program. Currently scheduled for 2010, this extra flight of the shuttle is going to launch a hunt for antimatter galaxies. 15)
In April 2010, the AMS project decided to switch from a superconducting magnet to a permanent magnet configuration. The design change of the AMS-02 instrument resulted in a launch delay from July 2010 to the fall of 2010 - and eventually to the spring of 2011.
Figure 2: Photo of the AMS-02 assembly in the Shuttle bay (image credit: AMS collaboration)
Launch: The STS-134 with the AMS-02 payload was launched on May 16, 2011. STS-134 (ISS assembly flight ULF6) was the last flight of Space Shuttle Endeavour. The AMS-02 payload was positioned on an external payload attach site of ISS. 17) 18) 19) 20) 21) 22) 23)
Orbit: The near-circular orbit of the ISS is at a nominal altitude of ~400 km with an inclination of 51.6º. The ISS maintains usually a torque equilibrium attitude (TEA) during the microgravity mode of its operation.
The objective of AMS-02 is to perform measurements of cosmic ray spectra of individual elements up to Z< 27 and up to the TeV region and of high energetic gamma-rays up to hundreds of GeV. The goal is to search for the 'missing universe' and to help scientists to understand better the fundamental issues on the origin and structure of the Universe by observing antimatter and ‘dark' matter. With a magnetic field 4000 times stronger than the magnetic field of Earth, this state-of-the-art particle physics detector will examine directly from space each particle passing through it in a program that is complementary to that of the LHC (Large Hadron Collider).
As a by-product, AMS-02 will gather a lot of other information from cosmic radiation sources on stars and galaxies millions of light years from our home galaxy. Not only astronomers, but also particle physicists, are eagerly waiting for its data.
Figure 3: Daniel Goldin, former NASA Administrator realized the unique potential of ISS for fundamental science and has supported AMS from the beginning. The image shows Daniel Goldin (left) and Samuel Ting of MIT on May 16, 2011 when AMS-02 was launched on the Space Shuttle Endeavour (STS-134), image credit: S. Ting (Ref. 34)
• December 8, 2016: The AMS (Alpha Magnetic Spectrometer) Collaboration announces the fifth anniversary of the AMS Experiment on the International Space Station (ISS) and summarizes its major scientific results to date. 24) 25)
- The AMS Experiment (shown on Figure 23) is the most sensitive particle detector ever deployed in space and is exploring a new and exciting frontier in physics research. As a magnetic spectrometer, AMS is unique in physics research as it studies charged particles and nuclei in the cosmos before they are annihilated in the Earth's atmosphere. The improvement in accuracy over previous measurements is made possible through its long duration time in space, large acceptance, built in redundant systems and its thorough calibration in the CERN test beam. These features enable AMS to analyze the data to an accuracy of ~1%. The first five years of data from AMS on the International Space Station are beginning to unlock the secrets of the cosmos.
- Since its installation on the ISS in May 2011, AMS has collected data from more than 90 billion cosmic rays with up to multi-TeV energies and published its major physics results in Physical Review Letters (Ref. 26).
- A note about cosmic rays: As the products of exploding supernovae, primary cosmic rays can travel for millions of years in the galaxy before reaching AMS. Secondary cosmic rays come from the interaction of primary cosmic rays with the interstellar media. Uniquely positioned on the International Space Station, AMS studies cosmic rays passing through its precision detectors, to define the charge, energy, and momentum of the passing particles in order to obtain an understanding of dark matter, the existence of complex antimatter in space, the properties of primary and secondary cosmic rays as well as new, unexpected phenomena. These are among the fundamental issues in modern physics.
- There are hundreds of different kinds of charged elementary particles. Only four of them – electrons, protons, positrons and antiprotons – have infinite lifetimes so they can travel through the cosmos forever. Electrons and positrons have much smaller mass than protons and antiprotons so they lose much more energy in the galactic magnetic field due to synchrotron radiation.
- As shown in Figure 4, AMS has observed that the electron flux and positron flux display different behaviors both in their magnitude and in their energy dependence.
- Most surprisingly, from 60 to 500 GeV, positrons, protons and antiprotons display identical momentum dependence but electrons exhibit a totally different dependence as shown in Figure 5. The reason that this observation is surprising is that both electrons and positrons lose energy (or momentum) equally when travelling through the galactic magnetic field and at a much higher rate than protons or antiprotons.
Figure 5: The positron, proton, and antiproton spectra have identical momentum dependence from 60 to 500 GeV. The electron spectrum exhibits a totally different behavior, it decreases much more rapidly with increasing momentum (image credit: AMS Collaboration)
- There has been much interest over the last few decades in understanding the origin and nature of dark matter. When particles of dark matter collide, they produce energy that transforms into ordinary particles, such as positrons and antiprotons. The characteristic signature of dark matter is an increase with energy followed by a sharp drop off at the mass of dark matter as well as an isotropic distribution of the arrival directions of the excess positrons and antiprotons.
- Figure 6 shows the latest results from AMS on the positron flux. As seen from the figure, after rising from 8 GeV above the rate expected from cosmic ray collisions, the spectrum exhibits a sharp drop off at high energies in excellent agreement with the dark matter model predictions with a mass of ~1 TeV. There is great interest in the physics community on the AMS measurements of elementary particles. For example, an alternative speculation for positron spectrum is that this rise and drop off may come from new astrophysical phenomena such as pulsars.
- AMS has also studied the antiproton to proton ratio. The excess in antiprotons observed by AMS cannot easily be explained as coming from pulsars but can be explained by dark matter collisions or by other new astrophysics models. Antiprotons are very rare in the cosmos. There is only one antiproton in 10,000 protons therefore a precision experiment requires a background rejection close to 1 in a million. It has taken AMS five years of operations to obtain a clean sample of 349,000 antiprotons. Of these, AMS has identified 2200 antiprotons with energies above 100 billion electron volts. Experimental data on cosmic ray antiprotons are crucial for understanding the origin of antiprotons in the cosmos and for providing insight into new physics phenomena.
- Protons are the most abundant particles in cosmic rays. AMS has measured the proton flux to an accuracy of 1% with 300 million protons and found that the proton flux cannot be described by a single power law, as had been assumed for decades, and that the proton spectral index changes with momentum.
- AMS contains seven instruments with which to independently identify different elementary particles as well as nuclei. Helium, lithium, carbon, oxygen and heavier nuclei up to iron have been studied by AMS. It is believed that helium, carbon and oxygen were produced directly from primary sources in supernova remnants whereas lithium, beryllium and boron are believed to be produced from the collision of primary cosmic rays with the interstellar medium. Primary cosmic rays carry information about their original spectra and propagation, and secondary cosmic rays carry information about the propagation of primary and secondary cosmic rays and the interstellar medium.
- Helium is the second most abundant cosmic ray. Helium has been studied over the past century. Although lithium is a secondary cosmic ray, its spectrum behaves similarly to protons and helium in that none of the three fluxes can be described by a single power law and they do change their behavior at the same energy.
- Since protons, helium, carbon and oxygen are primary cosmic rays and produced at the same sources; thus their flux ratios should be rigidity independent. Rigidity is momentum per unit charge and is the metric by which magnetic fields, such as those experienced by cosmic rays between their origin and AMS, act on charged particles. From the AMS measurements, for carbon-to-helium and for carbon-to-oxygen these ratios are, indeed, independent of rigidity, i.e., flat, as expected. Unexpectedly, the proton-to-helium flux ratio drops quickly but smoothly with rigidity.
- Other secondary cosmic rays being measured by AMS include boron and beryllium. The unstable isotope of beryllium, 10 Be, has a half-life of 1.5 million years and decays into boron. The Be/B ratio therefore increases with energy due to time dilation when the Be approaches the speed of light. Hence, the ratio of beryllium to boron provides information on the age of the cosmic rays in the galaxy. From this, AMS has determined that the age of cosmic rays in the galaxy is ~12 million years.
- The flux ratio between secondary cosmic rays (boron) and primary cosmic rays (carbon) provides information on propagation and the average amount of interstellar material (ISM) through which the cosmic rays travel in the galaxy. Cosmic ray propagation is commonly modeled as a fast moving gas diffusing through a magnetized plasma. Various models of the magnetized plasma predict different behavior of the boron-to-carbon (B/C) flux ratio. Remarkably, above 65 GeV, the B/C ratio measured by AMS is well described by a single power law B/C= kRd with d = -0.333±0.015. This is in agreement with the Kolmogorov turbulence model of magnetized plasma where d = -1/3 asymptotically. Of equal importance, the B/C ratio does not show any significant structures in contrast to many cosmic ray models. — The carbon and oxygen fluxes, which are both primary cosmic rays, and the boron, lithium, and beryllium fluxes, which are secondary, have characteristically different rigidity dependences.
- The Big Bang origin of the Universe requires that matter and antimatter be equally abundant at the very hot beginning of the universe. The search for the explanation for the absence of antimatter in a complex form is known as Baryogenesis. Baryogenesis requires both a strong symmetry breaking and a finite proton lifetime. Despite the outstanding experimental efforts over many years, no evidence of strong symmetry breaking nor of proton decay have been found. Therefore, the observation of a single anti-helium event in cosmic rays is of great importance.
- In five years, AMS has collected 3.7 billion helium events (charge Z = +2). To date we have observed a few Z = -2 events with mass around 3He. At a rate of approximately one antihelium candidate per year and a required signal (antihelium candidates) to background (helium) rejection of one in a billion, a detailed understanding of the instrument is required. In the coming years, with more data, one of our main efforts is to ascertain the origin of the Z = -2 events.
- In five years, AMS on the ISS has recorded more than 90 billion cosmic ray events. The latest AMS measurements of the positron spectrum and positron fraction, the antiproton/proton ratio, the behavior of the fluxes of electrons, positrons, protons, helium and other nuclei provide precise and unexpected information on the production, acceleration and propagation of cosmic rays. The accuracy and characteristics of the data, simultaneously from many different types of cosmic rays require the development of a comprehensive model. In the coming years, with more data, one of our main efforts is to ascertain the origin of the Z = -2 events.
- Most importantly, AMS will continue to collect and analyze data for the lifetime of the Space Station. As the results to date have demonstrated, whenever a precision instrument such as AMS is used to explore the unknown, new and exciting discoveries can be expected.
• August 26, 2016: The AMS project is reporting on the measurement of the antiproton flux and of the antiproton-to-proton flux ratio in primary cosmic rays in the absolute rigidity range from 1 to 450 GV based on 3.49 x 105 antiproton events and 2.42 x 109 proton events collected by the AMS-02 (Alpha Magnetic Spectrometer-02), on the International Space Station, ISS, from May 19, 2011 to May 26, 2015. 26)
- Of the four charged elementary particles traveling through the cosmos—protons, electrons, positrons, and antiprotons — the experimental data on antiprotons are limited because for each antiproton there are approximately 104 protons. Since the observation of antiprotons in cosmic rays, many studies of cosmic ray antiprotons have been performed. However, to measure the antiproton flux to 1% accuracy requires a separation power of ~106. The sensitivity of antiprotons to cosmic phenomena is complementary to the sensitivity of the measurements of cosmic ray positrons. For example, AMS has accurately measured the excess in the positron fraction to 500 GeV. This data generated many interesting theoretical models including collisions of dark matter particles, astrophysical sources, and collisions of cosmic rays. Some of these models also include specific predictions on the antiproton flux and the antiproton-to-proton flux ratio in cosmic rays.
• May 20, 2016: The International Space Station's robotic assets paid a recent visit to AMS-02 (Alpha Magnetic Spectrometer -02) to survey the external health of the payload and gain imagery ahead of a potential servicing EVA. This week marked the fifth anniversary since AMS-02 was launched to the orbital outpost in the payload bay of Space Shuttle Endeavour, as it continues its investigation into cosmic rays.27)
- The SSRMS (Space Station Remote Manipulator System) and SPDM (Special Purpose Dexterous Manipulator) were used to perform a video survey of the AMS-02 ( Alpha Magnetic Spectrometer-02) payload. Such surveys also allow experts on the ground to check for potential MicroMeteoroid and Orbital Debris (MMOD) strikes, which are a constant threat to external payloads on the ISS.
- The notes also added that there is preliminary interest in "servicing" AMS-02 via a future spacewalk. "The AMS survey (was) performed to understand the state of the payload and gain detailed views in preparation for a possible EVA to service the payload."
- However, good news was reported, based on the initial evaluations into the survey imagery, adding the experiment payload appears to be in a good condition, with the notes adding: "There were no issues reported during the survey."
• September 2, 2015: The operators of the dark-matter experiment aboard the International Space Station are striving to figure out how to keep three crucial cooling pumps working after the failure of a fourth last year. The glitch raises the most serious concerns yet about whether the AMS (Alpha Magnetic Spectrometer), which probes cosmic rays for signs of dark matter being annihilated in deep space, will last until the space station's planned retirement in 2024. Originally designed for a three-year mission, the AMS is in its fourth year with nine to go. 28)
Figure 7: The Alpha Magnetic Spectrometer has flown on the International Space Station since 2011 (image credit: NASA)
- "We are analyzing a whole host of possibilities" for what went wrong with the cooling pump and how to fix it, says Mark Sistilli, the AMS program manager at NASA's headquarters in Washington DC. Tests have already ruled out one possibility, that radiation fried the broken pump's electronics.
- The AMS continues to gather science data using the three remaining pumps. They are part of a liquid carbon dioxide cooling system that is meant to dissipate heat as the AMS, which is on the outside of the space station, cycles in and out of sunlight during each 92 minute orbit of Earth.
- Only one pump is needed at any given time. One failed in February 2014 and at least one of the other three is showing possible signs of trouble.
- Since the 8.5 ton AMS-02 began operating in 2011, it has tracked more than 69 billion cosmic rays flying through its detectors. Its goal is to search for antimatter and dark matter. In 2013, AMS scientists reported measuring numbers and energies of positrons that hinted at, but did not confirm, the existence of dark matter. The experiment's scientific power depends on how many particles it tracks, so the longer it runs, the more solid the conclusions. "We've had some terrific science so far," says Sistilli. "We really do want to get to 2024 if we can."
- Meanwhile, a second and smaller dark-matter experiment, the CALET (Calorimetric Electron Telescope), arrived at the space station on August 19, 2015. Led by the Japan Aerospace Exploration Agency (JAXA), it will hunt cosmic rays at energies higher than those in the AMS studies (Ref. 28).
- MS is the only major particle physics experiment on the ISS. In its first four years on orbit, AMS has collected more than 60 billion cosmic ray events (electrons, positrons, protons, antiprotons, and nuclei of helium, lithium, boron, carbon, oxygen, ....) up to multi-TeV energies. As an external payload on the ISS through at least 2024, AMS will continue to collect and analyze an increasing volume of statistics at highest energies which, combined with in-depth knowledge of the detector and systematic errors, will produce valuable insight.
- The AMS results on the positron fraction, the electron spectrum, the positron spectrum, and the combined electron plus positron spectrum are consistent with dark matter collisions and cannot be explained by existing models of the collision of ordinary cosmic rays. There are many new models showing that the results may be explained by new astrophysical sources (such as pulsars) or new acceleration and propagation mechanisms (such as supernova remnants).
- To distinguish if the observed new phenomena are from dark matter, measurements are underway by AMS to determine the rate at which the positron fraction falls beyond its maximum, as well as the measurement of the antiproton to proton ratio. As seen in Figure 8, the antiproton to proton ratio stays constant from 20 GeV to 450 GeV kinetic energy. This behavior cannot be explained by secondary production of antiprotons from ordinary cosmic ray collisions. Nor can the excess of antiprotons be easily explained from pulsar origin. The latest results on these studies will be reported by the AMS Collaboration during "AMS Days at CERN" and in future publications.
- In addition, a thorough understanding of the process involved in the collision of ordinary cosmic rays is a requirement in understanding the AMS results mentioned above. The AMS Collaboration will also report on the most recent results on the precision studies of nuclei spectra (such as protons, helium and lithium) up to multi-TeV energies.
- The latest data on the precision measurement of proton flux in cosmic rays from 1 GV to 1.8 TV rigidity (momentum/charge) will appear shortly in Physical Review Letters. These results are based on 300 million proton events. AMS has found that the proton flux is characteristically different from all the existing experimental results. As seen in Figure 9, the AMS result shows the measured flux changes its behavior at ~300 GV rigidity. The solid line is a fit to the data. The dashed line in Figure 2 is the proton flux expected with no change in behavior; as seen, it does not agree with the data.
- Most surprisingly, AMS has also found, based on 50 million events, that the helium flux exhibits nearly identical and equally unexpected behavior as the proton flux (Figure 10). AMS is currently studying the behavior of other nuclei in order to understand the origin of this unexpected change.
- These unexpected new observations provide important information on the understanding of cosmic ray production and propagation.
- The latest AMS measurements of the positron fraction, the antiproton/proton ratio, the behavior of the fluxes of electrons, positrons, protons, helium, and other nuclei provide precise and unexpected information. The accuracy and characteristics of the data, simultaneously from many different types of cosmic rays, require a comprehensive model to ascertain if their origin is from dark matter, astrophysical sources, acceleration mechanisms or a combination.
• March 2015: Nearly four years into the mission, the Alpha Magnetic Spectrometer is delivering precise data and arguably providing a few hints about the nature of dark matter. But it's unclear whether the mission will ever deliver on its ambitious goals. Cosmic rays are charged particles that get whipped around by magnetic fields, so they don't travel in straight lines and cannot be traced back to their source. To pin the origin of particular cosmic rays to dark matter, scientists will have to rule out every other possible explanation. Critics say the chances of identifying dark matter are very slim. And finding primordial antimatter, they say, is nearly impossible. 31)
- Such criticism barely registers with the mission's leader, particle physicist Samuel Ting. The 79-year-old Nobel laureate has made a career of designing elegant experiments and, despite frequent opposition, successfully lobbying to get them built. Then he has patiently collected and analyzed data, meticulous to the extreme, before revealing the often-impressive findings. Though results may come later than most scientists would prefer, Ting is confident that conducting a powerful particle physics experiment in space will expand scientists' understanding of the cosmos.
• January 21, 2015: AMS-02 is operating stable since May 2011. So far, the project collected 60 billion events, out of which 10.6 million were identified as leptons. The flux of positrons up to 500 GeV was measured; electrons up to 750 GeV, electrons+positrons up to 1 TeV and the positron fraction up to 500 GeV. 32)
- The positron fraction is compatible with a turnover beyond 200 GeV. The electron spectrum hardens beyond 30 GeV.
- All measurements are compatible with a common contribution to the positron and electron flux, which starts to dominate over the positron flux in the range 1 GeV to 10 GeV.
• Sept. 18, 2014: After 40 months of operations in space, AMS has collected 54 billion cosmic ray events. To date 41 billion have been analyzed. The data is analyzed at the AMS SOC (Science Operations Center) located at CERN as well as AMS universities around the world. Over the lifetime of the Space Station, AMS is expected to measure hundreds of billions of primary cosmic rays. Among the physics objectives of AMS is the search for antimatter, dark matter, and the origin of cosmic rays. The AMS collaboration will also conduct precision measurements on topics such as the boron to carbon ratio, nuclei and antimatter nuclei, and antiprotons, precision measurements of helium flux, proton flux and photons as well as the search for new physics and astrophysics phenomena such as strangelets. 33) 34)
- Of the 41 billion primary cosmic ray events analyzed so far, 10 million have been identified as electrons and positrons. AMS has measured the positron fraction (ratio of the number of positrons to the combined number of positrons and electrons) in the energy range 0.5 to 500 GeV. The project observed that the energy at which the fraction starts to quickly increase is 8 GeV (Figure 12) indicating the existence of a new source of positrons.
Figure 13 shows that the exact rate at which the positron fraction increases with energy has now been accurately determined and the fraction shows no observable sharp structures. The energy at which the positron fraction ceases to increase (corresponding to the turning point energy at which the positron fraction reaches its maximum) has been measured to be 275+32 GeV as shown in Figure 13 (upper plot). This is the first experimental observation of the positron fraction maximum after half a century of cosmic rays experiments. The excess of the positron fraction is isotropic within 3% strongly suggesting the energetic positrons may not be coming from a preferred direction in space.
Precise measurement of the positron fraction is important for understanding of the origin of dark matter. Dark matter collisions will produce an excess of positrons and this excess can be most easily studied by measuring the positron fraction. Ordinary cosmic ray collisions result in the positron fraction decreasing steadily with energy. Different models on the nature of dark matter predict different behavior of the positron fraction excess above the positron fraction expected from ordinary cosmic ray collisions. Depending on the nature of dark matter, the excess of the positron fraction has a unique signature. The characteristic features are highlighted in Figure 11.
The new results from AMS (published in Physical Review Letters) 35) show that items 1-4 in Figure 11 have been unambiguously resolved and are observations of a new phenomena. They are consistent with a dark matter particle (neutralino) of mass on the order of 1 TeV. To determine if the observed new phenomena is from dark matter or from astrophysical sources such as pulsars, measurements are underway by AMS to determine the rate of decrease at which the positron fraction falls beyond the turning point, (item 5), as well as the measurement of the anti-proton fraction (anti-proton to proton plus anti-proton ratio). These will be reported in future publications.
Secondly, AMS reports the precise measurements of the electron flux and the positron flux, i.e. intensities of cosmic ray electrons and positrons. These measurements show that the behavior of electrons and positrons are significantly different from each other both in their magnitude and energy dependence. Figure 14 (upper plot) shows the electron and positron fluxes multiplied by the energy cubed (E3, for the purpose of presentation). The positron flux first increases (0.5 to 10 GeV), then levels out (10 to 30 GeV), and then increases again (30 to 200 GeV). Above 200 GeV, it has a tendency to decrease. This is totally different from the scaled electron flux.
The behavior of the flux as a function of energy is described by the spectral index and the flux was expected to be proportional to energy E to the power of the spectral index. The result shows that neither flux can be described with a constant spectral index ( Figure 14, lower plot). In particular, between 20 and 200 GeV, the rate of change of the positron flux is surprisingly higher than the rate for electrons. This is important proof that the excess seen in the positron fraction is due to a relative excess of high energy positrons, as expected from dark matter collisions, and not the loss of high energy electrons. These results are published in Physical Review Letters in a separate article. 36)
This new observation of the electron and positron fluxes also demonstrates, as pointed out by Dr. Michael S. Turner, that there is a fundamental difference between matter (electrons) and antimatter (positrons).
In 1932, Carl Anderson discovered the positron in cosmic rays. Non-magnetic detectors in space and on the ground can measure the flux of the sum of electrons plus positrons. Over the last 50 years, there have been many experiments that measured the combined flux of electrons plus positrons in cosmic rays. These measurements have yielded interesting results and few of them indicated the possible existence of a structure at 300-800 GeV.
AMS, being a particle physics detector, provides many independent measurements of electrons, positrons, and electrons plus positrons. After collecting 41 billion cosmic ray events, AMS has been able to provide a measurement of the flux of electrons plus positrons, shown in Figure 14 (upper plot). The combined flux is smooth and reveals new and distinct information. Most interesting is the observation that, at high energies and over a wide energy range, the combined flux can be described by a single, constant spectral index (Figure 14, lower plot).
The precision measurements of the positron fraction, the individual fluxes and the combined flux are complementary to one to another. Together they will provide a deeper understanding of the origin of high energy cosmic rays and shed more light on the existence of dark matter.
During the life time of ISS, the AMS collaboration expects to obtain 300 billion events.
Figure 12: The positron fraction measured by AMS (red circles) compared with the expectation from the collision of ordinary cosmic rays showing that above 8 billion electron volts (8 GeV) the positron fraction begins to quickly increase. This increase indicates the existence new sources of positrons (image credit: AMS collaboration)
Figure 13: Upper plot shows the slope of positron fraction measured by AMS (red circles) and a straight line fit at the highest energies (blue line). The data show that at 275±32 GeV the slope crosses zero. Lower plot shows the measured positron fraction as function of energy as well as the location of the maximum. No sharp structures are observed (image credit: AMS collaboration)
Figure 14: The upper plot highlights the difference between the electron flux (blue dots, left scale) and the positron flux (red dots, right scale). The lower plot shows the spectral indices of the electron flux and of the positron flux as functions of energy (image credit: AMS collaboration).
• Sept. 2013: The 33rd International Conference on Cosmic Rays (ICRC 2013) – The Astroparticle Physics Conference – took place in Rio de Janeiro on 2–9 July, 2013 and provided a high-profile platform for the presentation of a wealth of results from solar and heliospheric physics, through cosmic-ray physics and gamma-ray astronomy to neutrino astronomy and dark-matter physics. A full session was devoted to the presentation of new results from the AMS-02 (Alpha Magnetic Spectrometer). Sponsored by the US Department of Energy and supported financially by the relevant funding and space agencies in Europe and Asia, this experiment was deployed on the ISS (International Space Station) on 19 May 2011. 37)
• July 2013: To date, the AMS-02 detector has collected over 40 billion cosmic ray events. Using the data collected during more than 2 years the AMS Collaboration presented at the ICRC 2013 (International Cosmic Ray Conference) new physics results on: 38) 39)
1) Proton spectrum from 1 GV to 1.8 TV and searches for spectral breaks
2) Helium spectrum from 2 GV to 3 TV and searches for spectral breaks
3) Electron spectrum from 1 GeV to 500 GeV
4) Positron spectrum from 1 GeV to 350 GeV
5) Positron+Electron spectrum from 0.5 Gev to 700 GeV
6) B/C ratio from 0.5 GV to 500 GV
7) Positron fraction and Positron fraction anisotropy.
Most of these results are based on statistical and systematic errors at O(1%).
Figure 15: New results from the first 2 years of AMS (image credit: AMS Collaboration, ICRC 2013)
Figure 16: New results from AMS: Proton flux (image credit: AMS Collaboration, ICRC 2013)
Figure 17: New results from AMS: Proton flux, comparison with past measurements (image credit: AMS Collaboration, ICRC 2013)
Figure 18: New results from AMS: Helium flux (image credit: AMS Collaboration, ICRC 2013)
Figure 19: New results from AMS: Helium flux, comparison with past measurements (image credit: AMS Collaboration, ICRC 2013)
• April 3, 2013: The AMS Collaboration announces the publication of its first physics result in Physical Review Letters. In the initial 18 month period of space operations, from May 19, 2011 to December 10, 2012, AMS-02 analyzed 25 billion primary cosmic ray events. Of these, an unprecedented number, 6.8 million, were unambiguously identified as electrons and their antimatter counterpart, positrons. The 6.8 million particles observed in the energy range 0.5 to 350 GeV are the subject of the precision study reported in this first paper. 40) 41)
Electrons and positrons are identified by the accurate and redundant measurements provided by the various AMS instruments against a large background of protons. Positrons are clearly distinguished from this background through the robust rejection power of AMS of more than one in one million. Currently, the total number of positrons identified by AMS, in excess of 400,000, is the largest number of energetic antimatter particles directly measured and analyzed from space. 42)
AMS has measured the positron fraction (ratio of the positron flux to the combined flux of positrons and electrons) in the energy range 0.5 to 350 GeV. The collaboration observed that from 0.5 to 10 GeV, the fraction decreases with increasing energy. The fraction then increases steadily between 10 GeV to ~250 GeV. Yet the slope (rate of growth) of the positron fraction decreases by an order of magnitude from 20 to 250 GeV. At energies above 250 GeV, the spectrum appears to flatten but to study the behavior above 250 GeV requires more statistics – the data reported represents ~10% of the total expected. The positron fraction spectrum exhibits no structure nor time dependence. The positron to electron ratio shows no anisotropy indicating the energetic positrons are not coming from a preferred direction in space. Together, these features show evidence of a new physics phenomena (Ref. 40).
Figure 20: The positron fraction measured by AMS-02 (image credit: AMS Collaboration, CERN)
Additionally, the energies of these positrons suggest they might have been created when particles of dark matter collided and destroyed each other. The AMS results are consistent with the findings of previous telescopes, like the Fermi and PAMELA gamma-ray instruments, which also saw a similar rise, but Samuel Ting (PI) said the AMS results are more precise.
Figure 21: A comparison of AMS results with recently published measurement (image credit: AMS Collaboration, CERN)
The discovery of of the more than 400,000 positrons, the antimatter equivalent of electrons, is sending shock waves through the scientific community. Where do the positrons come from? The Universe is almost completely devoid of antimatter, so the positron fraction of cosmic ray electrons — as much as 10% — is a little surprising. 43)
One idea is dark matter. Astronomers know that the vast majority of the material Universe is actually made of dark matter rather than ordinary matter. They just don't know what dark matter is. It exerts gravity, but emits no light, which makes it devilishly difficult to study.
A leading theory holds that dark matter is made of a particle called the neutralino. Collisions between neutralinos should produce a large number of high-energy positrons, which the AMS-02 should be able to detect with unprecedented sensitivity.
• May 19. 2012. Today one year ago, AMS-02 began the data taking operations from the ISS. During this first year, AMS-02 registered 17 billion cosmic ray events, smoothly operating in space under extreme thermal conditions. These events have been used to carefully calibrate the particle detectors in order to fully exploit their sensitivity to search for the most rare events. During this period AMS directly measured high energy cosmic rays from space: for the first time electrons of energy exceeding 1 TeV and positrons with energies exceeding 200 GeV have been recorded before entering the atmosphere. 44) 45)
Figure 22: Schematic view of high energy electrons and positrons observed by AMS-02 (AMS collaboration, Ref. 44)
• In April 2012, AMS-02 is 11 months in orbit since its installation on the ISS. AMS-02 is smoothly collecting Cosmic Rays with energies ranging from 100's of MeV to a several TeV: event number 14.000.000.000 (14 billion) was recorded on March 19, 2012. 46)
• In June 2011, AMS-02 is detecting particles smoothly and continuously: at a rate of about 50 million of cosmic rays/day it has already taken more than one billion of events! While the spectrometer was happily collecting particles in space, the AMS-02 team managed to transfer the POCC (Payload Operation Control Center) from NASA/JSC, Houston to a new building at CERN, Geneva, Switzerland. Since June 27, 2011, AMS-02 is controlled from the newly built POCC at CERN. 47)
• On June 1, 2011, the space shuttle Endeavour landed safely in Cape Canaveral, FLA (touchdown at 6:35 GMT) for the last time, ending the 16-day journey of STS-34 — and beginning its retirement from now on. 48) 49)
• On May 19, 2011, the AMS-02 detectors were activated and started operating smoothly and nominally since the beginnng of its life on ISS. The first event of AMS-02 on the same day was a 20 GeV electron while the second was a 42 GeV Carbon nucleus. The data of AMS-02 is being sent to the POCC (Payload Operation Control Center) at NASA/JSC in Houston, TX. 50)
• On May 19, 2011, the AMS-02 was anchored to the starboard truss of the ISS - the largest and most complex scientific instrument of the ISS. The AMS-02 will sift ~10,000 cosmic-ray hits every minute, looking for nature's best-kept particle secrets. By collecting and measuring vast numbers of cosmic rays and their energies, particle physicists hope to understand more about how and where they are born. 51) 52) 53)
Legend to Figure 24: A fish-eye lens attached to an electronic still camera was used to capture this image of NASA astronaut Michael Fincke (top center) during the fourth EVA of the STS-134 mission as construction and maintenance continue on the International Space Station. The docked space shuttle Endeavour is visible at left. The blackness of space and Earth's horizon provide the backdrop for the scene. The spacewalk on May 27, 2011 of astronauts Michael Fincke and Gregory Chamitoff was the last ever outing in the three decade history of NASA's Space Shuttle Program.
Figure 25: Photo of the installed AMS-02 with astronauts Andrew Feustel (right) and Greg Chamitoff (left middle on the truss) during a 6 hr 19 min EVA on May 20, 2011, (image credit: NASA, ESA, Ref. 49)
AMS-02 Instrumentation (main components - description prior to the permanent magnet swap):
Note: The description of AMS-02 with the SCM (Superconducting Magnet) configuration is kept in this file for better reference and for the description of those components (STD, TOF, TRD, RICH, and ECAL) which remained unchanged despite the swap to a permanent magnet.
The AMS-02 detector is a large acceptance (~0.5 m2 sr) spaceborne magnetic spectrometer designed to perform precise measurements of charged cosmic ray fluxes in a wide energy range (in the rigidity range from 0.5 GeV to 4 TeV). The main components of the detector are shown in Figures 26 and 27: 54) 55) 56) 57) 58) 59) 60) 61)
• A silicon tracker detector (STD) made of 8 double-sided silicon sensor layers, 6 of them contained inside the magnet, will measure the charged particle bending.
• A time of flight system (TOF) consisting of 2 double planes of scintillator counters which is able to reach a precision in the time of around 120 ps
• The anticoincidence counters (ACC) which ensure that only particles passing through the magnet aperture will be accepted.
• A transition radiation detector (TRD) which detects the transition radiation light produced by ultra-relativistic particles in a set of polypropylene fiber radiators by means of 5248 straw tubes operated at high voltage with a mixture of Xe and CO2.
• A ring imaging Cherenkov counter (RICH) will measure the velocity of relativistic particles from its Cherenkov cone opening angle. The Cherenkov radiator consists of a set of aerogel and NaF tiles, while the detection plane is instrumented with 680 R7600-M16 multianode Hamamatsu PMTs (Photomultiplier Tubes).
• An electromagnetic calorimeter (ECAL) that consists of layers of lead foils with glued scintillating fibers resulting into a total radiation depth of 17 Xo for shower development. Note: The Electromagnetic Calorimeter is also abbreviated as EMC.
• A cryogenic superconducting magnet (Cryomag) consisting of an arrangement of 2 main dipole coils and 12 racetrack coils designed to give the maximum field in the perpendicular direction, while minimizing the stray field outside the magnet. The magnetic flux density at the geometric centre of the system is 0.86 TeV.
• A system of two star trackers AST (AMICA Star Tracker) to allow the precise reconstruction of the direction of arrival of high energy gamma rays reconstructed in the detector. The AMICA (Astro Mapper for Instruments Check of Attitude) on AMS-02 is responsible for providing real-time information that is going to be used off-line for compensating the large uncertainties in the ISS flight attitude and the structural degrees of freedom. AST is an evolution of an existing pointing/tracking system flown three times on the Space Shuttle as part of the UVSTAR telescope. 62) 63)
The experimental approach of the AMS-02 mission is to look for antimatter in cosmic rays above the atmosphere (because antimatter annihilates in the atmosphere). The particle identification of cosmic rays relies on precise measurements of the rigidity, velocity, energy and electric charge.
The AMS-02 is an unpressurized, full truss mounted payload on the ISS that will utilize a Cryomag with planes of detectors above, inside and below the magnet. Electrically charged particles that pass through the magnetic field will curve. Charged particles made of matter will curve one way, and those of antimatter will curve the opposite way. The positions of the charged particles will be electronically recorded.
The large data sample collected after the experiment long exposure will allow a sensitive search for antimatter and dark matter signatures in cosmic rays as well as astrophysics studies regarding cosmic ray production, acceleration and propagation in the Galaxy.
Figure 28: The basic idea of the AMS-02 magnetic spectrometer measurement concept (image credit: MIT)
STD (Silicon Tracker Detector):
The STD, also referred to as the "silicon tracker", is the hyper-accurate center of AMS. The STD measures the particle's momentum in the strong magnetic (B) field of the AMS. It detects the trajectory of the incoming particles and identifies the magnitude and polarity of the particles' electrical charge (the higher the particle's momentum, the straighter the arc).
STD consists of 2300 double-sided silicon micro-strip sensors arranged in eight circular layers perpendicular to the magnet axis. STD provides a position resolution of 8.5 µm (30 µm) in the bending (non-bending) plane. The total instrumented surface area is 6.45 m2 with 196 k readout channels (Ref. 54). A laser alignment system is being used to ensure the long term stability of the resolution with position accuracy of better than 4 µm.
Figure 29: Two views of the Silicon Tracker, the right image shows the curvature of a cosmic ray track in the B field (image credit: AMS collaboration)
The 8 layers of silicon are arranged in 5 thin support planes. The external ones are equipped with a single silicon layer whereas the central ones, located inside the magnet volume, mount one silicon layer on each side. The presence of the superconducting magnet requires an active cooling system to evacuate the heat generated by the STD front-end electronics. A laser system, which provides optically generated signals in the 8 STD layers, will monitor the system alignment throughout the mission.
The bending power provided by the magnet and the precise particle trajectory measurements in the STD provide a resolution in the particle rigidity measurement (p/Z) of 1.5% at 10 GeV and a maximum detectable rigidity above 1 TeV for protons. In addition, the wide dynamic range of the tracker readout electronics provides charge separation of nuclei up to Z=26 (by dE/dx measurements). 64) 65)
The TOF Scintillator produces flashes of light when struck by particles or photons. The TOF serves as the fast trigger for the experiment. When an incoming particle crosses the bore of the cryo magnet, the TOF's resolution is sufficient to distinguish between upward and downward traveling particles (120 ps TOF resolution).
The four TOF detector planes are made of scintillator material. This detector measures the transit time of particles between two layers.
Figure 30: Schematic of TOF measurements (image credit: AMS collaboration)
ACC (Anti Coincidence Counter):
The ACC detects and identifies the particles entering or exiting through the side or that have not cleanly traversed the STD. The ACC provides a means of rejecting particles that may confuse the charge determination of the incoming particles that are of interest. 66) 67) 68) 69)
The ACC of AMS-02 surrounds the silicon tracker (STD) and can be used as a veto for the trigger decision made by the TOF. This is important for rejecting events with particles entering the detector from the side or with particles from secondary interactions inside the detector which could distort the charge measurement. To improve existing upper limits on antihelium an inefficiency of the ACC smaller than 10-4 is needed according to MC simulations. The inefficiency is the ratio of missed to the total number of particle tracks crossing the ACC.
A further important task of the ACC is to reduce the trigger rate during periods of very large flux activity, e.g. in the SAA (South Atlantic Anomaly). For this reason, it is important to use a detector with a fast response.
The ACC features a modular design, the cylinder has a diameter of 1.1 m and a height of 0.83 m and is made out of 16 scintillation panels (Bicron BC-414) with a thickness of 8 mm. The ultraviolet scintillation light through ionization losses of charged particles is absorbed by WLS (Wavelength Shifting Fibers) which are embedded into the panels. The WLS fibers are coupled to clear fiber cables for the final light transport to the photomultiplier tubes (Hamamatsu R5946).
A set of two panels is being read out by the same two photomultipliers, one on top and one on the bottom, via clear fiber cables (Y-shape) in order to have redundancy and to save mass (Figure 31, lower part). The slot between two panels is realized with tongue and groove and is crucial for the determination of the inefficiency because of less scintillator material and a smaller active (WLS) to passive (scintillator) material ratio.
The ACC power consumption is limited to 800 mW and the total mass of 54 kg.
Figure 31: The upper figure shows the ACC after integration (left), and the principle of component arrangement (right), the lower figure shows the ACC working principle (image credit: AMS Collaboration)
RICH (Ring-Imaging Cherenkov Detector):
RICH is used to measure the velocity of the charged cosmic particles that traverse the AMS-02. The RICH is able to determine the velocity of charged particles by measuring the vertex angle of the cone of Cherenkov light. The Cherenkov light is emitted as the particle passes through a tile of silica aerogel or sodium fluoride. The light guide material in the Unit Cell assembly is Polymethyl Methacrylate (PMMA), (PlexiglasTM). RICH measures the Cherenkov light using photomultiplier tubes.
RICH is a proximity focusing device with a dual radiator configuration on top made of 92 aerogel tiles of 25 mm thickness; the refractive index is 1.050. In addition, there are sodium fluoride (NaF) tiles with a thickness of 5 mm covering an area of 34 cm x 34 cm. The NaF placement prevents the loss of photons in the hole existing in the center of the readout plane (64 cm x 64 cm), in front of the ECAL device located below. 70) 71)
Figure 32: Schematic view of the RICH instrument (image credit: AMS collaboration)
ECAL (Electromagnetic Calorimeter):
The ECAL takes care of the electromagnetic particle identification, determining the electron energy with a resolution of 3% at 100 GeV. The ECAL measures the energy of electrons, positrons, and gamma rays up to 1 TeV and the direction with an angular resolution around 1º. The instrument has been constructed by an Annecy (France), Beijing (China) and Pisa-Sienna (Italy) collaboration team. 72) 73) 74)
ECAL is an imaging calorimeter consisting of 9 modules made of layers of lead and scintillating fibers. Its function is to completely stop particles. Each module has a 648 mm x 648 mm section and is 18 mm in depth, which corresponds to 1.8 radiation lengths. In two successive modules the fibers are rotated by 90º and follow in the x or y direction. The fibers of a module are being read only at one end of the PMT (Photomultiplier Tube) of Hamamatsu (R7600-00-M4) and placed alternatively on each side. One PMT consists of 4 independent pixels. In this way, the elementary cell of the calorimeter has the size of 648 mm x 9 mm in the x-y directions and 9 mm in the z direction.
Figure 33: ECAL inside the supporting structure (image credit: AMS collaboration)
The major challenge of the PMT and for its front-end electronics is related to the very large dynamic range of the light pulses created in the fibers by cosmic rays. The signal in the PMT ranges from a few photo-electrons for MIP (Minimum Ionizing Particles) to ~ 105 photo-electrons for electromagnetic showers corresponding to very high energy particles (for instance an electron of 1 TeV energy). The power consumption for all ECAL electronics is limited to 100 W.
The ECAL measurement requirements call for:
• .Precise measurement of the EM (Electromagnetic) shower energy from 1 GeV to few TeV
• 3D imaging of the EM and hadronic showers development. EM reconstruction of the shower direction and impact point angular resolution < 1º @ E > 50 GeV
• e/h discrimination from 1 GeV to 1 TeV (rejection power ~ 103)
• Stand-alone γ-trigger: gamma efficiency > 90% above 2 GeV.
Figure 34: Schematic view of the ECAL instrumentation (image credit: AMS collaboration)
The particle identification is done through its electric charge and its mass. The charge is determined by a combined measurement of the deposited energy in the TOF and STD planes and by the Cherenkov light detected in the RICH from Z=1 to Z ≤ 26 with very small charge confusion.
TRD (Transition Radiation Detector):
Transition radiation (TR) consists of soft x-rays which are emitted when charged particles traverse the boundary between two media with different dielectric constants. In the momentum range from 10 to 300 GeV/c, light particles such as electrons and positrons have much higher probability of emitting TR photons than heavy particles such as protons and antiprotons.
The objective of TRD is to distinguish an e+ or p- signal (electron or positron spectra) reducing the p+ or e- background by a rejection factor 10-3 to 10-2 in an energy range from 10-300 GeV. This will be used in conjunction with an electromagnetic calorimeter (ECAL) to provide overall p+rejection of 10-6 at 90% e+ efficiency. TRD identifies particles in addition to the ECAL. 75) 76) 77) 78) 79)
The detector consists of 20 layers of 6 mm diameter straw tubes alternating with 20 mm layers of polyethylene/polypropylene fleece radiator. The tubes are filled with a 80% / 20% mixture of Xe / CO2 at 1.0 bar absolute from a recirculating gas system designed to operate > 3 years in space. The layers are mounted to 0.1 mm precision in a stable carbon fiber composite/aluminum honeycomb octagonal mechanical support. The conical octagon structure has a width from 1.5 m at the bottom to 2.2 m at the top.
In the TRD design, the upper and lower four layers of tubes run in the x direction (oriented parallel to the magnetic field), the middle 12 layers run in the perpendicular y direction, to provide tracking in the bending and non-bending directions (3D tracking) of the 0.8 T superconducting magnet as well as particle identification.
Figure 35: Schematic view of the conical octagon with 328 modules in 20 layers (image credit: AMS Collaboration)
The straw tubes are built as modules of 16 tubes. The modules are arranged in the conical octagon structure, such that the upper and lower 4 layers of tubes run along the B-field direction and the 12 central layers in the perpendicular direction. In all, there are 328 modules, for a total of 5248 straws. The length of the straw modules varies from 0.8 m to 2.0 m. The wall material of the straws is a 72 µm kapton foil and the sense wires are 30 µm gold plated tungsten.
Figure 36: Complete TRD on top of upper magnet flange (image credit: AMS Collaboration)
The DAQ (Data Acquisition) system of the TRD is divided in two parts: The front end electronics which are mounted on the walls of the detector, and the first level of data acquisition which is hosted in two identical crates. The digitization of the signals from the straw tubes is done in the front end electronics. The crates hold the power supplies, the boards which collect and compress the data and the control of the whole TRD DAQ system.
Cryogenic superconducting magnet:
In AMS-02, the charged particle bending, provided by a superconducting magnet, is measured by a silicon tracker. The particle time of flight (TOF), and hence its velocity, is determined by a set of scintillation counter hodoscopes. Additional particle identification capabilities are provided by a TRD (Transition Radiation Detector), a RICH (Ring imaging Cherenkov Detector) and an ECAL (Electromagnetic Calorimeter). Finally, a shell of ACC (Anti Coincidence Counters), covering the inner surface of the magnet, provides a veto for non-fiducial particle trajectories.
The main goal of the superconducting magnet is to extend the energy range of particle and nuclei measurements to the multi-TeV region. The superconducting magnet was developed and built by Scientific Magnetics Ltd. (former Space Cryomagnetics Ltd.), Oxford, UK The cryogenic superconducting magnet consists of an arrangement of 2 main dipole coils and 12 racetrack coils, a superfluid helium vessel and a cryogenic system, all enclosed in an aluminum vacuum tank which serves also as the primary structural support of the experiment.
Supporting electronics, valves and cabling are located outside the vacuum tank. The vacuum tank itself has a toroidal shape with inner diameter 1.1 m, outer diameter 2.7 m and length of the central cylinder surrounding the STD 0.9 m. The geometry of the superconducting magnet defines the acceptance of the detector (0.5 m2 sr). 80) 81)
The magnet operates at a temperature of 1.8 K, cooled by means of the 2500 l of superfluid helium stored in the vessel. The higher latent heat and density of liquid 4He in its superfluid phase increases the total cooling capacity for a fixed helium tank volume. Moreover, the huge thermal conductivity of superfluid helium avoids significant temperature gradients. In zero gravity operation, this prevents temperature stratication which could result in unacceptable gradients across the magnet cold mass. Because of parasitic heat loads, the helium will gradually boil away throughout the lifetime of the experiment.
The dipole coils generate the magnetic field and the return coils control the stray field. The cryo-magnet design ensures an intense and quite uniform field perpendicular to the vertical axis, a negligible dipole moment and the fulfillment of NASA safety regulations concerning fringe magnetic fields on the ISS (Ref. 54).
Figure 37: Photo of the AMS-02 superconducting magnet (image credit: AMS collaboration)
Test preparations of AMS-02:
• Beam test at CERN (Feb. 4-6, 2010): The AMS-02 instrument was integrated at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. The first part of the tests was also conducted at CERN, when the detector was put through its paces using a proton beam from CERN's Super Proton Synchrotron accelerator to check its momentum resolution and its ability to measure particle curvature and momentum.
• Thermal vacuum and electromagnetic compatibility tests: In mid-February 2010, the AMS-02 instrument assembly arrived at ESA/ESTEC in Noordwijk, The Netherlands. The objective is to subject the AMS-02 to final qualification tests, in particular the overall thermal vacuum test. The LSS (Large Space Simulator) at ESTEC uses a huge vacuum chamber, complete with its own bright 'Sun' and walls cooled by liquid helium, to reproduce closely the conditions of space, while various shakers, acoustic chambers and electromagnetic tests are used to mimic conditions during launch and operation in space.
• Interface verification testing at KSC: Once the extensive testing is complete, AMS-02 will leave ESTEC at the end of May on a special US Air Force flight to KSC (Kennedy Space Center) in Florida.
• End-to-end testing on the launch pad: Operation centers at KSC and JSC (Johnson Space Center), Houston.
• Installation, activation and full operations on the Space Station ISS: Operations at JSC (~3 months), then shift to CERN
Figure 38: AMS-02 during integration activities at the CERN facility in Geneva (image credit: AMS collaboration)
Figure 39: Cutaway view of the AMS-02 instrumentation and components configured for the ISS (image credit: AMS collaboration)
• AMS-02 at ESA/ESTEC in Noordwijk: 82)
In March 2010, the AMS-02 payload was tested in the Maxwell Test Chamber at ESA/ESTEC to confirm its electromagnetic compatibility with the Shuttle and ISS systems. This demonstration test was necessary because the powerful magnet bends the path of incoming charged particles on their way through a stack of detectors.
The initial version of the magnet was so powerful, in fact, that it relied on superfluid helium to achieve superconducting temperatures, down to just 1.8º C above absolute zero.
Next came thermal-vacuum testing in ESA's the LSS (Large Space Simulator), to prove the thermal performance of all AMS-02 subsystems in space conditions.
AMS-02 Instrumentation (description after the change to a Permanent Magnet):
The planned extension of ISS operations from 2015 to 2020 and possibly to 2028 has a significant impact on the scientific perspectives of the AMS-02 experiment, which, being designed to search for rare events and to measure weak signals, could gain in sensitivity by a longer data taking in space. Hence, the AMS-02 will fly in the Permanent Magnet (PM) configuration. The SCM (Superconducting Magnet) version would not have had an endurance comparable to the extended lifetime of the ISS. (Ref. 1). 83) 84) 85)
The decision, made in the spring of 2010 - operating the ISS until at least 2020, made again the Permanent Magnet a very interesting option, and a consensus within the AMS Collaboration and NASA was quickly reached.
The AMS-02 PM configuration is technically simpler to operate with respect to the SCM one. There is no need for the Helium tank, for any cryogenic device, etc. There is also some gain in weight and the safety requirements are less stringent. Since the AMS-02 subsystems were originally designed to support both the PM and the SCM options, the two vacuum cases structures are mechanically identical and integration is very similar.
The AMS-02 PM configuration is technically simpler to operate with respect to the SCM one. There is no need for the Helium tank, for any cryogenic device, etc. There is also some gain in weight and the safety requirements are less stringent. Since the AMS-02 subsystems were originally designed to support both the PM and the SCM options, the two vacuum cases structures are mechanically identical and integration is very similar.
In the new PM design, the particles and anti-particles are bent by magnetic fields in opposite directions. A direct measurement of the curvature direction allows the separation between matter/antimatter (left-positive, right-negative). From an experimental point of view is straightforward to say that: the larger is the bending of the particle trajectory, the simpler is the determination of the sign of the curvature.
The trade off in using the PM is the fact that the magnetic field is 5 times weaker than the SCM one. This means that charged particles of the same energy (as the two depicted in the previous figure) are bent less by the PM field. This would limit the matter-to-antimatter separation to lower energies using the PM instead of the SCM.
To cope with this fact, an optimization of the geometry of the AMS STD (Silicon Tracker Detector) can be implemented. The basic idea is to extend the lever arm of the tracking measurement adding measurements above and below AMS-02. With a bigger lever arm also tiny angular deviation of the particle in the magnetic field can be appreciated. With such an "extended" STD, smaller curvatures can be measured. With this technique the project can recover the full sensitivity of AMS-02 on matter-antimatter separation.
An approximate formula for the relative error (the percentage of error) on the rigidity evaluation in the STD, ΔR/R, is given in Figure 40. The term ΔR/R is considered the percentage of error on the curvature determination. When ΔR/R become 100% the curvature becomes so tiny that we cannot separate anymore positive from negative tracks. In both formulas ΔR/R increases – corresponding to a worse accuracy on curvature determination – if the magnetic field is reduced. We can improve the resolution extending the lever arm (L).
The optimization of the STD is obtained adding 2 more planes at the beginning and at the end of the detector, thus extending the lever arm from ~ 1 m to ~ 4 m. These two planes are built reshuffling some "ladders" already existing on the STD, then no new electronics or Silicon ladders are needed.
Figure 41: Illustration of the silicon tracker layers in the new design (right), image credit: AMS collaboration
Legend to Figure 42: The green line is the difference between the blue (PM) and red (SCM) resolutions. At high energies the PM and SCM accuracies are equivalent; at low energies the difference in accuracy is only 10% using 9 tracker planes.
Expected performances of the new design:
As a side effect of the extension of the lever arm, the number of cosmic-rays per second passing through the tracking volume is reduced. This is because longer cylinders – with the same diameter – have narrower angular field-of-views. However the number of CRs (Cosmic Rays) collected in 10 years in the PM scenario is greater than the number of CRs that could be collected in the SCM scenario. The larger number of particles collected increases the probability of discovery very rare events as traces of primordial antimatter or products of Dark Matter annihilation.
As an example the number of positrons collected in the PM configuration is expected to be a factor 2 to 6 greater to the SCM configuration, depending on the energy. Positrons are an important rare component of cosmic-rays. They could be an important marker of the Dark Matter indirect search.
Indeed, an important AMS-02 research topic regards the measurement of positron-to-electron ratio. PAMELA (Payload for AntiMatter Exploration and Light-nuclei Astrophysics) on Resurs-DK1 (launch June 15, 2006) has clearly pointed out an excess of positrons at high energies: a similar effect was previously measured with larger statistical uncertainties by HEAT and AMS-01. The debate on the nature of such an excess is still going on. One possible hypothesis is that it is resulting from Dark Matter (DM) annihilation producing electrons and positrons in the final state.
DM particles are everywhere and interact very weakly with normal matter. Direct detection is pursued at underground laboratories and is very difficult. In the galaxy DM particles could interact and sometime annihilate. Since DM is expected to have a large mass, the energy released in these annihilations is quite large. This energy can be converted in pairs of ordinary matter-antimatter particles (electron-positron, proton-antiproton) having quite an high energy. The excess of positrons in the spectrum seen in PAMELA data could then be explained considering that high energy positrons could be produced by DM annihilations taking place in our galaxy.
From the experimental point of view, in AMS-02 the identification of electrons and positrons against the large background of protons will be done using the TRD (Transition Radiation Detector) and ECAL (Electromagnetic CALorimeter). The new STD configuration will match the acceptance of ECAL, so that all particles passing trough the STD, are also measured by the ECAL maximizing the AMS-02 acceptance and identification power for electrons and positrons.
Figure 43: Photo of the AMS-2 assembly (image credit: AMS Collaboration)
Figure 44: AMS-02, a TeV precision, multipurpose spectrometer - configuration and major components after the change to a permanent magnet (image credit: AMS Collaboration)
Last beam test at CERN:
The new AMS-02 design/development with the permanent magnet was completed in July 2010 and a beam test of the new configuration was conducted at CERN in the period August 9-20, 2010. The test involved systematic calibration using high energy protons, positrons, electrons and pions. The CERN accelerator is able to provide particles and antiparticles up to 400 GeV/c2 of energy. These particles are used to align the various parts of the tracking system to microns level accuracy and to test the particle identification capability of the experiment. Literally thousands of different test beam incident angles and positions have been selected to align the geometry and measure the response of the detectors, collecting hundreds of million of particles which will be later analyzed to define the calibration constants of the experiment, that are all the numerical parameters needed to have accurate expressed values from the different instruments. 86)
After final testing at ESA/ESTEC in Noordwijk, the Netherlands, the AMS-02 assembly was delivered to the Kennedy Space Center (KSC) in Florida on August 26, 2010. 87)
Figure 45: AMS-02 loaded onto the US Air Force C-5 Galaxy aircraft (image credit: ESA)
AMS-02 PIH (Payload Integration Hardware)
The PIH of NASA/JSC provides the required structural, electrical, and C&DH (Command and Data Handling) interfaces between the components of the AMS-02 experiment and the STS for transfer to and from orbit, and the ISS during its on-orbit operational lifetime (Ref. 8). 88)
The USS-02 (Unique Support Structure) is used to support the AMS-02 magnet and detectors and to interface the entire AMS-02 Experiment with the Space Shuttle Orbiter and ISS. The Vacuum Case is an integral part of the USS-02. The USS-02 is comprised of the following five subassemblies: 1) Upper USS-02 Assembly, 2) Vacuum Case Assembly, 3) Lower USS-02 Assembly, 4) Keel Assembly, and 5) passive Payload Attach System (PAS) Assembly/Umbilical Mechanism Assembly (UMA). The USS-02 mechanically attaches to the Space Shuttle Orbiter with four longeron trunnions and one keel trunnion. The AMS-02 payload mechanically attaches to the ISS via the PAS Assembly.
Several AMS-02 experiment components are mounted to the USS-02. These components include: the Transition Radiation Detector (TRD), Time of Flight Scintillator Counters (TOF), Ring Imaging Cherenkov Counter (RICH), Electromagnetic Calorimeter (ECAL), TRD gas supply system, Main Crates/Radiators, RICH electronics boxes, ECAL electronics boxes, Cryomagnet Avionics Box (CAB), Cryomag rectifiers, electrical cables and components of the Thermal Control System (TCS).
In addition, the following hardware, provided by the Space Shuttle Program, will be attached to the USS-02. It includes a Flight Releasable Grapple Fixture (FRGF) and a Remotely Operated Electrical Umbilical (ROEU) Payload Disconnect Assembly (PDA). The ISS Program provided hardware that will be attached to the USS-02 includes a Power Video Grapple Fixture (PVGF), a passive Umbilical Mechanism Assembly (UMA), an External Berthing Camera System (EBCS), a Worksite Interface Fixture (WIF), Side Mount, and nine EVA Handrails.
Figure 46: USS-02 with PAS (Passive Payload Attach System), image credit: NASA/JSC)
Figure 47: Overview of the PIH system (image credit: NASA/JSC)
AMS-02 interfaces to the ISS
• The LRDL communications is based on the MIL-STD-1553B dual serial bus. Monitoring data pass over the bus, through ISS for transmission on the Ku-band via the TDRS satellites to the ground, through NASA centers and the Internet to the AMS-02 POCC (Payload Operations and Control Center) in real time. The available date rate is ~ 10 kbit/s with a duty cycle of 55 - 90%. 10 byte/s is transmitted continuously. The LRDL is also the command path from the POCC to AMS-02. At most one 120 byte command is allowed each second.
• The HRDL communications is based on an ISS specific implementation of the TAXI protocol over fiber optic cables operating at 125 Mbit/s. This data stream contains the event data and a copy of the monitoring data. Within ISS the data is transmitted to the ACOP (AMS Crew Operations Post), where it is archived for eventual retrieval by the shuttle. ACOP also provides crew access to the monitoring data.
The AMS-02 payload and monitoring data will be collected in the crew quarters of ISS by a dedicated computer, referred to as ACOP (AMS Crew Operation Post).The electronics for the readout of subdetectors is being build based on a total of 650 microprocessors (2-to-4 fold redundant design). The data is archived at ACOP for eventual retrieval by the shuttle. ACOP also provides crew access to the monitoring data (Ref. 90).
When the Ku-band is available, data can be replayed from ACOP or streamed live from the experiment to the POCC (Payload Operations Control Center) and to the AMS-02 science operations center. AMS-02 is allocated an orbital average of 2 Mbit/s in downlink. To allow the contingency loading of software into the experiment and as a backup command path, data can be transmitted over the HRDL from ACOP to AMS-02.
Figure 49: Mounting location of the AMS-02 on the ISS structure (image credit: AMS collaboration)
Figure 50: Computer animation of the AMS-02 on the ISS (image credit: AMS collaboration)
Figure 51: Schematic view of cosmic rays (image credit: AMS collaboration, Ref. 34)
AMS-02 Ground Segment
The AMS-02 payload data is communicated to the ground via the White Sands Facility to the GSC (Ground Support Computers) and POIC (Payload Operation Integration Center), both located at NASA/MSFC (Marshall Space Flight Center); Figures 52 and 53.
Figure 53: ISS to Remote AMS Centers Data Flow (image credit: NASA) 91)
For AMS-02, the ground computing system is conceptually divided into AMS Ground Centers: 92)
• GSC (Ground Support Computers) at NASA/MSFC Huntsville AL: Reception of monitoring and science data from POIC; buffer science, housekeeping and NASA ancillary data for transmission to the POCC. Two redundant GSC units will be running at MSFC.
• POCC (Payload Operations and Control Center): where AMS operations take place, including commanding, storage and analysis of housekeeping data and partial science data analysis for rapid quality control and feedback.
• SOC (Science Operations Center): receives and stores all AMS science and housekeeping data, as well as ancillary data from NASA, ensures full science data reconstruction, calibration and alignment; archives all data and keeps data available for physics analysis.
• RC (Regional Centers) contain analysis facilities to support physicists from geographically close AMS universities and laboratories, RC also provide access to the AMS-02 data for visualization, detector verification studies and status of data processing. In addition, RC constitute the processing sites for the AMS-02 Monte Carlo production as well as AMS-02 Data reproductions.
The POCC and SOC are being set up in the basement of building 892 (CERN- Prevessin), close to the AMS-02 detector integration facility. A backup POCC will be temporarily moved to KSC together with the detector itself for the pre-flight tests, then to JSC for the detector activation period. After successful installation on ISS and running-in of the experiment for 2-3 months, the backup POCC at JSC will be cold spared and the CERN POCC will take over. The data transmission and operations issues during JSC phase of the AMS-02 ISS mission were discussed (Ref. 91).
Figure 54: Final version of the AMS-02 data flow (image credit: AMS Collaboration)
The raw data volume transmitted from ISS to ground is estimated to be of 4 Mbit/s on average. In addition to the science data, 1 kbit/s of health and status data are received in real time. The data are buffered at POIC and transmitted in real time to POCC and SOC.
The public internet is being used for all data transmissions between NASA/MSFC and the various AMS-02 centers on a global scale.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).