ISS Utilization: ISS-CREAM (Cosmic Ray Energetics and Mass)
Cosmic rays are energetic particles from outer space. They provide a direct sample of matter from outside the solar system. Measurements have shown that these particles can have energies as high as 105 TeV (or 1017 eV). This is an enormous energy, far beyond and above any energy that can be generated with man-made accelerators, even the Large Hadron Collider at CERN in Geneva, Switzerland.
NASA plans to place CREAM aboard the space station, becoming ISS-CREAM. The instrument has flown six times for a total of 161 days on long-duration balloons circling the South Pole, where Earth's magnetic field lines are essentially vertical. 1) 2) 3) 4)
ISS-CREAM is being developed as an international collaboration, including teams from the United States, Republic of Korea, Mexico and France, led by Professor Eun-Suk Seo of the University of Maryland.
The ISS-CREAM collaboration comprises the following institutions: NASA, USA; JAXA, Japan; University of Maryland, College Park, MD,USA; Penn State University, University Park, PA,USA; Sungkyunkwan University, Suwon, Korea; UNAM (Universidad Nacional Autonoma de Mexico), Mexico; Laboratoire de Physique Subatomique et de Cosmologie, UJF - CNRS/IN2P3, Grenoble, France; Kyungpook National University, Daegu, Korea; Northern Kentucky University, Highland Heights, KY, USA.
Table 1: Some background on cosmic rays
A century-old mystery in astrophysics is the origin of cosmic rays, which are naked atomic nuclei accelerated in deep space that can damage electronics and humans alike. CREAM, which has been highly successful in several long-duration balloon flights, observes features in the energy spectra and populations of cosmic rays and helps establish limits on their acceleration by supernovae. 5)
The origins of cosmic rays and the mechanisms that accelerate them to high speeds are among the oldest questions in modern astrophysics. Results from CREAM bring the science community closer to answering those questions, and build a stronger understanding of the fundamental structure of the universe.
• Cosmic rays reach Earth from far outside the solar system with enormous energies well beyond what man-made accelerators can achieve. Space-based direct measurements of high-energy cosmic rays are difficult because of low particle fluxes in the most interesting regions, and there are no data sets with elemental charge resolution and adequate statistics.
• CREAM has accumulated ~161 days of data during six successful balloon flights over Antarctica. This longest known exposure for a single balloon-borne experiment verified the instrument design and reliability. The key value in residing on the ISS is the long exposure above the atmosphere and orders of magnitude greater statistics without the secondary particle background inherent in balloon experiments.
• One of the key discoveries CREAM made with balloon flights is spectral hardening, which contradicts the traditional view that a simple power law can represent cosmic rays without deviations below the "knee", around 3 x1015 eV, where the cosmic ray spectrum steepens. The current CREAM result provides important constraints on cosmic ray acceleration and propagation models, and it must be accounted for in explanations of the electron anomaly, which generated a lot of excitement in the science community, as well as the media, due to its possible dark matter explanation. CREAM on the ISS would greatly reduce the statistical uncertainties, and extend balloon-borne CREAM measurements, to energies beyond any reach possible with balloon flights. They will provide keys to understanding the origin, acceleration and propagation of cosmic rays.
Researchers are rearranging CREAM's existing hardware so it can attach to the Exposed Facility platform extending from Kibo, the space station's JEM /Kibo (Japanese Experiment Module), after its planned launch in 2015. The space station operates as a platform for instruments like CREAM that otherwise might not fly, due to the expense of dedicated satellites.
The CREAM mission utilizes the proven science instrument designs of the balloon-borne CREAM project to develop an instrument to fly on the U.S. share of the ISS Japanese Experiment Module Exposed Facility (JEM-EF) for a 3-year operation. Its long exposure above the atmosphere offers orders of magnitude greater statistics without the secondary particle background inherent in balloon experiments investigating the origin of cosmic rays. CREAM addresses, specifically, the science objectives of the ACCESS (Advanced Cosmic-ray Composition Experiment for the Space Station) prioritized in the Small Space-Based Initiative category of the 2001 Decadal Study Report "Astronomy and Astrophysics in the New Millennium".
Precise CREAM measurements of energy spectra of individual nuclei over the proton-to-iron elemental range, from 1012 eV to >1015 eV, address the long-standing fundamental science questions:
• Do supernovae really supply the bulk of cosmic rays?
• What is the history of cosmic rays in the Galaxy?
• Can the energy spectra of cosmic rays result from a single mechanism?
• What is the origin of the steepening ("knee") around 3 x 1015 eV in the cosmic ray all-particle energy spectrum?
These questions have been difficult to answer because no space mission capable of measuring the low fluxes of particle at energies approaching the cosmic ray "knee" has yet been flown. The CREAM mission is the first experiment to have the statistics needed to pursue them effectively.
The CREAM instrument consists of complementary and redundant particle detectors. An ionization calorimeter determines the energy of the cosmic ray particles, provide tracking, and event trigger. Silicon charge detectors provide precise charge measurements. Top/bottom counting detectors provide shower profiles for electron/hadron separation. The BSD (Boronated Scintillator Detector) provides additional electron/hadron discrimination using thermal neutrons produced by particles that interact within the calorimeter.
The objective of the CREAM instrument is to:
1) Determine how the observed spectral differences of protons and heavier nuclei evolve at higher energies approaching the "knee".
2) Measure the potential changes in the spectra of secondary nuclei resulting from the interactions of primary cosmic rays with the interstellar medium.
3) Conduct a sensitive search for spectral features, such as a bend in the proton spectrum.
The CREAM instrument is configured with the CREAM calorimeter including carbon targets for energy measurements and four layers of a finely segmented SCD (Silicon Charge Detector) for charge measurements. These detectors have already demonstrated their capabilities to determine the charge and energy of high-energy cosmic rays from 1010 to >1014 eV for the proton to iron elemental range with excellent resolution.
In addition, two new compact detectors are being developed: TCD/BCD (Top/Bottom Counting Detectors) and BSD (Boronated Scintillator Detector). The TCD and BCD each consists of a plastic scintillator and 400 photodiodes. As shown in Figure 2, the TCD is located between the instrument's carbon target and the calorimeter, and the BCD is located below the calorimeter. These detectors provide the capability for electron separation from protons, a redundant energy trigger for the calorimeter, and a cosmic ray trigger for test and calibration on the ground. 6) 7) 8)
Figure 1: Illustration of the CREAM instrument (image credit: NASA)
Legend to Figure 1:
- FRGF (Flight-Releasable Grapple Fixture): a passive fixture which is identical to the PDGF (Power Data Grapple Fixture) but lacks its power and data ports.
- PIU (Payload Interface Unit)
- SCD (Silicon Charge Detector)
- BCD (Bottom Counting Detector)
ISS-CREAM instrument accommodation parameters:
- Instrument mass: ~ 1300 kg
- Instrument size: 1.85 m (L) x 1 m (H) x 0.95 m (W)
- Power consumption: ~ 600 W
- Nominal data rate: ~ 350 kbit/s.
The hadron rejection power derived from the e/p shower shape difference can be significantly enhanced by making use of the thermal neutron activity at late (>400 ns) times relative to the start of the shower. Hadron-induced showers tend to be accompanied by significantly more neutron activity than electromagnetic showers. The ISS-CREAM BSD measures this late thermal neutron shower activity by detecting the boron capture of these thermal neutrons in a boron-loaded plastic scintillator (5% boron concentration by weight and the natural 10B abundance of 20%) located below the BCD under the calorimeter.
Figure 2: Exploded view of the ISS-CREAM instrument (image credit: ISS-CREAM collaboration)
Photodiode detectors TCD (Top/Bottom Counting Detectors) and BSD (Boronated Scintillator Detector): 9)
• Electron/proton separation for electron and gamma-ray physics occurs by using the difference between electromagnetic and hadronic showers.
• Provide redundant trigger for ISS-CREAM calorimeter and MIP (Minimum Ionizing Particle) trigger for calibration.
• The silicon photodiode converts scintillation light to electric current, and electronhole pairs are also produced by penetrating cosmic rays.
• The charge signals are amplified by VLSI (Very Large Scale Integration) charge amp/hold circuits (VA-TA).
Figure 3: Operational principle of the TCD and BCD (image credit: ISS-CREAM collaboration)
Figure 4: Schematic diagram of the TCD and BCD (image credit: ISS-CREAM collaboration)
Current results and expected performance:
One of the key results from the ongoing analysis of CREAM data is an observed spectral hardening for each element above ~200 GeV/nucleon, indicating a departure from a single power law. Proton and helium spectra in the energy range from 2.5 to 250 TeV are represented by power-law fits with spectral indices of -2.66± 0.02 and -2.58 ± 0.02 for protons and helium, respectively. Both spectra are harder than lower energy data from previous experiments, e.g., the AMS (Alpha Magnet Spectrometer) spectral indices of -2.78 ± 0.009 for protons and -2.74 ± 0.01 for helium. A broken power law fit for C, O, Ne, Mg, Si, and Fe with spectral indices γ1 and γ2, respectively, below and above 200 GeV/nucleon, resulted in γ1 = -2.77 ± 0.03 and γ2 = -2.56 ±0.04. As shown in Figure 5, the spectral index γ1 is consistent with the low energy helium measurements, e.g., the AMS index of -2.74± 0.01, whereas γ2 agrees remarkably well with the CREAM helium index of -2.58 ± 0.02 at higher energies.
A hardening of proton and Helium spectra around 240 GV, similar to the spectral hardening first reported by CREAM, has also been reported by PAMELA (Payload for AntiMatter Exploration and Light-nuclei Astrophysics), flown on Resurs-DK1 (launch June 15, 2006), using a permanent magnet spectrometer with a variety of detectors. The experimental uncertainties are too large to debate the exact starting point of the hardening, whether it is 240 GV or 200 GeV/nucleon. The exact cause of the spectral hardening is still under investigation, although a number of possible explanations of these results have been proposed. The hardening may result from modification of gas flow in the shock precursor by the cosmic ray pressure, which shapes the concave energy spectrum of cosmic rays. Alternatively, the observed hardening could be due to nearby sources, as suggested for the recent observations of an enhanced high-energy electron spectrum. A multi-source model by Zatsepin and Sokolskaya considered novae stars and explosions in super-bubbles as additional cosmic ray sources. Whether it results from a nearby isolated SNR (Signal- to-Noise Ratio) or the effect of distributed acceleration by multiple remnants embedded in a turbulent stellar association is another question.
Legend to Figure 5: Data from previous experiments include BESS (open squares), ATIC-2 (open diamonds), JACEE (X), and RUNJOB (open inverted triangles). Some of the overlapping BESS and AMS data points are not shown to achieve better clarity. The lines for helium represent a power-law fit to AMS (open stars) and CREAM (filled circles), respectively. The lines for C-Fe data represent a broken power-law fit to the CREAM heavy nuclei data: Carbon (open circles), Oxygen (filled squares), Neon (open crosses), Magnesium (open triangles), Silicon (filled diamonds), and Iron (asterisks).
Whatever the explanation, the CREAM results contradict the traditional view that a simple power law can represent cosmic rays without deviations below the "knee" around 3 x1015 eV. The pervasive discrepant hardening in all of the observed elemental spectra provides important constraints on cosmic ray acceleration and propagation models, and it must be accounted for in explanations of the electron anomaly and cosmic ray "knee". Donato & Serpico reported that the spectral hardening reported by CREAM would lead to appreciable modifications for the secondary yields, such as antiprotons and diffuse gamma rays, in the sub-TeV range. They concluded that using a simple power law to model the astrophysical background for indirect dark matter searches, as often done in the literature, might lead to wrong conclusions about the evidence of a signal. Or, if a signal should be detected, use of a power law could lead to bias in the inferred values of the parameters describing the new phenomena. Yuan and Bi have demonstrated how tension between the AMS positron fraction and the total electron (including positron) spectra detected by Fermi and HESS can be removed by taking a harder primary electron spectrum at high energies, similar to the nuclei spectral hardening, for either pulsar or dark matter annihilation/decay scenario as the primary positron sources.
CREAM has pushed direct spectral measurements of nuclei, including the important secondary elements (e.g., boron), to ever-higher energies with Antarctic LDB (Long Duration Balloon) experiments. For primary element spectra, the energy region around 1015 eV is challenging to explore, because direct measurements run out of statistics at such high energies. Indirect ground-based measurements cannot resolve individual elements, and they encounter systematic problems caused by uncertainties in modeling hadronic interactions in the atmosphere.
ISS-CREAM can take the next major step to 1015 eV, and beyond. A 3-year exposure on the ISS would greatly reduce the statistical uncertainties and extend the CREAM measurements to energies beyond any reach possible with balloon flights, as illustrated in Figure 5. Being above the atmosphere, ISS-CREAM would be far superior to multiple balloon flights.
Status and plan:
The CREAM instrument is being reconfigured for accommodation on NASA's share of the JEM-EF (Japanese Experiment Module - Exposed Facility) for at least an order of magnitude increase in the exposure factor. The scope of work required for the ISS investigation includes modification of instrument components for the ISS environment, in addition to assessing safety and mission assurance concerns. The instrument must be functionally tested and qualified to meet the launch vehicle and on-station requirements for operations on the ISS. The instrument needs to be repackaged within a structure that meets the JEM-EF interface requirements.
The basic design of the instrument is mature, and it has heritage operating over many years in the near-space environment. The radiation effects on electronic circuits need to be adequately addressed for ISS-CREAM. Components are selected and utilized in a manner to prevent the possibility of failures as a result of SEL (Single Event Latchup), and to assure that SEU (Single Event Upset) and SET (Single Event Transient) effects will have minimal impact on data collection. The issue of SEU could result in occasional corrupted data, and relatively infrequent reboots of the computer. The power supplies were designed with overcurrent trip circuits in the power distribution sections to rapidly remove power from any subsystem that exhibits a high current condition, which might be caused by a SEL. The instruments parts and components were evaluated for any destructive SEL failures by the Radiation Effects and Analysis Group at GSFC (Goddard Space Flight Center).
Replacement parts used to mitigate effects of space (e.g., radiation) were taken from NASA-approved parts lists and/or are undergoing rigorous environmental tests. Where the design includes FPGAs (Field-Programmable Gate Arrays), the control logic is being modified to use triple mode redundancy to mitigate errors caused by SEUs. Related software updates are being made, and development testing was conducted at the NASA/MSFC (Marshall Space Flight Center) in the Spring of 2013. The actual C&DH (Command and Data Handling) setup on the ISS was simulated by connecting the ISS-CREAM Science Flight Computer to the Payload Rack Checkout Unit. During the testing, reliable flow of commands and telemetry between MSFC and the Science Operation Center at the University of Maryland was established.
The ISS Program Office at NASA/JSC (Johnson Space Center) completed an ISS and launch vehicle accommodation study for ISS-CREAM. The ISS-CREAM payload is about the size of a refrigerator with a mass of ~1,300 kg, including government furnished equipment such as grapple fixtures and a PIU (Payload Interface Unit). The estimated ~600 W power and nominal data rate of 350 kbit/s are all within the available JEM-EF resources. ISS-CREAM utilizes an active TCS (Thermal Control System), a fluor-inert fluid loop, provided by the JEM-EF through the standard PIU. Detailed thermal analyses of the ISS-CREAM payload are being performed.
ISS-CREAM is in its implementation phase to complete the detailed design, component fabrication, integration and testing of the fully integrated CREAM payload. As done for the ULDB (Ultra Long Duration Balloon) system flights, NASA/GSFC Wallops Flight Facility (WFF) is providing project management and engineering support for ISS-CREAM. Following environmental testing, the payload will be delivered to KSC (Kennedy Space Center).
Launch: The ISS-CREAM payload was delivered to NASA/KSC ( Kennedy Space Center). A launch of the ISS-CREAM instrument is scheduled for April 2017 on the SpaceX CRS-12 service flight to the ISS. The launch site is Cape Canaveral, FL. 10)
Orbit: Near-circular orbit, altitude of ~400 km, inclination =51.6º.
The CREAM instrument requires zenith viewing for optimal science results. The payload interface to JEM-EF (External Facility) is via the PIU (Payload Interface Unit), it utilizes the standard ISS resources via JEM-EF. There are no scientific samples that require preservation. The product of this payload is digital data. CREAM does not have a gimbal system and does not require off-nominal ISS attitudes. The payload should be notified if the ISS attitude is changed from the nominal XVV (X-axis in the Velocity Vector) attitude. There is no crew display associated with this payload. The payload will be operated continuously after initial check-out and on-orbit commissioning. The CREAM Investigation team continuously monitors payload health, status, and science telemetry. Crew training is not required for CREAM since there is no crew involvement for on-orbit operations. The nominal CREAM data rate is ~350 kbit/s. There are no safety critical on-orbit operations associated with this payload. There is no deployed payload equipment. There is no rotating equipment.
Operational protocols: The CREAM nominal flight operations are conducted after the payload is installed on JEM-EF. The flight operations are conducted and supported by three ground-based organizations:
- CREAM Project SOC (Science Operations Center) at the University of Maryland
- The POIC (Payload Operations and Integration Center) at NASA/MSFC
- The JAXA SSIPC (Space Station Integration and Promotion Center) at TKSC (Tsukuba Space Center). There is no on-orbit crew involvement for CREAM nominal operations. The crew may be involved in EPO (Education and Public Outreach) events related to CREAM, but, that is to be determined in future payload planning sessions, and is not confirmed at this time.
The data interface is Ethernet and 1553. CREAM uses Software Toolkit for Ethernet Lab-Like Architecture (STELLA) to comply with the CCSDS (Consultative Committee for Space Data Systems) protocol requirements as specified in the documents SSP (Space Station Program) 52050 and CCSDS-102.0-B-5. The CREAM software is designed to interface with the HOSC (Huntsville Operations Support Center Software) to retrieve the data from the payload and send commands in real-time.
Figure 6: ISS-CREAM data flow scheme (image credit: University of Maryland)
1) Dave Dooling, "ISS-CREAM to Tackle Century-Old Space Mystery," NASA, May 30, 2013, URL: http://www.nasa.gov/mission_pages/station/research/news/iss_cream.html
3) E. S. Seo, T. Anderson, D. Angelaszek, S. J. Baek, J. Baylon, M. Buénerd, N. B. Conklin, M. Copley, S. Coutu, L. Derome, L. Eraud, M. Gupta, J. H. Han, H. G. Huh, Y. S. Hwang, H. J. Hyun, I. S. Jeong, D. H. Kah, K. H. Kang, H. J. Kim, K. C. Kim, M. H. Kim, K. Kwashnak, J. Lee, M. H. Lee, J. Link, L. Lutz, A. Malinin, A. Menchaca-Rocha, J. W. Mitchell, S. Nutter, O. Ofoha, H. Park, I. H. Park, J. M. Park, P. Patterson, J. Wu, Y. S. Yoon, "Cosmic Ray Energetics And Mass for the International Space Station (ISS-CREAM)," 33rd International Cosmic Ray Conference (ICRC), Rio de Janeiro, Brazil, July 2-9, 2013, URL: http://www.cbpf.br/~icrc2013/papers/icrc2013-0629.pdf
4) "ISS-CREAM to Tackle Century-Old Space Mystery," NASA, May 30, 2013, URL: http://www.nasa-usa.de/mission_pages/station/research/news/iss_cream.html
5) Eun-Suk Seo, "Cosmic Ray Energetics and Mass (CREAM)," NASA, May 6, 2013, update: Jan. 1, 2015, URL: http://www.nasa.gov/mission_pages/station/research/experiments/1114.html
6) J. M. Park, T. Anderson, D. Angelaszek, J. B. Bae, S. J. Baek, J. Baylon, M. Copley, S. Coutu, M. Gupta, J. H. Han, H. G. Huh, Y. S. Hwang, H. J. Hyun, I. S. Jeong, D. H. Kah, K. H. Kang, H. J. Kim, K. C. Kim, K. Kwashnak, J. Lee, M. H Lee, J. T. Link, L. Lutz, A. Malinin, A. Menchaca-Rocha, J. W. Mitchell, S. Nutter, O. Ofoha, H. Park, I. H. Park, P. Patterson, E. S. Seo, J. Wu, Y. S. Yoon, "Results of Tests and Simulations for the Top Counting Detector and Bottom Counting Detector of the ISS-CREAM Experiment," 33rd International Cosmic Ray Conference (ICRC), Rio de Janeiro, Brazil, July 2-9, 2013, paper: 1015, URL: http://www.cbpf.br/~icrc2013/papers/icrc2013-1015.pdf
7) T. Anderson, D. Angelaszek, J. Baylon. M. Copley, S. Coutu, M. Gupta, J. H. Han, H. G. Huh, Y. S. Hwang, H. J. Hyun, H. J. Kim, K. C. Kim, K. Kwashnak, M. H. Lee, J. T. Link, L. Lutz, A. Malinin, A. Menchaca-Rocha, J. Mitchell, S. Nutter, O. Ofoha, J. M. Park, P. Patterson, E. S. Seo, J. Wu, Y. S. Yoon, "The ISS-CREAM Boronated Scintillator Detector," 33rd International Cosmic Ray Conference (ICRC), Rio de Janeiro, Brazil, July 2-9, 2013, paper: 0350, URL: http://www.cbpf.br/~icrc2013/papers/icrc2013-0350.pdf
8) Eun-Suk Seo, "Recent Discoveries of Cosmic Ray Anomalies," University of Virginia Physics Colloquium, November 15, 2013, URL: http://www.phys.virginia.edu/Announcements/Seminars/Slides/S2634.pdf
9) H. J. Hyun, T. Anderson, D. Angelaszek, J. B. Bae, S. J. Baek, M. Copley, S. Coutu, J. H. Han, H. G. Huh, Y. S. Hwang, D. H. Kah, K. H. Kang, H. J. Kim, K. C. Kim, K. Kwashnak, J. Lee, M. H. Lee, J. T. Link, L. Lutz, J. W. Mitchell, S. Nutter, O. Ofoha, H. Park, I. H. Park, P. Patterson, E. S. Seo, J. Wu, Y. S. Yoon, "Performances of the Photo-Diode Detectors for the T/BCD in the ISS-CREAM Experiment," 7th International Conference on New Developments In Photodetection (NDIP), Tours, France, June 30-July 4, 2014, URL: http://ndip.in2p3.fr/ndip14/AGENDA/AGENDA-by-DAY/Presentations/Posters/Session_blanc/ID34665_Hyun.pdf
10) Information provided by Nicholas Picot-Clemente of the University of Maryland.
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).