DAMPE (Dark Matter Particle Explorer) - nicknamed Wukong
The DAMPE mission is one of the scientific space science missions within the framework of the Strategic Pioneer Program on Space Science of CAS (Chinese Academy of Science).The main scientific objective of DAMPE is to detect electrons and photons in the range of 5 GeV–10 TeV with unprecedented energy resolution (1.5% at 100 GeV) in order to identify possible DM (Dark Matter) signatures. DAMPE is the first of four purely scientific satellites that add a new dimension to China's space efforts, which until now were strongly focused on engineering and applications.
The public (global campaign) gave DAMPE its nickname, Wukong after the Monkey King, who is the hero in the classic Chinese tale, "Journey to the West". Literally, "wu" means comprehension or understanding and "kong" means space, so "Wukong" the satellite has a mission to "understand the space," according to the NSSC (National Space Science Center). The Wukong mission is part of an outreach drive in China's space program. Wukong is also notable for being the first in a series of five space-science missions to emerge from the CAS (Chinese Academy of Sciences) ' Strategic Priority Program on Space Science, which kicked off in 2011.
The DAMPE research mission is the result of an international collaboration of institutions from China, Switzerland and Italy as well as a number of universities and particle physics institutions including support from CERN. The satellite finished its initial design phase in 2011 and was approved for construction for a late 2015 launch date. The DAMPE collaboration comprises four institutes under CAS, including the NSSC in Beijing; also involved are the University of Science and Technology of China in Hefei, the University of Geneva, Switzerland, and the Italian universities in Bari, Lecce, and Perugia. 1) 2) 3)
The research goal is to study electrons and gamma-rays, to examine the cosmic ray spectrum and to conduct high-energy gamma-ray astronomy. Dark matter is a hypothesized form of matter that is necessary to account for gravitational effects seen in extremely large structures, e.g. anomalies in the rotation of galaxies and gravitational lensing of light by galaxy clusters which can not be explained by the quantity of the observed matter alone.
Furthermore, DAMPE aims to extend the energy range of space-based particle detectors to the TeV (Tera electronVolt, 1012) region, also providing a higher energy resolution. DAMPE acts as a follow on mission or extension to the Fermi and AMS-02 missions and complements measurements from the CALET instrument deployed on the outside of the International Space Station in 2015. The study of the anisotropy of energetic particles in different energy ranges can deliver valuable insight in the nature of cosmic ray sources. Generating spectra of cosmic particles at lower energies and also provide information on the propagation and acceleration of cosmic rays.
Dark matter is believed to make up most of the matter in the universe. But it has never been detected directly; its existence is inferred from observed gravitational effects on visible matter and the structure of the universe. DAMPE is designed to observe the incoming direction, energy, and electric charge of extremely high-energy photons and electrons that result when dark matter candidate particles, called WIMPs (Weakly Interacting Massive Particles), annihilate. The satellite's payload is made up of a stack of thin criss-crossed strip detectors tuned to catch signals, created by photons and electrons as well as gamma rays and cosmic rays.
Hints of the true nature of dark matter have already emerged from some previous observations, including those conducted by the AMS -2 (Alpha Magnetic Spectrometer-2) onboard the ISS (International Space Station) and by the LHC (Large Hadron Collider) at the CERN physics research center near Geneva, Switzerland.
Figure 1: Artist's rendition of the DAMPE spacecraft in orbit (image credit: CAS)
DAMPE is an astrophysics observatory with a mass of about 1900 kg and a design life of 3 years (goal of 5 years). The spacecraft is outfitted with two-power generating solar arrays providing a total power of around 850 W. The satellite, using a combination of aluminum and carbon fiber reinforced plastic for its structural system, hosts a total payload mass of approximately 1,480 kg. About 12 GB of data are being downlinked/day. 4)
Figure 2: Photo of the DAMPE spacecraft (image credit: INFN/CAS/DAMPE collaboration)
Orbit: Sun-synchronous orbit, altitude = 500 km, inclination = 97.4º, period of 90 minutes.
• December 19, 2018: China's Dark Matter Particle Explorer, nicknamed "Wukong" or "Monkey King," will extend its service in space by two years, as it is still in good condition and collecting key scientific data. 7)
- The research team operating the satellite said Monday (17 December) that Wukong's key performance indicators have barely changed compared with three years ago when it was launched as China's first dark matter probe satellite.
- On 17 December, the satellite has reached its expected service life of three years, having orbited the earth 16,597 times in a sun-synchronous orbit at an altitude of 500 kilometers, detecting around 5.5 billion cosmic particles.
- "We hope that Wukong's 'sharp eyes' will detect 300 electrons that are obviously different from the normal energy spectrum by the end of 2019, which will provide theorists with sufficient data to study the nature of the electrons," said Chang Jin, chief scientist of the team.
- Chang said the research team is quite confident about the satellite working another two years in space.
- China launched Wukong at the end of 2015 to detect the high-energy electrons and gamma rays in space, which might be generated in the process of annihilation or decay of dark matter.
- The satellite's original objectives have been completed with some results exceeding expectations, according to the team.
Figure 3: Illustration of the deployed Wukong satellite (image credit: China Daily)
• November 30, 2017: The DAMPE mission published its first scientific results on Nov. 30 in Nature, presenting the precise measurement of cosmic ray electron flux, especially a spectral break at ~0.9 TeV. The data may shed light on the annihilation or decay of particle dark matter. 8) 9)
Figure 4: The electron plus positron spectrum measured by DAMPE (image credit: DAMPE collaboration)
- DAMPE is a collaboration of more than a hundred scientists, technicians and students at nine institutes in China, Switzerland and Italy, under the leadership of the PMO (Purple Mountain Observatory) of the CAS (Chinese Academy of Sciences). The DAMPE mission is funded by the strategic priority science and technology projects in space science of CAS.
- DAMPE, China's first astronomical satellite, was launched from China's Jiuquan Satellite Launch Center into sun-synchronous orbit on 17 Dec. 2015. At an altitude of about 500 km, DAMPE has been collecting data since a week after its launch.
- In its first 530 days of science operation through June 8 of this year, DAMPE has detected 1.5 million cosmic ray electrons and positrons above 25 GeV. The electron and positron data are characterized by unprecedentedly high energy resolution and low particle background contamination.
- Figure 5 shows the first published results in the energy range from 25 GeV to 4.6 TeV. The spectral data in the energy range of 55 GeV-2.63 TeV strongly prefer a smoothly broken power-law model to a single power-law model.
Figure 5: A comparison of electron plus positron spectrum measured by DAMPE with other published measurements (image credit: DAMPE collaboration)
- DAMPE has directly detected a spectral break at ~0.9 TeV, with the spectral index changing from ~3.1 to ~3.9. The precise measurement of the cosmic ray electron and positron spectrum, in particular the flux declination at TeV energies, considerably narrows the parameter space of models such as nearby pulsars, supernova remnants, and/or candidates for particle dark matter that were proposed to account for the "positron anomaly" revealed previously by PAMELA and AMS-02, according to FAN Yizhong, deputy chief designer of DAMPE's scientific application system.
- "Together with data from the cosmic microwave background experiments, high energy gamma-ray measurements, and other astronomical telescopes, the DAMPE data may help to ultimately clarify the connection between the positron anomaly and the annihilation or decay of particle dark matter," said FAN.
- The data also hint at the presence of spectral structure between 1 and 2 TeV energies—a possible result of nearby cosmic ray sources or exotic physical processes. Yet, more data are definitely required to explore this phenomenon.
- DAMPE has recorded over 3.5 billion cosmic ray events, with maximum event energies exceeding ~100 trillion electron volts (TeV). DAMPE is expected to record more than 10 billion cosmic ray events over its useful life—projected to exceed five years given the current state of its instruments.
• September 2016: DAMPE has a large geometric factor (~ 0.3 m2 sr) and provides good tracking, colorimetric and charge measurements for electrons, gammas rays and nuclei. This will allow precise measurement of cosmic ray spectra from tens of GeV up to about 100 TeV . In particular, the energy region between 1-100 TeV will be explored with higher precision compared to previous experiments. The various sub-detectors allow an efficient identification of the electron signal over the large (mainly proton-induced) background. As a result, the all-electron spectrum will be measured with excellent resolution from few GeV up to few TeV , thus giving the opportunity to identify possible contribution of nearby sources. 10)
- First on-orbit data and performances: The DAMPE detectors started to take physics data very soon after the launch. The performance parameters (temperature, noise, spatial resolution, efficiency) are very stable and very close to what is expected. The absolute calorimeter energy measurement has been checked by using the geomagnetic cut-off, its results are well calibrated. Also, the absolute pointing has been successfully verified. The photon-data collected in 165 days were enough to draw a preliminary high-energy sky-map where the main gamma-ray sources are visible in the proper positions.
- The energy released in the PSD allows to measure the charge and to distinguish the different nuclei in the CR (Cosmic Ray) flux. Figure 6 shows the result of this measurement for the full range up to iron (1 ≤ Z ≤ 26).
Figure 6: Very preliminary Z measurement up to iron with only 10 days of data (image credit: DAMPE collaboration)
- The DAMPE detector is expected to work for more than 3 years. This data-taking time is sufficient to investigate deeply many open questions in CR studies. In Figure 7 the possible DAMPE measurement of the all electron spectrum in 3 years is shown. The energy range is so large to observe a cut-off and a new increase of the flux due to nearby astrophysical sources, if present.
Figure 7: All-electron spectrum. The red dots represent the possible DAMPE measurements in 3 years assuming the power law suggested by the AMS-02 experiment, a cut-off at ~ 1 TeV and nearby astrophysical sources (image credit: DAMPE collaboration)
- In summary, the DAMPE program foresees important measurements on the CR flux and chemical composition, electron and diffuse gamma-ray spectra and anisotropies, gamma astronomy and possible dark matter signatures. This challenging program is based on the outstanding DAMPE features: the large acceptance (0.3 m2 sr), the "deep" calorimeter (32 X0), the precise tracking and the redundant measurement techniques (Ref. 10).
• March 18, 2016: China's first dark-matter detection satellite has completed three months of in-orbit testing, with initial findings expected to appear before the end of the year, according to CAS (Chinese Academy of Sciences). The DAMPE operations was handed over to the CAS/PMO (Purple Mountain Observatory) in Nanjing on March 17. 11) — Hence, the DAMPE mission started nominal operations as of March 18, 2016. Wukong will scan space in all directions in the first two years and then focus on sections in the sky where dark matter is most likely to be observed in the third year. 12)
- The DAMPE (Dark Matter Particle Explorer) satellite "Wukong" detected 460 million high energy particles in a 92-day flight, sending about 2.4 TB of raw data back to Earth, according to DAMPE chief scientist Chang Jin.
- The four major parts of the payload - a plastic scintillator array detector, a silicon array detector, a BGO calorimeter, and a neutron detector - functioned satisfactorily. The satellite completed all set tests, with all its technical indicators reaching or exceeding expectations.
• On December 24, 2015 DAMPE sent its batch of scientific data. The data was received at the Miyun Station under the RSGS (Remote Sensing Satellite Ground Station), and real-time transmitted to the NSSC (National Space Science Center of CAS in Beijing. 13)
- So far the experts are satisfied with results of preliminary analysis of data and have concluded that the payload is functioning properly as per expectations and they are not anticipating any problems.
- The satellite was launched at 00:12 UTC on 17 December 2015. The first-pass X-band data from the DAMPE was received by the Kashgar station in western China at 16:45 UTC. The data validation from the NSSC reveals that the DAMPE data was received in proper format and high quality, marking a sound operation of the satellite-to-ground data transmission network.
Sensor complement(PSD, STK, BGO, NUD)
DAMPE is a powerful space telescope for high energy gamma-ray, electron and cosmic rays detection. It consists of a double layer of plastic scintillator strips detector (PSD) that serves as anti-coincidence detector, followed by silicon-tungsten tracker-converter (STK), which is made of 6 tracking double layers; each consists of two layers of single-sided silicon strip detectors measuring the two orthogonal views perpendicular to the pointing direction of the apparatus. Three layers of Tungsten plates with thickness of 1mm are inserted in front of tracking layer 2, 3 and 4 for photon conversion. The STK is followed by an imaging calorimeter of about 31 radiation lengths thickness, made up of 14 layers of Bismuth Germanium Oxide (BGO) bars in a hodoscopic arrangement. A layer of neutron detectors is added to the bottom of the calorimeter. The total thickness of the BGO and the STK correspond to about 33 radiation lengths, making it the deepest calorimeter ever used in space. Finally, in order to detect delayed neutron resulting from hadron shower and to improve the electron/proton separation power a neutron detector (NUD) is placed just below the calorimeter. The NUD consists of 16, 1 cm thick, boron-doped plastic scintillator plates of 19.5 x 19.5 cm2 large, each read out by a photomultiplier (Ref. 3). 14) 15)
Figure 8: Cross-section of the DAMPE payload (image credit: CAS/NSSC)
PSD (Plastic Scintillator Detector)
The PSD is used to identify electrons and gamma rays. Simultaneously, as the back-up of the STK, it is also used to discriminate heavy ion species by measuring the energy loss of incident particles in the PSD.
The PSD instrument consists of one double layer (one x and one y) of scintillating strips detector made of scintillating strips of 1 cm thick, 2.8 cm wide and 82 cm long. The strips are staggered by 0.8 cm in a layer, thus fully covers an area of 82 cm by 82 cm. The PSD serves as anti-coincidence detector for photon identification, as well as charge detector for cosmic rays. The design specification is a position resolution of 6 mm, and a charge resolution of 0.25 for Z = 1 to 20.
STK (Silicon-Tungsten Tracker)
The STK, which improves the tracking and photon detection capability of DAMPE greatly, was proposed and designed by the European team and was constructed in Europe, in collaboration with CAS/IHEP (Institute of High Energy Physics), in a record time of two years. DAMPE became a CERN-recognized experiment in March 2014 and has profited greatly from the CERN test-beam facilities, both in the Proton Synchrotron and the Super Proton Synchrotron. In fact, CERN provided more than 60 days of beam from July 2012 to December 2015, allowing DAMPE to calibrate its detector extensively with various types of particles, with energy raging from 1 to 400 GeV. 16)
• DPNC (Département de physique nucléaire et corpusculaire), University of Geneva, Switzerland
• INFN (Istituto Nazionale di Fisica Nucleare) and University of Perugia, Italy
• IHEP (Institute of High Energy Physics), CAS, Beijing, China
• INFN (Istituto Nazionale di Fisica Nucleare) and University of Bari, Italy
• INFN (Istituto Nazionale di Fisica Nucleare) and University of Lecce, Italy.
STK consists of multiple layers of silicon micro-strip detectors interleaved with Tungsten converter plates. The principal purpose of the STK is to measure the incidence direction of high energy cosmic rays, in particular gamma rays, as well as the charge of charged cosmic rays. The STK identifies gamma rays by their conversions to charged particles in tungsten plates and infer their incident direction by the measuring with great precision the path of the charged particles within the STK.
The STK is made of 6 tracking planes each consists of two layers of single-sided silicon strip detectors measuring the two orthogonal views perpendicular to the pointing direction of the apparatus. Three layers of tungsten plates of 1 mm thick are inserted in front of tracking layer 2, 3 and 4 for photon conversion. The STK uses single-sided AC-coupled silicon micro-strip detectors. The sensor is 9.5 cm x 9.5 cm in size, 320 µm thick, and segmented into 768 strips with a 121 µm pitch. Only every other strip is readout but since analogue readout is used the position resolution is better than 80 µm for most incident angles, thanks to the charge division of floating strips. The photon angular resolution is expected to be around 0.2° at 10 GeV. The high dynamic range of the analog readout electronics of the STK allows to measure the charge of the incident cosmic rays with high precision. The full tracker uses 768 sensors, equivalent to a total silicon area of ~7 m2.
The Flight Model of STK has been built and delivered to China in April 2015, it has been successfully integrated and tested into with the full payload and the satellite. The total mass of STK is of 155 kg. The total power consumption is of 85 W. Its dimensions, including the outer envelop, are: 1.12 m x 1.12 m x 2.52 m.
The STK consist of twelve position-sensitive silicon detector planes (six planes for the x-coordinate, six planes for the y-coordinate). Three layers of tungsten are inserted in between the silicon planes (2, 3, 4 and 5) to convert gamma rays in electron-positron pairs. The specifications of the STK are given in Table 1 and a comparison with other experiments is shown in Figure 9 (Ref. 10).
Table 1: STK specifications
Figure 9: Comparison of STK features with other detectors (image credit: STK collaboration)
Figure 10: Schematic view of the STK layers (STK collaboration)
Figure 11: Photos of two development stages of STK (STK collaboration)
BGO (Bismuth-Germanium Oxide) calorimeter
The BGO calorimeter is used to measure the energy deposition of incident particles and to reconstruct the shower profile. The trigger of the whole DAMPE system is based on the signals from the BGO. The reconstructed shower profile is fundamental to distinguish between electromagnetic and hadronic showers.
The BGO calorimeter is made up of 14 layers of BGO bars in a hodoscopic arrangement. Each BGO bar is 2.5 cm x 2.5 cm in cross section and 60 cm in length, making it the longest BGO crystals ever produced. The bars are readout at both ends with PMTs (Photomultiplier Tubes), each PMT is readout from 3 dynodes (2, 5, 8) to extend the dynamic range. The total thickness of the calorimeter is equivalent to 31 radiation lengths and 1.6 interaction lengths. An excellent electromagnetic energy resolution of 1.5% above 100 GeV, and a very good hadronic energy resolution of better than 40% above 800 GeV can be expected.
Figure 12: Exploded view of the BGO scintllator (image credit: DAMPE collaboration)
Table 2: BGO specifications
NUD (Neutral Detector)
The NUD is a further device to distinguish the types of high-energy showers. It consists of four boron-loaded plastics each read out by a PMT. Typically hadron-induced showers produce roughly one order of magnitude more neutrons than electron-induced showers. The purpose of the NUD is to detect delayed neutrons resulting from a hadron shower in order to improve the electron/proton separation power, which should be 105 overall.
Typically hadron-induced showers produce roughly one order of magnitude more neutrons than electron-induced showers. Once these neutrons are created, they thermalize quickly in the BGO calorimeter and the neutron activity can be detected by the NUD within few µs (~ 2 µs after the shower in BGO).
Figure 13: Overview of the DAMPE detector assembly (image credit: DAMPE collaboration)
The DAMPE ground segment is composed of five parts, include the CCC (Chinese Control Center), the MC (Mission Center), the SSDC (Space Science Data Center) and three X-band stations. The X-band stations include the Miyun, Sanya and Kashi stations. They are responsible for scientific satellite tracking, science data reception, raw data recording and formatted outputting, and transferring the data to the Mission Center.
The CCC (Chinese Control Center), which also be known as Xi'An Center, manages and operates the satellite in each phase. CCC is in charge of the TT&C of the DAMPE, includes uplinking the telecommands to the satellite, downloading the S-band telemetry from the satellite, and determination of the satellite orbit parameters. The downloaded S-band telemetry raw data is transferred to the MC (Mission Center) in real-time for payload status monitoring. The telecommands for the payload are generated by the Mission Center.
The MC (Mission Center) is responsible for payload operations. The main functions are:
1) Mission planning and scheduling
2) Provide the schedule for X-band ground stations data reception
3) Payload telecommands management
4) Commands generation for the detectors PMS , and X-band subsystems
5) Command execution verification
6) Provide the TC plan (telecommands) to the CCC
7) Science data reception from X-band stations
8) Monitoring of the payload health & status.
The SSDC (Space Science Data Center) is responsible for science data processing, managing, archiving, permanent preservation and distribution.
The SC (Science Center) is also known as the Science Application System. The DAMPE Science Center proposes scientific exploration plan, generates high-level data products ,and organizes the research and application.
Figure 14: Overview of the DAMPE ground segment (image credit: CAS/NSSC)
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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).