- Email: [email protected]
First data from the DAMPE space mission
Available online at www.sciencedirect.com
Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65 www.elsevier.com/locate/nppp
First data from the DAMPE space mission Paolo Bernardini, on behalf of the DAMPE Collaboration Universit`a del Salento and INFN, Lecce, Italy
Abstract The DAMPE (DArk Matter Particle Explorer) satellite was launched on December 17, 2015 and is in smooth data taking since few days after. It was designed in order to work properly for at least three years and, thanks to its large geometric factor (∼ 0.3 m2 sr for protons and nuclei), is integrating one of the largest exposures for galactic cosmic ray (CR) studies in space. Even if primarily optimized to collect electrons and gammas, DAMPE provides good tracking, calorimetric and charge measurements also in case of protons and nuclei. This will allow precise measurement of CR spectra from tens of GeV up to about 100 T eV. In particular, the energy region between 1 − 100 T eV will be explored with higher precision compared to previous experiments: spectral indexes for individual species could then be well measured and the observed hardenings could be checked and better quantified. This would be very important for a comparison with present models of galactic CR acceleration/propagation mechanisms. The various subdetectors 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 T eV, thus giving the opportunity to identify possible contribution of nearby sources. A report on the mission goals and status will be presented, together with on-orbit detector performance and first data coming from space. Keywords: Satellite Detector, Electrons, Cosmic Rays, Gamma Astronomy, Dark Matter
1. Introduction The DArk Matter Particle Explorer (DAMPE) is a space mission supported by the strategic space projects of the Chinese Academy of Sciences with the contribution of Swiss and Italian institutions [1, 2]. The rocket has been successfully launched on December 17, 2015 (Fig. 1) and DAMPE presently flies regularly on a sunsynchronous orbit at the altitude of 500 km. Four different detectors are arranged on the satellite: a plastic scintillator array, a silicon-tungsten tracker, a BGO calorimeter and a neutron detector. They are devoted to measure the fluxes of charged CRs (electrons, protons and heavier nuclei), to study the high energy gammasignal from astrophysical sources and to search for indirect dark-matter signatures. Figure 1: Jiunquan base in China (December 17, 2015). The launch of the rocket with the DAMPE satellite [2].
https://doi.org/10.1016/j.nuclphysbps.2017.06.014 2405-6014/© 2017 Elsevier B.V. All rights reserved.
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P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
Here the main technical features of the detectors and a short review of the first preliminary results are presented. 2. On-board instruments DAMPE (Fig. 2) consists of a Plastic Scintillator strip Detector (PSD) that serves as anti-coincidence and charge detector, a Silicon-Tungsten tracKer-converter (STK) to reconstruct the direction of incident particles, a BGO imaging calorimeter (BGO) of about 32 radiation lengths that measures the energy with high resolution and distinguishes between electrons and protons, and a NeUtron Detector (NUD) that can further increase the hadronic shower rejection power.
Figure 2: Side view of the DAMPE detector [2].
2.1. Plastic Scintillator Detector (PSD) The high energy sky is mainly dominated by nuclei with different electrical charges. This charged flux is studied by DAMPE but it is also a background for gamma astronomy and dark-matter search. Therefore the PSD is designed to work as a veto and to measure the charge (Z) of incident high-energy particles up to Z = 26. According to these requirements PSD must have a high detection efficiency for charged particles, a large dynamic range and a relatively good energy resolution. Indeed Z is measured according to the energy release in the scintillator. PSD is the first layer of the detector. It has an active area of 825 × 825 mm2 , that is larger than the effective areas of other sub-detectors. The active medium is organic plastic scintillator, only light materials have been used in the mechanical PSD structure in order to avoid the development of electromagnetic showers due to electrons or gammas. The PSD geometry has been designed with the aim to reduce the PSD surface that can contribute to the so-called backsplash effect due to the heavy materials in the other sub-detectors. The PSD is arranged in a double layer configuration and has in total 82 detector modules. Each module has a long plastic scintillator bar of 825 mm with a 28 × 10 mm2 cross section. The signal is read by two PMTs a the ends of the bar. The modules are staggered by 8 mm in the layer, and the bars in the two layers are perpendicular. The single-module detection efficiency results 95% and it becomes 99.75% for the whole PSD. The Z-measurement is performed looking at the energy release due to ionization process in the PSD. Indeed the release is roughly proportional to Z 2 according to the Bethe-Bloch formula. To measure the charge from protons up to heavier nuclei a large dynamic range
Figure 3: Exploded view of the PSD [2].
from 0.1 to 1400 Minimum Ionizing Particles (MIPs) must be covered. Therefore a double dynode readout for each PMT has been implemented. The high-gain dynode covers the range from 0.1 to 40 MIPs, the low-gain one covers the range up to 1400 MIPs. The two-dynodes overlap is used for calibration. 2.2. Silicon-Tungsten Tracker (STK) The Silicon-Tungsten tracKer-converter (Fig. 4) is devoted to the precise reconstruction of the particle track, it is made by 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 STK are given in Table 1 and a comparison with other experiments is shown in Fig. 5 looking at active area and number of channels. The tracking resolution is expected to be 0.2◦ .
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
STK has been designed taking into account that power consumption and mass are limited on a satellite. It is a compact system where readout and power supply electronics (Tracker Readout Boards, i.e. TRB) are mounted on the sides of the trays.
61
Table 1: STK specifications
Active area of each silicon layer Silicon thickness Silicon strip pitch Tungsten thickness Fraction of radiation length Power consumption Mass
0.5534 m2 320 μm 121 μm 1 mm 0.976 82.7 watts 154.8 kg
Table 2: BGO specifications
Active area Depth Sampling Longitudinal segmentation Lateral segmentation
Figure 4: STK flight model [2].
DAMPE
Figure 5: Comparison of STK features with other detectors.
60 × 60 cm2 (on-axis) 32 radiation lengths > 90% 14 layers ∼ 1 Moliere radius
both views (x − z and y − z). The total vertical depth of the calorimeter is about 32 radiation lengths and 1.6 nuclear interaction lengths. This unprecedented depth ensures that almost 100% of the energy of electrons and γ-rays is deposited in the calorimeter, and about 40% of proton energy. The high limit (∼ 100 T eV) of the DAMPE measurement range is essentially determined by the overall geometric factor and the calorimeter dynamic range. Table 2 summarizes the key parameters of BGO calorimeter. Each crystal element is read out by two PMTs, mounted on both ends of the crystal (S0 and S1, respectively). The measured light asymmetry allows to further estimate where the energy has been deposited along the BGO bar. In order to cover a very large dynamic range (2 × 106 ), the signals are collected from three different dynodes covering different ranges on each side (S0 and S1). X Layer (22 BGO bars)
2.3. BGO Calorimeter (BGO) The BGO calorimeter is devoted 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 and the shower profile is fundamental to distinguish between electromagnetic and hadronic showers. The calorimeter is composed of 308 BGO crystal bars (2.5 × 2.5 × 60 cm3 is the volume of a single bar). The crystals are optically isolated from each other and are arranged horizontally in 14 layers of 22 bars (Fig. 6). The bars of a layer are ortogonal to those of the following plane in order to reconstruct the particle path in
Y Layer
14 Layers
Figure 6: Exploded view of the BGO scintllator [2].
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
62
2.4. Neutron Detector (NUD) Typically hadron-induced showers produce roughly one order of magnitude more neutrons than electroninduced showers. Once these neutrons are created, they thermalize quickly in the BGO calorimeter and the neutron activity can be detected by Neutron Detector within few μs (∼ 2 μs after the shower in BGO). Indeed neutrons entering the boron-loaded scintillator undergo the capture process 10
B + n →7 Li + α + γ
electron and proton beams. The wide energy range of electron beam and the high-purity of proton beam were sufficient to check energy resolution (Fig. 9), linearity and e/p separation. Also a beam of argon fragments was used for heavy-ion test. Details of the beam-test preliminary results as well as the features of the qualified module can be found in [1, 4, 5, 6]. Furthermore sea-level muon test have been performed during different stages of the DAMPE assembly. Then the energy response to MIPs, the efficiency and the detector alignment have been calibrated with a relatively large sample of CR muon events.
Its probability is inversely proportional to neutron velocity and the capture time is inversely proportional to 10 B loading. Roughly 570 optical photons are produced in each capture [3]. So the NUD is a further device to distinguish the types of high-energy showers. Fig. 7 shows the detailed structure of NUD, consisting of four boron-loaded plastics, each with a set of PMTs and auxiliary circuit. These plastics are wrapped with a layer of aluminium film for photon reflection, and anchored in an aluminium alloy framework by silicone rubber. The space between plastic scintillator and framework (1 mm on each side) is filled with silicone rubber to relieve the vibration during the launch of the satellite.
Figure 7: Structure of the Neutron Detector [2]. Figure 8: DAMPE prototype at CERN [2].
3. Detector design and ground tests An extensive Monte Carlo simulation activity was done during R&D phase in order to find a proper compromise between research goals and limitations on geometry, power consumption and weight. DAMPE performances were verified by a series of beam tests at CERN (Fig. 8). The PS and SPS accelerators provide
4. On-orbit operation After launch, the spacecraft entered the sky-survey mode immediately and the dedicated-calibration of the detector was performed in two weeks. The calibration included pedestal, response to MIPs, alignment, timing, and so on. Comparing on-orbit data with simulations
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
σE
=
0.0606 0.0203 ⊕ ⊕ 0.0082 E / GeV E / GeV
σE
=
0.0611 0.0147 ⊕ ⊕ 0.0088 E / GeV E / GeV
E
E
63
very stable and very close to what expected. The absolute energy measurement has been checked by using the geomagnetic cut-off and it results well calibrated. Also the absolute pointing has been successfully verified. Indeed the photon-data collected in 165 days were enough to draw a preliminary high-energy sky-map where the main gamma-sources are visible in the proper positions. Fig. 11 shows the MIP peak reconstructed with the energy released in the BGO crystals. It is evident the agreement with the distribution derived from the simulation.
Figure 9: Energy resolution for electromagnetic showers - Preliminary comparison of beam test data with simulation.
and on-ground tests we can conclude that the detector works very well. The satellite is on a solar-synchronized orbit lasting 95 minutes. The pedestal calibration is performed twice per orbit and the global trigger rate is kept at ∼ 70 Hz by using different pre-scale for unbiased and low-energy triggers at different latitudes (Fig. 10). In the absence of on-board analysis processing, the data are just packaged with timestamp and transmitted to ground (about 4 millions of events per day corresponding to 15 GB). After the event reconstruction the data are 100 GB per day.
Figure 11: MIP peak in the BGO calorimeter - Comparison of simulation with on-orbit data.
The energy released in the PSD allows to measure the charge and to distinguish the different nuclei in the CR flux. Figs. 12 and 13 show the result of this measurement for the light component (H and He) and for the full range up to iron (1 ≤ Z ≤ 26).
proton
Figure 10: Counting rate vs latitude and longitude (arbitary units on the color palette). The South Atlantic Anomaly is visible.
PSD high gain readout
helium
5. 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
Figure 12: Preliminary distribution for light CR component - Proton and helium peaks are well separated thanks to the energy measurement in PSD.
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
64
6. Expected measurements in 3 years 5
10
HistTotPsdMean_StkTrk
Entries 2946099 Mean
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 Fig. 15 the possible DAMPE measurement of the lepton spectrum in 3 years is shown without random fluctuations. 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.
2.174
RMS
104
2.47
3
Counts
10
102
10
1 0
5
10
15 Charge
20
25
DAMPE in 3 years
30
Figure 13: Preliminary Z measurement up to iron.
The measurement of electron and positron flux is one of the main goal of the DAMPE mission because some dark matter signature could be found in the lepton spectra. The shower development in the BGO provides the tool to distinguish leptons from hadrons. Then a shape parameter has been defined: Fi = spreadi ×
Ei , Etot
where i is the index of the BGO layer (1 ≤ i ≤ 14), spreadi is the shower width, Ei and Etot are the energy on the single layer and on all the layers, respectively. Using the shape parameters on the last BGO layers (13, 14) it is possible to separate leptons from hadrons with a rejection power higher than 105 (preliminary result in Fig. 14). The rejection capability will be further enhanced by means of the NUD.
Simulation assuming AMS-02 power law spectrum + cut-off + nearby sources
Figure 15: 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 T eV and nearby astrophysical sources. The random fluctuations are not shown.
F13 + F14 (mm)
proton
protons
Simulation assuming AMS-02 fit
e±
Figure 16: Proton spectrum in the range 10 GeV − 10 T eV. The red dots indicate the possible DAMPE measurement in 3 years assuming the AMS-02 data fit. The random fluctuations are not shown.
sum of spreads (mm) Figure 14: Preliminary result about e/p separation. The background on the right is due to CRs entering the satellite from the sides.
Many experiments [7, 8, 9, 10] observed a discrepant hardening of the CR elemental spectra at T eV-energies. This is another interesting topic related to CR origin and propagation. DAMPE will be able to perform significant measurements about it (Fig.s 16 and 17) and also
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
about the boron/carbon ratio (Fig. 18). Finally the large exposure will allow extending energy spectra measurements for protons and nuclei up to hundred T eV.
65
calorimeter (32 X0 ), the precise tracking and the redundant measurement techniques. References
Helium
Simulation assuming AMS-02 fit Figure 17: Helium spectrum in the range 10 GeV − 10 T eV per nucleon. The red dots indicate the possible DAMPE measurement in 3 years assuming the AMS-02 data fit. The random fluctuations are not shown.
Boron / Carbon ratio Simulation assuming AMS-02 fit
Figure 18: Boron/carbon ratio versus energy per nucleon. The red dots indicate the possible DAMPE measurement in 3 years assuming the AMS-02 data fit. The random fluctuations are not shown.
7. Conclusions The DAMPE satellite has been successfully launched in orbit on December 2015 and the preliminary data analyses confirm that the detectors work very well. The DAMPE program foresees important measurements about CR flux and chemical composition, electron and diffuse-gamma spectrum 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”
[1] J. Chang, Chinese J. Space Science 34 (2014) 550 (available at www.cjss.ac.cn/CN/10.11728/cjss2014.05.550) [2] G. Ambrosi et al. (DAMPE Collaboration), to be submitted [3] D.M. Drake et al., Nuclear Instrum. & Methods A 247 (1986) 576 [4] V. Gallo et al. ”The test results of the silicon tungsten tracker of DAMPE”, Proceedings of the 34th International Cosmic Ray Conference (2015) 1199 [5] P. Azzarello et al., Nuclear Instrum. & Methods A 831 (2016) 378 [6] Z. Zhang et al., Nuclear Instrum. & Methods A 836 (2016) 98 [7] A.D. Panov et al., Bull. Russ. Acad. Sci. 71 (2007) 494 [8] H.S. Ahn et al., Astrophys. J. Letters 714 (2010) L89; Y.S. Yoon et al., Astrophys. J. 728 (2011) 122 [9] O. Adriani et al., Science 332 (2011) 69 [10] L. Accardo et al., Phys. Rev. Lett. 114 (2015) 171103
Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65 www.elsevier.com/locate/nppp
First data from the DAMPE space mission Paolo Bernardini, on behalf of the DAMPE Collaboration Universit`a del Salento and INFN, Lecce, Italy
Abstract The DAMPE (DArk Matter Particle Explorer) satellite was launched on December 17, 2015 and is in smooth data taking since few days after. It was designed in order to work properly for at least three years and, thanks to its large geometric factor (∼ 0.3 m2 sr for protons and nuclei), is integrating one of the largest exposures for galactic cosmic ray (CR) studies in space. Even if primarily optimized to collect electrons and gammas, DAMPE provides good tracking, calorimetric and charge measurements also in case of protons and nuclei. This will allow precise measurement of CR spectra from tens of GeV up to about 100 T eV. In particular, the energy region between 1 − 100 T eV will be explored with higher precision compared to previous experiments: spectral indexes for individual species could then be well measured and the observed hardenings could be checked and better quantified. This would be very important for a comparison with present models of galactic CR acceleration/propagation mechanisms. The various subdetectors 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 T eV, thus giving the opportunity to identify possible contribution of nearby sources. A report on the mission goals and status will be presented, together with on-orbit detector performance and first data coming from space. Keywords: Satellite Detector, Electrons, Cosmic Rays, Gamma Astronomy, Dark Matter
1. Introduction The DArk Matter Particle Explorer (DAMPE) is a space mission supported by the strategic space projects of the Chinese Academy of Sciences with the contribution of Swiss and Italian institutions [1, 2]. The rocket has been successfully launched on December 17, 2015 (Fig. 1) and DAMPE presently flies regularly on a sunsynchronous orbit at the altitude of 500 km. Four different detectors are arranged on the satellite: a plastic scintillator array, a silicon-tungsten tracker, a BGO calorimeter and a neutron detector. They are devoted to measure the fluxes of charged CRs (electrons, protons and heavier nuclei), to study the high energy gammasignal from astrophysical sources and to search for indirect dark-matter signatures. Figure 1: Jiunquan base in China (December 17, 2015). The launch of the rocket with the DAMPE satellite [2].
https://doi.org/10.1016/j.nuclphysbps.2017.06.014 2405-6014/© 2017 Elsevier B.V. All rights reserved.
60
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
Here the main technical features of the detectors and a short review of the first preliminary results are presented. 2. On-board instruments DAMPE (Fig. 2) consists of a Plastic Scintillator strip Detector (PSD) that serves as anti-coincidence and charge detector, a Silicon-Tungsten tracKer-converter (STK) to reconstruct the direction of incident particles, a BGO imaging calorimeter (BGO) of about 32 radiation lengths that measures the energy with high resolution and distinguishes between electrons and protons, and a NeUtron Detector (NUD) that can further increase the hadronic shower rejection power.
Figure 2: Side view of the DAMPE detector [2].
2.1. Plastic Scintillator Detector (PSD) The high energy sky is mainly dominated by nuclei with different electrical charges. This charged flux is studied by DAMPE but it is also a background for gamma astronomy and dark-matter search. Therefore the PSD is designed to work as a veto and to measure the charge (Z) of incident high-energy particles up to Z = 26. According to these requirements PSD must have a high detection efficiency for charged particles, a large dynamic range and a relatively good energy resolution. Indeed Z is measured according to the energy release in the scintillator. PSD is the first layer of the detector. It has an active area of 825 × 825 mm2 , that is larger than the effective areas of other sub-detectors. The active medium is organic plastic scintillator, only light materials have been used in the mechanical PSD structure in order to avoid the development of electromagnetic showers due to electrons or gammas. The PSD geometry has been designed with the aim to reduce the PSD surface that can contribute to the so-called backsplash effect due to the heavy materials in the other sub-detectors. The PSD is arranged in a double layer configuration and has in total 82 detector modules. Each module has a long plastic scintillator bar of 825 mm with a 28 × 10 mm2 cross section. The signal is read by two PMTs a the ends of the bar. The modules are staggered by 8 mm in the layer, and the bars in the two layers are perpendicular. The single-module detection efficiency results 95% and it becomes 99.75% for the whole PSD. The Z-measurement is performed looking at the energy release due to ionization process in the PSD. Indeed the release is roughly proportional to Z 2 according to the Bethe-Bloch formula. To measure the charge from protons up to heavier nuclei a large dynamic range
Figure 3: Exploded view of the PSD [2].
from 0.1 to 1400 Minimum Ionizing Particles (MIPs) must be covered. Therefore a double dynode readout for each PMT has been implemented. The high-gain dynode covers the range from 0.1 to 40 MIPs, the low-gain one covers the range up to 1400 MIPs. The two-dynodes overlap is used for calibration. 2.2. Silicon-Tungsten Tracker (STK) The Silicon-Tungsten tracKer-converter (Fig. 4) is devoted to the precise reconstruction of the particle track, it is made by 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 STK are given in Table 1 and a comparison with other experiments is shown in Fig. 5 looking at active area and number of channels. The tracking resolution is expected to be 0.2◦ .
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
STK has been designed taking into account that power consumption and mass are limited on a satellite. It is a compact system where readout and power supply electronics (Tracker Readout Boards, i.e. TRB) are mounted on the sides of the trays.
61
Table 1: STK specifications
Active area of each silicon layer Silicon thickness Silicon strip pitch Tungsten thickness Fraction of radiation length Power consumption Mass
0.5534 m2 320 μm 121 μm 1 mm 0.976 82.7 watts 154.8 kg
Table 2: BGO specifications
Active area Depth Sampling Longitudinal segmentation Lateral segmentation
Figure 4: STK flight model [2].
DAMPE
Figure 5: Comparison of STK features with other detectors.
60 × 60 cm2 (on-axis) 32 radiation lengths > 90% 14 layers ∼ 1 Moliere radius
both views (x − z and y − z). The total vertical depth of the calorimeter is about 32 radiation lengths and 1.6 nuclear interaction lengths. This unprecedented depth ensures that almost 100% of the energy of electrons and γ-rays is deposited in the calorimeter, and about 40% of proton energy. The high limit (∼ 100 T eV) of the DAMPE measurement range is essentially determined by the overall geometric factor and the calorimeter dynamic range. Table 2 summarizes the key parameters of BGO calorimeter. Each crystal element is read out by two PMTs, mounted on both ends of the crystal (S0 and S1, respectively). The measured light asymmetry allows to further estimate where the energy has been deposited along the BGO bar. In order to cover a very large dynamic range (2 × 106 ), the signals are collected from three different dynodes covering different ranges on each side (S0 and S1). X Layer (22 BGO bars)
2.3. BGO Calorimeter (BGO) The BGO calorimeter is devoted 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 and the shower profile is fundamental to distinguish between electromagnetic and hadronic showers. The calorimeter is composed of 308 BGO crystal bars (2.5 × 2.5 × 60 cm3 is the volume of a single bar). The crystals are optically isolated from each other and are arranged horizontally in 14 layers of 22 bars (Fig. 6). The bars of a layer are ortogonal to those of the following plane in order to reconstruct the particle path in
Y Layer
14 Layers
Figure 6: Exploded view of the BGO scintllator [2].
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
62
2.4. Neutron Detector (NUD) Typically hadron-induced showers produce roughly one order of magnitude more neutrons than electroninduced showers. Once these neutrons are created, they thermalize quickly in the BGO calorimeter and the neutron activity can be detected by Neutron Detector within few μs (∼ 2 μs after the shower in BGO). Indeed neutrons entering the boron-loaded scintillator undergo the capture process 10
B + n →7 Li + α + γ
electron and proton beams. The wide energy range of electron beam and the high-purity of proton beam were sufficient to check energy resolution (Fig. 9), linearity and e/p separation. Also a beam of argon fragments was used for heavy-ion test. Details of the beam-test preliminary results as well as the features of the qualified module can be found in [1, 4, 5, 6]. Furthermore sea-level muon test have been performed during different stages of the DAMPE assembly. Then the energy response to MIPs, the efficiency and the detector alignment have been calibrated with a relatively large sample of CR muon events.
Its probability is inversely proportional to neutron velocity and the capture time is inversely proportional to 10 B loading. Roughly 570 optical photons are produced in each capture [3]. So the NUD is a further device to distinguish the types of high-energy showers. Fig. 7 shows the detailed structure of NUD, consisting of four boron-loaded plastics, each with a set of PMTs and auxiliary circuit. These plastics are wrapped with a layer of aluminium film for photon reflection, and anchored in an aluminium alloy framework by silicone rubber. The space between plastic scintillator and framework (1 mm on each side) is filled with silicone rubber to relieve the vibration during the launch of the satellite.
Figure 7: Structure of the Neutron Detector [2]. Figure 8: DAMPE prototype at CERN [2].
3. Detector design and ground tests An extensive Monte Carlo simulation activity was done during R&D phase in order to find a proper compromise between research goals and limitations on geometry, power consumption and weight. DAMPE performances were verified by a series of beam tests at CERN (Fig. 8). The PS and SPS accelerators provide
4. On-orbit operation After launch, the spacecraft entered the sky-survey mode immediately and the dedicated-calibration of the detector was performed in two weeks. The calibration included pedestal, response to MIPs, alignment, timing, and so on. Comparing on-orbit data with simulations
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
σE
=
0.0606 0.0203 ⊕ ⊕ 0.0082 E / GeV E / GeV
σE
=
0.0611 0.0147 ⊕ ⊕ 0.0088 E / GeV E / GeV
E
E
63
very stable and very close to what expected. The absolute energy measurement has been checked by using the geomagnetic cut-off and it results well calibrated. Also the absolute pointing has been successfully verified. Indeed the photon-data collected in 165 days were enough to draw a preliminary high-energy sky-map where the main gamma-sources are visible in the proper positions. Fig. 11 shows the MIP peak reconstructed with the energy released in the BGO crystals. It is evident the agreement with the distribution derived from the simulation.
Figure 9: Energy resolution for electromagnetic showers - Preliminary comparison of beam test data with simulation.
and on-ground tests we can conclude that the detector works very well. The satellite is on a solar-synchronized orbit lasting 95 minutes. The pedestal calibration is performed twice per orbit and the global trigger rate is kept at ∼ 70 Hz by using different pre-scale for unbiased and low-energy triggers at different latitudes (Fig. 10). In the absence of on-board analysis processing, the data are just packaged with timestamp and transmitted to ground (about 4 millions of events per day corresponding to 15 GB). After the event reconstruction the data are 100 GB per day.
Figure 11: MIP peak in the BGO calorimeter - Comparison of simulation with on-orbit data.
The energy released in the PSD allows to measure the charge and to distinguish the different nuclei in the CR flux. Figs. 12 and 13 show the result of this measurement for the light component (H and He) and for the full range up to iron (1 ≤ Z ≤ 26).
proton
Figure 10: Counting rate vs latitude and longitude (arbitary units on the color palette). The South Atlantic Anomaly is visible.
PSD high gain readout
helium
5. 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
Figure 12: Preliminary distribution for light CR component - Proton and helium peaks are well separated thanks to the energy measurement in PSD.
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
64
6. Expected measurements in 3 years 5
10
HistTotPsdMean_StkTrk
Entries 2946099 Mean
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 Fig. 15 the possible DAMPE measurement of the lepton spectrum in 3 years is shown without random fluctuations. 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.
2.174
RMS
104
2.47
3
Counts
10
102
10
1 0
5
10
15 Charge
20
25
DAMPE in 3 years
30
Figure 13: Preliminary Z measurement up to iron.
The measurement of electron and positron flux is one of the main goal of the DAMPE mission because some dark matter signature could be found in the lepton spectra. The shower development in the BGO provides the tool to distinguish leptons from hadrons. Then a shape parameter has been defined: Fi = spreadi ×
Ei , Etot
where i is the index of the BGO layer (1 ≤ i ≤ 14), spreadi is the shower width, Ei and Etot are the energy on the single layer and on all the layers, respectively. Using the shape parameters on the last BGO layers (13, 14) it is possible to separate leptons from hadrons with a rejection power higher than 105 (preliminary result in Fig. 14). The rejection capability will be further enhanced by means of the NUD.
Simulation assuming AMS-02 power law spectrum + cut-off + nearby sources
Figure 15: 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 T eV and nearby astrophysical sources. The random fluctuations are not shown.
F13 + F14 (mm)
proton
protons
Simulation assuming AMS-02 fit
e±
Figure 16: Proton spectrum in the range 10 GeV − 10 T eV. The red dots indicate the possible DAMPE measurement in 3 years assuming the AMS-02 data fit. The random fluctuations are not shown.
sum of spreads (mm) Figure 14: Preliminary result about e/p separation. The background on the right is due to CRs entering the satellite from the sides.
Many experiments [7, 8, 9, 10] observed a discrepant hardening of the CR elemental spectra at T eV-energies. This is another interesting topic related to CR origin and propagation. DAMPE will be able to perform significant measurements about it (Fig.s 16 and 17) and also
P. Bernardini / Nuclear and Particle Physics Proceedings 291–293 (2017) 59–65
about the boron/carbon ratio (Fig. 18). Finally the large exposure will allow extending energy spectra measurements for protons and nuclei up to hundred T eV.
65
calorimeter (32 X0 ), the precise tracking and the redundant measurement techniques. References
Helium
Simulation assuming AMS-02 fit Figure 17: Helium spectrum in the range 10 GeV − 10 T eV per nucleon. The red dots indicate the possible DAMPE measurement in 3 years assuming the AMS-02 data fit. The random fluctuations are not shown.
Boron / Carbon ratio Simulation assuming AMS-02 fit
Figure 18: Boron/carbon ratio versus energy per nucleon. The red dots indicate the possible DAMPE measurement in 3 years assuming the AMS-02 data fit. The random fluctuations are not shown.
7. Conclusions The DAMPE satellite has been successfully launched in orbit on December 2015 and the preliminary data analyses confirm that the detectors work very well. The DAMPE program foresees important measurements about CR flux and chemical composition, electron and diffuse-gamma spectrum 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”
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