The DAMPE detector - The DAMPE experiment

2.2 The DAMPE experiment

2.2.1 The DAMPE detector

DAMPE provides multiple measurements in order to identify the crossing particle thanks to several subdetectors: a double-layer Plastic Scintillator strip Detector (PSD), used for the measurement of the absolute charge (Z) and as anti-coincidence for photons;

a Silicon-Tungsten tracKer-converter (STK) that reconstructs the trajectory of the par-ticle, measures its charge and converts incoming photons into electron-positron pairs to estimate the photon direction; a Bismuth Germanium Oxide imaging calorimeter (BGO) of about 32 radiation lengths for precise energy measurements and for electron/photon identification, with respect to protons and heavy nuclei; a NeUtron Detector (NUD) to improve the electron/proton separation power. Fig. 2.8 gives a schematic view of

2.2. THE DAMPE EXPERIMENT

2016-11-01 Ruslan Asfandiyarov / University of Geneva 20

Particle Identification

• Tracker plays a crucial role in particle identification by providing a precise track of an incident particle

Figure 2.7: Picture of the DAMPE satellite (left) and a 3D event display of a candidate electron crossing DAMPE (right).

DAMPE, emphasizing the working principle of each subdetector. In the following sec-tion, we will give an overview of the DAMPE experiment and its sub-detectors. Since a part of this thesis was dedicated to the assembly of the STK and its charge measurement, we will discuss this sub-detector in more detail in chapter 3. More details about all the DAMPE sub-detectors can be found in [68].

z x PSD y

STK

BGO ECAL NUD

|Z|, γ anticoincidence

tracking, |Z|, γ conversion

E, e/p separation

Cosmic ray test (matching, MIPs, attenuation effect)

PMT stability test

PMT LED test dynode ratios

The light yield and uniformity of BGO crystal

Elements Test and ECAL Building

Neutron Detector

Energy fields : 2～60 MeV Energy resolution : <25%@30MeV

Weight:12.4 Kg Power:0.5W

e/p separation

Today's Date Sub Topic 3

Overview Layers Strips Copy Num SD and Hits

Type-A

Type-B

Strips Constructed in Geant4(length shortened)

➔Material: EJ-200,10mm thick

➔Wrapping material: Tyvek

➔Dimention(body):

Type-A: 824mm×28mm×10mm Type-B: 824mm×25mm×10mm

Figure 2.8: Layout of the DAMPE experiment.

CHAPTER 2. DARK MATTER PARTICLE EXPLORER 2.2.2 The Plastic Scintillation Detector (PSD)

The PSD provides the measurement of the absolute value of the charge (hereafter Z) of incident particles up to Z = 26. It consists of 2 double-layers, each one composed by 41 plastic scintillator bars. One double-layer is segmented in the X coordinate, the other in the Y. An exploded view of the PSD is reported in Fig. 2.9 and the position of the bars inside PSD is schematized in Fig. 2.10.

Figure 2.9: Exploded view of the PSD.

Figure 2.10: Side view of the arrangement of the scintillator bars inside the PSD. Picture taken from [69].

Each scintillator bar is read-out by two Photo Multiplier Tubes (PMTs) coupled to the two ends of the bar. An active area of 825 mm×825 mm is fully covered and every incident charged particle will cross at least two bars. According to the Bethe-Bloch formula, the energy deposition of proton MIPs in the PSD can be used to measure the energy depositions of high energy cosmic-ray particles. The charge spectrum measured by the first layer of the PSD is shown in this charge unit in Fig. 2.11. The PSD detec-tion efficiency of a single layer is more than 95% and with the combinadetec-tion of the two layers it provides an overall efficiency higher than 99% for charged MIP particles [70].

The segmented structure of the PSD is very important to discriminate photons from backsplash particles thanks to a position resolution lower than 2 cm.

Figure 2.11: Charge spectrum reconstructed with the first layer of PSD using one year of on-orbit data. Picture taken from [70].

2.2.3 The Silicon-Tungsten Tracker (STK)

The STK consists of 6 silicon layers measuring the X coordinate, 6 silicon layers measuring the Y coordinate of the particle, forming 6 tracking double layers. The layers are mounted on 7 carbon fiber reinforced polymer (CFRP) trays sandwitched with aluminium honeycomb. Each tracking layer is equipped with single-sided AC-coupled Silicon micro-Strip Detectors (SSD), manufactured by Hamamatsu Photonics [71]. The tracking layers are placed to measure the two orthogonal views perpendicular to the pointing direction of the apparatus (z axis in Fig. 2.8). Each silicon layer is made of 16 ladders, each formed by 4 SSD for a total of 192 ladders. The sensors are 320 µm thick with a n-doped silicon substrate, 9.5 × 9.5 cm² in surface, and segmented into 768 strips p-doped, with 121 µm pitch and 48 mm thickness. A schematic view of a single-sided micro strip detector is shown in Fig. 2.12. In order to limit the number of out channels and keep a good performance in terms of space resolution, the read-out is done only every other strip, for a total of 384 channels per ladder, read-read-out by 6 VA140 ASIC chips, produced by IDEAS [72] (Fig. 2.13). The bonding of two adjacent SSDs is shown in Fig. 2.14 in a picture and a scheme. The deposited ionization energy in a silicon sensor is proportional to the square of the particle charge (dE/dx∝Z²), therefore it can be used to distinguish different nuclear species. The particle direction in STK is obtained using a tracking algorithm, described in [73]. To improve the photon conversion rate, three layers of tungsten plates, with thickness of 1 mm, are inserted into the support trays of the second, third and fourth tracker plane. A schematic view of the behavior of various particles in STK is shown in Fig. 2.15. The STK construction phase was successfully finished in spring 2015. Its spatial resolution is expected from design to be better than 80 µm and it was measured with protons and helium (as an

CHAPTER 2. DARK MATTER PARTICLE EXPLORER

Figure 2.12: Schematic transverse view of the principle of operation of a single-sided silicon sensor.

Figure 2.13: Picture of the edge of one ladder read-out with 6 VA.

Figure 2.14: Bonding between two SSDs of one ladder, in a picture (left) and in a scheme (right).

example we report the result for protons in Fig. 2.16). The full thickness corresponds to 0.976 X₀ for a total weight of 154.8 kg. A picture of the tracker before the assembly of the last tray, the front-end electronics and the cooling and supporting structures is

W converter

y z

Charged particle !

Figure 2.15: Schematic view of the behavior of cosmic rays and photons traversing the STK.

Figure 2.16: Position resolution measured for the candidate protons in data (top) and the proton MC (middle) for the X and Y layers of the STK. The first layer is relative to the STK point closer to PSD. The external layers are grouped separately since their position resolution is lower due to higher uncertainty in the track projection for these layers. The difference between the resolution in data and MC is given in the bottom panel. Picture taken from [73].

shown in Fig. 2.17.

CHAPTER 2. DARK MATTER PARTICLE EXPLORER

been used for the readout of the double-sided micro-strip detectors of the tracker. The power consumption of one ladder is 116 mW.

2.3. Silicon ladder assembly

The ladder assembly consists of the front-end board glued onto the four silicon detectors, initially placed on a precision alignment jig.

After gluing, the position of the silicon detectors is measured with a coordinate measurement machine. A linearﬁt is applied to the mea-sured positions of the detector alignment patterns. The RMS of the residual distribution is then computed, to estimate the alignment error. Fig. 3shows the average alignment error of the 192 ladders installed on the STK. Thanks to the jigs, the alignment precision is about 4μm on average, much better than the required 40μ^{m .}

Part of the ladder quality assurance, a 12-h cosmic ray test is performed and the signal to noise ratio of cluster charges from minimum ionizing particle (MIP) is checked. On average theS/Nis about 15, excellent for the application of the STK. All 192 ladders needed for the STK Flight Model were produced by the end of March 2015, for a total production time of three months, shared between two production sites (University of Geneva and INFN Perugia). The silicon detector quality has been preserved during all the STK assembly steps, the 192 ladder leakage current at 80 V is 330 nA in average (Fig. 4).

2.4. Plane assembly

The 192 ladders are distributed on seven support trays. Five are equipped on both sides, while the front and the rear trays are equipped only on the side facing the interior of the tracker. A layer is composed of 16 ladders, arranged in two rows of eight. A layer is thus composed of an array of 8!8 detectors, as can be seen in Fig. 5. On the double-sided trays, the silicon layer strips are orthogonal to each other.

A support tray is made of an aluminum honeycomb structure sandwiched between two Carbon Fibre Reinforced Polymer (CFRP) face sheets of 0.6 mm thick each (1.0 mm for trays with tungsten).

It forms a light but rigid structure that can sustain the vibrations and the accelerations of a rocket launch. For the second, third and fourth trays, 1 mm thick tungsten plates are glued onto the lower CFRP sheet, inside the tray. The converter is thus located imme-diately above the corresponding tracking layer, to ensure good efﬁciency of conversion detection. The trays have been produced at Composite Design Sàrl [8]. The trays equipped with tungsten layer have been X-ray scanned at CERN, to check the alignment of the tungsten plates with respect to the four tray corners.

The ladder installation on each tracker tray has been done using an alignment and transfer jig, placed onto a precision frame holding the tray. The position of the 64 silicon sensors of the fully equipped tray was then measured with a coordinate measurement machine.

The trays have been stacked on top of each other to form the full tracker (Fig. 5). The silicon ladders on the bottom surface of a tracker tray and those on the top surface of the tray below form an X–Ytracking plane, the distance between theXand theYlayer of a tracking plane is "3 mm. The stack of the 7 trays forms 6 tracking planes that provide 12 measurement points (6Xand 6Y) when a charged track traverses the STK.

The STK has a total power consumption of 90 W. The tracker cooling is ensured with a network of heat conductors. Pyrolytic graphite sheets (PGS) connect the front-end lateral sides with a horizontal copper band installed on the tray edges. Copper straps are transversely glued to thermally connect the trays together, and to the aluminum radiators placed on the four lateral sides of the STK. During installation on the satellite, heat pipes have then been mounted, to ensure a constant radiator temperature of 10°C.

3. Signal digitization and readout

The VA chips are separated into two independent readout groups (VAs 1–3, and VAs 4–6). Both groups are readout in par-allel, but the VAs of a same group are serially readout. Thus 192 clock signals are necessary to transfer the 384 silicon strip signals.

Each group has its own ampliﬁcation circuit, which analogical outputs are then transferred to the main STK data acquisition boards (Tracker Readout Boards, TRB).

A TRB circuit reads out 24 ladders, and is composed of a stack of three boards. The ADC board, to which are connected the ladders, performs the analog to digital conversion of the ladder signals. The FPGA board has two FPGAs which manage the communication with the DAMPE DAQ system, and the generation of the control signals necessary for the 24 ladder readout and signal digitization.

The Power board produces the voltages necessary for the front-end electronics and the TRB circuit, as well as the silicon bias voltages. To reduce the data size, the FPGAs perform a data com-pression, using a zero-suppression and clusterﬁnding algorithm.

Eight TRBs are needed to readout the whole STK: two TRBs are h_lc

Entries 192 Mean 0.3255 RMS 0.2246

A) Ladder leakage current (µ

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Entries 192 Mean 0.3255 RMS 0.2246

Ladder leakage currents at 80V

Fig. 4.Total leakage current at 80 V bias of the 192 ladders of theﬂight model STK.

The average current is 330 nA, which means that the silicon detector quality has been preserved during the ladder assembly.

Fig. 5.The STK before the assembly of the last tray. The 64 silicon detectors are visible, together with the horizontal copper bands used for the heat transfer.

P. Azzarello et al. / Nuclear Instruments and Methods in Physics Research A∎(∎∎∎∎)∎∎∎–∎∎∎ 3

Please cite this article as: P. Azzarello, et al., Nuclear Instruments & Methods in Physics Research A (2016),http://dx.doi.org/10.1016/j.

nima.2016.02.077i

Figure 2.17: Picture of the STK before the assembly of the last tray. The 16 ladders forming one sensible layer of the STK are visible.

Data reduction and calibration

The 192 ladders of the STK are electronically read-out by 8 Tracker Readout Boards (TRB) [74]. Each ladder is connected with flexible cables to a TRB and each TRB reads out one quarter of each STK sensible layer for a total of 24 ladders as it is shown in the left part of Fig. 2.18. The location of the TRBs and the cooling radiators with respect to the whole STK are illustrated on the right side of Fig. 2.18. The average trigger rateSubmitted to ‘Chinese Physics C’

TRB x

y z

Ladder TFH

Fig. 2Ladder arrangement and connection of 6 X-view layers

The ladders are connected via flexible cables to the Tracker Readout Boards(TRBs) surrounding the tracking planes, where the ADCs, monitor circuits, high-voltage generator and control logics are situated. The TRBs are in charge of data acquisition and status monitoring of the silicon tracker. On one hand they receive triggers and configuration commands from payload DAQ, on the other hand, they transfer scientific data and house-keeping data back. There are eight TRBs in all at four vertical sides of the STK, as presented in the Fig.3. Each TRB reads out one quarter ladders of all six layers at the same side, as the Fig.2 shows. The system block diagram is shown in Fig.4. TRB nY1 TRB nY2

TRB pY2 TRB pY1 Silicon

Figure 2.18: Left: scheme of the connection between the ladders (the silicon wafers glued on the tracker front-end hybrid TFH) read-out by a TRB (Picture taken from [75]). Right: figure representing the location of the TRBs respect to the assembled STK.

for DAMPE is ∼ 70 Hz [76] therefore the STK will generate 637 GB of raw data per day whereas only 10 GB can be saved to on-board mass storage. So, already on-board is necessary to save only the important information received from the STK. In a calibration run, the pedestal calculation for each channel i of STK is performed:

pedi= 1 number of events used for the calibration, which is done twice per orbit. Then the data

acquisition and reduction is performed in STK by two FPGAs in each TRB. The FPGA reads 192 channels (one VA corresponds to 64 channels) of 12 ladders in parallel and stores the ADC values acquired in the so-called F RAM. The ADC values are replaced in the F RAM with pedestals subtracted ADC values. The common noise (CN) is attributed to the common behavior of the channels of the same chip, mainly due to the fluctuating bias voltage. For the event j the CN is defined as:

CN_j = 1 N_ch

Nch

i=1

(ADC_ij−ped_i) (2.2)

where N_chis the number of channels per chip (64). Noisy channels (strips in short circuit that give always a large ADC value) are excluded from this computation as well as a potential signal, requiring ADCij−pedi > 30 ADC. In this case, the CN is computed using 32 channels. If the noisy channels are more than 32, the CN is computed using 16 channels. Only if the noisy channels are more than 48 the entire chip information is not used in the subsequent step. The signal in the channel i for the event j is:

Sij = ADCij−pedi−CNj. (2.3) A calibration run is used to compute the intrinsic noise of channel i as:

σ_i= vu ut1

n XN

i=1

(ADC_ij−ped_i−CN_j)² (2.4) and this noise value is used in an on-board cluster finding algorithm. The ”seed” in the cluster is the channel that has a Sij/σiratio higher than 3.5. The seed signal is grouped together with the neighbor strips that have a signal higher than 5 ADC, forming in this way a cluster. The evolution of the noise σ in the STK channels is continuously monitored from the beginning of the data taking and the ratio of channels with different σ values after January 2016 is shown in Fig. 2.19. The correlation between the noise and the ladder temperature is shown in Fig. 2.20 and Fig. 2.21. The ladder temperature is stabilized through the cooling radiators. The ladder temperature is higher is higher than the radiators by 10 degrees on average (see Fig. 2.22).

2.2.4 The Bismuth Germanium Oxide imaging calorimeter (BGO) The BGO is an electromagnetic imaging calorimeter that provides the reconstruction of the longitudinal and lateral electromagnetic shower development. It is divided in 14 layers, 7 along the x axis and 7 along the y axis alternatively, to allow a 3D recon-struction of the shower development. Each layer contains 22 BGO bars with dimensions 25 mm×25 mm×600 mm for a total active area of 60 cm×60 cm. A schematic view of the BGO is shown in Fig. 2.23.

The main objective of the BGO is to reconstruct precisely the energy and to provide a high discrimination power between leptons and hadrons, based on the very different char-acteristic profiles of electromagnetic and hadronic induced showers. When photons and

CHAPTER 2. DARK MATTER PARTICLE EXPLORER

Figure 2.19: Ratio of channels in % withσ≤5 (green filled triangles), 5<σ≤10 (empty blue triangles) andσ>10 (empty blue circles) as function of time for the 192 ladders×6 VA×64 read-out strips = 73728 STK channels.

Figure 2.20: Mean ladder noise (points) and average ladder temperature (blue line) as function of time.

Figure 2.21: σ evolution as a function of time for several time intervals (top left) and as function of the ladder temperature (top right). Looking at the correlation between the noise and the temperature (top right), the first (red) interval showed a greater noise as function of temperature, due to an incomplete stabilization of the detector in the space environment. Two intervals are shown separately in the bottom panel and fitted with a linear function.

electrons reach the calorimeter, they interact with the BGO material through electromag-netic processes and produce an electromagelectromag-netic shower dominated by Bremsstrahlung and pair production. For electrons and photons entering the central part of the BGO with energies below 10 TeV, the shower is fully contained and the measured energy de-position is directly proportional to the energy of the primary particle, thanks to its 32 radiation lengths thickness. In the GeV-TeV energy range, the proton rejection is on the order of 10⁵, for a 90% electron identification efficiency. The energy measurement of hadrons is more complicated than the one of electrons and photons since the energy leakage is much higher (the thickness of the calorimeter corresponds to∼1.6 interaction lengths) and requires an unfolding procedure. The energy resolution of the calorimeter with respect to electrons and protons, was validated with beam tests at CERN and the results are shown in Fig. 2.24.

CHAPTER 2. DARK MATTER PARTICLE EXPLORER

Figure 2.22: Radiators (top left) and ladders (top right) temperatures. The average difference between the two temperatures is plotted as a function of the time period in the bottom left panel.

The distribution of these differences is shown as a histogram in the bottom right plot.

Figure 2.23: Scheme of the BGO calorimeter of DAMPE. Picture taken from [68].

Figure 2.24: Left: energy resolution for vertical and tilted electrons impinging DAMPE. The simulation for the two cases is shown in black and the vertical electron beam test data is shown with red points. Right: energy resolution for protons crossing vertically DAMPE. The dotted line represents the energy resolution for MC simulated protons while the red points correspond

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