Nuclear Instruments and Methods in Physics Research A

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Development of a composite large-size SiPM (assembled matrix) based modular detector cluster for MAGIC

A. Hahnâ,n, D. Mazinâ,b, P. Bangaleâ, A. Dettlaffâ, D. Finkâ, F. Grundnerâ, W. Habererâ, R. Maierâ, R. Mirzoyanâ, S. Podkladkinâ, M. Teshimaâ,b, H. Wetteskindâ

aMax Planck Institute for Physics (Werner-Heisenberg-Institut), Föhringer Ring 6, 80805 München, Germany

bInstitute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa City, Chiba 277–8582, Japan

a r t i c l e i n f o

Article history:

Received 25 March 2016 Received in revised form 14 June 2016

Accepted 16 June 2016

Keywords:

Silicon photomultiplier

Multi-pixel avalanche photodiodes

a b s t r a c t

The MAGIC collaboration operates two 17 m diameter Imaging Atmospheric Cherenkov Telescopes (IACTs) on the Canary Island of La Palma. Each of the two telescopes is currently equipped with a photomultiplier tube (PMT) based imaging camera. Due to the advances in the development of Silicon Photomultipliers (SiPMs), they are becoming a widely used alternative to PMTs in many researchﬁelds including gamma-ray astronomy. Within the Otto-Hahn group at the Max Planck Institute for Physics, Munich, we are developing a SiPM based detector module for a possible upgrade of the MAGIC cameras and also for future experiments as, e.g., the Large Size Telescopes (LST) of the Cherenkov Telescope Array (CTA). Because of the small size of individual SiPM sensors (6 mm6 mm) with respect to the 1-inch diameter PMTs currently used in MAGIC, we use a custom-made matrix of SiPMs to cover the same detection area. We developed an electronic circuit to actively sum up and amplify the SiPM signals.

Existing non-imaging hexagonal light concentrators (Winston cones) used in MAGIC have been modiﬁed for the angular acceptance of the SiPMs by using Cþ þbased ray tracing simulations. Theﬁrst prototype based detector module includes seven channels and was installed into the MAGIC camera in May 2015.

We present the results of theﬁrst prototype and its performance as well as the status of the project and discuss its challenges.

1. Introduction

The Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) are two 17 m diameter Imaging Atmospheric Cherenkov Telescopes (IACTs). They are situated at the Observatorio del Roque de los Muchachos on the Canary Island of La Palma at a height of 2200 m a.s.l. The mirror surfaces of 236 square meters and the PMT equipped cameras are used to measure Cherenkov light from extended air showers, induced by very high energy cosmic gamma rays in the energy range from 30 GeV to several 10 TeV. Both Cameras consist of 1039 PMTs (1 inchin diameter), put together in 169 modules[1,2].

The Cherenkov Telescope Array (CTA) is going to be the next

generation large-size instrument. The CTA northern array will be set up on the Canary Island of La Palma, close to the MAGIC telescopes. In the center of the telescope array, four Large Size Tele- scopes (LST), each equipped with an imaging camera based on the use of 1855 PMTs and a mirror diameter of 23 m will be placed.

CTA endeavors to increase the ﬂux sensitivity by an order of magnitude with respect to existing IACTS like MAGIC, H.E.S.S., or VERITAS[3].

SiPMs, as a novel technological development of a relatively small size, have never been used for composing large pixel size light detectors for IACTs. However, the suitability of SiPMs for detecting Cherenkov light of extended air showers has been proven by the First G-APD Cherenkov Telescope (FACT) collaboration, for small pixel sizes[4]. We developed a modular SiPM detector cluster for use in the MAGIC-I imaging camera alongside the existing PMT clusters. The mechanical construction, pixel count and shape as well as the shape of the light concentrators was matched to those of PMT modules. Our main goal for thefirst prototype was tofind out possible construction, controlling and electronics pro- blems in building a composite detector module suitable for the operation environment of the large size IACTs. In the development of this first prototype we did not focus on maximizing the fill Contents lists available atScienceDirect

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Nuclear Instruments and Methods in Physics Research A

nCorresponding author.

E-mail addresses:ahahn@mpp.mpg.de(A. Hahn),

mazin@mpp.mpg.de(D. Mazin),priya@mpp.mpg.de(P. Bangale), todettl@mpp.mpg.de(A. Dettlaff),ﬁnk@mpp.mpg.de(D. Fink),

grundner@mpp.mpg.de(F. Grundner),haberer@mpp.mpg.de(W. Haberer), rma@mpp.mpg.de(R. Maier),razmik@mpp.mpg.de(R. Mirzoyan), serp@mpp.mpg.de(S. Podkladkin),mteshima@mpp.mpg.de(M. Teshima), wet@mpp.mpg.de(H. Wetteskind).

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

nima.2016.06.102i

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factor of the multi-SiPM composite pixel active area. Its performance is not intended to compete with current PMT modules but will prove feasibility of large size composite SiPM-based pixels in IACTs. A mayor improvement of a telescope operation with semiconductor-based light detectors is anticipated in the enhanced robustness and the lack of aging at high photonﬂuxes, which is a typical problem of PMT-based cameras during moonlight observations[4].

2. Environmental conditions

The operating environment for light detectors used in IACTs differs signiﬁcantly from other applications, e.g., in accelerator physics. It is essential to consider the below-shown requirements for the design of a detector module:

Large active detection area.

High level background due to light from the night sky (LoNS).

Possible high ambient temperatures (about 28°C).¹

A single photoelectron rate induced by the Light of the Night Sky (LoNS) of 130 MHz was measured with the Hamamatsu PMT R10408, which is used in the MAGIC cameras. Because of the extended sensitivity towards longer wavelength for SiPMs, shown in Fig. 1, we expect a factor of about four higher number of background event rate for a SiPM-based detector module. To avoid pile- up, a small signal width of a few ns is needed. From this estimation one can easily see that the semiconductor device inherited dark count rate, even at these high temperatures, does only play a minor role in this application.

3. Mechanical structure

The detector module consists of seven pixel. Each pixel itself, is built up of seven 6×6 mm² Excelitas C30742–66 SiPMs. The sensors have a cell pitch of50 m, thus in total 14,400 cells. Theseμ devices have a typical breakdown voltage of 90–100 V and a nominal gain of around1×10⁶. On top of each pixel a non-imaging hexagonal light concentrator is placed to guide the light and reduce the necessary active area by a factor 2.25. In addition, the light concentrator reduces thefield of view of the pixel to cover the mirror area only. A sectional CAD drawing is plotted inFig. 2 left. Some parts of the SiPM pixel PCB are not covered by active area. Our pixel has afill factor of 69%, not including thefill factor of the SiPM. Because of the hexagonal shape of the light concentrator, 5% of the active SiPM area are covered under the light concentrator and can not be accessed by light. This covered area does only contribute to the dark count rate, which is of minor importance as discussed above.

4. Electronics

All seven pixels are biased by a single controllable HV DC/DC- converter inside the detector module housing. It provides a common bias potential for all seven pixels (49 SiPMs in total) and is connected to the SiPM cathodes. The seven SiPMs from Excelitas, that make-up one pixel, have a spread in their breakdown voltage.

Therefore we divided each pixel into three groups as seen inFig. 2

right. Within each of these groups the SiPMs have been selected to have a similar breakdown voltage. An individual overvoltage to a given SiPM group is provided by an controllable offset voltage fed to the SiPM anodes. The overall bias voltage applied to a SiPM group is the difference of the common HV and the individual offset voltage with a typical common HV of 106 V and an offset voltage of 6 V which results in a bias voltage of 100 V. The individual offset voltage is used for gain adjustments and enables us to switch off a particular group by increasing the offset voltage to a maximal value. In case of high light levels, switching off single groups of SiPMs allows a current limitation. A pixel has to be switched off in case a bright star is in itsﬁeld of view and the entire light is fo- cused onto that pixel. This can be achieved by increasing the offset voltages to their maximal value of 10 V, which results in an applied bias voltage below the breakdown voltage of the SiPMs. If a pixel is not switched off under this high light level conditions, it draws a high current and might saturate the HV power supply, which would cause a potential drop and unstable gains in the other pixels. In addition, the offset voltage path includes a current measurement.

A major challenge in achieving large active areas with semiconductor light detectors under the constraint of short signal width of a few ns, is the increase of its capacitance with the area of the devices which leads to slower response times. We use a composite matrix consisting of seven SiPMs. A common base transistor stage is used for fast summation of the individual signals to one combined output per pixel. The low input impedance of approximately7Ωprovides a fast signal response. The high output impedance allows us to sum the signals by simply connecting the outputs in parallel. The common base transimpedance circuit consumes50 mAat5 V. A single SiPM inside the pixel shows a rise time of1.8 nsand a FWHM of5.1 ns. The summed signal of seven SiPMs is wider in time, as expected, and has a rise time of2.2 ns and a FWHM of5.9 nsfor an input pulse with a FWHM of70 ps[6].

This proves that large size active surfaces of individual SiPMs, with a preserved fast signal response and a size comparable to a PMT photo-cathode can be achieved. The single photoelectron ampli- tudes can still be resolved.

A temperature sensor was added on top of each pixel, next to the SiPMs. The temperature coefﬁcient of the SiPMs is 90 mV/ C° [7]. No active parameter adjustment for compensating the temperature changes is foreseen. Temperature variations will be treated ofﬂine as discussed below.

5. Light concentrators

The currently used Hamamatsu PMTs have a hemispherical photo cathode whereas the SiPMs have aﬂat active area. SiPMs are Fig. 1.LoNS and Cherenkov spectra compared to a typical SiPM sensitivity curve in arbitrary units.[5], modiﬁed.

1The ambient temperature is not the temperature of the air outside but the temperature around the sensors inside the camera, which is difﬁcult (expensive) to cool down to temperatures of less than 28°C in our case.

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mainly sensitive to incident light up to an angel of70°at the active area. As a consequence a different design of light concentrators has to be used with respect to PMTs. For this we simulated a single light concentrator using the ROOT-based simulator for ray tracing (ROBAST)[8]. Light being reflected at the mirror has to be guided to the light sensors with maximum efficiency, whereas light reflected at the ground has to be cut. For the position of the SiPM prototype cluster this corresponds to a cutoff angle of approxi- mately31°.

The result is presented inFig. 3. The integrated collection efﬁ- ciency for photons is 92.1% and 91.5% for photons with an incident angle smaller than70°at the SiPM surface.

6. Operation

The accomplished detector module was installed in the MAGIC 1 camera, neighboring the PMT modules.

Most characteristic parameters of silicon detectors like gain and dark current, depend on the overvoltage. The overvoltage respectively the breakdown voltage depends on the temperature.

Therefore a constant operation temperature is favored for stable operating conditions and calibration. The temperature progress of a pixel during a night of observations is shown in Fig. 4. The temperature variations are within 1°C. With present moon the temperature is3,−4 C° increased due to higher signal currents.

The SiPM detector module is integrated in the standard MAGIC data acquisition system. The analog signals are guided via optical ﬁbers to the counting house and are recorded by a DRS4 chip. The prototype detector is not part of the trigger logic, events are only recorded when the camera is triggered by the central PMT area. A comparison between averaged SiPM and PMT calibration events, induced by a periodic light ﬂasher facing the MAGIC camera, are shown inFig. 5left. The signal characteristics are comparable to the lab measurements as previously discussed when the reduced time resolution is taken into account. The average calibration event of a PMT has 60 photoelectrons. The same pulse creates about 40 photoelectrons in a SiPM pixel. Please note that this is a preliminary result. It strongly depends on the calibration discussed below.

7. Calibration

A simple and reliable way to calibrate SiPMs is by using their pulse height spectrum. Various characteristic parameters can be

determined by it. The distance between the single photo-electron and the two photo-electron peak provides directly the conversion factor between ADC counts and photo-electrons. Because of the high LoNS induced signal rate, it is not possible to take low light level events with opened camera lids. With closed camera lids it is not possible to use a lightﬂasher with a correlated trigger for this measurement, as commonly used in lab measurements. Instead we use dark counts, which are either caused by thermal excitation or tunneling of free charge carriers. Dark counts are randomly distributed, therefore we use a sliding window peak search Fig. 2.Left: Sectional view of the non-imaging hexagonal light concentrator. It provides a light compression of 2.25 times and strongly attenuates the albedo beyond a cut-off angle. Right: Theﬁrst prototype pixel is built-up from seven SiPMs assembled into a matrix. The sensors are divided into three groups as indicated by the colored areas.

Fig. 3. Simulated collection efficiency of a light concentrator. The concentration ratio is 2.25. A wavelength dependent reflectivity for photons, sampled from the Cherenkov spectrum was included in this simulation. The integrated efficiency for all photons and for photons with an incident angle less than70°at the bottom of the detector surface is 92.1% and 91.5% respectively.

Fig. 4.SiPM pixel temperature during one night of observations. The temperature was measured with the sensor, next to the individual SiPMs.

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algorithm with a window width of 3, ns toﬁnd the dark count in a 30 ns event with 50 data points in it. The event trigger rate isﬁxed to 286 Hz. Because of the relatively slow signal fall time and the undershoot effect on the signal by the DRS4 chip (seeFig. 5left), only the data points in front of the pulse can be used for a baseline estimation and subtraction. This procedure is similar to the calibration procedure used by the FACT collaboration[9]and relays on the existence of cross talk.

The currently used Excelitas SiPMs, have to be operated at a low overvoltage, otherwise the dynamic range of photons is not high enough for detecting all air showers in the energy range of MAGIC. With this low bias voltage it is not always possible to re- cognize the two photo-electron peak in the charge histogram.

Fig. 5right displays such a dark count charge distribution at low overvoltage. Instead of using the distance between the single- and the two photo-electron peak, we use the difference in ADC counts of the pedestal and the single photo-electron peak. However, because of the applied sliding window peak search algorithm, there is a strong positive bias on the pedestal peak. The applied baseline subtraction is not sufﬁcient to fully eliminate this bias.

Each night of observations starts with a dark count run with closed camera lids. After that, before taking data of a source, the camera is opened and a calibration run is taken. In this, the telescopes are triggered only on the calibration lightﬂashes with a frequency of 300 Hz. We extract the calibration signals with the same algorithm as the dark counts andﬁt a Gaussian curve to the charge histogram. An example of this charge histogram is shown inFig. 5right. After that, we can associate the parameters of the Gaussian function with the calculated calibration factors from the dark count charge histogram.

During observations of sources, the lightflasher, produces calibration light pulses with a frequency of 25 Hz. We fill the ex- tracted charge of these calibration pulses in new a histogram and fit a new Gaussian curve to it. By comparing the Gaussian curve from the beginning of the night and the one calculated during the observations, we plan to estimate the change of the ADC counts to photo-electron calibration factor during the night. This change is expected to be small since the temperature variations are very small as previously shown inFig. 4.

This calibration technique is currently under development and

it depends on the solution of th bias treatment of the pedestal peak in the charge histogram.

8. Summary

We developed the first large size SiPM based pixel used in IACTs. We assembled and mounted it successfully in the MAGIC-I imaging camera. It was proven that large pixel sizes are possible to produce by assembling SiPMs into a matrix. The standard slow control was modified to control also the SiPMs. A dedicated light concentrator was simulated and produced. Thefirst prototype is integrated into the standard data taking but not in the trigger process nor in the standard analysis. A procedure for calibration is currently under development.

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Fig. 5.Left: Superimposed averaged calibration events of a SiPM and a PMT pixel as recorded by the MAGIC readout. Right: SiPM dark count charge histogram.

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