Advanced Lab Course (FOPRA):Light Sensors for γ-ray Astronomy

FOPRA #81

Abstract: Astroparticle physics commonly relies on the detection ofphotons which are emitted from the primary particle in a secondary process.Examples for such processes are fluorescence, scintillation and Cherenkovlight. The emitted light is then recorded with suitable photo sensors. Fromthe detector output the properties of the primary particle can be estimated.The most popular light sensors in particle physics are PhotoMultiplier Tubes(PMTs), Silicon PhotoMultipliers (SiPMs), and Positive Intrinsic Negative(PIN) photodiodes.

This advanced lab course offers the opportunity to undertake experimentswith these low light level (LLL) photo sensors, understand their workingprinciples and judge their suitability for γ-ray astronomy based on theirstrength and weaknesses.

Contents

1 Introduction 21.1 Astroparticle Physics . . . . . . . . . . . . . . . . . . . . . . . 21.2 Photo sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Photomultiplier Tubes . . . . . . . . . . . . . . . . . . 31.2.2 PIN Photodiode and APD . . . . . . . . . . . . . . . . 61.2.3 Silicon Photomultiplier . . . . . . . . . . . . . . . . . . 7

2 Experiments 102.1 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.1.1 Light Sources . . . . . . . . . . . . . . . . . . . . . . . 102.1.2 High Voltage . . . . . . . . . . . . . . . . . . . . . . . 102.1.3 Technical equipment . . . . . . . . . . . . . . . . . . . 102.1.4 Faraday Cage . . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Gain Measurement . . . . . . . . . . . . . . . . . . . . . . . . 112.3 Timing Precision . . . . . . . . . . . . . . . . . . . . . . . . . 122.4 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.5 PMT After-pulsing Measurement (depending on availability) . . . 132.6 SiPM Cross-talk Measurement (depending on availability) . . . . . 14

3 Evaluation and Report 153.1 Gain and Charge Resolution Measurement . . . . . . . . . . . 153.2 Timing Precision . . . . . . . . . . . . . . . . . . . . . . . . . 153.3 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.4 PMT After-pulsing Measurement . . . . . . . . . . . . . . . . 153.5 Cross-talk Measurement . . . . . . . . . . . . . . . . . . . . . 153.6 Application based Sensor Choice . . . . . . . . . . . . . . . . . 16

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1 Introduction

1.1 Astroparticle Physics

Astroparticle physics connects the smallest and the largest scales of physics:astrophysics and particle physics. Astroparticle physics tries to increase theunderstanding of astronomical objects like supernova remnants (SNRs) orrelativistic jets from black holes and also studies fundamental particle phys-ics at energies higher than possibly produced on earth.High-energy particles emitted from such astrophysical sources travel towardsEarth and can be recorded by detectors. The difficulty is that most of thiscosmic-rays are protons and ions. Due to their charge they do not pointback to their original sources because they get deflected in Galactic andextra-galactic magnetic fields during their transit. Only photons and neut-rinos do not suffer from this effect and therefore can be used to study theproperties of the emitting source (Figure 1). Optical and radio photons areeasily observable by ground based telescopes because those photons only suf-fer from moderate absorption until they reach the ground (see Figure 2). Forother wavelengths different detection techniques are required. X-rays andlow energy gamma rays are detected by calorimeters and particle trackersonboard satellites or balloons. But at even higher energies (& several tensof GeV) the fluxes become very low and the particle showers are too big tobe confined inside such small detectors. It would required detector sizes toolarge and too heavy for satellites or balloons.

Although such high-energy gamma rays do not reach the ground theycan be efficiently indirectly observed from ground. Ground based gamma-rayastronomy is based on a primary high-energy gamma ray (&1 GeV) creatingan extended air shower (EAS) of charged particles in the atmosphere. AnEAS can be directly detected by measuring the particle cascade in watertanks or scintillator plates. This technique is used for example by HAWC [1].Another technique is to detect the fluorescence or Cherenkov light an EASemits. Cherenkov light is emitted by charged particles that propagate fasterthrough a medium than the speed of light in that medium c = c0

nwith c0 the

speed of light in vacuum and n the refractive index of the medium. Cherenkov

photons are emitted under a characteristic angle Θ = acos(

1nβ

). Imaging

Air Cherenkov Telescopes (IACTs) collect this Cherenkov light with largemirrors and record it with sensitive fast cameras. Currently there are threelarge IACTs in operation MAGIC, H.E.S.S. and Veritas [2, 3, 4] and a newgeneration of IACTs is being build [5]. The cameras of these telescopes needto be composed of about 1000 fast and very sensitive pixels. The used photo

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sensors need to be able to resolve signals consisting of only several photonsand only few ns duration.

Figure 1: Artist image of cosmic-ray propagation from an astrophysicalsource to Earth. (credit: Helmholtz Alliance for Astroparticle Physics(HAP))

1.2 Photo sensors

Starting with the human eye over photographic emulsion up to PMTs andsemiconductor based sensors many different light detectors have been utilisedin particle and astroparticle physics. This section describes three commonphoto sensors that will be evaluated during this lab course for their use inIACTs.

1.2.1 Photomultiplier Tubes

The photomultiplier tube (PMT) was developed in 1930 by Leonid Kubetskyand is nowadays widely used in astronomy, medical imaging, and high energyphysics.It consists of a vacuum tube with a multi-dynode stage inside. The photo

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particle detectors

Earth surface

Figure 2: Height of 50% attenuation of incident photons of the electromag-netic spectrum in the Earth atmosphere. [8]

cathode is supplied with a high voltage (∼ 1000 V) which is shared with thedynodes via a resistive voltage divider. A schematic view is shown in figure3. An incident photon creates a free electron via photoelectric effect at thephoto cathode. This so called photo-electron (ph.e) is then accelerated in theelectric potential towards the first dynode. The number of released electronsover impinging photons is called quantum efficiency (QE).

QE =Nph.e.

Nphot

(1)

Focusing electrodes can be used optionally to increase the collection ef-ficiency of this first dynode. The overall photon detection efficiency (PDE)depends on the quantum efficiency QE(λ) and the ph.e. collection efficiencyα(U).

PDE(λ, U) = QE(λ) × α(U) (2)

The accelerated electron releases a bunch of new electrons at impact atthe dynode. This bunch of electrons is again accelerated towards the next

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dynode where the process of secondary emission repeats for each electron ofthe bunch thereby amplifying the signal at each dynode. The total chargegain η depends on the applied voltage and number of dynodes N and theirindividual gains ηi.

η =N∏i

ηi (3)

After several dynodes PMTs can reach a gain of 106 − 107 while still beingable to resolve single photo-electrons. PMTs can be manufactured with verydifferent active area sizes. Small and compact PMTs are for example usedin hand-held radiation detectors whereas huge PMTs with a size of 550 cm2

are used by the IceCube Neutrino Observatory and even bigger ones with2400 cm2 in the Super-Kamioka Neutrino Detection Experiment.

PMTs suffer from after-pulsing due to ion impact on the photo-cathode.A photo-electron that was created by normal photon impact on the photo-cathode hits an atom of the rest gas that is freely moving as part of theimperfect vacuum or adhesively bound onto the first dynode. This can createa partially ionized particle that is accelerated backwards towards the cathodedue to its positive charge. The impact of such a particle releases a bunchof electrons that are amplified by the dynode system and create big outputsignals. The time delay, in respect to the initial light pulse, of such after-pulses is characteristic, depending on their charge and mass.

Figure 3: Schematic view of a PMT.

The PMT’s overall photon detection efficiency is strongly dependent onmaterial used as photo cathode, its thickness, and on the mentioned collec-tion efficiency of the first dynode. Today’s PMTs used for the LST of CTAreach a quantum efficiency of 43 %.Because the first free electron is accelerated in vacuum its trajectory strongly

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depends on the surrounding magnetic field. The PMTs in the MAGIC cam-eras are therefore shielded with a mu-metal wrapping against disturbances.Ageing effects might reduce the detection efficiency with time by loweringthe conversion efficiency of the photo cathode or destruction of the dynodecoating. Powered PMTs show accelerated ageing until destruction when ex-posed to bright light and therefore need to be protected against high photoncurrents.

REMEMBER: Never expose a powered PMT to ambient light orit will be destroyed.

Because the photo-electrons need to travel different distances from thephoto cathode to the first dynode depending on the point of interaction ofthe incoming photon, there is a time spread in the outgoing signal. Thisso-called transit time spread (TTS), is gain dependent and usually in theorder of several ns.PMTs provide very fast signal shape with a width of a just few ns. Theconversion of recorded readout counts to photo-electrons is very easy byusing the so-called F-factor method which was described by Mirzoyan andSchweizer in [6] and [7].

1.2.2 PIN Photodiode and APD

A photo detector known since the 1960s is the Positive Intrinsic Negative(PIN) photodiode. It is still used in high energy physics experiments forthe readout of scintillating crystals. Another practical example is the use forsimple monitoring of light sources. As all pure solid state light detectors theyare insensitive to magnetic fields and are rather compact. Nevertheless it isnot possible to use PIN photodiodes in IACTs because they require between200 - 300 photons for a detectable signal and their peak sensitivity is in thered to infrared region. An additional drawback for their application is thelow bandwidth of just a few 100 kHz.The APD is a further development of PIN photodiodes. In contrast to PINdiodes, an APD has an intrinsic gain and much lower noise while providingsensitivity down to tens of photons. After the incoming photon created afree electron via internal photo effect, the free electron is accelerated in thep-n junction and creates more electron-hole pairs by impact ionization. Thisprocess is repeated several times thus achieving an internal gain of typically50 to 200. With a p-on-n layer structure sensitivity is highest for blue light.Their temperature dependence is very high even for indirect semiconductorbased photo sensors. Because they are operated in the linear gain regime

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below the breakdown voltage, the temperature has to be stable within afraction of a degree. This was realized for the Compact Muon Solenoid (CMS)experiment at CERN which uses APDs in the electromagnetic calorimeter.It requires the sensor temperature to be stable within ± 0.05 ◦C to preservethe energy resolution. This of course is impractical for the usage in telescopeswhich are operated at changing ambient temperatures.

1.2.3 Silicon Photomultiplier

If the bias voltage of an APD is increased one eventually reaches the break-down voltage where also the holes are accelerated enough to create new elec-tron hole pairs. Both charge carriers contribute now to the avalanche whichis therefore self-sustaining. This mode of operation is called Geiger-mode.To stop this avalanche the bias voltage needs to be lowered under the break-down voltage. After stopping the avalanche the APD needs to re-charge andis then able to trigger another time. This is only practicable at low trig-ger rates and cryogenic temperatures otherwise the thermally excited darkcounts would saturate the APD without any external light. One can howeverdivide the APD into a array of individual small APDs. The resulting deviceis called a Silicon Photomultiplier (SiPM). A microscope image is shown infigure 4.

The individual APD cells are again biased with a common voltage abovethe breakdown and are therefore called Geiger-mode APDs (G-APDs). Photonscan only be detected at the active area of each cell. The space between thecells, used for biasing the cells and shielding cross talk is field free and there-fore dead. A simplified drawing of the layer structure of a SiPM can be foundin figure 5. It shows the metal contact for biasing the cell and the trenchesbetween the cells to prevent cross-talk. Cross-talk is caused by photons emit-ted from the charge avalanche at break down. The emitted photons can reacha neighbouring cell and trigger a breakdown in this cell. The output signalis indistinguishable from the real detection of two photons. Therefore thecross-talk plays an important role in the reconstruction of the number ofincident photons from the measured signal.There are many other names for this device like: MAPD, SSPM, DAPD,PPD but SiPM is slowly ousting the other names. Sometimes G-APD wasused to refer to the full sensor instead of only a single cell. Throughout thislab course we will use the above-mentioned definitions of SiPM as the deviceand G-APD as a single cell.While a regular APD has a big capacitance and therefore a long rechargetime after breakdown, SiPM cells are much smaller and therefore have muchsmaller recharge times. Standard cell sizes or better cell pitches are 50µm,

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Figure 4: Microscope image of a SiPM. The individual G-APD cells areclearly visible.

75µm and 100µm. The number off cells is giving the dynamic range but onthe other hand the dead area is increasing with the number of cells as well.Each G-APD gives an identical signal when being triggered. At low lightlevels the information from each cell can be considered as a binary: cell firedor not fired. The consequence is that SiPMs have an extraordinary goodcharge resolution.

Because of the use of silicon, which is an indirect band gap semicon-ductor, SiPMs show a temperature dependent gain. Even in absence of light,thermally excited electrons can trigger a breakdown. Due to the binarynature of the cell’s signal, there is no way to distinguish them from real lightinduced signals.One of the advantages of SiPMs is their compactness and their low operationvoltage which is usually between 25 V and 100 V. The spectral sensitivity ofSiPM ranges from 350 − 900 nm which makes them suitable for air showerdetection but one must keep in mind that more noise (from the Light of theNight Sky (LoNS)) than with a PMT will be collected as well.

The PDE of a SiPM is given as a function of the ratio of light sensitivearea to the total SiPM area Geomeff , the wavelength dependent QE(λ) of the

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G-APD cell, and the probability of staring a Geiger avalanche Geigereff(U)which is voltage dependent.

PDE(λ) = Geomeff ×Geigereff(U) ×QE(λ) (4)

SiPMs are very robust and do not show any ageing, not even when ex-posed to high levels of light while under power.

Figure 5: Schematic view of the SiPM layers.

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2 Experiments

In this lab course you will run a few experiments with different photo sensorsto evaluate their usability in Cherenkov telescopes.

2.1 Safety

You must have attended the TUM physics department safety instructionsfor the advanced lab courses not more than one year ago. If this safetyinstructions include a test you must have passed it.You must have read and understood this document completely and complywith all instructions by the lab personal and instructors. If you have anyquestions or concerns regarding the safety of people or instruments you haveto ask the instructor prior to any further action. If you observe misconductagainst safety and behaviour rules you have to report it to the instructorimmediately.You must never work alone in the lab. The minimum number of people inthe lab to run or change the experiments is two at any time.

2.1.1 Light Sources

During these experiments you will use LED based light sources or Lasers.The used wavelengths are in the visible range. You must never directly lookinto the light source or attached optics and ensure the light source is turnedoff before opening the dark box.

2.1.2 High Voltage

For measurements with the PMTs you will use high voltage (HV) of up to 1kV. You must ensure that the high voltage is switched off before opening thedark box or reconnecting any cable. HV components must only be touchedby one hand the other hand must not touch the setup or table to avoidpotential currents through the heart.

2.1.3 Technical equipment

After human safety, the safety of the equipment is the second highest concern.You must ensure the safety of all equipment you use, use it only for the desireduse and within the design ranges.You must not eat or drink near the setup or bring liquids close to electronicsin the lab.

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It’s advised not to bring cell phones close to the setup, leave them at thedoor. Cell phones introduce a huge pickup noise in the setup.

2.1.4 Faraday Cage

Part of the experiment might be conducted inside a Faraday cage for bettershielding against electronic noise. The cage can be opened from the insideand is ventilated. Still, when working inside the cage you must have a secondperson outside or the door mechanically blocked from closing accidentally.Do not eat or drink inside the Faraday cage.

2.2 Gain Measurement

In this first experiment you will measure the gain and charge resolution ofdifferent sensors. The sensors get illuminated by short light pulses from aLED of < 1ns. The signal is recorded with an oscilloscope which can directlyperform the charge integration. The oscilloscope will be triggered with thefrequency of the light source. The resulting distributions are saved as textfiles for the later calculation of gain and charge resolution.

1. The PMT is installed into the dark box and the light source is aimed atit. Then the typical HV value for the PMT model is applied after thebox is sealed light-tight (Again, be very careful with the HV source.Only switch it on after you sealed the box and double checked thecabling. That is for your own safety as well as for the safety of theequipment).

2. The PMT signal output is connected to a charge sensitive amplifier. Itis powered with ±15V. The output of the amplifier is connected to theoscilloscope and the trigger input is connected to the pulse generatorfor the light source.

3. Adjust the light pulse intensity to a value that gives one photon orless on average for the gain measurement. Use a slightly higher lightintensity for the charge resolution measurement.

4. Now the signal integration mode of the oscilloscope is selected (yoursupervisor will help you) and a large number of waveforms are analysedand summed up. You should should get a histogram with a pedestalpeak (zero charge + noise) from trigger events with no photo-electrongenerated) and one or more peaks originating from one or more photo-electrons. The distance between the peaks can be converted to the gain

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and the separation of the peaks to the charge resolution.Make sure you save the histogram to a new text file without overwritingprevious measurements and copy this new file to a USB stick.

5. Perform this measurement for at least 4 different HV values.

6. Repeat the whole procedure for the other light sensors.

2.3 Timing Precision

In the next experiment you measure the timing precision of the differentdetectors.

1. The light detector is installed into the dark box together with the lightsource.

2. The light detector is connected to a charge sensitive amplifier. It ispowered with ±15V. The output of the amplifier is connected to theoscilloscope.

3. Use the external signal generator to trigger light source and oscillo-scope.

4. Make sure the dark box is tightly closed before switching on the detectoror light source.

5. Then the light detector bias voltage is applied.

6. Use the peak search method of the oscilloscope to record the arrivaltimes of the pulses.

7. Make sure you save the resulting histogram to a new text file withoutoverwriting previous measurements and copy this new file to a USBstick.

8. The TTS is calculated as the full width at half maximum (FWHM) ofthe arrival time distribution.

9. Repeat this measurement for at least 4 different bias voltage values.

10. Repeat the whole measurement for the next detector type

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2.4 Linearity

In the next experiment you measure the timing precision of the differentdetectors.

1. The light detector is installed into the dark box together with the lightsource.

2. The light detector is connected to a charge sensitive amplifier. It ispowered with ±15V. The output of the amplifier is connected to theoscilloscope.

3. A calibrated reference detector is used to calibrate the light intensity.

4. Connect the triggers of light source and oscilloscope.

5. Make sure the dark box is tightly closed before switching on the detectoror light source.

6. Then the light detector bias voltage is applied.

7. Use again the signal integration method of the oscilloscope to recordthe signal charge.

8. Make sure you save the resulting histogram to a new text file withoutoverwriting previous measurements and copy this new file to a USBstick.

9. Make sure you noted all relevant settings like bias voltage etc. for theevaluation.

10. Repeat this measurement for several different light intensities, increas-ing one by one.

11. Repeat the whole measurement for the next detector type

2.5 PMT After-pulsing Measurement (depending on availab-

ility)

1. The PMT is installed into the dark box and the light source is aimed atit. Then the typical HV value for the PMT model is applied after thebox is sealed light-tight (Again, be very careful with the HV source.Only switch it on after you sealed the box and double checked thecabling. That is for your own safety as well as for the safety of theequipment).

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2. The PMT signal output is connected to a charge sensitive amplifier. Itis powered with ±15V. The output of the amplifier is connected to theoscilloscope and the trigger input is connected to the pulse generatorfor the light source.

3. Adjust the light pulse intensity to a value that gives a few photons.

4. Use the persistence mode of the oscilloscope to stack many waveforms.

5. Save the waveform or read the values from the plot and save a screen-shot for the later evaluation.

6. Repeat this measurement for at least 4 different HV values.

2.6 SiPM Cross-talk Measurement (depending on availability)

1. The SiPM is installed into the dark box without a light source.

2. The SiPM is connected to a charge sensitive amplifier. It is poweredwith ±15V. The output of the amplifier is connected to the oscillo-scope.

3. Make sure the dark box is tightly closed before switching on the detectoror light source.

4. Then the SiPM bias voltage is applied.

5. Make sure you see the distinct line of the first phe.

6. Set the trigger of the oscilloscope on the level of about 0.5phe.

7. Use the charge integration mode of the oscilloscope to record the pulsecharges.

8. Make sure you save the resulting histogram to a new text file withoutoverwriting previous measurements and copy this new file to a USBstick.

9. Repeat this measurement for at least 4 different bias voltage values.Use the voltages you calibrated the sensor with.

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3 Evaluation and Report

For your report please work out the tasks described in this section and an-swer the questions below. The report should include all relevant plots, ifyou perform fits these should be drawn as well. Please provide uncertaintycalculations/estimates for all extracted values. All results should be givenwith their respective uncertainty or at least an estimate of their uncertainty.Please provide all names, matriculation numbers and the date of the labcourse.Afterwards submit you report in pdf format and English language to thesupervisor by email.

3.1 Gain and Charge Resolution Measurement

Calculate the gain and plot is versus the applied voltages for the differentdetectors. Compare the charge resolutions and discuss the differences qual-itatively.

3.2 Timing Precision

Give the TTS (FWHM of the recorded distribution) for the different sensortypes. How and why does it change with different operation voltages?

3.3 Linearity

Use the gain calibration of the first experiment to convert the recorded chargedistributions to a number of photo-electrons. Plot the resulting number ofphoto-electrons charge of the measured photo sensors versus the referencesensor. Can you see a difference of the trends? Can you explain their origin?

3.4 PMT After-pulsing Measurement

Mark all the visible characteristic after-pulse populations in the saved screen-shot or waveform. Can you calculate/estimate which elements of the rest gascause these after-pulses?

3.5 Cross-talk Measurement

Integrate the number of events with more than one phe. and calculate theratio to the total number of counts. Plot the result versus the applied voltage.Explain the trend.

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3.6 Application based Sensor Choice

Based on the measurements you conducted and your gained experienceschoose a suitable sensor for the following applications and justify your choicein one or two sentences.

1. For an IACT with large pixels (several cm2), like MAGIC or the LSTfor a new moon night

2. For an IACT with large pixels (several cm2), like MAGIC or the LSTfor a full moon night

3. For an IACT with small pixels (tens of mm2), like the FACT telescope

4. For a water Cherenkov tank like in HAWC

5. For a neutrino telescope like IceCube

6. For a low background dark matter detector

7. For an application onboard a small satellite

8. For a detector in a high radiation environment

9. For a combination of PET and MRI detectors for medical application.

10. For a handheld, portable radiation monitor

References

[1] HAWC Observatory, www.hawc-observatory.org/

bibitemmilagro

[2] H.E.S.S. observatory, www.mpi-hd.mpg.de/hfm/HESS

[3] VERITAS, veritas.sao.arizona.edu

[4] MAGIC, magic.mppmu.mpg.de

[5] Cherenkov Telescope Array, www.cta-observatory.org

[6] On the Calibration Accuracy of Light Sensors in Atmospheric Cherenkov,Fluorescence and Neutrino Experiments, Mirzoyan, R. and Lorenz, E.,Proceedings of the 25th International Cosmic Ray Conference, 1997, 265–268

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[7] The optical calibration of the MAGIC telescope camera, Schweizer, T. etal., 5, 2497–2503

[8] Measurement of Very High Energy Gamma-Ray Emission from Four Blaz-ars Using the MAGIC Telescopes and a Comparative Blazar Study, Wag-ner, R., 2006, phd thesis

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