A Temperature Compensated Power Supply for Silicon-Photomultiplier Pankaj Rakshe WAPP – 2014 20th December 2014

Acknowledgment Prof. S. R. Dugad (TIFR) Prof. P. D. Khandekar (VIIT) Mr. Sergey Los (FNAL) Mr. Raghunandan Shukla (TIFR) Ms. Sarrah Lokhandwala (TIFR) Prof. C. S. Garde (VIIT) Prof. S. K. Gupta (TIFR)

Introduction Sensitive Photo-detectors like PMT’s are of great interest to scientific community due to their use for examining processes that emit very low photon signal. For example, In GRAPES-3 experiment PMT’s are used to detect scintillation light from wavelength shifting fibres.

Photo-Multiplier Tube (PMT) Gain ~ 106 Response time ~ 2 ns Size : Dia 2”, Length 6” Operating Voltage ~ 2000 V Quantum Efficiency < 20 % Affected by Magnetic fields Expensive !

Silicon Photo-Multiplier (SiPM) Compact Device Operating voltage (30-120V) Resolution - Single photon detection Response time – ~100 ps High gain - 10 6 High Quantum Efficiency – 90% High Photon Detection Efficiency – 60% Immunity to Magnetic Field 1-3 mm 1-3 mm

SiPM SiPM is a 2-D array of Avalanche Photo Diode’s (APD’s) , all resistively coupled together. SiPM is generally biased above its breakdown voltage , called as Geiger mode. Each pixel (APD) acts as a binary device, indicating presence or absence of photon. Device as whole gives analog signal indicating number of pixels fired. Typical size of each APD (pixel) is 50 µm × 50 µm and a typical gain of ~ 106

Limitations of SiPM Dark Counts After Pulsing Cross-Talk Thermally Generated Carriers will give signal that appears as genuine photon signal After Pulsing Carrier Trapping and Delayed Release Cross-Talk Pixel Firing Due to Photon Detection in Adjacent Pixel Temperature Dependence of Breakdown Voltage SiPM Gain is Affected by Changes in Applied Over-Voltage (Applied Voltage – Breakdown Voltage)

SiPM Temperature Dependence Geiger mode: above breakdown VS = VBR + Over-Voltage Gain dependant on amount of Over-voltage applied But, Breakdown Voltage is dependent on temperature Effectively, gain changes by 3-5%/˚C Over-Voltage should be constant for constant gain 𝑽 𝑺 𝑻 = 𝑽 𝑩𝑹 𝑻 +𝑶𝒗𝒆𝒓 𝑽𝒐𝒍𝒕𝒂𝒈𝒆 * Bajarang Sutar * S. R. Dugad and K. C. Ravindran

Solutions to Temperature Dependency Constant Temperature Indoor Applications - applicable Temperature control of detectors in outdoor environment is not possible e.g. GRAPES-3 experiment containing 400 detectors in large area outdoor field of about 25000 m2 with temperature variations 5 - 30 ˚C Temperature Dependant Biasing Conditions The Bias Voltage of SiPM can be controlled for changing the operating point Temperature dependent Power Supply that will keep over-voltage constant (i.e. Gain constant) Two solutions to Gain stability Constant temperature : possible for indoor application But for outdoor applications: biasing change is option

Approaches To Temperature Compensation Method Used Author Year Remarks Limitations Dark Current Control Miyomoto et al. 2009 Dark Current and Temperature relation approximated to exponential function and use of Thermistor (similar relation) for compensation Uses Expensive Commercial Power Supply Limited to one/two channels External control required for temperature compensation Li et al. 2012 Dark current and Bias Voltage relation used for designing the voltage controlled current sink that changes bias condition Bias Voltage Control Bencardino et al. Use of Temp. to voltage converter and a Op-amp based circuit for changing the bias return potential w.r.t. temperature variations Licciulli et al. 2013 Blind SiPM used as a temperature sensor and amplitude of Dark pulses of Blind SiPM is maintained constant using Op-amp based feedback circuit for constant gain of other SiPM in parallel Gil et al. 2011 External Input of the power supply is controlled by a micro-controller based system to change the output voltage w.r.t. temp. Dorosz et al. LabVIEW based feedback system for controlling the power supply output Explain the approaches used till now Commercial power supply is used. --- costly, single channel, no temperature compensation provision, high current (not necessary) SiPM is in still research stage. New structures are coming everyday. Hence only one or two channels are sufficient for purpose of research. But if one needs to replace the PMT with SiPM, then for large scale experiment (~1000 detectors), mutiple commercial power supplies are not an option. Very costly and not feasible (3.5 Lacs cost) Hence need of multichannel power supply with temperature compensation at low cost

Specifications Sr. No. Parameter Value 1. Output Voltage 0 to 100V 2. Temperature Reading Resolution 0.1 ˚C 3. Temperature Compensation Factor 10 to 100 mV/˚C 4. Number of Channels 8 5. Output Voltage Resolution 10 mV 6. Maximum Current Limit per Channel 100 µA 7. Full Scale Leakage Current per Channel 40 µA

Block Diagram High Voltage Generation (Voltage Multiplier Chain) PC USB Control Unit (Micro-controller) Voltage Regulation Scheme Temperature Sensors DAC Divided into following parts: High Voltage generation High Voltage Regulation Control Unit: microcontroller and PC Current Sense Current Sense SiPM

Prototype Power Supply (SiPM-PPS-v1)

Features of SiPM Power Supply Programmable Output Voltage (0 - 100 V) with resolution of ~12 mV Programmable Temperature Compensation Factor (~12 – 100 mV/˚C) In-built Data Acquisition System for recording Temperature and Leakage Current via USB

Ripple = 5 mVP-P 0.04113% for 10% change in line voltage No load to Full load (100uA) regulation 0.6025% Ripple = 5 mVP-P Stable within 10 mV for 0 – 100 V

Compensation Algorithm Verification Comp. factor = 240 mV/˚C Comp. Factor = (0.029x2-1.4x) /˚C

SiPM Experimental Setup

SiPM Test: Without Compensation Gain Variation of about 4 %/˚C

SiPM Test: With Compensation Gain Variation of 0.8 %

In Conclusion Without compensation gain variation nearly 4 %/˚C With compensation gain variation 0.08 %/˚C Output Ripple of 5 mV with the Resolution of 12.5 mV in 100 V with temperature correction at each 0.2 ˚C change

Compared to Keithley 6487 Unit SiPM Programmable Power Supply Temperature Compensation Feature Small Size and Light Weight (portable) Multi-channel (8/16 channels) Cost – Inexpensive !!! (₹ 750/channel) which is 400 times less Voltage Source and Pico-ammeter

SiPM-PPS-v2 Number of channels = 16 Better output resolution = 6.25 mV DAC resolution = 14 bits Better current sensing resol. = 1 nA ADC resolution = 16 bits Provision for expandability I2C for multiple boards

Specifications Comparison Sr. No. Parameter Required Value Prototype Board SiPM_PPS v2 1. Output Voltage 0 to 100V 0 to 100 V 2. Number of Channels 8 16 3. Output Voltage Resolution 10 mV 12.5 mV 6.25 mV 4. Maximum Current Limit per Channel 100 µA 5. Full Scale Leakage Current per Channel 40 µA 35 µA 6. Temperature Reading Resolution 0.1 ˚C 7. Temperature Compensation Factor 12.5 to 100 mV/˚C 6.25 to 100 mV/˚C

Summary SiPM has some outstanding features that in some respect could replace PMT in near future. Newer versions of SiPM are being developed to overcome its limitations. The temperature dependence is one of the major limitation preventing the use of SiPM in outdoor applications. The programmable temperature compensated power supply is useful for operating the SiPM in different environmental conditions.