1. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Based Focal Plane Instrumentation Prototype for the MAGIC Telescopes D. Fink, A. Hahn, D. Mazin, J. Hose, P. Bangale, R. Mirzoyan

2. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 The MAGIC Telescopes Source: MAGIC Website, magic.mpp.mpg.de MAGIC: Gamma-ray astronomy at “low” energies with high sensitivity (~50 GeV energy threshold) Two Imaging Atmospheric Cherenkov Telescopes (IACTs) Located at the IAC site on La Palma, Canary Islands Operation in stereoscopic mode, 85m between telescopes MAGIC II installed and commissioned in 2009, MAGIC Upgrade including MAGIC I camera done in 2012

3. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 The MAGIC Telescopes – Camera Description Camera Camera: Located at the focus of the parabolic segmented mirror (17m dia.) 1039 pixels grouped in 139 modules of 7 pixels each Super-Bialkali type PMTs manufactured by Hamamatsu are used as the photodetectors Active area is ~1m in diameter, pixel separation is 30 mm (center to center) Only the central disk is populated, six module locations on the corners of the hexagon can be used for experimental investigation

4. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 MAGIC PMT Camera Module – Components PMT Input Channel Block Diagram Light Guide PMT Amplifier Slow Control PCB Fiber Connectors Test Pulse PCB

5. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 The MAGIC Telescopes – Signal Examples Sample images taken using the operational PMT based cameras: Top images are as recorded in stereo mode in the two cameras Bottom images are after image “cleaning” done offline during software analysis Events near the lower energy threshold contain fewer photons and are of short duration (1 to 2 nsec) Charge integration time of 3 nsec

6. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 Light Detector Design Constraints for IACTs Source: arXiv: 1406.0622 Operation at ambient temperature as opposed to cryogenic experiments Pixel sensor area driven by telescope optical properties (~400 mm 2 in MAGIC) -> Larger than optimal for SiPMs based on dynamic range requirements and cell size by several factors. Must also deal with bright objects in or near the field of view (Moon, Stars) Night Sky Background (NSB) is the largest contribution to unwanted noise Signal at right was taken during telescope operation, observation of a dark sky patch (lowest background rate, ~200 MHz/pixel) Negative peaks correspond to NSB events

7. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 MAGIC experimental SiPM Module – Requirements and Goals Summary of key requirements for using SiPM based sensors at the camera image plane: Sum of several sensor outputs to achieve the required active area Fast timing of the output sum (ideally <2 nsec FWHM) to minimize integrated background noise Provisions for disabling some or all sensors to limit response to stars in the FOV or in bright moonlight conditions based on slow control monitoring of sensor DC current Experimental SiPM Module Goals: First iteration of a fast summing scheme as proof of concept Operation alongside standard PMT modules for comparison and analysis Fill factor (ratio of active to total area), sensor detection efficiency, and optimization are secondary goals. Investigate the module concept first, parameter optimization can take place in subsequent revisions as better SiPM technology becomes available. *Note: PMTs are still the best sensor at present, but SiPMs are making rapid advances in QE, fill factor, cost

8. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Pixel Sensors, Configuration Source: Excelitas C30742-66 Series Data Sheet PCB configuration of 7 summed 6x6mm 2 sensors (Excelitas C30742-66) per pixel chosen grouped in three groups of 2, 3, and 2 devices: 50 µm cell pitch, nominal gain of 1.5x10 6, nominal 100V bias voltage common to all devices, nominal 5V overvoltage operation Control of bias voltage per group -> disable sensors for high background (e.g. moonlight) operation, adjust gain per group 30%@420nm

9. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Pixel Principle of Operation Common base discrete current sum stage was chosen to achieve fast summation Low input impedance for fast signals, high output impedance allows summation by connecting the ouputs in parallel All SiPMs have a common bias potential (~100V), individual overvoltage provided as an offset from 2 to 10V for gain adjustment and enable/disable

10. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Common Base Summation Circuit Common base discrete NPN transistor input stage: Uses commonly available BFR92 RF NPN transistor. Operating Current is 7 mA@5V (35 mW power consumption per device) Simulated input impedance @ 7 mA is low (~5 Ω), ouptut impedance high (several kΩ) for fast signal timing and current summation Input Impedance Smith Chart (Simulation)Output Impedance Smith Chart (Simulation) Time Domain Response (Simulation)Time Domain Response (Measured Average) FWHM~5 nsec

11. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Module Pixel PCB Design Proper consideration of PCB layout is necessary to avoid stray inductance in the low impedance input path 2 ½ D EM simulation was performed on layout models before fabrication to validate the design.

12. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Pixel Light Guides Light guides (modified Winston Cones) are used to provide a contiguous input area and concentrate incoming light on the sensor area: Simulated using ROBAST (ROot BAsed Simulator for ray Tracing) Light guide output incidence angle restricted to <70 o based on laboratory sensor measurements Vertical Polarization Acceptance Horizontal Polarization Acceptance

13. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Pixel Light Guides Hexagonal-to-hexagonal design modified from the original PMT module units Input acceptance angle matched to telescope mirror dimensions

14. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Module Components Analog Optical Transmission SiPM Pixel PCB Slow control (set bias, monitor SiPM current, temp.) SiPM HV DC/DC converter, 0-110V Slow Control Microcontroller board

15. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Module Laboratory Performance Measurements Sum of all 7 sensors Measurement done in a dark box with events acquired at low light levels using a fast LED optical input Tektronix DPO 7254C, 5 nsec/div, 20 GS/sec Persistence color plot of 244,619 events Separation of single photons qualitatively visible

16. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Module Laboratory Performance Measurements Measurement done in dark box, events acquired at low light levels using fast LED optical input Top left plot shows individual photon peaks for one sensor group, integrated signal in pVs Top middle plot shows signal shape averaged over all events. Baseline determined between the blue lines, signal integrated between red lines Top right plot shows histogram of event amplitudes Bottom left plot shows optical crosstalk determined by distribution fit

17. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Module Installation The SiPM module was installed in the middle right corner of the MAGIC I Camera in May 2015 Now operational during data collection enabling comparison with the PMT camera

18. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Module Sample Recorded Events

19. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 Summary First experimental SiPM Module has been successfully designed, constructed, and installed in the MAGIC I Camera Data collection and analysis is in progress Next Step: Improved Performance Selection of sensors with better fill factor and detection efficiency Aim for 2 nsec fwhm pulse width timing Modify sensor area coverage and/or light concentrator design for improved photon collection Lower power consumption while maintaining dynamic range, improve thermal stabilization Parallel Activity: Cooperation with INFN Padova, Italy, module with FBK SiPMs

20. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 Backup Slides

21. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 Linear focused type 3:1:1:1:1:1:1 voltage divider

22. International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Module Offset Schematics SiPM Offset and current measurement circuit Low offset voltage op-amps used for DC current measurement accurate to >12 bits, < 1µA to several mA Current measurement resistor included in the feedback loop, offset voltage is independent of sensor current