Si PM Based Focal Plane Instrumentation Prototype for
Si. PM Based Focal Plane Instrumentation Prototype for the MAGIC Telescopes D. Fink, A. Hahn, D. Mazin, J. Hose, P. Bangale, R. Mirzoyan International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
The MAGIC Telescopes MAGIC: Gamma-ray astronomy at “low” energies with high sensitivity (~50 Ge. V energy threshold) • Two Imaging Atmospheric Cherenkov Telescopes (IACTs) • Located at the IAC site on La Palma, Canary Islands • Operation in stereoscopic mode, 85 m between telescopes • MAGIC II installed and commissioned in 2009, MAGIC Upgrade including MAGIC I camera done in 2012 Source: MAGIC Website, magic. mpp. mpg. de International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
The MAGIC Telescopes – Camera Description Camera: • Located at the focus of the parabolic segmented mirror (17 m dia. ) Camera • 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 ~1 m 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 International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
MAGIC PMT Camera Module – Components Test Pulse PCB PMT Input Channel Block Diagram Light Guide PMT Amplifier Slow Control PCB Fiber Connectors International Conference on New Photo. Detectors (PD 15), 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 International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Light Detector Design Constraints for IACTs • 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 Si. PMs 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) Source: ar. Xiv: 1406. 0622 • 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 International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
MAGIC experimental Si. PM Module – Requirements and Goals Summary of key requirements for using Si. PM 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 Si. PM 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 Si. PM technology becomes available. *Note: PMTs are still the best sensor at present, but Si. PMs are making rapid advances in QE, fill factor, cost International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM Pixel Sensors, Configuration 30%@420 nm PCB configuration of 7 summed 6 x 6 mm 2 sensors (Excelitas C 30742 -66) per pixel chosen grouped in three groups of 2, 3, and 2 devices: • 50 µm cell pitch, nominal gain of 1. 5 x 106, nominal 100 V bias voltage common to all devices, nominal 5 V overvoltage operation • Control of bias voltage per group -> disable sensors for high background (e. g. moonlight) operation, adjust gain per group Source: Excelitas C 30742 -66 Series Data Sheet International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM 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 Si. PMs have a common bias potential (~100 V), individual overvoltage provided as an offset from 2 to 10 V for gain adjustment and enable/disable International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM Common Base Summation Circuit Input Impedance Smith Chart (Simulation) Common base discrete NPN transistor input stage: Time Domain Response (Simulation) • Uses commonly available BFR 92 RF NPN transistor. Operating Current is 7 m. A@5 V (35 m. W power consumption per device) FWHM~5 nsec Output Impedance Smith Chart (Simulation) Time Domain Response (Measured Average) • Simulated input impedance @ 7 m. A is low (~5 Ω), ouptut impedance high (several kΩ) for fast signal timing and current summation International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM 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. International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM 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 International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM Pixel Light Guides • Hexagonal-to-hexagonal design modified from the original PMT module units • Input acceptance angle matched to telescope mirror dimensions International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM Module Components Slow Control Microcontroller board Si. PM HV DC/DC converter, 0 -110 V Analog Optical Transmission Slow control (set bias, monitor Si. PM current, temp. ) Si. PM Pixel PCB International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM 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 7254 C, 5 nsec/div, 20 GS/sec • Persistence color plot of 244, 619 events • Separation of single photons qualitatively visible International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM 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 p. Vs • 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 International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM Module Installation • The Si. PM 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 International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM Module Sample Recorded Events International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Summary • First experimental Si. PM 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 Si. PMs International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Backup Slides International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
• Linear focused type • 3: 1: 1: 1 voltage divider International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
Si. PM Module Offset Schematics • Si. PM Offset and current measurement circuit • Low offset voltage op-amps used for DC current measurement accurate to >12 bits, < 1µA to several m. A • Current measurement resistor included in the feedback loop, offset voltage is independent of sensor current International Conference on New Photo. Detectors (PD 15), Moscow, July 2015
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