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SiPM 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 PhotoDetectors (PD15), Moscow, July 2015
The MAGIC Telescopes
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
Source: MAGIC Website, magic.mpp.mpg.de
International Conference on New PhotoDetectors (PD15), Moscow, July 2015
The MAGIC Telescopes – Camera Description
Camera:
•Located at the focus of the
parabolic segmented mirror
(17m 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 ~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
International Conference on New PhotoDetectors (PD15), 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 PhotoDetectors (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
International Conference on New PhotoDetectors (PD15), 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 mm2 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)
Source: arXiv: 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 PhotoDetectors (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
International Conference on New PhotoDetectors (PD15), Moscow, July 2015
SiPM Pixel Sensors, Configuration
30%@420nm
PCB configuration of 7 summed 6x6mm2 sensors (Excelitas
C30742-66) per pixel chosen grouped in three groups of 2, 3, and 2
devices:
•50 µm cell pitch, nominal gain of 1.5x106, 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
Source: Excelitas C30742-66 Series Data Sheet
International Conference on New PhotoDetectors (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
International Conference on New PhotoDetectors (PD15), Moscow, July 2015
SiPM Common Base Summation Circuit
Input Impedance Smith Chart (Simulation)
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)
Time Domain Response (Simulation)
Output Impedance Smith Chart (Simulation)
Time Domain Response (Measured Average)
FWHM~5 nsec
•Simulated input impedance @ 7 mA is low (~5 Ω),
ouptut impedance high (several kΩ) for fast signal
timing and current summation
International Conference on New PhotoDetectors (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.
International Conference on New PhotoDetectors (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 <70o based
on laboratory sensor
measurements
Vertical Polarization
Acceptance
Horizontal Polarization
Acceptance
International Conference on New PhotoDetectors (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
International Conference on New PhotoDetectors (PD15), Moscow, July 2015
SiPM Module Components
Slow Control Microcontroller board
SiPM HV DC/DC converter, 0-110V
Analog Optical Transmission
Slow control (set bias, monitor
SiPM current, temp.)
SiPM Pixel PCB
International Conference on New PhotoDetectors (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
International Conference on New PhotoDetectors (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
International Conference on New PhotoDetectors (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
International Conference on New PhotoDetectors (PD15), Moscow, July 2015
SiPM Module Sample Recorded Events
International Conference on New PhotoDetectors (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
International Conference on New PhotoDetectors (PD15), Moscow, July 2015
Backup Slides
International Conference on New PhotoDetectors (PD15), Moscow, July 2015
•Linear focused type
•3:1:1:1:1:1:1 voltage divider
International Conference on New PhotoDetectors (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
International Conference on New PhotoDetectors (PD15), Moscow, July 2015