International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Based Focal Plane Instrumentation Prototype for the MAGIC Telescopes D

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


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


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


MAGIC PMT Camera Module – Components

PMT Input Channel Block Diagram

Light Guide

PMT

Amplifier Slow Control PCB

Fiber Connectors

Test Pulse PCB


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


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 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)

• 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


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


SiPM Pixel Sensors, Configuration

Source: Excelitas C30742-66 Series Data Sheet

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

30%@420nm


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


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


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.


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

measurementsVertical Polarization

AcceptanceHorizontal Polarization

Acceptance


SiPM Pixel Light Guides

• Hexagonal-to-hexagonal design

modified from the original PMT

module units

• Input acceptance angle matched

to telescope mirror dimensions


SiPM Module Components

Analog Optical Transmission

SiPM Pixel PCB

Slow control (set bias, monitorSiPM current, temp.)

SiPM HV DC/DC converter, 0-110V

Slow Control Microcontroller board


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


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


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


SiPM Module Sample Recorded Events


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


Backup Slides


• Linear focused type

• 3:1:1:1:1:1:1 voltage divider


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

Documents

International Conference on New Photo- Detectors (PD15), Moscow, July 2015 SiPM Based Focal Plane Instrumentation Prototype for the MAGIC Telescopes D