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Photosensors

Noah Hood, Kaixuan Ni

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Outline

Photomultiplier Tubes (PMTs)*
Avalanche Photodiodes (APDs)*
Silicon Photomultipliers (SiPMs)*
Applications of photosensors

* Based on book: Particle Detectors: Fundamentals and Applications by Hermann Kolanoski and Norbert Wermes, 2020.

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Introduction

Particle detectors play a crucial role in studying fundamental particles and their interactions.
Photosensors are essential components of particle detectors, converting light into electrical signals.

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Photosensors used in the Summer School

Scintillator/SiPM Lab
Liquid Argon Lab
Inorganic Scintillator Lab

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Photomultiplier Tubes (PMT)

PMTs are vacuum-based devices with high gain and excellent timing resolution.
They consist of a photocathode, dynodes, and an anode.
PMTs are widely used in experiments requiring high sensitivity and single-photon detection.

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PMT Window

borosilicate glass most common largely due to price
transmission cuts off at 300nm

MgF2	115 nm
sapphire	140 nm
synthetic silica (fused quartz)	∼ 150 nm
UV-borosilicate	∼ 185 nm

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Photoelectric Effect

Photon with energy greater than the work function ionizes an electron
3 steps for emission in cathodes (Spicer Model)

Photon is absorbed and kinetic energy transferred to an electron,
The electron travels to the surface of the material,
The electron escapes from the surface of the cathode into the vacuum.

Quantum efficiency, function of the three steps:

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Quantum Efficiency

Metals - High reflectance, lots of electrons in valence band, high work function, generally not ideal
Usually compound semiconductors are used
Can coat surface with electropositive material (e.g. Cs), leaves positive ions on surface, effectively decrease vacuum energy/work function

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Photocathode

Monoalkali (e.g. Cs–I, Cs–Te, K–Br, Sb–Cs3)
Bialkali (e.g. Sb–Rb–Cs3)
Multialkali materials, (e.g. Sb–Na–K–Cs3)
Less frequently III–V semiconductors, ‘activated’ (doped) with caesium, like GaAs(Cs), GaP(Cs), or InGaAs(Cs).

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Photocathode

Monoalkali sensitive to UV and VUV (solar blind)
Bialkali and multialkali have higher QE (up to infrared sensitivity)
Expense of higher noise because longer wavelengths → smaller energy level gaps → possibility of thermal emission

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Electron Multiplier

Dynodes + Anode
Linearly focusing

Highest gain/linearity

“Venetian Blinds”

Optimal shape for tube
Allows large cathode
Small afterpulsing

Both are very sensitive to external magnetic fields – can be mitigated by mu-metal housing (high permeability, redirects mag fields)

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Voltage Divider – Base

Voltage of dynodes is controlled through a series of resistors
Choose resistances to balance load vs. stable potentials on dynodes
Capacitors help control effect of large currents on final dynodes

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Efficiency

Can also define radiant sensitivity similar to efficiency

Note relatively low efficiency, at most ~ 40% for PMTs

Dashed line without UV transparent window

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Single PE resolution

arXiv:1202.2628

R11410-10 (used in XENON1T/nT/LZ/PandaX)

XENON100, R8520

arXiv:1107.2155

Peak/valley: 1.54

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PMT Summary

high gain (~ 10^6),
very low noise,
single photon detection capability,
small quantum efficiency by comparison (typically about 25%),
good time resolution in the order of 200 ps,
commercially available in many variants for a range of application scenarios,
difficult to handle regarding size and shape in comparison to alternatives (e.g. photodiodes).

Super-Kamiokande

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Other Similar Detectors

Microchannel Plate

Channels coated with photocathode material
More position resolution, lower gain

Vacuum Phototriode (VPT)

Useful when strong magnetic fields present
Photoelectrons pass through anode first, some are returned, smaller multiplication

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Photodiodes

APDs are semiconductor-based devices that provide gain through avalanche multiplication.
They offer moderate gain and sensitivity, and are often used in applications with low-light levels.
Work using internal photoeffect, i.e. valence electrons are excited to conduction band instead of vacuum
Can be made much smaller than PMTs at the expense of amplification

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Photodiodes

PIN photodiode is most common
Undoped region is placed between p- and n- doped regions
Reverse bias voltage applied, field extends into intrinsic region

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Efficiency

Much higher than for PMTs, ~70% for photodiodes
Lower time resolution (~2 ns compared to ∼200 ps for PMTs)
No intrinsic amplification → harder for low intensity applications
Most of the noise depends on amplification

For comparison, a typical emission spectrum of a CsI (Tl) scintillation crystal (dotted line and right-hand y-axis).

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Avalanche Photodiodes (APD)

Add an additional “metallurgical junction” (strong p and n doping)
This creates a stronger field which creates more e/h pairs
N region much bigger than metallurgical region
“reach through” photons travel through strongly n-doped (450nm to 1000nm, absorption depths from 1μm to 1 mm)
For sensitivity to lower wavelengths, can go through p+ side

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Noise in APDs

Avalanche creates its own ‘excess noise’ originating from the statistical fluctuations in the avalanche amplification process,

Surface leakage current unaffected, bulk current multiplied by gain and excess noise.
APDs can perform better than photodiodes for low intensity

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Hybrid Detectors

Good for high magnetic field setups
Various possibilities:

Exchange dynode system for an (A)PD
Grids of APDs (pixel)

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G-APD

Best for detecting single photons due to high gain
Good time resolution (<1ns)
Lose energy resolution in geiger mode
Often arranged as pixel detectors

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Silicon Photomultiplier (SiPM)

SiPMs are solid-state devices with high photon detection efficiency and compact size.
Despite the name, this is an APD
operated in (limited) Geiger mode at an amplification gain of about 10^6
In this mode each APD cell is a binary (yes/no) operating photodetector
The pixel cells are small with typical dimensions in the range 15–70 μm
SiPMs are suitable for applications that demand high photon counting capability and low power consumptions

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SiPM Design

Wavelength affects penetration depth
E- drifted to amplification zone
Resistors between cells to decouple/quench current from avalanche
Rise time ~0.5 ns from avalanche
Decay time 30-100 ns from quench

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Dark Pulses

Leakage current, highly temperature and gain dependent
‘After-pulsing’: delayed pulses created by charge carriers trapped during the avalanche amplification – released after a characteristic trapping time of typically some hundred nanoseconds, thereby initiating new avalanches.
Optical cross talk: These are photons created during the amplification process reaching across boundaries into a neighbour SiPM cell where they initiate another avalanche.

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Analog SiPM

All cells are joined to a single output node
Max intensity
Beyond this, lose linearity as multiple photons can hit a single cell

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Digital SiPM

Each cell is read individually
MOS transistor circuits replace the passive quench resistors
Allows for disabling individual noisy cells
Could increase possible cross-talk
First developed for high intensity, but can also detect single photons
QE slightly smaller than analog due to transistors and other metals absorbing some photons
Digital readout simplifies design of the readout electronics

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Comparison

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Applications of photosensors

Photosensors are used in various particle physics experiments, including:

High-energy physics experiments such as colliders (e.g., Large Hadron Collider).
Neutrino detectors (e.g., neutrino oscillation experiments) and dark matter detectors.
Cosmic ray detection and astroparticle physics experiments.
Medical imaging applications like positron emission tomography (PET).

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Photosensors for low-background noble liquid experiments

Noble liquids (liquid argon, liquid xenon) are widely used in dark matter and neutrino experiments (lectures by Pedro Ochoa & Aaron Manalaysay)
Choice of photosensors play a critical role in the detector design and sensitivity
In addition to the photon detection efficiency or QE, considerations also on dark noises, power consumption, background radioactivity etc.

?

SiO2

Alexander Neumeier: Optical Properties of Liquid Noble Gas Scintillators, TUM PhD thesis

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Photon detection in nEXO

Energy resolution dominated by light

Need 3% efficiency of detecting scintillation photons for 1 % energy resolution
With negligible noise for light detection

Need at least 4 m² of detection area
Need reflective electrodes
Need low radioactivity

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Fabrice Retiere , June 8, 2023, XeSAT2023

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SiPMs, baseline photo-detector solution for nEXO

High gain (low noise)
Large manufacturing capabilities
But efficiency and radioactivity need work

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1.3x1.3 mm² T2K Multi-Pixel Photon counter

Pictures courtesy of Kyoto University

2mm

50μm

p-substrat

n-well

n⁺

p^-

p⁺

Cathode

Anode

Photons

Discovery,

accelerated

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Two SiPM options for nEXO (baseline)

Fondazione Bruno Kessler

Development chain driven by nEXO:

VUV-HD1
VUV-HD2, 1x1cm², did not work
VUV-HD3, 6x6mm², good performances
VUV-HD4, 2022-2023, 1x1cm², does not seem to work well

Hamamatsu

VUV4 Multi-Pixel Photon Counter
Single 6x6mm². Appears to have worse performance
Quad 2x2 6x6mm²
1x1cm² integrated on nEXO tile

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June 8, 2023

Technology decision in 2024

Discovery,

accelerated

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SiPM for lowest radioactivity content

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	²³⁸U	²³²Th	⁴⁰K
Prelim. nEXO requirements for 4m²	< 0.1 nBq/cm²	<1 nBq/cm²	< 10 nBq/cm²
FBK SiPM (bare wafers)^A	<0.4 nBq/cm²	~0.6 nBq/cm²	~3 nBq/cm²
SensL SiPM (packaged) ^B	<1.1 mBq/cm²	<33 μBq/cm²	<69 μBq/cm²
Hamamatsu PMT R11410-21 ^c	<0.4 mBq/cm²	0.016 mBq/cm²	0.37 mBq/cm²

^ACounting at U.Alabama after nuclear activation at MIT

^b NEXT Ge counting. http://arxiv.org/abs/1411.1433

^CE. Aprile et al. Material radioassay and selection for the XENON1T dark matter experiment. Eur. Phys. J., C77(12):890, 2017, https://arxiv.org/pdf/1705.01828.pdf

Discovery,

accelerated

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Hamamatsu Low RI MPPCs

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SiPM nuisances

Dark noise

Thermal. At room temperature ~100kHz/mm²

Carrier trap and release => after-pulsing
Light emission during avalanche

Direct cross-talk
Delayed cross-talk
External cross-talk, aka hit another SiPM

Large capacitance ~50pF/mm²

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External cross-talk

Discovery,

accelerated

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Correlated avalanches

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FBK

Dominated by cross-talk (prompt additional avalanche)

Hamamatsu

Dominated by after-pulsing

Significant batch to batch variation

Gallina, G., Guan, Y., Retiere, F. et al. Performance of novel VUV-sensitive Silicon Photo-Multipliers for nEXO. Eur. Phys. J. C 82, 1125 (2022), https://arxiv.org/pdf/2209.07765.pdf

Discovery,

accelerated

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Photo-detector performance comparison

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Parameters at LXe temperature for cm² scale channel size	PMT R11410-21^a	FBK VUV-HD3 @ 3V ^b	HPK VUV4 MPPC @ 3V ^b
Single channel active area	128 cm²(?)	25 cm^{2 c}	6 cm^{2 d}
Efficiency at 175nm	34%	24.4 ± 1.4%	20.5 ± 1.1%
Single avalanche charge resolution	25%	5%^c	5%^d
Dark noise rate (Hz/cm²)	1.3 ± 0.4	19 ± 1	35 ± 1
# correlated avalanche in 1 μs	0.02 ± 0.005	0.23 ± 0.06	0.06 ± 0.02
# Photons emitted per avalanche	N/A	1 ± 0.5	1 ± 0.5
Single photon timing resolution, σ	3.9 ± 0.6ns	~10 ns^c	~100 ns^d
Radiopurity per active area	~mBq/cm²	Medium ^c	< 10 nBq/cm^{2 d}
Power consumption in LXe	0.75 mW/cm²	2 mW/cm² ^c	2 mW/cm^{2 d}

^a Massaged from P. Barrow et al., https://arxiv.org/pdf/1609.01654.pdf

^bG.Gallina et al., https://arxiv.org/pdf/2209.07765.pdf

^c DarkSide-20k readout scheme for 25 cm² channel size

^d nEXO readout scheme for 6 cm² channel size (can be applied to FBK)

Discovery,

accelerated

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Motivation: higher performances, lower power, simpler

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~0.1nF/mm²

Analog to Digital

Conversion

Comparator (more power)

Waveform digitizer

Amplifier

Need power

to buffer

capacitance

Photon search engine

In software or firmware

Photon time stamp

Timing performance

Limited by size

Large pulse very messy

AQ

Active

Quench

Digital processing

Photon time stamp

Photon

Tag for every photon

Or some simpler aggregate

Bias

Transition for analog to digital SiPM (non nEXO baseline)

Discovery,

accelerated

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Photon to Digital Converter

Designed by Sherbrooke (Canada) and built at Teledyne-DALSA (Canada)
The good

Can optimize sensor and readout separately
Excellent fill factor – possibly 100% in back-side configuration

The bad

Tricky to build

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June 8, 2023

Discovery,

accelerated

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Photon to Digital Converter – complete system

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Discovery,

accelerated

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Current development state

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FPGA-based Controller

45 mm

TOP

BOT

57 mm

ASIC-based Controller

Discovery,

accelerated