CTA report 145:

Feasibility study on a future
upgrade of PMT camera pixels
with silicon photomultipliers
Anatolii Zenin, Hiroyasu Tajima, Akira Okumura,
Yuki Nakamura

ISEE, Nagoya University

Outline
Introduction• CTA and atmospheric imaging• Motivation• PMT and SiPM
• MST and light concentrators
Main Part• Simulated setup• Simulation andmeasurement results• Discussion
2

Credit: CTA

CTA and atmosphericimaging

Air Cherenkov technique3

Motivation
Why upgrade:• The duty cycle of telescopes with photomultiplier tube (PMT)cameras is ~10%, strongly limited by moonlight.

Photocathodes of PMTs degrade over time, especially when theoperating current is high, such as under high night sky
background conditions. It is necessary to change PMTs every 10years• Silicon photomultipliers (SiPMs) are background-tolerant, meaning
we can extend observation time by using them instead of PMTs.
SiPMs do not degrade, so there will be no decrease in telescopes'
performance over time4

To ensure that theperformance of SiPM
cameras will not be inferior
to the current PMT cameras,
we have to compare:• QE / PDE• Sensitivity to signal• Sensitivity to noisearb. unit

PMT and SiPM
54.5

CTA PMT QECTA SiPM PDE

43.5
32.5

100 GeV γ
Cherenkovspectrum

21.5• High NSB (moonlight)

tolerance and performance
under high NSB conditions• Overall telescope

performance (the end goalof this work)

1

La Palma
NSB spectrum

0.5
0200

300

400

500

600

700
wavelength, nm

PMT and SiPM sensitivity comparison(not to scale)5Medium-sized telescope
Cutoﬀ angle
26˚≤αc≤28˚(technical requirement)

αc

ROBAST simulation of MST optical system

6Simulated setup

ROBAST tool was used for thesimulation

Silicone (100µm)
Si3N4
SiO2Si50mm

PMTSiPM+ QE data

Based on the latest Hamamatsu
S14520-3050VS model+ Angular dependence+ Positional dependence? Anode collection eﬃciency(95% assumed)+ Geometry, incl. separate 50µm cells+ 64 ch. 3x3mm, 0.2mm gap+ Refractive indices of the materials+ Resin absorption length

Light concentrator+ Si absorption in UV region? Reflectance assumed? QE: 98.5% assumed up to 450nm,Hamamatsu measurement data used
for longer wavelengths
7

Simulated setup
PDE

NOTE: The simulation is calibrated to be in good agreement with the data
0.6100µ m silicone

Hamamatsu data(no gaps betweenchannels)

0.5

0.4

no coating
300µ m Hamamatsu datascaled Hamamatsu data

Simulation(100µm coating)

0.3

Simulation(No coating)

0.2
same Hamamatsu
data(90% fill factor)

0.1

0200

300

400

500

600

SiPM PDE

700

800

900
1000
wavelength, nm

8Results integrated over the whole range ofazimuthal angle φSimulation results
collection efficiency

365 nm0.5PMT
SiPM 100µm coating

0.45
0.4

αc0.35
0.3
0.25
0.2
0.15
0.10.05

310 nm

0.5PMT

Preliminary

0.45

SiPM 100µm coating0.4

αc0.35collection efficiency

collection efficiency

00.2

0.2

0.15

0.15

0.1

0.1

0.05

0.05

15

2025303540angle, deg

2025303540angle, deg

465 nmPMT
SiPM 100µm coating

αc

0.35

0.25

1015

0.4

0.25

5100.45

0.3

050.5

0.3

0000510152025303540angle, deg

9Results for azimuthal angle φ = 0(see the figure on the left)
collection efficiency

Measurement results

365 nm0.5

PMT simulation
PMT measurement

0.45

SiPM simulation0.4

αc0.35
0.3
0.25
0.2
0.15
0.1
0.05

310 nm0.5

PMT simulation
PMT measurement

0.45

SiPM simulation0.4

αc0.35collection efficiency

collection efficiency

00.2

0.15

0.15

0.1

0.1

0.05

0.05
2025

303540angle, deg

25303540angle, deg

465 nm
PMT simulation
PMT measurement

αc0.350.2

1520

SiPM simulation

0.25

1015

0.4

0.25

5100.450.3

050.50.30000510152025303540angle, deg

10Measurement results
We have created a functioning setup for
SiPM measurements, so the data necessary
for the comparison will be obtained in thenearest future.

SiPM under a light concentrator

Readout module (TARGET)
on a rotating stage

11

collectedefficiency
fractioncollection

Simulation results
0.25

Integration over theangle from 0 to αc
results in:

PMT
SiPM 100µm coating

Y(MPPC)= 1.00Y(PMT)

0.2

0.15

0.10.0500510152025303540angle, deg

Signal collection eﬃciency convolved with Cherenkov
spectrum (300~550 nm) and angular photon distribution
at the camera module

12

Discussion
From the simulation we can see that:• SiPM oﬀers roughly the same signal collection eﬃciency whencompared to PMT even with the light concentrator designed forPMT• SiPM is very sensitive to stray light when used with current light
concentrator• A new light concentrator designed specifically for SiPM might
improve its performance13

To doSeveral things have to be done before making any solid conclusions:• Comparing the simulation results to experimental data and revising
the simulation if necessary(in progress)• Designing a new light concentrator(in progress)

Comparing the performance of PMT and SiPM by a simple raytracing simulation(after designing a new light concentrator)• Full telescope simulation with CORSIKA and sim_telarray(the final step)

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