July 3 (2008) - Helsinki Institute of Physics

RAPSODI: 

SiPM development for applied 

research in radiation 

protection 

Fabio Risigo 

Universita’ dell’Insubria in Como (Italy) 

on behalf of 

The RAPSODI collaboration 

10 th iWoRiD, Helsinki June 29 – July 3, 2008

RAPSODI: RAdiation Protection with 

Silicon Optoelectronic Devices and 

Instruments 

Funded by the EC under the Sixth Framework Program (Co-operative research) 

Start-time Oct 2006; End-time Jan 2009 

Main objectives: Silicon Photo Multipliers development and optimization for three 

well defined applications: Dosimetry in Mammography, Radon Monitoring, illicit 

traffic of radioactive material (homeland security) 

4 Small and Medium Enterprises + 3 R&D performers 

SensL (IE) 

Specialized in SiPM development 

PTW (DE) 

Leader in EU market of radiation monitor & dosimeter 

Plch SMM (CZ) 

Leader in Radon meter construction 

ForimTech (CH) 

Small start-up with a focus on photon sensing 

technologies and detectors assembly 

UNICO (IT) 

Leading partner 

AGH (PL) 

Microelectronic Institute 

ITEP (RU) 

Long standing tradition in SiPM 

Significant characterization parameters of SiPM and the dosimetry measurements in mammography 


Silicon Photon Multiplier 

SiPM = High density (~10 3 /mm 2 ) matrix of diodes with a common output, working in 

Geiger-Müller regime 

advantages over traditional photo-detectors: 

 

 

 

high sensitivity (single photon discrimination) 

high speed (T rise 

~ 1 ns; T fall 

~ 50 ns) 

compactness, robustness, low operating 

voltage and power consumption, low cost 

Matrix of binary elements allows an analog 

measurement, counting the number of hit cells 

Producer 

Area (mm 2 ) 

Pixel size 

(µm) 

No. cells 

V working 

DCR 

GAIN 

PDE (%) 

(green) 

SensL 

3 x 3 

20 x 20 

8640 

30 

6 MHz 

10 6 

10 

Hamamatsu 

1 x 1 

17 x 14 

1600 

77 

220 kHz 

3 x 10 5 

20 

CPTA 

1 x 1 

30 x 30 

500 

24 

3 MHz 

10 6 

30 

SensL is developing custom oriented SiPM 


Full characterization protocol 

 

 

 

 

I-V measurements (leakage current, quenching resistor, breakdown voltage) 

Noise measurements (vs over voltage and vs temperature): 

 

 

dark counting rate (DCR) vs bias voltage 

optical cross-talk (DCR vs threshold) 

Analysis of (Poissonian photon) spectrum (vs temperature) 

 

 

 

 

resolution power (how many photons can I distinguish?) & gain 

working point optimization (at low and large flux) 

noise measurement (not DCR) 

optical cross-talk (deviations from the Poissonian distribution) 

linearity & dynamic range 

Spectral response (PDE vs , PDE vs temperature) 

 

timing properties and time resolution (not relevant for RAPSODI but in progress) 


Photon Spectrum & Gain 

Light source: Pico Quant PDL 800 - light at ~ 510 nm (green) 

G 

= ∆ 

peak − 

QDC 

e ⋅ k 

cal 

AMP 

peak 

 

 

 

good resolution power 

Gain 

Noise (NOT DCR!) 

measurement (width of 

single pulse) 

width count 

> 20 peaks 


Dark Count Rate & X talk 

DCR @ different thresholds on the output signal to be digitalized 

Dark Counting Rate (DCR) is the 

rate at which a Geiger avalanche is 

randomly initiated by thermal 

emission. 

> 0.5 ph 

> 1.5 ph 

Threshold scan 

0.5 ph 

1.5 ph 

2.5 ph 

DCR @ fixed threshold (0.5 phe-) 

> 2.5 ph 

Increasing the bias, the GM probability increases 

10 th iWoRiD, Helsinki June 29 – July 3, 2008 

the avalanche generation can generate an 

avalanche in a near cell through a photon; 

measuring the DCR for different thresholds is 

possible to define and evaluate the Optical 

Cross talk as: 

X talk 

= 

DCR(1.5 

ph) 

DCR(0.5 

ph)

Photon Detection Efficiency 

n 

_ cell 

= 

n 

ph 

⋅ QE 

⋅ 

FillFactor 

⋅ 

p 

GM 

PDE 

Pico Quant 

PDL 800 

Ligth guide 

Ø 1 mm 

clear fiber 

(divergence 15°) 

FC 

connector 

SiPM 

sensor 

Light calibrated against PMT (H5783) and 

tuned to have 200 ph/burst 

FC to guarantees the reproducibility of the fiber tip to SiPM 

surface distance and alignment (acceptance control) 

The PDE has to be corrected for the 

Optical Cross Talk as: 

n = n ⋅ 1+ 

_ meas 

_ cell 

( X ) 

talk 


Temperature Dependence 

Pico Quant PDL 800 – light @ ~ 510 nm 

Cooling Box + Temperature control 

Gain vs bias voltage 

Gain vs temperature 

for any specific working conditions you have a non trivial depend on the 

temperature, due to the variation of the V BD 

(against temperature) 

re-scaling the 

breakdown 

voltage 

dV 

V 

BD 

= ( T −TRT 

) + V 

dT 

RT 

normalizing all the measurements, you obtain a remarkable linear 

dependence against over voltage and a very little spread vs temperature 

Gain independent from 

temperature as function of 

over voltage 

Results particularly important once you have to guarantee the stability of an instrument as the environmental 

conditions change 


An Application: Dosimetry in 

mammography 

Dosimetry in mammography is 

utmost important and this is 

somehow proven by the ongoing 

debate on the relevance of 

mammography screening 

…but currently existing 

instruments are limited: 

Standard Termo-Luminescent 

Detectors require to be analyzed after 

examination, degrade with time 

MOSFET detectors suffer from low 

stability and degrade with each irradiation 

Ionization chamber devices need 

relatively high voltage (cannot be used in 

contact with the patient), not tissue 

equivalent 


precise measurements of the 

actual dose being received by a 

patient without distorting the X-ray 

beam and introducing any 

artefacts in the image 

Some functional requirements: 

dose rate range (2 ÷ 150 mGy/s) 

dose range minimum (0.5 mGy) 

sensitivity (5%) 

overall accuracy (±10%) 

tolerance to environmental 

variation & stability

The Prototype 

Designed and tested prototype @ PTW – secondary standard lab for dosimetry: 

Scintillator 

(tile or fiber) 

Ligth guide 

Ø 1 mm 

clear fiber 

FC connector & 

SiPM 

Electronics 

PHISYCAL OBSERVABLE: 

“buffered” signal sum 

Sum of samples signals selected 

by an edge detector algorithm + left 

& right buffer 

-> proportional to the DOSE 

Trace plot: typical mammo SiPM output 

(continuous photons flux – pulse duration of each sample100 ms) 

BUFFERED 

SUM 


~TIME

Result summary 

Two different set-up (optimized for dynamic range & ): 

1mm scintillator tile coupled with SensL SiPM (9k cells, 3x3 mm 2 ) 

Blue scintillator fiber coupled with Hamamatsu (400 cells, 1x1 mm 2 ) 

Irradiation: 0,22 ÷ 217 mGy/s 

Tile + 

SensL 

Fiber + 

Hamamatsu 

Precision(%) 

1.95 ± 0.05 

2.31 ± 0.03 

Sensitivity A 

(mGy/s) 

2.60 ± 0.05 

2.05 ± 0.03 

MDS B (mGy/s) 

0.524 ± 0.009 

0.458 ± 0.007 

Linear 

Dinamic range 

(mGy/s) 

160 

> 200 

A 

Sensitivity: Precision / system gain 

B 

MDS: minimum signal distinguishable from the noise 


Conclusions 

 

 

 

Within the RAPSODI project, a laboratory for SiPM and light sensors 

characterization has been setup at the University of Insubria. The lab 

has developed both the expertise and the instrumentation to perform 

all the measurements to fully characterize SiPM. 

This expertise has been used also in the test of a mammography 

dosimeter instrument being developed in collaboration with PTW. 

The performances of the first MammoDos prototype are according to 

specification and this will lead to the final product before the end of 

the project. 

! THANK YOU ! 


Backup slides

Temperature Dependence 

SensL 9k: 23.2 mV/°C ± 1.4 mV/°C 

Gain independent from 

temperature 

as function of over voltage

Equipment 

• USB-VME Bridge CAEN 

• V171816ch QDC CAEN V792N 

• DC power supply Agilent 6645A 

• Scope Agilent 54624A 

• Pulse Generator Agilent 33250A [50 MHz] 

• Lecroy 821 NIM discriminator 

• CAEN NIM level translators 

• SensL Board + ZFL-500-BNC [20 dB gain] 

• PDL800-B PicoQuant (green LED) 

• OZ optics coupler & focuser 

• Dark Box +XY stage and manual Z 

• Keithley 4200 SCA 

• 590 + 595 C-V meters QS+HF

RAPSODI project 

 

 

 

Real-time dosimeter for mammography (MAMMODOS): design, construction and 

commissioning of a real-time instruments to measure the dose released to the breast 

during a mammographic procedure. 

Apparatus conceptual design, study of the interaction of X-ray with plastic plate, 

dedicated SiPMs Electronics Development & production, first prototype construction and 

characterisation and software development, final system integration with dedicated SiPM 

and laboratory and hospital tests. 

Radon concentration meter (RADONPROBE): design, construction and commissioning 

of a portable real-time detector for Radon indoor and water concentration measurements. 

Apparatus conceptual design, gas collector & measuring chamber study, dedicated 

electronics and software development, construction and characterization of the Radon 

Probe first prototype and preliminary quality assessment, system integration with 

dedicated SiPMs and qualification tests. 

Portable radioactivity monitor (MICROSNOOPER): design, construction and 

commissioning of a very small and cheap portable real-time device to detect and identify 

any type of radiation. 

Apparatus conceptual design, study of the interaction of different types of radiation with 

the scintillator, development and tests of the prototype device, development and 

production of a dedicated integrated circuit, development of the software for device 

control and construction qualification and tests of the MICROSNOOPER prototype.

I-V measurements 

Remarks: 

quenching resistor 

breakdown voltage 

leakage current 

DCR

One Application: Dosimetry in 

mammography 

Real-time dosimetry in mammography: development of a dose-meter for 

mammography (MAMMODOS), which will allow real time and precise 

measurements of the actual dose being received by a patient without distorting the 

X-ray beam and introducing any artefacts in the image 

Mammography is the process of using low-dose X-rays (usually around 0.7 mSv) to examine 

the human breast. It is used to look for different types of tumors, cysts and micro-calcifications. 

Existing devices: 

Standard Thermo-Luminescent Detectors: they required to be analyzed after the examination; 

they degrade with time and need regular replacement (time consuming calibration procedure); 

furthermore, since they tend to be positioned upstream of the object, they create artifacts on the 

image. 

MOSFET detectors: they suffer from low stability and degrade with each irradiation. Being 

positioned upstream the object they also create artifacts on the image. 

Ionization chamber devices: they do not suffer of any degradation effect, their outputs is 

reliable and stable, but they need relatively high voltage and therefore cannot be used in contact 

with the patient during examination, besides that they are definitely not tissue-equivalent.

One Application: Dosimetry in 

mammography 

SetUp: scintillator (Tile or 

Filer) coupled with SiPM 

Trace plot: typical mammo SiPM output 

(continuous photons flux – pulse duration 100 ms) 

boundary conditions: 

dose range rate 

minimum detectable signal 

sensitivity 

overall accuracy 

Tolerance to environmental variation 

& stability 

SIGNAL region 

PEDESTAL region 

~TIME 

Numeri e plot linearita’ 

Minimum Signal Distinguishable from the noise floor ~0.5 mGy/s 

Sensitivity ~2 mGy/s

July 3 (2008) - Helsinki Institute of Physics

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