SiPM as Gamma Scintillation Detector

The Silicon Photomultiplier (SiPM) can be conveniently coupled to a scintillator crystal to obtain a gamma radiation detector. The SiPM has already been presented in the post : Silicon Photomultiplier.

As scintillator we have used Lutetium-yttrium oxyorthosilicate, also known as LYSO, is an inorganic chemical compound with main use as a scintillator crystal. Its chemical formula is Lu2(1-x)Y2xSiO5. It is commonly used to build electromagnetic calorimeters in particle physics and in the PET tomography. LYSO crystals have the advantages of high light output and density, quick decay time and excellent energy resolution.

LYSO crystal contains the element lutetium which is composed by two isotopes, but only one is stable, ¹⁷⁵Lu (natural percentage 97,41%) while the other, ¹⁷⁶Lu is a beta emitter and it decays with half-life of 3,78 × 10¹⁰ years (2,59% natural percentage).
Due to the content of Lutetium, the crystal LYSO appears to be weakly radioactive.

LYSO crystal radioactivity
Mass = Volume x Density = 0.42 x 2,2 x 7.4 = 2.6 g
24% * is Lutetium = 0.625 g
~ 2.5% è Lu-176 with halp-life of 3.78 x 1010 years
There are 6.02 x 1023 x 0.625 x 0.025/176 = 5.34 x 1019 nuclei
Disintegration constant is ln2/(3.154 x 107 x 3.78 x 1010) = 5.81 x 10-19 s-1
From which λN = 31 cps o Bq

Gamma Detector SiPM + LYSO

In order to use SiPM + LYSO as gamma detector it is better to keep the pulse signal as short as possible. One resistor of 47ohm and a capacitor to extract the signal of 1nF are a good compromise which allow to obtain pulses of about 200mV and about 200ns long. These pulses are amplified with a not-buffered CMOS  inverter with 100kohm resistor as a feedback. The amplified signal are squared by other two inverter CMOS which act as triggers. A positive CMOS signal of 3,3V and duration of about 400ns is obtained.
The CMOS inverters have been chosen because of their high speed compared to standard Op Amp and because they do not need special double voltage power supply.

The output pulses are sent to a Theremino Master Module, the PIN has to be configured as “Fast Counter” in order to perform the counting operations.

The upper line shows a SiPM pulse of about 200ns. The lower line shows a 3,3V pulse of about 400ns generated from inverter chain.

Theremino Geiger has to be calibrated with sensitivity and background level of SiPM / LYSO based detector. After calibration good results can be obtained with this detector. Due to the scintillation crystal the sensitivity of the detector is very high. A little defect is the high count value that is recorded idle, approximately 100CPS, this is due to the radioactivity of crystal LYSO due to content in lutetium.

In the chart below are shown the measures of some radioactive sources.

Gamma Spectrometer SiPM + LYSO

Using the SiPM adapter described in the post : Silicon photomultipliers we can achieve a gamma spectrometer based on the SW Theremino MCA. Below some gamma spectra are shown.

The spectrum of background includes natural background radiation and the contribution of the natural radioactivity of the crystal LYSO due to content in Lutetium.
The part of the background radiation has its maximum in the range between 100 and 150keV, while the part due to the crystal LYSO is mainly comprised between 200 and 500 keV.

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