Radon Detection with Si-PIN Photodiode

 

Abstract: in this post, we aim to examine the possibility of using the BPX61 photodiode as an alpha particle detector for quantifying the presence of radon gas. To enable sensitivity to alpha particles, the photodiode’s protective glass must be removed, exposing the underlying silicon chip. The Micod module, which includes both a charge amplifier and a shaping amplifier, was used for the tests. To concentrate the alpha-emitting particles resulting from radon decay onto the photodiode’s silicon chip, an electric field directed toward the sensor was used. The results obtained are positive and suggest the concrete possibility of utilising the photodiode in low-cost applications for radon gas detection.

Introduction

The decay products of radon gas (or thoron) are themselves alpha-emitting isotopes; furthermore, due to the emission of the alpha particle, these isotopes are positively charged because some electrons are captured by the alpha particle emitted by the nucleus. The main idea is therefore to use an electric field to concentrate these positively charged isotopes on the surface of the detector. When these isotopes decay, the alpha particles are detected by the detector without being absorbed by the surrounding air.

We have already utilised the photodiode as an alpha radiation detector; details can be found in the article “Alpha Detector with BPX61 Photodiode.” As front-end electronics, we used Micod’s CSP and SA, as described in the article Alpha Detector with Micod CSA-SA. The analogue signal generated by the electronics was acquired by the Red Pitaya board, as described in the article Improved Alpha Spectroscopy with Red Pitaya.

This is a work in progress, so we present only some preliminary results and some images of the apparatus.

Fig.1 – PIN photodiode scheme

Fig.2 – BPX61 photodiode detail

Fig.3 – BPX61 photodiode without protection screen glass

Fig.4 – Measure chamber with detector in the centre

An important part of the apparatus is the aluminum dome surrounding the detector, which is located at the centre of the hemisphere. The dome is negatively charged to a high potential: 1000–2000 V. The detector photodiode, on the other hand, is at zero potential. An electric field is thus created in the space between the dome and the detector, pushing the positive ions toward the detector surface.

Fig.5 – Alluminum dome for the generation of the electric field

Fig.6 – Interior of the instrument with the aluminum dome installed on the sensor

Fig.7 – Front-end electronics

Fig.8 – Finished apparatus

The first tests were performed by placing a thoron source (thoriated gas mantle) directly inside the aluminum dome and measuring the pulses detected by the detector. The following image shows the decay chain of thorium 232.

Fig.9 – Chain decay of Thorium 232

With the thoron source placed inside the measuring chamber, we obtain the spectrum shown in the following graph. The alpha peaks corresponding to the isotope Po-216 (6900 keV), the isotope Bi-212 (6200 keV), and the isotope Po-212 (8900 keV) are clearly visible.

Fig.10 – Alpha spectrum of Thoron progeny – I

By removing the source, the spectrum obtained lacks the Po-216 peak at 6900 keV, due to its short decay time, while the Bi-212 and Po-212 peaks remain, which have longer decay times.

Fig.11 – Alpha spectrum of Thoron progeny – II

Conclusions

In this post, we described some alpha spectrometry measurements performed with a Si-PIN photodiode to assess the amount of radon gas. The initial data are preliminary but encouraging because they allowed us to obtain alpha spectra that can be used to determine the amount of radon gas present in the measurement chamber.

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