Alpha Detector with BPX61 Photodiode

Abstract : in this post we aim to examine the possibility of using the BPX61 photodiode as an alpha and beta particle detector. In order to allow sensitivity to alpha particles, it is necessary to remove the protective glass in order to leave the underlying silicon chip exposed. The alpha spectrometer previously developed in PhysicsOpenLab : DIY Alpha Spectrometer, was used for the tests. The results obtained are very positive and suggest the concrete possibility of using the photodiode in “low-cost” applications for the specific detection of alpha particles, for example in the detection of Radon gas.


The BPX61 photodiode chip can be exposed by removing the protective glass. The operation is not difficult but it is a bit delicate, with pliers the metal case is bent producing four notches and thus breaking the slide, placing the component upside down the glass fragments will fall, it is not recommended to remove the fragments with pliers because you risk damaging the chip. The maximum reverse bias voltage is 30V, for safety we have used a reverse voltage of 25V, at which the thickness of the depletion area is between 50 and 100μm.
The images below show the photodiode without the protective glass and the photodiode placed in the measuring vacuum chamber.

The Spectrometer

We briefly report some information on the construction of our Alfa spectrometer. We decided to use a commercial CSP, the model CR-110 made by Cremat. It is a hybrid CSP preamplifier with a feedback capacity of 1.4 pF and a feedback resistance of 100 MΩ, the time constant of the amplifier is 140 μs. We decided to adopt a commercial component (it is not expensive) because the CSP is a critical component and the functioning of the system depends greatly on the good performance of the CSP in terms of gain and low noise. The diagram below shows the connections of the component :


In particular the bias resistance and filter resistance were chosen of 10 MΩ. These values, taking into account a dark current of less than 100nA, guarantee a good compromise between the need to limit the drop in the bias voltage and that of having an adequate amplitude signal on the coupling condenser towards the CSP.

The shaping amplifier instead was “homebrew”, as already described in the post PMT Pulse Processing. The images below show the finished circuits inside a metal box that constitutes a shield against RF interference.


The pulses generated by the shaper are acquired by MCA Theremino software through a USB sound card. Theremino MCA is the software that we have extensively used in gamma spectroscopy studies, widely reported on this blog. In Theremino web site there is a whole section on this application, with a rich set of documentation.

The version that we used is 7.2. This version has been specially modified to enable the usage also for alpha spectroscopy. In particular has been expanded the scale of energies up to 10 MeV, it has also been expanded adjustment range of MinEnergy and EnergyTrimmer parameters.


Using the BPX61 photodiode and the spectrometer described above, we made some measurements of alpha and beta sources, acquiring the energy spectra. We used an alpha Am241 source, an Sr90 beta source and radium-painted hands (alpha + beta). The following graphs show the energy spectra obtained.

Radium – Alpha peaks
Radium – Beta background


The results obtained with the BPX61 photodiode used as an alpha particle sensor are very good. The energy resolution is comparable to that of dedicated sensors. The low cost and the good energy resolution make it a valid alternative for environmental radioactivity monitoring applications such as the detection of Radon gas.

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