PSoC Controlled SiPM detector

The front-end electronics for the acquisition of the signals produced by SiPM (post Front-end Electronics for SiPM) can be conveniently interfaced with a PSoC microcontroller that performs the following functions:

• Analog signal reading from potentiometer for SiPM bias voltage configuration;
• Analog reference signal generation for the generation of SiPM bias voltage;
• Acquisition of the amplified signal produced by the SiPM, generation of the logic impulse and LED driving;
• ADC conversion of the pulse max amplitude and reset of the Peak & Hold;
• Pulse count in CPM;
• Measurement of the time interval between one pulse and the next;
• Management of display for data visualization and management of command buttons;
• Management of serial interface of data transmission to computers;
• Temperature sensor management for gain compensation of the SiPM (not yet implemented);

In the image below you can see the prototype including the power section of the SiPM, the amplification and Peak & Hold part and the prototyping board of the PSoC.

PSoC Programming

In the diagram below you can see the clock generation section : the clock master is configured at 48 MHz, while the Sample & Hold clock for the digital conversion of the signal is configured at 10 MHz. Then there are the “service” clocks for the LEDs, for the anti-bounce buttons and for the 1 Hz timer.

The diagram below shows the parts for the ADC conversion of the “trimpot” potentiometer of the board, used for the configuration of the SiPM bias voltage level. Then there is the DAC component for the generation of the reference voltage for the setting of the SiPM voltage. There is the component for the LCD and the timer for measuring the time interval between one pulse and the next.

The diagram below shows the acquisition section of the pulse generated by SiPM. The pulse is sent to an ADC converter for reading the signal amplitude. The pulse is also sent to a comparator for generating a logic pulse when the threshold set via software on the VDAC component is exceeded.

The diagram below shows the pulse count section, with the Counter_Pulse counter, and the section for generating holding and resetting signals for the Peak&Hold circuit. Holding and resetting signals are generated by two PWMs. The PWM_Hold generates the holding signal (this signal is not really used but serves for timing) and is triggered directly by the impulse that originates from the threshold comparator. The PWM_Reset generates the reset signal for the peaking capacitor and enables the analogue switch AMuxHw_RS which physically makes the connection of the capacitor with the discharge circuit.

The diagram below shows the components for managing the control buttons and the component dedicated to USB-UART communication with the PC for data transmission.

ϒ Spectroscopy Measures

With the SiPM + BGO probe (post SiPM with BGO Scintillation Crystal) and with the front-end electronics (post Front-end Electronics for SiPM) interfaced to a PSoC microcontroller, as described above, we have performed gamma spectroscopy measurements on a series of sample sources in order to test performance of the equipment. In the PSoC microcontroller a program has been implemented that performs the acquisition of each pulse and the digital conversion of the amplitude value. The acquired data are transmitted to a PC via the USB-UART interface (emulation of a serial terminal).
The acquired data can be processed further using Excel in order to create a histogram and evaluate the gamma emission spectrum of the source under examination.

In the image below you can see the measurement setup with the probe and a sample source positioned in front of the probe at a fixed distance.

Americium 241 (Am241) γ Source

The Americium 241 isotope has a low energy gamma emission with the main peak at 59.5 KeV. The spectrum obtained by means of the SiPM + BGO + PSoC spectrometer is shown in the graph below, in which the peak at low energies is evident. For the detection it is necessary to configure the bias voltage at 31 V and place the threshold around 150 mV, with these settings the peak is placed on 200 mV.

Sodium 22 (Na22) γ Source

The sodium 22 isotope is characterized by β decay with positron emission which annihilate and generate gamma radiation at 511 KeV, and there is also gamma emission at 1274 KeV. The graph below shows the spectrum obtained with our probe: we see the main peak at 511 KeV and the peak at 1274 KeV. The linearity of the system is quite good and the 511 KeV resolution is about 20%: value in line with what is expected for a small BGO crystal. The “shoulders” to the left of the two peaks are due to the Compton scattering, quite relevant due to the small size of the crystal.

Cesium 137 (Cs137) γ Source

The Cesium 137 isotope is characterized by intense gamma emission at 662 KeV. The graph below shows the spectrum obtained with the evident peak and the Compton continuous, the resolution is about 18%.

Strontium 90 (Sr90) β Source

We have also tested with our probe a pure β source such as strontium 90. The obtained spectrum is shown in the graph below, in which we see the characteristic continuous spectrum of the β emission.

Conclusions

The tests carried out have demonstrated the possibility of using the PSoC microcontroller, interfaced with a PC, and a SiPM – BGO probe, for the realization of an amateur gamma spectrometer. The resolution obtained is in line with what is expected for a small BGO crystal. Further tests can be made with other crystals (for example CsI(Tl) or NaI(Tl)) with which we expect far better performance.

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