Silicon photodiode gamma ray detector

Silicon photodiodes are the cheapest type of detector device that can be used to detect gamma rays. Theoretically, they can achieve low resolution energy spectrum detection. However, due to their original design not being used to detect gamma rays, special bias structures and sensitive signal extraction circuits are required in practical applications to extract nuclear pulse signals from noise. This article will use silicon photodiodes to detect gamma rays and extract nuclear pulse signals.

1. Detection circuit framework structure

The gamma ray detection circuit based on silicon photodiodes mainly includes two main modules: the reverse bias circuit and the signal extraction and processing circuit, as shown in Figure 1. There are two main schemes for gamma ray sensing: 1. Use scintillators to convert the rays into visible light, and then use silicon photodiodes to convert low light into electrical signals. 2. Direct detection allows the radiation to directly enter the depletion layer of the silicon photodiode to generate an electrical signal. The input of the signal extraction circuit is processed using JFET operational amplifiers to obtain useful nuclear pulse signals.

The design of scheme one is relatively complex, and the design architecture has stronger scalability. Its reverse bias voltage can be adjusted between 2.5V-30V, and it can be adapted to silicon photomultiplier tubes (SiPM) devices by modifying circuit parameters. Under the guidance of model simulation, Scheme 2 can obtain better output pulse waveforms through device selection optimization. After a large number of experiments, the cost of gamma ray detectors can be reduced to within 60 yuan.

2. Modeling of silicon photodiode circuits

The reverse bias mode of the silicon photodiode is shown in Figure 2, where the diode is connected to the circuit with reverse bias, and the reverse bias voltage is provided by the power supply VCC; The resistor RA serves as a current limiting resistor to prevent diode breakdown, while also preventing the signal generated by the diode from entering the power supply and causing attenuation; C1 is a DC isolated capacitor used to extract nuclear pulse signals on diodes; RL is the output load resistance.




The simplified internal model of the photodiode in reverse bias mode is shown in Figure 3, where I2 is the current generated by the optical signal; Rs is the reverse bias equivalent resistance of the photodiode, which is related to the self excitation of the depletion layer in the detector. The lower the temperature, the fewer charge carriers generated by self excitation, and the higher the resistance value; Ct is the interpole capacitance of the detector [1], which is proportional to the thickness of the depletion layer between the two stages and the area of the flat plate. The thicker the depletion layer and the smaller the plane area, the smaller the interpole capacitance. The internal ideal diode is omitted in the figure because under reverse bias, the ideal diode will not have any current and will increase the computational burden.



The magnitude of the reverse bias current (also known as dark current) I1 of a photodiode is related to the reverse bias voltage and operating temperature. The higher the temperature and the larger the reverse bias voltage, the greater the value of I1 [2]. Taking S2387 as an example, the relationship between dark current and reverse bias voltage is shown in Figure 4.


When measuring, it is necessary to use aluminum foil to avoid light on the photodiode, and obtain the actual measurement value by taking the average value after multiple measurements.
The measurement of the reverse bias voltage at both ends of the diode requires the use of the trigger mode of the oscilloscope to collect data. At the moment when the oscilloscope probe contacts the two ends of the photodiode, its voltage value will decrease. The reason for this phenomenon is that the input impedance of the oscilloscope probe is much smaller than that of the photodiode. Typically, the input impedance of the oscilloscope is 1M Ω -10M Ω, but the internal resistance of the detector is usually in G Ω units [3].
The junction capacitance Ct under reverse bias can be obtained by measuring the capacitance of C1 open circuit terminal to ground. Because the capacitance value of C1 is much greater than that of Ct, the tested capacitance can be approximated as the capacitance value of Ct [4].
When estimating, it is possible to simplify the dark current and internal resistance. When considering variables such as temperature in static mode, the dark current constant current source will change, and the internal resistance will change with the variation of voltage during signal transmission. In the actual derivation process, it was found that the size of the junction capacitance Ct has a significant impact on the amplitude of the output signal, while the influence of internal resistance and constant current source on the amplitude of the output signal is relatively small. When the temperature and bias voltage remain constant, the phenomenon of dark current can be simplified using internal resistance current to reduce the difficulty of modeling.


In order to obtain the X-ray capture efficiency of the detector, a self-made low-energy gamma ray spectrometer was used here. The instrument will be published on the Science and Technology Innovation Forum after layout and organization in the later stage. For everyone's reference design, all relevant design source files will also be open sourced to the forum.
Experimental details: The 241Am source was placed at 8cm, 9cm, 12cm, and 17cm for 200 seconds of counting statistics. The average counting results per second are shown in the table below. The high-energy gamma ray spectrometer was unable to detect the low-energy gamma rays released by 241Am in this test.
Counting of detectors at different positions
detector
8cm
9cm
12cm
17cm
Si PIN detector
zero point seven six five
zero point six five five
zero point three one five
zero point two five five
Low energy gamma ray spectrometer
five hundred and fifteen point two
four hundred and forty-eight point seven
three hundred and twenty-three point seven
one hundred and ninety-three point five
The effective detection area of the low-energy gamma ray spectrometer is 706mm2, and the effective detection area of the Si PIN detector is 4mm2. In this experiment, based on the test data of a low-energy gamma ray spectrometer at 8cm, it can be inferred that the ratio of the spherical crown area to the entire spherical surface is 1:116.77, and the radioactive activity of the radiation source can be estimated to be 60159.4Bq. The comparison of the equivalent detection area count of 4mm2 between Si PIN detector and low-energy gamma ray spectrometer is shown in the table below.
Equivalent Counting Rate of 4mm2 Detection Area in Low Energy Gamma Ray Testing
Detector (4mm2 detection area)
8cm
9cm
12cm
17cm
Low energy gamma ray spectrometer
zero point seven five
zero point five nine three
zero point three three three
zero point one six six
Si PIN detector measured value
two point nine one nine
two point five four two
one point eight three four
one point zero nine six
It can be seen from the comparative data in the table above. When detecting 241Am low-energy rays, the detection efficiency of Si PIN detectors is about 4 times lower than that of low-energy gamma ray spectrometers. The low-energy gamma ray spectrometer is made using cesium iodide (thallium)+PMT+special window, and has extremely high detection efficiency for low-energy gamma rays (X-ray energy end).
Summary:
This design is basically hand soldered without a PCB provided for everyone, but a detailed modeling of this type of detector has been carried out. The modeling process has used a lot of engineering simplification. Due to my limited ability, there may be inappropriate simplifications in the derivation, but it can provide a reference of an order of magnitude for everyone. It is known that the signal processed when designing this type of detector is extremely small.
The overall spectral performance of this detection is lower than that of the scintillator photomultiplier tube scheme, and the detection efficiency is also slightly lower than that of the scintillator scheme, especially when detecting high-energy rays.
This type of detector is suitable for detecting low-energy rays, and its detection efficiency is about 4 times lower than that of scintillator schemes when detecting Am241 rays, but its cost is 57 times lower.