CC BY
4330
PIN Photodiodes For Gamma Radiation
Measurements
M.A. Khazhmuradov, N.A. Kochnev, D.V. Fedorchenko
Abstract - We consider usage of the commercial PIN photodiodes as detectors for gamma-radiation. We describe the low-noise electronic circuit for detector module using BPW-34 photodiode. Theoretical and experimental results for counting and spectrometry modes using the developed detector module are presented. Parameters of BPW-34 photodiode and compact Geiger-Mtiller tube are compared.
I. Introduction
The modern industrial applications of radiation technologies require compact, fast and low cost radiation detectors. For the appliances where spectral measurements are not required the Geiger-Muller tubes are often used. This includes radiation level control, contamination measurements, well logging. The shortcomings of such detectors are well known and include rather low sensitivity especially for compact detectors, limits on count-rate and lifetime, high operational voltage.
The possible alternatives to Geiger-Muller tubes for dosimetry applications are PIN photodiodes. While these silicon detectors are designed to have very high sensitivity to visible light or infrared radiation they are also capable to register X-ray and low energy gamma radiation. Numerous studies of commercial visible light and infrared PIN photodiodes have proved that they could be used for spectrometry and counting applications for photon energies up to 100 keV [1-4]. In the work [2] PIN photodiodes were tested for 661 keV gamma photons and exhibited enough sensitivity for counter applications.
The main purpose of this paper is to study the BPW-34 commercial photodiode as a replacement of compact Geiger-Muller tube. This includes studies of both counting mode and spectrometry mode that is absent in Geiger-Muller tube.
II. Experimental setup
As radiation detector we have chosen commercial BPW-34 PIN photodiode. This photodiode usually is part of photo-interrupters or infrared control devices.
BPW-34 photodiode was intensively studied in the works [1-4] as gamma radiation detector. This diode has a plastic packaging (see Fig. 1). The sensitive component of the detector is 3x3 mm silicon die placed inside the packaging.
Fig. 1. BPW-34 PIN photodiode
The structure of sensitive area is typical to PIN photodiodes and is shown in the figure 2. It has a thick layer of intrinsic semiconductor i placed between the n and p layers. This layer is the primary region where incident photons are captured and photocurrent is generated. Additional n+ layer improves electric contact between sensitive area and metal electrode.
Manuscript received November 3, 2012.
M.A. Khazhmuradov is with the National Science Center Kharkov Institute of Physics and Technology (corresponding author to provide e-mail: khazhm@kipt.kharkov.ua).
N.A. Kochnev is with the National Science Center
Kharkov Institute of Physics and Technology (e-mail:
nickolya_k@mail.ru).
D.V. Fedorchenko is with the National Science Center
Kharkov Institute of Physics and Technology (e-mail:
fdima@kipt.kharkov.ua).
Fig.2. Structure of the sensitive layer
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For the BPW-34 photodiode width of the intrinsic layer is 210 pm [4]. When photon is captured in this area charge carriers emerge. The reverse voltage applied to the photodiode drives them to the corresponding conductivity areas. As a result a current pulse is generated.
Generation of the single electron-hole pair requires approximately 3.6 eV. To estimate the typical values of generated charge let us consider full capture of the single
59.2 keV photon that corresponds to the principal line of the 241Am spectrum. In this case the total charge of 2.6 fC is generated in the photodiode.
To register such a low charge one needs a low-noise electronic circuit with high input impedance and low input capacitance. These requirements are satisfied by amplifiers using junction gate field-effect transistors (JFET). We have developed and manufactured electronic circuit for source follower based on low-noise JFET transistor (see Fig. 3). Source follower circuit was chosen due to very low input capacity. In this case under constant gate-source voltage gate-source capacitance becomes zero, and as the drain potential is constant no increase of gate-drain capacitance due to Miller effect occur.
Fig. 3. JFET based source follower circuit
Now we estimate the total input capacitance of the whole circuit. For the low-noise JFET transistors 2SK152 input capacitance is less than 2 pF. Under turn-off voltage of 40 V PBW-34 photodiode has capacitance 10 pF, and with account for stray wiring capacitance the total input capacitance is less than 15 pF. Hence for input charge of 2.6 fC the output pulse amplitude is 0.17 mV.
The output from the source follower is transmitted to the driver amplifier with amplification 2000 and shaping time 50 ps (Fig. 4.). Driver amplifier has three stages: the first is differentiating stage and he second and third stages are integrating stages.
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After amplification and sharpening the output signal goes to analog-digital convertor (ADC) and microcontroller circuit. The AD conversion is performed by 10-digit ADC AD9200 and then data is preprocessed by Atmel Mega88PA microcontroller. Every 300 ns microcontroller reads ADC conversion data and determines the pulse maximum and duration. Microcontroller program also performs additional signal filtering from the random noise. After preprocessing microcontroller transmits data to personal computer PC through the UART-USB bridge based on the CP2102 module.
Due to the low input capacitance and very high input impedance the source follower circuit from the figure 3 has high sensitivity to electric interferences. Also it is subjected to microphone effect in photodiode and ceramic capacitors in differentiating and integrating stages. Besides that the circuit exhibits very high sensitivity to visible, infrared and ultraviolet light. Thus we have provided electrical and light shielding of the source follower circuit using the 35 pm copper foil.
III. Mathematical simulation
From the viewpoint of gamma radiation detection BPW-34 photodiode has relatively low sensitive layer thickness. Additionally, due to low atomic number of silicon the photo effect cross-section is rather low, especially for energies above 100 keV. At the same time registration of such photons is still possible due to the Compton scattering and consequent registration of secondary electrons.
The theoretical studies of the above scattering mechanism were performed using GEANT 4.9.6.1 software [5]. This software was developed by CERN and Geant4 is aimed on the simulation of the passage of particles through matter.
We have developed the solid model for BPW-34 photodiode (Fig. 5).
Fig. 5. GEANT4 model of BPW-34 photodiode
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It included epoxy case, silicon sensitive area with variable base thickness, and copper shielding with variable thickness. The case was approximated by a solid box with the following dimensions: 4.5 *4*2 mm. The size of sensitive area was 2.75*2.75*0.21 mm.
Although it was possible to change the base thickness preliminary simulations showed that for lower width detection efficiency degrades. For the real photodiode base thickness depends on the applied voltage and for 40 V turnoff voltage we have used it has the maximum value of 210pm [4]. The typical shielding thickness used during the simulation was 35 pm.
Within the GEANT 4.9.6.1 framework several models for simulation of photon and electron passage are available. For actual simulation we used G4EmStandardPhysics (option 4) package which includes the most exact models for the electromagnetic processes in the low energy range. Incident photon energy spectra corresponded to the spectra of real radiation sources: Cs, Ta, Am. Radiation properties
of these sources are given in the Table I.
Table I
Nuclide Energy, keV Relative intensity, % Backscattering energy, keV
137Cs 31,817 2,110 28,294
32,194 3,850 28,591
36,304 0,368 31,787
36,378 0,711 31,844
37,255 0,225 32,514
661,000 90,000 184,272
179Ta 54.611 13.818 44.99
55.79 24.158 45.79
63.2 8.093 50.67
65 2.053 51.82
241Am 13.9 12.5 13.183
17.8 18.0 16.64
20.8 4.7 19.234
26.35 2.4 23.887
59.54 35.9 48.287
For the real-world dosimetry applications detector performance for 137Cs radiation source is of particular interest. We have calculated the energy absorption spectrum for this source (Fig. 6). 200
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As we have stated above the photo effect cross-section for 137Cs primary line 662 keV is rather low and photodiode detects only secondary electrons from Compton scattering with energies less than 114 keV with constant probability about 1.0-10-5. The calculated total efficiency (ratio of the number of detected photons to the number of incident photons) of the BPW-34 photodiode for 137Cs source is 2.0-10-3. This efficiency is sufficient for counting applications.
At the photon energies less than 60 keV BPW-34 sensitivity is enough both for counting and spectrometry modes. We have calculated energy absorption spectra for 179Ta and 241Am nuclides which were used for experimental measurements. Figures 7 and 8 show the calculated energy absorption spectra.
Fig. 7. Calculated energy absorption spectrum for 179Ta
Fig. 8. Calculated energy absorption spectrum for 241Am
In the figures 7 and 8 dashed lines denote spectrum lines for the corresponding nuclide (see Table I). Note that absorption lines are shifted to lower energies. This arises due to photo effect on the K and L shells of the silicon with binding energies 1839 eV for K shell and 149.7 eV, 99.8 eV, 99.2 eV for Lb Ln and Lm shells [5].
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IV. Results and discussions
For the counting mode measurements we have used the 137Cs source with activity of 55.5 kBq. The BPW-34 photodiode was placed at the distance of 15cm from the source. During the 330 seconds of exposure we have registered 275 counts. This gives the detector sensitivity of 1.4-106 counts/R and efficiency 2.2-104. The later value is close to the calculated value 2.0-104.
As we consider the PIN photodiode as a replacement for Geiger-Muller tube we have to compare BPW-34 and tube with same size. Here we take for comparison the compact counter SBM-21 (see Table II).
Table II
Parameter: BPW-34 SBM-21
Sensitivity for 137Cs, counts/R 1.4 30
Operation voltage, V 30-40 260-320
Dimensions, mm 4.65x4.3x2 length: 21 diameter: 6
Although BPW-34 has lower sensitivity, due to small size one can create package from the several photodiodes to compensate this deficiency. Also it has lower operational voltage which is more suitable for practical applications.
For the spectrometry mode we have used 179Ta and 241Am sources. The measured spectra are shown in the Figs. 9 and 10.
Fig. 9. Experimental energy spectrum for 179Ta
In the Figs. 9 and 10 dashed lines denote spectrum lines and dotted lines denote backscattering peaks (see Table I). According to the obtained spectra the backscattering intensity is comparable to the intensity of the primary radiation lines. This shows that detector module needs additional shielding to absorb the photons from Compton scattering on the surrounding construction elements.
Analysis of the experimental spectra gives the working parameters of the BPW-34 in spectrometry mode with the electronic circuit described in the section II. The achieved characteristics are summarized in the Table III.
Table III
Operational energy range 15-60 keV
Energy resolution (FWHM) at 55.79 keV Noise level 6 keV (10%) 6 keV
V. CONCLUSION
In the framework we have studied commercial PIN photodiode BPW-34 in counting and spectrometer mode. We have developed and tested low-noise electronic circuit with ADC and microcontroller capable to preprocess data from photodiode and to transmit it to computer.
Our theoretical calculations and experimental measurements actually show that PIN photodiodes are reliable alternative for compact Geiger-Muller tubes. This is valuable for practical purposes of development compact dosimetry devices for radiation monitoring, medical and industrial appliances.
References
[1] Kainka B. Measure Gamma Rays with a Photodiode.Radiation detector using a BPW34 // Elektor Magazine, 2011, No.6, pp.22-26.
[2] Beckers T. Verification of radiation meter //Elektor Magazine, 2011, No.10, pp. 44-45.
[3] Da Silva I. Spectometry of X-Rays and Low Energy Gamma Using Silicon Photodiodes, Universidade Federal De Pernambuco, Departamento De Energia Nulear, Brasil, 2000, 94 p.
[4] Ram’irez-Jim’enez F.J., Mondrag'on-Contreras L., Cruz-Estrada P. Application of PIN diodes in Physics Research //AIP Conf. Proc. Vol. 857, pp. 395-406.
[5] Agostinelly S. et al Geant4 - a simulation toolkit // Nuclear Instruments and Methods in Physics Research A, 2003, Vol.56, No.3, pp.250-303.
[6] Clementi E., RaimondiD.L., Reinhardt W.P. Atomic Screening Constants from SCF Functions. II. Atoms with 37 to 86 Electrons // J. Chem. Phys., 1967, Vol. 47, pp. 1300-1307
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