In this research, for the first time, the detection response of Polycarbonate/Bismuth oxide composite to a pure beta-emitter 90Sr with two energies of 546.2 keV, and 2.28 MeV is studied. Firstly, the range and stopping power of the electrons of 90Sr in the composite at various concentrations of 0, 10, 20, 30, 40 and 50 wt% were calculated using the ESTAR program. Results of simulation demonstrated that the concentration of the heavy metal oxide particles into the polymer matrix played an important role to evaluate the range and stopping power of the electrons in the composite. Secondly, at the experimental phase, the sample of 50 wt% composite with dimensions of 4 cm×4 cm×0.1 cm3 was prepared. Afterwards, the sample was irradiated by 90Sr and the amount of electric current was measured using an electrometer at voltages of 100-1000 V. Additionally, the I-V plot exhibited a linear response at different voltages in the fixed source surface distance. Results of this study showed that this composite can serve as a novel beta detector.
Research Article
Introducing a Novel Beta-ray Detector Based on Polycarbonate/ Bismuth Oxide Nanocomposite: Simulation and Experiment
https://doi.org/10.21203/rs.3.rs-1043040/v1
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Beta Detector
PC-Bi2O3 nanocomposite
Dose Rate
Strontium-90
ESTAR Program
Detection and dosimetry of ionizing radiation are of important issues in the nuclear industry. Recently, polymer-nanocomposites have been used as radiation sensors, detectors, dosimeters, and shielding materials [1-15].
The mechanisms of interaction of beta particles with matter is categorized in two sections, electron excitation and ionization, in which electrons interact with the particles traversing the material via the Coulomb electric field [16-18]. Electrons lose their energy by friction attributed to the CSDA, or continuous slowing-down approximation [19-21]. The collisions of electrons with the particles are including hard collisions or inelastic scattering with orbital electrons produces excitation or ionization of electrons, and secondary electrons, inelastic scattering with nuclei leads to produce Bremsstrahlung, and soft collisions or elastic scattering, in which electrons lose a small fraction of their energies [16].
Some radioisotopes decay via beta-minus emission producing the fast electrons [22]. Several pure beta-emitters are 3H (18.6 keV), 14C (156 keV), 32P (1.71 MeV), 33P (248 keV), 35S (167 keV), 36Cl (714 keV), 345Ca (252 keV), 63Ni (67 keV), 90Sr/90Y (546 keV/2.27 MeV), 147Pm (224 keV), and 204Tl (766 keV) [22].Various types of scintillators are commonly used to detect beta-rays. In addition, low Z materials including organic polymers are excellent absorbers of charged particles such as beta-rays, which will provide high sensitivity for charged particle detection [20, 23-26]. A disadvantage of some scintillators and gas-flow type proportional counters is limitations due to their hygroscopicity, and scalability [26].
The main purpose of this research is to design and fabricate of a novel beta solid-state detector to measure the dose rate of beta-emitter sources in real-time. The material used is new in its kind and uses Polycarbonate/Bismuth oxide (PC-Bi2O3) composite. The reason for choosing polycarbonate as a polymer matrix is that PC is essentially an amorphous polymer that is expected to have a more suitable bond with bismuth oxide nanoparticles, because the crystalline regions of the polymer often repel nanoparticles, so the nanoparticles are dispersed in amorphous regions. In general, a polymer is composed of two crystalline and amorphous phases in terms of molecular structure. In the crystalline phase, the crystals are arranged in regular polymer chains called lamellae with a length of 20-60 nm and a distance of 0.736 nm from each other [27]. Since in this research, Bi2O3 fillers with the sizes of 90-210 nm are used, so the probability of Bi2O3 nanoparticles entering the crystalline region of the polymer is very low and these particles tend to be placed in the amorphous part and the amount of polymer crystallinity increases through the nucleation process. Therefore, the possibility of increasing the crystallinity of polycarbonate is enhanced by adding Bi2O3 nanoparticles to it. For the PC-Bi2O3 composite detector, the amount of sensitivity or minimum detectable dose rate (MDDR) for the detection of beta-rays can be controlled via heavy metal oxide inclusions into the polymer matrix. Generally, various factors affect the detector response, including polymer crystallinity, the weight percentage of metal oxide nanoparticles, nanocomposite thickness and other factors. Also, the homogeneous dispersion of metal oxide nanoparticles into the polymer matrix plays an important role in the detection response of this material.
The selection of suitable thickness for a solid detector, considering the charge particle equilibrium (CPE) is a key factor in the radiation ionizing detection. So, calculation of range and stopping power of electrons at different energies and weight fractions of the inclusions in the polymer composite using the ESTAR program should be carried out. After evaluation of the electron range in the media, it is possible to calculate the exact amount of optimal detector thickness at certain energies.
In this research, Strontium-90 (90Sr) was chosen as a pure beta-emitter source. The 90Sr source is generated in the reactor by the fission reaction of the 235U nuclei as [28]:
The decay mechanism of 90Sr is as follows [28]:
In which, 90Sr decays to 90Y with a half-life of 28.78 years, with beta particle energy of 546.2 keV; the 90Y, which emits beta and gamma, converts to 90Zr with a half-life of 64 hours, with beta particle energy of 2.28 MeV [28].
In this research, for the first time, a novel beta-ray detector based on PC-Bi2O3 nanocomposite was introduced, and the detection response of this material was investigated theoretically and experimentally to a pure beta particle source of 90Sr.
2.1 Sample preparation
The PC-Bi2O3 nanocomposite at a concentration of 50 wt% was synthesized using the solution casting method. The details of the synthesis have been described in our previous work [1]. The densities of PC matrix and Bi2O3 filler particles were 1.2 g/cm3 and 8.9 g/cm3 respectively. The average size of Bi2O3 particles in the prepared material was 90-210 nm. In this experimental work, to irradiate PC-Bi2O3 nanocomposite with beta-rays, a beta irradiation system model Buchler BSS-BA containing 90Sr reference source with an initial activity of 50 mCi (production date 1978) located in Secondary Standard Dosimetry Laboratory (SSDL) Karaj-Iran was used at different source-surface distances (SSDs) according to Table 1. Also, as exhibited in Fig. 1, the Supermax Standard Imaging electrometer was used to measure the electric charge during irradiation at fixed time steps of 15 Seconds. Also, to fabricate the electrodes on two surfaces of the top and bottom of the nanocomposite, copper sheets with dimensions of 3.5 cm×3.5 cm, and 4 cm×4 cm with a thickness of 100 µm adhered to the top and bottom surfaces of the 50 wt% PC-Bi2O3 nanocomposite respectively using the silver paste.
Table 1. The amounts of SSDs for 90Sr and corresponding dose rates
|
SSD (cm) |
Dose Rate (mSv/h) |
|
30 |
102.436 |
|
35 |
75.259 |
|
40 |
57.620 |
|
45 |
45.527 |
|
50 |
36.877 |
|
55 |
30.477 |
2.2 Simulation methodology
In this research, a pure beta-emitter of 90Sr with two energies at 546.2 keV, and 2.28 MeV was chosen for the measurements. To calculate the two key factors namely range and stopping power of the produced electrons in the composite via beta irradiation at different weight fractions of the heavy metal oxide fillers, the ESTAR program was applied [29].
The performance of any ionizing radiation detector essentially depends on the approach in which the radiation to be detected interacts with the material of the detector itself [22]. The linear stopping power for charged particles in a particular absorber is defined as the energy loss for that particle within the material divided by the corresponding path length via the Bethe-Bloch formula [22]:
In which v, ze are the velocity and charge of the particles, N and Z are the number density and atomic number of the absorber atoms, m0 is the electron rest mass, and e is the electronic charge, also I indicates to average excitation and ionization potential of the absorber material [22].
3.1 FESEM analysis
Fig. 2 depicts the Field Emission Scanning Electron Microscopy (FESEM) image of the prepared 50 wt% PC-Bi2O3 nanocomposite. For this purpose, the FESEM device model MIRA3TESCAN-XMU was used. As can be observed in Fig. 2, a cross-sectional view of the sample showed a suitable dispersion of Bi2O3 nanoparticles into the PC matrix. FESEM analysis approved the presence of the Bi2O3 nanoparticles in the polymer-nanocomposite.
3.2 Results of simulation
As can be seen from Fig. 3 (A, B), simulation results of the beta-emitter source of 90Sr at two energies of 546.2 keV and 2.28 MeV for 50 wt% PC-Bi2O3 composite at various weight fractions are depicted. It can be mentioned that increasing the weight fraction of the bismuth oxide particles into the polycarbonate matrix, led to a linear decrease of the range of beta particles in the composite material. This phenomenon is probably due to the increasing probability of occurring Bremsstrahlung secondary radiation by adding the Bi2O3 wt% in the polymer matrix. The energy dissipation of incident beta particles via interaction with this composite material is increased by Bremsstrahlung radiation and therefore the range of the particles will be decreased subsequently.
Also, according to Fig. 4, the total stopping power of electrons at various energies up to 3 MeV for PC-Bi2O3 composite at different weight fractions was calculated using the ESTAR program. It can be deduced that at the specific and constant energy, with increasing the weight percentage of bismuth oxide particles in the polycarbonate matrix, the amount of stopping power of electrons in the composite will be increased. This phenomenon could be attributed to the increment of the effective atomic number of the composite in higher amounts of the reinforcement phase regarding the Bethe-Bloch formula. Results of simulation showed that the optimal thickness for detection of predominant beta particles of 546.2 keV in 90Sr source for 50 wt% PC-Bi2O3 composite was estimated to be approximately 1.2 mm. It seems that a thickness of 1 mm for this detector would be suitable for measuring beta-rays with the energy of 546 keV, and also it can be mentioned that the CPE phenomenon would be established. However, detecting the higher energy electrons including 2.28 MeV will contribute to relatively poor efficiency. To improve the efficiency of the detector, it is possible to increase the thickness of the material, nevertheless, at higher thicknesses, the applied electric field will be decreased and it will practically lose its uniformity which leads to reduced sensitivity of the detector. Thus, selecting the optimal thickness for this detector is essential.
As can be seen from Fig. 5, for 50 wt% PC-Bi2O3 nanocomposite, the values of photocurrent at a fixed voltage of 400 V was increased gradually from 30-75 mSv/h and finally tend to saturate afterwards at 102 mSv/h. It seems this saturation is related to the recombination of beta induced-charged particles, in which, the detection response should be measured at higher voltages.
One of the most important quantities in determining the sensitivity of a detector is the signal to noise ratio. This ratio is obtained by dividing the photocurrent by the amount of dark current. The dark current and photocurrent values in the measurement phase were in the order of pA, and nA respectively. In Fig. 6, signal to noise ratio of the 50 wt% PC-Bi2O3 nanocomposite for detection response to 90Sr beta-rays is exhibited. As can be seen from Fig. 6, with increasing the amount of dose rate ranging from 30-102 mSv/h, the signal to noise ratio will be improved subsequently. This phenomenon might be pertinent to atomic displacements in the nanocomposite considering the interaction of beta particles with the atomic structures of this material. It is worth pointing out here that ionizing radiations including electrons through the interaction with the nanostructured materials, due to atomic displacement can alter the crystal structure and induce various events such as emission of secondary electrons, excitation, ionization, and bond breakage of the material atomic structures [30].
Generally, finding the suitable operating or working voltage plays an important role in radiation detectors. In Fig. 7, the I-V plot of the detector based on 50 wt% PC-Bi2O3 nanocomposite in the presence of beta radiation field of 90Sr source is exhibited, in which I is photocurrent in terms of nA which is measured by the electrometer. It is obvious from Fig. 7 that the detector response (photocurrent) is linear at various voltages range from 100 V to 1000 V. This linearity means that there is no saturation in the detector response of this nanocomposite in the aforementioned voltage range. So it is possible to choose a suitable operating voltage to achieve a significant sensitivity to detect the beta-rays efficiently.
In this research, for the first time, the detection response of a novel beta-ray detector based on Polycarbonate/Bismuth Oxide composite was studied via the simulation and experiment. Firstly, at the simulation phase, the range and stopping power of electrons related to 90Sr beta-emitter were calculated for various weight fractions of PC-Bi2O3 composites up to 50 wt% using the ESTAR program. Results of simulation showed that the amount of heavy metal oxide inclusions in the polymer composite had a substantial influence on the range and stopping power quantities of electrons in the composite detector. So that, increasing the weight fraction of bismuth oxide particles in the polycarbonate matrix, led to a decrease in the range of beta particles, and increase the amount of stopping power of the composite detector linearly. So, the optimal thickness for detection of predominant beta particles of 546.2 keV in 90Sr source for 50 wt% composite was estimated to be approximately 1.2 mm.
At the experimental phase, the 50 wt% PC-Bi2O3 nanocomposite was irradiated by a beta-emitter of 90Sr source and the amount of electric current passing through the nanocomposite was measured using an electrometer at various voltages of 100-1000 V. besides, I-V plots exhibited a linear response at various voltages. Also, the measured dose rate was linear with a linear correlation coefficient of ~0.9933 ranging from 30-75 mSv/h.
Results of this exploration showed that the cost-benefit Polycarbonate/Bismuth Oxide nanocomposite can be considered as a novel real-time beta detector to be used in radioactive monitoring systems for medical and industrial applications.
Acknowledgements
This work was extracted as part of a Master's thesis by Mr S. M. Safdari, MSc student at Islamic Azad University, Science and Research Branch, Tehran. We appreciate those who supported us in handling this research, especially the staff of NSTRI which resulted in the success of this study. Moreover, we would like to appreciate the efforts of Ms R. Mehrara and Professor S. M. Saleh Kotahi at the Physics Department, K. N. Toosi University of Technology for preparing the nanocomposite.
Contributions
S.M. and S.K. prepared the nanocomposite and developed the idea, S.M.S did the measurement and all authors contributed manuscript writing.
Conflict of Interest
The authors declare no conflict of interest.
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No competing interests reported.