|Year : 2022 | Volume
| Issue : 1 | Page : 22-27
A study on the optimization of processing parameters of boron-doped CR-39 solid-state nuclear track detectors for response to thermal neutrons
Deepa Sathian, Rupali Pal, AK Bakshi, BK Sapra
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Submission||02-Jul-2021|
|Date of Decision||17-Aug-2021|
|Date of Acceptance||29-Sep-2021|
|Date of Web Publication||28-Jun-2022|
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, R.No. 305, CT and CRS Bldg., Anushakti Nagar, Mumbai - 400 094, Maharashtra
Source of Support: None, Conflict of Interest: None
The methodology for personnel monitoring of neutrons in the intermediate and fast energy range using CR-39 solid-state nuclear track detectors (SSNTDs) is well established. This study presents the dosimetric response of boron-doped (carborane) CR-39 detector for the measurement of thermal neutrons. It includes optimization of etching process, minimum detection limit (MDL), and thermal neutron sensitivity. Duration of low frequency (100 Hz) and high frequency (3.5 kHz) for elevated temperature electrochemical etching was optimized as 4 h and 50 minutes respectively, and used for the response characteristic study. The measured thermal neutron sensitivity and MDL were 743 ± 13 tracks cm−2 mSv−1 and 0.02 mSv, respectively. This study is useful for thermal neutron monitoring in reactor, accelerator, and all other nuclear facility environment.
Keywords: Boron-doped CR-39, solid-state nuclear track detector, carborane, electrochemical etching, image analysis system, neutron dosimetry, thermal neutrons, tracks
|How to cite this article:|
Sathian D, Pal R, Bakshi A K, Sapra B K. A study on the optimization of processing parameters of boron-doped CR-39 solid-state nuclear track detectors for response to thermal neutrons. Radiat Prot Environ 2022;45:22-7
|How to cite this URL:|
Sathian D, Pal R, Bakshi A K, Sapra B K. A study on the optimization of processing parameters of boron-doped CR-39 solid-state nuclear track detectors for response to thermal neutrons. Radiat Prot Environ [serial online] 2022 [cited 2022 Aug 8];45:22-7. Available from: https://www.rpe.org.in/text.asp?2022/45/1/22/348727
| Introduction|| |
Personnel monitoring for neutrons in the energy range of 100 keV to 14 MeV is carried out with CR-39 poly allyl di-glycol carbonate (PADC) for radiation workers engaged in nuclear power plants, fuel reprocessing plants, oil-well logging facilities, industries, and other research organizations in India. The chemical formula of PADC is (C12H18O7)n. When irradiated with neutrons, elastic collision with H, C, and O gives rise to fast neutron-induced recoil proton tracks along with carbon and oxygen recoil tracks in the detector material. Further, nonelastic (n,α) reactions can occur with high-energy neutrons (>3 MeV), and these alpha can produce tracks depending upon the sensitivity of the polymer. Measurements of neutron dose for energies <100 keV is particularly challenging in personnel neutron dosimetry, as several nuclear reactions are operable in this energy range with wide variations in the interaction cross-sections. Generally, personnel neutron monitoring detectors coated with 6Li or 10B on CR-39 can be used for the measurement of lower energy neutrons as demonstrated by Harvey and Weeks for thermal neutron exposure over a wide dynamic range. Oda et al. have used BN ceramic convertor with CR-39 for use as a thermal neutron detector..Neutrons of energy <100 keV interact with 10B-based detectors by the reaction 10B(n,α)7Li, which is finding increasing use in the thermal neutron personnel dosimetry. For this purpose, the CR-39 PADC detector can be used with a radiator such as lithium tetraborate (Li2B4O7) which serves as a neutron to alpha converter. Luszik-Bhadra et al. used boron convertors with 0.1%, 0.4%, and 1% combinations of o-carborane over CR-39 to observe the thermal neutron sensitivity. Tsuruta et al. also carried out studies with the ortho-carborane–doped CR-39 and etched in solution of 30% KOH, at 60°C for 2–16 h for revelation of alpha tracks. Boron-doped PADC solid-state nuclear track detector (SSNTD) is one of the commercially available detectors, which is suitable for monitoring of thermal neutrons due to the presence of boron. The CR-39 doped with boron has a high thermal neutron cross-section of 10B (3840 barns). It has a small elastic scattering cross-section (n, p) for fast-energy neutrons as well; however, in comparison with 10B(n, α)7Li reaction cross-section, it is insignificant.
In reactor environment, in addition to fast neutron, neutrons in the energy range from thermal to 100 keV are also present due to moderation in moderator of the reactor and subsequent multiple scattering in the reactor hall. Generally, the neutron spectrum at a location in the reactor hall is unknown, and the component of pure thermal neutrons may not be straightforward. In such a case, boron-doped thermal CR-39 detectors can be used along with a cadmium cover to estimate the pure thermal neutron (<0.4 eV) fraction by utilizing the cadmium-subtraction method. At present, fast neutron monitoring of >100 keV neutrons is carried out with normal (not doped) CR-39 SSNTD. Hence, there is a need to have dosimeter for monitoring of neutrons lower than 100 keV, especially in reactor environment. In the present study, an effort was made to optimize the etching process of boron-doped CR-39, also known as carborane, to evaluate its thermal neutron sensitivity for low energy neutron measurements. In case of electrochemical etching (ECE), parameters such as etchant concentration, temperature of etching, etching time, and high voltage need to be optimized before it is introduced in the monitoring program. In the present study, the etching time was optimized to get best signal-to-noise ratio for carborane (C2B10H12)n with concentration of 0.5% by weight (bare detector without any radiator), keeping the etchant concentration, temperature, and applied voltage constant. Further, dosimetric characteristics such as sensitivity, minimum detection threshold, and field study following the optimized etching condition were carried out.
| Materials and Methods|| |
Description of thermal neutron irradiation facility
To detect the thermal neutron-induced alpha tracks produced in the reaction 10B(n,α)7Li in the boron-doped CR-39 detector,,, the detectors were first irradiated in the standard thermal neutron assembly in graphite (STAG), which serves as the primary standard for the thermal neutron fluence rate., This system consists of six 241Am-Be neutron sources having total neutron emission rate of 6.6 × 106 n.s−1. The sources are embedded in a graphite pile of dimensions 163 cm × 122 cm × 152 cm. This pile is assembled from a number of graphite bars each having the dimensions of 10 cm × 10 cm × 76 cm. The individual neutron sources are staggered to obtain better uniformity of neutron fluence rate over the air cavity of dimensions 5 cm × 5 cm × 15 cm. The distance of each source from the air cavity is 29 cm. The measured thermal neutron fluence rate at the center of the air cavity is 6327 n.cm−2.s−1. For this measurement, cadmium-subtraction method of gold foil (cadmium ratio of gold foil was five) activation was used. Using the fluence-to-dose equivalent conversion coefficient at 0.025 eV, thermal neutron dose equivalent was obtained.
Irradiation of carborane with 241Am-Be fast neutron source
For evaluating the sensitivity of carborane detectors to fast neutron, ten detectors were irradiated in a low-scatter irradiation laboratory with a 1 Ci 241Am-Be fast neutron source (standardized with primary standard) having a neutron yield of 2.5 × 106 n.s−1. The detectors were exposed to 3.30 mSv of fast neutron dose equivalent. Hp(10) conversion factor of Am-Be spectrum is used for the calculation of neutron dose equivalent from source yield.,
Optimization of the electrochemical etching parameters
To optimize the high frequency of electric field used in ECE for thermal neutron sensitivity of the carborane, these detectors (4 numbers) were exposed to thermal neutron dose of 1.25 mSv using the STAG facility. Dual-frequency ECE (elevated temperature electrochemical etching [ETECE]), method was used for processing the detectors with applied constant electric potential of 1360 V. The time of etching for lower frequency (100 Hz) was 4 h whereas for high frequency (3.5 kHz) was varied from 40 to 60 min in steps of 5 min. KOH solution of concentration 7N was used as etchant maintained at 60°C throughout the etching process. Four un-irradiated carborane detectors along with four irradiated detectors were etched.
Evaluation of track density using an image analyzer
Image analyzers are generally used for the evaluation of track detectors for routine and research applications. In the present work, an indigenously developed image analyzer system was used, which is being routinely used for ECE-based fast neutron monitoring with CR-39 detectors., The system consists of an analog B/W CCD video camera, interfaced to a PCI frame grabber with video for windows driver for Pentium-IV computer. It takes 15 s per detector for counting of tracks and estimating the track density. The digitized picture is of 768 × 576 × 8-bit resolution (1 pixel equals to 15 μ). The camera with a zoom lens and a suitable light source is fixed in a metal box, practically cutting off the effect of any ambient light on the scene. The box has a small sliding tray on which the processed detector foil is mounted. In this system, before starting the actual counting, the track counting parameters are selected. Subsequently, tracks in control detector are counted and stored. The net track count of the irradiated detectors can be determined by the software (subtracting the control reading from the track density for the irradiated detectors).
Track counting parameters for electrochemical etching technique
The software of the image analysis system converts the captured image of the track of the detector into a binary image. The basic parameters that are computed for the tracks in the binary image are area, perimeter, and roundness factor. The tracks that are produced due to irradiation should ideally be round in shape and have a roundness factor close to that of a circle. Roundness factor is computed as:
and is 1.0 for the perfect circle. The track parameters are set in the software such that only the tracks that fall within the set range of area and the roundness are counted. This method of shape-based filtering is employed to filter out the tracks that are elongated. The lower and upper limits of diameter and roundness factor of acceptable tracks are 60–250 μm and 0.7 and 2.0, respectively. [Figure 1] shows the image of thermal neutron-induced alpha tracks of boron-doped CR-39 detector, irradiated to 1 mSv thermal neutron dose equivalent, as seen through the image analysis system. The elongated blue track is eliminated based on the criteria of area and the roundness factor, and the green tracks are eliminated based on the set area range. The images and data can be stored against the detector number in a file in the present system.
|Figure 1: Thermal neutron (1 mSv) induced alpha tracks in boron-doped CR-39 solid-state nuclear track detectors as seen by the Image Analyzer. The blue line represents a scratch and green represents overlapped tracks both rejected by imaging software based on shape size diameter range criteria of actual tracks|
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| Results and Discussion|| |
Optimized elevated temperature electrochemical etching parameters of carborane
[Table 1] presents the results of the experiments carried out toward optimization of the etching parameters. The track density and signal-to-noise ratio for different etching times are presented (40–60 min) at 3.5 kHz frequency for the carborane detector. As seen from [Table 1], the signal-to-noise ratio was nearly constant up to an etching time of 50 min and reduced above this time. Further, the background track density beyond 50 min of etching duration increased significantly. Hence, the etching time was optimized to 50 min based on these results and also considering the shape and size of the tracks and clarity of the tracks formed. Low-frequency optimization was not carried out as it is just the track marking stage of ETECE. Accordingly, the optimized parameters used for further processing of the detectors are (i) applied voltage of 1360 V, (ii) concentration of 7 N KOH solution maintained at 60°C, (iii) time of etching at low frequency (100 Hz) for 4 h, and (iv) 50 min at high frequency (3.5 kHz).
|Table 1: Optimization of high-frequency etching time for boron-doped (carborane, 0.5%) CR-39 solid-state nuclear track detector for thermal neutron dose evaluation|
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Dosimetric characterizations of carborane
Calibration factor and minimum detection limit
Ten detectors of carborane were exposed to 1 mSv of thermal neutron dose and processed along with 15 numbers of control detectors using ETECE technique as described in the section “Optimization of the electrochemical etching parameters.” The average net track density of the exposed detector was found to be 741 ± 8 tracks.cm−2 which represents the total thermal neutron sensitivity and control detector was 97 ± 4 tracks.cm−2. A calibration factor (CF) of 0.00135 mSv.tracks−1.cm−2 is arrived from the inverse of the thermal neutron sensitivity. MDL was then calculated using the following relation:
MDL = CF (mSv.tracks−1.cm−2) ×3 σ (background tracks) tracks.cm−2
= 0.016 mSv ~ 0.02 mSv
where “σ” is the standard deviation (1 SD) of 15 control (unirradiated) foils calculated using the SD equation.
Dose response of carborane
To study the dose response, the carborane detectors were irradiated with thermal neutron doses of 0.25, 0.5, 0.75, 1, and 1.25 mSv. The irradiated and the control detector were etched together as per the method described in the section “Optimized elevated temperature electrochemical etching parameters of carborane.” The results on dose response of carborane are presented in [Figure 2]. From [Figure 2], the intercept is the control detector (track density for zero dose) which is 100 ± 7 tracks.cm−2 and the slope is the response factor, which is obtained as 743 ± 13 tracks.cm−2.
|Figure 2: Thermal neutron dose response curve of boron doped CR-39 solid-state nuclear track detectors (carborane)|
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Fast neutron sensitivity of carborane
As carborane detectors are normal CR-39 doped with boron-10, it can register fast neutron induced tracks also through elastic collision along with thermal neutron non-elastic collision tracks. To compare this sensitivity of carborane detectors for thermal and fast neutrons, the carborane detectors were exposed to a fast neutron dose equivalent of 1 mSv of on a phantom at a distance of 75 cm away from a 1 Ci 241Am-Be neutron source with a neutron source yield of 2.48 × 106 n.s−1. The detectors were etched and evaluated for their track densities. The net sensitivity after subtracting the background was evaluated as 42 ± 11 tracks.cm−2 mSv−1. From this study, it was observed that the fast neutron sensitivity of carborane is three times lower than that of CR-39 (not doped with boron), used for fast neutron personnel monitoring. It is concluded that the thermal neutron sensitivity of carborane is ~18 times greater than that of its fast neutron sensitivity (741/42 = 17.64). Since the purpose of this work was to see the feasibility of boron-doped CR-39 SSNTD for thermal neutron measurements, the sensitivity of fast neutrons for different doses was not attempted.
Sensitivity of boron-doped CR-39 solid-state nuclear track detectors to pure thermal, epithermal, and intermediate neutrons
To assess the sensitivity of boron-doped CR-39 to pure thermal (<0.4 eV), epithermal (0.4 eV–1000 eV), and intermediate energy neutrons (1000 eV–100 keV), five boron-doped detectors were separately irradiated in thermal STAG facility as bare (without cadmium) and another set of 5 detectors inside 1 mm thick cadmium box for a thermal neutron dose of 1 mSv. The bare boron-doped detectors were observed to give thermal neutron-induced alpha tracks of 741 ± 8 tracks.cm−2 and the cadmium-covered detectors gave track density of 148 ± 6 tracks.cm−2, which is attributed to epithermal + intermediate neutrons. The subtraction of tracks due to bare and cadmium-covered CR-39 detector therefore gives the tracks due to pure thermal neutrons as 593 ± 12 tracks.cm−2. Normal (undoped) CR-39 detector, irradiated to 1 mSv in thermal STAG, gave only its background track density (30 tracks.cm−2.mSv−1) and zero net track density (no additional tracks due to fast neutrons if present as the cadmium ratio of the STAG is 5 with gold foil activation method), which confirms the thermal calibration of boron-doped CR-39 using STAG.
Since the boron-doped CR-39 detector showed higher sensitivity to thermal neutrons, these detectors can be used for the measurement of thermal neutron dose with an MDL of 0.02 mSv. In an unknown neutron field, cadmium-subtraction method can be used for the measurement of accurate pure thermal neutron dose by using pure thermal calibration factor as 1/593 = 0.0017 tracks.cm−2.mSv−1. For total thermal neutron measurement, bare carborane detectors can be used with calibration factor 1/741 = 0.00135 tracks.cm−2.mSv−1. Since the fast neutron sensitivity of these detectors is 18 times lower than that of thermal neutrons, in case of measurement of leakage neutron spectra outside the reactors, the response of these detectors can be assumed to be mainly due to thermal, epithermal, and intermediate energy neutrons.
Field trial of boron-doped CR-39 in research reactor
Field trials with carborane and normal CR-39, together as a dosimeter, were carried out in a research reactor with heavy water as moderator, light water as coolant, and natural uranium as fuel at a reactor power of 20 MW. The locations of the reactor for neutron measurements with CR-39 detectors were chosen based on Rem counter measurements. Doses estimated from both thermal and fast CR-39 detectors at different locations around beam holes of a research reactor are shown in [Table 2], which is arrived from the calibration factor obtained from dose response curve for boron doped [Graph 1] detectors and fast neutron calibration factor of usual CR-39 detectors. 3He-based neutron Rem counter readings taken in these locations are instantaneous and are considered to be from thermal to fast neutron energy (15 MeV). The dose rates measured by the Rem counter follow similar trend with respect to locations observed with CR-39 detectors. Both CR-39 detectors and Rem counter gave comparable neutron dose rates. Slight variation can be attributed to the instantaneous nature of Rem counter measurements, which is an active and bulky neutron survey meter, whereas CR-39 is a compact passive neutron detector. CR-39 detectors (fast neutrons) along with boron doped CR-39 detector (thermal + epithermal + intermediate) can therefore be used as a personal dosimeter as well as area dosimeter for neutrons from thermal to fast in a mixed neutron environment.
|Table 2: Field trials with boron-doped CR-39 and normal CR-39 for thermal and fast neutrons|
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| Conclusions|| |
The optimum ETECE etching conditions for boron-doped CR-39 was observed to be 4 h low (100 Hz) frequency and 50 min high (3.5 kHz) frequency at 1360 V constant potential at 60°C in 7 N KOH solution. The minimum measurable dose of 0.02 mSv for thermal neutrons was determined under the optimized etching conditions using ETECE. Cadmium-subtraction method has been suggested for the measurement of pure thermal neutrons using boron-doped CR-39 SSNTD. The carborane detector will serve as a thermal neutron personal as well as area dosimeter (with phantom/air calibration) in reactor, accelerator, and all other nuclear facilities environment where both fast- and lower-energy neutrons are present, to bridge the gap of the neutron monitoring.
The authors wish to thank Shri. R. M. Suresh Babu, Director, Health, Safety and Environment Group, Bhabha Atomic Research Centre, for his keen interest and encouragement for publishing this paper. The authors also wish to thank Neutron Standards Group of Radiation Standards Section for providing the STAG facility and many useful discussions. Thanks are also due to Dr. T. P. Selvam, RP and AD for reviewing the manuscript.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]