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Year : 2014  |  Volume : 37  |  Issue : 1  |  Page : 30-34  

Measurement of neutron energy spectrum from 241 Am-B source using CR-39 detectors and an in-house image analysis program (autoTRAK_n)

Accelerator Radiation Safety Section, Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Web Publication8-Dec-2014

Correspondence Address:
Sam Tripathy
Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-0464.146461

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An attempt has been made in the present work to estimate the neutron spectrum of a standard 241 Am-B source using CR-39 solid polymeric track detector and an in-house image analysis program autoTRAK_n. The program works on the principle of greyscale variations in and around the recoil tracks and calculates the track parameters such as diameter, major, minor axes and estimates three dimensional parameters like recoil track depth and angular incidence. It is also capable of counting overlapping tracks without any segregation procedure. In the present study, the standard 241 Am-B neutron spectrum generated from the recoil track parameters with the autoTRAK_n program is compared with the reported values measured using the time of flight method and found to be matching well, mainly at the characteristic peak. The total neutron fluence and dose equivalents are also determined with the program and are compared with the standard source measurements. The results obtained from autoTRAK_n are found to be in close agreement with the standard measurements with less than 5% error for both the parameters. Hence, this methodology can be used as a backup technique for neutron spectrum measurements along with other techniques considering its low cost, small size, easy irradiation, and processing.

Keywords: CR-39 detector, image analyzing program, neutron spectrometry and dosimetry

How to cite this article:
Paul S, Tripathy S, Sahoo GS, Joshi DS, Bandyopadhyay T. Measurement of neutron energy spectrum from 241 Am-B source using CR-39 detectors and an in-house image analysis program (autoTRAK_n). Radiat Prot Environ 2014;37:30-4

How to cite this URL:
Paul S, Tripathy S, Sahoo GS, Joshi DS, Bandyopadhyay T. Measurement of neutron energy spectrum from 241 Am-B source using CR-39 detectors and an in-house image analysis program (autoTRAK_n). Radiat Prot Environ [serial online] 2014 [cited 2022 Jun 28];37:30-4. Available from: https://www.rpe.org.in/text.asp?2014/37/1/30/146461

  Introduction Top

The knowledge of neutron spectrum is crucial for radiation protection, dosimetric studies and nuclear physics applications. [1],[2],[3],[4],[5] Several techniques for direct [6] and indirect [7],[8],[9] measurement of neutron spectrum are available, which are either bulky or expensive detectors coupled with complex spectrum unfolding techniques. [10],[11] In this aspect, the solid state nuclear track detectors like CR-39 (polyallyldiglycol carbonate, C 12 H 18 O 7 ) serve as an effective neutron detector due to the following features, (a) insensitivity to low linear energy transfer radiations, (b) permanent mode of registration, (c) low cost, (d) small size, (e) convenient for irradiation with easier processing, etc. [12] The integral mode of signal registration makes the detector useful for personal and area monitoring specially around particle accelerators, where the field is mostly pulsed and mixed in nature. [13],[14]

The nano-scale damages (latent tracks) due to neutron-induced recoils need to be developed up to the micrometer level, so that the tracks can be viewed and analyzed with the optical microscope. The track parameters depend on the particle type, the mass and the trajectory of recoils. However, the generation of neutron spectrum from the two-dimensional track parameters such as track diameter, major axis and minor axis is not possible because the particle energy is not linearly dependent on the track diameter. [15] Hence, different types of approaches with suitable parameters have been applied to obtain the neutron spectrum from CR-39 detectors [16],[17] by determining the track depth and the angle of incidence of the recoils in the detector.

In the present study, a direct and simple approach has been used for measuring the neutron spectrum from 241 Am-B (α, n) source using CR-39 detectors and an in-house image analyzing program autoTRAK_n. [18],[19],[20] This program estimates the track profiles like track depth and incidence angle of the recoil using the grey value variations within and around each track in the two-dimensional images and then correlates the track depth distribution with energy of the recoil particles. Finally, the total particle fluence and neutron dose equivalents are estimated by folding with track to neutron response function [21] and energy dependent ICRP-74 [4] dose conversion coefficients.

  Materials and methods Top

Irradiation and chemical etching

A set of CR-39 detectors (intercast, 37 mm × 25 mm × 1.5 mm) were irradiated with a 241 Am-B source at 4π geometry in the Radiation Standards Section, RP and AD, Bhabha Atomic Research Centre, India. The detectors were irradiated with a total neutron fluence of 1.03 × 10 9 cm−2 . The detector was very close to the source (<6 cm), much smaller in size compared to the source and was kept at a height of about 2 m from the floor and far away from walls and roof. So, the contributions from scattered components were considered to be almost negligible. The irradiated detectors were then developed with chemical etching using 6.25 N NaOH, 70°C for 9 h. These etched detectors were then imaged using an optical microscope attached with a 5  MP camera (Carl Zeiss, Germany) at a magnification of ×200. For the reference measurement, a pristine detector was also etched and imaged in similar conditions to obtain the background spectrum for the correction of systematic errors.

Image processing and analysis

The acquired two-dimensional track images were processed using the newly developed autoTRAK_n program [18] which works on the principle of grey level variations in and around the tracks. The variations are more inside the tracks and based on these changes the track depth and angle of incidence of the particles are calculated. The detailed algorithm of the program is reported elsewhere. [18] However, a brief discussion of the program is described below.

The algorithm processes the images in two separate phases. In the first phase, the recoil tracks are detected using the algorithm presented in [Figure 1]a. The point in the track boundary (P) is detected first based on the value of its grey level, then the algorithm generates a box considering P as the initial point and then size of the box increases progressively until the complete track is identified. Once recognized, the track is patched with the background color to avoid its recounting during further scanning of the image, which also reduces the computing time. Then, the program starts scanning for a new track as described in the [Figure 1]b. As shown in this figure, the track is located initially (red) and then a bounding box (yellow) around it is created, and then with progressive searching algorithm the final boundary (purple) covering the entire track contour is generated. Once the track is selected, and the boundary is marked, the track information is stored, and the track is filled with the background color to avoid repetition of its counting. This algorithm of searching tracks is based on an inside-out approach unlike other image analyzing software, thereby ensuring easy counting of extremely overlapping tracks.

In the second phase of the program, individual track parameters such as track diameter (for circular) or major and minor axes (for elliptical), area, eccentricity, track depth, angle of recoil etc., are calculated. The detailed algorithm for calculation of these parameters was discussed elsewhere. [18] The track depth calculated with the autoTRAK_n algorithm was then used to generate depth distribution of the recoil tracks, which was then correlated with the recoil particle energy using the range-energy relations from SRIM code. [22] A similar analysis was carried out to generate the depth distributions for the pristine detector. This background spectrum was then subtracted from the spectra obtained with irradiated detectors to obtain the net number of neutron-induced recoil tracks. The neutron energy was then calculated from recoil energy using the relation Eq. (1), where θ, ER stands for incidence angle and energy of the recoil particles respectively. The En indicates the neutron energy, and A is the mass number of the recoil.
Figure 1: (a) First phase of the algorithm in detecting the track boundary and patching (b) execution of phase-1 with simulated images of circular and elliptical tracks

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The neutron energy distributions were then folded with the track to neutron response function [21] to obtain the total number of neutrons in each energy bin. The neutron spectrum thus obtained is again folded with the energy dependent ICRP-74 dose conversion coefficients [4] [Figure 2] to estimate the dose equivalent H* (10).
Figure 2: Energy dependence of ICRP-74 dose conversion coefficients

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The total dose was estimated by summing the dose equivalents over the entire energy range using relation Eq. (2). [19],[20]

The dose equivalent is estimated in Sv by multiplying the factor 10−12 as the Dose Conversion Coefficient values are provided in pSv cm 2 and particle fluence (j) in cm−2 .

  Results and discussion Top

Fast neutron spectrum measurement and estimation of dose equivalent using CR-39 detector and autoTRAK_n program is successfully demonstrated in this work by reproducing the spectrum of a standard 241 Am-B source. In 241 Am-B system, the α-induced reactions produce neutrons with interaction with the isotopes of [10],[11] B, as given in the following nuclear reactions, given below in relation Eq. (3).

All the tracks (circular as well as elliptical) were considered for spectrum generation purpose. However, more than about 90% tracks were found to be circular in nature as can be seen from the track size distribution presented in [Figure 3]. The encircled portion contains the circular tracks at one sigma confidence interval. The inset figure shows the distribution of the track size with respect to the major axis.
Figure 3: Size distribution of neutron-induced recoil tracks in CR-39 detector irradiated with 241Am-B source

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The 241 Am-B spectrum obtained with the autoTRAK_n program is given in [Figure 4]. The net particle fluence was obtained by subtracting the background spectrum determined from pristine detector. As can be seen in the figure, the spectrum has a low energy contribution at about 1 MeV region, along with a peak at 3 MeV. The normalized spectrum is compared with the spectrum reported by Marsh et al. [23] obtained from their time of flight (TOF) measurements. Both the spectra matched reasonably well, especially at the characteristic 3 MeV peak, within an error of <1%. The bin-wise error bars given in the spectrum [Figure 4] indicate the statistical error in the total number of neutrons per unit energy bin. The errors were estimated by regenerating the spectrum for about 30 times with statistically significant number of tracks, each time considering a new set of tracks (~5,000 tracks for every set of spectrum generation).
Figure 4: 241Am-B neutron spectrum obtained using CR-39 detector and autoTRAK_n is compared with a similar spectrum reported by Marsh, et al.[23] which was measured using time of flight method

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The total neutron fluence and the dose equivalent were estimated automatically by the autoTRAK_n program which was found to be close to the standard reference measurements [Table 1]. The fluence was estimated to be 9.91 × 10 8 neutrons cm−2 compared to the standard value of 1.03 × 10 9 neutrons cm−2 and the estimated dose equivalent was found to be 400.4 mSv compared to the standard value of 418.6 mSv at the detector distance from source.
Table 1: Comparison of the total neutron fluence and doe equivalent values obtained using the present methodology with the reference values obtained for the source used in this study

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  Conclusion Top

In the present work, a simple and effective algorithm (autoTRAK_n) is validated by reproducing the neutron spectrum of a standard 241 Am-B source using CR-39 detectors. The program can also be applied for automatic counting of high density overlapping tracks without involving any segregation procedure. The spectrum is compared with that obtained by Marsh et al. using the TOF measurements. [23] The distributions agree well particularly for the characteristic peak at 3 MeV, though differ in detail at other energies. The fluence was estimated within 4% error and the estimated dose equivalent was also found to be within 4.3% compared to the standard values at the detector distance from the source. The program effectively counts the tracks due to its inside-out approach of detecting the tracks and can be applied for counting of highly dense overlapping tracks. It is important to note that the algorithm does not require any complex unfolding technique for estimation of neutron spectrum.

  Acknowledgements Top

The authors are thankful to staff members of Radiation Standards Section, RP and AD, BARC for providing source and their support in conducting the irradiations. The authors sincerely acknowledge Dr. R. M. Tripathi, Head, HPD and Dr. D. N. Sharma, Director, HS and EG, BARC for the inspiration in carrying out these studies.

  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1]


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