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Experimental Determination of Slow-Neutron Detection Efficiency and Background Discrimination in Mixed Radiation Fields Using Differential CR-39 Track Detectors

Ankit Kumar, Tushar Verma, Pankaj Jain, Raj Ganesh Pala, K. P. Rajeev

TL;DR

The paper develops a differential CR-39 detector method to quantify slow neutrons in mixed radiation fields by pairing a boron-coated CR-39 detector (BCR) with an uncoated control (CCR). By measuring track densities and computing $D = B - C$, the authors demonstrate a linear response $D = R t$ and extract a slow-neutron rate $R = 5.84 \pm 0.18$ tracks min$^{-1}$ (per 4.8 mm$^2$), with a corresponding detection efficiency $\varepsilon \approx (1.60 \pm 0.09) \times 10^{-3}$ using a known source flux $N = (3658 \pm 183)$ neutrons min$^{-1}$ (4.8 mm$^2$). The uncoated CCR provides a direct measure of fast-neutron and charged-particle backgrounds, enabling robust discrimination of slow-neutron signals and offering a conservative estimate of slow-neutron fluence even in unknown or complex mixed fields. The differential BCR–CCR approach eliminates reliance on external absorbers, improves spectrally faithful neutron measurements, and is applicable to plasmas, liquids, and other challenging environments where conventional detectors struggle.

Abstract

Measuring slow neutrons is difficult when the radiation field also contains charged particles and fast neutrons, especially when the radiation composition is not known in advance. In this work, we present a tested method to measure slow neutron fluence using CR-39 solid state nuclear track detectors. Two detectors are used together: a boron coated CR-39 detector and an uncoated CR-39 detector.The uncoated detector records tracks from charged particles and fast neutrons but does not respond to slow neutrons. The boron coated detector additionally detects charged particles produced when slow neutrons react with boron and generate lithium and alpha particles. Subtracting the track density of the uncoated detector from that of the boron coated detector provides a reliable and conservative measure of slow neutrons.Experiments using a reference thermal neutron source show that the difference between the two detectors increases linearly with exposure time. Statistical analysis gives a slow neutron equivalent track rate of 5.84 plus or minus 0.18 tracks per minute, clearly different from zero. The slope of this response is used to determine the detection efficiency of the boron coated detector. The uncoated detector measures the background caused by fast neutron leakage from the source. These results show that boron coated CR-39 detectors cannot be used alone for accurate slow neutron measurements. Reliable neutron fluence determination requires the simultaneous use of an uncoated detector. The difference between the two detectors provides a correct estimate of the thermal neutron flux in mixed radiation fields and where conventional neutron detectors cannot be used.

Experimental Determination of Slow-Neutron Detection Efficiency and Background Discrimination in Mixed Radiation Fields Using Differential CR-39 Track Detectors

TL;DR

The paper develops a differential CR-39 detector method to quantify slow neutrons in mixed radiation fields by pairing a boron-coated CR-39 detector (BCR) with an uncoated control (CCR). By measuring track densities and computing , the authors demonstrate a linear response and extract a slow-neutron rate tracks min (per 4.8 mm), with a corresponding detection efficiency using a known source flux neutrons min (4.8 mm). The uncoated CCR provides a direct measure of fast-neutron and charged-particle backgrounds, enabling robust discrimination of slow-neutron signals and offering a conservative estimate of slow-neutron fluence even in unknown or complex mixed fields. The differential BCR–CCR approach eliminates reliance on external absorbers, improves spectrally faithful neutron measurements, and is applicable to plasmas, liquids, and other challenging environments where conventional detectors struggle.

Abstract

Measuring slow neutrons is difficult when the radiation field also contains charged particles and fast neutrons, especially when the radiation composition is not known in advance. In this work, we present a tested method to measure slow neutron fluence using CR-39 solid state nuclear track detectors. Two detectors are used together: a boron coated CR-39 detector and an uncoated CR-39 detector.The uncoated detector records tracks from charged particles and fast neutrons but does not respond to slow neutrons. The boron coated detector additionally detects charged particles produced when slow neutrons react with boron and generate lithium and alpha particles. Subtracting the track density of the uncoated detector from that of the boron coated detector provides a reliable and conservative measure of slow neutrons.Experiments using a reference thermal neutron source show that the difference between the two detectors increases linearly with exposure time. Statistical analysis gives a slow neutron equivalent track rate of 5.84 plus or minus 0.18 tracks per minute, clearly different from zero. The slope of this response is used to determine the detection efficiency of the boron coated detector. The uncoated detector measures the background caused by fast neutron leakage from the source. These results show that boron coated CR-39 detectors cannot be used alone for accurate slow neutron measurements. Reliable neutron fluence determination requires the simultaneous use of an uncoated detector. The difference between the two detectors provides a correct estimate of the thermal neutron flux in mixed radiation fields and where conventional neutron detectors cannot be used.
Paper Structure (16 sections, 31 equations, 2 figures, 2 tables)

This paper contains 16 sections, 31 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experimental Setup
  3. Material and Equipments
  4. Detector Preparation
  5. Neutron Exposure
  6. Chemical Etching and Track Counting
  7. Results and Discussions
  8. Model predictions and the statistical significance of the fitted rate R
  9. Slow neutron detection efficiency of BCR
  10. Experimental significance
  11. BCR-CCR as an suitable detector system for slow neutron measurements
  12. Measurement of the slow-to-fast neutron flux ratio in mixed neutron fields
  13. Slow neutron measurement in an unknown field
  14. Conclusions
  15. Raw Data for BCR and CCR exposed to reference thermal neutron source
  16. ...and 1 more sections

Figures (2)

  • Figure 1: Optical micrographs of etched tracks on boron-coated CR-39 (BCR) and control CR-39 (CCR) detectors exposed to a thermal neutron source for 40 minutes. For each detector of 6 mm $\times$ 4 mm area, four equal non overlapping regions, each of area 1.8 mm $\times$ 2.5 mm (4.8 mm$^2$) were imaged: left-top (LT), right-top (RT), right-bottom (RB), and left-bottom (LB). Images (a)–(d) show BCR tracks counts of 507 (LT), 504 (RT), 505 (RB), and 533 (LB), yielding a mean of $512 \pm 6$ tracks per 4.8 mm$^2$. Images (e)–(h) show the corresponding CCR track counts of 288 (LT), 270 (RT), 290 (RB), and 283 (LB), yielding a mean of $283 \pm 4$ tracks per 4.8 mm$^2$. The actual number of mean tracks recorded during exposure on either detector is obtained after deduction of baseline value from each, which gives $500 \pm 6$ and $271 \pm 4$ for BCR and CCR respectively. Raw data is provided in Sec. I of the Supplemental Material.
  • Figure S1: Optical micrographs of boron-coated CR-39 (BCR) and uncoated control CR-39 (CCR) detectors exposed to a reference thermal neutron source for different durations are shown. Each row of images corresponds to a single BCR/CCR detector pair. From left to right, the panels represent the left-top (LT), right-top (RT), right-bottom (RB), and left-bottom (LB) regions of the detector surface. The image labeling is as follows: 10-min exposure---BCR (images 1--4) and CCR (images 5--8); 20-min exposure---BCR (images 9--12) and CCR (images 13--16); 30-min exposure---BCR (images 17--20) and CCR (images 21--24); and 40-min exposure---BCR (images 25--28) and CCR (images 29--32).