Neutron Detection

Passive Neutron Detection with CR-39

Measure neutron fields without power supplies, cables, or active electronics. CR-39 solid-state nuclear track detectors record every neutron interaction as a permanent, etchable damage trail in the polymer — ready for analysis days, weeks, or months after exposure.

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The Physics

How CR-39 Detects Neutrons

Neutrons carry no electric charge, so they cannot ionize CR-39 directly. Detection relies on nuclear reactions that convert a neutron into one or more charged particles heavy enough to break polymer bonds and leave an etchable latent track.

Thermal & Epithermal Neutrons

Boron-Coated CR-39 — the 10B(n,α)7Li Reaction

A thin boron or boron nitride coating is applied to one face of the CR-39 chip. When a thermal or epithermal neutron is captured by a 10B nucleus, the reaction produces an alpha particle (1.47 MeV) and a 7Li ion (0.84 MeV) traveling in opposite directions. One of these charged products enters the CR-39 substrate and creates a latent damage track that is revealed by chemical etching in NaOH solution.

By comparing the coated face against a nearby uncoated reference pane exposed at the same location, the true neutron signal can be isolated from the ambient alpha background. BSI manufactures boron-coated CR-39 in-house using a non-toxic, water-soluble adhesive that dissolves cleanly during the etch step without affecting bulk etch rate. Our published work on boron nitride coatings for CR-39 in Nuclear Engineering and Technology details coating uniformity and detection efficiency.

Fast Neutrons (MeV Range)

Proton Recoil in Bare CR-39

Fast neutrons — such as the 14 MeV neutrons produced by D-T fusion or the broad spectrum from an AmBe source — undergo elastic scattering on hydrogen nuclei within the CR-39 polymer itself. The recoiling proton carries enough energy and has a high enough linear energy transfer (LET) to create a visible track after etching, no converter coating required.

Track geometry reveals the recoil angle, and because the proton energy depends on scattering kinematics, experienced analysts can extract neutron energy estimates from track diameter and depth profiles. BSI's SEM large-area mapping workflow captures thousands of tracks per chip, and our AI classifier distinguishes proton-recoil tracks (labeled in magenta) from fission fragments (cyan) and surface defects with high confidence.

Comparison

Passive vs Active Neutron Detectors

Active detectors (3He tubes, BF3 proportional counters, scintillator arrays) give real-time count rates but require power, shielding, and calibration electronics. Passive CR-39 detectors trade real-time readout for simplicity, cost, and the ability to deploy hundreds of detectors simultaneously.

Attribute CR-39 Passive Detector Active Detector
Power requirement None Continuous (HV bias + readout)
Electronics None — polymer chip only Preamplifier, MCA, cabling
EMI sensitivity Immune Can cause false counts or dead time
Deployment scale Hundreds of chips simultaneously Limited by channel count and cabling
Record type Permanent physical record, re-analyzable Digital data; no physical artifact
Per-detector cost $2 – $6 per chip $500 – $10,000+ per channel
Readout speed Post-exposure (etch + microscopy) Real-time pulse output
Particle-type ID Track morphology + AI classification Pulse height discrimination

BSI Capabilities

AI-Enhanced Neutron Track Analysis

Traditional optical microscopy limits analysis to small fields of view and manual counting. BSI replaces this bottleneck with a two-stage workflow published in Nuclear Engineering and Technology: SEM large-area mapping combined with an AI-driven track classifier.

First, a scanning electron microscope tiles the entire etched CR-39 surface into a seamless high-resolution mosaic covering areas up to several square centimeters. Then a convolutional neural network trained on labeled datasets of alpha tracks, proton-recoil tracks, fission fragments, and surface artifacts classifies every feature in the image. The result is a per-chip report with particle type identification, track density maps, energy estimates derived from track geometry, and source-versus-noise discrimination that would take weeks to produce by hand.

This methodology grew from research at Texas Tech University, where BSI's founding team developed CR-39 analysis techniques across neutron, alpha, and fission-fragment experiments. Both the boron nitride coating process and the AI analysis pipeline are documented in peer-reviewed publications.

AI Output Examples

  • 01Proton-recoil tracks boxed in magenta — fast neutron signature
  • 02U-238 fission fragments boxed in cyan — confirmed by track length and LET
  • 03Surface defects and scratches rejected by the classifier — reducing false positives
  • 04Track density heat maps across the full chip area for spatial fluence profiling

Peer-Reviewed Publications

Common Questions

Neutron Detection Q&A

How does CR-39 detect neutrons?

CR-39 is a thermoset polymer that records charged-particle damage as latent tracks. Because neutrons are electrically neutral, they must first produce a charged secondary particle. For thermal and epithermal neutrons, a boron-10 converter coating captures the neutron and emits an alpha particle and a lithium-7 ion via the 10B(n,α)7Li reaction. For fast neutrons in the MeV range, elastic scattering on hydrogen in the polymer itself produces energetic recoil protons that leave tracks directly. After exposure, the CR-39 is etched in a heated NaOH solution to enlarge the latent tracks into pits visible under a microscope or SEM.

What is boron-coated CR-39?

Boron-coated CR-39 is a standard CR-39 chip with a thin layer of natural boron or boron nitride deposited on one surface. The coating acts as a neutron converter: when a thermal neutron is absorbed by 10B (which comprises roughly 20% of natural boron), the resulting nuclear reaction sends charged particles into the underlying polymer. BSI applies coatings using a non-toxic, water-soluble binder that dissolves during the NaOH etch without altering the bulk etch rate. We recommend pairing each coated chip with an uncoated reference pane at the same location so that neutron-induced tracks can be separated from background alpha activity. See our neutron-sensitized CR-39 page and best practices guide for handling and etching protocols.

How does passive detection compare to active neutron detectors?

Active neutron detectors — 3He proportional counters, BF3 tubes, lithium-glass scintillators — provide real-time count rates but require high-voltage bias, preamplifiers, multi-channel analyzers, and continuous power. They are sensitive to electromagnetic interference and represent a significant per-channel investment. Passive CR-39 detectors, by contrast, are small polymer chips that cost a few dollars each, require no power or electronics, are immune to EMI, and leave a permanent physical record that can be re-analyzed years later. The trade-off is that readout is not instantaneous: exposed chips must be etched and imaged post-exposure. For applications where real-time feedback is not essential — area monitoring, personnel dosimetry, field surveys, and large-scale spatial mapping — passive CR-39 is often the more practical and economical choice.

Get Started

Ready to Measure Neutrons?

Whether you need boron-coated chips for thermal neutron dosimetry or bare CR-39 for fast-neutron surveys, BSI can supply detectors, process your exposed samples, or help you build an in-house analysis capability. Tell us about your application and we will recommend the right configuration.

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Explore

Related Resources

Product

CR-39 Track Detectors

Product

Neutron-Sensitized CR-39

Guide

CR-39 Best Practices

Research

Published Research