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Direct Neutron Detectors based on Carborane Containing Conjugated Polymers

Aled Horner, Fani E. Taifakou, Choudhry Z. Amjad, Filip Aniés, Elizabeth George, Chris Allwork, Adrian J. Bevan, Martin Heeney, Theo Kreouzis

TL;DR

This work demonstrates a new class of thermal neutron detectors based on carborane-containing conjugated polymers, addressing the $^{3}$He shortage by leveraging intrinsic $^{10}$B in the polymer backbone (oCbT2-NDI) and comparing with boron-sensitized PNDI(2OD)2T. The detectors operate via direct conversion, with thermal neutrons captured by boron and fast neutrons detected by hydrocarbon interactions, enabling linear response up to $1.796 imes10^{7}$ cm$^{-2}$s$^{-1}$ and bias-dependent saturation. Quantitatively, the boron-rich devices show comparable or improved signal relative to sensitised controls, achieving QE values around 0.69–0.89% and thermal-conversion efficiencies near 0.14–0.20%, while maintaining solution-processable, scalable fabrication. The results establish the viability of carboranyl polymers for low-cost, large-area neutron detectors and highlight tunability via boron content or side-chain design to boost performance for practical applications.

Abstract

Thermal neutron detectors are crucial to a wide range of applications, including nuclear safety and security, cancer treatment, space research, non-destructive testing, and more. However, neutrons are notoriously difficult to capture due to their absence of charge, and only a handful of isotopes have a sufficient neutron cross-section. Meanwhile, commercially available $^3$He gas filled proportional counters suffer from depleting $^3$He feedstocks and complex device structures. In this work, we explore the potential of a carborane containing conjugated polymer ($o$CbT$_2$-NDI) as a thermal neutron detector. The natural abundance of $^{10}$B in such a polymer enables intrinsic thermal neutron capture of the material, making it the first demonstration of an organic semiconductor with such capabilities. In addition, we show that thermal neutron detection can be achieved also by adding a $^{10}$B$_4$C sensitiser additive to the analogous carborane-free polymer PNDI(2OD)2T, whereas unsensitised PNDI(2OD)2T control devices only respond to the fast neutron component of the radiation field. This approach allows us to disentangle the fast and thermal neutron responses of the devices tested and compare the relative performance of the two approaches to thermal neutron detection. Both the carborane containing and the $^{10}$B$_4$C sensitised devices displayed enhancement due to thermal neutrons, above that of the unsensitised polymer. The detector response is found to be linear with flux up to $1.796\,\times\,10^7\,$cm$^{-2}$s$^{-1}$ n$_{th}\bar{v}$ and saturates at high drive biases. This study demonstrates the viability of carboranyl polymers as neutron detectors, highlights the inherent chemical tuneability of organic semiconductors, and opens the possibility of their application to a number of different low-cost, scalable, and easily processable detector technologies.

Direct Neutron Detectors based on Carborane Containing Conjugated Polymers

TL;DR

This work demonstrates a new class of thermal neutron detectors based on carborane-containing conjugated polymers, addressing the He shortage by leveraging intrinsic B in the polymer backbone (oCbT2-NDI) and comparing with boron-sensitized PNDI(2OD)2T. The detectors operate via direct conversion, with thermal neutrons captured by boron and fast neutrons detected by hydrocarbon interactions, enabling linear response up to cms and bias-dependent saturation. Quantitatively, the boron-rich devices show comparable or improved signal relative to sensitised controls, achieving QE values around 0.69–0.89% and thermal-conversion efficiencies near 0.14–0.20%, while maintaining solution-processable, scalable fabrication. The results establish the viability of carboranyl polymers for low-cost, large-area neutron detectors and highlight tunability via boron content or side-chain design to boost performance for practical applications.

Abstract

Thermal neutron detectors are crucial to a wide range of applications, including nuclear safety and security, cancer treatment, space research, non-destructive testing, and more. However, neutrons are notoriously difficult to capture due to their absence of charge, and only a handful of isotopes have a sufficient neutron cross-section. Meanwhile, commercially available He gas filled proportional counters suffer from depleting He feedstocks and complex device structures. In this work, we explore the potential of a carborane containing conjugated polymer (CbT-NDI) as a thermal neutron detector. The natural abundance of B in such a polymer enables intrinsic thermal neutron capture of the material, making it the first demonstration of an organic semiconductor with such capabilities. In addition, we show that thermal neutron detection can be achieved also by adding a BC sensitiser additive to the analogous carborane-free polymer PNDI(2OD)2T, whereas unsensitised PNDI(2OD)2T control devices only respond to the fast neutron component of the radiation field. This approach allows us to disentangle the fast and thermal neutron responses of the devices tested and compare the relative performance of the two approaches to thermal neutron detection. Both the carborane containing and the BC sensitised devices displayed enhancement due to thermal neutrons, above that of the unsensitised polymer. The detector response is found to be linear with flux up to cms n and saturates at high drive biases. This study demonstrates the viability of carboranyl polymers as neutron detectors, highlights the inherent chemical tuneability of organic semiconductors, and opens the possibility of their application to a number of different low-cost, scalable, and easily processable detector technologies.

Paper Structure

This paper contains 8 sections, 5 equations, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Organic semiconductor detectors
  3. Experimental setup
  4. Results
  5. Polymer definitions
  6. Sample fabrication
  7. Calculation of the $^{10}$B content in the oCbT$_2$-NDI
  8. Quantum efficiency and thermal neutron conversion efficiency

Figures (3)

  • Figure 1: a) Chemical structure of the oCbT$_{2}$-NDI and PNDI(2OD)2T polymers. b) Structure of devices fabricated on substrate. The overlap area between the ITO anodes and Al cathode defines an individual device. c) Top panel: optical reflection micrograph of a PNDI(2OD)2T:B$_4$C device showing B$_4$C microparticles in the form of dark spots. The fast and thermal neutron interactions are superimposed graphically and do not correspond in scale to the bar shown in the micrograph. Bottom panel: Schematic cross section through a single device. The current direction is from anode to cathode. Ionising (charged) neutron reaction products increase the charge carrier density within the device thus increasing the device current.
  • Figure 2: a) Device current versus time plots in the alternating absence and presence of neutrons at $+10$ V bias. The current enhancement due to the $1.537$$\times$$10^7$ cm$^{-2}$s$^{-1}$ n$_{th}\bar{v}$ neutron fluence rate is clearly distinguishable as a sharp rise on exposure, followed by a sharp drop when the neutron field is turned off. The figure shows responses from two individual $4$ mm$^2$ devices for each of the following compositions: PNDI(2OD)2T:B$_4$C (green), oCbT$_2$-NDI (red), and PNDI(2OD)2T (grey). b) The current enhancement ($\Delta I$) upon neutron exposure at $+10$ V bias versus neutron fluence rate for a single device of each composition with linear fits to each data set. Both plots share the same legend shown in a).
  • Figure 3: a) Current enhancement versus bias characteristics for PNDI(2OD)2T:B$_4$C (green), oCbT$_2$-NDI (red), and PNDI(2OD)2T (grey) $4$ mm$^2$ devices of similar thickness. b) Current enhancement versus bias characteristics for $36$ mm$^2$oCbT$_2$-NDI (orange) and $4$ mm$^2$oCbT$_2$-NDI (red). All data sets were obtained at a fluence rate of $1.537$$\times$$10^7$ cm$^{-2}$s$^{-1}$ n$_{th}\bar{v}$. The dashed lines are fits to Eq. \ref{['eq:DI']} for each data set in both figures.