Neutrons interact with atomic nuclei whereas photons (gamma rays and X rays) interact via the electron shell structure of atoms. Consequently neutrons are scattered more readily by light isotopes (being similar in terms of mass to themselves) whilst photons are scattered more effectively by heavier elements. X-rays in particular are well-known for their imaging applications in industry and medicine. However, the use of neutrons can be complicated by the requirement to have an installation to provide a neutron source (such as a reactor or particle accelerator) whilst until recently the best way to detect the neutrons was via a zinc-sulphide scintillation screen in tandem with a light intensifier such as a CCD; such an arrangement is rarely industrially-compatible nor readily portable.
Based on the real-time pulse-shape discrimination capability for organic scintillators pioneered by Hybrid Instruments Ltd. and embodied in the mixed-field analyser, neutron tomography can be realised with a small number of detectors and a relatively small, isotopic neutron source such as 252Cf or 241AmBe.
The following features distinguish this approach from previous attempts:
The neutron conversion/light intensification stage is replaced by a small number of self-contained organic scintillation detectors. This is effectively a single-stage detection system that does not require beam transfer systems to protect the detectors from direct exposure to the source.
Fast neutrons can be detected directly without the uncertainties in terms of trajectory that arise from thermalisation and beam transfer.
The system is readily portable using transportable isotopic sources and is not critically dependent on optical standards of set-up and alignment.
The fast neutron tomography arrangement (left) and the results with a two-hour scan with a 75 MBq 252Cf source and 7 EJ309 organic scintillation detectors,
(Joyce et al., IEEE NSS (2015)).
K. Mitton, A. Jones and M. J. Joyce, (2014)
Digital fast neutron radiography of steel reinforcing bar in concrete. Journal of Instrumentation, 9 C12045 doi:10.1088/1748-0221/9/12/C12045.
M. J. Joyce, S. Agar, M. D. Aspinall, E. Colley, M. Colling, J. Dykes, P. Kardasopolous and K. Mitton, (2015)
Portable, Fast-Neutron Tomography with an Isotopic Source and Organic Scintillation Detectors. IEEE Nuclear Science Symposium, Abstract #2147, San Diego, USA.
M. J. Joyce, S. Agar, M. D. Aspinall, J. S. Beaumont, E. Colley, M. Colling, J. Dykes, P. Kardasopoulos and K. Mitton. (2016)
Fast neutron tomography with real-time pulse shape discrimination in organic scintillation detectors. Nuclear Instrument and Methods in Physics Research Section A (NIM A), DOI 10:1016/j.nima.2016.07.044