Practical implications of neutron survey instrument performance

Fast-Neutron Survey With Compact Plastic Scintillation Detectors

With the rise of the Silicon Photomultiplier (SiPM), it is now practical to build compact scintillation detectors well suited to portable use. A prototype survey meter for fast-neutrons and gamma-rays, based around an EJ-299-34 plastic scintillator with SiPM readout, has been developed and tested. A custom digital pulse processor was used to perform pulse shape discrimination on-the-fly. Ambient dose equivalent H*(10) was calculated by means of two energy-dependent 'G-functions'. The sensitivity was calculated to be between 0.10 and 0.22 cps/(µSv/hr) for fast-neutrons with energies above 2.5 MeV. The prototype was used to survey various laboratory radiation fields, with the readings compared with commercial survey meters. The high sensitivity and lightweight nature of this detector makes it promising for rapid survey of the mixed neutron/gamma-ray fields encountered in industry and homeland security.

Development of a Geant4 application to characterise a prototype neutron detector based on three orthogonal 3He tubes inside an HDPE sphere

The Bonner Sphere Spectrometer (BSS) system is a well-established technique for neutron dosimetry that involves detection of thermal neutrons within a range of hydrogenous moderators. BSS detectors are often used to perform neutron field surveys in order to determine the ambient dose equivalent H*(10) and estimate health risk to personnel. There is a potential limitation of existing neutron survey techniques, since some detectors do not consider the direction of the neutron field, which can result in overly conservative estimates of dose in neutron fields. This paper shows the development of a Geant4 simulation application to characterise a prototype neutron detector based on three orthogonal ³He tubes inside a single HDPE sphere built at the Australian Nuclear Science and Technology Organisation (ANSTO). The Geant4 simulation has been validated with respect to experimental measurements performed with an Am-Be source.

Modelling of neutron survey instrument performance and experimental validation of those calculated response data

Three moderator-type neutron survey instruments have been modelled for energy and angle dependence of the response, in greater detail than before. These response data have been verified by comparison with published experimental measurements and measurements made specifically for this project. Influences on the instrument response have also been investigated. These have included its mode-of-use and perturbations caused by variations in the instrument manufacture. The implications of these new response data have been assessed by an extensive programme of folding the responses with workplace energy distributions.