Measurement of gamma rejection of solid state neutron sensors to maximise neutron and minimise gamma sensitivities

Cambridge Microfab Ltd

National Physical Laboratory
Round 3

The aim of this Microfab/NPL  project is to learn how well Microfab HT series neutron sensors and electronics survive within harsh gamma radiation fields and to establish whether Microfab HT series sensors  can be used for reliable neutron detection in environments where the gamma flux is many orders higher than the neutron flux.

This project relies on the unusual  ability of NPL to simulate various environments where a small amount of fissile material needs to be detected by its neutron emission in an environment where there is also a very high gamma radiation background.

Neutron detectors of all types are  invariably affected by other types of radiation so there is normally  interference caused by gamma radiation when trying to detect neutrons. Under many circumstances it is easy to deal with this problem  by placing thick lead or tungsten shielding so as to block the gamma radiation. Such shielding does not block neutrons but can be used to remove or greatly reduce the gamma flux.

The neutron detection problem exists in various unusual situations at locations where there has been an accident involving fissile material. For instance at Fukushima very compact, light weight, low power, robot borne, instrumentation is essential due to access restrictions, the severity of the radiation fields and multiple complications arising from damage to the original buildings and equipment.  “Normal” neutron detectors fitted with enough shielding to remove the background gamma radiation are unsuitable for this application.

Microfab HT sensors are candidates for this type of measurement instrumentation because they are very compact, low power, low weight solid state devices. Because of their design we expect that Microfab  sensors should show only a very small response to gamma radiation.  And Microfab sensors are capable of detecting single neutrons with unusually high probability: up to 8% of thermal neutrons crossing the detector will be registered. This project will quantify how well our sensors reject gamma radiation and how well they and our associated electronics withstand intense gamma radiation.

With the help of NPL scientists and their facilities we will learn how well our sensors can detect individual neutrons  while operating in various high intensity gamma radiation fields, up to several 10’s of Gy/h,  with and without the introduction of an additional small neutron field via a separate source as well.

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