Delayed Neutron Counting System for Jet Plasma Neutron Yields Diagnostics

Abstract

Two fission reactions, ²³⁸U(n,f) and ²³²Th(n,f) have been identified as ideal fusion plasma neutron yield diagnostics monitors based on detection,within a time span of ~ 10 to 100 s, of their delayed - neutron signals; these responses, entirely decayed between two successive plasma pulses, are induced during exposure of the sensors in the vicinity of tokamak containment walls, using a fast pneumatic transfer system as vehicle between the machine and the remote counting stations.

The dynamic range of this method is very large and its sensitivity unchallengeable, reaching down to neutron fluences as low as a few 10⁶cm^-2. Routine and automatized application is straightforward while encompassing a minimal inventory of recycable and well-assayed targets.

At neutron fluences > 10⁷ cm^-2, the accuracy reachable is estimated as 5% in D, D operation provided a direct calibration be performed relative to absolute fission chambers in the MOL cavity uranium-235 fission spectrum standard neutron field.