Design and Alignment of Neutron Time-of-Flight Shadowbar Detector to Gate a Neutron Camera on the MJOLNIR Dense Plasma Focus.

The neutron time-of-flight (n-ToF) measures the time response of the incident x-ray and neutrons generated by the dense plasma focus (DPF). A shadowbarred detector isolates the prompt neutrons from the scattered neutrons in order to gate a neutron camera for flash neutron radiography, of only the prompt neutrons. To achieve this, an array of four n-ToF detectors, two of which are shadowbarred with 25cm of polyethylene, have been set up 2 meters from the DPF. The avalanche photodiodes are in two identical pairs with 10x different gains for high dynamic range. Data from the DPF trains the scaling and subtraction algorithm to produce the profile of the prompt neutrons. Using alignment lasers, the shadowbar must be spatially aligned with a precision of 1 cm and 1 degree to accurately obscure the DPF.