In highly compressed NIF capsules deuterons and tritons can be imparted high kinetic energy by elastic scattering with 14 MeV neutrons. The energetic ions can then interact with stationary fuel resulting in fusion while emitting a neutron from a moving reference frame. This results in neutrons with a kinetic energy in excess of 14.1 MeV and up to 22 MeV. Thulium is mono-isotopic and has two threshold reactions that can be used for neutron spectral measurement. The (n,2n) reaction has a threshold of 8 MeV and produces Tm-168 that decays with a 93.1 day half-life. This reaction is used to measure the 14.1 MeV neutron fluence. The (n,3n) reaction has a threshold of 15 MeV and produces Tm-167 that decays with a 9.24 day half-life. This reaction is used to measure Reaction-in-Flight, RIF, neutrons. The difficulty in making this measurement is observing Tm-167 in the presence of 5 orders of magnitude more Tm-168 atoms. The Los Alamos 4-Pi clover gamma ray spectrometer is uniquely able to discriminate gamma ray on the basis of decay scheme and energy. The measurements, the method and the results of RIF neutron production as a function of different laser pulse structures will be described.

Thulium activation by reaction-in-flight neutrons

248th National Meeting (2014)

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