Measuring the neutron energy spectrum of laser fusion targets with CR-39

Abstract

Listen

Translate article

We are developing a detector capable of measuring the neutron energy spectrum from a laser fusion target containing DT fuel. From such a spectrum the compressed areal density of the DT can be inferred by observing the fraction of 14.1 MeV neutrons down-shifted in energy by elastic scattering. The detector consists of a 0.1 cm thick Ta x-ray and debris shield backed by a 50–200 μm polyethylene radiator followed by layers of CR-39. The energy of each neutron producing a knock-on proton in the radiator, that in turn creates a damage track in the CR-39, can be derived from the resultant track diameter, location, and orientation. We have analyzed the proton sensitivity and sample readability of 5 types of CR-39 in the energy range 3–11 MeV and have found a type fabricated by American Acrylics from a monomer made by a French company, Allymer, to be the most acceptable. Calibration curves were obtained for this plastic at energies of 3–15 MeV and dip angles ranging from 75 to 90 degrees. These curves were subsequently used to unfold a 14.7 MeV spectrum generated at the Livermore Rotating Target Neutron Source.