Counting System for the Carbon Activation Diagnostic

Multi-step nuclear reactions in the core of an OMEGA implosion can be used to infer the density of the deuterium-tritium (DT) fuel at peak compression, an important measure of the implosion performance. A three-step process leads to the production of tertiary neutrons with energies up to 30 million electron volts (MeV), more than twice the energy of the primary neutrons (14.1 MeV) from the direct fusion of DT. The tertiary-toprimary neutron yield can be measured using a neutron-induced reaction on C that is energetically not possible at the primary neutron energy. The (n, 2n) activation reaction on C leads to the production of C, which is unstable and has a half-life of 20.3 minutes. The C decays to B and emits a positron (the antimatter equivalent of an electron) and a neutrino. The positron immediately annihilates with a neighboring electron, in the process emitting two 511-keV (thousand electron volts) gamma rays in opposite directions. LLE is developing a high-counting-efficiency detector system (in collaboration with the SUNY Geneseo physics department) to measure the number of activated C atoms in a sample of graphite exposed to the tertiary neutron flux on the OMEGA laser. As part of this development process, measurements were made of the background sensitivity of the detectors to determine the shielding requirements. Counting efficiency in normal and Compton coincidence modes was explored. The first activated Cu-diamond mixture to be used for an absolute calibration of detector sensitivity was also part of this project.