Session GP1 - Poster Session IV.
POSTER session, Tuesday afternoon, October 30
Exhibit Hall B,
The yield of tertiary neutrons with energies greater than 20 MeV has been proposed as a method to determine the \rhoR of ICF targets. Carbon activation is an appropriate measurement technique because of its high reaction threshold and the availability of high-purity samples. The ^12C (n, 2n) ^11C reaction has a threshold of 18.7 MeV well above the 14.1 MeV primary DT neutron energy. The isotope ^11C decays with half-life of 20.3 min and emits a positron, resulting in the production of two 511-keV gamma rays upon annihilation. The positron decay of ^11C is identical to the copper decay used in the activation measurements of 14.1 MeV primary DT yields; therefore, the present copper-activation gamma-detection system can be used to detect the tertiary-produced carbon activation. Because the tertiary neutron yield is more than six orders of magnitude lower than primary neutron yield the carbon activation diagnostic requires very pure carbon samples, free from any positron emitting contamination. In recent years we have developed carbon purification, packaging, and handling procedures that minimize the contamination signal to a level low enough to use carbon activation for tertiary neutron measurements in direct-drive implosion experiments with DT cryogenic targets on OMEGA. A concept of implementing a carbon activation system on NIF will be also discussed.