The Laboratory for Laser Energetics at the University of Rochester has been conducting experiments using laser induced nuclear fusion as a possible alternative energy source. The Ariel density of an inertial confinement fusion (ICF) reaction can be determined by calculating the ratio of the tertiary neutron yield to the primary neutron yield. During an ICF reaction, 14.1 MeV neutrons emitted from the T(d,n) fusion reaction strike fuel deuterons causing them to accelerate. These deuterons then collide with tritium fuel to produce tertiary T(d,n) reactions that produce high energy neutrons in the range of 18 to 32 MeV. A pure carbon sample is placed near the reaction where it becomes activated through the ¹²C(n,2n)¹¹C reaction. This reaction has a high neutron threshold near 17 MeV and the primary 14.1 MeV neutrons cannot activate the carbon. Once activated, the sample is removed from the reaction area. The ¹¹C consequently beta decays by emitting positrons. NaI detectors can then count, in coincidence, the back to back 511 keV gamma rays emitted from the positron annihilation. The number of gamma rays counted is directly related to the tertiary neutron yield of the fusion reaction. The chief concern of using this method is that back to back 511 keV gamma rays may be produced from reactions other than the ¹²C(n,2n)¹¹C reaction. These reactions include ¹⁴N(n,2n)¹³N and ¹²C(p,g )¹³N, among others. This recently was investigated at the University of California at Berkley using the Rotating Target Neutron Source (RTNS) which produces copious amounts of mono energetic 14 MeV neutrons. Further studies will be conducted using the accelerator at the Triangle University Nuclear Lab, which is capable of producing large amounts of high-energy gamma rays. Those gamma rays can be used to study the ¹²C(g ,n)¹¹C reaction which also produces unwanted back to back 511 keV gamma rays. Eventually the carbon activation diagnostic will be used at the National Ignition Facility in Livermore, California.

Research funded in part by the Department of Energy.

RTNS pictures