Lead activation neutron yield measurement system used in ICF experiment

Summary form only given. Lead activation system is used to measure very low yield (above 10⁶) of DT reaction, and it can also be applied to measure DD reaction neutrons (2.45 MeV). These neutrons produce ^207mPb radioactive nuclide by the reaction ²⁰⁸ Pb(n,2n)^207mPb and ²⁰⁷Pb(n,n)^207mPb in Pb sheath that surrounds a NaI(Tl) detector or 1.5 cm thick Pb disks in front of NaI(Tl) detector. Using this detector, adding amplifier, single-channel and multi-channel scaling counting system in computer can measure the 1.06 MeV and 570 keV gamma rays from the ^207mPb atoms with 0.8 s half-life decay, and then the yield may be calculated. Because half-life decay of ^207mPb atoms is too short to move activated sample to measure room manually or mechanic, the detection system is placed as close as 32 cm from the laser target, the X-rays burst from the laser irradiated target generates about 10⁶ times more light intensity in NaI(Tl) detector than does an individual gamma ray from the ^207m Pb. To minimize this effect, a gate circuit removes the detector high voltage 200 ms to 1 s adjustable, then restores it less than 100 ms after the shot. The drift of the peak of the pulse-height spectrum is unavoidable because of the NaI(Tl) detectors and the electronic system, it will affect right use of the system. We used an embedded multi-channel pulse height analyzer in multi-channel scaling counting system to adjust the peak of the multi-channel pulse height spectrum in time. It is very easy and fast to adjust the system by this way. This system is calibrated by pulse neutron tube, and it is used successfully in ICF experiment on Shenguang-II facility in 2005. Its rate of signal and noise is above 200, and uncertainty is less than 20% when it is used in measuring neutron yield above 1times10⁷