In the inertial confinement fusion (ICF) implosion experiment, the 16.7 MeV deuterium-tritium (DT) fusion gammas provide a high-accuracy alternative to 14.1 MeV fusion neutrons for fusion reaction width and bangtime measurements. Gas Cherenkov detector (GCD) can be used to measure DT fusion gammas, which has the advantage of energy threshold to eliminate the interference of low-energy gamma photons. Previous studies mainly focus on optimizing system efficiency or time response of GCD. However, the system time delay and shield size of GCD are lacking in optimal design by simulation method. In the present study, we build a GCD simulation model using the Geant4 software, so as to optimize its structure considering the environment boundary of installation on the 100 kJ level laser facility. The influences of precursor signal and background interference on the fusion gamma measurement are analyzed. The GCD structure is optimized to increase the system sensitivity, and the system time delay and shield size are optimized to reduce the interference background. The measurement signal and performance changes of GCD are calculated by using the simulation model, which is helpful for configuring measurement parameters and estimating signal amplitude in implosion experiments conducted on the 100 kJ level laser facility.
A whole three-dimensional model of GCD is built by using the Geant4 software, including the conversion processes of "gamma photon-Compton electron-Cherenkov photon" and the collection process of Cherenkov photons. First, the electron conversion efficiency changing with converter material and thickness is studied to obtain more high-energy electrons within a small emission angle. The Cherenkov photons arriving at the end of the gas cell are calculated according to the gas length and gas diameter, so as to optimize the structure of the CO2 gas cell. Meanwhile, the photon collection efficiency and the time waveform of collection photons are studied by changing the curvatures of the primary reflector
As the atomic number of material increases, the outcoming electrons within a small emission angle decrease (Fig. 3). A 15 mm thick carbon is selected as the gamma convertor according to the calculated electron conversion efficiency changing with the carbon thickness (Fig. 4). The CO2, as the radiation medium, is optimized as that with a length of 100 cm and a diameter of 15 cm according to calculated curve of collected Cherenkov photons (Fig. 5). The optimal curvatures of the primary reflector and the secondary reflector are chosen as 34 cm and 600 cm, respectively, according to the calculated collection photons and the signal frontier proportion
A whole three-dimensional model of GCD is built by using the Geant4 software, including the processes of "gamma photon-Compton electron-Cherenkov photon" and the boundary processes of photon reflection and transmission. The gamma converter and the CO2 gas cell, as the radiation medium and the tungsten shield size, are optimized. A detector sensitivity of 0.21 photons per incident gamma photon and an intrinsic time response of 16 ps are achieved. The measurement signal and performance changes of GCD are calculated by using this simulation model, which is helpful for configuring measurement parameters and estimating signal amplitude in implosion experiments. The time response of GCD-coupled PMT can reach about 108 ps. The amplitude of the simulated signal is about 0.7 V, while the neutron yield is 1013 with a PMT gain of 5×103 and a threshold energy of 6 MeV. The FWHM of the measured signal is about 164 ps after convoluting the fusion reaction width of 100 ps. The numerical calculation results indicate that the optimized GCD can meet the requirements of fusion gamma diagnostic in current implosion experiments on the 100 kJ level laser facility. In implosion experiments with high areal density, the instantaneous gammas activated by neutrons on the diagnostic devices will be strong. The influences of background interferences on the main Cherenkov signal are worth further study.