Charge Carrier Density and signal induced in a CVD diamond detector from NIF DT neutrons, x-rays, and electrons

Abstract

This report investigates the use of x-rays and electrons to excite a CVD polycrystalline diamond detector during a double pulse experiment to levels corresponding to those expected during a successful (1D clean burn) and a typical failed ignition (2D fizzle) shot at the National Ignition Facility, NIF. The monitoring of a failed ignition shot is the main goal of the diagnostic, but nevertheless, the study of a successful ignition shot is also important. A first large neutron pulse is followed by a smaller pulse (a factor of 1000 smaller in intensity) after 50 to 300 ns. The charge carrier densities produced during a successful and failed ignition shot are about 10{sup 15} e-h+/cm{sup 3} and 2.6* 10{sup 12} e-h+/cm{sup 3} respectively, which is lower than the 10{sup 16} e-h+/cm{sup 3} needed to saturate the diamond wafer due to charge recombination. The charge carrier density and the signal induced in the diamond detector are calculated as a function of the incident x-ray and electron energy, flux, and detector dimensions. For available thicknesses of polycrystalline CVD diamond detectors (250 {micro}m to 1000 {micro}m), a flux of over 10{sup 11} x-rays/cm{sup 2} (with x-ray energies varying from 6 keV to about 10 keV) or 10{sup 9} {beta}/cm{sup 2} (corresponding to 400 pC per electron pulse, E{sub {beta}} > 800 keV) is necessary to excite the detector to sufficient levels to simulate a successful ignition's 14 MeV peak. Failed ignition levels would require lower fluxes, over 10{sup 8} x-rays/cm{sup 2} (6 to 10 keV) or 10{sup 6} {beta}/cm{sup 2} (1 pC per electron pulse, E{sub {beta}} > 800 keV). The incident pulse must be delivered on the detector surface in several nanoseconds. The second pulse requires fluxes down by a factor of 1000. Several possible x-ray beam facilities are investigated: (1) the LBNL Advanced Light Source, (2) the Stanford SLAC and SPEAR, (3) the BNL National Synchrotron Light Source, (4) the ANL Advanced Photon Source, (5) the LLNL Janus laser facility. None of the cyclotrons/synchrotrons (1) through (4) are bright enough. The maximum monoenergetic x-ray flux available at the energies of interest (6-10 keV) is about 10{sup 4} x-rays/ns/cm{sup 2} at 10 m from the source. The maximum white beam x-ray flux (thus all energies of x-rays are used) is about 10{sup 6} x-rays/ns/cm{sup 2} at 10 m from the source. These numbers are well below the necessary 10{sup 11} x-rays/cm{sup 2} produced in a few ns. Also, producing double pulses separated from 50 to 300 ns with a factor of 1000 contrast between the first and second pulses seems very challenging using a cyclotron/synchrotron. The Janus laser-based x-ray facility (5) can generate over 10{sup 11} x-rays/cm{sup 2} at 10 cm from the target (nickel or zinc target, 7.5 keV to 8.6 keV x-rays lines) and double pulses are possible. Electron beams at the linac facility at LLNL can deliver from 5 to 100 pC double pulses, with electron energies varying from 15 to 90 MeV. Use of a 5 pC pulse could achieve the failed ignition densities, and a 100 pC pulse is just short of satisfying the densities of a successful ignition shot. Other linacs with higher current (400 pC per shot would be necessary) could satisfy both ignition densities.