Commissioning an X-Ray Detector System for Spectral Analysis of Tritium-Filled Targets

March 2017

The x-ray detection system

The Laboratory for Laser Energetics (LLE) at the University of Rochester uses glass and plastic targets filled with deuterium–tritium mixtures (DT) for research into inertial confinement fusion experiments. The 60-beam OMEGA Laser System is employed to implode these targets. Because targets require pressurizing to tens of atmospheres without crushing the fragile, thin-wall shells, the permeation-filling process can take several days. Typically, it takes five or six permeation time constants to fill targets to the desired pressure. An x-ray detection system (XDS), originally developed to measure bremsstrahlung of tritium β decay from the surface of metals, has been modified to nondestructively measure the pressure of DT fuel inside a target just prior to a shot.

The XDS comprises three primary components: a high-resolution, dual-axes imaging system for repeatable, accurate target positioning; a helium enclosure with triple-axes micrometer positioning; and an Amptek silicon drift detector (SDD). The SDD was fit with a silicon nitrate (Si3N4) window to measure x-ray energies from 200 eV to 40 keV. The detector features a 25-mm2 silicon drift diode with a measured 130-eV full-width-at-half-maximum resolution at 5.7 keV.

Two factors can lead to underfilling a target: (1) The permeation time constant of the shell wall is underestimated so sufficient time is not allowed for the gas pressure inside the target to equilibrate with the charging pressure, or (2) a defect develops in the shell, causing the target to depressurize more rapidly than predicted by the permeation time constant. The target-filling process starts with an evaluation of the D2 permeation rate of the target at General Atomics after fabrication. Upon arriving at LLE, the targets are charged with a desired fill pressure of DT. The β particle decay of tritium atoms within the DT fuel generates both fluorescent and bremsstrahlung x rays as the β particles interact with the shell material. The T2 permeation half-life can be determined for each target by measuring the x-ray activity over a period of a few minutes. In test cases, the premeation time constant of filled targets were measured over several days to compare with the D2 and DT time constants. After considering the root-mass difference for DT and T2, the measured half-lives were observed to be between 2 and 7 times longer than the corresponding D2 half-lives, depending on the age of the DT-filled target. The activity of the tritium bound to the shell was observed to be proportionate to the initial fill pressure. The shell activity was ~30% of the total activity in the target.

University of Rochester graduate, Rory Hamilton,
working on the commissioning of an x-ray detection system

A study was undertaken where the target fill pressure at the time of x-ray measurement for two glass targets was calculated considering the different half-lives, fill pressures, and storage times of the targets The SDD-measured activities for these targets were then calculated to have a Si Kα activity rate within 5% of each other. Both targets were then imploded on OMEGA using the same laser conditions (energy on target, pulse shape). The measured neutron yields were noted to be within 10% of each other. As such, the development of the XDS allows for an expedient, nondestructive verification of the tritium content in a glass or plastic target prior to it being loaded into the OMEGA target chamber. The XDS contributes improved efficiency and productivity while reducing the number of misfires that occur on OMEGA with underfilled ICF targets.

This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.