An improved method for the nondestructive assay of the tritium content of glass microballoon laser fusion targets

Abstract

In laser fusion experiments employing deuterium-tritium filled microballoons, it is essential that the fill pressure be known for each target prior to its use. While most targets retain their original fill pressure (5–50 atm) indefinitely when stored at liquid nitrogen temperature, some leakage has been observed in random samples. Since we have found that tritium and deuterium leak from glass microballoons at essentially the same rates, a measurement of only the tritium content is sufficient to characterize completely the target fill. The tritium content is determined by measuring the flux ofβ particles (from the decay of tritium) which emerge from the target wall. To relate this flux to the target fill pressure, a model has been developed based upon a Monte Carlo simulation of the transport of theβ particles through the target's gaseous interior and glass wall. The model is incorporated into an accurate and self-consistent procedure for the nondestructive determination of the tritium content of laser fusion targets with a wide range of radii and wall thicknesses. This paper contains a detailed description of the model, and its implementation for assaying the fuel content in such targets. Also included is a discussion of how to compensate for the counts due to tritium trapped in the shell material. This correction is especially important when the target walls are thicker than 3μm when the shell diameters lie in the range 50–300μm. Limitations of this procedure, associated errors, and general guidelines are provided in order to indicate when this technique may be preferable over alternate methods, such as x-ray counting.