PNNL

Abstract

We used two methods to attempt to measure the lifetime and energy level of the 229mTh low-lying nuclear state. Both methods used a 233U source to implant 229Th into a VUV transparent substrate. The uranium source had a low concentration of 232U (0.08 ppm) and was chemically purified prior to the start of the experiment. In the first method, the time decay method, sensitive to 229mTh lifetimes from 10 minutes to 15 hours, the VUV substrate was irradiated for 3 to 36 hours to ensure a sufficient transfer of 229mTh ions into the substrate. Then the signal from the substrate was observed using a PMT for 15 hours. A bandpass filter was placed between the substrate and PMT to provide a means of measuring the 229mTh photon energy. We used nine filters with center transmissions ranging from 140 nm to 200 nm. In the second method, direct detection, sensitive to lifetimes from zero to 10 minutes, the substrate was observed directly with the PMT and filter during the deposition process. For both methods our results were limited by the strong background fluorescence and phosphorescence due to the alphas and betas intrinsic to the decay of the 233U and its daughters. We did not see unambiguous signs of photons from 229mTh. This may be due to one or more of the following: lifetime of 229mTh too short or too long, branching ratio to produce 229mTh from 233U is too small, energy of 229mTh transition is outside of our observed range of 4.1 to 9.5 eV, and/or most likely, 229mTh ions quench too quickly.