Recent Developments Regarding a Time Measurement System Based on Isotopic Ratios

A time-measurement system under development at Los Alamos National Laboratory is based on measurements of the ratio of gamma-ray peak intensities from two radioactive isotopes. This system can be applied in cases where the requirement for data protection precludes the use of self-contained power sources such as batteries. The time-measurement system operates by determining the change in the ratio of the 47- keV peak of 210Pb to the 60-keV peak of 241Am. These gamma-ray peaks are relatively close in energy and, hence, any changes in their efficiencies will be similar. Furthermore, they are low enough in energy that the detector can be easily shielded from interfering peaks generated by sources near the detector. Although an ideal time-measurement system would be based on the decay of a parent isotope with a short half-life decaying to a long half-life daughter, nature has not provided any acceptable mother/daughter pairs. The practical system utilizes two unrelated isotopes, 210Pb and 241Am, with half-lives of 22 y and 432 y respectively. We will present recent data and design developments, including an analysis algorithm intended to improve the robustness of this system, as well as results reflecting the sensitivity of the system to interfering sources of radiation.