A Time-Measurement System Based on Isotopic Ratios

摘要

A time-measurement system can be built based on the ratio of gamma-ray peak intensities from two radioactive isotopes.The ideal system would use a parent isotope with a short half-life decaying to a long half-life daughter.The activities of the parent-daughter isotopes would be measured using a gamma-ray detector system.The time can then be determined from the ratio of the activities.The best-known candidate for such a system is the ~241 Pu- ~241 Am parent-daughter pair.However,this ~241 Pu- ~241 Am system would require a high-purity germanium detector system and sophisticated software to separate and distinguish between the many gamma-ray peaks produced by the decays of the two isotopes. An alternate system would use two different isotopes,again one with a short half-life and one with a half-life that is long relative to the other.The pair of isotopes ~210 Pb and ~241 Am(with half-lives of 22 and 432 years,respectively)appears suitable for such a system.This time-measurement system operates by measuring the change in the ratio of the 47-keV peak of ~210 Pb to the 60-keV peak of ~241 Am.For the system to work reasonably well,the resolution of the detector would need to be such that the two gamma-ray peaks are well separated so that their peak areas can be accurately determined using a simple region-of-interest(ROI)method.A variety of detectors were tested to find a suitable system for this application.The results of these tests are presented here.