Simultaneous determination of radionuclides separable into natural decay series using time interval analysis (TIA)

Full text: 1. A delayed coincidence method, called a time interval analysis (TIA) method, has been successfully applied to selective determination of the correlated α-αdecay events in millisecond order life-time. The decay processes are including 220Rn→216Po(T1/:145ms)→ {Th-series}, and 219Rn→215Po(T1/2:1.78ms)→{Ac-series}. The TIA is fundamentally based on the analysis of time intervals due to pulse from the decay events in the liquid scintillation counting (LSC). In the LSC, non-correlated decay events and other events became background or random events when they were compiled the time interval data within a fixed time (for example, a tenths of concerned half lives). Time intervals of the correlated or successive decay events could be distinguished because the exponential part of the correlated events is superimposed on those of a variety of random events as a horizontal line. This is so called the multiple time interval analysis (MTA). The detection efficiency of the TIA-analysis due to correlated α decay events could be subsequently improved in respect of background elimination using the pulse shape discrimination technique (PSD with PERALS counter) to reject α-pulses and applying solvent extraction of Ra-1) 2. In the present paper, the determination of another α-emitter nuclide, 22Ra, which belong to Np-decay series, could be also carried out using the TIA-method using the growth relation of the correlated α-decay events due to 221Fr→217At(T1/2:32.3ms) process when 225Ra was employed as a chemical yield tracer. TIA-measurement of 225Ra was required after almost maximal growth of progenies about 20 days later because of keeping transient equilibrium between 225Ra and 225Ac. During such waiting period, the growth of 222Rn, 218Po, 214Pb from 226Ra-progenies always disturbs the α-spectrum in the LSC and brings on the increment of random events. To improve this situation, the purging effects of 222Rn with N2-gas were effectively accomplished just before the TIA-measurement. As a result, the simultaneous determination of Th and Np-series (chemical yield tracer; 225Ra) was achieved by applying the TIA methods, while 226Ra (uranium decay series) was determined from the α-LSC spectrum. This method is proved to be useful for the environmental samples. Furthermore, this TIA-method has been tested for the determination of descendants of Rn-gas which was collected on a dust filter in the environment. (author)