Determining the half-life of polonium-215 by means of liquid scintillation counting

With a half‑life of less than 2 ms, polonium‑215 is a short‑lived, alpha‑emitting isotope in the decay chain of actinium‑227 (also known as the actinium series). Exact knowledge of the half‑life of polonium‑215 is indispensable, for instance, for correctly determining the activity of radium‑223, thorium‑227 and actinium‑227 samples using liquid scintillation counting (LSC). Since polonium‑215 may decay during the detector dead time that begins when the preceding decay of radon‑219 is detected, a correction is necessary. This correction must be taken into account when determining activity.

However, measuring the half‑life of polonium‑215 accurately is not easy. A particular challenge arises due to the short‑lived nature of polonium‑215, which makes it impossible to chemically isolate it from the other isotopes of the decay chain. Moreover, the actinium series contains several alpha‑emitting radionuclides that cause high background or interferences during measurements. Despite this difficulty, Working Group 6.11 has recently succeeded in experimentally determining the half‑life of polonium‑215 with high accuracy.

The relevant measurements were performed while commissioning and validating a new combined TDCR-CIEMAT/NIST measuring system. These measurements were designed, among other things, to check the new system’s long‑term stability. The measurement setup consists of a liquid scintillation counter equipped with three photomultipliers. In the course of the 80‑day measurement campaign, the detector signals were acquired by a CAEN N6751C 4‑channel 10‑bit digitizer and recorded in list‑mode format. The half‑life was determined by an offline analysis based on the distribution of the time differences between consecutive alpha decays.

The rather long total measuring time of the campaign was necessary because the actinium‑227 sample used had an activity of just 0.5 becquerels. The considerably reduced count rate meant that the time intervals between two random alpha decays were on average much longer than the half‑life of polonium‑215. This allowed the correlated radon‑219‑polonium‑215 decay pairs to be identified based on their time signature.

A histogram was compiled from the time differences observed between α‑α reference detection pairs. It was possible to model the histogram as a combination of an exponential function (generated by the Poisson statistics of the radon‑219‑polonium‑215 decay pairs) and a constant background due to uncorrelated decay events. The exponential part of the model function was directly parameterized with the half‑life of polonium‑215.

The obtained result of T_1/2(Po‑215) = 1.781 (5) ms [1] is in outstanding agreement with the value currently recommended by the Decay Data Evaluation Project (DDEP) for the half‑life of polonium‑215. Furthermore, the measurement procedure described here represents a novel and independent method that complements former determinations performed with alpha‑spectroscopic measurement techniques.

Reference

[1]   Takács, M.P.: Measuring the half‑life of Po‑215 by low‑level liquid scintillation counting, Applied Radiation and Isotopes, submitted on 30 May 2022.

Contact

M. P. Takács, Department 6.1, Working Group 6.11