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Determination of the half-lives of short-lived isomeric states


For the representation of one of the legal units, the becquerel (Bq), accurate knowledge of the decay data of the radionuclides under study is essential. For decades, the Radioactivity Department has therefore successfully committed itself to determining the half‑lives of relevant radionuclides. In the past few years, many detector systems have undergone extensive technical modernizations, so that measurements of very short half‑lives can now also be performed. In this way, it has recently been possible to determine with high accuracy the half‑lives of the isomeric states of Np‑237, Pa‑233, and Ac‑227, which result from the alpha decays of Am‑241, Np‑237, and Pa‑231, respectively: T1/2(Np‑237, E = 59.54 keV) = 67.86 (9) ns, T1/2(Pa‑233, E = 86.47 keV) = 36.44 (10) ns, and T1/2(Ac‑227, E = 27.37 keV) = 38.56 (15) ns [1].

The measurements were performed with digital 4πα(LS)‑γ coincidence counting in a new transportable coincidence system of the department. The measurement setup consists of a dedicated liquid scintillation counter (LS) equipped with three photomultipliers and a fast inorganic CeBr3 scintillator serving as an external gamma detector. This configuration enables the largely independent detection of the primary particle from the decay process and the subsequent gamma emission. The signals from all detectors are initially acquired independently by a CAEN N6751C 4‑channel 10‑bit digitizer with a sampling rate of 1 GHz and recorded in list‑mode format.

The determination of the half‑lives was performed by using offline analysis. The special feature of this method is that some settings, which were defined by the measurement setup in earlier classical experiments, can now be changed even after the actual measurement. These include discriminator settings that define thresholds or energy windows. The evaluation was used to determine the time interval between the alpha decay (occurrence in the LS detector) and the subsequent detection of a gamma emission (event in the CeBrdetector). This measurement was repeated for each α‑γ coincidence pair, and then a histogram was formed using all observed time differences. Due to the Poisson statistics of the decay process, the histogram could be modeled as a combination of an exponential function and a constant background, where the exponential part can be directly parameterized with the half‑life.

In addition to the experimental redetermination of the half‑lives described above, an extensive literature search was also performed, and the available historical measured values were compiled with the new results. The combined data set was evaluated according to the guidelines of the Decay Data Evaluation Project (DDEP), and a mean half‑life was calculated for each isomer state.


[1]    Takács, M.P., Kossert, K., 2021, Half‑life determination of short‑lived nuclear levels in Np‑237 (59.54 keV), Pa‑233 (86.47 keV) and Ac‑227 (27.37 keV), Applied Radiation and Isotopes 176, 109858.


Opens local program for sending emailM. P. Takács, Department 6.1, Working Group 6.11