Elsevier

Applied Radiation and Isotopes

Volume 108, February 2016, Pages 143-147
Applied Radiation and Isotopes

Half-life determination of the ground state decay of 111Ag

https://doi.org/10.1016/j.apradiso.2015.12.044Get rights and content

Highlights

  • Radioactive decay of 111Ag measured by HPGe γ-ray spectrometry.

  • Measured half-life of 7.423 (13) days.

  • Evaluated recommended half-life of 7.452 (12) days.

Abstract

The radioactive decay half-life of the β-emitter 111Ag has been measured using decay transitions identified using a high purity germanium γ-ray spectrometer. The time series of measurements of the net peak areas of the 96.8 keV, 245.4 keV and 342.1 keV γ-ray emissions following the β decay of 111Ag were made over approximately 23 days, i.e. ~3 half-life periods. The measured half-life of the ground state decay of 111Ag was determined as 7.423 (13) days which is consistent with the Evaluated Nuclear Structure Data File (ENSDF) recommended half-life of 7.45 (1) days at k=2. Utilising all available experimental half-life values, a revised recommended half-life of 7.452 (12) days has been determined.

Introduction

The short-lived 111Ag is a product of the nuclear fission of uranium and plutonium isotopes, either during nuclear power generation or in the explosion of a nuclear weapon. Silver-111 is the longest lived member of the A=111 isobaric chain formed in nuclear fission and βdecays to excited states in 111Cd, which is radioactively stable. As a product of nuclear fission, 111Ag is of interest to nuclear forensics and to the Comprehensive Test-Ban Treaty Organisation (De Geer, 1999). The 111Ag nucleus decays by the emission of a β particle to the stable nucleus of 111Cd, with 92% of decays direct to the ground state via a first forbidden Gamow-Teller β- transition 1212+, with the remainder via excited levels of 111Cd. These subsequently decay by the emission of characteristic γ ray emissions to the ground state of 111Cd (Blachot, 2009).

Previously, four measurements of the half-life (Baerg et al., 1960, Johansson, 1950, Roche, 1968, Rothman et al., 1974) have been performed between 1950 and 1974, as summarised in Table 1. An evaluation has been performed by Blachot (2009), with a recommended value of 7.45 (1) days based on two publications (Baerg et al., 1960, Rothman et al., 1974). Whilst the currently published data are consistent, recent measurements of the γ ray emission nuclear data of 111Ag (Collins et al., 2014) and the observed discrepancies to the previously published values warranted a renewed investigation of the 111Ag half-life.

The measurement of the radioactive decay was tracked over time using the 96.8 keV, 245.4 keV and 342.1 keV γ ray emissions from excited states in 111Cd populated following the βdecay of 111Ag using high purity germanium (HPGe) γ ray spectrometry. A detailed uncertainty budget is presented, using the uncertainty propagation formula (Pommé, 2015):σT1/2T1/22λT2n+1σAAWhere λ is the decay constant, T is the duration of the measurement period, n is the frequency of the occurrences of the uncertainty component and σA/A is the relative uncertainty of the activity due to the considered component. The standard uncertainties are quoted at k=1 throughout.

Section snippets

Sample preparation

The 111Ag source material used in the current measurements was produced by the neutron irradiation of a palladium oxide (PdO·H2O) target via theP110d(n,γ)P111dA111g+β reaction at Technische Universiteit Delft, Netherlands. Naturally-occurring palladium consists of the stable isotopes of 102Pd (1.09%), 104Pd (11.1%), 105Pd (22.3%), 106Pd (27.3%), 108Pd (26.5%) and 110Pd (11.7%) that under neutron irradiation produces 103Pd (T1/2=16.991 (19) d (De Frenne, 2009)), 107Pd (T1/2=6.5×106 y (3) (

Measurements and results

A time series of 13 separate measurements were made over a period of 23 days (~3 half-lives of 111Ag). The measurements were performed with a typical live time of 86,400 s, with dead times ranging from 0.66% to 0.27%. The initial measured count rates at t=0 for the 96.8 keV, 245.4 keV and 342.1 keV transitions were 1.2 s−1, 7.3 s−1 and 31.3 s−1 respectively.

The individual datasets of the 96.8 keV, 245.4 keV and 342.1 keV γ rays were fitted using a weighted least-squares fit of an exponential decay curve

Uncertainty

The uncertainty components were divided into high-, medium- and low-frequency uncertainty components (Pommé, 2015), a summary of the uncertainty components are provided in Table 2. The high-frequency components were composed solely of the standard deviation of the residuals of the least squares fit, and constituted a negligible contribution to the final standard uncertainty.

The medium-frequency components were composed of the geometric reproducibility of the sample. Due to the precision

Results and literature

The value of the half-life of 111Ag reported in this work is in statistical agreement with the evaluated half-life reported by Blachot (2009), where the values are within the bounds of their respective uncertainties at k=2. When compared to the currently published half-lives, the value reported in this work is the lowest half-life value of 111Ag.

The length of the measurement campaign in this work is significantly shorter than that performed by Baerg et al. (1960) and Rothman et al. (1974),

Conclusion

The measurement of the radioactive half-life of 111Ag has been performed using a HPGe γ-ray spectrometer to observe the rate of change in activity as a function of time of the 96.8 keV, 245.4 keV and 342.1 keV γ-ray emissions. A weighted least-squares fit of the exponential decay curve was performed for the measured data of each γ-ray emission, with a final determined half-life of 7.423 (13) days. This value was found to be in agreement at k=2 with the currently recommended half-life of 7.45 (1)

Acknowledgements

This work was supported by the National Measurement System Programmes Unit of the UK's Department for Business, Innovation and Skills and from the Science and Technology Facilities Council (UK).

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