Title: Search for the Radioactivity of 180mTa Using an Underground HPGe Sandwich Spectrometer
Citation: APPLIED RADIATION AND ISOTOPES vol. 67 no. 5 p. 918-921
Publication Year: 2009
JRC N°: JRC47724
ISSN: 0969-8043
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC47724
DOI: 10.1016/j.apradiso.2009.01.057
Type: Articles in periodicals and books
Abstract: Tantalum-180m is a very special radionuclide, which has generated a lot of interest in recent years. The ground state of 180Ta has a short half-life of only 8.1 h. Tantalum-180 could be said to be nature's rarest primordial "stable" isotope and it has the most long-lived metastable state know to man. Recent interests for this nuclide has been due to its potential use in gamma-ray lasers and the debate on its production in the stellar nucleosynthesis as well as its wide use in studies of nuclear structures of nuclei with high spin state. The radioactivity of 180Ta is yet to be detected. There are eight detection attempts reported in literature staring shortly after it was first discovered in 1955. Presently the highest value for the lower bound of the half-life is 1.2 x 10.16 years. It was determined in the first underground measurement of the nuclide in an experiment that was not optimised for this task. The experiment reported here was the first ever underground measurement specially designed for searching for the decay of 180mTa. The measurements were carried out using a specially developed HPGe sandwich spectrometer equipped with an active muon shield and a list mode data acquisition system. The system was installed in the underground laboratory HADES, which is located 225m underground in Mol, Belgium. The sample consisted of 6 disks of high purity tantalum of natural isotopic composition. The disks were 10 cm in diameter and together they had the mass 1500g. The mass of 180Ta was calculated using a natural isotopic abundance of 0.012%, which gave a total 180Ta mass of 180mg. The sample was measured for 80 days and the resulting lower bound for the half-life of 180mTa was 5.0 x 10.16 years. This half-life is composed of the half-lives of the two decay modes, electron capture and Beta-decay. in the gamma-ray spectrum there is a possible peak structure at 350.4 keV, which is a gamma-ray expected from the Beta-decay. This indicates that the decay of 180mTa could be detected given that this sample is measured for a longer period of time. it is estimated that another half a year of measurements would suffice to clarify if this structure is a peak or not.
JRC Directorate:Health, Consumers and Reference Materials

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