Geochemical estimation of the half-life for the double beta decay ofZr96

Abstract

An excess amount of Mo found in a 1.7X10 yr zircon sample from Cable Sands' Western Australia, yielded a half-life of (3.9+0.9}X 10' yr for the double beta decay of Zr to Mo.PACS number(s): 23.40.s, 27.60.+jThe precise determination of the decay constant of double beta decay has put a constraint on, and is expected to verify, the mass of the neutrino [1]. The half-life for this type of decay is estimated to be of the order of 10' -10 ' yr, as shown in the half-lives of 1 X 10 yr for s2Se and SX10 yr for ' Te [2]. Accordingly, it is difficult to directly detect the radioactivity accompanying double beta decays using counting methods. An alternative approach to counting methods is the detection of the decay product accumulated in very old natural minerals. In the present work, we have attempted to estimate the half-life of double beta decay of Zr from the amount of radiogenic Mo found in the Zr mineral. It is noted, however, that the concentration of molybdenum in natural zircon (ZrSi04) appears to be in the ppm range, which is too high a background to enable us to detect the Mo "anomaly. " Furthermore, the sensitivity of molybdenum in thermal ionization mass spectrometry (TIMS) is relatively low because of the high ionization potential of Mo. These difficulties hampered the success of a previous attempt [3]. Therefore, it was necessary to develop sophisticated empirical techniques in order to detect the positive anomaly of Mo in the Zr mineral.We analyzed 5.39 g of placer zircon found in a region south of Bunbury, Western Australia, which was provided from Cable Sands Pty. Ltd. The geological age was determined based on a measurement of the isotopic composition of lead and on the method by Kroth [4], and found to be 1.67X10 yr. The samples contained over 99% of zircon. This was confirmed by an analysis of zirconium with an ICP-MS after chemical dissolution of zircon (see below).Natural zircon (ZrSi04) usually contains a few ppm of molybdenum.Molybdenum, however, is believed to be present as a mineral inclusion (perhaps molybdenite, MoSz) and the proportion of Mo substituting for Zr ions in the crystal lattice is anticipated to be low. Thus it is expected that radiogenic Mo is not present in molybdenite inclusions but can be detectable in intrinsic portions of zircon. In order to reduce the Mo background and to find out the isotopic effect on Mo due to double beta decay of Zr, molybdenite inclusions must be re-moved from the zircon mineral. For this purpose, the crystals of zircon were crushed to a fine powder and treated with aqua regia to remove the molybdenite inclusions. Zircon is not eroded by this treatment, and the molybdenum produced by the decay of zirconium is considered to be maintained in the pure zircon crystal grains.Zircon, which had been treated with aqua regia, was then dissolved by hydrofiuoric acid (ten times the weight of the zircon sample) and a few drops of nitric acid in acid digestion bombs. (Prior to treatment with aqua regia, the sample had been heated overnight with HF + HC104 to remove s...

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State-of-the-art and progress in precise and accurate isotope ratio measurements by ICP-MS and LA-ICP-MS : Plenary Lecture

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