Terrestrial xenon anomaly and explosion of our galaxy

Abstract

The Xe129-Xe131 excess in geologically old tellurium ore is an important problem, as yet unresolved. In this letter we attempt to interpret the anomaly pattern of xenon isotopes by muon interactions on tellurium ore and its environmental rocks. The source of high muon flux necessary to explain the large excess amounts of the xenon isotopes will be discussed in terms of the explosion of our galaxy in the past. In studying double beta-decay of Te130, Inghram and Reynolds [1950] found the excesses of Xe129 and Xe131 over atmospheric abundances in an old tellurium ore. They explained the anomaly by the (n, γ) reactions on Te128 and Te130 with the neutrons originating from uranium. This explanation is based on the probable presence of unusually large amounts of uranium mineral in the neighborhood of the tellurium ore. According to a further experiment by Hayden and Inghram [1953], a tellurium ore of similar age that has not been in association with uranium does not contain excess Xe129 and Xe181 to the same extent. The recent work of Takaoka et al. [1965] shows however, the existence of excess Xe129 and Xe181 in tellurium ores that have been in quartz vein embedded in a granite mass whose radioactivity level is quite low [Ogata et al., 1966]. The reported values, of the anomalies are summarized in Table 1. The absence of excess in heavier xenon isotopes shows that the investigated ores themselves contain little uranium. If a few ppm of uranium were present in the ores, fissiogenic Xe136 and Xe184 should have been detected with their apparatus. If the anomalies are caused by the neutrons from uranium, the ratio of Xe129 to Xe131 should fall near 0.55. As most of the neutrons must be thermalized during their travel to the tellurium minerals in question, the value of 0.55 can easily be calculated by thermal neutron cross sections [Hughes and Schwartz, 1958] and isotopic abundances of Te128 and Te130. The observed values of the ratio are 1.8 and 2.7. Inghram and Reynolds ascribed this discrepancy to the decay of primordial I129. But this conclusion is quite improbable in view of low iodine content of usual tellurium ores and the short life (T1/2 = 1.7 × 107 yr) of I129 [Pepin, 1964]. I129 produced by neutron capture on tellurium during the hydrothermal stage would cause Xe129 anomaly, if I129 could be efficiently incorporated in tellurium ores during crystallization of tellurides. However, abnormally high concentration of uranium and tellurium in the hydrothermal solution must be assumed to explain the present Xe129 anomaly. Attempts to interpret the anomalies by the capture reactions of fast neutrons from thorium and its daughters are also unsuccessful because the thorium content required amounts unreasonably high. Here we conclude that the neutron-induced anomalies are quite small, if they exist at all. The only other possible nuclear process for the production or Xe129 and Xe131 from tellurium in deep-seated mineral is by negative muons and their secondaries.