For the analysis of the long-term safety of a repository for nuclear waste containing 79Se, as exact a knowledge of the half-life as possible is of great importance. In this connection, 79Se is one of the few nuclides which are responsible for the long-term, potential radiological hazard to the environment. As a branching point in the so-called s-process, 79Se plays - in addition - a key role for the understanding of the nucleosynthesis of heavy elements. Furthermore, the experts have great interest in investigating this isotope - which is difficult to access in experiments - in closer detail. To obtain a sufficient 79Se activity without radioactive impurities is an essential challenge.

The radioactive isotope 79Se is a radionuclide which only emits beta-rays and decays to 79Br, emitting low-energy electrons. Values published so far for the half-life extend over an order of magnitude between approx. 1.24•105 years and 1.13•106 years. Here, two historical values have not been taken into account.

Now, researchers of the Lehrstuhl für Radiochemie of the Technische Universität München, of the Institute for Transuranic Elements in Karlsruhe, of the Paul Scherrer Institute in Villigen (Switzerland) and of the Physikalisch-Technische Bundesanstalt in Braunschweig have redefined the half-life of 79Se [1]. Base material was a highly radioactive fission-product solution of spent nuclear fuel which contained - in addition to a small quantity of 79Se - considerable undesired activities of other radionuclides of several mega becquerels.

In preliminary tests, various radiochemical working steps were tested to realize the 79Se as purely as possible. Recently, two procedures - which are highly selective for selenium, but to which little attention has so far been paid in radiochemical analyses - have, among others, been applied. The procedures concerned are, on the one hand, the Reinsch test which has historically been developed in forensic medicine and was used to detect selenium, arsenic, antimony and mercury. This test is based on the reductive deposition of selenium on metallic copper and served - in the case of the activities presented here - to extract selenium from the fission-product solution. On the other hand, the 79Se was sublimated to complete the procedure. With the aid of these procedures, a 79Se solution of very high radiochemical purity - which is essential for the subsequent measurements - could finally be obtained.

The half-life can be calculated from the number of 79Se nuclei and from the activity. The number of 79Se nuclei, in turn, results from the number of selenium atoms which was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and by the isotope composition of the selenium. The latter was determined by a mass spectrometry procedure referred to as Hydride Generation Multicollector Inductively Coupled Plasma Mass Spectrometry (HG-MC-ICP-MS).

The activity was measured at PTB with the aid of liquid scintillation counting, and the detection probability was determined with an efficiency tracing method [2]. For radionuclides which only emit beta-rays - such as 79Se - this method allows activities to be determined with small uncertainties and is still well suited for measurements of a few becquerels. By a variation of the detection probability, information about the parameterization of the beta spectrum could be obtained.

The half-life determined in the new experiment amounts to T ½ = (3.27±0.08)•105 years [1]. It clearly deviates from the value of T ½ = (3.77±0.19)•105 years which has recently been published by a group of French researchers [3]. Up to now, the cause of the discrepancy could not be clarified. It is possible that it is due to substantial radionuclide impurities detected in the sample material of the French group.