TOTAL ABSORPTION · Selected applications of the total absorption technique

Final Activity Report Summary - TOTAL ABSORPTION (Selected applications of the total absorption technique)

Considering that beta decay is the process that governs the transmutation of most of the nuclear species, the importance of a full understanding of the physics which lies underneath becomes clear. Beta decay studies can reveal information about the beta-decay process itself as well as information on nuclear masses and on the properties of the nuclear states involved. One particular advantage of these studies is that, from a theoretical point of view, the process is governed by a very simple operator, namely the sigma-tau operator in the case of Gamow-Teller (GT) decay and the tau operator in the case of Fermi (F) decay. This apparent simplicity from the theoretical point of view meets difficulties on the experimental side. The BGT of an individual level in a daughter nucleus is determined by the balance of the gamma feeding and de-exiting the level. Common in such investigations is the use of semiconductor detectors to measure the gamma-ray intensities. Two main factors contribute to making such measurements difficult: 1. the fragmentation of gamma intensity and 2. the primary gamma-rays, being usually of high energy. Therefore, much of the GT feeding at high excitation energy is not observed and is thus incorrectly assigned to low-lying levels. This leads to a large and systematic error in the total BGT and in the BGT distribution, which consequently can lead to the misinterpretation of the underlying nuclear structure. The solution to this experimental problem is to use a device, a total absorption gamma spectrometer (TAGS), which is sensitive to the beta population of the nuclear levels rather than to the individual gamma rays. A TAGS can be constructed using a large NaI(Tl) scintillator which covers 4 pi in solid angle relative to the source. Such a device will absorb all the energy of the gamma rays that are produced in the de-excitation of a level fed in beta decay. Hence, instead of having peaks of the individual gamma rays, we will have sum peaks corresponding to the energy of the gamma cascades that follow the decay, and this gives direct information on the levels fed in the decay. The high efficiency of the NaI(Tl), as well as the reasonable energy resolution obtained with this kind of material make the TAGS an ideally suited device for the measurement of GT strength. The main goal of this project was the application of the TAGS technique to various problems of physical interest. This was part of a long term research plan in the framework of an international collaboration, between Valencia, Surrey, Strasbourg and Madrid, which recently installed a new TAGS at the on-line isotope mass separator ISOLDE, at the ‘Conseil Europeen pour la Recherche Nucleaire’ (CERN). The researcher was heavily involved in this research topic in recent years. The long term research project addressed the study of a series of problems related to the following lines: 1. ÿ-decay data for reactor heat calculations, 2. studies of p- pairing through the ÿ -decay process 3. search for shape effects from ÿ -decay studies, and 4. ÿ -decay studies of neutron-rich nuclei of astrophysical interest. This project covered a wide area of experimental studies ranging from practical applications to fundamental research in nuclear physics. The project had short term goals, to be performed during the 12 months of the grant, and long term goals, to be performed in a longer time scale. One of the main goals of the present project was the realisation of a measurement of the beta decay of the 102,104,105Tc isotopes using the TAGS technique topic a. This measurement required setting a new experimental setup at the IGISOL facility at the University of Jyväskylä in Finland. The measurement was performed and the data analysis was underway by the time of the project completion. From the results of this measurement we anticipated to answer a question related to a long standing discrepancy in the gamma component of the decay heat in the cooling time period of 300 to 3000 sec. Relative to the topic of search for shape effects from ÿ -decay studies, we performed a measurement of the beta decay of the 78Sr nucleus at ISOLDE (CERN) and finished the analysis of the data on 76Sr.

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