Physics Division Seminars bring us speakers on a variety of physics related subjects. Usually these are held in the Building 6008 large Conference Room, at 3:00 pm on the chosen day, but times and locations may vary. For more information, contact our seminar chairman,
Tel (Office): (865) 574-6124 (FAX): (865) 574-1268
The half-lives of electron capturing nuclei are expected to depend on the surrounding environment. This effect was first observed for 7Be. I shall discuss recent measurements of change of 7Be decay rate in different media and how our understanding of such variations in decay rate affect predicted 8B solar neutrino flux. A required input for calculating 7Be electron capture rate at the solar core is the relevant nuclear matrix element which is extracted from terrestrial 7Be decay rate measurements, assuming that beryllium atomic wave function is the same as if the beryllium atom is in free space and it has two full 2s electrons. However, the effect of in-medium loss of 7Be valence electrons should be considered for accurately extracting the nuclear matrix element from terrestrial measurements as all these measurements to date have been performed by placing 7Be in a medium or by forming a compound of 7Be. We have quantitatively understood all recent measurements on the change of decay rate of 7Be implanted in different media (metals, insulators, fullerene C60 etc.) using linear muffin-tin orbital (LMTO) method calculations. Considering the loss of 2s electrons due to in-medium effect, the extracted nuclear matrix element of 7Be electron capture reaction has been estimated to increase by 2-2.7%, thus decreasing the predicted 8B solar neutrino flux from Standard Solar Model (SSM) by the same amount (2-2.7%). This result would be important as the calculation of 8B solar neutrino flux would be refined in the future.
The study of change of half-life of heavier (110Sn, 109In) nuclei in different environments is important for understanding the nucleosynthesis process in general. Earlier efforts to observe the change of decay rate of heavier (heavier than 7Be) electron capturing nuclei in different media were not very successful. Recently we performed high precision measurements to measure the change of half-life of 110Sn and 109In implanted in Au and Pb. It was observed that half-lives of 110Sn and 109In are longer by approximately 0.4% and 1%, respectively, in Pb compared to those in Au. A qualitative explanation of this observation based on the electron affinity of the host medium and screening of inner shell electrons of tin and indium by valence electrons will be discussed.