核科学与技术系学术报告

作者: 时间:2010-06-07 点击数:

报告人:Yuri Litvinov 博士 德国赫姆霍兹重离子研究所

报告题目:Trapped ions for nuclear physics and astrophysics

报告时间:2010年6月8日 星期二 上午10:30—11:30

报告地点:澳门新萄京6692 cc学术交流厅(主513)

报告摘要:Stellar nucleosynthesis proceeds by nuclear fusion until iron in massive stars (in our medium-sized sun it ceases already at carbon) and stops there, because the fusion of still heavier nuclei needs energy. To overcome this borderline, Nature has invented a ’trick’: atomic nuclei can get heavier by capturing free neutrons; they can also alter their nuclear charge by transforming a neutron into a proton or vice versa. The latter processes are called beta decay. Now, by a subtle interplay of neutron capture and beta decay, the atomic nuclei can become continually heavier, and their nuclear charge may increase until the limit of nuclear stability. In this way, beta decay and neutron capture are at once the driving forces and the fuel for the synthesis of ever heavier nuclides. They determine the timescale of nuclear transmutations as well as the pathways of stellar nucleosynthesis and, thus, the final abundance of the atomic nuclei we face wherever in the world.

One of the major tasks of nuclear astrophysics consists in revealing these pathways by the artificial synthesis of those unstable nuclei in terrestrial laboratories and by precisely elucidating their main properties, in particular their masses and beta-decay probabilities. For accurate mass measurements the exotic nuclei are trapped and stored in an ion trap or a storage ring. The masses are obtained from the revolution frequencies of the stored ions.

An important fact is that the stellar nucleosynthesis proceeds in a hot environment. Therefore, the atoms undergoing beta decay in the stellar plasmas are as a rule not neutral but highly ionised, which means that the number of bound electrons is significantly smaller than the nuclear charge number. This can lead to considerable or even dramatic changes of their decay properties with respect to neutral atoms. The storage ring facilities provide the first opportunity to explore beta decay of highly-charged nuclides. An irrevocable prerequisite, however, for conducting these experiments is the capability to create and to preserve high atomic charge states for extended time periods, i.e. for a couple of minutes or even hours.

A sketch of the experimental techniques, the main results and their impact for nuclear physics and nuclear astrophysics will be given in this lecture.

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