Luận án Study of nuclear reactions for astrophysics

Nuclear physics plays an important role in the improvement of the world. There are many useful applications of the nuclear physics in industry, agriculture, medicine, etc. Besides, nuclear physics is a powerful tool to study astrophysics. Since all materials are constructed from nuclei, it is possible to study stars, supernovae and cosmological phenomena by using nuclear reactions in laboratories on the Earth. Therefore, the study of nuclear reactions is important not only for physics but also for astrophysics, so-called nuclear astrophysics. According to the cosmic observation and nuclear mechanisms, the stellar evolution models including a lot of nuclearprocesses are supposed [1, 2]. There are many reaction chains during the nucleosynthesis in stars, which include sensitive reactions at which the evolution can change its behavior to grow upon other branches. The implication of nuclear physics for astrophysicswas thought to have been taken place since the late of 1950s from the seminal works of Burbidge, Burbidge, Fowler, and Hoyle in their famous paper [3] and independently by Cameron[4]. However, these works were relied on theoretical prediction of astronomy, astrophysics and nuclear physics. It is necessary to perform experimental research to confirm the theory. There are many accelerator facilities with modern spectrometers which were built for measurements of nuclear astrophysics, such as TRIUMF [5] in Canada,JINA [6] and NSCL [7] in the United States of America, CRIB [8] in Japan, GSI [9] in Germany, etc In Vietnam, a Tandem accelerator located at Hanoi University of Science is being constructed to use for undergraduate training and study of nuclear astrophysics. In the rp-process of the nucleosynthesis in Supernovae [10, 11] and X-ray Bursts [12], the stellar reaction 22 Mg(α,p) 25 Alis a significant link. This reaction is very meaningful because it relates to not only the 26 Si structure but also the celestial phenomena as well as the experimental technique, as described in Abstract 2 section 1.5. There were two efforts to study the rate of 22 Mg(α,p) 25 Alreaction [13, 14]. However, the results are still uncertain since the observed data relied on the beta decays of 26 P or (p,t)reaction are far from the Gamow window (see section 1.7), which corresponds to the temperature range of Supernovae and Xray Burst environments. The excited states of 26 Si obtained by 26 P could not be used to calculate reaction rate of 22 Mg(α,p) 25 Alin the temperature region T 9 >1 GK because the energy levels are still low. The work in ref.[13] included a large uncertainty above the alpha threshold of 26 Si since the reaction rate was determined by the resonances that were assigned indirectly by using spinparities of the mirror nucleus, 26 Mg. In addition, the S-factor [15] needed for the reaction rate calculation was calculated from the quantum parameters of the mirror nucleus. Because the information above the alpha threshold of 26 Si corresponding to the region T 9 > 0.5 GK seems to be empty up to date, the calculated rates of the stellar reaction is still uncertain. In such research scenario, we decided to perform a direct measurement of the 22 Mg+α reaction by using CRIB facility located at RIKEN, Japan. The reaction energy corresponded to the stellar condition of T 9 > 0.5 GK. This work investigated the 26 Si structure above the alpha threshold and the rate of the stellar reaction 22 Mg(α,p) 25 Al. Because the resonances of nuclei may be caused by the cluster structure [16, 17, 18], the α-cluster structure of resonance states in the 26 Si nucleus was evaluated. For astrophysical aspects, the potential waiting point of 22 Mg in the nucleosynthesis [19], the existence of the gamma ray 1.275 MeV as well as the anomalies in the Ne-E problem [20, 21] and the abundance of 22 Na in meteorites could be revealed based on the rate of 22 Mg(α,p) 25 Al obtained in this study.