Origin of the element in the universe is one of the big questions in modern physics. The study on the origin is boosted by a recent finding of kilonova following the discovery of the gravitational wave. To understand the r-process, several nuclear physics parameters are demanded. Although the neutron capture rate is the one of the most important to determine the neutron density at the final stage, they have not been well studied experimentally because both the neutron and the nuclei are short-lived. 

In particular, the peak structure in the abundance is governed by the neutron capture reaction around in the freezing-out phase, where the compound neutron (CN) capture reaction plays an important role. In the CN reaction, the gamma emission probability around the neutron threshold is a key quantity. To determine the CN reaction rate via studying the gamma emission probability, we conducted the (d,p) surrogate reaction on ⁷⁹Se at the Optimized Energy Degrading Optics (OEDO) beamline – a deceleration and focusing element – at RIBF. 

The decelerated ⁷⁹Se was focused on a CD2 target. The recoil particles were detected by the SSD-CsI array. The outgoing residues were identified by the first half of the SHARAQ spectrometer. The gamma emission probability was obtained directly from the residual nuclei. This was the first experiment to determine the gamma emission probability without detecting gamma rays. In spring 2022, we applied this method to the (d,p) reaction with the decelerated ¹³⁰Sn beam at OEDO to reveal the condition to form the second peak at r-process.  

In the talk, the speaker will introduce the system of OEDO and discuss the surrogate reactions on ⁷⁹Se and ¹³⁰Sn. 

About the speaker:

Nobuaki IMAI, Dr. Sci. 

Position: Associate Professor at CNS, the University of Tokyo since June, 2014 – present; Assistant Professor at Institute of Particle and Nuclear Study, KEK from October 2004 to May 2014. 

Education: Doctorate Degree in Science from The University of Tokyo in April 2004. 

Community service: Hold several prestigious positions in Japan, which include Chairman of the RCNP P-PAC (2019-2021), Board Member of the RCNP P-PAC (2016-2017), Chairman of the Unstable Nuclei Division of the Future Project Steering Committee, Japan Nuclear Physics Forum (2016-present), Scientific secretary of JPS Division of Experimental Nuclear Physics (2018-2019), Chairman of RIBF Users Executive Committee (UEC) (2014-2016), Member of RIBF Users Executive Committee (UEC) (2013-2017). 

Awards: Japan Nuclear Physics Forum Award for Outstanding Young Physicists in the Experimental Nuclear Physics (2005).