The structure of 20C using proton removal reactions

The carbon isotopic chain is accessible experimentally up to the neutron drip line and provides a unique opportunity to study the evolution of shell structure with isospin asymmetry. Of particular interest is the proton component of the 2+ state along the carbon isotopic chain, which can shed light to the evolution of the Z=6 spin-orbit shell gap. The bound state of 20C, following a proton knockout reaction from a 21N beam can yield information on the proton component of the first 2+ state. The experiment was performed at RIKEN during the Day-One campaign and the SAMURAI setup is used to measure the inclusive and exclusive cross section of the 2+ state
from the C(21N,20C)X reaction.

The model used to interpret the results describes the excited 2+ state in terms of a mixing of pure proton and pure neutron excitation, where the proton amplitude is a measure of the amount of proton excitation. Following a proton removal reaction from 21N which populates both the ground and excited 2+ state in 20C, the proton amplitude was determined by measuring the inclusive cross section and exclusive cross section of the 2+ state.

Previous work has shown the existence of the Z=6 sub-shell closure, with evidence suggesting a weakening of the sub-shell closure as the neutron drip line is approached because of a reduction in the splitting of the 1p1/2 and 1p3/2 proton orbits. The deduced proton amplitude of (β^2) = 13.7 ± 1.2(stat) ± 0.3(sys)% shows that the amount of proton excitation of the first excited 2+ state in 20C is moderately increased. This value represents a moderate increase when compared to the proton amplitude of lighter neutron rich even-even 16,18C, supporting the previous work suggesting a weakening of the Z=6 sub-shell closure. This thesis presents the analysis of the data obtained during the Day-One campaign at SAMURAI.