Electromagnetic Properties of Light Neutron-Rich Nuclei - Lifetime Measurements of 16C and 23Ne

Abstract

The lifetimes of higher-lying excited states were measured for 16C and 23Ne using a target and degrader setup made of beryllium and gold to apply the Doppler-shift attenuation method. The experiment was performed at the Argonne National Laboratory exploiting the reaction 9Be(9Be,2p)16C* as well as 9Be(16O,2p)23Ne*, which was available due to oxidation of the target. The emitted gamma rays were measured with Gammasphere while the charged particles were detected with Microball. By comparing the measured gamma-ray spectra for different detection angles and 2p events with Geant4 simulations, lifetimes can be obtained. An elaborated explanation of the used analysis method and potential uncertainties is given in this work. The simulations were carried out for two different sets of stopping powers to check their influence on the results. For 23Ne the lifetimes of two higher-lying states could be measured for the first time. The lifetime of the (5/2+,7/2+) state at 2517 keV was obtained to 641(79)fs +16fs-6fs(syst,target), while the lifetime of the (5/2+, 7/2+) state at 1702 keV was obtained to 168(55)fs +8fs-1fs(syst,target) +72fs-80fs(syst,feeding). To consider the feeding for the lower state it was assumed that the angular distributions are the same for both observed transitions. Theoretical USDB calculations were able to reproduce the level energies well, while they significantly underestimate the experimental lifetimes. For 16C the lifetime of the 4_1+ state could be constrained between 1.9ps +0.0ps-0.1ps(syst,target) and 4ps. This results in 2.74e^2fm^4 ≤ B(E2;4_1+→2_1+ ) ≤ 5.78e^2fm^4 +0.32e^2fm^4 -0.00e^2fm^4(syst,target) as a transition strength limit for the 4_1+ state. Theoretical predictions from no-core shell model calculations with NN+NNN interactions and p-sd shell model calculations for several effective two body interactions fulfill this constraint. The most likely lifetime range for the 2_2+ state was obtained to be 244 fs to 376 fs. Together with branching ratio limits for this state, which are known from previous measurements, the transition strengths for the 2_2+ state could be constrained. No-core shell model, recent in-medium no-core shell model, and p-sd shell model calculations are compared to these results. In the second part of this work a conceptual design of a 14C electron scattering experiment at the QCLAM Spectrometer at the S-DALINAC is presented.