Lifetime measurements of first excited states inC16,18

Abstract

The electric quadrupole transition from the first 2 + state to the ground 0 + state in 18 C was studied through a lifetime measurement by an upgraded recoil shadow method applied to inelastically scattered radioactive 18 C nuclei. The measured mean lifetime is 18.9 ± 0.9(stat) ± 4.4(syst) ps, corresponding to a B(E2; 2 + 1 → 0 + g.s. ) value of 4.3 ± 0.2 ± 1.0 e 2 fm 4 , or about 1.5 Weisskopf units. The mean lifetime of the first 2 + state in 16 C was remeasured to be 18.3 ± 1.4 ± 4.8 ps, about four times shorter than the value reported previously. The discrepancy between the two results was explained by incorporating the γ -ray angular distribution measured in this work into the previous measurement. These transition strengths are hindered compared to the empirical transition strengths, indicating that the anomalous hindrance observed in 16 C persists in 18 C.

Shell-model calculations for neutron-rich carbon isotopes with a chiral nucleon-nucleon potential

“…n a l a j a n b l b j b a||M1||b 0d 5/2 0d 5/2 −2.553 0d 5/2 0d 3/2 2.743 0d 3/2 0d 5/2 −2.743 0d 3/2 0d 3/2 1.414 0d 3/2 1s 1/2 0.179 1s 1/2 0d 3/2 −0.179 1s 1/2 1s 1/2 −2.298 are reproduced nicely. We also report our predicted excitation energy, 4.661 MeV, for the unbound J π = 2 + 1 state in 22 C. The behavior of the theoretical 2 + excitation energies confirms that there is no N = 14 subshell closure. We obtain a ground-state wave function in 20 C with only 14% of the (νd 5/2 ) 6 configuration (calculated using the OXBASH shellmodel code [20]).…”

“…We see that the agreement is quite good, providing evidence for the reliability of our calculated effective operator which takes into account microscopically core-polarization effects up to third order in the NN potential. It is well known [10,[22][23][24] that these observed transition strengths are strongly hindered when compared with the values obtained by an empirical formula based on the liquid-drop model [25]. In this connection, we point out that our effective reduced single-neutron matrix elements (see Table IV) correspond to a neutron effective charge of about 0.4e with a harmonic-oscillator parameter b = 1.72 fm.…”

“…This is a major goal of nuclear physics, because these studies may reveal exotic features, such as shell modifications, that go beyond our present understanding of the structure of nuclei. In a recent Letter [1] Tanaka et al have reported on the observation of a large reaction cross section in the drip line nucleus 22 C. The structure of 22 C is quite interesting, because it can be considered the heaviest Borromean nucleus ever observed [2]. This nucleus is weakly bound, with a two-neutron separation energy S 2n evaluated to be 420 ± 940 keV [1], and may be seen as composed of three parts: two neutrons plus 20 C. These three pieces must all be present to obtain a bound 22 C because of the particle instability of 21 C. This is the structure of a Borromean nucleus, which has only one bound state but, considered as three-body system, admits no bound states in the binary subsystems [3].…”

“…online) Experimental[22,24] and calculated excitation energies of the yrast J π = 2 + states for carbon isotopes from A = 16 to 22. N is the number of neutrons.…”

Surface-enhanced pair transfer amplitude in quadrupole states of neutron-rich Sn isotopes

“…For instance, the ratio between the neutron and proton amplitude becomes significantly different from the liquid drop ratio N/Z [42][43][44][45][46][47]. New aspects of the quadrupole vibration may emerge also in the pair transfer transitions feeding the 2 + states.…”