Anomalously HinderedE2StrengthB(E2;21+→0+

Abstract

The electric quadrupole transition from the first 2 + state to the ground 0 + state in 16 C is studied through measurement of the lifetime by a recoil shadow method applied to inelastically scattered radioactive 16 C nuclei. The measured lifetime is 75 ± 23 ps, corresponding to a B(E2; 2 + 1 → 0 + ) value of 0.63 ± 0.19 e 2 fm 4 , or 0.26 ± 0.08 Weisskopf units. The transition strength is found to be anomalously small compared to the empirically predicted value.PACS numbers: 23.20. Js, 21.10.Tg, 29.30.Kv Quadrupole strengths are fundamental quantities in probing the collective character of nuclei. The enhancement of the electric quadrupole (E2) transition strength with respect to that of single proton excitation may reflect large fluctuation or deformation of nuclear charge [1]. One of the important E2 transitions in an even-even nucleus is that from the first 2 + (2 + 1 ) state to the ground state (0 + g.s. ), the reduced transition probability B(E2) of which has long been a basic observable in the extraction of the magnitude of nuclear deformation or in probing anomalies in the nuclear structure. With recent advances in techniques for supplying intense beams of unstable nuclei, several exotic properties such as magicity loss [2,3,4] have been discovered in neutron-rich nuclei through measurements of E2 strengths.The present Letter reports lifetime measurements of the 2 + 1 state of the neutron-rich nucleus 16 C. The lifetime is inversely proportional to B(E2). A simple model of a nucleus as a quantum liquid-drop well describes the systematic tendency that B(E2) varies in inverse proportion to the excitation energy E(2 + 1 ) of the 2 + 1 state [5]. For carbon isotopes, when N changes from the magic number 8 to 10, E(2 + 1 ) decreases dramatically from 7.012 to 1.766 MeV [6]. It can then be anticipated that 16 C (N = 10) would have a much larger B(E2) than that of * Electronic address: imai@rarfaxp.riken.go.jp † On leave from ATOMKI, Debrecen, Hungary. ‡ Present address: NSCL Michigan State University, East Lansing, Michigan 48824, USA.14 C (N = 8). Unexpectedly, a remarkably small B(E2) was found for 16 C in the present work. The observed value, in Weisskopf units, turns out to be far smaller than any other B(E2) measured on the nuclear chart.In the present experiment, a new technique was employed to measure the lifetime of an excited state populated in inverse-kinematics reactions. The technique essentially followed the concept of the recoil shadow method (RSM) [7], in which the emission point of the de-excitation γ-ray is located and the γ-ray intensity is recorded as a function of the flight distance of the deexciting nucleus. As the flight velocity of the de-exciting nucleus is close to half the velocity of light, the flight distance over 100 ps corresponds to a macroscopic length of about 1.7 cm. Thus, the present shadow method provides a wide range of applicability, extending to lifetimes of as short as a few tens of ps. In particular, the method is useful for determining the B(E2) value of Z < 10 nucl...

Erratum: Reaction cross sections of carbon isotopes incident on a proton [Phys. Rev. C 77, 034607 (2008)]

“…Several works have been done already experimentally [3,4,5,6] and theoretically [7,8,9,10,11]. For example, the structure of 22 C has been studied by two (W.H.…”

Scaling of Charge-Changing Interaction Cross Sections and Point-Proton Radii of Neutron-Rich Carbon Isotopes

“…In this Letter, we report on precision measurements and a phenomenological analysis of cc for stable and unstable nuclei, namely, for 9-11 Be, 14-16 C, and [16][17][18] O nuclei reacting with a carbon target at approximately 300 MeV=nucleon. The known charge radii of Be and O isotopes were used as an input in the Glauber-type calculations in order to obtain the ratios of experimental and calculated cc .…”

“…(1), the charge-changing cross sections calc cc have been calculated for [9][10][11] Be, 14 C, and [16][17][18] O and are presented in Table I. The ratios of experimental expt cc and calculated calc cc values are indicated with filled symbols (circles: 9-11 Be; square: 14 C; triangles: [16][17][18] O isotopes) in Fig. 2.…”

“…The first measurement of BðE2Þ in 16 C suggested an anomalous deformation [18]. This stimulated intensive studies of the BðE2Þ values [19,20] as well as inelastic scattering experiments with 16 C [21].…”

“…In summary, we have performed the precision measurements of the charge-changing cross sections cc of 9-11 Be, 14-16 C, and [16][17][18] O nuclides on carbon at energies around 300 MeV=nucleon. Based on the Glauber-type analysis with the phenomenological correction, we have discovered an approximate, but universal, scaling of cc values over a wide range of A=Z.…”