H. Hasegawa

DALI2: A NaI(Tl) detector array for measurements of

A NaI(Tl) detector array called DALI2 (Detector Array for Lo w Intensity radiation 2) has been constructed for in-beam γ-ray spectroscopy experiments with fast radioactive isotope (R I) beams. It consists typically of 186 NaI(Tl) scintillator s covering polar angles from∼15◦ to ∼160◦ with an average angular resolution of 6 ◦ in full width at half maximum. Its high granularity (good angular resolution) enables Doppler-shift correcti ons that result in, for example, 10% energy resolution and 20 % full-energy photopeak e fficiency for 1-MeVγ rays emitted from fast-moving nuclei (velocities of v/c ≃ 0.6). DALI2 has been employed successfully in numerous experiments using fast RI beams wi th velocities ofv/c = 0.3 − 0.6 provided by the RIKEN RI Beam Factory.

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The electric quadrupole transition from the first 2(+) state to the ground 0(+) state in 16C is studied through measurement of the lifetime by a recoil shadow method applied to inelastically scattered radioactive 16C nuclei. The measured mean lifetime is 77+/-14(stat)+/-19(syst) ps. The central value of mean lifetime corresponds to a B(E2;2+(1)-->0(+)) value of 0.63e(2) fm(4), or 0.26 Weisskopf units. The transition strength is found to be anomalously small compared to the empirically predicted value.

rays from fast nuclei

Abstract The neutron-rich nucleus 13 B was studied via the proton transfer reaction 4 He( 12 Be,  13 B γ ) at 50 A MeV . The known 4.83-MeV excited state was strongly populated and its spin and parity were assigned to 1 / 2 + by comparing the angular differential cross section data with DWBA calculations. This low-lying 1 / 2 + state is interpreted as a proton intruder state and indicates a deformation of the nucleus.

Anomalously hindered E2 strength B(E2;2+(1)-->0+) in 16C.

We have measured Coulomb excitation of 2 4 Si to its first excited state to obtain the E2 transition probability. This is the first measurement of B(E2) for a T z = -2 nucleus, which enables us to compare their transitionprobabilities between the mirror pair of T = 2, namely 2 4 Si and 2 4 Ne.

Low-lying proton intruder state in 13B

By taking advantage of the high efficiency of the Coulomb dissociation method, we studied the stellar 22Mg(p, γ)23Al reaction. The radiative width Γγ of the first excited state in 23Al was determined by measuring the intermediate-energy 23Al → 22Mg + p breakup process with a lead target.

Coulomb Excitation of 24Si

The Coulomb dissociation of the proton-rich nuclei {sup 27}P was studied experimentally using {sup 27}P beams at 57 MeV/nucleon with a lead target. The radiative widths of the low-lying excited state in {sup 27}P were deduced. The resonant capture reaction rate of stellar {sup 26}Si(p,{gamma}){sup 27}P through these states was estimated using the measured radiative widths. The astrophysical implications obtained from the extracted reaction rate will be discussed.

Coulomb dissociation experiment for explosive hydrogen burning: Study of the 22Mg(p, γ)23Al reaction

The Coulomb dissociation of the proton-rich nuclei {sup 27}P was studied experimentally using {sup 27}P beams at 57 MeV/nucleon with a lead target. The radiative widths of the low-lying excited state in {sup 27}P were deduced. The resonant capture reaction rate of stellar {sup 26}Si(p,{gamma}){sup 27}P through these states was estimated using the measured radiative widths. The astrophysical implications obtained from the extracted reaction rate will be discussed.

Investigation of Stellar 26Si(p,γ)27P Reaction via Coulomb Dissociation

The Coulomb dissociation of the proton‐rich nuclei 27P was studied experimentally using a 27P beam at 57 MeV/nucleon with a lead target. The γ decay widths of the low‐lying excited states in 27P were deduced. The resonant capture reaction rate of stellar 26Si(p,γ)27P through these states was estimated using the measured γ decay width. The astrophysical implications derived from the estimated reaction rate is discussed.

Investigation of Stellar 26 Si (p,γ) 27 P Reaction via Coulomb Dissociation

We performed an experiment of the inelastic scattering of the two neutron halo nucleus 14Be at 68 A MeV on a carbon target. Since 14Be has no bound excited state, 14Be breaks up into 12Be and two neutrons. Using the invariant-mass method, the first excited state of 14Be is clearly observed at Eex = 1.56(13) MeV. A distorted-wave Born approximation analysis is performed and the spin-parity corresponding to the peak is assigned to be Jπ = 2+. Deformation length of 14Be is extracted to be ∂C = 1.53(10) fm. These experimental results are compared with shell-model calculations.

Coulomb Dissociation of 27P for Study of 26Si(p,γ)27P Reaction

Stucture of N = 17,18 nuclei has been investigated via proton inelastic scattering and neutron removal reactions up to the neutron dripline in inverse kinematics by use of radioctive ion beams provided by the RIKEN isotope separator, RIPS. Low energy excited states have been found in all the 26,27F and 27Ne nuclei, which are out of the neutron sd shell configuration space and are considered as intruder states arisng from cross shell excitations. Monte-Carlo shell model calculations with the sdpf-m interaction predict low energy negative parity excited states from neutron cross shell excitation at N = 17,18, which is in agreement with our experimental results. Observation of intruder states even at N = 17, especially the p3/2 state in 27Ne at 765 keV is a clear indication of a vanishing N = 20 shell gap at Z = 8 as predicted by the sdpf-m shell model calculations.