D. R. Chakrabarty

Electromagnetic transition from the 4+ to 2+ resonance in 8Be measured via the radiative capture in 4He + 4He.

An earlier measurement on the 4+ to 2+ radiative transition in 8Be provided the first electromagnetic signature of its dumbbell-like shape. However, the large uncertainty in the measured cross section does not allow a stringent test of nuclear structure models. This Letter reports a more elaborate and precise measurement for this transition, via the radiative capture in the 4He + 4He reaction, improving the accuracy by about a factor of 3. Ab initio calculations of the radiative transition strength with improved three-nucleon forces are also presented. The experimental results are compared with the predictions of the alpha cluster model and ab initio calculations.

Direct observation of the 4+-to-2+ gamma transition in 8Be

Electromagnetic transition rates provide an important test for nuclear structure models. The nucleus 8Be exhibits a pronounced alpha-cluster structure and is a building block for more complex cluster nuclei. Here we report on the first observation of the gamma transition between the 4(+) and 2(+) states of 8Be in the 4He+4He reaction. The measured on-resonance cross section of 165+/-54 nb leads to a B(E2) of25+/-8e(2) fm(4), in good agreement with alpha-cluster models and sophisticated ab initio structure calculations.

Direct Observation of the4+-to-2+Gamma Transition inBe8

Electromagnetic transition rates provide an important test for nuclear structure models. The nucleus 8Be exhibits a pronounced alpha-cluster structure and is a building block for more complex cluster nuclei. Here we report on the first observation of the gamma transition between the 4(+) and 2(+) states of 8Be in the 4He+4He reaction. The measured on-resonance cross section of 165+/-54 nb leads to a B(E2) of25+/-8e(2) fm(4), in good agreement with alpha-cluster models and sophisticated ab initio structure calculations.

Inhomogeneous and intrinsic damping of giant dipole resonance in hot rotating nuclei with A ~ 150

High-energy gamma rays in the range of ~4–28 MeV were measured in the reaction 28Si+124Sn at E(28Si) ~ 185 MeV in coincidence with low-energy gamma ray multiplicities and evaporation residues. The centroid energy and width of the giant dipole resonance were extracted for various multiplicity windows from the statistical model analysis. These extracted widths, along with those from an earlier measurement at E(28Si) ~ 149 MeV, show a discrepancy with the results of a calculation under the thermal shape fluctuation model which describes the inhomogeneous damping of the resonance. An empirical form of the temperature and angular momentum dependence of the width, describing the data at both the beam energies, has been derived. The present results suggest that the contributions from both the inhomogeneous damping and the intrinsic collisional damping processes should be included.

Measurement of the damping of the nuclear shell effect in the doubly magic 208Pb region.

The damping of the nuclear shell effect with excitation energy has been measured through an analysis of the neutron spectra following the triton transfer in the (7)Li induced reaction on (205)Tl. The measured neutron spectra demonstrate the expected large shell correction energy for the nuclei in the vicinity of doubly magic (208)Pb and a small value around (184)W. A quantitative extraction of the allowed values of the damping parameter γ, along with those for the asymptotic nuclear level density parameter ã, has been made for the first time.

Structures in angular momentum gated proton and alpha particle spectra in low-energy 12C and 16O induced reactions

Gamma-ray multiplicity gated proton spectra have been measured in the 12C + 93Nb and 16O + 89Y reactions producing the same compound nucleus 105Ag at an excitation energy of ~39.8 MeV, and in the 12C + 89Y reaction at E(12C) = 40 MeV. Multiplicity gated alpha particle spectra have been measured in the 12C + 93Nb reaction at E(12C) = 37.5–45 MeV. All these measurements were aimed at gaining more insight into the origin of the unexpected broad structures seen earlier in the high energy part of the proton spectra at high gamma-ray multiplicities. Two possible explanations, one of the enhanced nuclear level density within the statistical model and the other of a massive cluster transfer process, were suggested by us to understand the earlier data. The former prescription appears to be favoured by the present proton data.

Fission time-scale from the measurement of pre-scission light particles and

An overview of the experimental result on simultaneous measurement of pre-scission neutron, proton, α-particle and GDR γ-ray multiplicities for the reaction 28Si +175Lu at 159 MeV using the BARC–TIFR Pelletron–LINAC accelerator facility is given. The data were analysed using deformation-dependent particle transmission coefficients, binding energies and level densities which are incorporated in the code JOANNE2 to extract fission time-scales and mean deformation of the saddle-to-scission emitter. The neutron, light charged particle and GDR γ-ray multiplicity data could be explained consistently. The emission of neutrons seems to be favoured towards larger deformation as compared to charged particles. The pre-saddle time-scale is deduced as (0–2) × 10−21 s whereas the saddle-to-scission time-scale is (36–39) × 10 −21 s. The total fission time-scale is deduced as (36–41) × 10 −21 s.

\gamma

Proton and {alpha}-particle spectra have been measured at backward angles in the {sup 12}C + {sup 93}Nb reaction at E({sup 12}C)=37.5-60 MeV and in the {sup 16}O+{sup 93}Nb reaction at E({sup 16}O)=54-75 MeV. The inclusive particle spectra generally are in good agreement with the statistical model calculations except for the {alpha} spectra in the {sup 12}C-induced reaction at beam energies of 37.5-42.5 MeV which are close to the Coulomb barrier in the entrance channel. In these cases the evaporation peak is up to 2 MeV lower than its expected position from the statistical model predictions. Gamma-multiplicity-gated {alpha} spectra, measured in the same reaction, also show the aforementioned discrepancy. These observations reveal that the conventional statistical model cannot explain the {alpha}-particle spectra in the {sup 12}C-induced reactions near the Coulomb barrier.