I. Mazumdar

Experimental study of nuclei in the vicinity of the 'island of inversion' through the fusion-evaporation reaction

We report the observation of high-spin states in nuclei in the vicinity of the island of inversion, populated via the {sup 18}O+{sup 18}O fusion reaction at an incident beam energy of 34 MeV. The fusion reaction mechanism circumvents the limitations of other reactions used so far to populate excited states in these nuclei. The level structures of {sup 33,34}P and {sup 33}S have been investigated and their level schemes were substantially extended. The intensities and branching ratios of the {gamma} transitions were measured. The observed {gamma} coincidence intensity anisotropy and polarization asymmetry were analyzed for a consistent assignment of the spins and parities of the nuclear levels. The 1876-keV transition in {sup 34}P that had an ambiguous multipolarity assignment has been confirmed to be a mixed transition. Truncated (1p-1h) shell model calculations within the sdpf valence space were performed. These calculations reproduce the experimental excitation energies of {sup 34}P but fail to explain the observed mixing ratio for the 1876-keV transition.

A large high-energy gamma-ray spectrometer at NSC

Abstract A large NaI HIgh energy Gamma Ray Spectrometer (HIGRASP) has been designed and constructed at NSC, New Delhi. The spectrometer has been working quite satisfactorily for the last two years and has achieved the set goals. Detailed design considerations have resulted in very good energy resolution (2.5% at 22.5xa0MeV) and line shape of the gamma-ray spectrum. Experiments to study giant dipole resonance decay from hot rotating nuclei and nuclear Bremsstrahlung gamma emission have been successfully carried out using this spectrometer. The design, construction and response of the detector is discussed in detail.

Characteristics of a two-fold segmented Clover detector

Abstract The characteristics of a Compton-suppressed, two-fold segmented, Clover detector have been studied in singles and add-back modes for using it in the study of heavy-ion fusion–evaporation reactions. The measurements have been performed with standard sources of 152 Eu , 60 Co and in-beam γ -rays from the fusion–evaporation reaction 28 Si on 27 Al at E lab =100 MeV . Reduction in γ -ray Doppler broadenings due to the two-fold segmentation is demonstrated through in-beam data.

Decoupling the effect of temperature on GDR widths in excited compound nucleus 144Sm

We measured giant dipole resonance (GDR) γ-rays in the 28Si + 116Cd reaction at ~105 and 120 MeV excitation energies. The resonance widths were extracted as a function of temperature at different angular momentum values to decouple the effect of these two variables on resonance widths. The high energy GDR γ-rays from the decay of compound nucleus (CN) 144Sm were measured using a large NaI(Tl) detector in coincidence with a sum-spin multiplicity filter. The experimental data were analyzed under the framework of statistical decay of CN. The GDR cross sections were also calculated with a thermal shape fluctuation model (TSFM), which incorporates the temperature and spin dependent shell corrections within the Nilsson–Strutinsky approach. The GDR widths were extracted in the temperature range 1.5–2.2 MeV. The measured widths indicate that the increase of GDR widths with temperature at different average spin values follows a similar trend and is borne out by theoretical calculations. Present calculation shows that TSFM satisfactorily reproduces the GDR cross section as well as widths in the temperature range 1–2.2 MeV over a wide range of angular momentum.

Efficiency calibration and coincidence summing correction for a large volume (946 cm3) LaBr3(Ce) detector: GEANT4 simulations and experimental measurements

The paper describes the studies on efficiency calibration and coincidence summing correction for a 3.5″×6″ cylindrical LaBr3(Ce)detector. GEANT4xa0simulations were made with point sources, namely, 60Co, 94Nb, 24Na, 46Sc and 22Na. The simulated efficiencies, extracted using 60Co, 94Nb, 24Na and 46Sc that emit coincident gamma rays with same decay intensities, were corrected for coincidence summing by applying the method proposed by Vidmar et al. (2003). The method was applied, for the first time, for correcting the simulated efficiencies extracted using 22Na that emits coincident gamma rays with different decay intensities. The measured results obtained using 60Co and 22Na were found to be in good agreement with simulated results.

GEANT4 simulations and experimental measurements of absolute source activity using modified sum-peak method

In the present work, we have made Monte Carlo simulations of the response of 1 × 1 LaCb(Ce) and 1 × 1 LaBn(Ce) scintillation detectors using the radioactive sources emitting two gamma rays in cascade, namely, 60Co, 94Nb, 46Sc and 24Na for different values of source-detector separation. These simulations were done using GEANT4 toolkit that include radioactive decay module and general particle source (GPS) module. Energy spectra were generated for each of these double gamma emitters and absolute activities were calculated using conventional sum-peak method and modified sum-peak method proposed very recently. In order to validate the simulated results, we have carried out experimental investigations using a calibrated 60Co source. The measured activities are found to be in very good agreement with the simulated activities.

Structure of dipole bands in {sup 106}In

High spin states in neutron-deficient {sup 106}In were investigated using {sup 78}Se({sup 32}S,p3n) reaction at 125 MeV. The level scheme is extended up to 7 MeV of excitation energy for the negative parity states constituting four dipole bands, and the positive parity states which mainly exhibit single-particle excitations are extended up to 5 MeV. Projected deformed Hartree-Fock calculations were carried out to understand the configurations of different bands in this nucleus.

GEANT4 simulation of a new 4π sum-spin spectrometer at TIFR

A 4π sum-spin spectrometer has been designed, constructed and commissioned at the Tata Institute of Fundamental Research (TIFR), Mumbai. The spectrometer is an array of 32 conical NaI detectors of pentagonal and hexagonal cross sections. The primary role of the array is to measure the multiplicity and sum-energy of the discrete low energy gamma rays emitted in heavy-ion induced fusion-evaporation reactions. The determination of the multiplicity and sum-energy of the low energy discrete gamma rays from evaporation residues is crucial for angular momentum gating or selection of narrow angular momentum windows in the phase space. This spectrometer is primarily to be used in conjunction with a high energy gamma ray spectrometer for detection of phase space selected Giant Dipole Resonance (GDR) gamma rays. Since its commissioning the array has been successfully used in in-beam experiments to study GDR decay from hot-rotating 192Pt nucleus. We have carried out extensive simulations for calculating the detection efficiencies of the individual detectors and the entire array including the scattering chamber. The total intrinsic and photo peak efficiencies have been measured accurately with calibrated low energy gamma ray sources and are found to be in very good agreement with the values obtained from GEANT4 simulations. We have also carried out efficiency measurements and GEANT4 simulations for a smaller array of 14 straight NaI detectors of hexagonal cross sections packed in castle geometry and the results have been compared with the 4π array. The fold distributions have also been calculated for different gamma multiplicities for both the 14 elements and the 4π array. The detailed results of the simulations and measurements for the 4π array and the results of the comparative studies with the 14 elements array in castle geometry are presented.

Search for chiral bands in 133Ce

High spin states in 133Ce studied studied through the 120Sn(18O,5n)133Ce fusion evaporation reaction show the presence of a nearly degenarate pair of positive parity bands. A comparision of their properties indicate that these bands could form a ’Chiral doublet.