Gamma Array Research Articles

Shape coexistence inHo153

The high-spin states in 153Ho, have been studied by 139 57 La(20Ne, 6n) reaction at a projectile energy of 139 MeV at Variable Energy Cyclotron Centre (VECC), Kolkata, India, utilizing an earlier campaign of Indian National Gamma Array (INGA) setup. Data from gamma-gamma coincidence, directional correlation and polarization measurements have been analyzed to assign and confirm the spins and parities of the levels. We have suggested a few additions and revisions of the reported level scheme of 153Ho. The RF-gamma time difference spectra have been useful to confirm the half-life of an isomer in this nucleus. From the comparison of experimental and theoretical results, it is found that there are definite indications of shape coexistence in this nucleus. The experimental and calculated lifetimes of several isomers have been compared to follow the coexistence and evolution of shape with increasing spin.

Structure ofTe8052132: The two-particle and two-hole spectrum ofSn8250132

High-spin states in Te132, an isotope with two proton particles and two neutron holes outside of the Sn132 doubly magic core, have been extended up to an excitation energy of 6.17 MeV. The prompt-delayed coincidence technique has been used to correlate states above the T1/2=3.70(9)μs isomer in Te132 to the lower states using Th232(Li7,f) at 5.4 MeV/u and the Indian National Gamma Array (INGA). With Be9(U238,f) at 6.2 MeV/u and EXOGAM γ-array coupled with the VAMOS++ spectrometer, the level scheme was extended to higher excitation energies. The high-spin positive-parity states, above Jπ=10+, in Te132 are expected to arise from the alignment of the particles in the high-j orbitals lying close to the Fermi surface, the πg7/22, and the νh11/2−2 configurations. The experimental level scheme has been compared with the large scale shell model calculations. A reduction in the p−n interaction strength resulted in an improved agreement with the measurements up to the spin of 15ℏ. In contrast, the comparison of the differences between the experiment and these calculations for the N=76,78 isotones of Te and Sn shows the increasing disagreement as a function of spin, where the magnitude is larger in Te than in Sn. This behavior could possibly be attributed to the deficiencies in the p−n correlations, in addition to the n−n correlations in Sn.1 MoreReceived 30 October 2015Revised 26 February 2016DOI:https://doi.org/10.1103/PhysRevC.93.034324©2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsEnergy levels & level densitiesFissionNuclear spin & parityRare & new isotopesTechniquesShell modelNuclear Physics

Geant4 simulation study of Indian National Gamma Array at TIFR

A Geant4 simulation code for the Indian National Gamma Array (INGA) consisting of 24 Compton suppressed clover high purity germanium (HPGe) detectors has been developed. The calculated properties in the energy range that is of interest for nuclear γ-ray spectroscopy are spectral distributions for various standard radioactive sources, intrinsic peak efficiencies and peak-to-total (P/T) ratios in various configurations such as singles, add-back and Compton suppressed mode. The principle of operation of the detectors in add-back and Compton suppression mode have been reproduced in the simulation. The reliability of the calculation is checked by comparison with the experimental data for various γ-ray energies up to 5 MeV. The comparison between simulation results and experimental data demonstrate the need of incorporating the exact geometry of the clover detectors, Anti-Compton Shield and other surrounding materials in the array to explain the detector response to the γ-ray. Several experimental effects are also investigated. These include the geometrical correction to angular distribution, crosstalk probability and the impact of heavy metal collimators between the target and the array on the P/T ratio.

Lifetime measurements in mass regions A=100 and A=130 as a test for chirality in nuclear systems

Two odd-odd nuclei from the A ~ 100 and A ~ 130 regions, namely ^102Rh and ^134Pr have been studied in search for chiral doublet bands via ^94Zr(^11B,3n)^102Rh and ^119Sn(^19F,4n)^134Pr reactions, respectively. Two nearly degenerate bands built on the pig_9/2 ⊗ vh_11/2 configuration have been identified in ^102Rh and on the pig_11/2 ⊗ vh_11/2 configuration for ^134Pr. Lifetimes of excited nuclear states were measured using Dopplershift attenuation method and recoil distance Doppler-shift method. The deexciting gamma rays were registered by the Indian National Gamma Array for ^102Rh and using the EUROBALL IV detector array with an inner Bismuth Germanate (BGO) ball for ^134Pr, respectively. Polarization and angular correlation measurements have been performed to establish the spin and parity assignments for these bands. The derived reduced transition probabilities are compared to the predicitons of the two quasiparticles + triaxial rotor and interacting boson fermion-fermion models.

Spectroscopy of fission fragments using prompt-delayed coincidence technique

The time-stamp structure of the digital data acquisition system of the Indian National Gamma Array (INGA) has been utilized to carry out prompt-delayed coincidence technique for the spectroscopic study of fission fragments. This technique was found to be useful to determine the states above the long-lived isomer (with half-life up to ∼5 μs), present in the fission fragments. The angular correlation of γ-rays, emitted by the fission fragments, has also been used in the present INGA geometry to determine the spins of the de-exciting states.

Antimagnetic rotation and sudden change of electric quadrupole transition strength in 143Eu

Lifetimes of the states in the quadrupole structure in 143Eu have been measured using the Doppler shift attenuation method and the parity of the states in the sequence has been firmly identified from polarization measurements using the Indian National Gamma Array. The decreasing trends of the deduced quadrupole transition strength B(E2) with spin, along with increasing J(2)/B(E2) values before the band crossing, conclusively establish the origin of these states as arising from antimagnetic rotation. The abrupt increase in the B(E2) values after the band crossing in the quadrupole band, a novel feature observed in the present experiment, may possibly indicate the crossing of different shears configurations resulting in the re-opening of a shears structure. The results are reproduced well by numerical calculations within the framework of a semi-classical geometric model.

Role of neutrons in the coexistence of magnetic and antimagnetic rotation bands inCd107

Negative parity high-spin states of $^{107}\mathrm{Cd}$ have been investigated using the reaction $^{94}\mathrm{Zr}({}^{18}\mathrm{O}$,5n), from the $\ensuremath{\gamma}$-ray coincidence events recorded by the Indian National Gamma Array. A magnetic dipole ($M1$) band structure was established for the first time in this nucleus decaying to the low-spin states via several paths. Lifetimes of five in-band levels in this band have been measured using the Doppler shift attenuation method. The experimentally deduced $B(M1)$ values are found to decrease with increasing spin. The experimental observations, interpreted by the tilted axis cranking calculations, suggest that the $M1$ band is developed from the shears mechanism based on the 5qp configuration $\ensuremath{\pi}({g}_{9/2}^{\ensuremath{-}2})\ensuremath{\bigotimes}\ensuremath{\nu}({h}_{11/2}{g}_{7/2}^{2})$, which is then crossed by another 5qp configuration $\ensuremath{\pi}({g}_{9/2}^{\ensuremath{-}2})\ensuremath{\bigotimes}\ensuremath{\nu}({h}_{11/2}^{3})$. The semiclassical model of the shears mechanism also reasonably reproduces the decreasing trend of the observed $B(M1)$ values as a function of spin, supporting the above interpretation. The present work highlights the unique coexistence of both magnetic and antimagnetic (observed by us earlier) rotation bands in one nucleus arising from the same proton configuration, but different neutron configurations.

Transfer reaction studies in inverse kinematics with the magnetic spectrometer PRISMA

The large solid angle magnetic spectrometers, in combination with large gamma arrays, allowed to perform reaction mechanism and nuclear structure studies in different regions of the nuclear chart, especially in the neutron-rich direction. By studying transfer of multiple pairs valuable information on nucleon-nucleon correlations can be derived, especially from measurements performed below the Coulomb barrier. There is growing interest in the study of the properties of the heavy binary partner, in the Pb and in the actinides regions, crucial also for astrophysics.

γ-ray production cross sections of inelastic neutron scattering on natural molybdenum

γ-ray production cross sections of inelastic neutron scattering have been measured for molybdenum using the (n,n’γ)-technique. The experiment was performed at the GELINA facility at the Institute for Reference Materials and Measurements (IRMM) with the Gamma Array for Inelastic Neutron Scattering (GAINS) setup. GAINS consisted of eight high purity germanium detectors at the time of this experiment. The sample was made of natural molybdenum, which includes seven isotopes (A = 92, 94, 95, 96, 97, 98, 100). The presence of so many isotopes in the sample leads to overlapping peaks in the spectra, which limits the amount of data that can be extracted from the analysis. Nevertheless, a total of 31 γ rays from the seven isotopes were analysed and γ-ray production cross sections were determined. Comparisons to other experimental results were made when such data was available. Also comparisons with model calculations were made with the Talys 1.6 code.

A New Front-End High-Resolution Sampling Board for the New-Generation Electronics of EXOGAM2 and NEDA Detectors

This paper presents the final design and results of the FADC Mezzanine for the EXOGAM (EXOtic GAMma array spectrometer) and NEDA (Neutron Detector Array) detectors. The measurements performed include those of studying the effective number of bits, the energy resolution using HP-Ge detectors, as well as timing histograms and ${\rm n}/\gamma $ discrimination performance. Finally, the conclusion shows how a common digitizing device has been integrated in the experimental environment of two very different detectors which combine both low-noise acquisition and fast sampling rates. Not only the integration fulfilled the expected specifications on both systems, but it also showed how a study of synergy between detectors could lead to the reduction of resources and time by applying a common strategy.

Recent results from digital INGA at BARC–TIFR Pelletron Linac Facility and future plans

An experimental campaign aimed at studying various high spin phenomena using the Indian National Gamma Array (INGA) at the BARC–TIFR Pelletron Linac Facility has been successfully completed. The array consisting of a large number of Compton-suppressed clover detectors was coupled to a digital data acquisition system with 96 channels. The present system provides higher throughput, better energy resolution and better stability of gain for the multidetector Compton-suppressed clover array compared to its previous conventional system with analog shaping. Selected results from this array are discussed which highlight the exotic shapes, novel excitation modes and interesting isomers of the nuclei. The preliminary results from the experimental efforts to improve the sensitivity and capability of the array by adding ancillary detectors are also described briefly.

Neutron inelastic cross-section measurements forMg24

The $\ensuremath{\gamma}$ production cross sections from neutron inelastic scattering on ${}^{24}\mathrm{Mg}$ were measured for neutron energies up to 18 MeV at GELINA (the Geel Linear Accelerator), the neutron source operated by EC-JRC-IRMM, Belgium. The level cross section and the total inelastic cross section were determined. We used the GAINS (Gamma Array for Inelastic Neutron Scattering) spectrometer with seven large-volume high-purity germanium (HPGe) detectors placed at ${110}^{\ensuremath{\circ}}$ and ${150}^{\ensuremath{\circ}}$ with respect to the beam direction. The neutron flux was determined with a $^{235}\mathrm{U}$ fission chamber. The results are compared with calculations performed with the talys 1.6 code using the default settings.

Cross-section measurements for theFe56(n,xnγ) reactions

A measurement was performed at the white neutron source Geel Linear Accelerator (GELINA) of the Institute for Reference Materials and Measurements to determine the cross sections for the ($n$, ${n}^{\ensuremath{'}}\ensuremath{\gamma}$) and ($n$, $2{n}^{\ensuremath{'}}\ensuremath{\gamma}$) reactions on $^{56}\mathrm{Fe}$. The Gamma Array for Inelastic Neutron Scattering (GAINS) was used. The results are scaled to the neutron-induced fission cross section of ${}^{235}\mathrm{U}$. The paper emphasizes the multiple checks performed to assure the consistency of the results. $\ensuremath{\gamma}$ production cross sections, total inelastic cross sections, and level production cross sections were determined. A good agreement exists with previous measurements. Theoretical calculations were performed with the talys 1.6 reaction code using default parameters, but also using a microscopic approach. These calculations are discussed in comparison to the experimental data.

Collective excitations inS33

High spin states of $^{33}\mathrm{S}$ populated through ${}^{27}\mathrm{Al}{(}^{12}\mathrm{C},\ensuremath{\alpha}\mathrm{pn})^{33}\mathrm{S}$ reaction at E$(^{12}\mathrm{C})\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}40$ MeV have been studied using the Indian National Gamma Array (INGA) facility. The level scheme was extended and modified utilizing data from the $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence, directional correlation, and linear polarization measurements. Three levels of the negative parity yrast sequence were found to be connected by strong E2 transitions. The lifetimes of these states determined by the Doppler shift attenuation method have been utilized to study the evolution of collectivity with spin. Large basis shell model calculations have been performed to understand the microscopic origin of these levels.

Multiple magnetic rotational bands based on proton alignment inEu143

High-spin dipole structures in $^{143}\mathrm{Eu}$ have been studied via the fusion-evaporation $^{116}\mathrm{Cd}$ $(^{31}\mathrm{P}$, $4n)$ reaction at a beam energy of 148 MeV using a clover detector array (Indian National Gamma Array). The parity of the two observed dipole bands has been firmly established from linear polarization measurements. Level lifetimes of these bands have been measured using the Doppler shift attenuation method. The drop in $B(M1)$ values with spin exhibits a clear signature of the magnetic rotational character of these two bands. Configurations were assigned to the bands in the framework of the shears mechanism with the principal axis cranking model calculation. Alignment of an additional proton in the ${h}_{11/2}$ orbital gives rise to the unique observation of a large increase in the $B(M1)$ values for the higher-lying band.

Yrast structure of the shell model nucleusNb89

Yrast and near-yrast states in $\text{odd-}A$ $^{89}\mathrm{Nb}$ were investigated using the $^{65}\mathrm{Cu}(^{28}\mathrm{Si}$,$2p2n)$ reaction at a beam energy of 105 MeV. The $\ensuremath{\gamma}\text{-ray}$ coincidence events were recorded with the Indian National Gamma Array spectrometer. About 30 new transitions have been observed extending the level structure of this nucleus up to spin $45/2\ensuremath{\hbar}$ and excitation energy of 10.5 MeV. Large-scale shell model calculations were performed using the effective interactions JUN45 and jj44b to understand the structure of the observed states. Reasonable agreement between the experimental observation and shell model calculations suggests that no cross-shell excitations are important up to the maximum spin observed in the current experiment.

The Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas jet target

New radioactive ion beam (RIB) facilities will push further away from stability and enable the next generation of nuclear physics experiments. Of great importance to the future of RIB physics are scattering, transfer, and capture reaction measurements of rare, exotic, and unstable nuclei on light targets such as hydrogen and helium. These measurements require targets that are dense, highly localized, and pure. Targets must also accommodate the use of large area silicon detector arrays, high-efficiency gamma arrays, and heavy ion detector systems to efficiently measure the reaction products. To address these issues, the Jet Experiments in Nuclear Structure and Astrophysics (JENSA) Collaboration has designed, built, and characterized a supersonic gas jet target, capable of providing gas areal densities on par with commonly used solid targets within a region of a few millimeters diameter. Densities of over 5×1018atoms/cm2 of helium have been achieved, making the JENSA gas jet target the most dense helium jet achieved so far.

Sharp knee phenomenon of primary cosmic ray energy spectrum

Primary energy spectral models are tested in the energy range of 1-200 PeV using standardized extensive air shower responses from BASJE-MAS, Tibet, GAMMA and KASCADE scintillation shower arrays. Results point towards the two-component origin of observed cosmic ray energy spectra in the knee region (GAPS spectral model) consisting of a pulsar component superimposed upon rigidity-dependent power law diffuse galactic flux. The two-component energy spectral model accounts for both the sharp knee shower spectral phenomenon and observed irregularity of all-particle energy spectrum in the region of 50-100 PeV. Alternatively, tested multi-population primary energy spectra predicted by non-linear diffusive shock acceleration (DSA) models describe observed shower spectra in the knee region provided that the cutoff magnetic rigidities of accelerating particles are 6.0+/-0.3 PV and 45+/-2 PV for the first two populations respectively. Both tested spectral models confirm the predominant H-He primary nuclei origin of observed shower spectral knee. The parameters of tested energy spectra are evaluated using solutions of inverse problem on the basis of the corresponding parameterizations of energy spectra for primary H, He, O-like and Fe-like nuclei, standardized shower size spectral responses in the 550-1085 g/cm^2 atmospheric slant depth range and near vertical muon truncated size spectra detected by the GAMMA array.

The Limits of the GAINS Spectrometer

The Gamma Array for Neutron Inelastic Scattering (GAINS) is currently operating at the GELINA (Geel Linear Accelerator) neutron source producing highly precise neutron inelastic scattering data. A series of recent investigations explored the limits of the experimental setup and technique. Measurements on 12C and 57Fe attempted the determination of very high and very low energy gamma production cross sections, respectively. An extended analysis was directed towards the generation of covariance matrices specifically for this experimental approach.

Nuclear structure at high spin using multidetector gamma array and ancillary detectors

A multidetector gamma array (GDA), for studying nuclear structure was built with ancillary devices namely gamma multiplicity filter and charged particle detector array. This facility was designed for in-beam gamma spectroscopy measurements in fusion evaporation reactions at Inter-University Accelerator Centre, New Delhi. Description of the facility and in-beam performance with two experimental studies done are presented. This array was used in a number of nuclear spectroscopic and reaction investigations.