Gamma Array Research Articles

Structure of A∼130 nuclei in La-Ce region

The variety of shapes and structures, observed in light rare earth A ∼ 130 nuclei, have been discussed in view of different angular momentum coupling schemes and their interplay that comes into effect at high spin. The N = 79 and 80 isotopes in La-Ce region, produced via fusion evaporation reaction, have been studied using the Indian National Gamma Array (INGA) consisting of 18 clover HPGe detectors. Two nearly degenerate ΔI = 1 bands have been observed at high spin of 137Ce and a triaxial deformation of γ = ±30° has been assigned to the bands, from the total Routhian surface (TRS) calculations. The high-spin candidates of the yrast band of 138Ce show signature splitting both in energy and B(M1)/B(E2) values. The bandcrossing due to the alignment of a pair of h11/2 proton particles has been conjectured at ℏω ∼ 0.3 MeV, from the single-particle Routhians obtained from TRS calculations. Lifetime measurements by Doppler shift attenuation method (DSAM) has been carried out and from the estimated reduced transition probability B(M1), the ΔI = 1 band in 138Ce has been characterized as a magnetic rotation (MR) band. The rise in the values of B(M1), for the higher spin candidates of the band, has been conjectured as the reopening of a different shear at the top of the Band B1. The characteristic of the MR bands in A ∼ 130 region has been discussed in the light of a phenomenological calculation and compared to the MR bands in other mass regions.

Indian National Gamma Array at Inter University Accelerator Centre, New Delhi

A 4π multi-detector gamma-ray spectrometer named the Indian National Gamma Array (INGA) has been set up at the Inter University Accelerator Centre, New Delhi, for nuclear structure studies. The array is designed to incorporate twenty four Compton-suppressed Clover germanium detectors with a total photopeak efficiency ∼5%. The spectrometer along with sub-systems developed in-house like, mechanical support structure, high voltage power supplies, automatic liquid nitrogen filling system, front-end electronics and data acquisition system are described. The mechanical support structure facilitates the use of the Clover Germanium array with a recoil mass separator. The array has been used in a number of nuclear spectroscopic investigations. The in-beam and off-beam performance of the array are reported.

Primary cosmic ray energy spectrum in terms of the GAMMA muon data

The energy spectrum of the primary cosmic rays is obtained by conversion from the truncated muon size spectrum of EAS studied with the GAMMA array. The previously observed dependence of E 0 on N μ tr at various zenith angles is used. It is shown that the present spectrum has the same structure (“bump”) as the spectrum determined by another independent multi-parametric energy estimation method confirming that the “bump” structure is not produced by uncertainties in our methods.

Level structure of the odd–odd nucleus 54Mn

The energy levels of 54Mn were populated by a 145 MeV 20Ne beam incident on a 51V target nucleus. This, difficult to access, N = 29 nucleus was produced by projectile break-up followed by incomplete fusion of an alpha particle. The de-exciting gamma transitions were detected using a multi-clover array: Indian National Gamma Array (INGA). The level structure of 54Mn has been extended up to E* ≅ 5 MeV and Jπ = 15+ with an addition of nine new γ-transitions. The absence of any regular band-like structure in 54Mn indicates the dominance of the single-particle nature of this odd–odd nucleus. The polarization measurements indicate the possibility of a negative-parity level at 1925 keV.

Future Plans for the GAMMA Array on Mt. Aragats

The GAMMA collaboration is studying the possibility of instrumenting the 40 × 40 meter hadron calorimeter structure at the center of the GAMMA array. This mechanical structure, built about 25 years ago, has never been instrumented, and if this could be done, it would provide a valuable addition to the GAMMA array for the investigation of the EAS hadron component and for multivariate analysis of the GAMMA data.

Indication of the onset of collectivity inP30

$^{30}\mathrm{P}$ has been studied by in-beam \ensuremath{\gamma}-spectroscopy following the fusion-evaporation reaction $^{16}\mathrm{O}$($^{16}\mathrm{O}$,$\mathit{pn}$) at ${E}_{\mathrm{lab}}=40$ MeV, using the Indian National Gamma (Clover) Array (INGA) up to moderate spins ($I=5$). Polarization measurements of seven gamma rays have been performed for the first time. To understand the underlying structure of the levels and transition mechanisms, experimental data have been compared with the results from large basis cross-shell shell model calculations. The results for the negative parity states are especially important in this respect. Positive parity states indicate an onset of collectivity, whereas the negative parity states are members of \ensuremath{\nu}-\ensuremath{\pi} multiplets.

High spin structure of 35Cl and the sd– fp shell gap

The high spin states of 35Cl have been studied by in-beam γ-spectroscopy following the fusion–evaporation reaction 12C( 28 Si , α p ) 35Cl at E lab = 70 and 88 MeV, using the Indian National Gamma (Clover) Array (INGA). Lifetimes of six new excited states have been estimated for the first time. To understand the underlying structure of the levels and transition mechanisms, experimental results have been compared with those from the large basis cross-shell shell model calculations. Involvement of orbitals from fp shell and squeezing of the sd– fp shell gap seem to be essential for reliable reproduction of high spin states.

Shape changes at high spin in 78Kr

High-spin states in 78Kr have been studied via the 63Cu (19F, 2p2n)78Kr reaction at a beam energy of 60 MeV using the Indian National Gamma Array (INGA). In this nucleus, lifetimes have been measured upto the I π=22+ level in the yrast positive-parity band and upto the I π=15- level in the negative-parity band using the Doppler Shift Attenuation Method (DSAM). The deduced transition quadrupole moments Qt's are found to decrease with rotational frequency for both the bands.

STUDY OF PRIMARY PARTICLES IN THE KNEE REGION WITH THE GAMMA ARRAY

Experimental data on extensive air shower characteristics obtained with the GAMMA array at Mt. Aragats (Armenia) at Ne ≥ 105 are used to calculate the so-called αe-parameter, which is directly connected with the energy of the primary cosmic radiation. It is shown that measurement of the αe-spectrum is an effective way to obtain the primary energy spectrum. Using the αe-parameter the primary all-particle energy spectrum of the cosmic radiation in the knee energy region is obtained and compared with the corresponding data of other experiments. Energy spectrum for "young" showers generated for the most part by protons is also obtained.

High-spin structures of 51 121, 123, 125, 127Sb nuclei: Single proton and core-coupled states

The 121,123,125,127Sb nuclei have been produced as fission fragments in three reactions induced by heavy ions: 12C + 238U at 90 MeV bombarding energy, 18O + 208Pb at 85 MeV, and 31P + 176Yb at 152 MeV. Their level schemes have been built from gamma rays detected using the EUROBALL III and IV arrays. High-spin states of 123,125,127Sb nuclei have been identified for the first time. Moreover isomeric states lying around 2.3 MeV have been established in 123,125,127Sb from the delayed coincidences between the fission fragment detector SAPhIR and the gamma array. All the observed states can be described by coupling a d5/2 or g7/2 proton to an excited Sn core involving either vibrational states or broken neutron pairs.

STRUCTURE OF POLONIUM ISOTOPES AT HIGH SPIN WITH RFD + EUROBALL IV

An experiment has been performed to investigate the structure at high spin of the neutron-deficient polonium isotopes using the Recoil Filter Detector coupled to the EUROBALL IV gamma array. High spin states of 200,201 Po have been populated by the 30 Si +176 Yb reaction at a beam energy of 160 MeV. We report here the performances of the RFD + EUROBALL IV coupling.

The present and planned recoil mass spectrometers at Nuclear Science Centre, New Delhi

Recoil mass spectrometers (RMS) are ideal instruments for identifying weakly populated reaction products in the forward angle from heavy-ion reactions amidst an intense beam background. The Heavy Ion Reaction Analyzer (HIRA) at the Nuclear Science Centre (NSC), New Delhi, is one of the few operating first generation RMSs. The features of and physics studies pursued using HIRA are covered in the paper. With the augmentation of the accelerator facilities at NSC, a second generation RMS combined with a gas-filled separator is planned. The design details of the new facility, Hybrid Recoil Mass Analyzer (HYRA), are presented. HYRA will be operated either stand-alone or in conjunction with a Large Gamma Array (LGA) at its target position. Physics studies planned with these forthcoming facilities are outlined.

New results on the EAS muon component in the GAMMA experiment at Mt. Aragats

We present analysis of the data connected with study of muon characteristics with 5 GeV energy threshold for EAS with N e ϵ [3 10 5−10 7] obtained with help of the new configuration of the muon underground detector on the GAMMA array. The results are compared with previous GAMMA results as well as with data of the Tien-Shan experiment. Comparison with the simulation data is also carried out.

Calibration and simulation of a gamma array for DRAGON at ISAC

A γ ray detector has been built for the DRAGON facility at TRIUMF to detect the γ ray emitted in astrophysically important proton and α radiative capture reactions. The γ detector was designed to balance cost with maximum solid angle coverage and efficiency. To study the properties of the current design, GEANT simulations are being carried out and compared with prototype measurements using calibration sources and radioactive beams supplied by ISAC. Simulations will be compared with data allowing a realistic simulation to be produced. This modified simulation will then be used to provide efficiency predictions of the γ array when an actual experiment’s parameters are inputted. Using the simulated efficiency of the array, cross sections for radiative capture can be calculated from the measured γ ray yields, for the individual reactions. The following will outline some initial results of background suppression of beam related experiments. Also shown, are some preliminary comparison of point source data and GEANT simulations.

The cosmic ray energy spectrum around the knee measured with the GAMMA array at Mt. Aragats

Experimental data on EAS characteristics with Ne > 105 are used to calculate the so-called αe-parameter which is directly connected with the energy of the primary cosmic ray radiation. It is shown that the distribution of showers selected by a constant value of this parameter is isotropic, and the measurement of the αe-spectrum is a direct way to obtain the primary energy spectrum. Using the αe-parameter, the primary all-particle energy spectrum of the cosmic radiation in the knee region is obtained. The energy spectrum is compared with the corresponding data of other experiments.

MEGHNAD — A multi element detector array for heavy ion collision studies

In the coming decade, the expanding field of experimental nuclear physics in our country is going to see a quantum leap in research and developmental activities with new accelerator facilities like the variable energy cyclotron with ECR heavy ion source, the upcoming K-500 superconducting cyclotron, both at VECC, Calcutta, and the superconducting linac boosters at both the Pelletron Accelerator Facilities at TIFR, Mumbai and NSC, New Delhi. When heavy ion beam available from such machines fall on a target and undergo collision, very rich and often pristine fields of research open up. In order to carry on such activities, we have taken up a project to build a multi element gamma, heavy ion and neutron array of detectors (MEGHNAD) to detect and study the properties of a wide variety of particles like neutrons, protons, light mass clusters, massive ejected fragments, and gamma rays with good solid angle coverage and efficiency. Design of the detector array, performance of the prototype detector and brief outline of the research programme to be undertaken with the detector array will be discussed.

Intruder bands in 113Sn

The level structure of 113Sn has been investigated. The data were obtained by in-beam γ-ray spectroscopy using the reaction 100Mo(18O,5n)113Sn at 70 MeV performed at the OSIRIS gamma array in the Institut fur Kernphysik FN Tandem accelerator. We report in this short note the observation of two intruder type rotational bands in this nucleus.

A facility for studying radiative capture reactions induced with radioactive beams at ISAC

The measurement of low energy fusion reactions of importance to nuclear astrophysics scenarios are a prime objective of the physics program of the new ISAC facility, located at TRIUMF in Vancouver, Canada. Intense radioactive beams of 19Ne, 14,15O, 20,21Na, 17,18F and other low Z species with energies in the range of 0.15 to 1.5 MeV/mass unit will be available to measure cross sections and resonance strengths of alpha and proton induced reactions. An important component of the experimental configuration will be a new Reaction Products Detection Facility (RPDF) consisting of a windowless gas target, surrounded by a gamma array, while the recoils are separated from the intense radioactive beam using a Recoil Mass Spectrometer (RMS). The RMS will be based on a Wien filter. The recoiling reaction products will then be detected using either a Si μ-strip array or a gas filled detector. Using these devices along with coincidence requirements and time of flight conditions a background reduction factor of the order of 10 +15 is the present goal.

A facility for studying radiative capture reactions induced with radioactive beams at ISAC

The measurement of low energy fusion reactions of importance to nuclear astrophysics scenarios are a prime objective of the physics program of the new ISAC facility, located at TRIUMF in Vancouver, Canada. Intense radioactive beams of 19Ne, 14,15O, 20,21Na, 17,18F and other low Z species with energies in the range of 0.15 to 1.5 MeV/mass unit will be available to measure cross sections and resonance strengths of alpha and proton induced reactions. An important component of the experimental configuration will be a new Recoil Product Detection Facility (RPDF) consisting of a windowless gas target, surrounded by a gamma array, while the recoils are separated from the intense radioactive beam using a Electro-Magnetic Separator (EMS) employing Wien filters. The recoiling reaction products will then be detected using either a Si μ-strip array or a gas filled detector. Using these devices along with coincidence requirements and time-of-flight conditions a background reduction factor of the order of 10 +15 is the present goal.

Experimental check of the new method of selection of EAS with constant energies at observation level 700 g/cm2

The experimental data obtained in the new EAS experiment Gamma at Mt. Aragats (Armenia) allows to select showers generated by primaries with different masses but with the same primary energy. Such a selection gives the possibility to estimate the primary mass composition for given energies by much more simple and precise way. In this work, the recent experimental data obtained with the help of Gamma array are compared with the Tien Shan results. An excellent agreement is shown, which confirms our Monte Carlo calculations, carried out for EAS muon and electrons components.