Gamma Array Research Articles

Conceptual design and simulated performance of the Barrel part of gamma array for the experimental terminal at HFRS of HIAF

The Barrel of γ array, consisting of 640 CsI(Tl) crystals and covering a polar angle range from 36.4° to 135.6°, has been designed for the experimental terminals at High energy FRagment Separator of High-Intensity heavy-ion Accelerator Facility in Huizhou. Together with the existing Endcap, consisting of 1024 CsI(Tl) crystals and covering a polar angle from 15.6° to 36.4°, at the External Target Facility in Lanzhou to form a new γ array, it is expected to meet the experimental requirements for in-beam γ-ray spectroscopy technique. A Monte Carlo code based on Geant4 was implemented to study the performance of the new γ array. Taking into account the Doppler correction and the intrinsic energy resolution of the detector, the energy resolution is 5.4% (FWHM) for the Endcap and 9.8% (FWHM) for the Barrel when the energy of the emitted γ-ray is 1 MeV in the Center-of-Mass frame with the beam energy of 500 MeV/u (beam velocity β = 0.759). The full-space photopeak efficiency is greater than 40% when the energy of the emitted γ-ray is less than 5 MeV in the Center-of-Mass frame and the beam energy is less than 700 MeV/u (β = 0.821). Such performance can satisfy most of the physics requirements for in-beam γ-ray spectroscopy experiments.

Nucleon-nucleon correlations probed in sub-barrier transfer reactions and the nuclear Josephson effect

In recent years a series of near- and sub-barrier transfer experiments have been carried out at LNL, with reaction products detected in inverse kinematics and at forward angles with the large solid angle magnetic spectrometer PRISMA. Nucleon-nucleon correlation properties have been studied by measuring transfer cross sections far below the Coulomb barrier, making excitation functions down to very low energies and corresponding to very large distances of closest approach where the nuclear absorption is small. Such kind of studies are of general interest since one probes the tails of the density distributions. One subtle case is the possible manifestation of a nuclear Josephson effect. Predictions have been made of a specific gamma strength function associated with the dipole oscillations generated by the, mainly successive, two neutron transfer process. The coupling of PRISMA to the AGATA gamma array, recently installed at LNL, offered a unique opportunity to study such an effect. In a very recent experiment we directly tested for the first time the possible manifestation of Cooper pair oscillations, observed to date only in condensed matter physics.

The structure of low-lying 1− states in 90,94Zr from (α,α′γ) and (p,p′γ) reactions

The low-lying dipole strength was investigated in 90,94 Zr via the (p,p′γ) reaction at 80 MeV and via the (α,α′γ) reaction at 130 MeV. These experiments were performed at the Research Center for Nuclear Physics (Osaka University) and made combined use of the high-resolution magnetic spectrometer Grand Raiden and the CAGRA HPGe gamma array. The comparison of our results with existing (γ,γ′) reaction data shows differences in the excitation patterns. In the present experimental conditions, in fact, both α and p probes are exciting the investigated 1− states mainly through the short-range nuclear force, expected to favour the population of surface excitations. Distorted Wave Born Approximation (DWBA) calculations were made using form factors built assuming specific transition densities, characterized by a strong neutron component at the nuclear surface and based on Random Phase Approximation (RPA). A combined analysis of the (α,α′γ) and (p,p′γ) data was performed to investigate the isoscalar character of the 1− states in 90,94Zr. Although an overall general consistency was obtained in the studied energy intervals, not all (p,p′γ) cross sections were well reproduced using the isoscalar strength deduced from (α,α′γ) data. This might suggest that the mixing of isoscalar and isovector components and the degree of collectivity are not the same for all the 1− states studied.

Indications of octupole correlation in 84Sr

Excited states in the 84Sr nucleus were investigated via the 76Ge(12C,4n)84Sr reaction at a beam energy of 58 MeV. The de-excited γ-rays were detected using the Indian National Gamma Array (INGA) spectrometer at Inter-University Accelerator Center, New Delhi. Directional Correlation from Oriented (DCO) states ratio and the polarization asymmetry (Δ) measurements were done to confirm the spin-parity of the low-lying states. Eight new γ-ray transitions were placed in the level scheme of 84Sr. The systematic behaviour of energy staggering S(I) of the γ-band (Band 1 and Band 2) was compared with the γ-bands in the mass A ≈ 80 region and the nuclei of other mass regions with similar behaviour (odd-I down). The E1 character is confirmed for strong γ-ray transitions connecting Band 3 to the Yrast band. Comparison of new results such as B(E1)/B(E2) ratio, frequency ratio ω −/ω + and energy displacement ΔE in 84Sr with those of 72Se, 150Sm, 152Gd, 220Ra and 224Th nuclei suggests the presence of octupole correlations in 84Sr.

Neutron-induced inelastic γ-production cross sections on 58,60,64Ni

This paper reports partial results of a (n, n’γ) measurement on nickel. The inelastic channel was measured using the Gamma Array for Inelastic Neutron Scattering (GAINS) spectrometer at the 100-m measurement cabin of the Geel Electron Linear Accelerator (GELINA) neutron source of the European Commission’s Joint Research Centre (EC-JRC) in Geel, Belgium. Using γ spectroscopy, we were able to extract angle-integrated production cross sections for several γ rays but we report here only the results for the main transition in 58Ni. We discuss however in detail the observed discrepancy between our data and other experiments (especially the work of Voss et al.). We also shortly comment on the quality of the neutron-target optical model potential in describing the inelastic data in this mass region. The calculations were performed using the talys 1.9 code in the default settings.

A new measurement on 56Fe(n,inl) using GAINS@GELINA

The extended dataset of 56Fe(n,n’γ) cross sections measured by our group more than a decade ago at GELINA (Geel Linear Accelerator) was used in many recent evaluations like ENDF, JEFF and CIELO. Despite the special measures we took to ensure reliability and accuracy, concerns were raised by various groups with regard to several features of this dataset (absolute normalization and/or shape) and therefore the 56Fe(n,inl) cross section is still under the evaluation by the International Nuclear Data Evaluation Network (INDEN). Consequently, a new experiment is now under preparation aiming to take advantage of the numerous experimental improvements of the GAINS (Gamma Array for Inelastic Neutron Scattering) setup implemented over the years. While γ spectroscopy combined with the time-of-flight method will remain the main technique involved, several other experimental details will differ substantially.

The past and the future of the GAINS spectrometer @ GELINA

GAINS (Gamma Array for Inelastic Neutron Scattering), currently installed at the GELINA (Geel Electron Linear Accelerator) neutron source of the EC-JRC (European Commission-Joint Research Centre), is one of the best instruments available worldwide for measuring high resolution, low uncertainty neutron inelastic cross-section data relevant for both fundamental research and nuclear physics applications (like Generation IV facilities). It features 12 HPGe detectors coupled with a 235U fission chamber for neutron flux monitoring. The paper presents a short history of the GAINS development over the years and reviews some of the main experimental results together with plans for future experiments.

Evidence for prolate-oblate shape coexistence in the odd- ABr383573 nucleus

The excited states in $^{73}\mathrm{Br}$ nucleus have been investigated through the fusion evaporation reaction $^{50}\mathrm{Cr}(^{28}\mathrm{Si}$, $\ensuremath{\alpha}p)^{73}\mathrm{Br}$ at a beam energy of 90 MeV using the Indian National Gamma Array. The $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence technique has been used to add eight new $\ensuremath{\gamma}$-ray transitions in the level scheme. The mixing ratio of $\mathrm{\ensuremath{\Delta}}I=0$ (mixed with $E2$ and $M1$) transitions have been determined using angular distribution and ${R}_{\mathrm{DCO}}$-polarization measurement. The half-life of the ${9/2}^{+}$ isomeric state has been measured to be ${\ensuremath{\tau}}_{1/2}=52(2)$ ns from the variation in the intensity of delayed $\ensuremath{\gamma}$-ray transition as a function of coincidence time window. The two state mixing model calculations were performed to obtain the mixing amplitude, and mixing interaction of two different configurations of $^{73}\mathrm{Br}$. The calculated mixing amplitudes along with the deformations of two different configurations provide the monopole transition strength ${\ensuremath{\rho}}^{2}(E0)$ for Se, Br, and Kr isotopes in a semiempirical approach. These results support a prolate-oblate shape coexistence in the odd-$A\phantom{\rule{4pt}{0ex}}^{73}\mathrm{Br}$ nucleus. The observed structural properties have been discussed in terms of projected shell model calculations.

Experimental evidences of shape co-existence in 154Ho

The excited states of odd-odd 154Ho nucleus have been studied by the 141Pr(16O, 3n)154Ho reaction at Elab= 82 MeV utilizing the Indian National Gamma Array (INGA) setup at BARC-TIFR Pelletron facility, Tata Institute of Fundamental Research, Mumbai. Data from γ−γ coincidence, directional correlation (DCO), and polarization measurements have been analyzed to assign and confirm the spins and parities of the levels. The reported level scheme of 154Ho has been improved by including new levels and transitions. Mixing ratios for the mixed transitions extracted from DCO ratios and parity assignments from polarisation data are reported for the first time. The low-lying states in H154o, which are mostly collective, characterized by regularity in the excitation spectra, are proposed to be arising from neutron excitation. At higher excitation, there is a transition from collective to single-particle character, accompanied by the appearance of irregular and complex structure caused by proton excitation. The newly identified E1 gamma transitions connecting the low-lying positive and negative parity bands indicate octupole correlation in 154Ho. These new connections also support the phenomenological analysis to interpret the observed experimental features from a different perspective.

Structural evolution and K mixing in V49

Lifetimes of a few negative parity levels and multipole mixing ratios ($\ensuremath{\delta}$) for a few dipole transitions in $^{49}\mathrm{V}$, populated through the $^{48}\mathrm{Ti}(^{4}\mathrm{He},2np)^{49}\mathrm{V}$ reaction with a 48 MeV $^{4}\mathrm{He}$ beam, were measured. The Indian National Gamma Array (INGA) facility was used to detect $\ensuremath{\gamma}$ rays at three different angles. Large basis shell model calculations were performed to understand the microscopic origin of these levels and to interpret the observations. $K$ mixing between different $K$ bands in $^{49}\mathrm{V}$ were established from one-nucleon-transfer spectroscopic factor calculations and total Routhian surface (TRS) calculations.

Different manifestations of triaxial shapes of the positive and negative parity bands in Os187

The excited states of $^{187}\mathrm{Os}$ have been studied via $^{186}\mathrm{W}(^{4}\mathrm{He},3n)^{187}\mathrm{Os}$ reaction at a beam energy of 36 MeV. The $\ensuremath{\gamma}$ rays were detected using the Indian National Gamma Array at the Variable Energy Cyclotron Centre having seven Compton-suppressed clover high-purity germanium (HPGe) detectors and one low-energy photon spectrometer (LEPS) detector with a digital data acquisition system. The level scheme of $^{187}\mathrm{Os}$ has been extended substantially up to $\ensuremath{\approx}$3.86 MeV of excitation energy and $37/2\phantom{\rule{0.16em}{0ex}}\ensuremath{\hbar}$ of spin with the placement of more than 90 new $\ensuremath{\gamma}$ rays. All known bands have been extended and new band structures have been identified. The results show evidence of triaxial shapes for different configurations of $^{187}\mathrm{Os}$ and different manifestations of nonaxial shape have been observed in the same nucleus. A comparison of the observed band crossing frequency in $^{187}\mathrm{Os}$ with neighboring nuclei gives evidence of a deformed shell gap at $N=110$. The experimental results are well explained using total Routhian surface calculations.

Alignment effects in the medium-spin level structure of Se78

The excited states of $^{78}\mathrm{Se}$ were populated using the $^{76}\mathrm{Ge}(\ensuremath{\alpha},2n)$ fusion evaporation reaction. The deexcited gamma rays were detected using the Indian National Gamma Array (INGA) spectrometer comprising clover and low-energy photon spectrometer (LEPS) detectors. The ground state band of $^{78}\mathrm{Se}$ has been extended up to ${E}_{x}$ = 8.5 MeV and $J$ = $16\ensuremath{\hbar}$; this excitation regime is very close to the $\ensuremath{\pi}(1{g}_{9/2}^{2})$ crossing frequency. Several newly observed in-band and intraband transitions belonging to the other bands of $^{78}\mathrm{Se}$ have also been placed in the level scheme based on their coincidence relationships. From the newly identified $E1$ transitions, an enhancement in the $B(E1)$/$B(E2)$ values with spin has been observed which indicates the onset of octupole correlations in the band structure built on the ${3}^{\ensuremath{-}}$, 2508-keV state. Persistence of triaxial shape up to the highest observed spin of the ground state band has been observed and the feature of successive $1{g}_{9/2}^{2}$ quasiparticle alignments due to protons and neutrons has been unveiled. The consequences arising due to the occurrence of the neutron $1{g}_{9/2}$ quasiparticle alignments in the other observed correlated band structures are also presented. The results are interpreted in terms of total Routhian surface (TRS) calculations and the level structure systematic of the neighboring nuclei.

Shape coexistence and octupole correlations in Se72

In the present paper, we report the results from the study of excited states in the $^{72}\mathrm{Se}$ nucleus using the $^{50}\mathrm{Cr}(^{28}\mathrm{Si}, \ensuremath{\alpha}2p)^{72}\mathrm{Se}$ fusion evaporation reaction at a beam energy of 90 MeV. The deexciting $\ensuremath{\gamma}$ rays were detected using the Indian National Gamma Array (INGA). A total of ten new $\ensuremath{\gamma}$-ray transitions have been identified using the $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence technique. The ${K}^{\ensuremath{\pi}}={0}_{2}^{+}$ band based on an isomeric state has been extended up to a (${10}^{+}$) state at 5.473 MeV excitation energy, and four new interconnecting transitions have been placed between this band and yrast band. Further, the enhanced interconnecting $E1$ transitions between positive- and negative-parity bands suggest the existence of octupole correlations in this nucleus. The characteristics of the observed bands in the experiment have been interpreted in terms of the total Routhian surface (TRS) calculations.

Three-phonon multiplets in 116Sn

Low-lying states in the even-even 116Sn nucleus have been studied by populating them using the 114Cd(α, 2n)116Sn reaction at the beam energy of 34 MeV. γγ coincidences, directional correlation ratio, and polarization asymmetry measurements were performed using the Indian National Gamma Array (INGA) with a digital data acquisition system. Three-phonon multiplets in the 116Sn nucleus have been investigated. The experimental energy levels are analyzed using theoretical approaches of the interacting boson model (IBM) in the U(5) limit and large-scale shell-model calculations using two different model spaces.

Collective and noncollective states in Zn66

Excited states in the $^{66}\mathrm{Zn}$ nucleus were populated via a $^{56}\mathrm{Fe}(^{12}\mathrm{C},2p\ensuremath{\gamma}$) fusion-evaporation reaction at a beam energy of $\ensuremath{\approx}62$ MeV. The deexciting $\ensuremath{\gamma}$ rays were detected using the Indian National Gamma Array (INGA). The level scheme of the $^{66}\mathrm{Zn}$ nucleus has been updated by placing several new $\ensuremath{\gamma}$ rays as well as by assigning the spin and parity of various excited states from the present spectroscopic results. The microscopic structure of the observed states have been investigated in the light of large shell-model calculations. The shape of this nucleus in the low-spin regime has been studied under the framework of total Routhian surface (TRS) calculations. The lifetime of first ${3}^{\ensuremath{-}}$ state at 2826 keV is experimentally measured using the Doppler-shift attenuation method and the deduced $B(E1)$ value indicates the presence of octupole collectivity in this nucleus.

Evidence of antimagnetic rotational motion in Pd103

Lifetime measurements have been carried out for the levels of the negative parity yrast sequence in $^{103}\mathrm{Pd}$ nucleus using the Doppler shift attenuation method. The levels were populated via $^{94}\mathrm{Zr}(^{13}\mathrm{C}, 4n\ensuremath{\gamma})^{103}\mathrm{Pd}$ fusion-evaporation reaction at a beam energy of 55 MeV. De-exciting $\ensuremath{\gamma}$ rays were detected by utilizing the Indian National Gamma Array. The extracted transition probabilities and other auxiliary observations indicate that the sequence may be resulting from the antimagnetic rotational (AMR) motion of valence nucleons. The key characteristic feature of the AMR motion is the steady decrease of the $B(E2)$ transition probability with spin, which is seen in the present measured transitions for $^{103}\mathrm{Pd}$. The experimental results are compared with the theoretical predictions of tilted axis cranked approach based on the covariant density functional theory. It is noted that the properties of the AMR band structure for $^{103}\mathrm{Pd}$ predicted in this model analysis are in good agreement with the present experimental findings. Further, semi-classical particle-rotor model has been employed to substantiate the AMR interpretation of the observed band structure in $^{103}\mathrm{Pd}$ and it is shown that results are similar to the band structures observed in the neighboring isotopes, which have also been considered as candidates for AMR motion.

Onset of triaxial deformation in Zn66 and properties of its first excited 0+ state studied by means of Coulomb excitation

The electromagnetic structure of Zn66 at low excitation energy was investigated via low-energy Coulomb excitation at INFN Legnaro National Laboratories, using the Gamma Array of Legnaro Infn Laboratories for nuclEar spectrOscopy (GALILEO) γ-ray spectrometer coupled to the SPIDER (Silicon PIe DEtectoR). A set of reduced E2, E3, and M1 matrix elements was extracted from the collected data using the gosia code, yielding 12 reduced transition probabilities between the low-spin states and the spectroscopic quadrupole moment of the 21+ state. The B(E2) values for transitions depopulating the 02+ state have been determined for the first time, allowing for the lifetime of this state to be deduced and, consequently, the ρ2(E0;02+→01+) monopole transition strength to be extracted. In addition, the B(E3;31−→01+) value has been determined for the first time in a Coulomb excitation experiment. The obtained results resolve the existing discrepancies between literature lifetimes and demonstrate that Zn66 cannot be described by using simple collective models. Therefore, new state-of-the-art beyond-mean-field and large-scale shell-model calculations were performed in order to interpret the structure of this nucleus. Both the experimental and theoretical results suggest that the triaxial degree of freedom has an important impact on electromagnetic properties of Zn66, while the unique features of the 02+ state indicate its distinct and rather isolated structure.

Evolution of collectivity and shape transition in Zn66

Excited states in $^{66}\mathrm{Zn}$ were investigated through the in-beam $\ensuremath{\gamma}$-ray spectroscopic techniques using the $^{52}\mathrm{Cr}(^{18}\mathrm{O}, 2p2n$) fusion-evaporation reaction at a beam energy of 72.5 MeV. The $\ensuremath{\gamma}$-rays emitted by the de-exciting nuclei were recorded in coincidence mode using the 14 Compton suppressed Ge clover detectors of the Indian National Gamma Array. With 14 new transitions being identified, the level scheme of $^{66}\mathrm{Zn}$ has been extended up to the excitation energy $\ensuremath{\approx}12.3\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ and spin $\ensuremath{\approx}17\phantom{\rule{0.28em}{0ex}}\ensuremath{\hbar}$. A rotational band, associated with the two quasineutrons from the $1{g}_{9/2}$ orbital, has been found to exhibit a band crossing with the ground-state band at a spin of $6\ensuremath{\hbar}$. The evolution of the collectivity and shape transition in this nucleus have been discussed in the framework of the total Routhian surface calculations and in comparison with the neighboring $^{68,70}\mathrm{Ge}$ nuclei.

Interplay between single particle and collective excitation in 49V

High spin states of 49V populated through 48Ti(4He, 2np)49V reaction with 48 MeV 4He beam, have been studied using the Indian National Gamma Array (INGA) facility. The relative intensities, RDCO and polarization measurements have been carried out for a few transitions in 49V. The level lifetimes have been extracted for a few negative parity yrast levels using Doppler shift attenuation method. Large basis shell model calculations have been performed to understand the microscopic structure of these levels.

The MUGAST campaign at GANIL

The paper describes the MUGAST campaign at GANIL designed for direct reaction measurement. The physics cases addressed in this campaign deal with shell model evolution, nuclear astrophysics and unbound states. The set-up consist of state-of-art stripped silicon array, MLTGAST coupled with the large acceptance spectrometer, VAMOS and with the state-of-the-art gamma array AGATA. It also accommodates cryogenic targets and specific targets. The challenges of the measurements performed are described.