arXiv subjects

H. Pai

Publications and source records attributed to H. Pai.

At least 19 recordsLinked to original sources

arXiv2026

A detailed view at magnetic dipole strengths: The case of semi-magic $^{50}$Ti

Magnetic dipole, $M1$, strengths were studied in semi-magic $^{50}$Ti up to the neutron-separation threshold by combining data from $(d,p)$ one-neutron transfer, $(\gamma,\gamma')$ real-photon scattering, $(e,e')$ inelastic scattering at extreme backward angles, and $(p,p')$ at $E_p = 210$ MeV and extreme forward angles. The combination of all probes provided unique access to the neutron spin-flip contribution and the possibility to evaluate its role in generating the spin-flip $M1$ strengths. The small contribution of the neutron $(1f_{7/2})^{-1}(1f_{5/2})^{+1}$ spin-flip transitions, which were probed with the $(d,p)$ reaction, to the overall strength in $^{50}$Ti questions the standard picture for the microscopic origin of spin-flip strength in the $fp$ shell. For $^{50}$Ti, this letter shows that $J^{\pi} = 1^+$ states with larger neutron $(1f_{7/2})^{-1}(1f_{5/2})^{+1}$ spectroscopic factors do not correspond to the ones with the largest $B(M1;0^+_1 \rightarrow 1^+_i)$ strengths.

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arXiv2025

Precise measurement of the $\gamma$-decay probability of the Hoyle state with a new triple coincidence-detection method

We measured the $\gamma$-decay probability of the Hoyle state with a new method of triple coincidence detection of a scattered $\alpha$ particle, a recoil $\rm ^{12}C$ nucleus, and a $\gamma$ ray in inelastic alpha scattering on $\rm ^{12}C$. This method successfully enabled a low-background measurement and a precise determination of the $\gamma$-decay probability of the Hoyle state as $\Gamma_\mathrm{\gamma}/\Gamma=[4.00 \pm 0.22 \mathrm{(sta.)} \pm 0.18 \mathrm{(sys.)}]\times10^{-4}$, which is consistent with the previous literature value. Therefore, we concluded that the literature value can be reliably used in the study of nucleosynthesis in the universe.

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arXiv2025

Nuclear level density of ${}^{128}$Te from $(\mathrm{p},\mathrm{p}'\gamma)$ scattering and complementary photonuclear data

We have extracted the nuclear level density of ${}^{128}$Te from a $(\mathrm{p},\mathrm{p} '\gamma)$ scattering experiment using the large-volume \labr\ and \cebr\ detectors from ELI-NP at the 9~MV Tandem facilities at IFIN-HH. The decay data were normalised using photonuclear data, resulting in nuclear level densities without intrinsic model dependencies from the constant temperature or Fermi gas models. The deduced nuclear level density follows in between the expectations from these two models, but we observe a clear divergence from a microscopic model based on the Skyrme force.

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arXiv2024

Statistical properties and photon strength functions of the ${}^{112,114}$Sn isotopes below the neutron separation threshold

Here, we report on the measurements of the $\gamma$-ray strength functions and nuclear level densities of ${}^{112,114}$Sn performed for the first time at the 9~MV Tandem accelerator facilities at IFIN-HH using the Oslo method. We extract thermodynamic properties and gross and fine properties of the pygmy dipole resonance for systematic comparison in the chain of Sn isotopes. The results are compared with microscopic models implemented in the TALYS reaction code and the fully microscopic quasiparticle-phonon model for the underlying nuclear structure of the dipole strength in ${}^{112,114}$Sn. The quasiparticle-phonon model results show the importance of complex configurations to the low-energy dipole response in the pygmy dipole resonance energy region. The experimental data are further included in the cross-section and reaction rate calculations for the $(\mathrm{n},\gamma)$ reaction of the $p$-process nuclei ${}^{112,114}$Sn showing a significant increase in reaction rates at high temperatures compared to existing nuclear databases.

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arXiv2024

Existance of octupole correlation in 116Sn

The negative parity states in 116Sn have been investigated in terms of octupole correlation. The same is probed by using the Indian National Gamma Array (INGA) facility at Variable Energy Cyclotron Centre, Kolkata using the reaction, 114Cd({\alpha},2n) 116Sn at 34 MeV energy. Three new {\gamma}-transitions relevant to the present investigation are reported and the spin-parities of the associated levels are assigned based on the DCO-ratios and polarisation measurements. The interband transitions between the positive parity ground band and the negative parity band are newly observed and the corresponding extracted ratio of transition probability, B(E1)/B(E2) indicated the existence of octupole correlation in these nuclei. The enhanced transition rates for E1 and E3 transitions between these opposite parity bands and corroboration of soft octupole deformation ultimately aid the onset of octupole excitation in this isotope.

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arXiv2023

Search for the origin of wobbling motion in the $ A \approx 130 $ region: The case of $^{131}$Xe

In-beam $ \gamma $-ray spectroscopy of $^{131}$Xe has been carried out to study the structure of the intruder $ \nu h_{11/2} $ band. Excited states were populated via an $ \alpha $-induced fusion-evaporation reaction at E$ _{\alpha} = 38 $ MeV. Inspection of $ \gamma \gamma $-coincidence data resulted in the identification of a new rotational sequence. Based on the systematics of excitation energy, assigned spin-parity, decay pattern, and the electromagnetic character of the inter-band $ \Delta I = 1 $ $ \gamma $-transitions, this sequence is proposed as the unfavoured signature partner of the $ \nu h_{11/2} $ band. The structure of this band is further illuminated in the light of the triaxial particle rotor model (TPRM). The possibility of wobbling excitation in $ N = 77 $ Xe-Ba-Ce isotones has been explored in a systematic manner.

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arXiv2023

Rotation of pear-shaped $^{100}$Ru nucleus

Atomic nuclei in general can have deformed shapes and nearly all these shapes are symmetric with respect to reflection. Only a few Actinide nuclei have stable reflection asymmetric pear shapes in their ground state and exhibit characteristic rotational bands. In this article, we report on the observation of two alternate parity rotational bands in 100Ru, which are connected by seven interleaved electric dipole transitions and their rates are found to be enhanced. In addition, the moments of inertia associated with these two opposite parity rotational bands have been found to be similar. These experimental observations indicate the rotation of a stable pear-shaped 100Ru nucleus, which is the first such observation outside the Actinide mass region. This shape is built on an excited configuration and originates from the rotational alignment of the angular momenta of a pair of neutrons. This unique observation establishes an alternate mechanism by which an atomic nucleus can assume a pear shape.

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arXiv2022

A fast-rotating pear-shaped nucleus

The spectroscopic studies have identified few Actinide and Lanthanide nuclei of the periodic table, which can assume the pear shape. The low frequency collective rotation of these nuclei has been established by determining the band structure of the excited levels and their gamma decay rates. In this article, we report the rotation of a pear-shaped nucleus in 100 Ru, which rotates nearly three times faster than the previously known cases. The three novel consequences of this fast rotation are: the realization of a pear-shape in an excited state, its moment of inertia becoming a constant of motion and its shape evolution. These inferences have been arrived at by comparing the characteristics of the rotational band of 100 Ru with three of the best-known examples of a rotating pear-shaped nucleus and through the theoretical interpretation of the data. The observation of a pear-shaped nucleus in the lighter mass region opens up the possibility of a systematic study of the effects of fast rotation on this shape.

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arXiv2022

Collective enhancement in nuclear level density of $^{72}$Ga and its effect on $^{71}$Ga(n, $\gamma$)$^{72}$Ga capture cross-section

The $\gamma$-gated proton spectra measured in the reactions $^{64}$Ni($^{9}$Be, p2n)$^{70}$Ga and $^{64}$Ni($^{9}$Be, pn)$^{71}$Ga, have been utilized to obtain the nuclear level density (NLD) of $^{71}$Ga and $^{72}$Ga nuclei by using the statistical model (SM) calculations. It is seen that the $\gamma$-gated proton spectrum are reasonably explained by using the large value of the inverse level density parameter ($k$ = 11.2 MeV) in the NLD prescription of the Fermi gas (FG) model. The large value of $k$ is indicative of the rotational enhancement, which is consistent with the earlier results in other mass regions. Furthermore, a rotational enhancement factor has been included in the NLD and used in the SM calculation keeping the systematic value of $k$=8.6 MeV and it explains the $\gamma$-gated proton spectrum nicely. The result clearly indicates the presence of collective enhancement in NLD. Subsequently, the NLD with collective enhancement has been utilized in the TALYS calculation, for the first time, to calculate the $^{71}$Ga(n, $\gamma$)$^{72}$Ga capture cross-section. It is observed that, while the FG model without the collective enhancement in the NLD for $^{72}$Ga under predicts the capture data, with the rotational enhancement correction the FG model over predicts the data by similar amount at higher energies. However, in the energy range of 0.01 MeV to 0.1 MeV, the FG model corrected for rotational enhancement describes the data quite well. Thus, the present work indicates that collective enhancement, whenever required, should be taken into account fro proper description of low energy capture cross section data.

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arXiv2022

Shape evolution in the rapidly rotating $^{140}$Gd nucleus

Ground state band of $^{140}$Gd has been investigated following their population in the $^{112}$Sn($^{35}$Cl,~$\alpha$p2n)$^{140}$Gd reaction at 195 MeV of beam energy using a large array of Compton suppressed HPGe clovers as the detection setup. Apart from other spectroscopic measurements, level lifetimes of the states have been extracted using the Doppler Shift Attenuation Method. Extracted quadrupole moment along with the pairing independent cranked Nilsson-Strutinsky model calculations for the quadrupole band reveal that the nucleus preferably attains triaxiality with $\gamma$ = -30$^\circ$. The calculation though shows a slight possibility of rotation around the longest possible principal axis at high spin $\sim$ 30$\hbar$ which is beyond the scope of the present experiment.

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arXiv2021

Preparation of Isotopically enriched $^{112,116,120,124}$Sn targets at VECC

Resistive heating and mechanical rolling methods have been employed to prepare isotopically enriched thin target foils of 116Sn (~380 μg/cm2), 124Sn(~400 μg/cm2) and thicker foils of 112Sn (1.7 mg/cm2),120Sn (1.6 mg/cm2),respectively. Preparation of enriched targets with small amount of material, selection of releasing agent for thin targets and separation of deposited material insolvent were among the several challenges while fabrication of the thin targets. Uniformity of the targets has been measured using 241Am α-source. NaCl has been used as releasing agent in preparation of the thin targets. These targets have been successfully used in nuclear physics experiments at VECC.

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arXiv2020

First Observation of Multiple Transverse Wobbling Bands of Different Kinds in $^{183}$Au

We report the first observation of two wobbling bands in $^{183}$Au, both of which were interpreted as the transverse wobbling (TW) band but with different behavior of their wobbling energies as a function of spin. It increases (decreases) with spin for the positive (negative) parity configuration. The crucial evidence for the wobbling nature of the bands, dominance of the $E2$ component in the $ΔI = 1$ transitions between the partner bands, is provided by the simultaneous measurements of directional correlation from the oriented states (DCO) ratio and the linear polarization of the $γ$ rays. Particle rotor model calculations with triaxial deformation reproduce the experimental data well. A value of spin, $I_m$, has been determined for the observed TW bands below which the wobbling energy increases and above which it decreases with spin. The nucleus $^{183}$Au is, so far, the only nucleus in which both the increasing and the decreasing parts are observed and thus gives the experimental evidence of the complete transverse wobbling phenomenon.

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arXiv2020

Structure of high-lying levels populated in the $^{96}$Y $\rightarrow ^{96}$Zr $β$ decay

The nature of $J^π=1^-$ levels of $^{96}$Zr below the $β$-decay $Q_β$ value of $^{96}$Y has been investigated in high-resolution $γ$-ray spectroscopy following the $β$ decay as well as in a campaign of inelastic photon scattering experiments. Branching ratios extracted from $β$ decay allow the absolute $E1$ excitation strength to be determined for levels populated in both reactions. The combined data represents a comprehensive approach to the wavefunction of $1^-$ levels below the $Q_β$ value, which are investigated in the theoretical approach of the Quasiparticle Phonon Model. This study clarifies the nuclear structure properties associated with the enhanced population of high-lying levels in the $^{96}$Y$_{gs}$ $β$ decay, one of the three most important contributors to the high-energy reactor antineutrino spectrum.

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arXiv2020

Nuclear level density of $^{69}$Zn from gamma gated particle spectrum and its implication on $^{68}$Zn(n, $γ$)$^{69}$Zn capture cross-section

Evaporated $α$-spectra have been measured in coincidence with low energy discrete $γ$-rays from residual nucleus $^{68}$Zn populated in the reaction $^{64}$Ni($^9$Be,$α$n)$^{68}$Zn at $E(^9$Be) = 30 MeV producing $^{73}$Ge compound nucleus. Low energy $γ$-gated $α$-particle spectra, for the first time, have been used to extract the nuclear level density (NLD) for the intermediate $^{69}$Zn nucleus in the excitation energy range of E $\approx$ 5-20 MeV. The slope of NLD as a function of excitation energy for $^{69}$Zn matches nicely with the slope determined from RIPL estimates for NLD at low energies and the NLD from neutron resonance data. Extracted inverse NLD parameter (k = A/$\widetilde{a}$) has been used to determine the nuclear level density parameter value $a$ at neutron separation energy $S_n$ for $^{69}$Zn. Total cross-section of $^{68}$Zn(n,$γ$) capture reaction as a function of neutron energy is then estimated employing the derived $a(S_n)$ in the reaction code TALYS. It is found that the estimated neutron capture cross-section agrees well with the available experimental data without any normalization. The present result indicates that experimentally derived nuclear level density parameter can constrain the statistical model description of astrophysical capture cross-section and optimize the uncertainties associated with the astrophysical reaction rate

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arXiv2020

Quasi-$γ$ band in $^{\text{114}}$Te

The low-lying non-yrast states in $^{114}$Te have been investigated using the Indian National Gamma Array through the fusion-evaporation reaction $^{112}$Sn($^{4}$He, 2n) at a beam energy of 37 MeV. Eight new $γ$-transitions have been placed in the level scheme to establish the quasi-$γ$ band in this nucleus. Spin and parity of several excited states have been assigned from the present spectroscopy measurements. The comparison of experimental results on the observed bands with the Interacting Boson Model (IBM) and Triaxial Projected Shell Model (TPSM) confirming the existence of the quasi-$γ$ band structure in the $^{114}$Te nucleus.

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arXiv2018

Extremely asymmetric shears band in 143Sm

A dipole sequence has been observed and investigated in the 143 Sm nucleus populated through the heavy-ion induced fusion-evaporation reaction and studied using the Indian National Gamma Array (INGA) as the detection system. The sequence has been established as a Magnetic Rotation (MR) band primarily from lifetime measurements of the band members using the Doppler Shift Attenuation Method (DSAM). A configuration based on nine quasiparticles, with highly asymmetric angular momentum blades, has been assigned to the shears band in the light of the theoretical calculations within the framework of Shears mechanism with the Principal Axis Cranking (SPAC) model. This is hitherto the maximum number of quasiparticles along with the highest asymmetricity associated with a MR band. Further, as it has followed from the SPAC calculations, the contribution of the core rotation to the angular momentum of this shears band is substantial and greater than in any other similar sequence, at least in the neighbouring nuclei. This band can thus be perceived as a unique phenomenon of shears mechanism in operation at the limits of quasiparticle excitations, as manifested in MR band-like phenomena, evolving into collectivity.

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arXiv2017

Abrupt phase change of the core rotation in the 143 Sm nucleus

Dipole sequences in the 143 Sm nucleus have been investigated via the 124 Sn ( 24 Mg, 5n) reaction at E lab = 107 MeV using the Indian National Gamma Array (INGA). The spin-parity of the associated levels have been firmly established from the spectroscopic measurement. Level lifetimes of several levels in the dipole bands have been measured using the Doppler Shift Attenuation Method. The decreasing trend of the measured B(M1) and B(E2) transition strengths in one of the sequence (DB I) spells out its origin as Magnetic Rotation (MR). The trends of B(M1) and B(E2) in DB I are reproduced well in the theoretical calculations using the Shears mechanism with the Principal Axis Cranking (SPAC) model. However, the calculations fail to reproduce the sharp rise in the B(M1)/B(E2) ratio at the highest spins in DB I and the same has been interpreted from the decreasing of the core rotation along the sequence. The experimental observations along with the the theoretical calculations for the second dipole band (DB II), indicate that the core rotation, rather than the shears mechanism, is being favored for angular momentum generation. This represents a unique observation of forking of the shears band DB I from an abrupt phase change of the core from spherical into the deformed one.

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arXiv2016

Magnetic dipole excitations of $^{50}$Cr

The low-lying $M1$-strength of the open-shell nucleus $^{50}$Cr has been studied with the method of nuclear resonance fluorescence up to 9.7 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC and Compton backscattered photons at the High Intensity $γ$-ray Source (HI$γ$S) facility between 6 and 9.7 MeV of the initial photon energy. Fifteen $1^{+}$ states have been observed between 3.6 and 9.7 MeV. Following our analysis, the lowest $1^{+}$ state at 3.6 MeV can be considered as an isovector orbital mode with some spin admixture. The obtained results generally match the estimations and trends typical for the scissors-like mode. Detailed calculations within the Skyrme Quasiparticle Random-Phase-Approximation method and the Large-Scale Shell Model justify our conclusions. The calculated distributions of the orbital current for the lowest $1^{+}$-state suggest the schematic view of Lipparini and Stringari (isovector rotation-like oscillations inside the rigid surface) rather than the scissors-like picture of Lo Iudice and Palumbo. The spin M1 resonance is shown to be mainly generated by spin-flip transitions between the orbitals of the $fp$-shell.

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