Γ-ray Branching Research Articles

Search for the γ decay of the narrow near-threshold proton resonance in 11B

The γ decay of the elusive narrow, near-threshold proton resonance in 11B was investigated at Laboratori Nazionali di Legnaro (INFN) in a particle-γ coincidence experiment, using the 6Li(6Li,pγ) fusion-evaporation reaction and the GALILEO-GALTRACE setup. No clear signature was found for a possible E1 decay to the 1/21−, first-excited state of 11B, predicted by the Shell Model Embedded in the Continuum (SMEC) with a branching of 0.98−69+167×10−3 with respect to the dominant particle-decaying modes. The statistical analysis of the γ-ray spectrum provided an average upper limit of 2.37×10−3 for this γ-ray branching, with a global significance of 5σ. On the other hand, by imposing a global confidence level of 3σ, a significant excess of counts was observed for E=γ9300(20) keV, corresponding to a resonance energy of 11429(20) keV (namely 200(20) keV above the proton separation energy of 11B) and a γ-ray branching of 1.12(35)×10−3. This result is compatible with the SMEC calculations, potentially supporting the existence of a near-threshold proton resonance in 11B.

High-Precision Spectroscopy of ^{20}O Benchmarking AbInitio Calculations in Light Nuclei.

The excited states of unstable ^{20}O were investigated via γ-ray spectroscopy following the ^{19}O(d,p)^{20}O reaction at 8 AMeV. By exploiting the Doppler shift attenuation method, the lifetimes of the 2_{2}^{+} and 3_{1}^{+} states were firmly established. From the γ-ray branching and E2/M1 mixing ratios for transitions deexciting the 2_{2}^{+} and 3_{1}^{+} states, the B(E2) and B(M1) were determined. Various chiral effective field theory Hamiltonians, describing the nuclear properties beyond ground states, along with a standard USDB interaction, were compared with the experimentally obtained data. Such a comparison for a large set of γ-ray transition probabilities with the valence space in medium similarity renormalization group abinitio calculations was performed for the first time in a nucleus far from stability. It was shown that the abinitio approaches using chiral effective field theory forces are challenged by detailed high-precision spectroscopic properties of nuclei. The reduced transition probabilities were found to be a very constraining test of the performance of the abinitio models.

First Evidence of Axial Shape Asymmetry and Configuration Coexistence in Zn74 : Suggestion for a Northern Extension of the N=40 Island of Inversion

The excited states of N=44 ^{74}Zn were investigated via γ-ray spectroscopy following ^{74}Cu β decay. By exploiting γ-γ angular correlation analysis, the 2_{2}^{+}, 3_{1}^{+}, 0_{2}^{+}, and 2_{3}^{+} states in ^{74}Zn were firmly established. The γ-ray branching and E2/M1 mixing ratios for transitions deexciting the 2_{2}^{+}, 3_{1}^{+}, and 2_{3}^{+} states were measured, allowing for the extraction of relative B(E2) values. In particular, the 2_{3}^{+}→0_{2}^{+} and 2_{3}^{+}→4_{1}^{+} transitions were observed for the first time. The results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the N=40 gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize ^{74}Zn in its ground state. Furthermore, an excited K=0 band with a significantly larger softness in its shape is identified. A shore of the N=40 "island of inversion" appears to manifest above Z=26, previously thought as its northern limit in the chart of the nuclides.

A High-Resolution Study of the 110Tc → 110Ru → 110Rh → 110Pd Decay Chain with the GRETINA Array

Spectroscopic data, such as precise γ-ray branching and E2/M1 multipole-mixing ratios, provide vital constraints when performing multi-dimensional Coulomb-excitation analyses. Consequently, as part of our new Coulomb-excitation campaign aimed at investigating the role of exotic non-axial (triaxial) deformations in the unstable refractory Ru-Mo isotopes, additional beta-decay data was obtained. These measurements make use of ANL’s CARIBU facility, which provides intense beams of radioactive refractory isotopes along with the excellent efficiency and angular resolution of the GRETINA γ-ray tracking array. In this article, we report on the analysis of the A = 110 decay chain, focussing on the identification of previously unreported states in 110Ru following the decay of 110Tc.

Spectroscopy of Cd99 and In101 from β decays of In99 and Sn101

We report on new γ-ray spectroscopy results from β decays of In99 and Sn101. 30 new γ rays were observed following the β decay of In99, and inconsistencies in the literature with respect to the γ rays following the β decay of Sn101 were addressed with two confirmed cases and two new transitions. The experimental γ-ray energies, intensities, and coincidence relationships are discussed with shell model calculations, where theoretical β-decay branching ratios from the parent nuclei and γ-ray cascades of excited states from the daughter nuclei were combined to generate hypothetical βγ spectra and βγγ coincidence matrices. The most intense β-delayed γ-ray branches in both Cd99 and In101 were well reproduced with this approach, and several γ rays were assigned to new excited states based on their good agreement with shell model predictions.

Cross sections for thermal neutron capture by 180W and 184W

Based on activation measurements with W targets of natural isotopic abundance, the cross sections for thermal neutron capture by 180W and 184W have been determined by comparing the activities of 181W and 185W after activation with that of 187W. Relative to the known cross sections for capture by 186W, after correction of the latter for changes in the recommended γ-ray branching intensities, the values of the thermal cross sections have been determined to be 22.7(16) b for capture by 180W and 1.60(11) b for 184W. The half-life of 187W was re-determined to be 23.80(3) h.

Identification of significant E0 strength in the [formula omitted] transitions of 58,60,62Ni

The E0 transition strength in the 22+→21+ transitions of 58,60,62Ni have been determined for the first time following a series of measurements at the Australian National University (ANU) and the University of Kentucky (UK). The CAESAR Compton-suppressed HPGe array and the Super-e solenoid at ANU were used to measure the δ(E2/M1) mixing ratio and internal conversion coefficient of each transition following inelastic proton scattering. Level half-lives, δ(E2/M1) mixing ratios and γ-ray branching ratios were measured at UK following inelastic neutron scattering. The new spectroscopic information was used to determine the E0 strengths. These are the first 2+→2+E0 transition strengths measured in nuclei with spherical ground states and the E0 component is found to be unexpectedly large; in fact, these are amongst the largest E0 transition strengths in medium and heavy nuclei reported to date.

α decay of the T=1, 2+ state in B10 and isospin symmetry breaking in the A=10 triplet

Here, the rate of the T=1, 2+ to T=1, 0 + transition in 10B ( T=1, T z=0) is compared to the analog transitions in 10Be ( T=1, T z=–1) and 10C ( T=1, T z=+1) to provide constraints on ab initio calculations using realistic nuclear forces. The relevant state in 10B, at E x=5.164 MeV, is particle unbound. Therefore, a determination of the B( E2) electromagnetic transition rate requires a precise and accurate determination of the width of the state, as well as the α-particle and γ-ray branching ratios. Previous measurements of the α-particle branching ratio are just barely in agreement. We report on a new study of the α-particle branch by studying the 10B(p,p') 10B* reaction in inverse kinematics with the HELIOS spectrometer. The α-particle branching ratio that we observe, 0.144±0.027, is in good agreement with the evaluated value and improves the associated uncertainty. The resulting experimental B( E2) value is 7.0±2.2 e 2fm 4 and is more consistent with a flat trend across the A=10 triplet than previously reported. This is inconsistent with Green's function Monte Carlo predictions using realistic three-nucleon Hamiltonians, which overpredict the B(E2) value in 10C and 10B.

Isospin mixing from β -delayed proton emission

We present a general scheme of a shell-model analysis of a β-delayed proton emission. We show that the experimental proton to γ-ray branching ratio for the isobaric analog state (IAS) populated in β decay of a precursor, supplemented by theoretical proton and γ-ray widths, can be used to extract spectroscopic factors for isospin-forbidden proton emission. In the case of a well-justified two-level mixing approximation and a relatively well known spectroscopic factor of the admixed state, the proposed scheme provides a new way to determine the amount of the isospin mixing in the IAS. This conjecture is illustrated by the theoretical analysis of Cr44 and Fe48 decay.

High-Statistics Study of theβ+/EC-Decay of110In

A study of the 110In β+/EC decay was performed at the TRIUMF Isotope Separator and Accelerator (ISAC) facility to probe the nuclear structure of 110Cd. The data were collected in scaled-down γ-ray singles, γ − γ coincidence, and γ-electron coincidence mode. The data were sorted and a random-background subtracted γ − γ matrix was created containing a total of 850 million events. We expanded the level scheme of 110Cd significantly by identifying 75 levels under 3.8 MeV, including 12 new ones, and increased the number of previously observed transitions from these levels to 273. The γ-ray branching intensities have been extracted through an analysis of the coincidence intensities. The branching ratios were combined with a reanalysis of lifetimes measurements obtained in an (n, n'γ) reaction with monoenergetic neutrons for the calculation of B(E2) values and these results have lead to the proposal of a γ-soft rotor, or O(6) nucleus, rather than a vibrational, or U(5) pattern for the nature of the low-lying, low-spin levels in 110Cd.

Constraining nuclear photon strength functions by the decay properties of photo-excited states

A new approach for constraining the low-energy part of the electric dipole Photon Strength Function (E1-PSF) is presented. Experiments at the Darmstadt High-Intensity Photon Setup and the High Intensity γ→-Ray Source have been performed to investigate the decay properties of 130Te between 5.50 and 8.15 MeV excitation energy. In particular, the average γ-ray branching ratio to the ground state and the population intensity of low-lying excited states have been studied. A comparison to the statistical model shows that the latter is sensitive to the low-energy behavior of the E1-PSF, while the average ground state branching ratio cannot be described by the statistical model in the energy range between 5.5 and 6.5 MeV.

New approaches to assign configurations using low-statistic γ-ray spectra

Analytical methods to deduce single particle configurations from g K factors using branching ratios in rotational bands, especially in low-statistic cases for heavy nuclei, will be presented. Two methods utilising γ-ray branching ratios in rotational bands and also intensities in the K X-ray region of spectra are shown and results from these are given.

The 25Mg(p, γ)26Al reaction at low astrophysical energies

In the present work we report on a new measurement of resonance strengths ωγ in the reaction 25Mg(p, γ)26Al at Ecm=92 and 189 keV. This study was performed at the LUNA facility in the Gran Sasso underground laboratory using a 4π BGO summing crystal. For the first time the 92 keV resonance was directly observed and a resonance strength ωγ=(2.9±0.6)×10−10 eV was determined. Additionally, the γ-ray branchings and strength of the 189 keV resonance were studied with a high resolution HPGe detector yielding an ωγ value in agreement with the BGO measurement, but 20% larger compared to previous works.

Observation of γ-delayed 3α breakup of the 15.11 and 12.71 MeV states in 12C

The reactions B10(He3,pααα) at 4.9 MeV and B11(He3,dααα) at 8.5 MeV have been used to investigate the γ decay of states in 12C. By measuring the four-body final state in complete kinematics we are able to detect γ transitions indirectly. We find γ transitions from the 15.11 MeV state in 12C to the 12.71, 11.83, 10.3 and 7.65 MeV states followed by their breakup into three α particles. The relative γ-ray branching ratios obtained are (1.2±0.3), (0.32±0.12), (1.4±0.2) and (4.4±0.8)%, respectively, with the remaining (92.7±1.0)% of the γ decays going to the bound states. We obtain Γα/Γ=(2.8±1.2)% for the isospin-forbidden α decay of the 15.11 MeV state. From the 12.71 MeV state we find γ transitions to the 10.3 and 7.65 MeV states. The relative γ-ray branching ratios are (0.9−0.5+0.6) and (2.6−1.2+1.6)%, respectively, with the remaining (96.6−1.3+1.7)% of the γ decays going to the bound states. Finally, we discuss the relation between the β decay of 12N and 12B to states in 12C and the γ decay of the 15.11 MeV analog in 12C to the same states.

Decay of 1.643 h 95Ru and its daughter's level structure

The decay of 95Ru has been investigated by means of γ-ray spectroscopy. The 95Ru nuclei were produced by the reaction 92Mo( α, n) 95Ru at a beam energy of 17MeV. High-purity Ge detectors have been used singly and in coincidence to study γ-rays in the decay of 95Ru to 95Tc. 132 γ-rays are reported, among them, energies and intensities for 127 transitions have been determined. A decay scheme of 95Ru with 31 levels is proposed which accommodates 127 of these transitions. Spins and parities for three new levels are proposed from calculated log ft values, measured γ-ray branching ratios, and in-beam experiment results of the daughter nucleus 95Tc. Combining with the high-spin states observed by in-beam γ-ray spectroscopy of previous decay works, the structure of the excited states of 95Tc is discussed in the framework of the projected shell model.

The role of the intrinsic E2 matrix element between the first two 0 + states in their configuration mixing in 100Zr

Shape coexistence in 100Zr is a well-known phenomenon. However, a consistent description of the configuration mixing between two shapes with very different deformation has not been made. In this work, the B(E2,2 + 2→0 + 2) value in 100Zr has been inferred from the known γ-ray branching ratios between the intraband transition and the interband transitions to members of the ground-state band. The absolute scale of the latter was established under the assumption that adjustments to the interband transitions due to the coupling between the rotation and intrinsic motions can be approximated by a perturbation expansion of angular-momentum dependence. Correction terms up to the second order, which account for the deformation difference between two 0 + bands, were considered in the description. The intrinsic E2 matrix elements for the second 0 + state at 331 keV is derived to be ≈0.53 eb from the B(E2,2 + 2→0 + 2) value assuming a rotational relationship. This suggests that the weakly deformed 0 + 2 state coexists with the strongly deformed ground state of 〈0 1|E2|0 1〉≈1.06 eb. Configuration mixing between the two 0 + states has been studied using both the known E0 and the intrinsic E2 matrix elements derived from this work. A weak mixing of ≈7.7% was found, which is nearly a factor of two lower than suggested by a previous analysis. Quantitative evidence of the energy shift for the ground state, deduced from the systematics of transition energies for the yrast states, is consistent with this weak mixing scenario. Near-spherical and well-deformed shapes with the intrinsic E2 matrix elements of ≈0.37 eb and ≈1.14 eb, respectively, are identified as the basis states before the mixing takes place. The intrinsic E2 matrix element between those two unperturbed 0 + states, ≈0.19 eb, is required to describe their configuration mixing.

Decay of 1.643-hr 95Ru to 95Tc

The decay of 95Ru has been investigated by means of γ-ray spectroscopy. The 95Ru nuclei were produced by the reaction 92Mo(α,n)95Ru at the beam energy of 17 MeV. High-purity Ge detectors have been used singly and in coincidence to study γ-rays in the decay of 95Ru to 95Tc. 132 γ-rays are reported, among them, energies and intensities for 127 transitions have been determined. A decay scheme of 95Ru with 31 levels is proposed which accommodates 127 of these transitions. Spins and parities for three new levels are proposed from calculated logft values, measured γ-ray branching ratios, and in-beam experiment results of the daughter nucleus 95Tc.

LOW-SPIN STATES OF 83Rb EXCITED IN THE 83Sr(β++EC)83Rb DECAY

Gamma-rays from the 83 Sr (β++ EC )83 Rb decay have been investigated with Compton suppressed HpGe detectors. The activity was produced via the 85 Rb (p,3n) reaction at 27.1 MeV beam energy. Singles in multispectra mode and γ–γ coincidence experiments were performed. Approximately 190 transitions including 94 new transitions and 19 new levels were assigned to 83 Rb , based on their measured half-life and/or observation in coincidence with well-known lines. A decay scheme has been constructed consisting of 41 excited states and 180 transitions in 83 Rb . Additional new information has been obtained on γ-ray branching ratios, log ft values, spins and parities.

Non-yrast states and shape co-existence in light Pt isotopes

Low-lying states in the even-even light platinum isotopes 176Pt, 178Pt, 180Pt and 182Pt have been populated using β + /EC decay from parent gold nuclei, created in (HI, xn) reactions. State energies, spins and parities and γ-ray branching ratios were determined using γ-ray and electron spectroscopy. Whereas non-yrast states were observed in 178Pt, 180Pt and 182Pt, none were seen in 176Pt. The excitation energies of the observed states are analysed in terms of a band-mixing model, yielding the moments of inertia of the unperturbed bands. Branching ratios and ground-state-band quadrupole moments are calculated and compared with experimental values. The results indicate that the two lowest-lying 0 + states in each of the light Pt isotopes are formed from the mixing of two intrinsic states of different deformation, and other low-lying states can be described as admixtures of rotational states built on these intrinsic states, and on γ-vibrational states.

Measured magnetic moments in 169Tm and the particle-rotor model; implications for transient field calibration

Gyromagnetic ratios were measured in the 1 2 + [411] ground-state band of 169Tm by the transient field technique in such a manner that the extracted g-factors are independent of assumptions concerning the strength and velocity dependence of the transient field. Precise γ-ray branching intensities and multipolarity mixing ratios were determined from measured particle-γ-ray angular correlations. The electromagnetic properties of the low-lying natural parity states in 169Tm are compared with particle-rotor calculations based on the Woods-Saxon potential and the implications of the measured magnetic moments for the calibration of the transient field strength are discussed.