Gamma-ray Angular Distributions Research Articles

Recoil-distance measurements of the lifetimes of high-spin states in83Sr and85Sr

In-beam gamma -ray spectroscopy has been performed for high-spin states of 83Sr and 85Sr, populated through the 74,76Ge(12C, 3n gamma ) reactions, respectively. Gamma-ray angular distributions and recoil-distance Doppler-shift measurements provided branching ratios, multipolarity mixing ratios and lifetimes of the electromagnetic decays of these states. The results are discussed in terms of the existing nuclear model calculations. Also, calculations of positive-parity states are presented, performed on the basis of the asymmetric rotor model (with variable moment of inertia). This model gives a reasonable description of the experimental data for 83Sr, but it fails in the case of 85Sr.

Gamma-ray studies on the neutron-deficient isotope70Se

The level structure of 70Se has been studied with the aid of the 58Ni(14N, pn gamma )70Se and 60Ni(12C, 2n gamma )70Se reactions. A decay scheme has been established from results of gamma-gamma and neutron-gamma coincidence experiments. Spin and parity assignments of 2+ (945 keV), 2(+) (1600 keV), 4+ (2039 keV) and 6+ (3003 keV) have been deduced from the results of gamma-ray angular distributions and linear polarisation measurements. Spin restrictions have also been placed on two levels: 3, 5 (3524 keV) and 5, 7 (3915 keV). Comparisons are made with the structure of neighbouring even-even isotopes and their energy level systematics.

Heavy-ion gamma-ray spectroscopy of60Ni

Twenty excited states below 6.8 MeV and with spins up to J=(9) have been observed in 60Ni in the reaction 56Fe(7Li, p2n gamma ). Lifetimes of levels have been determined from DSA and RD measurements. Gamma-ray angular distribution and linear polarisation measurements have been made and unambiguous Jpi assignments deduced. Several new high-spin states above 4 MeV in excitation are proposed and the results have been compared with the predictions for high-spin states of various theoretical models.

An isospin-mixed triplet in 44Ti

Three levels in 44Ti at energies of 9215 ± 2, 9227 ± 2, and 9239 ± 2 keV have been studied in the 40Ca(α,γ) reaction. Gamma-ray branching ratios, angular distributions, and transition strengths have been measured for each level. Proton decay strengths have also been determined. Each level is assigned Jπ = 2+. The level at 9227 keV must be predominantly T = 1 because of its strong M1 decays, while the other two levels are believed to be predominantly T = 0 because of their stronger E2 transitions to the ground state. Treating the three levels as an isospin-mixed triplet gives a consistent interpretation for all the experimental data, and allows quantitative estimates to be made of the amount of isospin mixing and of the magnitude of the isovector matrix elements.

Investigation of Positive Parity Levels in 141Pr

The excited states of 141 Pr up to 1800 keV were studied by means of Coulomb excitation of 141 Pr with protons of energies from 4.0 to 5.5 MeV. The gamma-ray yields and angular distributions were measured with a Ge(Li) detector. The levels in 141 Pr at 1348, 1580.3, 1608.4 and 1785 keV were observed in Coulomb excitation for the first time. The results of gamma-ray angular distribution measurements establish spin values of 3/2 + , 5/2 + , 3/2 + , 3/2 + and 1/2 + for the 1126.9, 1292.6, 1348, 1608.4 and 1785 keV levels respectively. The reduced E2 and M1 transition probabilities were determined for the 1126.9, 1292.6, 1348, 1493, 1608.4 and 1785 keV levels. The results were compared with the available theoretical calculations.

Study of negative parity levels in 63Cu

Low-lying negative parity levels in 63Cu were Coulomb excited with 3.25 to 4.25 MeV protons to test the weak coupling core-excitation model. A Ge(Li) detector was used to measure the gamma-ray yields. The 1412, 1547, and 1861 keV levels in 63Cu were Coulomb excited for the first time. Gamma-ray angular distributions were measured at 4.25 MeV proton energy in deducing multipole mixing ratios and spin values. The E2 and M1 reduced transition probabilities were determined for the six states. The 669.6, 962, 1327, and 1547 keV levels have properties consistent with the interpretation of coupling a 2p3/2 proton to the first 2+core state. The present results were compared with the available particle–core and particle–phonon model calculations.

Gfactor of the 14 ms8+isomeric state inY88

The method of in-beam perturbation of gamma ray angular distribution by nuclear magnetic resonance was applied to remeasure the g factor of the I/sup ..pi../=8/sup +/,T/sub 1/2/=14 ms isomeric state in /sup 88/Y with the result g=0.609(6).

Effect of the tensor force on gamma-ray de-excitation angular distributions from dipole states populated in theC12(p,p′) reaction

Angular distributions have been measured for the de-excitation gamma rays from the isoscalar and isovector ${1}^{+}$ states populated in the $^{12}\mathrm{C}$($p$,${p}^{\ensuremath{'}}$) reaction at ${E}_{p}(\mathrm{lab})=22\ensuremath{-}27$ MeV. Fluctuations in the ${A}_{0}$ coefficients, which measure the total excitation cross sections, are observed to decrease with energy toward the high end of the region investigated. The ${a}_{2}$ coefficients are still fluctuating at 27 MeV, but exhibit reasonably well-defined average values of about -0.40 and 0.15 for the $T=0$ and $T=1$ levels, respectively. Microscopic distorted-wave calculations based on a realistic $G$ matrix interaction, while not sufficient to completely describe the present data, suggest that the isospin dependence of the ${a}_{2}$ coefficients is associated with the tensor force. The theoretical calculations have been extended to 50 MeV and this effect of the tensor force persists.NUCLEAR REACTIONS $^{12}\mathrm{C}$($p$,${p}^{\ensuremath{'}}\ensuremath{\gamma}$) ${E}_{p}=22\ensuremath{-}27$ MeV; measured gamma-ray angular distribution; $M1$ states miscroscopic DWBA analysis.

Levels and Electromagnetic Transitions in 51Cr

Gamma-ray angular distribution, linear polarization, Doppler-shift attenuation and γ-γ coincidence measurements have been made in the 48 Ti(α, nγ) 57 Cr reaction. In addition, excitation functions have been measured in the 51 V(p, nγ) 51 Cr reaction. A total of 115 γ rays have been assigned as transitions in 51 Cr, and 56 excited levels have been identified in 51 Cr below 4.5 MeV. Previously reported decay schemes of 51 Cr levels have been considerably modified. All the negative parity levels in 51 Cr below 2.8 MeV can be classified into two groups : those which belong to a strongly deformed 1/2 - band, and those which belong to the spherical f 7/2 . configurations. Electromagnetic transitions between these two groups of levels are hindered.

High spin states in 96Tc populated through the (α,n) reaction

High spin states of 96Tc populated by the (α,n) reaction using alpha beams from 13 to 27 MeV have been studied. Gamma-ray energies and intensities, gamma–gamma coincidence probabilities, gamma-ray angular distributions, and electron conversion coefficients were determined at 18 MeV. In addition, some results taken at 14 MeV are reported.The high spin structure observed in this work includes the following levels: 49.3(6+), 318.8(6+), 574.7(7+), 926.9(9+), 946.5(8+), 1062.1(8+), 1138.8(8+), 1447.2(9+), 1702.8(10+), 1861.6(9+), 1922.3(11+), 2147.5(11+), 2213.5(10(+)), 2317.2(12+), 2396.8(11(+)), 2599.0((13)+), 2642.4((14)+), and 3020.1(12(+)).These experiments also clarify and extend the information obtained by previous workers. In particular, evidence is adduced for low-lying states at 0.0(7+), 34.3(4+), 45.3(5+), 120.3(3−), 177.0(5+), 226.2(2−), 227.0(4+), and 254.3(3+).The work identifies many other states of intermediate energy.Attenuation coefficients for states in 96Tc are calculated using a theoretical model.

(n,n′γ) studies of 114,116Cd

Energy levels below 2.2 MeV excitation in 114,116Cd have been investigated using the (n, n′γ) reaction. Gamma-ray angular distributions were measured, and energy levels, spins, and gamma-ray multipole mixing ratios were deduced. Levels and their spins have been confirmed at 1134 (0), 1210 (2), 1283 (4), 1365 (2), 1733 (4), and 1843 (2) keV for 14Cd and at 1213 (2), 1220 (4), and 1381 (0) keV for 116Cd. New spin assignments have been made for the levels at 1861 (3) and 2049 (3) keV for 114Cd and at 1644 (2,3) and 1917 (3) keV for 116Cd. The states at 1959 keV in 114Cd and 1923 keV in 116Cd are shown to be consistent with assignments of 3.

Band-like structures in97Ru

The 88Sr(12C, 3n gamma ) reaction is used to populate high-spin states in the nucleus 97Ru. Gamma-ray excitation functions, angular distribution and recoil-distance measurements are used to determine branching ratios, multipolarities and lifetimes of the electromagnetic decays of these states. The lifetimes of the states at 2739.4, 2545.7, 1845.9, 1199.3, 879.1 and 421.6 keV have been determined for the first time as tau =12500+or-1300, <5, 21+or-3, 5+or-1, approximately=5 and 50+or-3 ps respectively. Some level sequences with Delta I=2 (band-like structures) are observed with relatively large 'intraband' B(E2) values as well as smaller B(E2) transition rates and hindered M1 transitions between 'bands'. The results are discussed in terms of current nuclear models.

Investigation of level properties of 139La

The excited states of 139La up to 1800 keV were studied by means of Coulomb excitation of 139La with protons varying in energy from 4.0 to 5.5 eV. The gamma-ray yields and angular distributions were measured with a Ge(Li) detector. The levels at 1715.0 and 1767.2 keV were Coulomb excited newly. The results of gamma-ray angular distribution measurements establish spin values of 3/2+, 9/2+, 7/2+, 5/2+, and 3/2+ for the 830, 1219.1, 1536.3, 1715.0, and 1767.2 keV levels, respectively. The reduced E2 and M1 transition probabilities were obtained for 11 transitions. The results were compared with the available theoretical calculations.

Coulomb excitation of low-energy levels in 45Sc

Low-lying negative parity levels in 45Sc were Coulomb excited with 2.5 to 3.5 MeV protons and 4 to 5 MeV 4He ions to test the weak coupling core-excitation model. A Ge(Li) detector was used to measure the gamma-ray yields. The 543, 976, 1408, and 1662 keV levels in 45Sc were Coulomb excited for the first time. Gamma-ray angular distributions were measured at 3.0 MeV proton energy in deducing multipole mixing ratios and spin values. Energy level measurements (in units of kiloelectronvolts) and spin values obtained are as follows: 976, 5/2, 7/2 and 1408, 7/2. The E2 and M1 reduced transition probabilities were determined for the six states. The 376, 720, 1237, 1408, and 1662 keV levels have properties consistent with the interpretation of coupling a 1f7/2 proton to the first 2+ core state.

High-spin states in43K

The nucleus 43K has been studied via the 40Ar( alpha ,p gamma )43K reaction, using solid targets of natural argon. A gamma - gamma coincidence experiment established five new levels at excitation energies of 1850, 2048, 2509, 3115 and 3139 keV. Gamma-ray angular distributions and linear polarisations were measured at an alpha-particle energy of 17.0 MeV. The analysis of these data yielded spin and parity assignments of Jpi =7-/2, 7+/2, 11-/2 and 15-/2 for the levels at 738, 1510, 1850 and 3115 keV respectively. Mean lifetimes were measured for four levels using the DSAM: 1510 keV, 8.2+or-2.1 ps; 2048 keV, 2.4+or-0.8 ps; 1850 keV, 6.7+or-1.7 ps; 3115 keV, 5.0+or-1.5 ps. A lower limit of 7 ps was obtained for the lifetimes of the levels at 1207 keV and 2509 keV.

Measurement of Cross Sections for the 27Al(n, n′γ)27Al Reaction Near Threshold

Cross-section ratios for production of 0.843-, 1.013-, and 2.209-MeV gamma rays at 55 deg by the 27Al(n,n′γ)27Al reaction relative to fast-neutron fission of 235U have been measured at intervals of ∼0.05 MeV from threshold up to ∼2.5 MeV, with an average neutron energy resolution of ∼0.08 MeV. These ratios and ENDF/B-IV fission cross sections were used to compute gamma-ray production cross sections. Gamma-ray angular distributions were measured at En = 1.112, 1.310, 1.512, 1.714, 1.914, 2.118, 2.310, and 2.512 MeV. These distributions were found to be isotropic within the experimental errors. The experimental results are compared with data from the literature and with results from optical/statistical model calculations of neutron inelastic scattering from aluminum.

Spin-alignment attenuation factors in mass 61–67 nuclei populated by ( α, n) and ( α, p) reactions

Gamma-ray angular distributions for 39 E2 transitions in A = 61–67 nuclei, excited in ( α , n) or ( α , p) reactions, are collected from the literature and reduced to spin-alignment attenuation factors α 2 and α 4 . The validity of the prediction for α 2 from statistical compound nucleus reaction theory is investigated. Also the sensitivity of this prediction to the optical-model parameters is examined. The present data support the rule 0 ⩽ α 4 ⩽ α 2 , but contradict the assumption of a Gaussian shape for the population of magnetic substates.

Radiative decays of unbound high spin states in 24Mg

Abstract The 20Ne(α, γ)24Mg reaction has been used to measure radiative decay rates for known high spin levels at 12.86(6+), 13.44(6+) and 14.15(8+) MeV in 24Mg, and for previously unidentified levels at 12.00, 13.04 and 14.08 MeV. Gamma-ray angular distributions indicate that the 12.00 and 14.08 MeV levels are probably 6+ levels, while the 13.04 MeV level is the lowest 0+ T = 1 state. A new level at 11.005 MeV, populated in the radiative decay of the 14.08 MeV level, is most likely the lowest 5+ T = 1 state in 24Mg. Alpha-particle widths have also been measured for several of these levels and for the known 6+ level at 13.85 MeV using the 20Ne(α, α′γ)20Ne reaction and previously determined branching ratios. The results for both radiative and particle decay widths are compared with recent shell-model calculations performed within a full (sd)8 basis.

Gamma-ray spectroscopy of high-spin levels in67Ga

Reactions 64Zn( alpha ,p gamma )67Ga and 53Cr(16O, pn gamma )67Ga populated the levels in 67Ga. Gamma-gamma coincidence experiments were carried out at Ealpha =13 and 15 MeV with the ( alpha ,p gamma ) reaction and E(16)O=42 MeV with the heavy-ion reaction. The Doppler-shift attenuation method was used with the ( alpha ,p gamma ) reaction to deduce mean lifetimes (or limits) for certain levels, along with the recoil-distance method utilised both in singles and gamma - gamma coincidence with the heavy-ion reaction. A mean lifetime of tau m=60+or-30 ns for the first excited state at 167 keV was measured using the delayed coincidence method. Gamma-ray angular distributions and linear polarisations were measured at Ealpha =13.5 MeV using a Ge(Li)-NaI(Tl) escape-suppressed spectrometer and a three-Ge(Li) Compton polarimeter and analysis resulted in spin and parity assignments. Transition strengths were calculated and the results are discussed in terms of the particle-cluster-vibrational-core coupling model.

Strongly Deformed Kπ=1/2- Bands in 51Cr and 53Fe

Gamma-ray angular distributions, γ-γ coincidences and Doppler-shift attenuations have been measured in the 48 Ti(α, nγ) 51 Cr and 50 Cr(α, nγ) 53 Fe reactions. Linear polarization of γ rays from the 48 Ti(α, nγ) 51 Cr reaction and excitation functions and angular distributions of γ rays from the 51 V(p, nγ) 51 Cr reaction have also been measured. Present data have established unambiguously the presence of a series of levels in 51 Cr and 53 Fe, which are most naturally interpreted as the members of K π =1/2 - bands. They are 1/2 - , 3/2 - , 5/2 - , 7/2 - , 9/2 - and 11/2 - levels at 777, 749, 1353, 1557, 2379 and 2705 keV in 51 Cr, and 1/2 - , 3/2 - , 5/2 - and 7/2 - levels at 774, 740, 1421 and 1695 keV in 53 Fe. Lifetimes, branching ratios and mixing ratios of the transitions from these levels have been measured. Obtained reduced transition probabilities indicate that the K π =1/2 - bands in 51 Cr and 53 Fe are more deformed than the yrast levels. It has been shown that there are many previously unresolved γ rays in 51 Cr and the decay scheme of 51 Cr levels has to be considerably modified.