Γ-ray Transition Probabilities Research Articles

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.

Nuclear Data Sheets for A=203

Evaluated nuclear structure and decay data for all nuclei with mass number A=203 (203Os, 203Ir, 203Pt, 203Au, 203Hg, 203Tl, 203Pb, 203Bi, 203Po, 203At, 203Rn, 203Fr, 203Ra) are presented. All available experimental data are compiled and evaluated, and best values for level and γ-ray energies, quantum numbers, lifetimes, γ-ray intensities and transition probabilities, as well as other nuclear properties, are recommended. Inconsistencies and discrepancies that exist in the literature are discussed. A number of computer codes (https://www-nds.iaea.org/public/ensdf_pgm/index.htm) developed by members of the NSDD network were used during the evaluation process. For example, the reported absolute γ-ray emission probabilities and their uncertainties were determined using the GABS code. The γ-ray transition probabilities were determined using the RULER code and the corresponding uncertainties were estimated by means of a Monte-Carlo approach. This work supersedes the earlier evaluation by F.G. Kondev (2005Ko20), published in Nuclear Data Sheets 105, 1 (2005).

Nuclear Data Sheets for A=219

Evaluated experimental structure and decay data are presented for 12 known nuclides of mass 219 (Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa, U, Np). Recommended values are given for level parameters, γ and α radiations, and other spectroscopic information. No excited states are known in 219Bi, 219Po, 219Pa, 219U and 219Np. Except for isotopic identification, no information about its half-life or decay characteristics is available for 219Pb. For 219Po, several γ rays have been reported from the decay of 219Bi β− decay, but no level scheme has been proposed. For 219At, only four excited states are known from α decay of 223Fr. For 219Rn and 219Fr, only the low-spin states are known from α decays of 223Ra and 223Ac, respectively. For 219Ra, 219Ac and 219Th, mainly the data for high-spin states are available from in-beam γ-ray studies. For 219Ra, high-spin data are available from 2017He15, 1992Wi02 and 1987Co36, but evaluators find significant differences in relative photon branchings between the three studies. Detailed comments are given in the Adopted dataset for this nuclide. Half-lives of excited states are known only for seven levels in 219Rn, thus there is a general lack of knowledge about γ-ray transition probabilities. This work supersedes data in the previous evaluations of A=219 published by 2001Br31, 1992Br10 and 1977Ma30.

Nuclear Data Sheets for A=190

The evaluated experimental data are presented and evaluated for 15 known nuclides of mass 190 (Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po). This work supersedes earlier full evaluations of A=190 by 2003Si05, 1990Si09, 1982Le22 and 1973Sc42. For 190Hf, only the isotopic identification has been made, with no information about the half-life of the ground state or its decay properties. For 190Ta, only one excited state as an isomer has been identified, but with no knowledge about its decay properties. For 190W, 190Re, 190Bi and 190Po, available spectroscopic data are scarce. The decay schemes of 190Ta, 190W, 190Au, 190Hg, 190Au isomer decay, 190Tl (both the activities), 190Pb, and 190Bi (both the activities) are considered incomplete by the evaluators. At present, extensive level structures, both the low-spin and high-spin, are known for 190Os, 190Ir, 190Pt, 190Au, 190Hg, 190Tl and 190Pb, but data for level lifetimes, and γ-ray transition probabilities are generally lacking.

The concept of nuclear photon strength functions: A model-independent approach via ([formula omitted]) reactions

Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink–Axel hypothesis and the related concept of so-called photon strength functions to describe γ-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasi-monochromatic photon beams and a newly implemented γ–γ coincidence setup. This technique does not assume a priori the validity of the Brink–Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus 128Te were obtained for γ-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink–Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink–Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold (Sn=8.78MeV) for the studied case of 128Te.

Thermal neutron capture cross sections forO16,17,18andH2

Author(s): Firestone, RB; Revay, Z | Abstract: Thermal neutron capture γ-ray spectra for O16,17,18 and H2 have been measured with guided cold neutron beams from the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) reactor and the Budapest Research Reactor (BRR) on natural and O17,18 enriched D2O targets. Complete neutron capture γ-ray decay schemes for the O16,17,18(n,γ) reactions were measured. Absolute transition probabilities were determined for each reaction by a least-squares fit of the γ-ray intensities to the decay schemes after accounting for the contribution from internal conversion. The transition probability for the 870.76-keV γ ray from O16(n,γ) was measured as Pγ(871)=96.6 ±0.5% and the thermal neutron cross section for this γ ray was determined as 0.164±0.003 mb by internal standardization with multiple targets containing oxygen and stoichiometric quantities of hydrogen, nitrogen, and carbon whose γ-ray cross sections were previously standardized. The γ-ray cross sections for the O17,18(n,γ) and H2(n,γ) reactions were then determined relative to the 870.76-keV γ-ray cross section after accounting for the isotopic abundances in the targets. We determined the following total radiative thermal neutron cross sections for each isotope from the γ-ray cross sections and transition probabilities; σ0(O16)=0.170±0.003 mb; σ0(O17)=0.67±0.07 mb; σ0(O18)=0.141±0.006 mb; and σ0(H2)=0.489±0.006 mb.

Light p-Shell -Hypernuclei by the Microscopic Three-Cluster Model

A systematic investigation of ^6_ΛHe, ^6_ΛLi, ^7_ΛLi ^8_ΛLi and ^9_ΛBe hypernuclei is carried out within the framework of the microscopic α+χ+Λ cluster-model dynamics (χ=n, p, d, t, or α). The positive and negative parity energy spectra are analysed in detail and are classified into several characteristic “bands” according to the underlying structures. The E2, E1 and M1 γ-ray transition probabilities and magnetic dipole moments are calculated. The intra-band B(E2)’s are obtained to be several times enhanced in comparison with the shell-model values. The level widths of the strong peaks observed at lower B_Λ are roughly evaluated on the basis of the calculated reduced width amplitudes. The existing data can be well explained by the present model. The cluster-model estimates of the effective neutron number Neff(θ=0°) for the (K^-, π^-) reaction are consistent with the experiments.

Transition probabilities and deformations in the doubly odd nucleus 152Eu

In the (n, γ) reaction, nanosecond lifetimes of excited states of the N = 89 transitional nucleus 152Eu have been measured by the delayed coincidence method using a low-energy Ge(Li) detector. From the analysis of relative intensities in prompt and delayed γ-ray spectra, the following half-lives of isomeric states could be obtained for the first time: T 1 2 (141.83 keV) = 2.5 ± 0.5 ns, T 1 2 (158.05 keV) = 1.8 ± 0.4 ns, T 1 2 (160.88 keV) = 2.3 ± 0.4 ns and T 1 2 (180.63 keV) = 2.1 ± 0.6 ns. Further, it has been confirmed that the levels at 77.26 and 89.85 keV have nanosecond lifetimes T 1 2 > 30 ns. Absolute γ-ray transition probabilities available so far in this nucleus are compared with Weisskopf and Nilsson estimates, the latter being deduced for quadrupole deformation parameters ε 2 = 0.15, 0.20, 0.23 and 0.26 with the pairing effect taken into account. From the B( E2) value of the 71.03 keV transition, the average quadrupole deformation of ε 2 ≲ 0.18 for the corresponding excited states is concluded, which is smaller than ε 2 ≈ 0.25 established earlier for the ground state. Experimental matrix elements of allowed El transitions deviate from the relevant systematics in strongly deformed nuclei. The suggestion that these deviations are associated with strong configuration admixtures is supported by a new graphic systematization of allowed and forbidden B( E1) values in deformed odd-odd nuclei.

Levels and transition probabilities in 120, 122, 124, 126, 128sn studied in the decay of In isotopes

The energy levels in even Sn nuclei, with A = 120–128, populated in the decay of In isotopes have been studied, using an on-line mass-separator. For A = 122, 124, 126 the mass separation was complemented with a chemical separation of Cd, which removed the high-spin isomers of In from the samples. Three isomers with suggested I π = 1 +, 4 + or 5 +, and (8) − were observed in 120, 122in isomers with I π = 3 + and (8) − were found in 124, 126, 128 In. Decay schemes for 10 previously unknown or little known isomeric decays are presented, together with information obtained in measurements of level half-lives in 122, 124, 126, 128Sn. The β- and γ-ray transition probabilities and the systematics of energy levels are used to identify levels in the Sn nuclei which contain large amplitudes of specific two-quasiparticle configurations.

Excited states of the doubly odd deformed nucleus 160Tm

Abstract The decay of 4.8 min 160 Yb has been investigated by means of Ge(Li) detectors, toroidal and magnetic-lens β-spectrometers and scintillation counters. The singles spectra of γ-rays and conversion electrons as well as prompt and delayed γ-γ and γ-ce coincidences have been measured. The 160 Tm level scheme is proposed including 94.5% of the total γ-ray intensity observed in the 160 Yb decay. The lifetimes of the 42.02, 174.40 and 215.78 keV states have been determined to be 1.6±0.3, 17±1 and 0.65±0.15 ns, respectively. From the analysis of the experimental data, the quantum characteristics of 160 Tm levels are proposed. Experimental γ-ray transition probabilities are compared with Weisskopf and Nilsson estimates, the latter also taking into account pairing correlations.

Electromagnetic transitions in some doubly odd deformed nuclei

Nanosecond lifetimes of excited states in doubly odd deformed nuclei have been determined by in-beam measurements applying the method of delayed γ-γ coincidences as well as by experiments in the radioactive decay with the method of delayed γ-ce coincidences, respectively. Analysing the time distributions and delayed γ-ray spectra, the following half-lives of isomeric states could be obtained for the first time: T 1 2 (63.7 keV in 160 Tb) = 60 ± 5 ns, T 1 2 (138.7 keV in 160 Tb) = 5.7 ± 0.5 ns, T 1 2 (82.0 keV in 156 Ho) = 1.25 ± 0.20 ns, T 1 2 (87.2 keV in 156 Ho) = 58.5 ± 3.5 ns, T 1 2 (139.2 keV in 158 Ho) = 1.85 ± 0.10 ns, T 1 2 (38.3 keV in 162 Ho) = 1.2 ± 0.2 ns, T 1 2 (179.9 keV in 162 Ho) = 8.7 ± 0.2 ns, T 1 2 (342.8 keV in 164 Ho) = 2.6 ± 0.2 ns, T 1 2 (295.1 keV in 166 Ho) = 1.10±0.15 ns, T 1 2 (44.6 keV in 162 Tm) = 1.40±0.15 ns, T 1 2 ,(163.4 keV in 162 Tm) = 1.1 ± 0.1 ns, T 1 2 (220.1 keV in 178 Ta) = 8.5 ± 1.0 ns, T 1 2 (289.5 keV in 178 Ta) = 2.0 ± 0.5 ns, T 1 2 (392.8 keV in 178Ta) ≈ 1 ns, T 1 2 (316.5 keV in 186 Re) = 0.20 ± 0.10 ns, T 1 2 (300.2 keV in 188 Re) = 1.5 ± 0.2 ns and T 1 2 (482.2 keV in 188 Re) = 0.26 ± 0.10 ns . Furthermore, upper limits for the half-lives of fifteen excited states in 160Tb, 164, 166Ho and 186, 188Re have been estimated. For eight isomeric levels in 186, 188Re, the lifetimes earlier determined have been remeasured. Unlike previous studies, the existence of isomeric states at 87.2 keV in 156Ho and at 179.9 keV in 162Ho is suggested. Absolute γ-ray transition probabilities are deduced and compared with single-particle estimates according to Weisskopf and Nilsson, the latter also including pairing correlations. The K-, Ω- and f-forbidden transitions can qualitatively be explained in terms of configuration mixings. Experimental El, ΔK= 1 transition matrix elements in odd-odd deformed nuclei are supposed to be appreciably influenced by higher-order vibrational admixtures coupled via RPC and p-n interaction mixings.

Gamma-gamma directional correlations in 90Zr

Directional correlations of γγ cascades in 90Zr have been measured employing a twelve-channel goniometer. The obtained correlation coefficients are: 141.2–1129.1 keV, A 2 = −0.070(12), A 4 = −0.003(10); 1611.8–1129.1 keV, A 2 = −0.088(20), A 4 = 0.013(18); 1716.4–1129.1 keV, A 2 = −0.011(75), A 4 = 0.041(60); 1984.7–1129.1 keV, A 2 = −0.006(54), A 4 = −0.036(50); 827.7–1913.3 keV, A 2 = 0.164(40), A 4 = 0.062(35); 132.7–2186.4 keV, A 2 = 0.187(38), A 4 = −0.063(36); 890.6−2186.4 keV, A 2 = 0.132(51), A 4 = −0.023(48). The spin and parity 7 + is assigned to the 5060.0 keV level. The multipole mixing ratios of the involved γ-ray transitions have been determined. The γ-ray transition probabilities for some transitions are calculated and compared with predictions of the shell model for the 90Zr nucleus.

Gamma-ray transition probabilities in 91Nb +

The lifetimes of 14 excited states in 91Nb and lower bounds for 5 additional states were measured with the Doppler-shift attenuation method, utilizing the 91Zr(p, nγ) 91Nb reaction. By using calibrated F( τ) curves, the uncertainty of the stopping power was reduced to about 15 %. Multipole mixing ratios were determined for several transitions from the γ-ray angular distributions. The experimental data on γ-ray transition probabilities were compared with theoretical values. These were calculated using shell-model wave functions based on a 88Sr core and active protons in the 2 p 1 2 and 1 g 9 2 orbitals. For the negative-parity states also excitations of protons from the 2 p 3 2 and 1 f 5 2 orbitals were taken into account.

Electromagnetic transitions in some odd-proton deformed nuclei

In-beam measurements of nanosecond lifetimes applying the method of delayed γ-γ coincidences were performed in the (p, n) reaction. Analysing the time spectra with the centroid shift method, the following half-lives of excited nuclear states in the subnanosecond region could be found: T 1 2 (353.2 keV in 161 Ho) = 0.52±0.15 ns , T 1 2 (252.7 keV in 161 Ho) ≦ 0.2 ns , T 1 2 (579.4 keV in 161 Ho) ≦ 0.2 ns , T 1 2 (431.2 keV in 163 Ho) = 0.37±0.15 ns , T 1 2 (439.9 keV in 163 Ho) = 0.35±0.15 ns , T 1 2 (471.3 keV in 163 Ho) ⪅ 0.2 ns , T 1 2 (612.8 keV in 163 Ho) ⪅ 0.3 ns , T 1 2 (295.6 keV in 171 Lu) = 0.85±0.20 ns , T 1 2 (469.2 keV in 171 Lu) ≦ 0.2 ns , T 1 2 (357.0 keV in 173 Lu) = 0.40±0.08 ns and T 1 2 (449.0 keV in 173 Lu) = 0.58±0.12 ns . Following half-lives in 173Lu have been remeasured: T 1 2 (425.3 keV) = 0.84±0.20 ns and T 1 2 (434.9 keV) = 0.38±0.10 ns . Absolute γ-ray transition probabilities are deduced and compared with Nilsson model predictions including pairing correlations. Coriolis mixing calculations are performed for K-allowed as well as for K-forbidden transitions.

Levels in 179Ta and γ-ray transition probabilities in 179Ta and 183Re

The level scheme of the deformed odd-proton nucleus 179Ta has been investigated by inbeam spectroscopic methods. Single γ-ray spectra as well as prompt and delayed γ-γ coincidences were measured in the reactions 179Hf(d, 2n) and 179Hf(p, n). The proposed level scheme of 179Ta contains, in addition to the ground state band 7 2 + [404], the rotational bands built on the intrinsic states 9 2 − [514], 5 2 + [402], 1 2 + [411] and 1 2 − [541]. By nanosecond lifetime measurements with Ge(Li) detectors the following half-lives have been obtained for 179Ta levels: T 1 2 (238.6 keV) = 95 ± 5 ns , T 1 2 (520.3 keV) = 350 ± 20 ns , T 1 2 (628.1 keV) = 80 ± 10 ns . A three-quasi-particle state of T 1 2 = 8.6 ± 1 ms with spin and parity tentatively assigned as I π = ( 19 2 , 21 2 ) − has been identified at an excitation energy of 1253.3 keV. For the 5 2 1 2 − [541] level in 183Re, excited in the reaction 181Ta(α, 2n), a lifetime of T 1 2 = 2.0 ± 0.1 ns was measured.

E1 transition probabilities between the nilsson states [formula omitted

The half-lives of the 1 2 , 1 2 +[411] level at 208.1 keV in 171Lu and of the 1 2 , 1 2 +[411] and 3 2 , 1 2 + [411] levels at 425.0 and 434.6 keV in 173Lu have been measured in the decay of 171Hf and 173Hf to be 29.7±1.1 ns, 0.82±0.20 ns and 0.46±0.20 ns, respectively. These half-lives yield the absolute γ-ray transition probabilities of the El transition from 1 2 , 1 2 + [411] to 1 2 , 1 2 −[541] in 171Lu and of the E1 transitions from 1 2 , 1 2 +[411] to the spin 1 2 and 1 2 members of the 1 2 − [541] band and from 3 2 , 1 2 + [411] to the spin 3 2 and 5 2 members of the 1 2 − [541] band in 173Lu. These absolute transition probabilities are found to be in good agreement with the Nilsson estimate, taking into account pairing correlations and Coriolis coupling.

Interprétation des niveaux de 57Co par l'interaction cȩur-particule

The experimental data for 57Co are interpreted on the basis of the particle-core coupling model. The core is assumed to be 56Fe (in the ground and the excited states), and the particle (proton) states taken into account are: 1 f 7 2 , 2 p 3 2 , 2 p 1 2 and 5 2 . Th factors, and γ-ray transition probabilities and mixing ratios for the first nine levels are fitted quite well with the model.

Absolute γ-ray transition probabilities in doubly odd deformed nuclei

A compilation is presented which contains data about the available absolute electromagnetic transition probabilities between two-quasiparticle states in doubly odd deformed nuclei of medium-mass number. Comparisons of dipole transitions between the same Nilsson states in doubly odd and odd-mass nuclei are performed on the basis of Weisskopf and Nilsson hindrance factors. Some level assignments are critically considered. Octupole transitions are also systematized.

Distribution of Partial Radiation Widths inU238(n,γ)U239

The resonant-neutron-capture γ-ray spectra for 28 resonances below 600-eV neutron energy are measured in U238(n,γ)U239 with improved neutron and γ-ray energy resolution to allow an accurate test of the predictions of the statistical model of neutron capture. No convincing departures from these predictions are observed. The variation over 23 S-wave neutron resonances of the γ-ray transition probabilities to 15 final states is consistent with the Porter-Thomas distribution for both E1 and M1 multipoles. There is also no statistically significant correlation between the different decay modes of the neutron resonances. A large correlation coefficient of + 0.81 is observed between the partial radiative widths of the 3991- and 3982- keV γ rays. This result, however, is not in violation of statistical independence for a sample size of 15 γ rays. The E1 and M1 γ-ray strength functions are (2.6 ± 0.4) × 10−3 and (8.1 ± 1.6) × 10−3, respectively, compared to 3 × 10−3 and 4 × 10−3 listed by Bartholomew as the median values of all nuclei. Three of the weaker resonances at 10.2, 89.5, and 263 eV are assigned to P-wave capture. The neutron binding energy is measured to be 4806.7 ± 2.0 keV, which is 5.0 keV higher than the previously accepted value. Previously unobserved γ-ray transitions are reported.

Lifetimes of excited levels and electromagnetic transition rates in 147Pm and 199Hg

The lifetimes of the 687.4, 531.9 and 91.03 keV levels in 147Pm and the 158.2 keV level in 199Hg are measured by the delayed coincidence technique using a time-to-amplitude converter. The results are: T 1 2 (687.4 keV level, 147 Pm) = 0.25±0.10 ns , T 1 2 (531.9 keV level, 147 Pm) = 0.133±0.030 ns , T 1 2 (91.03 keV level, 147 Pm) = 2.57±0.02 ns) , T 1 2 (158.2 keV level, 199 Hg) = 2.49±0.03 ns . The energies and relative intensities of γ-rays in the decays of 147Nd and 199Au are determined using high-resolution Ge(Li) detectors. The γ-ray transition probabilities for M1 and E2 multipolarities are deduced from the above results.