The coriolis-coupled bands in 231Pa and the decay of 231Th
Abstract
A re-investigation of the 231Th decay scheme has been performed with magnetic and semiconductor spectrometers. The Q-value of the decay was determined to be Qβ = 389 ±2 keV. For 231Pa the measurements suggest spins and parities of for the 318.0, 320.2, and 351.8 keV levels, respectively. Half-lives of 274 ± 10 ps and 45.1 ± 1.3 ns were found for the 58.6 keV () and the 84.2 keV () levels. Calculations, in which the Nilsson model was extended to include Coriolis coupling of the iOrbitals and two N = 4 Orbitals, successfully explain the peculiar structure of the positive-parity rotational bands in 231Pa. From these calculations and from the value of the ground-state quadrupole moment, derived from the rotational transition probability B(E2, 58.6 keV), the deformation parameters were determined to be (ε2, ε4) = (0.164,−0.100).
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Nuclear Structure and Decay Data for A=231 Isobars
2022, Nuclear Data SheetsThe present evaluation of currently known nine nuclides of A=231: (231Rn, 233Fr, 231Ra, 231Ac, 231Th, 231Pa, 231U, 231Np and 231Pu) represents an update and revision of previous A=231 evaluation by 2013Br04. All the known decay and reaction data are evaluated, with recommended properties given in the Adopted Levels, Gammas datasets, for gamma-ray energies, photon branching ratios, level half-lives, spins, parities, transition probabilities, and configuration assignments. For 231Rn, only an identification of the nuclide has been made without an experimental determination of its half-life. For 231Fr, no excited states are known. About 30 levels in 231Ra up to 1774 keV excitation are known only from β− decay of 231Fr with level half-lives determined for five excited states. For 231Ac, excited states up to 3122 keV are known from β− decay, (t,α), and fragmentary information for high-spin levels from two heavy-ion reaction studies, with lifetimes of excited states measured for five levels. Extensive structure and rotational band information for 231Th is available for a large number of levels up to 1714 keV from β− and α decays, (n,γ) thermal and resonance, single-particle transfer reactions (d,p), (d,t) and (3He,α), with limited data for high-spin levels from one secondary reference, and half-lives available for five excited states. Very detailed structure data are available for 231Pa up to 2139 keV from β−, ε and α decays, (α,t), (d,d′) and high-spin data from heavy-ion Coulomb excitation and (p,2nγ) reaction, with level half-lives known for 34 excited states, mostly deduced by evaluators from transition probabilities determined in Coulomb excitation. Only eight excited states are known in 231U up to 1268 keV from ε and α decays, with no data for half-lives of excited states. For 231Np and 231Pu, only the ground-state is known from α decays, with a tentative level at 324 keV in 231Pu. Measurements of nuclear rms charge radius and isotope shifts for 231Ra have been made by 2018Ly01, and that for rms charge radius for 231Fr by 2014Bu06.
235U is a well known nuclide for nuclear structure and reactor applications. There have been a large number of studies of its alpha decay to levels in 231Th, including some recent ones such as 2018Ma03 and 2017Le03, yet evaluators' analysis of the decay scheme suggests that, while α-particle transitions are well established, but several issues still remain about intricacies of gamma-ray and conversion electron spectroscopy which need to be resolved through dedicated experiments for a complete understanding of the decay scheme characteristics, although, this sort of research project would seem a challenging spectroscopic problem. As commented in individual datasets for other decays, important spectral information is lacking for most of the decay schemes, with almost no information available for some of these, resulting in many incomplete decay schemes in A=231 mass chain.
Nuclear Data Sheets for A = 231
2013, Nuclear Data SheetsSpectroscopic data for all nuclei with mass number have been evaluated, and the corresponding level schemes from radioactive decay and reaction studies are presented.
Highlights of this evaluation include:
A new interpretation of γ–ray spectroscopic results measured with the GAMMASPHERE spectrometer (2002AbZV, 2000JaZY, 1999Br17) has established the level structure of the ground state rotational band up to spin/parity in 231Ac.
Precise measurements of energies and cross sections of scattered tritons from the 232Th(d,t) reaction has produced or confirmed the identification of several rotational bands in 231Th.
The alpha hindrance factors (HF) presented in this evaluation were calculated using values of the radius parameter interpolated from those for even–even adjacent nuclei given in 1998Ak04.
The Limitation of Relative Statistical Weight (LWM) method (1985ZiZY) has been used for discrepant data throughout this evaluation.
Assessment of internal conversion coefficients for anomalous electric dipole transitions
2008, Applied Radiation and IsotopesSound quantitative knowledge of internal conversion coefficients (ICCs) is important when establishing decay schemes. ICCs are normally derived from efficiency tables and calculation, but for anomalous transitions and some electric dipole transitions (E1) in particular, theoretical values can differ considerably from the few available measured values. Experimentally measured ICCs and their ratios have been compiled, and these data have been used to propose a method based on the existence of measured ratios to determine the total ICC for such transitions. Comparisons have been made with the existing measured values, and good agreement was observed within the uncertainty limits.
Coulomb excitation and structure of the rotational bands in <sup>231</sup>Pa
2003, Nuclear Physics AThe level structure of 231Pa was investigated by Coulomb excitation using 148 MeV 32S and 255, 260 and 261 MeV 58Ni projectiles and the NORDBALL detector array. The 1/2[530] ground-state rotational band was excited up to spin 39/2−. The 3/2[651] band, established up to spin 37/2+, was observed to be populated apart from direct Coulomb excitation by the de-excitation of the 1/2[530] band. The 1/2[400] band could be followed up to spin 23/2+. Weak excitation of the bands 5/2[642], 5/2[512], 3/2[532] and 3/2[521] was seen up to spins 11/2+, 15/2−, 9/2− and 9/2−, respectively. Matrix elements were extracted from the γ-ray yields with the help of the semi-classical Coulomb-excitation least-squares search code GOSIA. The level scheme and transition intensities were analyzed in the framework of the quasiparticle-plus-phonon model including Coriolis-coupling (QPMC).
Nuclear data sheets for A = 215, 219, 233, 227, 231
2001, Nuclear Data SheetsThe evaluator presents in this publication spectroscopic data and level schemes from radioactive decay and nuclear reaction studies for all nuclei with mass numbers A=215, 219, 223, 227, and 231. Nuclei with mass number A=215 have spherical shape, and their nuclear structure have been interpreted in terms of the shell model. Nuclei with mass number A=219 belong to a transitional region with spherical, quadrupole, and possibly octupole deformations; those with mass numbers A=223 and 227 belong to a newly studied region of coexisting quadrupole with possible octupole deformations; and nuclei with mass number A=231 essentially have quadrupole deformation. Highlights of this publication are given below: A recent study of the 206Pb(13C,4nγ) reaction (1998St24) has extended up to Jπ=61/2+ the previously known level scheme from 208Pb(13C,6nγ) (1988Fu10,1983Lo16). This study has provided a new interpretation for all the spectroscopic data above the 2247--keV (Jπ=29/2--) level in 215Ra. A new isotope, 219U (42 μs), which was identified with the recoil separator VASSILISSA (1994Ye08,1994AnZY,1993An07), has been included in this evaluation. The first 223Rn β-- decay scheme (1992Ku03) and a significantly improved measurement of 227Ac α decay (1995Sh03) are presented here. A level scheme of 223Fr has been constructed on the basis of these studies, where most of the levels below 600 keV have been assigned to parity doublet bands and interpreted in terms of the reflection--asymmetric rotor model. This evaluation includes the first level structures of 227Fr and 227Th. Levels in 227Fr are from 227Rn β-- decay (1997Ku20); those in 227Th, are from 231U α decay, from 227Pa electron--capture decay, and from 226Ra(α,3nγ) (1997Mu08,1995Li04,1998Ma83). A study of the β-- decay of a mass--separated source of 231Fr has produced the first level scheme of 231Ra (2001Fr05). The only published data (1994Br36,1996Le01) on the electron--capture decay of 231U have provided much information on the level structure of 231Pa. Studies on 232Th(p,2nγ) and Coulomb Excitation of 231Pa (1996Le01,1999Wu05), which have complemented the existing spectroscopic data from these reactions, have suggested the possibility of weak octupole correlations in 231Pa. A new isotope of plutonium, 231Pu (1999La14), has been included in this evaluation.
Measurement of gamma-ray intensities of <sup>231</sup>Th using semiconductor detectors
1999, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentNuclide 231Th was yielded by the 232Th(n, 2n) reaction with neutron irradiation in the Kyoto University Reactor (KUR). Moreover, the thorium was purified chemically. Gamma-ray spectra of thorium have been measured using low-energy photon spectrometers and a high-purity germanium detector. Relative γ-ray intensities ranging from 25 to 352 keV in the decay of 231Th have been determined with satisfactory accuracy. The results are in very good agreement with those of earlier studies. We observe two new γ-rays at 77.69 and 177.66 keV, whose intensities are found to be (0.063±0.010)% and (0.00095±0.00020)%, respectively, relative to that of 84.21 keV taken as 100%. Absolute intensity of 84.21 keV γ-ray which is the most prominent one from the decay of 231Th and that of 185.739 keV following the decay of 235U are also determined from the secular equilibrium for 235U–231Th. The results obtained in two cases are (6.60±0.25)% and (58±2)%, respectively.