Anders Bäcklin

Collective and two-quasiparticle states in 158Gd observed through study of radiative neutron capture in 157Gd

Abstract The level structure of 158 Gd has been studied using the prompt γ-rays and conversion electrons emitted following neutron capture in 157 Gd. The γ-ray energy and intensity measurements were made using both Ge(Li) detectors and a curved-crystal spectrometer. Conversion-electron energy and intensity measurements were made using two separate magnetic spectrometers: one to measure the primary electron spectrum and the other to measure the lower energy secondary electron spectrum. Some γ-γ coincidence measurements were also made among the secondary γ-rays. From these data, a neutron separation energy of 7937.1 ± 0.5 keV has been determined for 158 Gd. A level scheme containing 59 excited states with energies 158 Gd. Many of these states have been grouped into rotational bands. A total of thirteen excited rotational bands with band-head energies below 2.0 MeV are contained in the level scheme. Features of the proposed level scheme include: the K π = 0 − , 1 − and 2 − octupole-vibrational bands with band-head energies of 1263, 977 and 1793 keV, respectively; the γ-vibrational band at 1187 keV; three excited K π = 0 + bands with band-head energies of 1196, 1452 and 1743 keV; several two-quasiparticle bands with band-head energies in keV (and K π assignments) of 1380 (4 + ), 1636 (4 − ), 1847 (1 + ), 1856 (1 − ), 1920 (4 + ) and 1930 (1 + ). An analysis of (d, p) reaction data is presented which permits definite two-quasiparticle configuration assignments to be made to most of these latter bands. Evidence is presented which suggests strong mixing of some two-neutron and two-proton bands. A phenomenological four-band mixing analysis is made of the energy and E2 transition-probability data for the ground-state band and the three lowest-lying excited collective positive-parity bands. Good agreement with experiment is obtained. A Coriolis-mixing analysis of the octupole bands has been carried out and good agreement with the data on level energies and E1 transition probabilities to the ground-state band has been achieved. Values of Z , the ratio of the E1 transition matrix element with ΔK = 1 to that with ΔK = 0, involving the octupole bands and the first four 0 + bands are derived. For three of these 0 + bands, absolute values of these matrix elements are deduced. An interesting alternation in the sign of Z is observed for these four 0 + bands.

Possible deformed states in 115In and 117In

Levels and transitions in 115In and 117In have been studied from the beta decay of 2.3 d 115gCd and 2.5 h 117gCd. Using a Ge(Li) detector and a double focussing beta spectrometer, energies, intensities, conversion coefficients and multipolarities were obtained for the following transitions (energies in keV and multipolarities are given): 115In: 35.63 (97.0% M1 + 3.0% E2), 231.47 (E1), 260.80 (M1), 267, 336.23 (M4+< 5% E5), 492.14 (96% E1 + 4% M2), 527.70 (E1). 117In: 71.0, 89.80 (E2+< 20% M1), 273.32 (M1, E2), 315.27 (M4+<7% E2), 344.29 (E1), 434.12 (E1). Using the delayed coincidence technique, half-lives were measured for two levels in 115In and for three levels in 117In. Energies, spins, parities and half-lives are given for the following levels: 115In: 597.03, 32−; 828.39, 32+, 5.4 ns; 863.95, 12+ or 32+, 1.1 ns. 117In: 588.59, 32−; 0.20 ns; 659.56, 32+, 58.7 ns; 749.37, 12+ or 32+, 4.3 ns. Reduced transition probabilities are given for several transitions in both nuclei. The E2 transition rates between the two excited positive parity states in both nuclei were found to be about 100 s.p.u. indicating a possible deformation of these states. The energy spacing and transition rates between these states can be well accounted for within the Nilsson model assuming the states to form a K = 12+ rotational band. A deformation δ of about 0.20 is obtained for both nuclei.

Quadrupole collective properties of 114Cd

Abstract The nucleus 114 Cd has been Coulomb excited using beams of 16 O, 40 Ca, 58 Ni and 208 Pb. Several new states have been observed and an almost complete set of reduced E2 matrix elements for the lowest-lying positive-parity states in 114 Cd have been measured. In total, about 40 E2 matrix elements have been determined in a model-independent way, including the static quadrupole moments of the 4 + 1 , 6 + 1 , 2 + 2 and 2 + 3 states. Large negative static quadrupole moments were found for the 4 + 1 and 6 + 1 states and a large positive quadrupole moment for the 2 + 2 state. All E2 matrix elements involved in the deexcitation of the quintuplet of states at an energy of about 1.2 Me V were determined. The data are compared with predictions by various models, including a configuration mixing calculation within the framework of the IBA model, the harmonic vibrator and a model in which the states of a near-harmonic vibrator are mixed with those of a rotational intruder band with a large deformation. Although no perfect match is obtained, the data favour a vibration-like structure involving levels up to the four-quadrupole phonon multiplet.

Possible rotational states in odd In nuclei

Abstract Levels and transitions in 107 In and 109 In have been studied in in-beam spectroscopy on the reactions 106 Cd(d, nγ) 107 In and 108 Cd(d, nγ) 109 In. Low-lying 1 2 + and 3 2 + states resembling the possible rotational bands observed in the heavier In isotopes are seen in both nuclei. The potential energy has been calculated for 107–121 In with the odd proton in different orbitals, using the Strutinsky normalization procedure. A prolate minimum at a deformation e ≈ 0.2 is obtained for the lowest 1 2 + orbital, which is in good agreement with the experimental data for 115–119 In. The excitation energy of the 1 2 + minimum shows a fairly good agreement with experimental data.

Internal conversion study of 113Cd(n, γ) 114Cd

Abstract The internal conversion spectrum from 113 Cd(n, γ) 114 Cd has been measured with a resolution of 0.2%. Conversion lines were found for 45 transitions, and the corresponding multipolarities were obtained with the aid of gamma ray intensities from earlier crystal diffraction measurements. In addition to the known E0 transitions from the 1134 and 1305 keV levels to the ground state, a line corresponding to an E0 transition between these levels was found. The conversion coefficients of four transitions between levels of equal spin and parity were found to be anomalously large, which can be ascribed to E0 admixture. Above 400 keV, the energy values were found to deviate in a systematic way from those obtained in the crystal diffraction measurements. The decay scheme was constructed by using a computer programme based on energy relationships. The possibility of obtaining accidental relationships was taken into account. Seven levels were established above 1.4 MeV.

Multiphonon vibrational states in 106,108Pd

Abstract The nuclei 106 Pd and 108 Pd have been Coulomb excited using beams of 16 O, 58 Ni and 208 Pb. The data determined 25 and 31 E2 matrix elements in 106 Pd and 108 Pd, respectively. The experimental data are qualitatively in agreement with the spherical-harmonic quadrupole vibrational model, but quantitatively there are large discrepancies, even for the states generally accepted to be the two-phonon vibrational states. A sum-rule method was applied to the data, the result of which implies that the vibrational quadrupole strengths of the 0 + and 4 + two-phonon states are fragmented and shared between several E2 matrix elements. The E2 matrix elements imply appreciable quadrupole triaxiality with a γ-centroid of about 20°. A systematic comparison is made with neighbouring Pd-isotopes and with 114 Cd.

The level structure in 193Ir

Abstract The decay of 193 Os has been studied using Ge(Li) detectors and a 50 cmπ√2 spectrometer. Multipolarities and E2/M1 mixing ratios have been determined for 23 transitions. Half-life measurements applying the delayed coincidence technique have given the following mean lives: τ (138.97 keV level) = 108 ± 15 ps and τ (180.17 keV level) = 80 +11 −21 ps. The M1 and E2 transition probabilities obtained are discussed in terms of the core excitation model and the Nilsson model.

A Tomographic Method for Verification of the Integrity of Spent Nuclear Fuel Assemblies—II: Experimental Investigation

Abstract A tomographic method for verification of the integrity of used light water reactor fuel has been experimentally investigated. The method utilizes emitted gamma rays from fission products in the fuel rods. The radiation field is recorded in a large number of positions relative to the assembly, whereby the source distribution is reconstructed using a special-purpose reconstruction code. An 8 × 8 boiling water reactor fuel assembly has been measured at the Swedish interim storage (CLAB), using installed gamma-scanning equipment modified for the purpose of tomography. The equipment allows the mapping of the radiation field around a fuel assembly with the aid of a germanium detector fitted with a collimator with a vertical slit. Two gamma-ray energies were recorded: 662 keV (137Cs) and 1274 keV (154Eu). The intensities measured in 2520 detector positions were used as input for the tomographic reconstruction code. The results agreed very well with simulations and significantly revealed a position containing a water channel in the central part of the assembly.

The low-energy level structure of 191Ir

Abstract The decay of 191 Pt to 191 Ir has been investigated using Ge(Li) detectors and a double focusing beta spectrometer. Thirty-five transitions were observed and most of them were placed in a level scheme. Special attention was given to the low energy level band structure. Several multipolarity mixing ratios were determined from L-subshell ratio measurements. Using the delayed coincidence technique the half-life of the 179.05 keV level was measured to 40±12 psec. The low-level decay properties are discussed in terms of the Nilsson model with the inclusion of Coriolis coupling.

Nondestructive Experimental Determination of the Pin-Power Distribution in Nuclear Fuel Assemblies

A need for validation of modern production codes with respect to the calculated pin-power distribution has been recognized. A nondestructive experimental method for such validation has been developed based on a tomographic technique. The gamma-ray flux distribution is recorded in each axial node of the fuel assembly separately, whereby the relative rod-by-rod content of the fission product 140Ba is determined. Measurements indicate that 1 to 2% accuracy (1σ) is achievable. A device has been constructed for in-pool measurements at reactor sites. The applicability has been demonstrated in measurements at the Swedish boiling water reactor (BWR) Forsmark 2 on irradiated fuel with a cooling time of 4 to 5 weeks. Data from the production code POLCA-7 have been compared to measured rod-by-rod contents of 140Ba. An agreement of 3.1% (1σ) has been demonstrated. It is estimated that measurements can be performed on a complete BWR assembly in 25 axial nodes within an 8-h work shift. As compared to the conventional method, involving gamma scanning of individual fuel rods, this method does not require the fuel to be disassembled nor does the fuel channel have to be removed. The cost per measured fuel rod is estimated to be an order of magnitude lower than the conventional method.