Bernd P. K. Maier

The double focusing iron-core electron-spectrometer “BILL” for high resolution (n, e−) measurements at the high flux reactor in Grenoble

A new high resolution iron-core electron spectrometer (BILL) has been constructed at the High Flux Reactor in Grenoble. It is currently being used for measurements of conversion electrons after thermal neutron capture. An internal target (maximal size 40 cm2) is irradiated in a thermal neutron flux of 3 × 1014 n cm−2 sec−1. The electrons pass through a vertical beam tube, 14 m long and 10 cm in diameter, defining the solid angle of 3.4 × 10−6 / 4π. The spectrometer itself consists of two independent flat electromagnets. Both magnets act as double focusing spectrometers, where the double focusing is achieved by the combination of homogeneous and 1r fields. The first, a 58° deflecting magnet, forms a 10:1 scaled down intermediate image of the large, 14 m distant target in the reactor hall. The dispersion of this magnet is 94 cm. The second magnet views this reduced image with a very high dispersion of 540 cm. The radius of the optic circle of both magnets is 50 cm. The energy range of the spectrometer is 15 keV < E < 10 000 keV. Multi-wire proportional counters are used as detectors. The best measured momentum resolution has been 8 × 10−5 at 300 keV with a target of 2 × 100 mm2. Routine measurements have been carried out with targets of 30 × 100 mm2 achieving a resolution of Δp/p = 4 × 10−4. The momentum precision of the instrument is 1 part in 105 and the sensitivity for neutron capture conversion electrons 300 μb at 400 keV.

ENERGY LEVELS AND GAMMA TRANSITIONS IN Dy 164

The neutron capture Gamma ray spectrum of Dy 164 was measured with the curved crystal spectrometer at the DR-3 reactor at Risø. A level scheme was constructed from the energies (30 keV−1 MeV) and the intensities of the detected lines. The states obtained are: the groundstate rotational band with the levels at 73.392 (0.2+), 242.230 (0.4+), and 501.32 keV (0.6+), the Gamma vibrational levels 761.80 (2.2+), 828.19 (2.3 +), 915.98 (2.4+) and 1024.63 keV (2.5+) and a band withK=2−having the energies 976.88 (2.2−), 1039.30 (2.3-), 1122.76 (2.4−) and 1225.15 keV (2.5−). The precisely determined energies are compared with the collective model. The intensities of the transitions agree with the branching ratios expected from this model.

Niederenergetische Gammalinien vom Neutroneneinfang in Lu 175 und Lu 176

The low energy gamma-rays from neutron-capture in Lu 175 and Lu 176 have been investigated by means of the bent crystal-spectrometer at the DR-3-reactor at Risø. From the transitions in Lu 177 3 rotational bands have been determined. The levels of the (404)K=7/2+ groundstate rotational band are: 121,62 keV (I=9/2), 268,79keV (I=11/2), 440,66 keV (I=13/2), 636,22 keV (I=15/2), 854,34 keV (I=17/2). The level-sequence of the (514)K=9/2−-band is: 150,39 keV (I=9/2), 288,99 keV (I=11/2), 451,49 keV (I=13/2), 637,05 keV (I=15/2) and 844,88 keV (I=17/2). At 457,92 keV is the basis for the (402)K=5/2+-band the higher levels of which are 552,05 keV (I=7/2), 671,89 keV (I=9/2), 816,63 keV (I=11/2), 985,23 keV (I=13/2), 1176,73keV (I=15/2) and probably 1389,5 keV (I=17/2). The energies of the levels apart from the 1389 keV-level have an accuracy of 7×10−5. The energy differences between the 3 bands agree very well with the values expected from the Bohr-Mottelson-formulaE=A·I(I+1)+B·I2(I+1)2. The calculated branching-ratios within the 3 bands are in fairly good agreement with the experimental values. ThegK-factors have been determined for 2 bands: It was found that for the (514)-bandgK=1,16±0,04 and for the (402)-bandgK=1,33±0,07.

GAMMA TRANSITIONS AND ENERGY LEVELS IN

220 neutron capture Gamma rays of Dy 165 were measured with the Riso curved crystal spectrometer. The intense transitions permitted the construction of a level scheme for this oddN nucleus. The rotational bandsK=7/2+[633],K=1/2−[521] andK=5/2−[512] were precisely localized, including the spin 11/2 states. Gammavibrational bands built on these Nilsson orbits were observed with their band heads at 538.6, 573.5 and 570.2 keV. TheI=5/2 term at 533 keV can be considered as the 5/2−[523] Nilsson state. The branching ratios allowed the determination of (gK−gR)2-factors for the [633], [521] and [512] bands and yieldedE 1-hindrance factors.Abstract220 neutron capture Gamma rays of Dy 165 were measured with the Risø curved crystal spectrometer. The intense transitions permitted the construction of a level scheme for this oddN nucleus. The rotational bandsK=7/2+[633],K=1/2−[521] andK=5/2−[512] were precisely localized, including the spin 11/2 states. Gammavibrational bands built on these Nilsson orbits were observed with their band heads at 538.6, 573.5 and 570.2 keV. TheI=5/2 term at 533 keV can be considered as the 5/2−[523] Nilsson state. The branching ratios allowed the determination of (gK−gR)2-factors for the [633], [521] and [512] bands and yieldedE 1-hindrance factors.

sup 165

Abstract The low-energy spectrum of the 55 Mn(n, γ) 56 Mn reaction has been studied with a γ-diffraction spectrometer. These data allowed the construction of a level scheme for 56 Mn with two previously unobserved doublets. High-energy γ-transitions to the low-energy states have been measured for different neutron resonances at the BNL fast neutron chopper. For most states a unique spin and parity could be proposed. A shell-model calculation of the structure of the low-lying states of 56 Mn has been carried out with the Oak Ridge-Rochester computer programme; there are plausible theoretical analogues for 11 of the 12 experimentally observed states. Calculated spectroscopic factors and M1 transition probabilities are in qualitative agreement with the experiment.

Dy

Theγ-ray spectrometer at the DR 3-reactor at Risø was used to investigate the low energyγ-lines of U 239 which are emitted during neutron-capture in U 238. Very closely lying lines have been found between 500 and 650 keV. The big intensity of the 133 keV line and the lack of other transitions of appropriate energy and intensity point out that this line corresponds to the hinderedE 2-transition from the (631)I=1/2+ level to the (622)I=5/2+ groundstate. A suggestion was made for the decay possibility of the octupole states at 700 keV found by N.Fiebiger.

Energy levels of 56Mn

Abstract Utilizing the Riso bent crystal spectrometer, 47 transitions have been observed from 54 keV to ≈ 1 MeV from the reaction Nb93(n,γ)Nb94. These transitions determine the following levels in Nb94 (energy in keV, spin-parity in parentheses): G.S. (6+, ≈ 40.7 (3+), 58.724 (4+), 78.675 (7+), 113.404 (5+), 314.64 (5+), ≈ 334 (2+) and 640.5 (5+). The 315 keV and 641 keV states probably correspond primarily to the J = 3 2 state of the three neutron configuration coupling to the g 9 2 proton configuration. The other six states result primarily from the coupling of the J = 5 2 state of the three neutron configuration and the g 9 2 proton configuration. Energies of the levels and transition probability ratios are consistent with this interpretation.

Bestimmung des (631)K=1/2, I=1/2-Zustandes im U 239

Abstract103 gamma transitions in Hg 200 have been observed. 37 of them were fitted into a level scheme which comprises 17 states. The energy of the one-phonon vibrational stateI=2+ has been determined to be 367.970±0.020 keV, that of the two-phonon vibrational triplet 947.34±0.07 keV, 1029.37±0.05 keV and 1254.20±0.10 keV. Further levels have been found at energies of 1570.30±0.10 keV, 1571.7±0.6keV, 1594.3±0.4 keV, 1631.5±0.2 keV, 1641.5±0.3 keV, 1718.5±0.2 keV, 1730.8±0.2 keV, 1775.7±1.1 keV, 1842.9±0.8 keV, 1882.9±0.3 keV, 2062.6±0.5 keV and 2082.9±1.0 keV.

Energies and character of low-lying levels in Nb94

A value ofσ=3, 5±1 is obtained for the spin cut off coefficient in the Fermi gas level density formula from a comparison of calculated and experimental (n, γ)-intensities feeding the observed levels of deformed nuclei in the rare earths region. The model used for the computations is tested at two nuclei with compound states of low spin (Ic=1/2) and high spin (Ic=13/2 or 15/2). The calculations can help to determine spins of nuclear levels in some cases.