H. Ohm

Investigation of beta strength functions by neutron and gamma-ray spectroscopy

Abstract Neutron spectra from the decay of the β− delayed neutron precursors 55.6 sec 87Br, 24.5 sec 137I, 2.05 sec 85As and 1.71 sec 135Sb have been studied with high resolution 3He ionization chambers. By γ-ray measurements, the partial neutron emission probabilities to excited states in 84Se and 134Te have been determined independently. For the neutron-emitter nuclei 87Kr and 137Xe, from which neutron decay proceeds only to the ground states of the final nuclei, 86Kr and 136Xe, unique information on the shape of the β− strength function Sβ(E) is obtained. For the emitter nuclei 85Se and 135Te, correlations between neutron transition energies and differences in level energies in 84Se and 134Te, have defined some levels in 85Se and 135Te which are strongly fed by β− decay. It is shown that the experimental shape of the β− strength function cannot be explained by the widely used assumptions Sβ(E) = const or Sβ(E) ∼ ϱ(E) or by the “gross theory” of β-decay, but that it is the result of structures in the low-energy tail of the Gamow-Teller giant resonance (GTGR) expected from general nuclear structure considerations.

Beta-delayed neutron emission from93–100Rb to excited states in the residual Sr isotopes

Beta-delayed neutron emission from the precursors93–100Rb to excited states in the residual nuclei92–99Sr has been measured by means ofγ-ray and neutron spectroscopy. In general, considerable neutron feeding of excited final states is observed. The experimental branching ratios (Pni) are compared to predictions from statistical model calculations. It is demonstrated that the main parameter affecting thePni-values is the shape of theβ-strength function (Sβ). For several Rb isotopes hindrance, respectively enhancement of neutron branches to specific final states is observed giving evidence for the persistence of intermediate structure of the neutron-emitting states. By the example of98Rb it is shown that a few integralβ-delayed neutron properties may be sufficient to derive first estimates on the real shape ofSβ above the neutron binding energy. This result is of importance for exotic nuclei where detailed spectroscopic investigations are precluded.

Delayed neutrons and high-energy γ-rays from decay of 87Br

Abstract Delayed neutrons and high-energy γ-rays following decay of 87Br have been studied to characterize the β-decay to and the decay of neutron unbound levels in 87Kr. The principal part of the neutron spectrum appears due to some 20–30 individual transitions and for seven of these, decay by γ-ray emission has been demonstrated. The β-strength function derived from these data and the decay scheme at lower energies is dominated by a broad resonance located near the neutron binding energy which can be identified with the main strength of a single-particle transition. The fluctuations in the reduced β-transition probabilities are found to be in accord with the Porter-Thomas distribution and the observed level density just above the neutron binding energy is consistent with the Gilbert and Cameron prediction after accounting for fluctuation in the β-decay transition probabilities.

The beta-decay of95Rb and97Rb

Theβ-decay of 377 ms95Rb and 168ms97Rb has been thoroughly investigated by means ofγ-ray, conversion electron andβ-delayed neutron spectroscopy. More than 97% of theβ-decay to particle-bound states has been placed in level schemes of95Sr and97Sr. High-resolution neutron spectroscopic studies have allowed to extend the knowledge of the excitation spectra up to 9 MeV, respectively 10 MeV. The density of neutron-unbound levels in95Sr and97Sr has been derived from peak stripping analyses of the neutron spectra. Beta-strength functions (Sβ(E)) have been investigated in detail. They indicate that Gamow-Tellerβ-decay of both precursors is dominated by nuclear structure. The discovered pronounced resonances and the variation in the shape ofSβ(E) are compared with predictions from the gross theory ofβ-decay and shell model calculations in the random phase approximation (RPA).

The 49K beta decay

Abstract The decay of 49 K has been studied using a mass-separated ion beam by neutron and gamma-ray singles and multiparameter spectroscopic measurements. The 49 K activity was produced by 600 MeV proton fragmentation reactions in a uranium target. The observed beta-strength function displays two resonances centered at about 6.5 MeV and 9.5 MeV in 49 Ca. This structure is discussed in simple shell-model terms.

Beta-delayed neutrons and high-energy gamma-rays from decay of137I

Beta-delayed neutrons and high-energyγ-rays following decay of137I have been studied in order to characterizeβ-decay to and deexcitation of neutron-unbound levels in137Xe. The totalγ-ray intensity observed accounts for at least 20% of theβ-intensity to levels aboveBn. No evidence was found for neutron andγ-ray emission from the same levels. With the experimentalβ-strength function aboveBn detailed tests were performed on the reproduction of the experimental neutron spectrum, the level density and Porter-Thomas fluctuations of the reducedβ-transition probabilities to highly excited states in137Xe.

Nonstatistical interpretation of delayed neutron emission — simple shell model approach

Abstract A simple shell model calculation has been shown to account for the main features of the β-strength function of the delayed neutron precursors 85 As, 87 Br, 135 Sb and 137 I, and also for the neutron emission probabilities of these and the precursors 88–92 Br. Preferential neutron emission from intermediate levels to excited states in the final nuclei is explained by considering the magnitudes of particle-hole excitations in the wave functions of levels accessible to neutron emission.

Systematic trends in the beta minus strength functions of neutron-rich rubidium isotopes

Abstract Experimental β − -strength functions ( S β ) are presented for the odd-mass Rb isotope sequence A =89to97. As expected from general nuclear structure considerations, but in contrast to present purely statistical concepts, these strength functions exhibit well separated “pygmy” resonances and a systematic behaviour as a function of mass number and β -decay energy.

Beta-delayed neutron emission following the decay of 17N

Abstract The delayed neutron spectrum following β − decay of the 4.17 sec 17 N has been measured with high-resolution 3 He spectrometers. The intensities of the neutron branches from levels in 17 O at 4549.3 ± 1.3, 5081 ± 21, 5387.1 ± 1.2 and 5949.9 ± 1.9 keV excitation energy result in 17 N β − transitions of (34.8 ± 2.6), (0.6 ± 0.4), (52.7 ± 3.5) and (7.0 ± 0.5)%, respectively. The neutron widths of these states were determined as 54.8 ± 0.4, 113 ± 55, 63.2±1.1 and 60.5 ± 3.2 keV, respectively, in disagreement with previously reported values. The results are compared with theoretical calculations and with the mirror decay of 17 Ne.

The analysis of delayed neutron energy spectra recorded with 3He ionization chambers

Abstract Details of the neutron detection efficiency of 3 He ionization chambers have been clarified. The usefulness of pulse rise-time analysis has been explored for the example of the 17 N delayed neutron spectrum. Because of obvious disadvantages, this method has been rejected, however, in favour of a complete response correction that removes all others than 3 He(n,p) 3 H events with great precision without disturbing the efficiency function. Peak fitting analysis is applied to structured delayed neutron spectra, and its physical significance is discussed.