We investigated decays of K 51 , 52 , 53 at the ISOLDE Decay Station at CERN in order to understand the mechanism of the β -delayed neutron-emission ( β n ) process. The experiment quantified neutron ...and γ -ray emission paths for each precursor. We used this information to test the hypothesis, first formulated by Bohr in 1939, that neutrons in the β n process originate from the structureless “compound nucleus.” The data are consistent with this postulate for most of the observed decay paths. The agreement, however, is surprising because the compound-nucleus stage should not be achieved in the studied β decay due to insufficient excitation energy and level densities in the neutron emitter. In the K 53 β n decay, we found a preferential population of the first excited state in Ca 52 that contradicted Bohr’s hypothesis. The latter was interpreted as evidence for direct neutron emission sensitive to the structure of the neutron-unbound state. We propose that the observed nonstatistical neutron emission proceeds through the coupling with nearby doorway states that have large neutron-emission probabilities. The appearance of “compound-nucleus” decay is caused by the aggregated small contributions of multiple doorway states at higher excitation energy. Published by the American Physical Society 2024
At CERN-ISOLDE, high-purity radioactive ion beams of 219Fr and 221RaF were investigated with α-decay spectroscopy at the CRIS and ASET experiments in the course of three different experimental ...campaigns. The half-life of 215At, α-decay daughter of 219Fr, is measured to be 36.3(3)9μs, and that of 221Ra was determined to be 26.2(1)6s, both of which are well in line with the trends in this region of the nuclear landscape but at odds with some of the reported literature.
The long-lived xenon isomers 129mXe and 131mXe are of interest for the GAMMA-MRI project, which aims at developing a novel imaging modality based on magnetic resonance of polarized unstable tracers. ...Here, we present the steps leading to and following the production of these two isomers via neutron irradiation of highly-enriched 128Xe and 130Xe gas samples at two high-flux reactors, the High-Flux Reactor (Réacteur à haut flux, RHF) at the Institut Laue-Langevin (ILL) and the MARIA reactor at the National Centre for Nuclear Research (NCBJ). We describe the experimental setups and procedures used to prepare the stable xenon samples, to open the irradiated samples, and to transfer xenon isomers into reusable transport vials. The activity of 129mXe and 131mXe was measured to be in the range of tens of MBq per sample of 0.8(1)mg, and was proportional to thermal neutron flux density. A small activity of unstable contaminants was also visible in the samples, but their level is not limiting for the GAMMA-MRI project’s objectives. In addition, the minimum thermal neutron flux density required to produce 129mXe and 131mXe sufficient for the project could be also determined.
•GAMMA-MRI project - a novel imaging modality.•Gaseous samples preparation for high-flux neutron irradiation.•Successful production of xenon isomers (129mXe and 131mXe) in high-flux nuclear reactors.•Neutron capture cross section for 128Xe and 130Xe in Mauwellian neutron flux.
Models of the β-delayed neutron emission (βn) assume that neutrons are emitted statistically via an intermediate compound nucleus post β decay. Evidence to the contrary was found in an 134In β-decay ...experiment carried out at ISOLDE CERN. Neutron emission probabilities from the unbound states in 134Sn to known low-lying, single-particle states in 133Sn were measured. The neutron energies were determined using the time-of-flight technique, and the subsequent decay of excited states in 133Sn was studied using γ-ray detectors. Individual βn probabilities were determined by correlating the relative intensities and energies of neutrons and γ rays. The experimental data disagree with the predictions of representative statistical models which are based upon the compound nucleus postulate. Our results suggest that violation of the compound nucleus assumption may occur in β-delayed neutron emission. This impacts the neutron-emission probabilities and other properties of nuclei participating in the r-process. A model of neutron emission, which links the observed neutron emission probabilities to nuclear shell effects, is proposed.
β-delayed γ-neutron spectroscopy has been performed on the decay of A = 84 to 87 gallium isotopes at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of 3He neutron ...counters (BRIKEN). β-2n-γ events were measured in the decays of all of the four isotopes for the first time, which is direct evidence for populating the excited states of two-neutron daughter nuclei. Detailed decay schemes with the γ branching ratios were obtained for these isotopes, and the neutron emission probabilities (Pxn) were updated from the previous study. Hauser-Feshbach statistical model calculations were performed to understand the experimental branching ratios. We found that the P1n and P2n values are sensitive to the nuclear level densities of 1n daughter nuclei and showed that the statistical model reproduced the P2n/P1n ratio better when experimental levels plus shell-model level densities fit by the Gilbert-Cameron formula were used as the level-density input. We also showed the neutron and γ branching ratios are sensitive to the ground-state spin of the parent nucleus. Our statistical model analysis suggested J ≤ 3 for the unknown ground-state spin of the odd-odd nucleus 86Ga, from the Iγ(4+ → 2+)/Iγ(2+ → 0+) ratio of 84Ga and the P2n/P1n ratio. In conclusion, these results show the necessity of detailed understanding of the decay scheme, including data from neutron spectroscopy, in addition to γ measurements of the multineutron emitters.
At CERN-ISOLDE, high-purity radioactive ion beams of
Fr and
RaF were investigated with α-decay spectroscopy at the CRIS and ASET experiments in the course of three different experimental campaigns. ...The half-life of
At, α-decay daughter of
Fr, is measured to be 36.3(3)9μs, and that of
Ra was determined to be 26.2(1)6s, both of which are well in line with the trends in this region of the nuclear landscape but at odds with some of the reported literature.
The β decay of the neutron-rich 134In and 135In was investigated experimentally in order to provide new insights into the nuclear structure of the tin isotopes with magic proton number Z=50 above the ...N=82 shell. The β-delayed γ-ray spectroscopy measurement was performed at the ISOLDE facility at CERN, where indium isotopes were selectively laser-ionized and on-line mass separated. Three β-decay branches of 134In were established, two of which were observed for the first time. Population of neutron-unbound states decaying via γ rays was identified in the two daughter nuclei of 134In, 134Sn and 133Sn, at excitation energies exceeding the neutron separation energy by 1 MeV. The β-delayed one- and two-neutron emission branching ratios of 134In were determined and compared with theoretical calculations. The β-delayed one-neutron decay was observed to be dominant β-decay branch of 134In even though the Gamow-Teller resonance is located substantially above the two-neutron separation energy of 134Sn. Transitions following the β decay of 135In are reported for the first time, including γ rays tentatively attributed to 135Sn. In total, six new levels were identified in 134Sn on the basis of the βγγ coincidences observed in the 134In and 135Inβ decays. A transition that might be a candidate for deexciting the missing neutron single-particle 13/2+ state in 133Sn was observed in both β decays and its assignment is discussed. Experimental level schemes of 134Sn and 135Sn are compared with shell-model predictions. Using the fast timing technique, half-lives of the 2+, 4+, and 6+ levels in 134Sn were determined. From the lifetime of the 4+ state measured for the first time, an unexpectedly large B(E2;4+ → 2+) transition strength was deduced, which is not reproduced by the shell-model calculations.
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