Trap-assisted studies of odd, neutron-rich isotopes from Tc to Pd

We review the present and future of trap-assisted structure studies of odd, neutron-rich Tc, Ru, Rh and Pd isotopes at the limits of present experimental techniques. These nuclei of refractory elements are produced in light-particle induced fission and filtered by their mass number with the IGISOL mass separator. Further mass separation with the JYFLTRAP Penning trap system provides a clean, monoisotopic beam perfectly suited for precise nuclear spectroscopy. Connecting the IGISOL and the JYFLTRAP facilities to the recently installed MCC30/15 cyclotron opens new prospects for post-trap spectroscopy of very exotic, neutron-rich nuclei.     

Nuclear structure studies of neutron-rich fission fragments performed at JYFLTRAP

The JYFLTRAP double Penning trap setup at the University of Jyväskylä, Finland has been used to perform a wide range of studies related to neutron-rich fission fragments that can be produced with the Ion Guide Isotope Separator On-Line (IGISOL) method. Experimental results from high-precision mass measurements and decay-spectroscopy measurements have allowed us to investigate the nuclear structure of exotic neutron-rich nuclei.     

Neutron-rich nuclei and fission; recent developments and future aspects

Production and studies of neutron-rich nuclei produced in fission are reviewed. Some recent experiments performed with the ISOL technique at the IGISOL and the ISOLDE facilities are reviewed. The manipulation of neutron-rich nuclei is discussed with a special focus on radioactive ion cooling and trapping techniques under construction worldwide. Perspectives of obtaining intense post accelarated beams of fission products are discussed. Received: 1 May 2001 / Accepted: 4 December 2001     

Symmetric and asymmetric fission of 226Ra induced by 5 to 15 MeV neutrons

Kinetic energy spectra of fragments in the 5–15 MeV neutron-induced fission of ²²⁶Ra are obtained. The fraction of fragments corresponding to the central peak of the mass yield curve is determined for each neutron energy used. It is shown that at low excitation energies of the ²²⁷Ra compound nucleus precluding second-chance fission the mass yield curve retains its triple-peaked nature. At still lower excitations, near the fission threshold, radium undergoes asymmetric fission, just as the heavier nuclei.     

Monte Carlo simulation of fast neutron-induced fission of ~(237)Np

The potential-driving model is used to describe the driving potential distribution and to calculate the preneutron emission mass distributions for different incident energies in the ~(237) Np(n, f)reaction. The potential-driving model is implemented in Geant4 and used to calculate the fission-fragment yield distributions, kinetic energy distributions, fission neutron spectrum and the total nubar for the ~(237) Np(n, f)reaction. Compared with the built-in G4 ParaFissionModel, the calculated results from the potential-driving model are in better agreement with the experimental data and evaluated data. Given the good agreement with the experimental data, the potential-driving model in Geant4 can describe well the neutron-induced fission of actinide nuclei, which is very important for the study of neutron transmutation physics and the design of a transmutation system.     

Angular distributions and anisotropy of the fragments from neutron-induced fission of <Superscript>233</Superscript>U and <Superscript>209</Superscript>Bi in the intermediate energy range of 1–200 MeV

New results of the neutron-induced fission experiments carried out at the neutron time-of-flight spectrometer GNEIS of the PNPI are given. Angular distributions of fission fragments from the neutron-induced fission of ²³³U and ²⁰⁹Bi nuclei have been measured in the energy range 1–200 MeV using position sensitive multiwire proportional counters as fission fragment detector. The recent improvements of the measurement and data processing procedures are described. The data on anisotropy of fission fragments deduced from the measured angular distributions are presented in comparison with the experimental data of other authors.     

Search for the emission of π<Superscript>0</Superscript> mesons from the neutron-induced fission of <Superscript>235</Superscript>U nuclei

A nuclear-reactor experiment on the search for the emission of neutral pions from the neutron-induced fission of ²³⁵U nuclei has been carried out. To this end, an experimental setup for searching for the emission of π⁰ mesons with high sensitivity to this process has been designed and produced. This setup consisted of two Cherenkov total-absorption spectrometers for the detection and measurements of the energy of two gamma-ray photons from π ⁰-meson decay. The spectrometers were placed at the exits of two coaxial horizontal experimental channels. To protect the detectors from low-energy β and γ rays and neutron fluxes, 3.5-m-long water filters were situated in the channels. To reject the cosmic background, large-area scintillation counters operating in the anticoincidence mode were placed over each spectrometer. The energy and time resolution of the spectrometers, as well as the efficiency of detecting charged particles by scintillation counters, were tested on the secondary electron beam of the Tomsk electron synchrotron Sirius. Runs of measurements of the effect on the operating reactor (duration of 805 h) were alternated with runs of measurements of background on the stopped reactor (duration of 403 h). Statistical processing of the experimental results yields an upper limit of ≤5.3×10 ^?12 with a 90% confidence level for the probability of the emission of neutral pions from the neutron-induced fission of ²³⁵U nuclei. This result and results of other works carried out with sources of spontaneously fissioning ²⁵²Cf nuclei show that the probabilities of the emission of π⁰ mesons from spontaneous fission and fission induced by fission-spectrum neutrons are equal to each other.     

Does the coherent bremsstrahlung of fission fragments exist?

The spectrum of γ rays from the neutron-induced fission of ²³⁵U nuclei has been studied in a range of 10–90 MeV. The experiment has been carried out in the horizontal experimental channel of a nuclear reactor. The energy spectrum was measured using a single-crystal NaI(T1) spectrometer. The data on the energy dependence of the yield of γ rays have been obtained in a range of 12–38 MeV. The upper limit on the probability of the emission of γ rays has been determined for a range of 38–90 MeV. It has been shown that the experimental data are inconsistent with available models attributing the yield of γ rays in this energy range to the coherent bremsstrahlung of fission fragments in the Coulomb field.     

Penning trap assisted decay spectroscopy of neutron-rich 115Ru

Exotic, neutron-rich ¹¹¹Mo and ¹¹⁵Ru nuclei, produced in proton-induced fission of ²³⁸U target, were separated with the IGISOL mass separator. The separator was coupled to the JYFLTRAP Penning trap to select the ions of a single, desired element out of the isobaric IGISOL beam. Monoisotopic samples of ¹¹⁵Ru and ¹¹¹Mo ions were observed with a microchannel plate detector after the trap or were implanted on a catcher foil for gamma- and beta-ray coincidence spectroscopy. In spite of short data taking time new gamma transitions were identified in the beta decay of very neutron-rich ¹¹⁵Ru.     

Isotopic distributions of Rb and Cs in the proton and neutron induced fission of 238U

The statistical model for fission is used to calculate the isotopic distribution of Rb and Cs in the fission of ²³⁸U induced by high-energy protons and reactor neutrons. The double-humped behaviour of the isotope yield curve as observed in high-energy proton-induced fission and the experimentally observed hump on the neutron-rich side in neutron-induced fission are semi-quantitatively reproduced.     

在线同位素分离器和丰中子核β衰变

简要介绍了在线同位素分离器的原理以及在奇异核β衰变核谱学中的应用。着重介绍了芬兰Jyv（ae）skyl（ae）大学的离子制导型在线同位素分离器,并对在该装置上开展的丰中子Ag同位素的β^-衰变研究进行了综述。Isotope Separator on Line is briefly introduced with emphasis on the basic concept and features related to the production of exotic nuclei and β decay spectroscopy. The Ion Guide Isotope Separator on Line （IGISOL）at the University of Jyv（ae）skyl（ae）, Finland is presented in detail with experimental results on the decay of neutron-rich silver isotopes produced in proton induced symmetric fission.     

Fission fragment track etching modelling for the cellular localisation of Neptunium-237 nuclei in rat tissues

Heavy ion recording in dielectric isotropic detectors has a wide range of applications in uranium cartography or dating. In this study, solid state nuclear tracks detectors (SSNTDs) have been applied to localise neptunium-237, in vivo, by means of the neutron-induced fission cartography of neptunium-237 nuclei in organ sections. At the cellular level, a precise localisation can only be achieved with a good understanding of the etch pit evolution during the chemical etching process.

A tailor-made software for modelling the etching process has been developed to simulate the profile of an etched track produced by a neutron-induced fission fragment. This software is based on a model that considers the evolution of the track etching velocity along the damage trail in order to perfectly model the shape of the track.     

Decay heat studies for nuclear energy

The energy associated with the decay of fission products plays an important role in the estimation of the amount of heat released by nuclear fuel in reactors. In this article we present results of the study of the beta decay of some refractory isotopes that were considered important contributors to the decay heat in reactors. The measurements were performed at the IGISOL facility of the University of Jyväskylä, Finland. In these studies we have combined for the first time a Penning trap (JYFLTRAP), which was used as a high resolution isobaric separator, with a total absorption spectrometer. The results of the measurements as well as their consequences for decay heat summation calculations are discussed.     

Study of the high-energy part of the spectrum of γ rays from the neutron-induced fission reaction of 235U nuclei

The spectrum of γ rays from the neutron-induced fission reaction of ²³⁵U nuclei has been investigated in the energy range 10–90 MeV. The experiment was performed in the horizontal channel of a nuclear reactor. The energy spectra was measured using a single-crystal NaI(Tl) spectrometer. The data on the energy dependence of the yield of γ rays have been obtained in the energy range 10–36 MeV. The upper limit of the probability of γ-ray emission has been determined for the range 36–90 MeV.     

β-delayed neutron emission studies

The study of β-delayed neutron emission plays a major role in different fields such as nuclear technology, nuclear astrophysics and nuclear structure. However the quality of the existing experimental data nowadays is not sufficient for the various technical and scientific applications and new high precision measurements are necessary to improve the data bases. One key aspect to the success of these high precission measurements is the use of a very pure ion beam that ensures that only the ion of interest is produced. The combination of the IGISOL mass separator with the JYFLTRAP Penning trap is an excellent tool for this type of measurement because of the ability to deliver isobarically and even isomerically clean beams. Another key feature of the installation is the non-chemical selectivity of the IGISOL ion source which allows measurements in the important region of refractory elements. This paper summarises the β-delayed neutron emission studies that have been carried out at the IGISOL facility with two different neutron detectors based on ³He counters in a polyethylene moderator: the Mainz neutron detector and the BEta deLayEd Neutron detector.     

Modeling a neutron-rich nuclei source

The deuteron break-up process in a suitable converter gives rise to intense neutron beams. A source of neutron-rich nuclei based on the neutron-induced fission can be realised using these beams. A theoretical optimization of such a facility as a function of the incident deuteron energy is reported. The model used to determine the fission products takes into account the excitation energy of the target nucleus and the evaporation of prompt neutrons. Results are presented in connection with a converter-target specific geometry. Received: 1 December 2000 / Accepted: 12 May 2001     

Radiochemical investigation of the mass distribution and probability in stopped μ− induced fission of 238U

Radiochemical measurements of 24 fission products show that the mass-split is asymmetric in stopped μ^? induced fission of ²³⁸U. The mass distribution is similar to 14 MeV neutron-induced fission. The main difference is a smaller peak-to-valley ratio no point indicating a mean excitation energy of about 20 MeV. The fission probability (prompt and delayed) is 0.15 ± 0.03 per stopped muon.     

Measurements of nuclide yields in neutron-induced fission of natural uranium for SPIRAL2

Cross-sections for nuclide production in fast-neutron induced fission of natural uranium are part of the input for predictions of yields of neutron-rich nuclides obtainable at Radioactive Ion Beam facilities. We first describe the neutron spectra produced according to the scheme once envisaged for SPES (protons on an enriched ¹³C target) and the one adopted for SPIRAL2 (deuterons on natural carbon), which both have been measured at JYFL. We then present the measurements of Z-splits in isobaric chains performed at IGISOL. When coupled with the fission cross-section and A-splits for the relevant neutron spectrum, they allow estimates of nuclide cross-sections. It looks that calculations, even those based on modern libraries, are too optimistic by about a factor of two.     

Multi-modal analysis for low energy neutron-induced fission of ~(235)U

Low energy neutron induced fission of 235U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of 235U with an improved version of the Los Alamos ...