Multimode approach to 241Am and 237Np fission induced by 660-MeV protons

The results obtained by measuring cross sections for the formation of fragments originating from ²⁴¹Am and ²³⁷Np fission induced by 660-MeV protons are presented. The charge and mass distributions of fragments are analyzed within the multimode-fission model, symmetric and asymmetric fission channels being separated. The contributions of various fission components are estimated, and the fission cross sections for the ²⁴¹Am and ²³⁷Np nuclei are calculated along with the fissilities of these nuclei.     

Investigation of the formation of residual nuclei from radioactive 237Np and 241Am targets in reactions with 660-MeV protons

The burning of radioactive waste is investigated. Targets from ²⁴¹Am and ²³⁷Np were irradiated with 0.66-GeV proton beams. The cross sections for the formation of 60 and 80 residual nuclei from ²³⁷Np and ²⁴¹Am are determined. The experimental results are compared with the theoretical cross sections calculated by the cascade-evaporation model.     

Independent yields of Kr and Xe fragments in the photofission of <Superscript>237</Superscript>Np and <Superscript>243</Superscript>Am odd nuclei

Results are presented that were obtained by measuring the independent yields of Kr (A = 89–93) and Xe (A = 135–142) appearing as fragments in the photofission of ²³⁷Np and ²⁴³Am odd nuclei. The respective experiments were performed in a beam of bremsstrahlung photons from electrons accelerated to an energy of 25 MeV at the microtron of the Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research (JINR, Dubna). Use was made of the procedure involving the transportation of fragments emitted from the target by a gas flow along a capillary and the condensation of inert gases in a cryostat at liquid-nitrogen temperature. The identification of Kr and Xe appearing as fragments was performed by the gamma spectra of their daughter products. The mass-number distributions of the independent yields of Kr and Xe isotopes were obtained, along with those for the complementary fragments (Y and La in the fission of ²³⁷Np and Nb and Pr in the fission of ²⁴³Am).     

Transmutation of 129I, 237Np, 238Pu, 239Pu, and 241Am using neutrons produced in target-blanket system ‘Energy plus Transmutation’ by relativistic protons

Target-blanket facility ‘Energy + Transmutation’ was irradiated by proton beam extracted from the Nuclotron Accelerator in Laboratory of High Energies of Joint Institute for Nuclear Research in Dubna, Russia. Neutrons generated by the spallation reactions of 0.7, 1.0, 1.5 and 2 GeV protons and lead target interact with subcritical uranium blanket. In the neutron field outside the blanket, radioactive iodine, neptunium, plutonium and americium samples were irradiated and transmutation reaction yields (residual nuclei production yields) have been determined using γ-spectroscopy. Neutron field's energy distribution has also been studied using a set of threshold detectors. Results of transmutation studies of ¹²⁹I, ²³⁷Np, ²³⁸Pu, ²³⁹Pu and ²⁴¹Am are presented.      

Muon attachment in prompt fission of237Np

The probabilities of muon attachment to fission fragments in the prompt fission of²³⁷Np have been determined as a function of fragment mass and total kinetic energy release in a ( ^–,f ₁ f ₂ e ^–)-coincidence measurement. A strong dependence of the attachment probability on the fragment mass is observed. The experimental results are compared with theoretical predictions.It is a pleasure to thank Professor T. Mayer-Kuckuk for his co-operation in performing the project. We gratefully acknowledge the permanent support of Professor J.P. Blaser and his staff at PSI. We thank Professor R. Engfer, Dr. A.v.d. Schaaf, and Dr. H.C. Walter for their support in using the electron spectrometer SIN-DRUM I. We are thankful to Professor S. Polikanov (GSI) and to Dr. T. Krogulski (University of Warsaw, Bialystok) for intensive and fruitful discussions. We appreciate the collaboration of Professor W. Müller and Dr. J. Pauwels (CBNM-Mol) in the target preparation. This work has been funded by the German Federal Minister for Research and Technology (BMFT) under the contract number 06BN271 (PD, HH, FR, ChR, WS). We thank the following institutions and organisations for financial support: Foundation for Fundamental Research on Matter (FOM), the Netherlands Organisation of the Advancement of Pure Research (NWO) (JK, CdL, AT), and the Swiss National Foundation (SNF) (WB, EH; HP, DV). One of the authors is grateful for a fellowship granted by the Studienstiftung des deutschen Volkes (FR).     

Isomeric state at 945.3 keV in237Np

A new isomeric state in²³⁷Np has been discovered at an excitation energy of (945.3±0.2) keV withT _1/2 =(711±40) ns. Six gamma-ray lines have been measured depopulating the isomeric state to known levels in²³⁷Np. The comparison of the measured absolute gamma-ray transition probabilities with empirical values yields as most probable spinI andK assignmentI=K=13/2 for this isomeric state. It is most likely a 1-proton-2-neutron quasiparticle state 13/2^?π5/2⁺ [642] +ν7/2^? [743] + ν 1/2⁺ [631]. However, a one-quasiparticle 11/2^? [505] configuration for the isomer cannot be excluded.     

237Np separation from environmental matrices by extraction chromatography method

Aliquat-336 anchored on hydrophobized silica gel was used for effective separation of ²³⁷Np from soil matrices and from model solutions. Alpha spectrometry for ²³⁷Np determination was used. The interfering radionuclides present in environmental samples such as ²³⁴U, ²³²Th, ²³¹Pa and ^239+240Pu were removed by separation with nitric acid solutions containing formic acid and ferrous sulfamate as reducing agents. Spiking samples with ^238,234U, ²³²Th and ²³⁸Pu were used to check separation effectiveness. A separation yield of ²³⁷Np from soil matrices was checked by measuring of added known amount activity and was found to be about 15%. The yield of separation for model solutions was about 56%. Different parameters of eluant on recovery of ²³⁷Np from column were studied.     

The probability of prompt and delayed fission of muonic237Np

Fission fragments from the reaction²³⁷Np(μ ^?,γ,f) have been measured in coincidence with muonic X-rays. The efficiency of the fission fragment detector is determined from (μ ^?,γ,f)-data of the same experiment. The total fission probability perμ-stopP _t has been measured as well as the fission probabilities P_f of the non-radiative muonic (3d→1s)- and (2p→1s)-transitions; the latter has been divided into two parts leading to different mean excitation energiesE:P _t =(54±17)%,P _f (3d→1s)=(41±21)%,P _f (2p→1s,E=6.218 MeV)=(61±19)%, andP _f (2p→1s,E=6.525 MeV)=(57±18)%. The influence of the muon on the fission barrier is discussed. The fission probability after muon capture is compared with a calculated value using a distribution of nuclear excitation energies following muon capture and the fission probability as measured in a²³⁸U(³He,αf)-reaction.     

237Np and 57Fe Mössbauer study of NpFeGa5

⁵⁷Fe and ²³⁷Np Mössbauer ōmeasurements have been performed for NpFeGa₅, which is one of the so-called neptunium 1-1-5 compounds. The ⁵⁷Fe Mössbauer spectra below T _N = 118 K show the magnetically ordered state. The magnitude of the hyperfine magnetic field at the ⁵⁷Fe nucleus is determined to be 1.98 ± 0.05 T at 10 K. From the ²³⁷Np Mössbauer spectrum at 10 K, the hyperfine magnetic field at the ²³⁷Np nucleus is 203 T and the hyperfine coupling constant is determined to be 237 T/μ_B using the Np atomic magnetic moment of 0.86 μ_B determined by the neutron diffraction study.     

Photon interaction studies using 241Am γ-rays

We have carried out some photon interaction measurements using 59.54 keV γ-rays from a ²⁴¹Am source. These include γ attenuation studies as well as photoelectric absorption studies in various samples. The attenuation studies have been made using leaf and wood samples, samples like sand, sugar etc., which contain particles of varying sizes as well as pellets and aqueous solutions of rare earth compounds. In the case of the leaf and wood samples, we have used the γ-ray attenuation technique for the determination of the water content in fresh and dried samples. The variation of the attenuation coefficient with particle size has been investigated for sand and sugar samples. The attenuation studies as well as the photoelectric studies in the case of rare earth elements have been carried out on samples containing such elements whose K-absorption edge energies lie below and close to the γ-energy used. Suitable compounds of the rare earth elements have been chosen as mixture absorbers in these investigations. A narrow beam good geometry set-up was used for the attenuation measurements. A well-shielded scattering geometry was used for the photoelectric measurements. The mixture rule was invoked to extract the values of the mass attenuation coefficients for the elements from those of the corresponding compounds. The results are consistent with theoretical values derived from the XCOM package.     

A study of the radiation from Am241

The radiation from the alpha decay of Am²⁴¹ has been investigated by studying the fine structure of the alpha spectrum and by performing alpha-gamma and gamma-gamma coincidence measurements. For the daughter nucleus Np²³⁷ a level scheme is proposed which comprises 21 excited states and 39 gamma transitions. Evidence was found for the existence of previously unknown gamma rays at (80), 82, 88, (90), 100, 150 (two transitions), 170, (~210), 295, (300), 360, 420, 460 and 690 keV, as well as for the probable occurrence of a new fine structure alpha group at 4754 keV corresponding to a level at 804 keV. For the alpha groups and for most of the gamma rays absolute intensities are given. For some transitions theK-conversion coefficients and multipolarities were determined. The results are discussed within the framework of the unified model. The level density agrees well with the predictions of the superfluid model and is approximately twice the density yielded fromNilsson's theory.     

237Np Mössbauer spectroscopy on neptunium doped borosilicate glasses

Mössbauer and magnetization measurements have been carried out on borosilicate glasses containing neptunium. Different melting and annealing times, in various crucibles have been investigated. Using the 59.5 keV Mössbauer resonance in²³⁷Np we have characterized valency states of neptunium ions in glasses and studied hyperfine interactions. Roughly, absorption spectra show two sites easily differenciated above 50 K by two lines. Isomer shifts (relative to NpAl₂) are respectively: – 10.0 (1.0) and + 32.5(.5) mm/s. These resonance lines can be assigned, the first at Np⁴⁺, the second at Np³⁺ species. The intensity ratio Np⁴⁺/Np³⁺ varies with elaboration conditions. At 4.2 K, Np⁴⁺ ions exhibit a hyperfine splitting arising from relaxation phenomenon. Magnetization measurements do not show any magnetic ordering.     

Measurement of cross sections for the reactions 241Am(n, 2n) and 241Am(n, 3n)

The cross sections for the reactions ²⁴¹Am(n, 2n)²⁴⁰Am and ²⁴¹Am(n, 3n)²³⁹Am are measured for several neutron-energy values in the range 13.42–14.86 MeV. An upper limit on the cross section for the reaction ²⁴¹Am(n, α)²³⁸Np is also obtained.     

Fission of237Np+n through a double humped fission barrier

The subthreshold fission cross section of neutron induced fission of²³⁷Np has been measured between 20 and 60 eV with the CBNM time-of-flight spectrometer. The resonances in this energy range have been analysed with a single level shape fitting computer program. The result is interpreted in the frame of Strutinsky's theory of nuclear deformation.     

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.     

Multimodal analysis of prompt neutron spectra for 237Np(n,f)

An attempt was made to analyzed the systematic variation of the prompt neutron spectra of ²³⁷Np(n,f) on the basis of the multimodal analysis of mass and kinetic energy distributions. The spectra of neutrons emitted from fragments of each mode (standard I, II, III, superlong) were calculated independently and the total spectra were synthesized. It was found that the partial spectrum for standard I mode is the softest, getting harder in ascending order for standard II, standard III and superlong modes. The calculated total spectra well represented the experimental data and were found to be more sensitive to the variation of the incident energy than the conventional treatment of the Madland-Nix model.