The influence of pairing and nuclear structure on the thermal-neutron-induced fission of235U

The mass separated fission product beam provided by the mass separator “Lohengrin” has been used to determine the nuclear charge distribution for the thermal-neutron-induced fission of²³⁵U for all light fission products in the region 80≦A≦107. The measurements were performed at the most probable kinetic energy of the fission products. By using the known fission product mass yields, the independent yields for a total number of 100 nuclides were obtained under the condition of the most probable kinetic energy. The proton pairing effect modulates the average nuclear charge of the fission fragments and the isobaric charge distribution widths in a regular fashion. The probabilities of breaking a pair and of forming fragments with an energetically unfavourable neutron-to-proton ratio are found to compete with each other. Both probabilities depend on the mass split and reach their maximum values in the region of the most probable masses. The odd-even-proton effect is found to vary smoothly between 16% for the most abundant mass splits and 40% for the rare mass splits. The odd-even-neutron effect exhibits maxima nearN=50 andN=60, where it reaches 16%. These maxima and the extremely low Tcyield (0.13±0.05%) are discussed with regard to fragment shell effects.     

Excitation energy dependence of fragment characteristics for the photofission of 232Th

Independent and cumulative product yields were measured for the photofission of ²³²Th with bremsstrahlung with endpoint energies 6.5, 7.0, 8.0, 11.0, 12.0, and 14.0 MeV, applying γ spectrometric techniques on catcherfoils and pneumatically transported ²³²Th-samples. The independent heavy fragment yields for the fission of the ²³²Th compound nucleus at excitation energies in the vicinity of the fission barrier were deduced. Postneutron mass, isobaric charge, isotopic mass distributions, isotonic and elemental yield distributions and proton odd-even effects were obtained from these independent yields. In the mass distributions a maximum yield is observed for mass splits with heavy fragments in the region of A = 142, corresponding with a high production of Ba(Z = 56) - isotopes. A slightly increased yield is also observed for mass splits with heavy mass in the vicinity of A = 134. The latter effect increases with increasing compound nucleus excitation energy. The similarity between the mass distributions of the N = 142 fissioning systems ²³²Th, ²³⁴U and ²³⁶Pu is striking. For low excitation energy the proton odd-even effect in the element distributions amounts to 30%, while on the other hand no sizeable neutron odd-even effect could be deduced from the isotonic distributions. The proton odd-even effects remain constant up to compound nucleus excitation energies of about 7.85 MeV. For higher compound nucleus excitation energies the proton odd-even effect drops rapidly. A possible explanation of these observations in terms of pair breaking at the outer barrier is proposed.     

Emission properties of fission fragments and their isomeric ratios

The properties of neutron emission from fragments formed in the spontaneous fission of ²⁵²Cf and in the thermal-neutron-induced fission of ²³⁵U are analyzed on the basis of the statistical model of nuclear reactions. Upon extracting the mean excitation energies of fission fragments from experimental data on the mean multiplicities of neutrons, the observables of neutron emission can be described over wide ranges of total kinetic energies and masses. The observed values of mean fragment spins are also reproduced. A method for calculating the isomeric ratios of the independent yields of fission fragments that is based on the cascade-evaporation model of excited-nucleus decay is employed to describe experimental data on ²³⁵U fission induced by thermal neutrons and on ²³⁸U fission induced by alpha particles. The effect exerted on the isomeric ratios for fission fragments by two different assumptions on the spin distributions of primary-fragment populations—the assumption of the distribution associated with rotational degrees of freedom and the assumption of the distribution associated with the internal degrees of freedom of fully accelerated fragments—is investigated.     

CHARGE DISTRIBUTION IN THE 14.7MeV-NEUTRON-INDUCED FISSION OF 232Th: INDEPENDENT YIELDS OF ISOTOPES OF Rh,Ag, In AND Sb

The 12 cumulative yields and 7 independent yields in the symmetric region were determined in the fission of ²³²Th induced by 14.7MeV neutrons with nuclear chemistry techniques. The independent yields of the present work, along with the independent yields obtained in our previous work, were used to calculate the empirical Z_p values on the basis of a Gaussian charge distribution function with a σ of 0.56. A mean value of 0.49±0.25 for ΔZ(=Z_p—Z_UCD) is obtained from all Z_p values in the asymmetric region. Abnormal ΔZ values in the symmetric region seem to suggest that the width of charge distribution for symmetric splits is larger than the value of 0.56. The results show strongly in favor of the Two-Mode-Fission-Hypothesis.     

Determining isotopic distributions of fission products with a Penning trap

A novel method to determine independent yields in particle-induced fission employing the ion guide technique and ion counting after a Penning trap has been developed. The method takes advantage of the fact that a Penning trap can be used as a precision mass filter, which allows an unambiguous identification of the fission fragments. The method was tested with 25MeV and 50MeV proton-induced fission of ²³⁸U . The data is internally reproducible with an accuracy of a few per cent. A satisfactory agreement was obtained with older ion guide yield measurements in 25MeV proton-induced fission. The results for Rb and Cs yields in 50MeV proton-induced fission agree with previous measurements performed at an isotope separator equipped with a chemically selective ion source.     

Proton induced fission of <Superscript>232</Superscript>Th at intermediate energies

The mass-energy distributions and cross sections of proton-induced fission of ²³²Th have been measured at the proton energies of 7, 10, 13, 20, 40, and 55 MeV. Experiments were carried out at the proton beam of the K-130 cyclotron of the JYFL Accelerator Laboratory of the University of Jyväskylä and U-150m cyclotron of the Institute of Nuclear Physics, Ministry of Energy of the Republic of Kazakhstan. The yields of fission fragments in the mass range A = 60–170 a.m.u. have been measured up to the level of 10?4%. The three humped shape of the mass distribution up has been observed at higher proton energies. The contribution of the symmetric component grows up with increasing proton incident energy; although even at 55 MeV of proton energy the shoulders in the mass energy distribution clearly indicate the asymmetric fission peaks. Evolution of shell structure was observed in the fission fragment mass distributions even at high excitation energy.     

Proton- and neutron-induced fission on uranium carbide target

In the frame of the R&D program for the SPES project of the Legnaro Laboratories, the production yields of Rb, In and Cs isotopes have bean measured at the synchrocyclotron of the Petersburg Nuclear Physics Institute (PNPI) of Gatchina (Russia). Production yields for direct proton fission and neutron-induced fission on a high-density UC₂ powder target are compared. The overall efficiencies as a function of the target temperature have been measured.Received: 3 July 2003, Revised: 2 October 2003, Published online: 27 January 2004PACS: 24.75. + i General properties of fission - 25.85.Ec Neutron-induced fission     

Post-neutron mass distribution for 238Pu(nth,f)

The yields of 51 fission products in 36 mass chains produced in ²³⁸Pu(n_th, f) have been determined via γ-ray spectroscopy. The mean number of neutrons per fission was found to be 〈ν_T〉 = 2.5 ± 0.2. A mean proton odd-even effect δ_p = (14 ± 3)% was also deduced for 9 mass chains. These results are discussed in terms of the existing data on other fissioning nuclei.     

Influence of the odd neutron on the fragment characteristics in the photofission of 235U

Cumulative and independent post-neutron product yields were measured for the photofission on ²³⁵U with 6.5, 7.0, and 8.4 MeV bremsstrahlung, applying γ-spectrometric methods on catcherfoils and pneumatically transported ²³⁵U-samples. The independent yields for the heavy (A = 128–149) fragments were deduced. Post-neutron mass and isotonic yield distributions were studied together with the elemental yield distributions and the proton odd-even effect. The mass distribution shows about an equal yield for the mass regions around A = 134 (N = 82) and A = 142 (N = 86–88). The proton odd-even effect for the fission of this odd-A nucleus remains at a constant value δ_p = 25% in the studied compound nucleus excitation energy region, indicating that the odd neutron does not influence the energy transfer between the different degrees of freedom during the descent from saddle to scission point in a substantial way.     

Cascade-exciton model analysis of level density parameter dependence in proton induced fission cross sections of some sub-actinide nuclei

Fission cross sections strongly depend on the ratio of the level density parameter in fission to neutron emission, a_f/a_n. In this work, a cascade-exciton model implemented in the code CEM95 has been used to observe this effect for proton induced fission cross sections of tungsten, lead and bismuth. The method was employed using different level density parameter ratios for each fission cross section calculation. The calculated fission cross sections are compared with the available experimental data in the literature. It has been observed that a change of the ratio of the level density parameter, a_f/a_n, is necessary with the incident energy of the proton, to best estimate the fission cross sections in CEM95.     

Fission of 232Th, 238U and 235U induced by negative muons

The absolute yields of prompt and delayed fission induced by negative muons in ²³²Th, ²³⁸U and ²³⁵U have been measured. The delayed fission yields are much lower than could be predicted from Γ_n/Γ_f systematics for 15–20 MeV nuclear excitation. The systematics of prompt fission yields are compared with recently obtained photofission data. It is suggested that prompt fission can be used for investigating the channel structure of the fission barrier.     

CiADS中LBE散裂靶的放射性核素研究

铅铋合金(LBE)作为中国加速器驱动系统(CiADS)散裂靶的候选材料,长期辐照使其具有很强的放射性。散裂靶放射性核素的研究仅考虑了质子束的散裂反应,而忽略了反应堆裂变中子的活化作用。本文采用FLUKA和MCNP耦合计算LBE和及其结构件的放射性产物。比较了裂变中子和高能质子在放射性产物的活度、主要放射性核素、毒性和衰变光子等方面的贡献。裂变中子的活化作用对主壳、导管和射束管有显著影响。当反应堆趋于临界状态时,裂变中子对LBE的活化作用是高于质子束流的。在LBE中,96.66%的 210Po是由裂变中子诱导的。这些结果表明,裂变中子在LBE及其结构部件的活化计算中是必不可少的。此外,本研究为CiADS的辐射防护提供了参考数据,也为ADS系统中散裂靶的放射性核素研究提供了更准确的方法。     

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

Low energy neutron induced fission of ~(235)U 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 ~(235)U with an improved version of the Los Alamos model for incident neutrons below the (n, nf) threshold. The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental data. The calculated results are in good agreement with the experimental data.     

Energy spectra of secondary electrons ejected by ions from foils

Secondary electron spectra in the energy range below 3.5 keV arising from the passage of α-particles and fission products through thin carbon and gold foils are presented and compared to gas target spectra and theoretical calculations which include the slowing down and the angular scattering of the electrons in the foil. A good agreement between the calculation and the experimental results is found. The shape of fission product induced electron spectra differs systematically from that of light ion induced spectra. Total yields and mean energies of the secondary electrons are derived from the spectra.     

On-line production of Rb and Cs isotopes from uranium carbide targets

A series of on-line mass separation experiments have been performed at the IRIS facility to measure the yield and release of Rb and Cs neutron-rich isotopes produced by fission reaction of ²³⁸U. A 1 GeV proton beam was used to bombard uranium carbide targets with the densities of 11 g/cm³ and 1.5 g/cm³ held at temperatures in the range (2000-2230) ^°C. The release curves of Rb and Cs long-lived isotopes were measured from both kinds of targets. The overall production efficiency was determined making use of experimentally measured cross-sections of that isotope production. Comparison of the experimental yields of Rb and Cs isotopes with the calculated ones after corrections for losses due to finite release times suggests that the diffusion is the dominating process reducing the efficiency for short-lived isotopes. When normalized to the same thickness, an enhancement for the high-density rod target of the measured isotope yields is observed when going far from stability. This is possibly explained by the reactions induced by secondary neutrons. A significant odd-even effect with higher yields of Cs even neutron isotopes has been observed, confirming a similar effect obtained in earlier experiments.     

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 ...     

Isotopic invariance of fission fragment charge distributions for actinide nuclei at excitation energies above 10 MeV

The fragment mass and energy distributions from the proton-induced fission of compound nuclei ²³³Pa, ^{234,236,237,239}Np, ^{239,240,241,243}Am, and ²⁴⁵Bk at proton energy E _p =10.3 and 22.0 MeV have been experimentally studied. It was revealed that the shapes of the asymmetric fission mass distributions are mainly defined by the proton numbers of compound nuclei and demonstrate only a weak dependence on the neutron ones. The detailed study of the fission fragment mass yields for compound nuclei Np and Am isotopic chains has shown that the asymmetric fission fragment charge distributions calculated within the unchanged charge density hypothesis for nuclei with equal Z _C practically coincide.     

Cross sections for interactions of 8.8 GeVα particles with Ag,Au, Bi and Th

Makrofol was used as 4π track detector to determine reaction cross sections of Ag, Au, Bi and Th induced by 8.8 GeVα particles. The variation of cross sections as functions of theZ ²/A parameter of the target has been investigated. Data concerning geometry of events, momentum transfer and fission probabilities were discussed in order to discern fission products from those originating from more violent processes. Comparison with proton data has been done and differences were discussed.