Effect of entrance channel parameters on fission fragment angular momentum in medium energy fission

Independent isomeric yield ratios of¹³²I were radiochemically determined in alpha particle induced fission of²³⁸U in the energy range 25–44 MeV. Fission fragment angular momenta were deduced from the measured isomeric yield ratios using spin dependent statistical model analysis. It was seen that angular momentum of¹³²I increases with increase of excitation energy and angular momentum of the fissioning nucleus. Comparison of the present data on¹³²I in²³⁸U(α,f) with the literature data for the same product in²³⁸U(p, f) and²³⁸U(γ, f) at various excitation energies show that fragment angular momentum strongly depends on the input angular momentum in the range of excitation energy considered. Experimental fragment angular momentum at all excitation energies were seen to be in agreement with the theoretical values calculated based on thermal equilibration of the various collective rotational degrees after considering the occurence of multichance fission. Thus, strong effect of input angular momentum as well as the statistical equilibration among the various collective rotational degrees of freedom in medium energy fission is corroborated.     

Correlation between fractional independent yields and neutron-to-proton ratio of fission products in low energy fission

Fractional independent yields of fission products in the thermal neutron-induced fission of²³³U,²³⁵U,²³⁹Pu,²⁴¹Pu and in the spontaneous fission of²⁵²Cf have been correlated with the neutron-to-proton ratio of the fission products. The yields of the products from a fissioning system, when plotted as a function of neutron-to-proton (N/Z) ratio of fission preducts, fall on two Gaussian distribution corresponding to light and heavy fission products. The centroids of the distribution or the most probable value of neutron-to-proton ratio is found to be very close to theN/Z of the fissioning nucleus. From the most probable value ofN/Z the various parameters of charge distribution e.g. most probable massA _p, most probable chargeZ _p, the mass dispersionσ _Aand the charge dispersionσ _Zhave been obtained and are in good agreement with the experimental values ofA _pandZ _p.     

Absence of delayed fission in theβ −-decay of 2.3 min238Pa

We have searched for beta-delayed fission in the decay of 2.3 min²³⁸Pa produced in the²³⁸U(n,p) reaction with 14.7 MeV neutrons. Through microprocessor-controlled chemical separations of protactinium about 10⁹ atoms of²³⁸Pa were isolated and exposed to fission track detectors. From the absence of fission tracks an upper limit for the betadelayed fission probability of²³⁸Pa, i.e.P _βf <2.6 10^?8, is obtained at 95% confidence level. This rules out positive evidence for this decay mode of²³⁸Pa reported elsewhere. Simple theoretical estimates ofP _βf range from 10^?7 to 10^?9.     

Fragment angular momenta in low and medium energy fission of242Pu

Independent isomeric yield ratios of¹²⁸Sb were determined radiochemically in the thermal neutron induced fission of²⁴¹Pu and 34 MeV alpha particle induced fission of²³⁸U, both involving the same compound nucleus (²⁴²Pu). Fragment angular momenta estimated from the measured isomer ratios using the statistical model analysis showed significantly larger fragment angular momenta in the medium energy fissioning system compared to the low energy fissioning system. This has been attributed to the effect of higher excitation energy and angular momentum in the entrance channel leading to increased fragment temperature, moments of inertia and angular velocity. An attempt was made to calculate the fragment angular momentum in the medium energy fission using the Fermi gas model for the fissioning nucleus, taking into account the multichance fission, saddle shapes of the fissioning nuclei and the angular velocity components of the fissioning nuclei both along and orthogonal to the fission axis. The calculated angular momenta agree well with the experimental results.     

Measurement of fast neutron induced fission cross section of thorium using Lexan plastic track detector

Fission-track registration characteristics of Lexan solid state nuclear track detectors have been used to measure the fast neutron induced fission cross section of²³²Th. The fast neutrons (?14.2MeV) were produced with the help of an AN-400 model Van-de-Graaff accelerator at Banaras Hindu University laboratory using³H(²H,n)⁴He reaction and were used to irradiate the fissile target deposited on the plastic detector. The track densityT, registered on the plastic detector is related to the fission cross sectionσ _f, through the relationT=knσ _føt wheren is the number of fissile atoms per cm² in the deposit, ø is the neutron flux,k is fission track registration efficiency andt is the time of irradiation. The fission cross sectionσ _f of²³²Th, relative to the well measured fission cross section of²³⁸U, was found to be 0.36±0.04 barn.     

Nuclear dissipation,fission probability and neutron multiplicity prior to fission

We discuss the effects of nuclear dissipation on fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratioa _f/a_n of the level density parameters for a given strength of the reduced dissipation coefficientβ. These low energy constraints ona _f/a_n andβ balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of¹⁵⁸Er formed in the reaction¹⁶O+¹⁴²Nd at 207 MeV we show that dueto transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis.     

Measurement of mass- and time-dependence of delayed fission neutrons by an on-line mass separator

Delayed neutron emission for thermal neutron fission of²³⁵U is investigated by means of a helium-filled mass separator. At the focus of the separator, the neutron activity is detected in coincidence with theΒ decay preceding the neutron emission. Measurements of the activity build-up give the total delayed neutron yield and the yields for different times after fission. Measurements of the activity decay give the half-life distribution. The mass-dependence of yield and half-life was investigated for the whole fission product mass region. The splitting of the total yield of delayed neutrons between light and heavy fission products has been determined. Based on Keepin's value of the total number of delayed neutrons per fission, a yield of 1.05 and 0.53 neutrons per 100 fissions has been obtained for light and heavy fission products, respectively. The experimentally determined mass- and time-dependences of delayed neutron emission are compared with those calculated from the known precursors. Unidentified neutron activities in the mass regions 85–88 (half-life 1 sec) and 96–100 (half-life 1.5 sec) are discussed. By means of theoretical considerations the yield difference for delayed neutrons between light and heavy fission products is interpreted and the new neutron activity is assigned to the precursors⁹⁸Y or⁹⁹Y.     

Half-lives,neutron emission probabilities and fission yields of neutron-rich rubidium isotopes in the mass regionA=96 toA=100

Neutron-rich rubidium isotopes were produced by thermal-neutron induced fission of²³⁵U and were isolated by surface ionization and mass separation. The new isotope (51 ±17) ms¹⁰⁰Rb was identified. The half-lives of the rubidium isotopes with mass numberA=96 toA=100 were measured using neutron multiscaling or gamma multispectrum analysis. Measurements of the delayed-neutron emission probabilityP _n of⁹⁹Rb and of the fission yields of⁹⁷Rb,⁹⁸Rb, and⁹⁹Rb were performed.     

Fission fragment angular distribution in alpha-particle-induced fission of actinide elements

Using Lexan polycarbonate plastic as the fission fragment track detectors, the fragment angular distributions have been measured in the cases of fission of²³²Th and²³⁸U induced by alpha particles of various energies ranging from 40 to 70MeV obtained from the 88″ variable energy cyclotron at Calcutta. The center-of-mass angular distributions have been calculated and fitted by a series of Legendre polynomials. TheW(10°)/W(90°) ratios (defined as anisotropy) were measured at several energies for both the targets. These data are utilized in calculation of the energy dependence ofK ₀ ² , the standard deviation of the distribution in the angular momentum projection on the nuclear symmetry axis at the saddle point. Values of Γ _f /Γ _η , i.e. the ratio of the fission width to neutron emission width have been determined for²³²Th and²³⁸U nuclei. The integral cross-section for alpha induced fission in each target was determined by numerical integration of the respective center-of-mass angular differential cross-sections. The results were compared with similar data available in the literature which served to resolve some of the discrepancies observed in earlier measurements. The results were also compared with theoretical cross-sections.     

Parity nonconservation and Brosa modes in nuclear fission

The parity nonconserving (PNC) asymmetry coefficient for the angular distribution of fragment emission from binary fission of²³³U irradiated by polarized thermal neutrons has been measured. Complementary fragments were detected by a double ionization chamber with high resolving power. The experiment was carried out at the high flux reactor of the ILL, Grenoble. Integrated over all masses and energies of light fragments from asymmetric fission of²³⁴U^h, the PNC asymmetry coefficient is found to be =(3.29±0.31) × 10^–4.The positive sign of indicates a preferential emission of light fragments parallel to the spin of neutrons inducing fission. Theory claims that PNC effects are sensitive to the properties of the fission barrier. On the other hand, one may wonder whether characteristics of eventual fragments are already specified at the barrier. The measurement of PNC asymmetries for distinct mass-energy ranges of fragments has been utilized to answer this question. Mass-energy distributions of fragments were analyzed along the lines of the Brosa model of fission as a superposition of individual modes. Within experimental uncertainty no mode dependence of asymmetric fission could be disclosed. The result is in keeping with the Brosa model where asymmetric modes bifurcate only once the standard fission barrier has been passed.     

Resonances in the isomeric and prompt fission probabilities of240Pu

The prompt and isomeric fission probabilities of²⁴⁰Pu have been studied using the²³⁹Pu(d,p) reaction. A number of resonances are observed in the subbarrier population of the 4 ns fission isomer for excitation energies between 4.0 and 6.2 MeV. Apart from a structure at 4.3 MeV, they do not coincide with transmission resonances found in prompt fission. Calculations with an extended doorway state model which simultaneously reproduce the measured delayed and prompt fission probabilities yield revised fission barrier parameters as well as spectroscopic information on fission and gamma widths of highly excited states in the second minimum.     

Ternary fission involving4He emission in the16O (144 MeV)+232Th and12C (108 MeV)+197Au reactions

The results of coincidence measurements of⁴He emission with fission fragments in reactions of¹²C(108 MeV) ions with a¹⁹⁷Au target and of¹⁶O(144 MeV) ions with a²³²Th target are presented. On the basis of a Monte Carlo kinematic simulation of nuclear reactions the experimental energy spectra and velocity distributions of alpha particles have been analyzed. A conclusion has been drawn that the main source of⁴ He emission is evaporation from the fissioning compound nucleus. Substantial part of alpha particles was emitted from fully accelerated fission fragments. Some of⁴He nuclei with an average energy of about 16 MeV (in the CM system) emitted mainly perpendicular to the fission axis were identified as being similar long-range alpha particles produced in ternary fission of heavy nuclei at a low excitation energy. The emission multiplicities of these particles are considerably higher than those observed at a low excitation energy. The experimental results are compared with the statistical model predictions.     

Dependence of light charged particles parameters on Z2/A in ternary fission

Emission of light charged particles from ternary fission of²⁴⁸Cm,²⁵²Cf and²³⁵U+n _th has been measured by a CsI(T1) counter. Relative probabilities and parameters of p, t,α energy spectra in triple fission were extracted and compared with earlier reference results.     

Determination of fission barrier heights from β-delayed fission

Beta delayed fission (βDF) gives a possibility to investigate the fission barrier for nuclei far off beta stability. However, before any information on the fission barrier can be extracted, the effect of low-lying structures in the beta-strength function (S _β) on theβDF branching ratio has to be considered. This is in general not done. In this paper the lowlying structures that occur inS _β are discussed and microscopic calculations for the Gamow-Teller strength function are presented for²³²Th (β ^?-decay) and²³²Pu,²⁴⁰Cm,^244,248Cf and²⁴⁸Fm (β ⁺-decay). Using the calculated strength functionsβ ⁺DF branching ratios are calculated and compared with the experimental ones. The sensitivity of the results to different shapes ofS _β is investigated. It is concluded that, when the expected structures inS _β are considered, there are at present no indications fromβ ⁺ DF measurements that the errors in the fission barrier calculations are larger than the uncertainty given for those calculations. The difference in magnitude between theβ ^? DF and theβ ⁺DF branching ratios is also explained by the occurrence of low-lying structures inS _β.     

Angular distribution in ternary fission

The angular distribution of long-range alpha particles emitted in keV-neutron induced fission of²³⁵U has been measured using a technique which employs only a particle telescope to derive the angular information. The neutron energy region investigated is 100 keV-1 MeV. The angular distribution oflras has been found to be peaked perpendicular to the neutron-direction with a substantial amount of anisotropy near 200 keV.     

Search for delayed fission products from superheavy nuclei in the reaction136Xe+208Pb

The production of superheavy elements in binary reactions of the type²⁰⁸Pb (¹³⁶Xe, X) Y was investigated atE _c.m.=470 MeV. The experiment was designed to search for delayed fission products from elements withZ between 108 and 116 and fission lifetimes ofΤ?10^?12 s. No fission events were observed the upper limit for the formation cross section being 1.2 Μbarn.     

Cross section for the subthreshold fission of 236U

The cross section for ²³⁶U fission in the neutron-energy range E _n = 0.001–20 keV was measured by using the INR RAS (Institute of Nuclear Research, Russian Academy of Sciences, Moscow) LSDS-100 neutron spectrometer of the lead slowing-down spectrometer type. The resonance fission areas of the resonances at 5.45 eV and 1.28 keV were found, and the fission widths of these resonances were evaluated. The cross section for the ²³⁸U(n, f) fission process was measured, and the threshold sensitivity of the LSDS-100 to small values of fission cross sections was estimated. The well-known intermediate structure in the cross section for the neutron-induced subbarrier fission of ²³⁶U was confirmed.     

A multiparameter system for heavy nuclei fission study

A multiparameter system designed to measure as many as six parameters of a single fission event on line with an EC 1010 computer is described. The kinematics of emitted fragments and the calibration of light particle and fragment spectra are discussed. Fragment energy spectra from the thermal neutron induced fission of²³⁵U have been measured. Results of light particle emission from spontaneous fission of²⁵²Cf and from neutron induced fission of²³⁵U are presented.Dedicated to Professor Ivan Úlehla on the occasion of his sixtieth birthday.We would like to thank the following collaborators for their help and support during the course of these experiments: E. Kuli for the improvement of various measuring devices, J. Hoffmann, J. Badura and J. Novotny for the construction of the stabilization system, and J. ech and M. Keller for the construction of the real time interface unit. We are thankful to Z. Kosina for information accorded during the preparation of the data processing codes.     

Correlations of fission fragment angular momentum with collective and intrinsic degrees of freedom

In the present work angular momenta of the fragments corresponding to¹³²I^m,g have been deduced from the radiochemically determined independent isomeric yield ratios and statistical model based analysis in neutron induced fission of²³⁵U,²³⁹Pu and²⁴⁵Cm and spontaneous fission of²⁵²Cf. These data along with similar data on¹³⁴I, reported earlier from this laboratory, bring out the effects of deformed 66n and spherical 82n shells on fragment angular momentum showing also an inverse correlation of the latter with elemental yields. Quantitative estimates of fragment scission point deformation and the coefficient of change of fragment angular momentum with kinetic/excitation energy have been deduced and are seen to be in good agreement with the expected theoretical estimates.     

Analysis of fission excitation functions and the determination of shell effects at the saddle point

A method is proposed to deduce the shell correction energy corresponding to the fission transition state shape of nuclei in the mass region around 200, from an analysis of the first chance fission values of the ratio of fission to neutron widths, (Γ _f /Γ _n )₁. The method is applied to the typical case of the fissioning nucleus²¹²Po, formed by alpha bombardment of²⁰⁸Pb. For the calculation of the neutron width, the level densities of the daughter nucleus after neutron emission were obtained from a numerical calculation starting from shell model single particle energy level scheme. It is shown that with the use of standard Fermi gas expression for the level densities of the fission transition state nucleus in the calculation of the fission width, an apparent energy dependence of the fission barrier height is required to fit the experimental data. This energy dependence, which arises from the excitation energy dependence of shell effects on level densities, can be used to deduce the shell correction energy at the fission transition state point. It is found that in the case of²¹²Po, the energy of the actual transition state point is higher than the energy of the liquid drop model (LDM) saddle point by (3 ± 1) MeV, implying significant positive shell correction energy at the fission transition state. Further, the liquid drop model value of level density parametera is found to be a few per cent smaller for the saddle point shape as compared to its spherical shape.