Thermal Neutron Induced Fission Of 235U Research Articles

Nucleon transport processes in fission and heavy ion reactions

The nucleon exchange process between two nuclei in close proximity and its application to an explanation of fragment mass and charge distributions in fission and in heavy ion deep inelastic collisions are reviewed. An analysis of the measured correlations between the energy loss from relative motion and the fragment mass and charge variances in the heavy ion deep inelastic collisions is presented. The recent data on fragment mass and charge variances as a function of the fragment kinetic energy in thermal neutron induced fission of235U, lends added support to the hypothesis that the nucleon transport process plays a similar role both in fission and in heavy ion deep inelastic collisions.

Isomeric yield ratio of fission product148Pr in235U(n th,f)

The independent isomeric yield ratio of148Pr in thermal neutron induced fission of235U has been determined experimentally. The fission product148Pr isomers, extracted directly by on-line mass separation technique, have high-spin (J=4) to low-spin (J=1) isomer ratio of 0.14±0.04 using growth and decay analysis. Statistical model calculation of isomeric yeild ratio using constant initial r.m.s. angular momentumJrms can not reproduce either present results or other recent measurements of isomer ratios. TheJrms derived from isomer ratio data in all thermal fissioning systems indicate a wide spread ranging from 2ħ to 13ħ. No clear correlation betweenJrms and isomeric spins or number of neutrons of isomers is found, thus, more model refinements and experimental works should be done in order to evaluate independent isomeric yields correctly.

A double ionization chamber for fission fragment detection

In the present paper some problems associated with studies of the spontaneous fission of transfermium elements are considered, and advantages of ionization chambers over other detectors for fission fragment detection are discussed. A double ionization chamber for detecting fission fragments has been built, and the energy and mass calibrations have been performed using the thermal neutron-induced fission of 235U. The use of a gas jet, in combination with a double ionization chamber as a detector, in experiments with heavy ions is discussed. Model experiments have been carried out.

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.

Independent yields of 92mNb in the thermal neutron-induced fission of 233U, 235U and 239Pu

Independent yields of 92 m Nb were measured in the thermal neutron-induced fission of 233U, 235U, and 239Pu, using radiochemical techniques. Upper limits of < 2.2 × 10 −8% and < 4.6 × 10 −8% were found for 233U and 235U, respectively. The independent yield for 239Pu was determined to be (11 ± 6) × 10 −8%. This yield is much higher than values predicted from models of charge and isotopic distributions. It probably reflects the influence of the nearby closed 50 neutron shell.

The thermal-neutron-induced alpha-accompanied fission of235U investigation of the low energy part of the alpha spectrum

The energy spectrum of the α-particles emitted in the thermal-neutron induced fission of235U was measured from 11.5 MeV down to 2 MeV using the parabola mass spectrometer Lohengrin at the I.L.L. high flux reactor. This low energy part of the energy spectrum presents a smooth connection with the energy spectra which have been recently reported above 7 MeV. The overall energy spectrum, which is known to be quasi-gaussian above 12 MeV, is slightly asymmetric at low energy, where the observed particles are 6% more than expected from a gaussian shape. As a consequence, all the values reported for235U for the rate of α-accompanied fission compared to binary fission have to be multiplied by 1.06. This asymmetry is about 2 times less important than the one reported for252Cf. No evidence was seen for any intense low energy component as reported before. The possible reasons for the existence of this asymmetry are discussed.

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.

Isotopic compositions of xenon and krypton in Belgian Congo pitchblende.

Isotopic compositions of xenon and krypton released from a sample of Belgian Congo pitchblende were measured mass-spectrometrically in stepwise heating experiments. Relative contributions from spontaneous fission of 238U, thermal neutron-induced fission of 235U and fast neutron-induced fission of 238U, to the production of fissiogenic xenon found in the ore were calculated to be 64.1, 18.8 and 17.1 percent, respectively.

Independent yields of 96Nb and 51 min 98Nb in the thermal neutron-induced fission of 233U, 235U and 239Pu

Abstract The independent yields of 96 Nb and 51 min 98 Nb were determined in the thermal neutron-induced fission of 233 U, 235 U and 239 Pu using radiochemical techniques. The results for these fissionable nuclides are as follows for 96 Nb: (5.3±0.3) × 10 −3 %, (5.2±0.2) × 10 −4 % and (3.6±0.1) × 10 −3 %, respectively, and for 51 min 98 Nb: (0.13± 0.03)%, (0.034±0.015)% and (0.16±0.03)%, respectively.

Decay properties of 144–146La

γ-ray singles and coincidence measurements have been performed on the neutron-rich nuclides 144La, 145La and 146La, produced in thermal neutron-induced fission of 235U. The studies were facilitated by the combination of a gas jet recoil transportation (GJRT) system with the rapid on-line chemical separation system SISAK. Half-lives of 42.1±0.7 s, 25.2±2.6 s and 8.5±1.0 s were obtained for 144La, 145La and 146La, respectively. Several new γ-ray energies have been assigned to each of the nuclides, and partial decay schemes are proposed for 144La and 146La together with a preliminary partial decay scheme for 145La.

Nuclear charge distributions from 4?-? measurements on mass separated fragments from235U(n th ,f)

A 4π-β detector was used at the Lohengrin mass separator to measure theβ-activity of mass chains 91, 93, 94, 95, 137 and 139 produced in the thermal neutron induced fission of235U. This activity was analysed intoZ-yields. The different possible corrections were considered and applied to the data. TheZ-yields from this work are compared with the diverse existing results obtained with other methods. The need for and significance of this method to determine theZ-distribution of fission fragments of the heavy group is discussed.

Studies of mass and energy correlations in thermal neutron fission of U-235 accompanied by long range alpha particles

Several characteristics of fission accompanied by long range alpha particles (LRA) have been studied in the thermal neutron induced fission of235U. The kinetic energies of fission fragments and the LRA were measured with a back-to-back ionization chamber and semiconductor detectors respectively. The kinetic energies of the two fragments and the LRA in LRA fission, along with the energies of pair fragments in the normal binary fissions, were recorded event by event on a magnetic tape by means of a four-parameter data acquisition system. The data were analysed to study the dependence of different quantities in LRA fission on the fragment mass ratio, LRA energy and the total kinetic energy of the fission fragments. It is seen that the most probable energy of LRA increases significantly for near symmetric mass divisions. The total kinetic energy for all mass ratios in LRA fission is found to be (2.6±0.7) MeV larger than that in binary fission. The difference in the total kinetic energies in LRA and binary fissions is seen to be dependent on mass ratio. This result may suggest that the scission configuration in LRA fission is different for different mass ratios. Correlations between the fission fragment and LRA energies have been studied for several mass ratios. It is seen that the most probable fragment kinetic energyĒ k varies nearly linearly with the LRA energyE a for various mass divisions but the variation of the most probable LRA energyĒ a with fragment kinetic energyE k is found to deviate from linearity for several mass ratios. From a least square fit to the variation ofĒ k withE a it is found that the slope (dĒ k/dEa) increases with the increase in mass ratio. The present results are discussed to arrive at a better understanding of the scission configuration in the fission accompanied by LRA emission.

Independent yields of 150Pm in the thermal neutron-induced fission of 233U, 235U, 239Pu and 249Cf and cross-section for the 149Pm( n, γ) 150Pm reaction

Independent yields for the shielded nuclide 150Pm were determined in the thermal neutron-induced fission of 233U, 235U, 239Pu and 249Cf using radiochemical techniques. They were found to be (2·6±0·2)×10 −4%, (3±1)×10 −5%, (1·15±0·07)×10 −3% and (1·9±0·2)×10 −2%, respectively; these values correspond to empirical Zp values of 58·66, 58·31, 58·79 and 59·18, respectively, if a Gaussian charge dispersion function with a σ of 0·56 is used. A comparison of the Zp values for 150Pm with Zp values for other nuclides in the thermal neutron-induced fission of several actinide isotopes is given. The cross-section for 149Pm( n, γ) 150Pm in a swimming pool reactor neutron spectrum was found to be (1630±160) barn.

Decay of the 4.2-min152Pm into levels of152Sm

After bombardments of enriched152Sm with 14-MeV neutrons, three isomers of125Pm reaction, have been observed with half-lives of 4.2±0.2 min, 7.5±0.2 min and 15.0±1.0 min. Sources of pure 4.2-min152Pm were obtained by separating the promethium from its parent 11-min152Nd produced by thermal-neutron induced fission of235U. The beta and gamma radiations of the 4.2-min152Pm have been investigated using semiconductor and scintillation spectrometers. Most of theγ-ray transitions could be placed in a level scheme for 152Sm. Especially interesting is a level at 1083 keV for which the assignmentI π=0+ is proposed. In a 4πβγ experiment, the ground state feeding of152Sm in theβ −-decay of the 4.2-min152Pm has been measured to be 61% of allβ-ray transitions. AQ β-value of 3.4±0.2 MeV has been obtained.

The investigation of LRA fission of235U by non-shielded semiconductor telescope

The LRA particles energy spectrum from ternary thermal neutron induced fission of235U has been measured. A nonshielded semiconductor telescope technique has been used. The experimental spectrum has been fitted by the Gaussian distribution with average alpha energy of 15·65 ±0·09 MeV and full width at half maximum of 9·56±0.10 MeV.

Systematics of neutron emission probabilities from delayed neutron precursors

A simple correlation based on the gross theory ofβ-decay is derived between the neutron emission probabilitiesPn of delayed neutron precursors, theirβ-decay energies and the neutron binding energies of the daughter nuclei. The correlation is shown to be valid for delayed neutron precursors among the fission products. ThePn-values of several expected but still unidentified neutron precursors are estimated together with their contributions to the delayed neutron groups in thermal-neutron induced fission of235U. Some aspects of theβ-strength function for transitions into highly excited states are discussed.

Der Zerfall des93Sr

The nucleide93Sr was isolated by chemical separation methods after thermal neutron induced fission of235U, and its decay properties were investigated employing scintillation and semiconductor spectrometers and coincidence techniques. A half-life of 7.43±0.03 min and aQβ-value of 4.3±0.2 MeV were found. Out of 74 gamma-rays, 55 transitions, representing 97% of the observed gamma-ray intensity, could be placed in a decay scheme comprising 22 excited states of93Y.

Fission fragment K x-ray emission and nuclear charge distribution for thermal neutron fission of 233U, 235U, 239Pu and spontaneous fission of 252Cf

The emission of K X-rays by fission fragments within ≈ 1 nsec after fission has been studied as a function of fragment mass and nuclear charge for thermal neutron-induced fission of 233U, 235U, 239Pu and spontaneous fission of 252Cf. This work is based on a simultaneous measurement of the fragment kinetic energies to obtain the fragment masses and high-resolution measurement of the characteristic energy spectra of the K X-rays emitted by the fragments to determine their nuclear charges. For all four fissioning systems the K X-ray yields per fragment were found to be strongly dependent on nuclear structure. Near closed nuclear shells the K X-ray yields for odd- Z nuclei are enhanced relative to even- Z nuclei. Away from closed shells the K X-ray yields increase rapidly with Z and A, and the odd-even Z-fluctuations diminish. With an approximate parameterization for the dependence of K X-ray yields on Z and A, the average nuclear charges of the primary fragments represented as a linear function of their pre-neutron-emission mass (charge division) and the width of the charge distribution about the average values (charge dispersion) were obtained. The linear charge division functions obtained in this work are in good agreement with existing radiochemical data and with recent β-decay chain-length measurements. The average charge dispersion (before neutron emission) found in this work for 235 U( n, f) ( σ z = 0.40 ±0.05 charge units) is narrower than the charge dispersion (0.56±0.06) determined radiochemically (after neutron emission).

Nuclear charge distribution of heavy fission fragments from thermal-neutron-induced fission of235U

Fragments from thermal-neutron induced fission of235U have been separated by a mass spectrometer with respect to their masses and kinetic energies within 1 μsec. The separation principles are briefly described. For masses 130 to 139 amu the charge distributions have been determined by counting the number of beta tracks emitted from the individual mass selected fission fragments in a nuclear photographic emulsion. In another method, the average number of beta particles for each fragment mass is determined by use of a 4π-proportional counter. The mean nuclear charge as a function of mass is compared with other experimental results and theoretical curves. Contradictory to the radiochemical results, this experiment yields a dip in the mean nuclear charge versus mass curve at mass 132 amu corresponding to the doubly magic nucleus (N=82,Z=50)132Sn. Recent theoretical calculations of Norenberg are in agreement with this finding.

Prompt fission gamma rays

The spectrum of prompt γ-rays emitted by the fission fragments in the thermal neutron-induced fission of 235U has been exactly calculated within the framework of the basic statistical theory assumptions for the emission of neutrons and γ-quanta from excited nuclei. The shape of the spectrum corresponds approximately with the shape obtained experimentally but the computed total energy carried away by the γ-quanta (⪅6·2 MeV) is appreciably less than the experimentally determined amount (8–9 MeV). It is possible to remove this discrepancy if, in calculating the residual excitations, one takes into account the ‘thinning out’ of the levels of the fission fragment nuclei in the vicinity of the ground state.