Electromagnetic and nuclear fission of238U in the reaction of 100, 500, and 1000 A·MeV208Pb with238U

The folding- and azimuthal-angle and velocity distributions for the²³⁸U fission fragments have been measured in reactions with 100, 500, and 1000 A·MeV²⁰⁸Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16±0.07, 0.48±0.08, and 0.60±0.04, respectively. The electromagnetic fission cross section as a function of the²⁰⁸Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (≈1.5 b) is approximately constant between 100 and 1000 A·MeV.     

Complete fusion studies of the 20Ne+235U system

The probability for complete fusion has been measured for the ²⁰Ne+²³⁵U system at bombarding energies of 175 and 252 MeV. The fission fragment angular correlation technique was used to obtain both the absolute cross section for complete fusion and relative cross sections for the ratio of complete fusion to incomplete fusion processes. Total reaction cross sections were also determined from elastic scattering measurements. Values of σ_R = 1900±40 and 2810 ± 140 mb were obtained at 175 and 252 MeV, respectively. These measurements yield if$\text{σ}{}_{\mathrm{<mtext>CF</mtext>}}\text{σ}{}_{\mathrm{<mtext>R</mtext>}}\text{= 0.74±0.05}\text{at}\text{175}\text{MeV and}\text{0.48 ±0.06}\text{at}\text{252}\text{MeV}$. The corresponding values of the critical angular momentum are compared with the predictions of several heavy-ion interaction models. It is observed that the l_c values obtained from these data agree well with the current models, although the energy dependence is not as strong as predicted. In addition, it is observed that these values of l_c exceed the limiting value for a charged rotating drop.     

Population of the 236U shape isomer in a photonuclear reaction

A fission isomer with a half-life of 115 ± 5 ns and a yield ratio Y_iso/Y_prompt = (2.02 ± 0.16) × 10^?5 was observed in bremsstrahlung-induced fission of natural uranium. The isomer is ascribed to ²³⁶U populated via a ²³⁸U(γ, 2n) reaction. The integrated cross section for isomeric fission is determined to be σ_int = 32 ± 6 μb MeV. Comparing this value with a calculated total isomer production cross section, a branching ratio of the isomer decay of Γ^γ_II/Γ^f_II ≈ 6 can be deduced.     

Determination of the fission barrier of232Pu from β-delayed fission and the problem of the first barrier

A delayed fission activity with a half-life of (55±7) s and a production cross section of (5±1)nb has been observed in the irradiation of²³⁷Np with 104 MeVα particles. It is tentatively assigned to the electron capture decay of²³²Am followed by fission of²³²Pu With a probability of (1.3 _?0.8 ⁺⁴ )· 10^?2 A barrier height of (5.3±0.4) MeV is extracted for²³²Pu exceeding theoretical barrier heights by about 2MeV. Systematic deviations between experimental and theoretical barrier heights previously observed in particular for U and Th isotopes are now also established for neutron deficient Pu isotopes indicating severe defects in the procedure of calculating static fission barriers.     

Fission barrier of 209Bi measured by electron induced fission

For ²⁰⁹Bi an electrofission cross section below 10^?36 cm² was measured by the observation of correlated binary fission tracks in mica detectors. From the cross-section data between 28 and 41 MeV electron energy, a fission barrier of 25.5 ± 1.5 MeV was deduced. The method offers the possibility to measure fission barriers ar low angular momentum and for nuclei not accessible otherwise.     

Messung der Neutronenpolarisation bei der Reaktion12C(d,n 0)13N mit Hilfe der Mott-Schwinger-Streuung

The polarization of neutrons produced in the reaction¹²C(d, n ⁰)¹³N was measured. Deuterons from the Karlsruhe isochronous cyclotron were used to induce this reaction at 51,5 MeV laboratory energy. The degree of polarization was determined by using the special features of Mott-Schwinger scattering. With an uranium scatterer analysing efficiencies of up to 0.92 can be obtained at very small angles (0.23°). The analysing efficiency can be calculated if the differential cross section at 0° and the total cross section is known. These quantities were experimentally determined. The differential cross section for 49.4 MeV neutrons, scattered by uranium, was measured between 0.88° and 2.10°. By an extrapolation the value 43.4±2.6 b/sr was found for the nuclear differential cross section at zero degree. A total cross section ofσ _t=4.80±0.22 b was obtained. The neutron polarization was measured at a reaction angle of 24.5° and the result isP=?0.45±0.07. This value is fairly above the semiclassical 1/3 limit and can be only explained, if spin orbit forces are taken into account. For (d, n) reactions this is the first neutron-polarization measurement above an energy of 20 MeV.     

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.     

The scission neutron spectrum of252CF (SF)

The scission neutron spectrum was obtained as a difference between the integral experimental fission neutron spectrum evaluated by Mannhart based on measurements in the laboratory reference system from several authors in the different neutron energy ranges, and the laboratory fission neutron spectrum stemming only from those neutrons which are evaporated from the fully accelerated fission fragments. The scission neutron spectrum was represented by a Weisskopf evaporation spectrump _s(? _l )=P ₀/(T _s)²·?_l·exp(??_l/T_s). A least squares fit gave for the fraction of scission neutrons with respect to all fission neutrons a value ofp ₀=(0.011±0.003) and for the pseudo scission neutron temperature a value ofT _s=(0.20±0.03) MeV. The low pseudo temperature is compatible with a cold nuclear matter of the prescission configuration. The low energy of the scission neutrons indicates that they come from a slowly moving source. The width of the distribution suggests that the source is small: radius of about 5 fm. This information conforms with the idea of satellite droplets which are formed when the neck snaps.     

Measurement of the π−n→π−π−p cross section near threshold

The cross section for the reaction π^?n→π^?π^?p has been measured in a deuterium bubble chamber. The cross section at E_π,lab = 226, 250 and 312 MeV was found to be equal to 9.4 ± 2.3, 25.0 ± 5.3 and 57 ± 10 μb, respectively.     

Energieverteilung und Wirkungsquerschnitt der Photoprotonen aus N15

The energy spectra of photo protons from highly enriched N¹⁵ were investigated with the bremsstrahlung from the Heidelberg betatron running at six different endpoint energies between 19 and 30.5 MeV. The protons were detected at 90° to theγ-beam by means of a CsJ-spectrometer with pulse shape discrimination. The energy spectra show pronounced maxima atE _p =3.2; 4.6; 9.5 and 13.3 MeV. Proton yields are given as a function of endpoint energy, the yield value atE ₀=30.5 MeV being (7.0±0.8) μb/MeV · ster. Because the first excited state in the daughter nucleus C¹⁴ lies 6.09 MeV above the groundstate, the cross section for groundstate transitions of the process N¹⁵(γ, p)C¹⁴ could be derived from the upper 6 MeV of the single proton spectra. The main contribution to the cross section comes from the region between 18 and 26 MeV excitation energy with maxima at 19.5; 20.4; 22.7 and 24.5 MeV. A “pygmy resonance” occurs at 15.2 MeV with further less pronounced structures at 13.6 and 17.0 MeV. The integrated cross section for groundstate transitions up to 30.5 MeV is (22±3) MeVmb assuming isotropic angular distribution. The ratios of protons from transitions to excited states and from the (γ, n p)-reaction to those of groundstate transitions rise from 0.45 atE ₀=24.5 MeV to 0.70 atE ₀=30.5 MeV.     

Electrofission of238U and232Th

Absolute electrofission cross sections for²³⁸U and²³²Th in the energy regionE _e =7 ?65 MeV and fission fragment angular distributions forE _e =7–30 MeV have been measured. The angular distributions show strong anisotropies for low energies. The relative dipole and quadrupole contributions as a function of excitation energy are discussed in terms of the low lying fission transition states above the fission barriers. The cross sections show significant deviations from the results of some earlier measurements, in particular in the energy region above the giant dipole resonance. From the difficulties of absolute electrofission cross section measurements and the ambiguities in their interpretation it is concluded that by this time the quantitative analysis of electrofission cross sections with respect to the contributions of the giant quadrupole resonances to the fission decay channel should be regarded as rather tentative.     

Fragment mass and kinetic energy distribution in isomeric fission of 240Pu

Fragment mass and kinetic energy distributions have been measured for isomeric fission of ²⁴⁰Pu. The mass distribution is asymmetric with the average heavy fragment mass nearly equal to that found for ground state spontaneous fission of ²⁴⁰Pu, but slightly lower than for n_th + ²³⁹Pu-fission. The average total fragment kinetic energy appears to be higher in isomeric fission (179.5_?0.7^+1.5 MeV) than in spontaneous fission from the ground state (176.8 ± 1.8 MeV).     

Fission cross section and fragment angular distribution in gold fission induced by 55 MeV alpha particles using solid state nuclear track detectors

The angular distribution of fission fragments in alpha induced fission has been studied at an incident energy of 55 MeV in¹⁹⁷Au target. The relative differential fission cross sections were measured at different angles between 10° and 170° and the resulting angular distributions fitted by least squares method with Legendre polynomials. In the present work, a correction for the self-scattering and self-absorption of fission fragments in the target itself was applied and a target of 3 mg/cm² was used to get good statistics. The anisotropy for 55 MeV alpha induced fission of gold was 2·83±0·43 and the fission cross section calculated by integrating the measured angular distributions over all the solid angles was 5·2±1.0 mb, confirming the value of 4·0±0·05 mb reported by Burnettet al but contrary to the high value of 10±3 mb reported by Ralarosyet al.     

Thorium fission by 4.8 and 14 MeV neutrons

The fission of natural Thorium byE _n =4.8 and 14.0 MeV neutrons has been investigated by measuring the kinetic energies of both fragments in surface barrier detectors. The fragments were detected forE _n =14.0 MeV neutron energy at 90 ° to the neutron beam, forE _n =4.8 MeV at 0 ° and 90 °. The results given are the distributions of mass and of energy correlative to the fragment mass, and the correlation between anisotropy and fragment mass atE _n =4.8 MeV. The average total kinetic energy before prompt neutron emission atE _n =4.8 and 14.0 MeV neutron energy has been found to be \(\overline {E_K^* } \) =170.47±0.03 and 168.1 ± 0.05 MeV respectively. Within the limits of statistical error the anisotropy atE _n =4.8 MeV is independent of the fragment mass. The results are analysed in the framework of the Two-Mode-Fission-Hypothesis. It appears, that the behaviour of the kinetic energy is too complicated as to be well described by the Two-Mode-Fission-Hypothesis. Both the Cluster Model of fission and the Fragment Shell Theory are suited to reproduce qualitatively the observed energetics of fission.     

Induced nuclear fission accompanied by pion emission

Basic features of the nuclear-fission process induced by protons of incident energy in the range 150<E _p<600 MeV and accompanied by pion emission are predicted on the basis of the cascade-evaporation-fission model. Specific calculations are performed for the total cross section; and the angular and double-differential distributions of pions; excitation-energy,mass number, and charge-number distributions of compound nuclei; and the mass-energy distributions of fission fragments. Various lines of possible experimental investigations into this fission channel are discussed, including searches for the pionic channel of nuclear decay induced by protons of energy close to the meson-production threshold, advancements to the energy region E _p<100 MeV in order to study of new mechanisms of pion production in nuclear fission, and an extension of investigations to the case of nuclear fission accompanied by kaon emission.     

Der Kernphotoeffekt am3H

The nuclear photoeffect in tritium was investigated by irradiating a gaseous³H-target with bremsstrahlung of 32,5 MeV endpoint energy in the collimated beam of the Heidelberg betatron. Protons and deuterons from the reactions³H(γ, n)d and³H(γ, p)2n were measured independently at (90±15)° with respect to the γ-beam by adE/dx-E-semiconductor telescope. The cross section of the twobody disintegration³ H(γ, n)d is derived forγ-energies fromE _γ=18 to 31 MeV. The integrated cross section from threshold to 32 MeV is (11.8±1.4) MeVmb. The cross section of the threebody disintegration³ H(γ, 2n)p is obtained by making assumptions regarding the angular distribution and energy split up amongst the three disintegration products, which appear plausible on theoretical grounds. Within experimental errors the results reported here seem to be in agreement with a theoretical calculation byFetisov.     

Ausbeute der Reaktion Ag(γ,t) mit Bremsstrahlung bis zu 55,7 MeV

The yield of triton production from silver irradiated with bremsstrahlung ranging in maximum energy between 31.2 and 55.7 MeV was measured relative to the C(γ, n)C¹¹-reaction. The silver targets were dissolved in nitric acid, the water converted to hydrogen gas, and the tritium activity counted in a low-level Geiger counter. Above threshold, the quantityσ _y (approximately equal to the bremsstrahlung-weighted cross section) increases roughly linearly with maximum energy and reaches (19±1) μb at 55.7 MeV. A comparison with the statistical theory of nuclear evaporation shows that the experimental yields exceed the theoretical predictions by a factor of 100 to 1000.     

Prompt γ-rays emitted in the thermal-neutron induced fission of 233U and 239Pu

The average number and average energy of γ-rays emitted within ≈ 5 nsec after fission have been determined as functions of fragment mass and as functions of total kinetic energy. They were obtained from a four-parameter experiment that recorded, event-by-event, correlated of γ-rays and of fission-fragment pairs and the time, relative to fission, at which a γ-ray was detected. For ²³³U(n_th, f) the average total number and energy emitted per fission were found to be 6.31 ± 0.3 and 6.69 ± 0.3 MeV, respectively, giving an average photon energy of 1.06 ± 0.07 MeV. The results for ²³⁹Pu(n_th, f) given in the same order, are 6.88 ± 0.35,6.73 ± 0.35 MeV, and 0.98 ± 0.07 MeV.     

Structure in the subthreshold 232Th(n,f) cross section

The fission cross section of ²³²Th has been measured over the neutron energy range 0.1 MeV to 30 MeV at the Livermore 100 MeV electron linac. The observed structure in the cross section between 0.7 MeV and 1.4 MeV, attributed to vibrational states in the second well of the double-humped fission barrier, is clearly correlated with the angular anisotropy of fission fragments measured by others.     

Differential cross section,analyzing power and phase shifts for p-3He elastic scattering below 1.0 MeV

Differential cross sections and analyzing powers for the elastic scattering of polarized protons by unpolarized ³He nuclei have been measured at eight energies between 0.3 MeV and 1.0 MeV for scattering angles θ_c.m. = 52.4°–173.3°. The cross-section values were normalized to the Rutherford cross section for proton-krypton scattering. The analyzing powers have been measured with a statistical accuracy of about 0.001. The phase-shift analysis based on these data included all phases for orbital angular momenta l ≦ 1 and the channel-spin mixing parameter for the P waves. An energy parametrization of the phase shifts by an effective-range approximation allowed a simultaneous utilization of all data.