Measurement Of Fission Cross Section Research Articles

Nuclear spectroscopy of highly deformed 231Th

A measurement of the neutron-induced fission cross section of 230Th has been made in the neutron energy range 625 keV to 1.4 MeV with a neutron energy resolution of 5 keV. At selected energies near a prominent resonance at 715 keV, the angular distribution of fission products with respect to the neutron beam has been measured with neutron energy resolution of about 18 keV. The results are interpreted in terms of a β-vibration in the secondary minimum of the fission potential barrier and allow a direct estimate of the effective moment of inertia of the 231Th nucleus in its shape isomeric state as well as other spectroscopic information on the nuclear levels associated with this shape.

Measurement and normalization of the relative 239Pu fission cross-section in the low resonance region

Measurements of the 239Pu fission cross-section were performed at a well-collimated short flight-path of the C.B.N.M. Linac from 30 eV to below thermal energy for a direct normalization to the 2200 m/s reference cross-section. The fission reaction rate and the neutron spectrum were determined simultaneously with solid state detectors placed on each side of a back-to-back 239Pu- 10B-layer. From the counting-rate ratios σ f -curves were calculated assuming a 1/v behaviour of the 10B(n, α) 7Li cross-section. Several fission and resonance integrals are calculated from the normalized σ f -curve and compared with other published results. It is shown that the large discrepancies between the reported results (up to more than a factor of 2) can be considerably reduced by applying a common normalization procedure, e.g. to the fission integral ∫ 9ev 20ev σ f(E)d E=1046·6±10·4 barn ev obtained from these measurements.

Neutron total cross-section fluctuations of 235U in the keV range

Relative neutron total cross-sections of 235U have been measured in the neutron energy range between 10 and 100 keV with a time-of-flight resolution of 0·3 nsec(m. Strong structure is observed in the total cross-section, which is at least partly in agreement with the structure observed recently in high resolution fission cross-section measurements, thus indicating that the latter is at least in part due to the entrance rather than the fission channel. A Monte Carlo simulation of the total cross-section shows that its structure may be fully explained in terms of pure resonance parameter statistics.

Measurement of fission cross sections of U235 and Pu239 in a neutron spectrometer by means of the moderation time

Measurement of fission cross sections of U235 and Pu239 in a neutron spectrometer by means of the moderation time

Measurement and normalization of the relative 235U fission cross-section in the low resonance region

Measurements were performed with the Linac of the C.B.N.M. to below thermal energy for a direct normalization of fission cross-sections to the 2200 m/sec reference cross-section. The measurements were performed at a well-collimated short flight-path with low repetition-rate and long burst-width of the Linac. The fission reaction rate and the neutron spectrum were determined simultaneously with solid state detectors on each side of a back-to-back 235U and 10B layer. From the counting-rate ratios σ f -curves were calculated assuming a 1 ν behaviour of the 10B( n, α 7Li cross-section. Several fission integrals are calculated from the normalized σ f curve and compared with other published results. It is shown that the large discrepancies (up to 35 per cent) which exist between the different results can be considerably reduced by a common normalization procedure e.g. using the fission integral 11eV 7·8eVσf(E) dE=240.2±2.1 barn eV obtained from this measurement.

Fast Fission Chamber Amplifier-Discriminator System

To measure the fission cross section of 233U as a function of neutron energy, instrumentation with fast pulse rise-time and short resolving time characteristics is required. The efficient detection of fission-fragment pulses in the presence of a high alpha background requires these characteristics. A fission detector with an electron collection time of 25 nsec and containing 1 g of 233U, and an amplifier with a risetime of 7 nsec and an over-all gain of 1000 were developed for this measurement. This system, detecting the fast rising current pulse from a parallel plate ionization chamber, is an integral part of the instrumentation used by a group at ORNL engaged in the measurement of the neutron fission and capture cross sections as a function of neutron energy. With this system, the fission cross section of 233U was successfully measured over a neutron energy range from 0.4 to 2000 eV.

Neutron Fission and Capture Cross-Section Measurements for Uranium-233 in the Energy Region 0.02 to 1 eV

The relative neutron capture and fission cross section in the neutron energy range 0.02 to 1.0 eV have been simultaneously measured. The data are normalized by means of the previously reported tota...

Measurement of the Prompt Neutrons Emitted in the Fission ofBi209andU238Induced by 155-MeV Protons

The energy distribution of the neutrons emitted in the fission of $^{209}\mathrm{Bi}$ and $^{238}\mathrm{U}$ induced by 155-MeV protons was measured at 0 and 90\ifmmode^\circ\else\textdegree\fi{} with respect to the fission fragment direction. The neutron velocity was measured by the time-of-flight method in coincidence with the measurement of the kinetic energy of the two fission fragments. The number of postfission neutrons as a function of the fragment mass and the energy distribution of the postfission neutrons were determined without reference to any theoretical model. The number of prefission neutrons was determined on the assumption of isotropic emission from the excited nucleus. With this assumption we find that 5.8\ifmmode\pm\else\textpm\fi{}1.0 prefission neutrons are emitted from U and 6.9\ifmmode\pm\else\textpm\fi{}1.0 from Bi. The number of postfission neutrons is 5.1\ifmmode\pm\else\textpm\fi{}0.5 for U and 4.2\ifmmode\pm\else\textpm\fi{}0.5 for Bi. We compare our experimental results with the results of calculations based on the usual model of a prompt intranuclear cascade followed by neutron evaporation-fission competition with several assumptions for $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$ (ratio of fission width to neutron emission width). We are unable to find a consistent set of parameters which would give good agreement between the calculated results, our experimental results, radiochemical results for spallation residues, and total fission cross-section measurements. Our experimental results indicate that $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$ decreases at high excitation energies. We find that the number of postfission neutrons increases rapidly with increasing fragment mass for both Bi and U. When our results for U are compared with low-energy fission data for this element, it is found that almost all the additional excitation energy available in high-energy fission is concentrated in the heavy fragment.

A technique for the absolute counting of nuclear fission events

A technique has been developed for the absolute measurement of nuclear fission events which is virtually independent of any of the usual computed corrections for losses. The two fission products arising from each event are detected singularly and in coincidence, and the absolute fission rate is determined from the three recorded counting rates. The technique was developed as part of a programme to determine ν for 252Cf. With a redesigned counter the method should be adaptable to the measurement of fission cross-sections.

A new technique for capture and fission cross-section measurements

A technique is described for obtaining, simultaneously, capture and fission cross sections with the same detector-a deuterated-benzene-based liquid scintillator. Neutron and γ-ray events are separated by pulse-shape discrimination, and on-line pulse-height weighting is used to insure spectral independence for capture events. With the present technique, the latter requirement is satisfied for detector arrays of large solid angle.

Fission cross-section measurement on 235U

The cross section of the 235U(n f) reaction has been measured with a high-energy resolution for neutrons between 6 eV and 3 keV. A fission fragment and a fission neutron detector were used simultaneously. Resonance parameters for 78 levels between 6 and 150 eV are given. The statistical distributions of Γ f, Γ γ and α have been studied.

A measurement of the 235U fission cross section at 30 AND 64 keV

The fission cross section of 235 was measured absolutely at 30 keV neutron energy using kinematically collimated neutrons at the threshold of the 7Li(p, n) 7Be reaction. The number of fission events in a 1·2 mg/cm 2 U 3O 8 foil was determined by counting fission fragments in a gas scintiliation detector. The neutron flux was measured both by counting of the induced 7Be activity in the target and through a parallel measurement of the induced 198Au activity in a gold foil. Another measurement was made at 64 keV using the 8H(p, n) 3He reaction at threshold as the neutron source. The flux was normalized in this case by assuming a value for the 197Au capture cross section ratio at the two energies. Values obtained for the fission cross section are 2·19 ± 0·06 barns at 30 keV and 1·78 ± 0·13 barns at 64 keV.

A large area fission fragment detector with fast response

The performance of a fission fragment detector is described which can be used for high resolution fission cross section measurements. It consists of two very thin plastic scintillator foils of large areas sandwiching a fissile layer. Coincidence techniques applied to the signals of two photomultipliers which view these scintillators allow to discriminate fission fragments against alphas and other background sources. Amplitude spectra and the dependence of fission detection efficiency from location on the detector surface have been studied. As a result of an application of the detector, 235U fission reaction rates are shown in function of neutron time of flight.

Measurements of the 235U neutron fission cross section in the energy range 0·04–14 MeV

The neutron fission cross section of 235U has been measured relative to the n-p scattering cross section at a number of neutron energies in the range 0·04–4 MeV to an accuracy of about 3 per cent. Hydrogen and methane gas proportional counters and a Polyethylene radiator semiconductor counter were used as recoil proton flux measuring instruments in the appropriate parts of this neutron energy range. Good agreement was obtained between the shapes of the recoil proton spectra and those calculated. The effect of neutron scattering and the loss of fission fragments in the uranium foil were investigated experimentally. At 1·0, 2·25 and 14·1 MeV the present results agree reasonably well with a currently accepted best curve through previous measurements of this cross section (BNL-325). At energies less than 500 keV however the cross sections measured here are about 7 per cent below this curve and at an isolated measurement at 5·4 MeV the present result is 1 l per cent below previous measurements.

Subthreshold Neutron-Induced Fission Cross Section ofAm241

The corona-type spark chamber has been adapted for detection of fission fragments and has been used as the detector for neutron-induced fission cross-section measurements of ${\mathrm{Am}}^{241}$. Measurements were made over the neutron energy range from 0.032 to 80 eV. The ratio of average fission width to spacing is in good agreement with that predicted from the shape of the fast-neutron fission threshold. The number of effective fission channels obtained from an analysis of the distribution of fission widths was found to be 2.9\ifmmode\pm\else\textpm\fi{}0.6 channels.

The measurement of the neutron fission cross section of 240Pu in the energy range 60–500 keV

The neutron fission cross section of 240Pu has been measured relative to that of 235U in the energy range 60–500 keV. The cross section rises from a mean value of 75 ± 7 mb below 160 keV to 286 ± 54 mb at 485 keV. The results have been compared with earlier measurements.

Fission cross sections of 232Th, 233U, 235U, 237Np and 238U for 5–37 MeV neutrons

Results are presented of fission cross section measurements on 232Th, 233U, 235U, 237Np and 238U for neutrons in the energy range 5–37 MeV. The measurements were made using the 1·5 m cyclotron of the I. V. Kurchatov Order of Lenin Institute of Atomic Energy. The neutrons were produced by the D(d, n) 3He and T(d, n) 4He reactions and the time-of-flight method was used to effect the neutron energy selection. Gas scintillation fission counters were employed as detectors.

Electrodeposition of thorium oxide on aluminium

For measurements of the neutron-induced fission cross section of 242Pu, large-area (42 cm2) 242Pu targets were prepared on Ti-coated Si wafers by means of constant current density molecular plating. Radiochemical separations were performed prior to the platings. Quantitative deposition yields (>95%) were determined for all targets by means of alpha-particle spectroscopy. Layer densities in the range of 100–150 μg/cm2 were obtained. The homogeneity of the targets was studied by radiographic imaging.A comparative study between the quality of the layers produced on the Ti-coated Si wafers and the quality of layers grown on normal Ti foils was carried out by applying scanning electron microscopy and energy dispersive X-ray spectroscopy. Ti-coated Si wafers resulted clearly superior to Ti foils in the production of homogeneous 242Pu layers with minimum defectivity.

SECTION OF PHYSICAL SCIENCES: A GRADUATE RESEARCH PROGRAM IN NUCLEAR ENGINEERING AND SCIENCES*

Research problems in reactor physics, nuclear spectroscopy, and radiation effects on materials are discussed. The R.P.I. linac and associated facilities are described. The time dependence of flux on space, energy, and angle transients is being studied. Precision measurements of asymptotic and time- dependent spectra in moderators and subcritical assemblies are reported. Measurements of neutron capture and fission cross sections and the detailed study of certain photonuclear disintegrations are reported as in progress. Some results are given for isotopic assignments of hafnium energy levels. Other programs reported are molecular energy levels and neutron cross sections from low- - energy neutron inelastic scattering, plans for mass spectrometric isotopes laboratories, and radiation damage in semiconductors and thin metal films. (R.E.U.) H03 H029803 No genetic effects were observed in Drosophila on exposure to homogeneous and inhomogeneous magnetic fields of high field strength and gradient. Synergistic effects of magnetic fields in combination with x radiation, starvation, hyperoxia, and hypoxia were also not observed. (auth)

A Thermal Neutron Fission Counter for Nuclear and Analytical Measurements

A fission counter which has been used at the Argonne CP-5 reactor is described. It has been applied to the measurement of thermal neutron fission cross sections and to the determination of very small quantities of fissionable materials. The counting system consists of a dual chamber fission counter, the associated electronic apparatus, a mechanism for positioning the counter in the thermal column, and the protective neutron shielding. The design and performance of the system and the operating procedures are discussed in detail.