keV Neutron Energy Research Articles

The 186W(n, γ) 187W reaction at 24 keV, hexadecapole deformations, and fragmentation of Nilsson model strength

The (n, γ) reaction at 24 keV and thermal neutron energies was performed on enriched samples of 186W to determine specific spin-parity limitations on states in the 700–1600 keV energy region in 187W. A number of these states are populated by some of the strongest transitions previously observed in the 186W(d, p) 187W reaction. Comparison of the results of the two reactions yields information on the distribution and fragmentation of Nilsson single quasiparticle strengths. As in the odd Hf nuclei and in 183, 184w, significant fragmentation of strength was observed in 187W. The systematics of the fragmentation is presented and it is shown that a substantial loss of strength occurs in 183, 184W but not in 179Hf or 187W. The fragmentation, and its unexpected systematics, are not explained by previously available calculations. It is proposed that large hexadecapole deformations may be important in understanding the fragmentation. Such shape components lead to single particle energy shifts, enhanced ΔN = 2 and Coriolis mixing as well as other j-dependent effects that tend in the direction of the experimental results. Calculations are performed to illustrate these effects.

Neutron-Induced Fission Cross Section ofCf252

The neutron-induced fission cross section of Cf252 was measured from 20-eV to 5-MeV neutron energy, using neutrons from the Physics-8 nuclear explosion. Pronounced sub-barrier resonance fission is observed below a few keV neutron energy. The fission threshold is found to occur at ∼900 keV. Analysis of the data indicates that the fission barrier is quite transparent, giving an average fission width comparable to the value of 0.02-0.05 eV expected for the average radiative capture width in the resonance region.

Neutron flux measurement by counting the associated 3He particles from the T(p,n) reaction corresponding to 500 keV neutron energy

This paper continues an earlier publication 1) about counting associated 3He particles from the T(p,n) reaction. Even with a simpler experimental set-up than described there (without electrostatic separation of the Coulomb scattered protons) but with Ti-T targets having thinner Al backings (200 μg/cm 2 Al) it is possible to shift the energy of the associated neutrons down to 500 keV with a slight increase in accuracy. The results of this method were compared with those from n-p scattering in a methane-filled proportional counter. Agreement was found within 0.8%.

Precise measurements of the average kinetic energy of fragments in the fission of 235U by fast neutrons

The average total kinetic energy of fragments in the fission of 235U by neutrons of energies ranging from 200 to 900 keV has been measured using two solid-state detectors to detect the the fission fragments in coincidence. The average total kinetic energy of fragments is independent of the incident neutron energy and, within experimental error, does not show the reported discontinuity around 300 keV neutron energy.

Spin cut-off factors and the independent-pairing model

A brief resume of the various models used for the description of the nuclear-level density and the relevant expressions for the calculation of the spin cut-off factorσ based on each of these models are presented. The experimental values of the isomeric ratios in the reactions68Zn(n, γ)69Zn,79Br(n, γ)80Br,80Se(n, γ)81Se,115In(n, γ)116In and130Te(n, γ)131Te at 24 keV neutron energy reported in literature are used to extract the spin cut-off factors for the isotopes69Zn,80Br,81Se,116In,131Te. The values ofσ predicted by the independent-pairing model are found to be nearest to these experimental values.

Nuclear explosion experiments to determine nuclear properties of heavy isotopes

Results of experiments to determine the properties of heavy isotopes from studies of heavy-element yield curves obtained in nuclear explosions, which cannot be performed in the laboratory, are reported. In most cases, 238U was used as the target element. Measurements of uranium neutron capture cross sections around 20 keV neutron energy in the mass range 242–249 reveal a decrease at mass 244 corresponding to 152 neutrons, presumably due to the “shell effect”. These results agree well with original calculations. No fission competition from mass 242–248 is evident, but a fission-to-capture ratio of 5 for mass 249 appears probable. There is some evidence of thermal fission in the uranium capture chain at 239, 241U and strong evidence of thermal fission in the plutonium capture chain. Strong thermal fission competition is indicated in the neptunium capture chain.

Instrumentation to measure the ratio of neutron capture to fission cross sections at keV neutron energies

Instrumentation for measurement of α (the ratio of the neutron capture cross section to the fission cross section) was designed, assembled, and calibrated, and measurements were made for 239Pu, 233U, and 235U at neutron energies from 10 to 600 keV. The detector consisted of eight photomultiplier tubes located around the periphery of a 210-gal tank of gadolinium-loaded liquid scintillator. The time resolution of the system was 6.5 × 10 −9 sec under operating conditions. The energy resolution at the 2.5-MeV sum peak of 60Co was 35%. Neutron energies were measured by time-of-flight techniques using a flight path of 1 meter. Fission events were distinguished from capture events by the detection of thermalized fission neutrons following the primary events. Measurement of the relative efficiency of the large scintillator tank for detecting capture and fission events was made by simultaneously compiling the background and foreground pulse-height spectra for both the capture and fission events. Typical spectra for the neutron time-of-flight for capture and fission events along with the background and foreground spectra for both types of events are shown. The techniques and adjustments used to obtain the timing resolution on the large tank are discussed. The logic necessary for distinguishing fission events and the foreground and background pulse-height spectra associated with both types of events are described.

Neutron radiative capture in Mo

Neutron radiative capture in natural Mo has been studied in the neutron energy range from 10 eV to 25 keV. An area analysis has been applied to part of the observed resonances which yields gΓ n for small resonances or Γ γ for large resonances. Above 1 keV neutron energy, the observed capture rate has been averaged to give the mean capture cross section which is in good agreement with earlier measurements.

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.

The (n, γ) cross section of 197au at 30 and 64 keV neutron energy

The neutron capture cross section of 197Au was measured at 30 and 64 keV neutron energy using the activation method. Two different methods were used at 30 keV for the absolute neutron flux determination. The results are σ n,γ Au (30 keV) = (0600 ± 0·009) barn and σ n,γ Au (64 keV) = (0·357 ± 0·009) barn.

Ratio of Capture to Fission in U235 at keV Neutron Energies

The ratio of the neutron-capture cross section to the fission cross section (α) for U235 has been measured for incident neutron energies from 12 to 690 keV by a large gadolinium-loaded liquid-scintillator technique. Additional measurements at 30 and 64 keV were made by a method employing a liquid scintillator and a fission chamber. The experimental values of α can be approximately described by a linear decrease from 0.374 at 10 keV to 0.177 at 210 keV followed by a less rapid linear decrease to 0.095 at 700 keV. The results of these experiments are consistent and in reasonable agreement with other reported values of α in this energy range.

Total neutron cross section of carbon

An accurate determination of neutron total cross section of carbon has been made by the transmission method in the energy region 3 keV to 660 keV. No structure is observed in the total cross section over the whole energy region, and it is concluded that the upper limit of the width of the proposed level at 610 keV neutron energy must be 10 eV in order to escape detection in the present experiment. The observed total cross section has been analysed in terms of the effective range theory as well as the resonance theory, and the results are found to be in good agreement. The relative reduced width for the bound level at −1.86 MeV (i.e. the 3.09 MeV 2 s 1 2 state in C 13) is found to be θ 2 = 0.2, and the interaction radius is found to be 4.7 fm.

Neutron-capturing processes in stars

The cross sections for neutron absorption by N/sup 14/, O/sup 17/ and Ne/ sup 21/ are given for 10, 15, and 60 kev neutron energies, and the products of the abundances expected in stellar interiors and the capture cross sections for 15 kev neutrons are listed. At low temperatures in stellar irteriors the neutron capture process, starting with Ne/sup 20/ as seed nucleus, is discussed and seen to progress in a cycle through Ne/sup 21/, O/sup 18/, O/sup 19/, F/sup 19/, F/sup 20/, back to Ne/sup 2 0/. This cyclic reaction is seen to establish a definite dynamic equilibrium between the species of atomic nuclei involved in the cycle. Abundance ratios of Ne/sup 20//Ne/sup 21/ and O/sup 18//F/sup 19/ are calculated and compared with observed ratios. Some possible neutron and heavy seed nuclei production processes are discussed. (H.D.R.)

The polarization of neutrons from the Li 7(p, n)Be 7 reaction (I)

Previous measurements on the polarization of neutrons from the Li 7(p, n)Be 7 reaction have been extended. Left-right asymmetries in the scattering of neutrons emitted from the reaction at a laboratory angle of 50° have been measured usinng scatterers of Li 6, Li 7, O and Mg. Measurements were carried out for laboratory scattering angles near 90°, and for neutron energies between 85 and 420 keV. Data obtained using the Li 7, O and Mg scatterers were analyzed to give the polarization of the neutrons emitted from the reaction. The measured polarizations are of the order of 0.4 down to 250 keV neutron energy, below which they decrease abruptly. Relatively small asymmetries observed using Li 6 as a scatterer suggest the antiparallel spin S-wave interaction may be the stronger for this nucleus.

The 2.85 keV neutron resonance of sodium

The measurement of the total neutron cross-section of sodium in the region 600 eV to 15 keV using the Harwell 15 MeV linear electron accelerator is described. Analysis of the results in conjunction with the known values of the low energy elastic and radiative capture cross-sections leads to the following conclusions. The cross-section up to 15 keV may be described in terms of a potential scattering cross-section of 2.4 barn and two s-wave resonances, one at 2.85±0.04 keV neutron energy and the other at negative energy. The 2.85 keV resonance has parameters σ 0 = 370±5 barn, Γ n = 405±12 eV, Γ γ ≦ 0.34±0.01 eV and spin J = 1+. The negative energy resonance has a ratio of reduced width to resonance energy of γ n 2/ E R = −0.5 while | E R| ⪢1 keV. The spin of this resonance is J = 2+.

Investigation of Nuclear Energy Levels in Sulfur

The total cross section of sulfur has been measured for neutrons of energies from 15 to 1450 kev with energy spreads between 1.5 and 9 kev. Below 750 kev, nine well-defined maxima are observed, three of which are interpreted as caused by $s$-neutrons; the others are believed to be produced by $p$- or $d$-neutrons. The $s$-resonances have natural widths of about 15 kev, while one of the other resonances which was studied in detail has a width of 1.5 kev. In the case of the latter resonance, the compound nucleus appears to be formed with spin of $\frac{3}{2}$. Above 750 kev neutron energy the density of observed levels increases, and no interpretation of the resonances was attempted.