Neutron Production Cross Sections Research Articles

The 11B(α, n) 14N cross section for α-energies up to 8 MeV

The three neutron groups, populating the ground state, the 2.313 and the 3.945 MeV states of 14N were studied. More than 30 resonances were seen in the 0° excitation curve. Angular distributions were taken at 58 α-energies, between 3.7 and 7.9 MeV. Neutron production cross sections, derived from these distributions, were compared by the method of detailed balance to the previously known 14N(n, α) 11B cross section. Compound nucleus formation involving overlap of several resonances appears dominant for the emission of all three neutron groups.

Fast forward neutron production in the 7Li(p,n) 7Be reaction for 41- and 64-MeV protons

Measurements have been made by the time-of-flight technique of the neutron energy spectra observed at zero degrees from the 7Li(p,n) 7Be reaction using 41- and 64-MeV incident protons. The neutron production cross sections for the fast forward peak (corresponding to excitation of the ground and first excited states of 7Be) has been found to be 33±6 mb/sr at a mean energy of 39 MeV and 36±4 mb/sr at 60 MeV. These results are compared with those reported elsewhere, and the suitability of such 7Li targets as a source of monoenergetic neutrons is discussed.

Total Neutron Yields from the Proton Bombardment ofO17,18

The $^{17}\mathrm{O}(p,n)^{17}\mathrm{F}$ reaction has been observed for the first time in the energy region from threshold to 5 MeV. Total neutron-production cross sections measured with good resolution are given. The reaction threshold is determined to be 3743 \ifmmode\pm\else\textpm\fi{} 6 keV. High-resolution total neutron cross sections are also given for the reaction $^{18}\mathrm{O}(p,n)^{18}\mathrm{F}$ from threshold to 4 MeV.

2H + reactions from 1.96 to 6.20 MeV

The differential cross sections of 2H(d, d) 2H, 2H(d, 3He)n, 2H(d, p) 3H and 2H(d, t)p have been measured in a gas scattering chamber at ten bombarding energies between 2.0 and 6.2 MeV with accuracies ranging from ±1.7 % to ±3.4 %. The differential cross section of 2H(d, n) 3He has been measured at forward angles to an accuracy of ±2.5 % at the same energies using a time-of-flight detection system. The neutron detection efficiency was calibrated from the 2H(d, 3He)n cross sections, so the neutron production cross sections reported here do not depend on any previous neutron measurements. The 2H(d, 3He)n cross sections have been converted into 2H(d, n) 3He cross sections and Legendre polynomial fits are given for the complete angular distributions of 2H(d, n) 3He.

The 7Li(α,n) 10B differential cross section for α-energies of up to 8 MeV

Three neutron groups, populating the 1.74 MeV, the 0.72 MeV and the ground state of 10B were studied. The 0° differential cross section was determined in 0.1 MeV intervals for E α = 4.8 to 7.9 MeV. Angular distributions for the n 0 group, populating the ground state of 10B, were taken at fifteen α-energies and for the n 1 group at five energies. Neutron production cross sections derived from these distributions were compared to the well-known 10B(n, α 0) cross section, using the reciprocity theorem, and to the total yield as published by other authors. Except near the 7Li(α, n) threshold where the angular distribution is isotropic, all distributions show asymmetry about 90° and generally peak backwards. Compound-nucleus formation involving overlap of several resonances appears dominant for the emission of the ground state neutron group, whereas for the n 1 group certain evidence of the interference between compound-nucleus formation and direct reaction is seen.

The rate of energy loss and neutron yield for protons on thick lithium targets at energies from 3–10 mev

A continuous-flow manganese bath has been used to measure the neutron yield per unit current from a thick lithium target under proton bombardment, over a proton energy range from 3–10 MeV and to an accuracy of 2–3 per cent. The results at low energy have been analysed to provide information on the rate of energy loss of protons in lithium, dE dx , while those at high energy have been used to estimate the cross-section for neutron production by multi-body break-up of lithium-7.

Production of medium-energy neutrons from proton bombardment of light elements

Cross sections for neutron production from targets of 2H, 6Li, 7Li, 9Be, 12C and 27Al have been measured at 0° for incident proton energies of approximately 30, 40 and 50 MeV. For production of a monoenergetic neutron beam in this energy region 7Li is the best practical target. 12C has a low neutron production cross section and is particularly useful for proton beam collimation.

The [formula omitted] cross section between 1.5 and 7.8 MeV

Three neutron groups, populating the 7.65 MeV, 4.44 MeV and the ground state of 12C were separated by proton recoil detection in an organic scintillator. The 0° differential cross sections were measured in 0.1 MeV intervals between 1.5 and 7.8 MeV and angular distributions were determined at eight α-energies. For the ground state neutron group, angular distributions at eight additional energies were taken. Neutron production cross sections from the reaction were derived using these distributions and those of previous authors. Most of the angular distributions measured are asymmetric about 90°, showing various degrees of fore- and aft-peaking. It appears that compound nucleus formation in 13C, involving several overlapping resonances, plays a dominant role in some cases, in others there is evidence for direct reaction processes.

Structure in the giant resonance of 108Pd and of 110Pd

The 108Pd photoneutron cross section has been measured from threshold to 24.5 MeV by direct neutron detection. The 108Pd(γ, p) and 110Pd(γ, n)109mPd cross sections have been measured from threshold to 29 MeV by counting residual γ-activity.The dynamic collective model predictions of Huber for 108Pd and 110Pd are in reasonable agreement with these cross sections in the giant resonance region, but a large amount of observed strength at energies less than 14 MeV and greater than 20 MeV is not accounted for by the model.The relative strengths of the proton and neutron production cross sections in 108Pd are shown to be consistent with Morinaga's concept of an isospin-split giant resonance. Similar agreement is demonstrated for 103Rh, 90Zr and 65Cu. However, the relative strengths in 58Ni are found to be inconsistent with this concept.

The Neutron-Production Cross Section of 238Pu in a Fast Spectrum

The production cross sections where α = σc/σf for 238Pu and 239Pu, have been compared in the fast neutron flux at the center of a bare spherical critical assembly of 239Pu. These quantities averaged by the fast-neutron spectrum indicate nearly the same properties for the even-even 238Pu nucleus as for the odd-even 239Pu nucleus. The ratio measured in a neutron flux peaked at 0.25 MeV with an average neutron energy Ē = 1.67 MeV is (238Pu)/(239Pu) = 1.01 ± 0.06, and (238Pu) = 3.76 ± 0.23 b. The results of this study indicate that 238Pu metal probably has a critical mass of the same order of magnitude as 239Pu metal.

Neutrons emitted at 0° from nuclei bombarded by 143 MeV protons

A neutron time-of-flight spectrometer has been used to measure the energy spectra of neutrons emitted in the forward direction when various nuclei are bombarded by 143 MeV protons. The elements studied are deuterium, lithium, beryllium, carbon, aluminium, copper, lead and uranium, the relative neutron yields being compared with cross-section values deduced from a simple classical calculation. A value is derived for the forward neutron production cross-section for inelastic p-d scattering which is in disagreement with the prediction of an impulse approximation calculation. Finally the neutron spectra from lithium and beryllium are discussed in terms of nuclear shell structure.

Neutron Emission from Compound Nuclear Systems of High Angular Momentum

Measurements were made of the energy spectra and angular distributions of neutrons emitted with laboratory energies in the range from 1 to 10 Mev in reactions induced by 160-Mev oxygen-16 ions incident on thin targets of aluminum, nickel, copper, and gold. Spectra were obtained (at laboratory angles between 30\ifmmode^\circ\else\textdegree\fi{} and 150\ifmmode^\circ\else\textdegree\fi{}) by analysis of the proton-recoil pulse-height spectra produced by the neutrons in a stilbene scintillation crystal. All spectra and angular distributions at these angles are consistent with the hypothesis of evaporation from compound nuclei. The energy spectra of neutrons emitted from the initial compound nuclei were calculated by use of statistical evaporation theory, both with and without modification to include (in a classical approximation) the effects of conservation of angular momentum. The dominant effect of conservation of angular momentum upon the calculated neutron spectra from the initial compound nuclei is a lowering of the nuclear emission temperature for the cases and parameters chosen. The measured spectra are compared to the spectra calculated both with and without the modification. The angular distributions are symmetric about 90\ifmmode^\circ\else\textdegree\fi{} and peaked forward and backward. The total cross section for neutron production increases with increasing mass of the target atom, and is roughly 70% greater for copper than for nickel. This sudden increase is probably related to the greater neutron excess of copper.

Energy distribution of neutrons produced by a thermonuclear reaction

The energy distribution of neutrons in a thermonuclear reaction is calculated. It is assumed that the reacting mixture of gases is in thermodynamic equilibrium and that the differential cross section for neutron production is isotropic in the center of mass system.

A Versatile, High Efficiency 4π Neutron Detector

A high efficiency 4π neutron detector has been constructed which consists of 12 B10 F3 proportional counters imbedded in a two-foot cube of paraffin. The response of this detector as a function of neutron energy has been measured in the range 0.1 to 2.5 MeV by observing the yield curves for the T(p,n) and Li7(p,n) reactions, the absolute total neutron production cross sections for which have been accurately measured by Macklin and Gibbons. These measurements indicate a detection efficiency of approximately 10 per cent and a response which is flat to within ± 5 per cent over the entire energy range studied. The efficiency for the detection of neutrons from a Ra-α-Be source (average neutron energy approximately 5 MeV) was found to be 6.5 per cent. The efficiency was observed to decrease for neutron energies below about 0.1 MeV by measuring the yield from the V51(p,n) reaction. The efficiency for the detection of 50-keV neutrons is approximately 4 per cent. The detector has also been used to detect neutron thresholds for nuclear ground states and excited states by observing the slow threshold neutrons in the forward direction with a Li6I scintillator surrounded by a B4C shield. This "closed geometry" technique appears capable of only slightly less sensitivity than the “open geometry” method and is somewhat superior in energy resolution.

Fast photoneutrons from tantalum and gold

The photonuclear cross sections for the production of neutrons with energies greater than the Si 28(n, p)Al 28 threshold have been determined for tantalum and gold from a maximum bremsstrahlung energy of 18 to 65 MeV. The integrated cross section to 65 MeV is about 800 MeV · mb for tantalum and 700 MeV · mb for gold. The fast neutron integrated cross section is compared with total neutron production cross section at several energies and is about 10% at all energies.

Splitting of the Giant Resonance for Deformed Nuclei

Photoneutron yield measurements have been made for terbium, tantalum, and gold with good energy resolution from threshold up to 25 Mev. Neutron-production cross sections were obtained directly from the experimental points without smoothing the raw data. Corrections were made for the multiple production of neutrons above the ($\ensuremath{\gamma}, 2n$) thresholds. The giant resonances for terbium and tantalum were found to be split into two resonances as predicted by Okamoto and Danos. The giant resonances for all three nuclei were fitted by the superposition of two Lorentz shape resonance lines. The intrinsic quadrupole moments determined from these fits to the experimental data were: terbium, +5.6\ifmmode\pm\else\textpm\fi{}0.6 barns; tantalum, +5.7\ifmmode\pm\else\textpm\fi{}0.3 barns; and gold, +1.6\ifmmode\pm\else\textpm\fi{}0.6 barns.

T(d, n)He4Reaction

The differential cross sections for neutron production in the $\mathrm{T}(d, n){\mathrm{He}}^{4}$ reaction have been measured over the deuteron bombarding-energy range 0.25 to 7.0 Mev. Legendre polynomial fits to the data are presented and compared with existing data. The total cross section plotted as a function of deuteron energy has an anomalous behavior which may indicate a broad level in ${\mathrm{He}}^{5}$ near 20-Mev excitation energy.

CIII. The neutrons and alpha-particles from the disintegration of9Beby6mevgamma-rays

Abstract By measuring the total photo-neutron yield, the cross section for neutron production from 9Be under irradiation with the 19F(p, ξ, γ) radiation is found to be σ t =(12·5 ± 1·2) × 10-28 cm2. The energy spectrum of the two alpha-particles, which are emitted in coincidence in the breakup, has also been studied ; the cross section for all beryllium photo-processes which lead to the emission of two alpha-particles, both of whose energies exceeds 400 kev, is found to be σα=(12·8±2·5) × 10-28 cm2. The near equality of σ t and σα shows that the disintegration rarely proceeds through the 8Be ground state, while a detailed examination of the coincident alpha-particle spectrum suggests that (γ, n) and (γ, α) reactions are present. The (γ, n) transitions are mainly through the 2·9mev excited state of 8Be and the (γ, α) transitions mainly through the 5He ground state, the branching ratio being roughly 1·2 : 1.

FAST NEUTRONS FROM THE T + D AND T + Li REACTIONS

The fast neutrons produced by the triton bombardment of LiF, Li2CO3, LiOH∙H2O, LiOD, and LiOD∙D2O + D2O have been detected, and the thick target yields and their slowing down lengths in graphite measured. The tritons were produced by irradiating the compounds in the high thermal neutron flux of the thermal column of the NRX pile. The spatial distributions of the fast neutrons from the T + D and T + Li reactions were measured in the graphite with indium resonance detectors. The numbers of fast neutrons emitted per second were obtained by comparing the space integrals of the indium activity with that measured when a standard Ra-α-Be source of neutrons was in the position of the lithium compounds. The number of incident tritons was estimated from the measured flux of thermal neutrons at the lithium compound and the total capture cross section of the lithium. The thick target yields were thereby derived. The average cross sections for neutron production by 2.6 Mev. tritons are 1.5 × 10−24cm.2 per atom of lithium and 0.41 × 10−24cm.2 per atom of deuterium. The slowing down lengths in graphite of density 1.64 gm. per cc. are 19.2 and 23.4 cm. for the neutrons from the lithium and deuterium respectively. The smallness of the latter indicates that inelastic scattering of these 14 Mev. neutrons occurs in graphite.