Fast Neutron Cross Sections Research Articles

The Production of N16 and N17 in the Cooling Water of the NRX Reactor

Measurements have been made of the fast neutron cross sections for the production of N16 and N17 in the uranium rod cooling water of the NRX reactor by the reactions O16(np)N16 and O17(np)N17. The N16 and N17 were detected by their gamma and fast neutron activities, respectively, in a continuously flowing sample of the effluent water from a uranium rod. The cross sections (fission neutron spectrum) for the reactions O16(np)N16 and O17(np)N17 are 1.85 ± 0.15×10−2 millibarns and 9.3 ± 0.9×10−3 millibarns, respectively.

Total cross sections for fast neutrons by a time of flight method

A time of flight method for the determination of total cross sections for fast neutrons has been realized by detecting neutrons twice along their path by stilbene scintillation counters. The time interval between the pulses of the two counters is measured by a delayed coincidence circuit and determines the energy of the neutrons between the counters. The experimental arrangement and its properties with respect to efficiency, energy resolution and background are discussed. The time resolution was limited by the low efficiency of the coincidence circuit for resolving times smaller than 4 nanoseconds. The method has been tested by measuring with an average energy resolution of 20% the total cross section of sulphur between 400 and 1200 keV. The results are compared to earlier measurements and to the optical model. Further possibilities of the method are discussed.

Correction de diffusion lors des mesures de sections efficaces totales pour neutrons rapides

A general formula is given for the calculation of the inscattering correction which is to be applied to the total cross section for fast neutrons, as measured by transmission experiments. It is shown that inelastic scattering and (n, 2n) reactions are negligible for this evaluation: only elastic scattering is to be taken into account. This influence can be estimated by comparison with some experimental results or by application of the diffraction theory of neutron scattering using, as nuclear radius, R = r 0A 1 3 where r 0 = 1.25 × 10 −13 cm; the maximum error involved in this evaluation is ± 20%.

The characteristic level and the nuclear level density

The suggestions put forward by Bethe and Hurwitz that level densities ought to be measured from a standard energy, such as is given by the semi-empirical formula, and not from nuclear ground states is considered. The semi-empirical formula is corrected, following P. Fong, and this “characteristic level” is determined for nuclei from A = 10 to A = 250. The experimental data of D. Hughes et al. for the fast neutron cross sections are used to determine the level densities for the corresponding nuclei. These were found to be in agreement with the Fermi-gas model.

Fast-Neutron Cross Sections of Ge, As, Se, and Br

The total cross sections of Ge and Se were measured for neutron energies between 60 and 650 kev, and those of As and Br in the energy range from 60 to 3000 kev. A comparison with calculated values is made.

Nonelastic Scattering Cross Sections for Fast Neutrons

Nonelastic Scattering Cross Sections for Fast Neutrons

Measurements of Inelastic Scattering Cross Sections for Fast Neutrons

Measurements of Inelastic Scattering Cross Sections for Fast Neutrons

26. A time of flight method for the measurement of total cross-sections for fast neutrons

26. A time of flight method for the measurement of total cross-sections for fast neutrons

Nonelastic Scattering Cross Sections for Fast Neutrons

Measurements have been made of the nonelastic cross sections of neutrons in Be, C, Al, Ti, Cr, Fe, Ni, Cu, Ag, Sn, Pb, and Bi. These cross sections include all processes except elastic scattering. The experiments have been carried out with monoenergetic neutrons of 3.5, 4.7, 7.1, 12.7, and 14.1 Mev. The neutrons were produced with a Van de Graaff accelerator using the nuclear reactions: ${\mathrm{T}}^{3}(p, n){\mathrm{He}}^{3}$, $\mathrm{D}(d, n){\mathrm{He}}^{3}$, and ${\mathrm{T}}^{3}(d, n){\mathrm{He}}^{4}$. Cross sections were determined from transmissions through spherical shells which were 3.8 cm in radius and 2 cm thick. A new type of neutron detector which gives energy information about neutrons and is insensitive to $\ensuremath{\gamma}$ radiation was placed at the center of the spherical shell and was used as a biased detector of neutrons. The results of these experiments show that the nonelastic neutron cross section for 12.7- and 14.1-Mev neutrons is nearly geometrical and is closely approximated by $\ensuremath{\sigma}=\ensuremath{\pi}{(R+\ensuremath{\lambda})}^{2}$, where $R=1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{A}^{\frac{1}{3}}$. The nonelastic cross sections are roughly constant in the energy range 5 to 14 Mev; below about 5 Mev the cross sections begin to decrease.The elastic cross sections can be obtained by subtracting the nonelastic cross sections from the total cross sections. The variations with energy of the nonelastic and elastic cross sections are quite different.Data with 12.7-Mev neutrons and low counter bias, give the relative number of $\ensuremath{\gamma}$ rays given off per inelastic collision of these neutrons. The number of $\ensuremath{\gamma}$ rays decreases rapidly with increasing atomic number and there are less than 5% as many $\ensuremath{\gamma}$ rays from Bi as from Al, Ti, Cr, and Fe.

Pile Production of Radioisotopes by Threshold Reactions

A GREAT advantage of production of radioisotopes by threshold reactions (n, p) and (n, α), relative to neutron capture (n, γ), is that the radioisotope can be separated chemically from the target material, and hence be obtained in high specific activity, in principle completely carrier-free. The rate of threshold reactions in the pile, however, is much smaller in general than that of capture reactions, not only because of the low cross-sections for neutrons in the MeV. range but also because of the low flux of neutrons of that energy in the pile. As measurement of the small yields of such reactions is often difficult, especially for long-lived radioisotopes, it is very useful to be able to predict approximate yields in advance. An example is furnished by the reaction 54Fe(n, p)54Mn, recently discussed by Stafford and Stein1; in this case some experiments had indicated production of the 324-day manganese-54 by pile neutrons, whereas others showed none. The most recent experiments1, however, utilizing samples enriched in iron-54, definitely established the reaction with a cross-section for fast neutrons of 11 millibarns (10−27 cm.2). It is the purpose of the present communication to point out that the approximate yields for threshold reactions in the pile can be predicted, and to illustrate the method for 54Fe(n, p)54Mn as an example.

Validity of the inverse velocity law for the fission cross section of 235U

Synopsis Cadmium ratio measurements were used in order to check the validity of the 1/ v law for fission of 235 U in the resonance region. The fast fission effect was eliminated by measuring the transmission of epicadmium neutrons through nickel, which has different cross sections for resonance and fast neutrons. It was found that the 1/ v law does not hold quite good in the resonance region, the resonance integral being ∼ 20 barns.

Fast Neutron Cross Sections and Nuclear Level Density

Fast Neutron Cross Sections and Nuclear Level Density

Methods for Measuring Fast Neutron Cross Sections

Methods for Measuring Fast Neutron Cross Sections

Regularities in the Total Cross Sections for Fast Neutrons

Regularities in the Total Cross Sections for Fast Neutrons

Nuclear Cross Sections for Fast Neutrons and Interaction between High Energy Nucleons

Nuclear Cross Sections for Fast Neutrons and Interaction between High Energy Nucleons

Nuclear Cross Sections for Fast Neutrons and Interaction between High Energy Nucleons

Recent observation of proton-proton scattering shows that the agreement with existing phenomenological theories based on static potential is rather poor, and considerations of velocity dependent forces are required. It will be shown, however, that the only familiar velocity dependent force, spin-orbit coupling introduced by Case and Pais, are not acceptable from the consideration of high energy neutron-nucleus scattering.

Emission of slow neutrons by a radium-beryllium source.

KNOWLEDGE of the emission of slow neutrons by sources is essential wherever effective slow-neutron cross-sections much exceed fast-neutron cross-sections. The investigation of the Szilard–Chalmers effect on manganese and arsenic with fast neutrons is an example1–4. We have now measured the emission of slow neutrons, in three energy channels (resonance bands of dysprosium, indium, iodine), by a ‘natural source’ of the common type. The source consisted of 300 mgm. radium as the chloride, mixed with an excess of beryllium metal (2–3 gm., but exact amount unknown), and was contained in a platinum capsule of 0.025 cm. wall thickness. Total neutron emission was of the order of 3 × 106 sec.−1, that is, equivalent to that of about 200 mC. radon–beryllium5.

Capture Cross Sections for Fast Neutrons

The fast neutron capture cross section of ${\mathrm{Al}}^{27}$ was measured by observing the 2.3-minute activity of ${\mathrm{Al}}^{28}$ for neutron energies between 10 and 550 kev. Resonances were observed which showed some correlation with scattering resonances so that radiation widths of some of the levels could be estimated. Measurements of activation cross sections were also carried out for F, V, ${\mathrm{Cu}}^{65}$, Ag, I, ${\mathrm{In}}^{115}$, and Au.

Capture Cross Sections for Fast Neutrons

The neutron capture cross sections at an effective energy of one Mev have been measured for 32 isotopes, including 4 isomeric pairs. Unmoderated fission neutrons were used as the source, and the high intensities available by this method have made it possible to measure fast cross sections as low as ${10}^{\ensuremath{-}4}$ barn. The cross sections (except those for a few "magic number" nuclei) show a smooth variation with atomic weight, ranging from ${10}^{\ensuremath{-}1}$ b at $A=100$ to ${10}^{\ensuremath{-}4}$ b at $A=20$. The rapid change in cross section is a measure of the change in level spacing of the compound nucleus formed after neutron capture. A survey is made of all the existing data on absorption cross sections as functions of $A$ and neutron energy.

Measurement of Transport and Inelastic Scattering Cross Sections for Fast Neutrons. I. Method

An experimental method for measuring transport and inelastic scattering cross sections for fast neutrons is described. The cross sections are obtained from poor geometry and back scattering measurements by use of detector, the energy response of which rises above an adjustable neutron energy from zero to an almost constant value. A method of evaluation of the data is discussed.