J.E. Monahan

The angular distributions of neutrons scattered from various nuclei

Abstract The angular distributions of neutrons scattered from Li6(96%), Li(normal), Be, C, Si, Ca, Cu, Sn, Pb, and U238 have been measured for incident energies in the range from 50 kev to 2300 kev. In these measurements the energy spread of the incident neutron beam was sufficiently large to average out most of the pronounced fluctuations due to individual scattering resonances. In addition, a series of angular distributions of neutrons scattered at resonances were measured for the zero-spin nuclei C12, O16, Si, and S32 as well as for the isotopes Li6 and Li7. The results are presented in terms of the least-square values of the coefficients in a Legendre-polynomial expansion of the differential scattering cross section relative to the laboratory system of coordinates. The various corrections which have been made in order that these data represent a microscopic cross section are discussed.

Level scheme of Au198 determined by analysis of high-precision capture gamma-ray measurements☆

Abstract The gamma-ray spectrum resulting from the capture of thermal neutrons by Au 197 has been investigated by use of the Argonne 7.7-meter bent-crystal spectrometer. A total of 122 lines corresponding to transition energies less than 835 kev were observed. Their energies were determined with an average precision of 1 part in 5000. The method of generating a “most probable” level scheme for Au 198 from these measurements is described. A scheme containing 25 states is obtained which shows an unusual “fine structure” grouping of several levels.

NEUTRON CROSS SECTIONS OF THE BORON ISOTOPES FOR ENERGIES BETWEEN 10 AND 500 kev

Abstract The total neutron cross sections of 10 B, 11 B, C and O and the absorption cross section of 10 B have been measured for neutron energies between 10 and 500 keV. The 10 B absorption cross section does bot deviate appreciably from a 1/ν dependence in this interval of neutron energies. The present values of the 10 B absorption cross section are systematically larger than measured values of the 10 B(n, α) cross section; but the evidence presented indicates that this excess cannot be explained in terms of other neutron-absorbing reactions.

Perturbation of the statistical properties of nuclear states and transitions by interactions that are odd under time reversal

Abstract It is found that a rather small time-reversal-odd part in the nuclear Hamiltonian would produce an appreciable perturbation in the Porter-Thomas distribution of widths and also in the distribution of the spacings between energy levels. A comparison between the experimentally determined variances for some published neutron widths and partial radiation widths and the calculated variance allows one to place an upper limit on the magnitude of the time-reversal-odd part. The shift of the spacing distribution is particularly large on a percentage basis in the region of small spacings. As a measure of the time-reversal-odd part of the wave function, the overlap |( Ψ , TΨ )| has been evaluated. All conclusions are based on a Monte-Carlo calculation with random Hermitian matrices having a small imaginary part.

Possible doorway-state structure in the scattering of neutrons by Fe

Abstract An analysis of 150 keV wide structure observed in the scattering of neutrons by Fe at energies between 0.35 and 0.96 MeV is based on the doorway-state interpretation of such structure. The methods of analysis are presented in some detail. The distribution of compound-nucleus widths and spacings as well as the measured differential cross sections and polarizations are shown to be reasonably consistent with predictions that result from the doorway-state hypothesis.

Extrapolation of low-energy reaction cross sections☆

Abstract Many applications in astrophysical and controlled-fusion research require values of nuclear-reaction cross sections at interaction energies considerably below the range in which absolute measurements are practical. Measured cross sections thus must be extrapolated to the energies of interest. For exoergic charged-particle reactions, which are not dominated by resonances within the range of interest, it is shown that a physically meaningful extrapolation formula can be derived from the R -matrix theory. For energies such that only incident s-waves contribute to the reaction, the resultant formula contains at most three “free” parameters. These results are used to extrapolate cross sections for various d+ 6 Li reactions, measured at energies above 100 keV, to energies in the range of 1 keV.

Neutron scattering from nuclei near A = 20 at energies below 2.2 MeV

Abstract The polarization and differential cross sections for neutrons scattered from F, Na, Al and P have been measured at five scattering angles, at energies from 0.2 to 2.2 MeV, with incident-energy spreads of 100 to 150 keV. Structure, characterized by peaks that are separated by energy intervals of 300 to 400 keV and have widths from 100 to 200 keV, is seen in the total and differential cross sections and in the polarization for these nuclei. Calculations based on a nuclear optical model are in reasonable agreement with an average through the measured total and differential cross sections, but give somewhat poorer fits to an average through the measured polarizations. The observed fluctuations cannot be reproduced by any calculations based on an optical-model potential with fixed or slowly varying parameter values. The possible interpretations of this structure are discussed.

Nuclear Cross Sections for Light Ions on 6Li

We review our measurements of total and differential cross sections for protons, deuterons and 3He incident on 6Li. These are the reactions which are important for evaluating a fusion reactor based on burning 6Li. Several of these reactions had not previously been measured, and among those that had there were large discrepancies in some cases. We employed several checks in our measurements to verify that the errors (including systematic errors) were 10 to 20 percent.

Possible resonance structure in the elastic scattering of neutrons by Y near En = 1 MeV

Abstract The total cross section as well as the differential cross section and polarization in the elastic scattering of 0.8–1.4 MeV neutrons by Y have been measured with neutron beams of energy spread less than 20 keV. Rather weak structure with widths ≈ 50 keV was observed at a few energies within this range. The data were analyzed by use of a model in which the scattering process is described in terms of resonance amplitudes superimposed on an optical-potential background. Although not completely definitive, this analysis indicates the existence of three intermediate-width resonances (two 1− and one 1+) at neutron energies between ≈ 1.0 and 1.2 MeV. The properties of the 1− resonances suggest that these are the parent states of the proposed T> components of the El giant resonance observed near 21 MeV excitation energy in 90Zr produced in the 89Y(p,γ0) reaction. The resolved resonance structure in this energy region is in reasonable agreement with a recent calculation of the energies and widths of negative-parity states in 90Y.

Criteria for the design of experiments to measure differential cross sections and other polynomial distributions

Abstract The criteria presented in this paper enable an experimenter to select a set of positions at which a distribution should be measured in order that the resulting set of measurements should determine the distribution most efficiently, i.e., with minimum uncertainty for a given expenditure of time, effort, or money. The results are applicable to the measurement of any distribution that can be represented by a finite-degree polynomial in the independent variable or in some function of the independent variable. Thus, for example, the criteria can be used to select a set of angles at which measurements of the particle flux would most efficiently determine the corresponding differential cross section or polarization distribution. In particular, we determine the positions at which a polynomial distribution should be sampled in order to minimize a given linear combination of the sample variances in the resultant estimated values of the polynomial coefficients. It is shown that an approximate minimum of this quantity can be obtained even when the only a priori information about the distribution to be measured is that it is a polynomial of finite degree. Criteria are given also for the selection of measurements that provide “most sensitive” tests of hypotheses about the degree of the distribution. The analysis makes possible the use of a computer to program (and control) suitable experiments in order to attain a desired objective most efficiently.