W.A. Schier

Spin and parity assignments for 35Cl levels from the 31P(α, p)34S reaction

Abstract 35 Cl states at excitation energies between 9.9 and 11.8 MeV have been identified through sharp resonances in the 31 P(α, p o ) 34 S excitation functions at 25°, 105° and 155° for E α = 3.25–5.50 MeV. Forty-eight on-resonance angular distributions, normalized to an absolute cross section scale, have been subjected to single-level and two-level analyses resulting in spin and parity assignments for each resonance. Approximately half the resonances were of the pure single-state type, having unique angular distribution shapes. Data from 12 resonances of an earlier experiment 1 ) were analyzed with the same theory, extending the diagnostics down to a 35 Cl excitation energy of 9.1 MeV. A set of optical potentials consistent in all four reactions that this experimental program encompasses has been incorporated in the present analysis. Validity of the optical potential is demonstrated for α-particles elastically scattered by 31 P.

New experimental system for measuring composite delayed-neutron spectra following fission☆

Abstract We describe a new experimental approach for measuring composite delayed-neutron spectra as a function of delay time following fission. Fission fragments are transported from the fission chamber to a low background counting room by helium jet and tape transport systems. A β-neutron time-of-flight spectrometer incorporating both 6Li-glass and Pilot U scintillators is used to measure the spectra in the neutron energy range 10–2000 keV. Delay times can be varied from 0.17 s to several minutes. Fission is induced with either thermal or fast neutrons.

Spin and parity assignments for 23Na levels from the 19F(α, p)22Ne reaction

Abstract Resonances were found in the 120° and 160° excitation functions for the 19 F(α, p o ) 22 Ne reaction between E = 2.5 and 5.0 MeV corresponding to 23 Na levels at excitation energies between 12.56 and 14.51 MeV. Twenty-one on-resonance angular distributions were analysed with single-level and two-level theory to extract 23 Na spin and parity information. The results of an earlier experiment were analysed by the same procedure, extending the diagnostics down to a 23 Na excitation energy of 11.55 MeV. The analysis incorporated an optical potential for the α-particle consistent with previous channel-spin- 1 2 reaction studies and α-induced reaction data.

Programs in C for parameterizing measured 5″ × 5″ NaI gamma response functions and unfolding of continuous gamma spectra

A 5″ × 5″ NaI(Tl) detector has been used to measure gamma-ray spectra resulting from the decay of aggregate fission products. In order to extract the true gamma-ray energy distribution from the measured spectra, the detector response functions for monoenergetic gamma rays spanning the energy range of the measurements must be determined. At present we have measured 13 such response functions in the energy range 0.081–6.13 MeV. NGRC is a program in C written to implement an interpolation scheme for estimating the response function at any other intermediate energy. This program takes a library of response function tails and constructs a response function matrix which is used as input to a second program CRSUP written for obtaining gamma-ray energy distributions. It assumes the measured spectrum consists of a superposition of a specified number of response functions placed at energies determined by the program according to the detector resolution and spectrum end point energy. The program then computes the distribution of the strength of the response functions in a least-squares fashion. This program is designed to maximize the number of response functions that can be used in modeling the measured spectrum without reducing the number of bins used in each response function. The response functions constructed by the interpolation procedure have been used in the program SPEC-FIT to fit in a least-squares fashion the gamma-ray spectrum of 152Eu. The fit is an excellent reproduction of both the photopeak and continuous regions of the entire measured spectrum. Finally the validity of the least-square method implemented by CRSUP has also been tested by using this program to unfold an analytically constructed continuous spectrum. The results obtained were in excellent agreement with the assumed distribution function, illustrating the applicability of CRSUP for unfolding other types of continuous spectra as encountered in beta, neutron-time-of-flight and Rutherford-backscattering spectroscopy.

SIX-GROUP DECOMPOSITION OF COMPOSITE DELAYED NEUTRON SPECTRA FROM 235U FISSION

In this paper, a constrained least-squares method is developed for deducing six-group delayed neutron energy spectra from composite spectra measured at six or more delay time intervals following fission. The constraining condition is chosen to yield stable solutions that also provide good fits to the measured spectra. The method is applied to previously measured composite spectra of {sup 235}U to obtain six-group delayed neutron energy spectra. The solutions are unique for a large range of constraint spectra. The dependence of the solutions on the choice of six-group parameters {beta} {lambda} is also examined.

55Mn(n, n'γ) cross-section studies for En = 1.0–3.6 MeV

Abstract Absolute 55 Mn(n, n′γ) γ-ray production cross sections have been measured for 19 transitions from levels up to and including the 2429 keV state in 55 Mn over the energy range E n = 1.0–3.6 MeV. Angular distributions were also measured for 6 of the transitions. Branching ratios were extracted and total inelastic neutron cross sections were inferred for these 55 Mn excited states. The measured and inferred cross sections are compared with calculated cross sections using the statistical compound nucleus theory.

Search for an energy dependence in /sup 235/U delayed neutron spectra

A sensitive search for a dependence of composite (aggregate) delayed neutron spectra on the energy of neutrons inducing fission of /sup 235/U was performed for eight nearly contiguous delay time intervals between 0.17 to 85.5 s. The experimental arrangement combined a helium jet and tape transfer system with a beta-neutron time-of-flight spectrometer. Thermal and fast neutron measurements were performed successively. Results are compared with the spectra derived from individual precursor data complemented by theoretical estimates of missing spectra.

Composite delayed-neutron spectra from U-235

Abstract Delayed-neutron spectra from thermal neutron-induced fission of U-235 have been measured over the neutron energy range, .01–2.0 MeV, for delay times following fission ranging from 0.17–85.5 s. A helium jet system was used for the rapid transport of fission products to a low-background area, where the spectra were determined from beta-neutron correlations using the neutron time-of-flight method.

Recent Results of Neutron Inelastic Scattering to Higher-Excited Levels in 232Th and 238U

Neutron inelastic scattering cross sections for levels of 232Th and 238U between 680 and 1200 keV excitation energy have been measured in the neutron energy range 0.9–1.5 MeV. Nearly monoenergetic neutrons were generated using the 7Li(p,n) reaction initiated by a pulsed proton beam produced by a Van de Graaff accelerator using terminal pulsing in conjunction with a Mobley bunching system. Scattered neutrons were observed with a time-of-flight spectrometer with experimental parameters chosen to yield an overall energy resolution of 12–15 keV for inelastically scattered neutrons. Level cross sections were deduced from measured 125°-differential scattering cross sections. Excitation functions were compared with our own previous (n,n′γ) measurements, recent theoretical calculations and the ENDF/B-V evaluations.

Stripping features of 6Li(d, n)7Be∗ (0.43 MeV) at Ed from 0.2 to 0.8 MeV

Abstract Angular distributions of neutrons from the 6Li(d, n)7Be∗ (0.43 MeV) reaction were obtained over the angular range 0° to 160° at deuteron energies of 0.24, 0.34, 0.44, 0.54, 0.64, and 0.84 MeV with an associated gamma time-of-flight measurement. All display lp = 1 stripping patterns and are fitted with PWBA theory using a constant r0 = 5.7 fm. The shapes of the 0.24 and 0.34 MeV distributions are fully reproduced with the PWBA distributions. A dispersion-theory interpretation of the break angle between PWBA theory and all available experimental data up to Ed = 3.0 MeV is in good agreement with the break angle calculated for a constant energy denominator of 8.16 MeV.