G.P. Couchell

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.

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.

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.

Beta particle spectrometer for measuring aggregate beta spectra following fission

Abstract The beta spectrometer consists of a gated plastic scintillator suitable for measuring aggregate beta energy spectra following fission. This general utility spectrometer is very insensitive to the gamma rays accompanying the fission products, has good linearity, and adequate energy resolution for these continuous beta energy distributions. Response functions are measured below 1 MeV and trial sets of response functions are tested with known beta spectra extending to 5 MeV.

Relative efficiency measurements of fission fragment transfer with a helium jet

Abstract The helium jet system was designed to transfer different mass fission fragments with nearly equal efficiency. Relative efficiency measurements made with an X-ray detector comparing helium-jet transferred activity to “rabbit” -shuttle transferred activity verify that the transfer efficiency is essentially a constant except for isotopes of noble gases (krypton and xenon) which do not “stick” to the catcher at the exit of the He-jet.

Search for energy dependence among composite delayed neutron spectra of U-235

Abstract A sensitive technique is described to search for a dependence of delayed neutron spectra on fast vs. thermal fission of U-235. The results of such a search are given.

Data reduction and analysis in composite delayed-neutron time-of-flight studies

Abstract A method is described for converting a continuous neutron time-of-flight (TOF) spectrum into an energy spectrum by representing the TOF spectrum as a linear combination of spectrometer time-response functions.

Nuclear Spectroscopy Utilizing an (Alpha, Nucleon) Resonance Reaction

Resonance reactions offer that vital element of simplicity required in nuclear spectroscopy, especially when they involve entrance and exit channels of channel spin 0 or 1/2, since then the formation and decay of any one of several possible intermediate states Jπ in a compound nucleus can take place only via a unique pair of incoming and outgoing partial waves 1, 1′ associated with angular distributions of highly individualistic shape and magnitude. The potentialities have already been intensively, and fruitfully, exploited [1] in studies of the 13C(α,no)16O reaction to arrive at spin and parity assignments to high-lying states of 17O occupying the region of level overlap where coherent interference between a pair of levels of unlike parity manifests itself in disruption of the 90° symmetry of the ensuing angular distribution.