J.R. Beyster

MEASUREMENTS OF NEUTRON SPECTRA IN WATER, POLYETHYLENE, AND ZIRCONIUM HYDRIDE

An experimental arrangement designed for accurate measurements of low- energy neutron spectra was assembled and tested. A pulsed high-current electron linear accelerator was used to produce short bursts of fast neutrons that were introduced into a moderating and absorbing assembly. The steady-state energy spectrum of neutrons in the assembly was determined by pulsed-beam time-offlight techniques. Hydrogen-moderated systems poisoned with a number of common neutron absorbers (boron, cadmium, samarium) were studied, and the resulting spectra were compared with theoretical predictions using both free and bound hydrogen scattering kernels. In general, a marked difference existed between measured spectra and spectra calculated using a free hydrogen kennel. In the case of water where a detailed scattering kemel is available for room temperature, theory and experiment are in reasonable agreement. (auth)

MEASUREMENT OF NEUTRON DIFFUSION PARAMETERS IN WATER BY THE PULSED NEUTRON METHOD

Neutron diffusion parameters in water were measured at 26.7 deg C with the pulsed neutron technique. The results are 210 plus or minus 1 mu sec for the neutron mean lifetime, 37503 plus or minus 366 cm/sup 2/ sec/sup -1/ for the average diffusion coefflcient, and 5116 plus or minus 776 cm/sup 4/ sec/sup - 1/ for the diffusion cooling constant. From these values the thermal absorption cross sectlon of hydrogen and the thermal diffusion length in water can be lnferred to be 325 plus or minus 2 mb and 2.83 plus or minus 0.02 cm, respectively. With a pulsed high-intensity neutron source provided by an electron linear accelerator, neutron lifetime measurements were performed on small and large water samples wlth values of the geometrlcal buckling from 0.014 cm/sup -2/ to 0.59 cm/sup -2/. Effects of harmonic modes in the large water geometrles, whlch were determined by measurements of the time-dependent spatial flux distributlons resulting from an external pulsed source of fast neutrons, were found to be adequately predictable with slmple diffuslon theory. (auth)

Neutron-Spectrum Measurements in H 2 O, CH 2 and C 6 H 6

AbstractNeutron-spectrum measurements in several moderators have been made with geometrical conditions approximating an infinite medium. The moderators studied were water, polyethylene and benzene, and the absorbers dispersed in the moderators were boron, samarium, erbium and gadolinium. The measurements are compared with predictions of the neutron spectra utilizing scattering models which take into consideration chemical binding. Measurements of scalar and angular neutron-energy spectra were made in a slab geometry water-moderated multiplying assembly at room temperature. The measured spectra are compared with DSN transport-theory calculations utilizing the Nelkin bound-hydrogen scattering model for water.

Neutron Thermalization in Zirconium Hydride

Molecular models used to explain neutron thermalization in zirconium hydride are reviewed, and results obtained with the models are compared with recent experimental results. The Einstein oscillator model and the phenomenological description of Miller each explain some of the data, but they predict the temperature dependence of the neutron spectra quite differently. This dependence for zirconium hydride poisoned with boron was measured by time-of- flight means. The spectrum was found to shift markedly with temperature, almost as predicted by the Einstein models. (D.C.W.)

The General Atomic Coupled Electron Linear Accelerator

The General Atomic Linear Accelerator Facility consists of two L-band linear accelerators which can be operated independently or coupled together to form one machine. The first accelerator is a single section powered by a 20-MW klystron and is capable of operating over an energy range of one MeV to 17 MeV. Peak currents of 16 A at 12 MeV and 40 ns pulse length have been obtained with the first Linac. The second Linac, which is built in line with the first machine, consists of three sections powered by one 10-MW and two 20-MW klystrons. The second Linac is capable of producing a 35-MeV electron beam with an intensity of 2 A at pulse length less than 0.1 ?s. By rotating the injector of the second Linac it accepts the beam from the first machine, thereby creating a higher energy machine capable of delivering energies of up to 80 MeV. Other features of the facility include six independent irradiated cells, 15 target positions, and a high power positron converter; as well as monoenergetic gamma ray sources, high intensity neutron targets, and a single particle per pulse capability. In addition, a special cell contains the APFA-III accelerator-pulsed fast burst reactor, which is operated at a multiplication of 1000 to obtain neutron fluences of 1012 - 1013 n/cm2 in 5 ?s.

Accelerator Pulsed Fast Assembly

An accelerator pulsed fast reactor has been constructed and placed in operation at General Atomic. A 20 megawatt L-band electron accelerator drives a super-critical U-235 metal reactor. This device has the unique characteristic that peak power and pulse width can be varied independently by adjusting various accelerator and reactor parameters. Pulse widths range from less than 0.1 ?sec to 5 ?sec and the fast fluxes attainable are in excess of 5 ? 1015 n/cm2/sec at the surface and about an order of magnitude higher internally. Further improvements are planned to increase these yields by an order of magnitude. In addition to single pulse operation, these devices can be repetitively pulsed up to several pulses per second to provide high intensity fast or thermal fluxes if some means of cooling the fuel is provided.

NEUTRON SPECTRA IN GRAPHITE POISONED WITH SAMARIUM

ABS>From American Nuclear Society Meeting, New York, Nov. 1963. Some previous experimental and theoretical investigations of neutron scattering from graphite are reviewed, and some spectra obtained with poisoned graphite are presented and discussed. The spectra were measured in a Sa-poisoned graphite cube at 80, 300, and 600 deg K and in an assembly representing the core-reflector interface in a heavily poisoned graphite reactor. The spectra were calculated in adequate agreement with experiment. (D.C.W.)