MEASUREMENT OF THE TEMPERATURE DEPENDENCE OF NEUTRON DIFFUSION PROPERTIES IN BERYLLIUM USING A PULSED NEUTRON TECHNIQUE (thesis)

Technical Report · Mon Aug 01 04:00:00 EDT 1960

OSTI ID:4124630

Andrews, W M

The pulsed-neutron technique was used to measure the temperature dependence of neutron-diffusion properties in beryllium. Essentially the pulsed technique consists in (1) introducing a short pulse of neutrons into a volume of the material under study, (2) waiting long enough for the neutrons to thermalize and reach a normal mode distribution, and (3) measuring the decay rate of the nornal-mode density distribution. The decay constant is a function of the neutron absorption in the material and leakage from the volume. The leakage is modified by the diffusion-cooling effect; this effect is due to a preferential leakage of higher energy neutrons which leaves the equilibrium distribution "cooled" below that which would exist in an infinite medium. Experimentally. the decay constant is measured for a series of buckling values and then the decay constant vs. buckling data are fitted to a quadratic expression in terms of the buckling. The values of the absorption cross section, the diffusion coefficient, and the diffusion cooling coefficient are obtained from this. In the case of temperature-dependent measurements, the decay constant is measured for each buckling value at a series of specified temperatures and the decay constant vs. buckling data are analyzed at each temperature. Thus the values of the neutron- diffusion parameters are obtained at each of the temperatures investigated. Both Van de Graaff and Cockcroft-Walton accelerators were used as sources of pulsed neutrons. The time analyzer was a 256channel pulse-height analyzer converted to a time-base analyzer. Two Li/sup 6/I detectors were used to measure the decay of the neutron density. The high-temperature facility was a closed-cycle forced-hot- air system. The temperature range investigated was 292 to 478 deg K. The value of the absorption cross section of the high-purity commercialgrade beryllium used was found to be about 10 mb. At room temperature the value of the diffusion coefficient was found to be 0.50 cm. With increasing temperature the value of the diffusion coefficient decreased about 6%. The value of the diffusion-cooling coefficient was found to be 2.3 cm/sup 2/ at room temperature, decreasing to a value of 1.4 cm/sup 2/ with an increase in temperature. A study of the time necessary after a pulse for the normal mode to be established is discussed. It was found that this time is independent of the relative source-detector position for an externally located source. (auth)

• Research Organization: California. Univ., Livermore. Lawrence Radiation Lab.
• DOE Contract Number: W-7405-ENG-48
• NSA Number: NSA-15-000831
• OSTI ID: 4124630
• Report Number(s): UCRL-6083
• Country of Publication: United States
• Language: English

Related Subjects

ABSORPTION
ACCELERATORS
BERYLLIUM
BUCKLING
COCKCROFT-WALTON ACCELERATORS
COOLING
CROSS SECTIONS
DECAY
DENSITY
DIFFUSION
LITHIUM IODIDES
MEASURED VALUES
NEUTRONS
PHYSICS
PULSES
SCINTILLATION COUNTERS
TEMPERATURE
THERMAL NEUTRONS
VAN DE GRAAFF ACCELERATORS