William R. Mills

CYCLIC ACTIVATION ANALYSIS.

Abstract Cyclic activation analysis is a technique which utilizes very efficiently a low-output, pulsed source of neutrons and short-lived radioactivities for analytical purposes. With this technique radioactivities having half lives of a few minutes to as short as a few milliseconds can readily be used as a basis for elemental analysis. This paper presents a mathematical analysis of cyclic activation, some experimental studies verifying the mathematical predictions, application of the technique to a specific analysis problem, and some suggested areas where cyclic activation could possibly be used to good advantage.

Dual-spaced neutron logging for porosity

Two-group neutron diffusion theory has been applied to two concentric cylindrical regions representing borehole and formation to show that the ratio of thermal neutron flux at two appropriately large distances along the axis is virtually identical to the epithermal flux ratio, and is a measure of a single epithermal parameter of the formation, viz., the slowing down length. The slowing down length is most strongly dependent on hydrogen content. It is shown that variations in salinity of the liquid in the formation or the borehole and uncertainties in borehole size have relatively little effect on the flux ratio. Experiments in a borehole model showed that the thermal flux ratio does not depend on eccentricity of location of the logging tool in the borehole. Fits of theory to experiment allowed determination of slowing down lengths for Pu-Be neutrons in salt water filled sandstone at two porosities for which experimental values have not previously been available. These investigations suggest that an improved porosity log can be obtained by use of 70- and 90-cm source-detector spacings with a neutron source of 5 X 10 8 neutrons/sec or greater. Proportional counters containing He 3 are recommended as detectors. Such a system employing flux ratios is expected to be insensitive to casing and cement in cased holes.

Uranium assay logging using a pulsed 14-Mev neutron source and detection of delayed fission neutrons

An in‐situ uranium assay logging system has been developed that measures directly and quantitatively the uranium concentration in the formation surrounding a drill hole. System operation is based on the DFN (delayed fission neutron) method which involves (1) bombarding a formation with short duration bursts of neutrons from a pulsed‐neutron generator to induced fission in any uranium present; (2) separating delayed fission neutrons, from source and prompt fission neutrons, by waiting a few milliseconds after each neutron burst before activating the neutron counter system; (3) deactivating the neutron counter system before the beginning of the next neutron burst; and (4) repeating this bombard‐wait‐count cycle a sufficient number of times to accumulate a statistically acceptable number of the delayed neutron counts. The DFN logging method has been used routinely in our field operations for several years, and it has successfully overcome one of the most perplexing problems encountered in uranium exploration...

THE EFFECTS OF FLUID INVASION IN PULSED NEUTRON LOGGING

The weak dependence of the results of pulsed neutron logging on the presence of the oil well borehole suggests that thermal neutron die‐away measurements in an open hole may be capable of “seeing” beyond a limited amount of borehole fluid invasion. In order to obtain an estimate for the depth of investigation which can be expected from pulsed neutron logging when some invasion has occurred, multigroup neutron diffusion theory was used to calculate thermal neutron lifetimes for the open‐hole logging geometry. The transient, three‐group equations were solved with the aid of an IBM 7090 computer. Two cases of open‐hole fluid invasion were investigated: Fresh‐water filtrate invading a salt‐water‐saturated formation, and salt‐water filtrate invading a formation saturated with fresh water (or oil). In both cases the formation is assumed to be a 20 percent porosity sandstone and the fluid‐filled borehole is taken to be 20 cm in diameter. It is shown that for any given depth of borehole fluid invasion, up to the ...

GAMMA RADIATION FROM INELASTIC SCATTERING OF 14-Mev NEUTRONS BY THE COMMON EARTH ELEMENTS

Gamma rays in the energy range 2 to 11 Mev produced by inelastic scattering of 14-Mev neutrons by nine elements were measured at a mean angle of 90 deg . Excluding carbon and oxygen, the maximum energy gamma rays varied from about 8 Mev for phosphorus to about 10.5 Mev for magnesium and 1l Mev for silicon. Resolved gamma rays were observed from carbon (4.43 Mev), oxygen (6.1 and 7 Mev), silicon (1.78 Mev), aluminum (2.2 Mev), phosphorus (2.2 Mev), sulfur (2.2 Mev), and calcium (3.7 Mev). In the energy range 4 to 6 Mev there are indications of individual gamma rays in silicon; no resolved gamma peaks above 2 Mev were observed for iron and magnesium. Except for carbon and oxygen, the intensity of gamma rays decreases with increase in energy and varies from about 3 to 9 times higher at 2 to 3 Mev than at 5 to 6 Mev. Gamma production cross sections are given for each element, relative to the known cross section for carbon. The ratio of the integrated cross section for gamma-ray production above 2 Mev to the nonelastic neutron cross section varies from 0.59 for sulfur to 0.99 for iron. (auth)

Determination of the thermal neutron absorption cross section of 1-liter rock samples by a pulsed source technique

Abstract A method is presented for measuring the thermal neutron absorption cross section of geological rock samples. Neutrons from a pulsed 150 kV ion accelerator are thermalized in a 1 1 sample surrounded by a water reflector. The water is in a cylinder concentric with the sample, and it aids in reducing neutron leakage from the system. A He-3 detector inserted in the sample monitors the thermal neutron die-away. The thermal neutron cross section is calculated from the die-away rate using diffusion theory in a fundamental mode analysis for the concentric cylinder geometry of the sample and water reflector.