George E. Thomas

Measurement of the Time Dependence of Scintillation Intensity by a Delayed‐Coincidence Method

A new method for the measurement of the time dependence of the light intensity from scintillators is described. An important aspect of the method is the ease with which it can be applied to all modes of excitation. The results of measurements on the time dependence of the scintillators for several modes of excitation are presented.

Neutron detection with glass scintillators

Abstract A systematic study is made of the characteristics for neutron detection of boron- and lithium-containing glass scintillators. The results of measurements of the optical transmission, the pulse-height distribution, and the time dependence of scintillation intensity are presented. A careful study of the pulse-height distributions for thermal neutrons shows that a major part of the width of the distributions can be accounted for in terms of well-understood statistical fluctuations. Problems associated with the application of the scintillators are considered, with special emphasis on application to time-of-flight neutron spectroscopy. As part of these considerations, a Monte Carlo calculation is performed to obtain a complete understanding of the time uncertainties involved in the capture of neutrons in the scintillators. It is concluded that the lithium glass in an excellent detector of neutrons when large uncertainties in time ( > 0.3 >sec) can be tolerated, the boron glass excels when intermediate accuracy (≈ 30 nsec) in timing is required, but neither kind of glass is suitable for most experiments that require fast timing.

High-sensitivity neutron-capture gamma-ray facility

Abstract An exceptionally sensitive experimental system for the measurement of neutron-capture gamma rays is described and the characteristics of the system are examined critically. The results of measurements on the gamma-ray spectra from neutron capture in 6 Li, 10 B, 12 C, 13 C and 14 N are reported. Some of these data are used to derive an accurate calibration of the efficiency of the Ge(Li) gamma-ray spectrometer used in the measurements.

Boron‐Loaded Liquid Scintillation Neutron Detectors

The general problems involved in constructing boron‐loaded liquid scintillation neutron detectors are considered. The characteristics of particular counters which have been successfully used in neutron transmission measurements by the time‐of‐flight method are then described. The design of these counters was guided by the results of a Monte Carlo study of neutron capture in a boron‐poisoned medium. This calculation gives the probability of neutron capture as a function of neutron energy, counter thickness, and time. The calculated results are compared with experimentally determined efficiencies. The advantages and problems encountered in using the boron‐loaded liquid scintillator with the Argonne fast neutron chopper are discussed.

Gamma-ray intensity standards: The reactions 14N(n,γ) 15N, 35Cl(n,γ) 36Cl and 53Cr(n,γ) 54Cr

Abstract The relative and absolute intensities of γ-rays from thermal-neutron capture in 53Cr, 52Cr, 50Cr and 35Cl have been determined over the energy range 1.8–11 MeV. The absolute intensities were measured by a method in which transition intensities are compared with the well-known 100% intensity of the 7368-keV transition in 208Pb. Detector-efficiency curves were computed from the previously reported relative intensities for transitions in 14N(n,γ) 15N and 53Cr(n,γ) 54Cr, and the good agreement between their shapes over a wide energy range suggests that both sets of intensities are reliable.

Glass Scintillators for Neutron Detection

The characteristics for neutron detection of some clear boron-containing glass scintillators are discussed. The pulse-height response of the glasses GL- 127 and GI-55 to thermal neutron capture is given. Tests on small samples of the two glasses indicate they can be used as detectors for slow neutrons. (C.J.G.)

Thin carbon foil breakage times under ion beam bombardment

Abstract We have measured the breakage times of thin carbon foils of different thicknesses under bombardment by Ar+ ions of 1–3 MeV energy. The breakage times are independent of foil thickness over the areal density range of 2–22 μg/cm2. Our results are in reasonable agreement with previous measurements of foil breakage. We propose an empirical formula for the foil breakage time τ in terms of the ion energy E: τ( p μ A · min / mm 2 ) = A(∼10)·E 1.15 ( MeV / amu ) .

Boron‐Loaded Neutron Detector with Very Low γ‐Ray Sensitivity

A boron‐loaded liquid scintillator is described which gives a much greater difference in the pulse shapes for γ rays and for slow neutrons than any of the liquids currently used. A detailed study of the pulse shapes indicates that the γ‐ray sensitivity of a neutron detector filled with this liquid can be reduced by a factor of 500 with only a 5% decrease in the neutron sensitivity by application of pulse‐shape discrimination. A survey of the pulse‐shape characteristics of a number of scintillators currently used, together with the results for the new liquid, is presented, and the performance of the new detector with a simple system of pulse‐shape discrimination is described.

A boron-loaded liquid scintillation neutron detector using a single photomultiplier☆

Abstract One of the unique features of this detector is that the cell containing the liquid is mounted directly onto a single photo-multiplier, a selected high-gain tube with a favorable signal-to-noise ratio. This detector is relatively simple to construct and use as is the associated circuitry.

Thin film coatings which inhibit spin relaxation of polarized potassium atoms

Abstract A new technique is being developed at Argonne National Laboratory to produce a beam of polarized deuterium atoms. A part of the apparatus consists of a small cell which contains potassium vapor. The potassium atoms are polarized by circularly polarized light from a high-power cw laser. A port feeds deuterium into the cell and the polarization of the potassium is transferred to the deuterium by spin-exchange scattering. It is important that the potassium and deuterium atoms do not lose polarization by scattering from the walls of the exchange cell. We have tried various coatings of the exchange walls in order to inhibit losses from spin relaxation. Methods used to produce these coatings as well as their success will be described.