P. R. Bell

A General Purpose Linear Amplifier

An amplifier that is suitable for use in a variety of nuclear‐particle counting experiments is described. The high and low frequency response of the amplifier can be varied by means of a three‐position switch. In the wide‐band position the transient response to a step function is a voltage pulse with a rise time of 0.15 μsec. and a total duration of less than 0.5 μsec. In the narrow‐band position a signal of 3 microvolts produces a voltage pulse at the output equal to r.m.s. noise. A pulse‐height selector is included for measuring pulses up to 100‐volts amplitude.

Intrinsic Scintillator Resolution

Measurements of spectrometer resolution using an electron gun and phosphor type of light source yield values considerably better than those obtained from scintillations in a phosphor. Line widths of about 3.4 per cent have been obtained at a pulse height equivalent to 661 kev in NaI. An investigation is being made to determine the causes of this difference. A number of possibilities have been eliminated. Flasher light pulses may be collimated or diffused at the photo surface with no difference in result. They may be mixed with random scintillation pulses giving simultaneously the relatively poor resolution of the scintillator and the good resolution of the flasher. Optical filters do not affect the result when the flasher is used, but do give a relative improvement with a scintillator. It is found that as the light is attenuated in any way from a scintillation crystal, its relative resolution is improved. The improvement is compatible with the concept of an intrinsic crystal resolution. In other words, the difference of the squares of crystal width and flasher width remains constant with varying attenuation. It remains constant also for a given crystal with photomultipliers of different types and resolutions. The intrinsic width indicated for a good crystal is about 6 per cent. Results of experiments attempting to locate the cause of this effect are reported.

Response of "Total Absorption" Spectrometers to Gamma Rays

A single crystal of sodium iodide 9-1/4 inches in diameter and 9 inches high with the top bevelled at 450 was recently obtained from Harshaw Chemical Company. A half-inch hole was drilled in the bevelled end to the center of the crystal. Three 5-inch diameter Dumont 6364s were optically connected directly to the flat end of the crystal. Sources were placed inside the crystal and γ spectra obtained, taking advantage of full geometry. A 4-3/4 inches diameter sodium iodide crystal 5-1/2 inches high with the top bevelled at 45° has a 1/4 inch hole drilled to the center. The crystal is optically connected to a Dumont 6364. The crystal is placed inside a 28-inch tank of liquid scintillator which uses four Dumont 6364s to detect pulses in the liquid. Samples are placed inside the crystal and the pulses from the crystal and liquid scintillator are placed in anticoincidence to reduce the Compton portion of the γ spectra. Typical spectra with these counters are shown along with ae comparison of data taken with these and smaller crystals.

Electronic Pile Simulator

A device has been made that follows the same set of kinetic equations as a chain‐reacting pile. The voltage output varies as a function of time as the neutron flux would vary in a pile. A potentiometer varies the effective multiplication factor and can be considered as a control rod. Five delayed neutron periods are simulated. A high precision electronic integrator is used, and the instrument is capable of an accuracy much better than one percent.

Transmission Characteristics of Light Pipers

The transmission characteristics of light pipers used in scintillation counting are very important, especially in situations in which the light piper must be long and thin, as in medical probing counters. This paper discusses several of the important factors involved in the transmission of light through pipers. The superiority of specular reflectors as compared with diffuse reflectors, the superiority of dry reflectors as compared with optically coupled reflectors, and the importance of surface polish are discussed. The use of total-internal-reflection couplers between scintillation crystal and light piper is described. Relative pulse heights obtained with certain crystal-piper-reflector combinations are given, and transmissions as functions of piper length and configuration are shown.

Improved Overload Response of the A‐1 Amplifier

It is frequently desirable to use a linear amplifier to study signals having a wide range of amplitudes. Modifications have been made to the A‐1 amplifier which permit it to be used with signals up to 300 times the normal overload value.

Peak Efficiency of NaI(Tl) Crystals for Gamma Rays from 0.150 to 7.5 Mev

The peak efficiencies of cylindrical 1-1/2 X 1-inch, 3 X 3-inch and 3 X 3-inch bevelled crystals have been determined by the multiplication of the theoretical total efficiency for a particular energy gamma ray by the experimentally determined ratio of the areas under the full energy peak and under the total pulse height spectrum for the gamma ray. Care was taken to eliminate spurious effects in the pulse height spectrum such as scattering, bremsstrahlung, and other interfering gamma radiation from the source, etc., in the experimental measurements. The peak efficiencies have also been determined at certain energies by ß-γ coincidence experiments and yield the same results within 5 per cent for energies to 2.76 mev. The intensities of gamma rays determined in this way have been compared to the disintegration rates obtained from 4π ß-counting techniques from sources with known decay schemes, and the results agreed within the limits given above. The extension of these efficiencies to crystals of different composition or to crystals with the same shapes but different dimensions will be discussed.

Electron Component in Geiger Discharge

Observation of the wave form of current during a Geiger discharge indicates the presence of a component due to electron collection ammounting to as much as 50 percent of the total energy involved in the discharge. It appears, therefore, that the initial ion sheath can have a mean radius of the order of 10 to 15 times the radius of the center wire. Apparatus is described, wave forms are shown and the method used to evaluate electron component and mean radius of ion sheath is indicated. Results for two tubes with different quenching vapors are tabulated.

FILAMENTARY SUBMINIATURE SCALERS

In order to reduce the power requirements and heat dissipation in multichannel pulse‐height analyzers with parallel digital storage, we have devised and tested a low‐wattage scale‐of‐two employing filamentary subminiature tubes. These units are designed for use in scales of 16 or 32 and exhibit great reliability and long life. The resolving time is 5 μsec with grid‐catching diodes and 13 μsec unaided. For greater reliability in very long cascades and for battery operation, a Higinbotham version has been devised. Both designs promise extremely long life and may be useful wherever bistable switching circuits are used.