Volume 28, Issue 7, July 1957
Index of content:
28(1957); http://dx.doi.org/10.1063/1.1715918View Description Hide Description
The general problems involved in constructing boron‐loaded liquid scintillation neutrondetectors are considered. The characteristics of particular counters which have been successfully used in neutrontransmission 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.
28(1957); http://dx.doi.org/10.1063/1.1715919View Description Hide Description
An automatic adiabatic calorimeter shield control has been built and operated successfully for approximately 700 hours. It has been used with an existent low temperature (10 to 300°K) intermittently heated calorimeter in runs of up to 100 hours duration. Each control unit consists essentially of a difference thermocouple, high sensitivity taut suspension galvanometer, a set of four photomultiplier tubes, a relayservo mechanism, a reversible ac motor and gear train and two ON‐OFF control relays. The observed behavior of the control and the internal consistency of the data obtained during its use indicate that the precision with which it controls the shield temperature is comparable with that obtainable from a trained human operator.
28(1957); http://dx.doi.org/10.1063/1.1715920View Description Hide Description
Troublesome changes of the bridge balance in a twin‐T nuclear resonance bridge have been eliminated by the application of a voltage‐sensitive condenser. A small amplitude, high‐frequency modulation voltage applied to the condenser senses any deviation from balance, and a servo loop provides a dc correction voltage which maintains the bridge balance. No apparent deterioration of the spectrum results from modulation of the bridge.
28(1957); http://dx.doi.org/10.1063/1.1715921View Description Hide Description
A description is given of the dielectric recording technique wherein information is stored as a surface charge on a rotating dielectric coated drum. The specific application of the technique to a multiple channel recorder is shown. A general comparison between the properties of analog dielectric and magnetic drum recording is presented. The comparison shows dielectric recording to have important advantages for some applications.
28(1957); http://dx.doi.org/10.1063/1.1715922View Description Hide Description
Theory of design and construction of a field stabilizer for high‐resolution nuclear magnetic resonance spectrometers is described. Achievable time stability is 1:108. A voltage induced by field variations in suitably located coils is amplified by a galvanometer amplifier, integrated, and fed back to a system of coils biasing the field. The feedback ratio is 70 db from dc up to 0.2 cps. The stabilizer allows measurement of nuclear magnetic resonance signals with resolution of 108 under field conditions, independent of field fluctuations produced by temperature variations and stray fields.
28(1957); http://dx.doi.org/10.1063/1.1715923View Description Hide Description
A neutron velocity spectrometer with a 100‐Mev betatron as a source of neutron pulses has been in use at the Knolls Atomic Power Laboratory since 1953, for cross section measurements up to about 50 000 ev. An internal, thick uranium target is bombarded by 80‐Mev electrons when the betatron orbit is rapidly contracted, giving approximately 0.1‐microsecond bursts of neutrons at a 60‐pps repetition rate with a peak intensity of about 7×1015neutrons per second. Measurements of total, capture, and fission cross sections are made with high‐resolution detecting equipment located at points up to 20 meters from the target. The timing, registering, and data‐processing techniques are designed to achieve maximum benefit from the short burst duration.
28(1957); http://dx.doi.org/10.1063/1.1715924View Description Hide Description
Data are presented on the effect of voltage‐divider circuits on the operating characteristics of multiplier phototubes. Linear, superlinear, and saturated behavior of output current vs light flux are related to divider current. Characteristics are shown for varying sensitivity by controlling interdynode voltage. Divider circuits are described, which protect the multiplier phototube from excessive current flow.
28(1957); http://dx.doi.org/10.1063/1.1715925View Description Hide Description
The principal hazards of operating a liquid hydrogen bubble chamber are failure of equipment (due to overpressure) and uncontrolled escape of hydrogen gas, which may cause an explosion. If safety considerations are incorporated in planning from the job beginning, components can be designed to reduce or eliminate the probability of accidents arising from the known hazards.
The degree of safety, the hydrogen safety‐vent system, and the hazards, and operation of the liquid hydrogen bubble chambers at UCRL are discussed.
28(1957); http://dx.doi.org/10.1063/1.1715926View Description Hide Description
A description is given of the mechanical gauge and electrical circuit employed for measuring millisecond pressure pulses in a liquid hydrogen bubble chamber, in a pulsed magnetic field region.
The pressure pulse produces a deflection of a diaphragm that acts as one electrode of a capacitor. The change in capacitance is measured in an electrical circuit by comparison with a reference capacitor.
A common source feeds two cables, one of which is terminated by the pressure‐measuring capacitor(``transducer''), the other by the adjustable comparison unit. Pulses from the common source are reflected from the two terminations, and their difference is amplified and displayed. The circuit employed is such that the distributed capacitance of the cables does not affect the sensitivity of the system. Hence, the interconnecting cables are not limited in length up to several thousand feet.
The frequency‐response limit is about 10 kc for units currently used, because of the diaphragm characteristics. Steady‐pressure calibration can be employed in order to determine the pressure and output relation.
28(1957); http://dx.doi.org/10.1063/1.1715927View Description Hide Description
A device is described for producing a uniform distribution of a solution of radioactive material over a large area source.Droplets of controlled size are expressed from a syringe at a constant rate and systematically and uniformly placed over an area up to 50 centimeters square.
28(1957); http://dx.doi.org/10.1063/1.1715928View Description Hide Description
A testing machine is described in which a specimen contained within a water‐jacketed vessel is heated in vacuum or in a gas atmosphere by a tantalum‐tube resistance heater, and is then subjected to static tensile loads up to 100 000 psi. This machine is now in routine use at Los Alamos Scientific Laboratory for short‐time creep and stress‐rupture testing of metallic and nonmetallic specimens at 2000 to 2500°C. With minor modifications, it can also be used for longer‐period creep and stress‐rupture studies, for ordinary tensile, compressive, or shear testing, and for operation at temperatures substantially above 2500°C or down to room temperature.
28(1957); http://dx.doi.org/10.1063/1.1715929View Description Hide Description
An apparatus is described which permits the direct reading of minority carrier lifetimes greater than 10 microseconds. The decay of optically injected excess minority carriers is compared with voltage decay in an RC circuit and lifetimes are read directly from a calibrated dial. Measurements can be made quickly without critical adjustment.
An alignment chart for rapid calculation of surface recombination velocity from lifetime data is included.
28(1957); http://dx.doi.org/10.1063/1.1715930View Description Hide Description
Technical design and construction details for a 6.8‐meter grazing incidence vacuum spectrograph are discussed. The spectrograph is built into a 30‐in. by 12‐ft steel tube evacuated by a high‐speed pumping system capable of evacuation from atmospheric pressure to 5×10−5 mm in thirty minutes. Operating at an incidence angle of 82°, a spectral range from 50 to 2000 A is covered in steps on a 30‐in. plateholder. Provision is made for changing entrance slit widths quickly and easily through rotary vacuum seals. The wavelength region may be changed from outside the instrument while under vacuum. Spectra from 540 to 1800 A are shown.
28(1957); http://dx.doi.org/10.1063/1.1715931View Description Hide Description
By a method of successive approximation the index of refraction of polystyrene films has been determined from two photographs of continuous spectra. It is shown that the method is valid if the dispersion is known within several percent as a starting assumption.
28(1957); http://dx.doi.org/10.1063/1.1715932View Description Hide Description
A microwave measurement system has been built which makes possible the quick measurement and automatic recording of the phase and amplitude response of a general four‐terminal network. This system was set up to measure in particular the phase and attenuationproperties of samples of ferritematerials in wave guide for two opposite directions of applied dc magnetic field at both X and S bands. The system is built around a traveling wave tube used in a ``frequency offset'' application. The system has been used to make rapid measurements of phase and amplitude of a device as a function of frequency.
28(1957); http://dx.doi.org/10.1063/1.1715933View Description Hide Description
Resistance heating is employed to obtain temperatures up to 3000°C in a graphite tube furnace, and up to 2200°C in a molybdenum tube furnace, both operating from the same controls. Partial control of the atmosphere is accomplished in the former case; the latter furnace may be operated in vacuo, or selected atmospheres. Provision is made for the application of pressure to samples at high temperature. The furnaces have been found to be useful and reliable tools in refractories laboratories, and examples of their application are given.
28(1957); http://dx.doi.org/10.1063/1.1715934View Description Hide Description
By plotting E/Iresistance of commercial ballast tubes against voltage it can be seen that ballasting action is a typical ``growth'' phenomenon. It is concluded that the requirements for ballasting action are (1) nonuniform temperature distribution in a current‐heated resistor and (2) a high‐temperature coefficient of resistance that increases with temperature.