Volume 12, Issue 12, December 1941
Index of content:
12(1941); http://dx.doi.org/10.1063/1.1769804View Description Hide Description
A Comparison Between a Geiger‐Mueller Counter, a Secondary Electron Multiplier Tube, and Photographic Film for Detecting Weak X‐Rays12(1941); http://dx.doi.org/10.1063/1.1769805View Description Hide Description
A comparison has been made between three methods, given in the title, for the detection of weak x‐rays. The secondary electron multiplier tube has a lower resolving time than the Geiger‐Mueller tube, but the G‐M tube is from 5 to 10 times as sensitive to x‐rays of the wave‐lengths here studied, 0.63A to 1.54A. A diffraction pattern of a liquid can be obtained with a G‐M counter in roughly one‐twentieth the time taken for a photographic exposure.
12(1941); http://dx.doi.org/10.1063/1.1769806View Description Hide Description
An artificial neutron source has been built which is very stable in operation and flexible in use. Using the D—D reaction, target currents of more than 200 microamperes at 300 kv give a neutron yield of 10 grams radium‐beryllium equivalent. It has been found that voltage stability at these currents may be greatly improved by the use of special diaphragms, and that a reflux target cooling system is very satisfactory in maintaining a heavy water‐ice target. Electrostatic deflection of the ion beam has produced a neutron source which may be modulated at high frequencies or otherwise time‐controlled.
12(1941); http://dx.doi.org/10.1063/1.1769807View Description Hide Description
Apparatus is described by means of which a modulated source of slow neutrons is produced and their intensity determined as a function of time. The deuteron beam of a high voltage tube is deflected by a modulated electrostatic field in such a way as to produce square bursts of fast neutrons of any desired length. After being slowed by hydrogenous material a selected beam of neutrons falls on a suitable detector at a measured distance from the source. Pulses from the detector are impressed on an oscilloscope screen, together with a sharply defined time scale and photographic records made. The time analysis of the neutron distribution is then made by a numerical count of the neutron pulses as a function of the time interval in which they occur.
12(1941); http://dx.doi.org/10.1063/1.1769808View Description Hide Description
A new device which performs the photometry of stellar images on a photographic plate and which segregates the images according to size is described. The starimages to be photometered are manually shifted into the optical focus of the device. The results of tests indicate greater accuracy in counting than is attainable by visual means. No systematic deviations of the counts so obtained from catalog compilations or from the results deduced visually were found.
12(1941); http://dx.doi.org/10.1063/1.1769809View Description Hide Description
A search was made for liquid mixtures giving low expansion ratio, low background fog, and good electron tracks in cloud chambers filled with air at near atmospheric pressure. As was expected from theory, mixtures of any lower alcohol with water gave a minimum in the curve of expansion ratio versus concentration, while no minimum was found in the curve for mixtures of any two alcohols. The optimum mixture found for chamber use was 50 percent ethyl or normal propyl alcohol, 25 percent water, and 25 percent acetone; the presence of acetone definitely increases the contrast between tracks and background fog. A brief study of photochemicalfog, with a quartzmercury arc, showed that wave‐lengths below 3900A are chiefly responsible for such fog. Irradiation of pure alcohols gives no perceptible fog, but irradiation of the optimum mixture above produces dense fog. However the clean‐up after irradiation is sufficiently rapid to offset the disadvantage of its sensitivity to light.
12(1941); http://dx.doi.org/10.1063/1.1769810View Description Hide Description
12(1941); http://dx.doi.org/10.1063/1.1769811View Description Hide Description
Some precautions necessary for accurate measurements of volume with the McLeod gauge are discussed, particularly the necessity of determining the capillary depression from each set of readings. Applications to the determination of small quantities of helium are given.