Volume 8, Issue 12, 01 December 1937
Index of content:
8(1937); http://dx.doi.org/10.1063/1.1710257View Description Hide Description
8(1937); http://dx.doi.org/10.1063/1.1710259View Description Hide Description
The photographic intensifying action of metal screens has been known for a number of years but it is only with the advent of high voltages and thick materials of industrial radiography that their use has become especially advantageous. Lead is the material now commonly employed. The present work is an attempt to extend our quantitative knowledge of this subject by studying the various phenomena separately. The higher atomic number elements are shown to be the most suitable for intensifying screens and of these lead provides the best combination of properties. The front screen is more effective in its intensification than the back screen. This is explained on the basis of present‐day knowledge of electron emission under x‐ray excitation. Most of the intensifying action seems to be due to the electrons emitted and only a small part to characteristic and scattered x‐rays. The intensification factor increases with kilovoltage and filtration between 160 and 200 kv and from 0.6 to 2.5 cm of steel, but apparently with sufficient decrease in x‐ray wave‐length, the intensification of lead foil decreases, because it is much less in gamma‐ray radiography. The effect on intensification of variation in thickness of lead screens is very slight but, for practical reasons, it is desirable to have the back screen fairly thick. The thickness of the front screen is determined largely by its reduction of secondary radiation and a value from 0.010 to 0.015 cm is satisfactory. The experiments indicate that, in the reduction of secondary radiation by lead screens, differential intensification of the primary and secondary radiation is of major importance. Differential absorption by the front screen is also a factor. Lead screens are of advantage in welding and casting inspection but one of their most striking properties is shown in the radiography of small objects that normally require masking in which masking may often be eliminated because of the characteristics of the lead screens. The screens should be kept clean, since light materials absorb the intensifying radiation considerably. Visually, definition with the lead screen is just as good as it is with a nonscreen exposure but intimate contact in the cassette is essential. There seems to be little hope of making important improvements in the radiographicproperties of lead screen, as e.g., by the addition of a fluorescing surface layer.
8(1937); http://dx.doi.org/10.1063/1.1710260View Description Hide Description
This paper gives the results of an investigation of the voltage characteristic of the copper arc in air during the process of arc formation. A pair of copperelectrodes, initially in contact and carrying current, were separated and the voltage arising across the arc gap was studied. Cathode‐ray oscillograms indicate that at the instant of contact separation there develops across the gap a voltage of about 12.2 volts. This voltage was attained in less than 10−5 seconds. This rapidly developed voltage was found to be independent of arc current for current values up to 100 amperes, which suggests that the rapidly developed initial voltage corresponds to the cathode fall of the arc.