Double Shield Research Articles

A search for the decay of Vanadium-50 with a low-level gamma-spectrometer

The spectrometer consists of a 4″×4″ NaJ-crystal with a double lead shield — the inner one being of selected lead — and a guard anticoincidence ring in between. The apparatus is situated in a cellar covered by 3 meters of soil. The total background above 0.03 MeV is 120 cpm, being extremely stable with an intrinsic r.m.s. variation below ±2 per mil. The crystal is surrounded by a sample container of volume 1.8 liters. An energy dependent gamma detection limit (2σ) ranging between 0.3 and 0.8 dpm has been obtained. This corresponds to a maximum detectable halflife of 1.0 to 2.7× 1018 years per mole of the radioactive nuclide in the sample container. In the case of vanadium-50 (isotopic abundance 0.25%) a lower limit of the half-life was found to be 9.0×1016 and 6.9×1016 years respectively for the two modes of decay V50-EC-Ti50*-γ(1.58 MeV) — Ti50 and V50-β−-Cr50*-γ(0.78 MeV)-Cr50.

Recovery from Partial-Body X-Irradiation as Measured by the Lethality of Two Exposures

For the purpose of studying recovery from partialbody irradiation, the efficiencies of various shielding procedures have been evaluated. A double wraparound shield, which was the most efficient, reduced the rate to about 1% of the outside dose at a distance of approximately 6 cm from the edge of the shield. These studies, together with appropriate dissections, have permitted estimates of dosages to various organs under the conditions of exposure employed. In confirmation of previous work, the LD/sub 50/'s for upper-body, lower-body, and whole-body irradiation were found to be 1750 r, 1080 r, and 750 r, respectively. By using two partial-body exposures to x radiation, separated by various time intervals, the pattern of recovery (as measured by lethal effects) has been determined for lower- and upper-body irradiation. The recovery from a lower-body exposure of 648 r can be expressed by a single exponential function with a half- recovery time of 1.7 days. The recovery from an upper-body exposure of 1300 r can be expressed by a single exponential with a half-recovery time of 7.0 days or by two exponentials, one with a half-recovery time of 3.8 days and the second with a half-recovery time of 7.5 days. (auth)

A Small High Frequency Microphone

A small rochelle-salt crystal probe microphone and preamplifier have been developed for use in air, or water, at frequencies up to 100 kc. The microphone response is reasonably flat up to 50 kc in air, with a sensitivity of approximately −100-db re 1 volt per dyne per cm2. Beyond this frequency the response varies, but is satisfactory and usable up to 100 kc at least. In water the sensitivity is about −100 db and is relatively flat to 100 kc. At 100 kc there is a variation in response of about 5 db as the microphone is rotated through 360°. This variation is less than 2 db at 20 kc. The crystal ensemble is coated with Glyptal, and has two copper shields, insulated from each other, plated over it. Its dimensions are 14″×516″×38″, and it is mounted on tubing 7″ long and 18″ diameter. The preamplifier was designed to have very high input impedance so as to be independent of capacity changes in the crystal, and a low output impedance. This is accomplished by using two stages—a cathode-follower input and a pentode output with 500-ohm output impedance. The use of a double shield and a high impedance load on the cathode follower results in a gain of 2.5 db, flat within 0.5 db from 2 kc to 200 kc, the lower limit being imposed by coupling-condenser size rather than input impedance. The microphone has had considerable use in air and water media during the past year and has stood up well.

The heat liberated by the beating heart

Since myothermic methods have been improved several attempts have been made to measure the heat liberated by the beating heart. Two observers, both experienced in the difficulties and limits of myothermic experiments, A. Y. Hill (1) and K. Bürker (2), were unable to decide how far their results showed the real change of temperature of the heart, and how far they were the effects of several sources of error. On the other hand, Herlitzka (3), Snyder (4), and Bohnenkamp (5) believed themselves entitled to draw important conclusions from their experimental results. At the suggestion, therefore, of Prof. A. V. Hill, I undertook to investigate the heat production of the heart with the various means available in his laboratory. Since, as will be shown below, my results were not in accordance with those of the last named investigators, I was forced to compare my methods with theirs. According to the most reliable of my observations I believe the warming of the heart (eel or tortoise) for a single beat to be not higher than 0·00012°. For the heart of the rabbit Herlitzka found a rise of 0·004° to 0·012°; for the terrapin’s heart Snyder found 0·00075° to 0·00255°: and for the frog’s heart Bohnenkamp found 0·001 to 0·01°. Apparatus and Methods. Galvanometer .—The galvanometer employed was designed and constructed by A. C. Downing (6). The “figure of merit” of this instrument (23,000) permitted the use of a high sensitivity, without making the deflection time when critically damped too great. With a sensitivity from 1·2 X 10 -9 to 1·3 X 10 -10 amps. per mm. the galvanometer had a complete period from 0·6 to 2·5 secs. The four coils, each of resistance 6·6 ohms, could be placed in series, or parallel, or series-parallel, so that the resistance of the galvanometer as a whole could be made to approximate to that of the thermopile, or thermocouple, used in the different experiments. The galvanometer was protected by a double shield of high permeability steel, as described by Hartree and Hill (7). Records were made photographically, as described by them, but with the difference that the distance of the drum was 1·30 metres and that the light from the lamp was interrupted each half-second by a shutter carried on an electro-magnet actuated by a half-second pendulum.