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in this experiment. It should be noted, however, that the earlier experiments yield, in general, still higher values for the stopping powers than are obtained from Thompson's / values and Eqs. (2) and (3). Some of these earlier results represent average values over the whole range of the particle rather than instantaneous values. The discrepancies in earlier results are the chief justification for the use of Thompson's accurate work as a standard of comparison in this experiment. It is of interest to note that if /Ai=170ev, following the suggestion of Sachs and Richardson, 9 a calculated 9 D. C. Sachs and J. R. Richardson, Phys. Rev. 89,1163 (1953.

By photographing an oscilloscope it is possible to measure short periods ranging from 0.1 second to 5 seconds. The pulses from a Geiger counter are fed into the x axis amplifier of an oscilloscope using no sweep. An oscillograph recorder is used to photograph the cathode-ray tube so that a record of all the pulses from the counter is obtained. The activated target is moved from the bombardment position to the counting position in about 0.2 second by means of a pneumatic tube. Exposure of calcium to a 23-Mev alpha-particle beam resulted in an activity with a half-life of 0.58±0.04 second. This activity may be assigned to Ti 43 produced by the reaction Ca 40 (a,n )Ti 43 • W HEN one attempts to measure the halflives of radioactive isotopes which are rather short, say less than ten seconds, several difficulties arise. One is the problem of minimizing the time between the end of the activating bombardment and the beginning of the counting period. Another is the problem of handling the high density of pulses issuing from a Geiger counter when it is exposed to a rapidly decaying source. If a conventional scaling circuit and mechanical register are used, it becomes necessary to take photographs of the register, the scaling lights, and a clock. This method is limited by the accuracy to which the clock can be read on the film and the uncertainty of the true number of scaling lights because of the finite exposure time.The method to be described in this paper allows not more than about 0.2 second to elapse between the end of irradiation and the start of counting. The Geiger counter pulses are fed directly into an oscilloscope and photographed so that a record of all the pulses is obtained.The radioactive sample was prepared by bombardment in a beam of alpha-particles produced in the 45-inch cyclotron. Because of the high FIG. 1. A section of one of the records obtained by photographing an oscilloscope with an oscillograph recorder. The time interval between arrows is 0.05 second during which 153 pulses issued from the Geiger counter.

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The nuclear radiations from Ba 139 have been investigated with the help of a magnetic lens spectrograph and scintillation counters in coincidence. Beta rays of energy 2.380, 2.227, and 0.822 Mev have been found together with an internal conversion line from a gamma ray of energy 0.163 Mev. The photoelectron spectrum, taken in the lens, showed gamma rays of energies 0.163 and 1.43 Mev. Beta-gamma coincidence experiments, performed with scintillation counters, showed that the 2.23-Mev beta-ray group is in coincidence with the 0.163-Mev gamma ray. The 0.163-Mev line has a K/(L-\-M) ratio of 7.0 and an internal conversion coefficient a K of 0.22, both of which correspond to an Ml transition. The connection with the shell model is discussed.