Oct., i94o. ] T H E BARTOL RESEARCH FOUNDATION. 5 I I

were almost entirely caused by alpha-particles ejected from the radioactive contamination in the counter walls, although some of the smallest ones were caused by slow electrons. Curve B gives the distribution of pulse sizes when two milli- grams of radium were placed 5 ° cm. from the counters. We see that while the number of small pulses was much increased, the number of large pulses was considerably decreased, as a result of the decrease in size of the pulses by the presence of the large amount of background ionization. From these data it is evident that care should be exercised in the use of a counter operating in this manner to insure that either the background radiation has no effect upon the size of the larger pulses which one desires to observe or that the background is kept constant during the measurements, particularly so when the counters are used near a cyclotron where the intensity of the gamma-radiation is extremely high. The actual magnitude of the suppression of the gas amplification of the large pulses depends considerably upon the experimental conditions. The suppression is reduced by using counters of small diameter, at low potentials, with large wire capacities. It is evident that the magnitude of the effect is almost independent of the circuit used to record the pulses, since the effect depends upon the reduction of the field strength near the counter wire, and not upon the wire potential or any of the potentials of the circuit components as is the case in the phenomenon of the paralysis of an ordinary Geiger counter.

SHOWER PRODUCTION BY MESOTRONS IN DIFFERENT MATERIALS.*

BY

W. F. G. SWANN AND W. E. RAMSEY.

The apparatus used has already been described 2 and com- prises a number of Geiger counter trays, each forming a sensitive area 2o by 2o cm., mounted in a vertical column and arranged in relation to the associated electrical circuits in such manner that the apparatus is only allowed to record when a ray passes through all of the trays. Below the top

* Reprinted from The Physical Review, 57, 749 (I94O).

5 1 2 T H E BARTOL RESEARCH FOUNDATION. [J. F. i.

t ray is placed a block of lead 18 cm. thick, or its mass equiva- lent in other materials, so that all records by the apparatus are initiated by a ray which is capable of passing through at least this thickness of material. When a ray passes through all of the trays, any shower rays accompanying it are recorded separately by the individual counters which they excite, the record being a photograph of spots of light from the mirrors of a number of electroscopes--one for each counter. The

FIG. I.

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,~TWO ELECTRONS ' 0 T ,0 r.~THREEI ELECTRONS I

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whole apparatus is mounted below a 3o-foot column of water contained in a large water tank.

With the above apparatus the authors measured the num- bers of one-ray showers, two-ray showers, three-ray showers, etc., produced in lead last August, and the present experiments extend the measurements to lead, tin, iron, and magnesium.

In dealing with the various materials, it was necessary to have a thickness sufficiently great to insure tha t shower pro- duction through mesotrons had attained equilibrium. Thus, it was necessary to have the thickness comparable with or greater than the range of the electrons. In order tha t one might be free from complications involved in variation of

Oct., 194o.] THE BARTOL RESEARCH FOUNDATION. 513

mesotron intensity by different absorptions in the different elements, it was arranged that, in the case of each element, the total equivalent thickness as regards mass absorption should be the same, the balance being adjusted for conveni- ence in each case by the choice of a Suitable thickness of lead above the material under examination.

Figure I s h o w s the results. Horizontally are plotted t h e atomic numbers of the elements concerned. The ordinates represent the numbers of showers, of the kind cited, associated with the passage of IOOO mesotrons through the apparatus.

The two-electron, and perhaps the three-electron, showers showed frequency of occurrence which is independent of the atomic number within the limits of accuracy of the experi- ments; but the one-electron showers show a marked depend- ence upon atomic number, a dependence sufficiently strong to dominate the situation even if we should plot a curve representing the total number of shower rays associated with i ooo mesotrons. These results are not in harmony with the elementary conclusions from Bhabha's theory, which predicts results practically independent of atomic number. As shown in our former paper, 1 however, the data for the actual num- bers of showers agree, as regards order of magnitude, with the theoretical predictions.

It is a curious fact that the regularity of the graphs for the actual data is greater than one might expect on the basis of the statistical uncertainty indicated in the usual way by the vertical lines. This is a mat ter which may have a rather profound significance in relation to processes involved.

1 ['resented at the New York Meeting of the American Physical Society, February 23-24, 194o (_Phys. Rev., 57, 554(A) (194o)).

= W. F. G. Swann and W. E. Ramsey, Phys. Rev., 56, 378 0939); W. F. (;. Swann, Rev. Mod. Phys., II, 242 (I939).