224 B. ROSSI AND D HALL buted to the decay, or an increasing fur the aly attributed to the polarizat S An attempt to determine the rates of decay of mesotron of different momenta has been re- ported by Nielsen, Ryerson, Nordheim and Mesotron groups of diffe 30 ected by taking the diffe between the intensities of the mesotron beam after filtration through various thicknesses of lead By this method, however, it is hardly possible to reach a sufficient accuracy, since the Y STEEL difference is small compared with the quantities directly measured. In our experiments the sta- E LEAD tistical precision was greatly improved by meas- uring the difference directly; i.e., by recording only mesotron which can traverse a given thick opped by a here u is the mass and p=uB/(1-B) is the ditional absorber lomentum of the mesotron. The probability of decay per centimeter path is obviously 1/L. It is EXPERIMENTAL METHOD seen that the average range is directly pro- The experimental arrangement is schematically portional, and the probability of decay inversely represented in Fig. 1. The Geiger-Muiller counter proportional, to the momentum ubes were of the self-quenching type. Their The experiments described in the present internal diameter was 4 cm and their effective paper were primarily designed to test the de- lengths were as follows: counters A, B, cand E disintegration probability on mo- 27 cm: counter D, 20 cm: counters f, 60 cm. The mentum expressed by Eg.(2). The to provide an additional check of the disintegra- connected in parallel. The counter battery F relativistic transformation formula for time inter- counters 4. B, C and D. In order to cut off the vals. Further experimental evidence on the sub- soft component, 5 cm of lead was permanently ject of the decay particularly desirable in placed above counter A and 10 cm of lead be- view of Fermis recent theory showing that the tween counters B and C. Including the material energy losses of fast particles in condensed of the frame, the permanent absorber above or materials are appreciably reduced by the dielectric between counters A, B, C and D was equivalent polarization of the medium. According to this in absorption power to 186 g/cm2 of lead, while theory even stable mesotron are absor bed by that between D and F was equivalent to 10 g/cm2 of lead. Counters A, B, C and D were pro materials of the same mass per cm. The differ- tected on the side by lead walls 11.5 cm thick ence in absorption due to polarization increases An additional lead absorber 2 of 115 g/cm2could with increasing mesotron momentum; i.e., varies oppositely from the difference due to decay. The be introduced between D and F and an absorber polarization effect, as calculated by Fermi, was Smade of iron plates could be arranged above the quantitatively inade equate to account for the apparatus so as to cover the whole solid angle experimental results already obtained. Yet it was subtended by counters A, B,Cand D.The interesting to investigate whether the observed apparatus was set up in a moving van which absorption anomaly was a decreasing function of could be taken to different altitudes on mountain the mesotron momentum, as the anomaly attri roads. The whole syste pt for the absorber S, was enclosed in a thermostatic box. E. Fermi, Phys. Rev. 57, 485 (1940 By means of an appropriate vacuum-tube