THE PHYSICAL REVIEW A Jounal of Experimental and Theoretical Physics Established by E. L. Nichols in 1893 VoL. 59. No. 3 FEBRUARY 1, 1941 SECOND SERIES Variation of the Rate of Decay of Mesotrons with Momentum BRUNo ROSSI* AND DAVID B. HALL Universily of Chicago, Chicago, Illinois (Received December 13, 1940) In order to determine the dependence of the probability of decay on momentum, mesotron h range between 196 and 311 g/cm2 of lead and mesotron with than 311 g/cm of lead were investigated separately. The softer group of mesotron disintegrate at a rate about three times faster than the more penetrating group, retical predictions based on the relativity change in rate of a movin ew value of proper lifetime of mesotron of (2.4 0.3)X10-6 sec is determined measurement with particles with momentum of approximately 5X105 ev/c INTRODUCTIO traveling in the atmosphere No appreciable RECENT experiments on the variation of lumber of mesotron, however, will disintegrate ray intensity with altitude have within a condensed absorber, even equivalent in mass to the whole thickness of the atmosphere shown that the rate of decrease of the mesotron because the time required for the traversal of component with increasing atmospheric depth such an absorber is very short compared with the cannot be accounted for completely by ordinary lifetime of mesotron ionization losses. It has been established, namely, A simple relativistic consideration shows that that the number of mesotron is much more if the absorption anomaly of mesotron is due to strongly reduced by a layer of air than by a \me for meneous decay it must be more pronounced air layer with regard to ionization losse The anomalous absorption in air is interpreted high energy. In fact, let To be the "proper life- timeof mesotron: i.e., the lifetime measured of a few microseconds. According to this as- rest, and T the lifetime measured in a frame of sumption, a considerable fraction of the mesotron beam will disappear by disintegration while velocity B 6 Then Ithaca, New York oag,(a)Phys. Rev. and the" average range before decay'"L; i.e., the L=r=力T0/k 6 A. Ehmert, Zeits. f. Physik 115, 333(1940)