STUDY OF STOPPED $\mu$-MESONS BY THE CRYSTAL COUNTER METHOD*)

The paper under review is of considerable interest as the first attempt to use a crystal counter for the study of cosmic rays. At the same time, in addition to a definite conclusion about the average value of the energy released in the capture of stopped negative $\mu$-mesons, the authors obtain a number of methodological results characterizing the crystal counter and the possibilities of its use in coincidence circuits.

As the crystal counter, a specially prepared and carefully selected specimen of an AgCl crystal was used—

) H. G. Voorhies and J. C. Street, Phys. Rev. 76*, 1100—1105 (1949).

measuring \(75 \times 33 \times 6\) mm, placed in an electric field of intensity \(2400\) V/cm. To eliminate ionic conductivity, the crystal had to be cooled with liquid nitrogen to \(-196^\circ\)C. With the aid of a “telescope” with anticoincidence counters, cases were selected in which mesons stopped in an AgCl crystal (or in the platinum walls near it). Pulses from the “telescope” started the oscilloscope sweep only under the condition of a certain prescribed minimum amplitude (threshold) of pulses from the crystal. By photographing, on this sweep, individual pulses from the crystal having a width of \(0.7\ \mu\)sec, it was possible to study cases of delay associated with decay, measuring these delays with an accuracy of up to \(0.03\ \mu\)sec.

The amplitude spectrum of the pulses given by the crystal counter was studied by changing the threshold value both for fast mesons (with the anticoincidence counters switched off) and for stopped mesons. In the latter case, from among all pulses (the so-called group \(A\)) a special group \(B\) was selected, accompanied by delayed pulses and therefore known to belong to positive stopped mesons.

It is shown that the form of the amplitude spectrum of the pulses is determined by fluctuations of ionization losses and by the angular distribution of the mesons; the corresponding quantitative comparisons made it possible to calculate the counter sensitivity, amounting to about \(50\ \mu\)V per \(1\) MeV of energy released through ionization in the AgCl crystal. Taking into account the average energy (\(7\) eV) expended in transferring each \(\delta\)-electron to the conduction band, the authors also calculate the mean path of a conduction electron in the crystal, equal to only \(0.6\) mm. The latter means that the pulse amplitude should in practice not depend on the localization of the energy release in the crystal.

Finally, comparing the number of registered pulses with the flux of mesons through the crystal, the authors show that at a threshold of \(200\ \mu\)V the counter “senses” practically all relativistic particles.

The energy release in the capture of negative mesons was studied by comparing the amplitude spectra for the indicated higher pulses of group \(A\) and group \(B\). It turned out that both the shapes of these spectra and the mean pulse amplitudes practically coincide; moreover, quantitative estimates show that even in the most extreme assumptions the additional energy release associated with capture of the \(\mu\)-meson by the nucleus does not exceed, on average, \(3\) MeV. Those few cases (15 out of 300) in which the energy release exceeded the mean value (equal to \(12\) MeV) by no less than \(18\) MeV could be wholly attributed to stopped protons. The result obtained is new evidence that almost all the rest energy of the captured \(\mu\)-meson is immediately carried away from the nucleus by some neutral particle.

V. A.