A detailed investigation has been made of a nuclear disintegration produced by a charged particle, almost certainly a proton, of energy ~20 000 GeV. The disintegration, of type 22 + 76
p
, was recorded in a stack of plates exposed at ~90 000 ft. The jet of secondary charged particles, and the associated cascade of electrons and photons, can be followed through the emulsion of twenty-two plates. The observations lead to a value 0⋅25 for the ratio of neutral
π
-particles to charged shower particles produced in the disintegration. If
π
0
-mesons are produced in high-energy nuclear interactions with a frequency half that of the charged
π
-mesons, and if 10% of the shower particles are assumed to be protons, the present results indicate that 40 % of the charged shower particles are other than
π
-mesons. As there is no evidence for the existence of nucleon pairs, it is reasonable to identify these other particles with the heavy
k
-particles. The corresponding ratio of the numbers of charged
k
- to π-mesons,
N
K
±/
N
π
±, is equal to 0⋅80±0·4. The interaction length of the shower particles from the event, measured without distinction between the different types of mesons which may be among them, is shown to be very nearly equal to the value for nucleons. It follows that the
k
-mesons, which constitute a large fraction of the shower particles, interact strongly with nuclei.
Dependence of the short-wavelength cutoff in the mid-IR pulse spectrum on the interaction length in SiO2 and CaF2
