Particelle pesanti instabili in emulsioni nucleari

Riassunto

Si presentano i risultati di una analisi sistematica sulle particelle pesanti instabili effettuata mediante pellicole di emulsione nucleare esposte alla radiazione cosmica durante la Spedizione Internazionale di Sardegna del 1953. Dopo una breve introduzione sui metodi sperimentali usati, vengono esposti dati riguardanti i mesoni pesanti, gli iperoni ed i frammenti instabili. Nelle conclusioni del lavoro sono presentate alcune considerazioni sulle frequenze relative dei vari eventi e sulla attendibilità degli schemi di decadimento proposti.

Summary

During the past year a systematic investigation of heavy unstable particles has been carried out at the University of Padua using two stacks of 40 Ilford G5 nuclear research stripped emulsions each 600µ thick, exposed at balloon altitudes. In Table III is given a summary of the events observed during the first scanning as well as those found during the systematic re-examination. Later a detailed analysis of all unstable particle events was made. For particles whose ranges terminated in the emulsion, the mass values were obtained from scattering measurements applying both the method of «constant sagitta» and a method similar to the «P» method of the Bristol group, as well as the method of «comparative grain densities» based on measurements of specific ionization. The masses of those particles which did not come to the end of their ranges in the emulsion were determined from measurements of multiple scattering and specific ionization. These mass values were based on calibration curves determined for particles of known mass and charge. 12 K _l mesons decaying at rest have been observed (Tab. IV). An average mass value of 985±30 m_e has been obtained and the mass distribution about this mean is consistent with a unique mass value for all K _l particles observed. In four of the events the minimum secondary particles were emitted at very small angles with respect to the emulsion plane. In Fig. 1 is shown apβ vs.g* plot of the four K-meson secondaries and of 20 longπ-meson tracks. It would seem that 3 of the events listed in the table could be satisfactorily explained by the scheme: K ^±_π2 →π ^±+π ⁰+Q in that the 3 secondaries seem to be very probablyπ-mesons, each of the same energy. Using for thepβ of the secondary particle the value of 163±7 MeV/c, and for the primary mass a value of 962±38 m_e and applying the momentum laws, we have calculated the mass of the neutral secondary to be 296±60 m_e. Sixτ-mesons decaying at rest have been observed in this investigation; three of theτ-events are «complete» and were produced by a positively charged primary. The average value for theQ is 75.4±0.4 MeV which corresponds to a mass of 967±0.8 m_e (see table V). In another event a K-particle comes to rest in the emulsion layers and decays into a single positive pion of 6 MeV. This event has also been interpreted as the decay of aτ-meson, in which theτ-decays by an alternative mode into a charged pion and two neutral particles. In addition an event was found during the re-examination of «little stars», which was very probably due to the disintegration in flight of aτ-meson (see photograph 1 and Table VI). Six events attributed to the capture of K-mesons have been observed (see Table VIII). The weighted average of measurements of the masses of the primaries is 1 007±74 m_e; which is very close to that of the K _l . In the search for Λ⁰, 292 negative pions were followed back, four of them being observed to come from stars in which only one other charged particle was involved. Three of these two prong stars (see Table XII) seem to have been produced by the decay in flight of a neutral hyperon according to the scheme: Λ⁰→π ⁻+p+37 MeV. Four decays of charged hyperons have been observed. Two of the «primary» particles come to rest in the emulsion, both decaying into a low energy proton; the energies of these protons being 18.8±0.2 MeV and 18.2±0.4. MeV respectively. In the remaining two events the «primaries» decay in flight; one into a proton and the other into a pion. In another event a particle of near-protonic mass comes to the end of its range and is apparently captured by a nucleus of the emulsion producing a σ_Y-star (see Tables IX, X and XI). Five clear examples of the delayed disintegration of a heavy fragment emitted from a nuclear disintegration have been observed. In one of these events the disintegration of a He-nucleus is accompanied by the emission of a negative pion and has been interpreted as being the result of the mesonic decay of a Λ⁰-particle, whose binding energy has been calculated to be 1.7 MeV. In another event we are dealing with the disintegration of a probable Tritium nucleus, with the emission of a 40.3 MeVπ-meson and a 2.5 MeV He-nucleus. For this event the binding energy of the Λ⁰ is found to be less than 1 MeV. The stars from which K-particles are emitted generally exhibit a rather low multiplicity: we are therefore led to the conclusion that K-meson production at «primary» energies of less than 5 GeV is an important phenomenon. We have no examples of the double emission of heavy unstable particles. By making an analysis of the sources of loss during the scanning, we have tried to estimate the frequencies of the various types of events. These frequencies, together with the proposed observational efficiencies for each class of event, are presented in Table XIV.