P. H. Fowler

XV. Nuclear transmutations produced by cosmic-ray particles of great energy

Summary A new method of measuring the scattering of charged particles in their passage through, photographic emulsions has been developed. It has been employed to determine the mass of charged particles emerging from nuclear “explosions” produced by cosmic radiation. Of the mesons among the “shower” particles, produced in these explosions with momentum less than about 180 MeV./c, it is shown that at least 90 per cent are π-particles; and that less than 5 per cent, and possibly none, are μ-particles, electrons or other particles of small mass. If mesons of greater mass are sometimes created in nuclear interactions with an energy less than 300 MeV., they must be produced with a frequency less than 1 per cent of the protons and less than 5 per cent of the μ-particles. The observations therefore prove that the great majority of the charged particles created in nuclear interactions are π-particles. The greatly increased rate of measurement with the new method and the fact that it allows the determination of th...

XXXVII. Nuclear transmutations produced by cosmic-ray particles of great energy.—Part IV. The distribution in energy, and the secondary interactions of the particles emitted from stars

Summary The measurements of “multiple scattering” and “grain density” discussed in Parts II. and III. of the present series of papers, have now been extended to include a total of 1000 tracks of particles associated with “stars”. The method of measurement and the criteria for the selection of tracks were similar to those described previously (III.). The new material provides additional evidence in support of the view that most of the mesons ejected from nuclear explosions with kinetic energy less than 150 MeV. are π-particles.

LXXIX. Nuclear transmutations produced by cosmic-ray particles of great energy.-Part I. Observations with photographic plates exposed at an altitude of 11,000 feet

Summary In this paper a phenomenological description is given of the results of observations made with “electron-sensitive” emulsions exposed at a height of 11,000 feet. It is shown that approximately half the nuclear disintegrations in which many charged particles are emitted are produced by particles of charge |e|, moving at relativistic velocities. The other half of these “stars” must be attributed to a neutral radiation. The directions of motion of most of the “primary” particles are inclined to the vertical at angles less than 40°. The particles appear to be able to pass through nuclei, imparting energy to the nucleons so that the nuclei evaporate, and having a 50 per cent probability of suffering a change of charge |e| in each encounter. If the energy of the incident particle is sufficiently great, “showers” of charged particles are produced. Photo-micrographs are given of characteristic examples of “stars” of different types.

LXVIII. Measurement of ionization in nuclear emulsions

Summary The gap-length distribution along the tracks of ionizing particles in nuclear emulsions has been investigated, and shown to be exponential for all values of specific ionization. The coefficient of the exponential, g, is suggested as the most useful measure of the ionization. In the first part of the paper, practical visual methods of measuring g are discussed. Accuracies not far inferior to the maximum possible are obtained by counting the small proportion of gaps of lengths exceeding two or three times the average value. The ratio of the coefficient g in a given track, to that in the track of a relativistic electron, is shown to be independent of degree of development, for particles below six times plateau ionization. In the seoond part of the paper, a detailed model is proposed, which is capable of explaining the dependence of g on the velocity and charge of the ionizing particle, and on the development and sensitivity of the emulsion.

CXXIV. Nuclear transmutations produced by cosmic-ray particles of great energy.—Part VI. Experimental results on meson production

Summary An analysis has been made of the secondary particles ejected from nuclear disintegrations observed in electron sensitive emulsions exposed to the cosmic radiation at 68,000 ft. Scattering and grain density measure-ments have been carried out on the tracks of 2000 particles associated with these stars. In addition, the grain density and angular distribution of 3070 shower particles and 1508 “grey” tracks have been measured. For 200 stars, the energy of the primary particle which produced the disintegration was measured. A detailed analysis was made of such events. Single fast τ-mesons of kinetic energy less than 1 BeV. are found to interact strongly with nuclear matter. An estimate of the frequency of occurrence of neutral mesons was made from a consideration of the energy balance in stars of low multiplicity, n s .

Observations With Electron-Sensitive Plates Exposed to Cosmic Radiation

WE have recently made observations with the new Kodak nuclearresearch emulsions, described by Dr. R. W. Berriman1, which record the tracks of particles of charge e, even at minimum ionization. The new plate represent a technical advance of great importance ; they allow us to obtain a much deeper insight into nuclear processes than was possible withytfhe older emulsions, and they greatly extend the field of application of the photographic method to physical and biological problems.

The flux of υ- rays at high altitudes, and comparison with the muon flux at sea-level

SummaryThe flux of υ-rays and/or electrons of energies (360 ÷ ÷ 5000) GeV has been investigated at atmospheric depths of between 8 gm/cm2 and 450 gm/cm2. The flux of primary υ-rays and electrons ofE > 470 GeV is found to be≤10-3 of the flux of nucleons above the same energy. The measured energy spectrum of muons at sea-level has been compared with that deduced from observations on the y-rays. There is good agreement between the two estimates of flux if directlyproduced pions are the sole source of both υ-rays and muons. An upper limit is placed on the ratio of numbers of K-mesons and pions produced in high-energy nuclear collisions. Even at the highest energies investigated (50000 GeV primary energy), the energy going into creation of K-mesons in shown to be much less than that of pions.RiassuntoSi è analizzato il flusso di raggi υ e/o di elettroni di energie (3605 000) GeV a profondità atmosferiche comprese fra 8 g/cm2 e 460 gkcm2. Si è trovato che il flusso di raggi υ primari ed elettroni conE > 470 è ≤ 10-3 volte il flusso dei nucleoni con energie maggiori di quella. Si è confrontato lo spettro energetico misurato dei muoni al livello del mare con quello dedotto dalle osservazioni dei raggi υ C’è un buon accordo fra le due stime del flusso se i pioni prodotti direttamente sono la sola sorgente sia dei raggi υ che dei muoni. Si pone un limite superiore al rapporto fra i numeri di mesoni K e pioni prodotti in collisioni nucleari di alta energia. Si mostra che, anche aile più alte energie studiate (50 000 GeV di energia primaria), 1’energia impiegata nella creazione dei mesoni K è molto inferiore a quella dei pioni.

SPURIOUS SCATTERING IN NUCLEAR EMULSIONS

SummarySpurious scattering has been investigated in three stacks of stripped emulsions; one of these has been exposed to the 4.5 GeV π-meson beam at Berkeley, the other two to cosmic radiation. Multiple scattering measurements performed on flat tracks of high energy particles show that the spurious scattering in two of these stacks is appreciably smaller than that obtained byBiswaset al., Lohrmann andTeucher, andFay and that reliable measurements up to 5 GeV (for singly charged particles) can be made using a basic cell size of one mm. Clearly spurious scattering is a function of the quality of the stack. A procedure for eliminating spurious scattering is suggested which in particular cases permits measurements to be extended to higher energy regions.RiassuntoLo scattering spurio è stato esaminato in tre pacchi di emulsioni pelate; uno di questi è stato esposto al fascio di mesoni π di 4.5 GeV di Berkeley, gli altri due alla radiazione cosmica. Misure di scattering multiplo eseguite su traiettorie piane di particelle di alta energia mostrano che lo scattering spurio in due di questi pacchi è considerevolmente minore di quello ottenuto daBiswaset al., Lohrmann eTeuchee, eFay, e che si possono eseguire misure attendibili flno a 5 GeV (per particelle a carica singola) servendosi di una cella base di 1 mm. Lo scattering spurio è chiaramente funzione della qualità del pacco. Si propone un procedimento atto ad eliminare lo scattering spurio che in casi particolari permette di estendere le misure a energie superiori.

LXXI. The abundance of lithium, beryllium, and boron in the primary cosmic radiation

Summary In a previous paper, a method of determining the charge, of heavy nuclei in the cosmic radiation was described which depended on measuring the δ-ray density along a track in a photographic emulsion and the multiple scattering of the particle. The results suggested the presence of a large proportion of the light elements lithium, beryllium and boron in the radiation, a result in contradiction with that of other workers. The first part of the present paper describes a re-examination of the original method and evidence for its reliability. It is shown that an alternative method, based on observations of grain-density in the tracks of heavy particles recorded in less sensitive emulsions, gives results in agreement with those obtained by the δ-ray method. In the second part, the results of a more extended series of measurements by the, δ-ray method are described. The results indicate that at geo-magnetic latitude 55° N., the flux of lithium, beryllium and boron nuclei in the cosmic radiation is equal t...

XCVIII. Nuclear transmutations produced by cosmic-ray particles of great energy

Summary In Part I. of the present series of papers (Brown et al. 1949), which we shall refer to as (I.), an account has been given of experiments with “electron-sensitive” photographic emulsions exposed to the cosmic radiation on the Jungfraujoch at 11,000 feet. In this second part we describe similar observations made at altitudes up to 130,000 feet by means of free balloons. In analysing the results, it will be convenient to adopt the same nomenclature for the classification of events of different types that was employed in (I.). It is shown that, at ∼75,000 feet, the radiation which leads to the production of “stars” is made up of charged and neutral particles of which the directions of motion are more widely dispersed than at lower depths in the atmosphere; and that high energy processes are relatively more frequent than at 11,000 feet. By making measurements of the deviations in the tracks of fast particles due to Coulomb scattering, it has been possible, in cases favourable for measurement, to give ...