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From Current Literature
Investigation of Nuclear Disintegrations in Cosmic Rays Using Photographic Plates
Recently photographic plates have been successfully used for the investigation of nuclear disintegrations in cosmic rays[^1]. The determination of the mass of a particle is based on measuring the number of silver grains developed per unit length of track. By determining this number for protons of a given energy, one can calibrate the plates, i.e., correlate the specific number of grains with definite specific ionizations. If the track of a particle ends in the emulsion, then both its range and its specific ionization are known[^2]. From its range one determines the energy of the particle, and the specific ionization determines its velocity. Knowledge of these two quantities is sufficient for determining the mass of an unknown particle, on the assumption that its charge is equal to unity. The errors arising from the fading of the image in the interval between the passage of the particle and the development of the plate are comparatively small. This makes it possible to distinguish meson tracks from proton tracks, although individual measurements of the meson mass cannot be considered especially accurate.
As a result of the study of photographic plates exposed at an altitude of 2800 m on the Pic du Midi in the Pyrenees, a number of new interesting phenomena were discovered. Of 65 meson tracks ending in the photographic plates, 40 disappear without forming any other tracks. It may be thought that in this case a meson decay occurs, and the resulting electron with an energy of the order of \(5 \cdot 10^7\) eV leaves no track on the plate. At the end of 15 meson tracks, stars are observed, i.e., nuclear disintegrations with 2 or more tracks; the remaining 10 tracks end with the formation of one ionizing particle. The probability of an accidental coincidence of the end of a meson track (ionization along the meson track increases up to the point where the tracks meet) with the beginning of another track is \(10^{-9}\).
In two cases the meson track ends with the formation of another meson track. The masses of the initial and final mesons are of the same order \((350 \pm 5)\) and \(330 \pm 50\) electron masses. Since both tracks arose simultaneously, the fading of the plate must have affected them to the same extent, and the difference in the masses in any case does not exceed 100 electron masses. If it is assumed that the mass of the initial and final mesons is identical, then the final meson must receive from somewhere its final energy equal to 2 MeV (the kinetic energy of the initial meson is equal to zero). The nuclear reactions
\[ X_Z^A + \mu_- \to X_{Z-2}^A + \mu_+ \quad \text{and} \quad X_Z^A + \mu_+ \to X_{Z+2}^A + \mu_-, \tag{1} \]
where \(X\) is a nucleus of atomic weight \(A\) and atomic number \(Z\), and \(\mu_-\), \(\mu_+\) are negative and positive mesons, proceed not with the release but with the absorption of energy and cannot cause the indicated transformation of one meson
FROM CURRENT LITERATURE
into another. Fission of a silver nucleus upon absorption of a meson according to the scheme
\[ \mathrm{Ag}^{107}_{47}+\mu_- \to X^{Y}_{Z}+V_{45-Z}+\mu_+ \tag{2} \]
is energetically possible; however, its probability must be very small.
There are therefore grounds to suppose that the masses of the initial and final mesons are different \(^{3,4}\). In this case, both the decay of the heavier meson into a lighter one and a light quantum \(^{5}\), and a reaction of type (1), say with a \(\mathrm{C}^{12}_{6}\) nucleus, are possible:
\[ \mathrm{C}^{12}_{6}+\mu_- \to \mathrm{Be}^{12}_{4}+\mu_+ . \tag{3} \]
Such a reaction will occur if the mass difference is of the order of 60.
If the process is a decay, then the energy of the quantum, taking its momentum to be equal and opposite to the momentum of the secondary meson, is 25 MeV. Examination of other plates has shown two cases of meson formation in nuclear disintegrations. The total number of observed disintegrations is 1600; of these, 170 have sufficiently high energy for a meson to be able to form in them. Thus, only in 1 out of 80 nuclear disintegrations is meson formation observed; however, this may be connected with the impossibility of recording, by means of photographic emulsions, mesons with energy \(>5\) MeV. Since, of 1600 nuclear disintegrations, only 15 were due to mesons, one may conclude that the bulk of nuclear disintegrations is caused not by mesons, but by protons and neutrons. A ratio of the same order (1:100) for the number of stars formed by mesons to the total number of stars is obtained by comparing the absorption of mesons in lead \(^{6}\) at an altitude of 4200 m and the number of stars formed in a lead plate of the same thickness at this altitude. The authors attempt to determine the mass of the meson on the basis of the assumption that the disintegration caused by the meson is the disintegration of a nitrogen nucleus according to the scheme \(^{8}\),
\[ \mathrm{N}^{14}_{7}+\mu_- \to 2\mathrm{He}^{4}_{2}+2\mathrm{H}^{1}_{1}+4\mathrm{n}^{1}_{0}. \tag{4} \]
The maximum energy of the neutrons required from considerations of conservation of momentum is then calculated, and for the mass of the meson a value of \(240 \pm 50\) electron masses is found, which agrees with the observation from the number of grains. However, these conclusions, as the authors themselves acknowledge, may prove to be incorrect, since in the disintegration of a bromine or silver nucleus by a meson with the emission of the observed particles, energy of the same order is absorbed, and according to the latest data the disintegration of heavy nuclei is more probable than the disintegration of light ones \(^{9,12}\).
In another paper, Occhialini and Letts \(^{13}\) report on a method for detecting neutrons in cosmic rays using the same photographic plates. It is known that boron, under the action of fast neutrons, disintegrates according to the scheme
\[ \mathrm{B}^{10}_{5}+\mathrm{n}^{1}_{0}\to 2\mathrm{He}^{4}_{2}+\mathrm{H}^{3}_{1}. \]
The authors first irradiated plates coated with boric-acid salts with laboratory neutrons of energy 13.4 MeV. This produced triple forks. Measurement of the energy of the particles in one such fork from their ranges gives 13.8 MeV, which agrees well with the sum of the neutron energy and the energy released in the reaction, equal to 0.4 MeV.
Comparison of the vector sum of the momenta with the neutron momentum also gives a good result. The authors then exposed similar plates, coated with boron salts, at an altitude of 2800 m and found a number of disintegrations closely resembling those observed under laboratory conditions. One of these disintegrations is examined in greater detail.
All three tracks end in the emulsion and give \(26.5 \pm 1.5\) MeV and \(2.9 \pm 0.2\) MeV for the energies of the \(\alpha\)-particles and \(7.4 \pm 0.1\) MeV for the energy of the triton \((\mathrm{H}^3_1)\).
Let us construct the momentum diagram and suppose that the generating particle is a neutron; then we obtain for its energy \(45 \pm 8.5\) MeV, whereas the sum of the kinetic energies of the charged particles is \(36.8 \pm 1.8\) MeV. If, conversely, one calculates from the total momenta and energies of the particles the mass of the generating particle, it proves to be equal to \(1.23 \pm 0.30\), i.e., within the limits of error, equal to the mass of the neutron.
All this gives grounds for considering the identification of the neutron as beyond reproach. The authors believe that the indicated method is applicable up to neutron energies of 100 MeV.
A large number of such disintegrations is therefore direct proof of the presence of an appreciable number of high-energy neutrons in cosmic rays at an altitude of 3 km, and the considerable probability of such a process once again convinces us of the neutron nature of the cosmic-ray component that generates star disintegrations.
References Cited
- Lattes, Muirhead, Occhialini and Powell, Nature, 159, 694 (1947).
- Lattes, Fowler and Cuer, Nature, 159, 301 (1947).
- Hughes, Phys. Rev., 69, 371 (1946).
- Leprince, Ringuet et L’Heritier, Journ. Phys. et Rad., 7, 65 (1946).
- Wentzel, Rev. of Mod. Phys., 9, 4 (1947).
- Hall, Phys. Rev., 66, 320 (1944).
- Powell, Phys. Rev., 6, 385 (1946).
- P. I. Lukirskii, DAN, 54, 219 (1946).
- Fermi, Teller and Weisskopf, Phys. Rev., 71, 314 (1947).
- Wheeler, Phys. Rev., 71, 320 (1947).
- Conversi, Pancini and Piccioni, Phys. Rev., 71, 209 (1947).
- Sigurgeirson and Yamakawa, Phys. Rev., 71, 319 (1947).
- Lattes and Occhialini, Nature, 159, 332 (1947).
P. Nemirovskii