Interaction between Cosmic Rays and Matter

B. Rossi

Editor’s Note

Italian physicist Bruno Rossi had recently shown that cosmic rays passing through matter often generate secondary particles. He had developed a sensitive technique to investigate the secondary “particle showers” by detecting the near-simultaneous passage of particles at three different detectors. Here he reports that showers seems to be more abundant in materials of higher atomic number, and that the particles penetrate lead for shorter distances than primary cosmic rays. Rossi’s technique spurred the further development of “coincidence-detection” devices for high-energy and nuclear physics. Rossi himself later showed that cosmic rays near the Earth’s surface have two components. The first component was ultimately identified as electrons and photons, the second with a new type of particle called the muon.ft　　中文

LAST year I showed by means of a coincidence method that a secondary corpuscular radiation is generated when cosmic rays pass through matter1. From the beautiful experiments of Blackett and Occhialini we know now that these secondary particles are produced in so-called “showers”. This can also be shown by the method of coincidences; moreover, if a coincidence is observed between counters arranged in a triangle, we may conclude with a high degree of certainty that we have to do with a shower originating from a point in the neighbourhood of the counters2. The method of triple coincidences therefore offers a very useful means for investigating the frequency of occurrence of the showers.ft　　中文

Up to the present, the following results have been obtained:

(1) The showers occur more frequently in elements of high atomic number; the ratio of the numbers of coincidences caused by thin layers of lead, iron, aluminium of the same weight per cm.2 is 4:2:1 approximately3.

(2) The number of showers emerging from a layer of lead, as a function of the thickness of this layer, increases at first, reaches a maximum at a thickness of about 20 gm./cm.2 and then decreases very rapidly; at 100 gm./cm.2, for example, the frequency of the coincidences is less than one half of the maximum value. We conclude that the radiation which causes the showers has a mean range of a few centimetres in lead. It follows that this radiation cannot be identical with the primary cosmic rays.

(3) When the thickness of the layer is further increased, the frequency of the emerging showers decreases very slowly. The most probable hypothesis to explain this seems to be to assume that a further production of the rays which cause the showers takes place in the layer; these rays are therefore to be regarded as a secondary radiation of the primary cosmic rays, the equilibrium value of which is roughly three to four times greater in air than in lead.

(4) The shower-producing rays are more readily absorbed by elements of higher atomic number. When 24.5 gm./cm.2 of lead is placed over the counters, 70±3 coincidences per hour are observed; this number was reduced to 36.7±1.4 by a further sheet of lead of 39 gm./cm.2 on top of the first one but only to 52.3±1.7 by a sheet of aluminium of the same weight per cm.2 and in the same position. From this and from (1) we conclude that the absorption of these secondary rays by an element and the number of showers which they produce depend in the same way on the atomic number. Thus it seems that the production of showers must be the main reason for their absorption. This is in agreement with the consideration that these rays should have an energy of at least some milliards of electron-volts, which could not be absorbed by a few centimetres of lead in the ordinary way.

(5) That the equilibrium value of the secondary radiation is lower in elements of high atomic number may be explained by their greater absorption, if we assume that the rate of production is roughly the same in all elements; which seems plausible from the experiments on the absorption of the primary rays.ft　　中文

(132, 173-174; 1933)

Bruno Rossi: Physical Institute, University of Padova, Italy, July 3.

References:

  1. Rossi, B., Phys. Z., 33, 304 (1932).

  2. Rossi, B., Atti. R. Acad. Naz. Lincei (in the press).

  3. Rossi, B., Z. Phys., 82, 151 (1933).