STUDY OF THE ALBEDO OF THE EARTH’S ATMOSPHERE FOR COSMIC RADIATION USING THE CHERENKOV EFFECT

A noticeable fraction of cosmic radiation in the upper layers of the atmosphere is directed from below upward. This reflected cosmic radiation arises from the fact that particles generated in nuclear disintegrations fly out in all directions, and some of them move in directions opposite to the direction of the main flux of primary radiation in the stratosphere. The existence of such an “albedo” of the Earth’s atmosphere for cosmic radiation has a strong effect on the results of experiments carried out in the stratosphere.

It is known, for example, that in the stratosphere there is a considerable east–west effect of cosmic radiation[^1]. However, the observed magnitude of this effect is much smaller than that predicted by theory[^2]. The discrepancy is especially large at a geomagnetic latitude close to 40°, at a zenith angle of about 60°. In this case, a fourfold discrepancy between experiment and theory is observed. It is quite possible that such a strong reduction of the expected east–west asymmetry is caused by the presence of a reverse flux of secondary particles, which greatly increases the weak intensity in the eastern direction and has little effect on the large intensity of particles arriving from the west. The papers reviewed[^3][^4] are devoted to a preliminary study of this problem. In paper 3 the experiment was carried out with the aid of a telescope of counters (Fig. 1), inclined at an angle of 60° to the vertical. The telescope was lifted by balloons to an altitude of 25 km (pressure 27 g/cm²). Triple coincidences 1—2—3 measured the intensity of single particles, while quadruple coincidences 1—2—3—4 measured the intensity of showers formed in lead absorber g. Comparing the intensities of 1—2—3 and 1—2—3—4 for the positions A and B of the lead g, one can try to determine the influence of the reverse flux of particles absorbed in g on the intensity recorded by the telescope. It turned out that the intensity 1—2—3 is almost independent of the position of the lead. From this, however, it still cannot be concluded that the albedo is small: although the thickness of the lead g is sufficient to remove a considerable part of the reverse flux of particles, the lead itself, irradiated by the nuclear component of cosmic radiation, is a source of secondary particles, some of which are directed upward. The author therefore concludes that such an experiment does not make it possible to determine the albedo for particles absorbed by 17 cm of lead.

Fig. 1.

In the second paper[^4] he estimates the albedo of cosmic radiation, using for this purpose the Cherenkov radiation of the charged component of cosmic radiation.

This method had not previously been applied and, in its possibilities, is of considerable interest. The layout of the apparatus, raised into the stratosphere (pressure 17 g/cm²) by balloons, is shown in Fig. 2. A telescope of counters registered charged particles. Between the counters of the telescope a block of transparent organic glass was placed.

Cherenkov radiation, emitted at a small angle \(\vartheta\) to the direction of motion of the charged particle \(\left(\cos \vartheta = \frac{1}{\beta n},\right.\) where \(n\) is the refractive index of glass, \(\left.\beta = \frac{v}{c}\right)\), undergoes internal reflection at the boundaries of the glass block and was detected by a photomultiplier switched on in coincidence with the telescope.

Fig. 2.

At sea level, such a system records all charged particles passing through the telescope with an efficiency of 94%. If the whole instrument is turned through \(180^\circ\) about the axis \(AA'\), then only a small fraction of the light scattered in the opposite direction will reach the photomultiplier. In fact, the efficiency of the instrument then decreased to 21%. It is evident that, with the aid of such an instrument, one can detect primarily, in the flux of cosmic rays, the admixture of particles moving in the opposite direction. The author found that, at an altitude corresponding to a pressure of \(17\ \mathrm{g/cm^2}\), the albedo for relativistic particles is about 7%. It is obvious that the described instrument is still far from perfect. Nevertheless, it is an interesting example of the use of Cherenkov radiation for investigations in the field of cosmic rays.

A. B.

CITED LITERATURE

1. D. V. Skobel’tsyn, UFN 41, 331 (1950).
2. Winckler, Stix, Dwight and Sabin, Phys. Rev. 79, 656 (1950).
3. Winckler, Phys. Rev. 85, 1053 (1952).
4. Winckler, Phys. Rev. 85, 1054 (1952).