Hillas / Robinson / Athelstan Spilhaus Cosmic Rays

The Commonwealth and International Library: Selected Readings in Physics
1. Auflage 2013
ISBN: 978-1-4831-5192-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 308 Seiten

Cosmic Rays is a two-part book that first elucidates the discovery, nature, and particles produced by cosmic rays. This part also looks into the primary cosmic radiation; radio waves from the galaxy; extensive air showers; origin of cosmic rays; and other cosmic radiations. Part 2 consists of reprinted papers involving cosmic rays. Papers 1 to 10 treat the nature of the radiation, arranged chronologically; in Papers 11 to 16 the scene moves away from the Earth.

Autoren/Hrsg.

Hillas, A. M.

Robinson, Robert

Athelstan Spilhaus, Dean

Ter Haar, D.

Leseproben

I

Discovery

Publisher Summary

This chapter reviews the discovery of cosmic rays. Elster and Geitel in Berlin had pioneered the study of electrical conduction in the open air and had deduced that this resulted from the presence of positive and negative ions in the air. Attempts to interpret variations of the conductivity with geographical location, altitude, time, and atmospheric conditions were complicated by the fact that it depended not only on the concentration of ions but also on their variable mobility. However, Geitel and C.T.R. Wilson found that some ionization was present even in closed vessels containing air. When much of the ionization had been traced to radioactive impurities and deposits, but the residual amount was uncertain, Hess in Vienna and Kolhörster in Berlin undertook the preparation of very remarkable ascents in open balloons to find how high the radiation penetrated. The chapter also discusses a few experiments that were carried out by Millikan and Cameron that involved measurements of the ionization produced in electrometers lowered into snow-fed lakes in California, which were thought to be free from radioactive contamination.

THE history of the investigation of cosmic rays is remarkable for the variety of the new phenomena revealed or illuminated, so one is prepared for some original turn of events whenever a new method or domain of observation is opened up. It is of interest to see this happening as we pass through the eras of Geiger–Müller counters, cloud chambers, emulsions, radio-telescopes, and so on, and as the site of the observations is extended. But first, we see what can be done with an electroscope.

1.1 The penetrating radiation

Around 1900 the phenomena of ionization and electrical conduction in gases were under investigation in several laboratories; the electron was a new discovery, the phenomena of radioactivity were becoming established, though without a fundamental understanding in terms of atomic nuclei, and the effectiveness of certain radiations in producing ionization in air was known.

Elster and Geitel in Berlin had pioneered the study of electrical conduction in the open air and had deduced that this resulted from the presence of positive and negative ions in the air. Attempts to interpret variations of the conductivity with geographical location, altitude, time and atmospheric conditions, and so to trace its source, were complicated by the fact that it depended not only on the concentration of ions but also on their variable mobility. This bedevilled many experiments for the next decade. However, Geitel and C. T. R. Wilson, also interested in atmospheric electricity, found that some ionization was present even in closed vessels containing air, even in the dark, Wilson concluding that 20 ion pairs must be formed per cm3 per second. These conclusions were drawn from observations of the rate at which a charged gold leaf electroscope lost its charge even when protected against surface leakage. Wilson’s account continues (Wilson, 1901):

“The experiments with this apparatus were carried out at Peebles. The mean rate of leak when the apparatus was in an ordinary room amounted to 6·6 divisions of the micrometer scale per hour. An experiment made in the Caledonian Railway tunnel near Peebles (at night after the traffic had ceased) gave a leakage of 7·0 divisions per hour, the fall of potential amounting to 14 scale divisions in the two hours for which the experiment lasted. The difference is well within the range of experimental errors. There is thus no evidence of any falling off of the rate of production of ions in the vessel, although there were many feet of solid rock overhead.

“It is unlikely, therefore, that the ionization is due to radiation which has traversed our atmosphere; it seems to be, as Geitel concludes, a property of air itself.”

A worthy attempt—but it turned out (Elster and Geitel) that air in cellars and caves was especially conducting due to traces of radioactive emanation constantly seeping from the ground. It was clear that radioactive contamination played an important part in these observations. Perhaps all matter was to some extent radioactive.

Rutherford, then working in Montreal, himself observed the residual ionization in electroscopes, and found that some could be cut out by a lead shield (Rutherford and Cooke, 1903) indicating that part was due to ?-rays from the walls of his building, but inside a 5-ton mass of lead there remained 6 ions cm-3 s-1. As late as the first edition of his book on radioactive substances (Rutherford, 1913) he concludes, however, that “the very weak activity actually observed is in all probability to be ascribed to the presence of traces of radioactive matter as impurities” —in the walls of the vessel. It may seem that Rutherford missed the mark, but the conclusion appears to be essentially correct, in so far as it refers to observations near sea level. Most of the effects observed were not due to an external radiation (which, it now appears, contributes one-quarter of Rutherford’s residual ionization). It seems fortunate that most experiments around this time showed sufficient inaccuracy and unrepeatable results to stimulate further investigation over varying conditions and especially at higher altitudes. One observation in particular which spurred the search for an external radiation was a considerable diurnal variation in the ionization observed by several workers, pointing to the possibility of an extra-terrestrial source. However, the results were not reproducible: the variation was affected by atmospheric conditions and precipitation, and was much reduced as observing methods improved.

It should be remembered that the known ionizing agents arising from radioactive materials were a-particles, which leave a very heavy trail of ions along their tracks, and are stopped by ~ 5 cm of air or a sheet of paper, ß-particles, which expend their energy in ionization more gradually, and ?-rays, much the most penetrating, which would be cut down by a factor 10 in intensity by a few centimetres of lead, their ionization resulting only from secondary fast electrons ejected from materials they traverse.

To make more accurate measurements of the small amount of ionization Wulf developed a very sensitive electrometer in which the main part consisted of two quartz fibres fastened together at their ends, made slightly conducting, and carrying a small weight. When charged, the fibres bowed apart, and the very small electrical capacitance made the instrument sensitive to very small charges; and patient investigation showed that careful cleaning of zinc vessels gave less radioactive contamination and lower ionization rates.

This was the instrument used by most workers. It was possible to detect changes in ionization of a few ions cm-3 s-1.

The ionization fell considerably over lake water or over a glacier (the experiments gave considerable scope for travel). As both should be relatively free from radioactive contamination, it became clear that much of the ionization arose from ?-rays emitted from radioactive trace elements in the earth, and quantitative estimates did not seem unreasonable. Although 3 ions cm-3 s-1 remained over a glacier, and all estimates showed that the ionization to be expected from emanation escaping into the air should be at most one-tenth of the direct effect of ?-rays from the earth, it was hard to be certain that deposits did not form on the apparatus. However, Gockel did not find evidence of any decrease in the effect with altitude above a glacier. Experiments on towers showed a partial reduction as expected, but were inconclusive: in 1910 Wulf found that the ionization fell from 6 to 3·5 as he ascended the Eiffel Tower (330 m), whereas ?-rays should be halved in 80 m of air.

1.2 Manned balloon ascents

At this point, when much of the ionization had been traced to radioactive impurities and deposits, but the residual amount was uncertain, Hess in Vienna and Kolhörster in Berlin undertook the preparation of very remarkable ascents in open balloons to find how high the radiation penetrated.

Hess made two ascents to 1000 m in 1912, and after finding no drop in ionization realized that there must be a different cause. He then checked that at least up to 90 m from 1 g of radium C the ?-rays had an absorption coefficient of 4·5 × 10-5 cm-1 in air. He also analysed carefully the sources of radioactive materials in the air, and found that they could account for at most one-twentieth of the ionization seen between 1 and 2 km up (Hess, 1913).

In September 1912 Hess reported at a meeting in Münster the results of seven balloon flights, now reaching over 5 km, in which he found that after an initial reduction as expected, the intensity of ionization became very much greater with altitude. He concluded that there was a very penetrating radiation coming through the atmosphere from above. This remarkable observation went almost unrecorded in the British scientific literature. Extracts from Hess’s report are reprinted in Part 2 (Paper 1).

Kolhörster checked the effects which low temperature would have on the Wulf electrometers, and then in 1913 and 1914 made his dangerous ascents to 6 km and then 9 km,...