Jan Lacki, Uni Genève
The year 2012 marks the hundredth anniversary of the discovery of cosmic rays by the Austrian Victor Franz Hess. It gives the physics community the opportunity to look back at a century of scientific investigations in a field that offered physics some of its most exciting discoveries, making early research in particle physics possible and offering today a way to extend it beyond energies achievable in our accelerators.
As it happened often in the history of discoveries, that of cosmic rays came as a surprise, uncovering a new realm of physical phenomena way beyond what was initially imagined. It was nonetheless the outcome of a sustained effort following a clear rationale. It originated in the field of atmospheric electricity pioneered mainly by Austrian and German investigators at the turn of the 19th century. It was known since at least the observations of Charles Augustin Coulomb (1736-1806) that charged electroscopes loose spontaneously their charge (1785) but the phenomenon went under close scrutiny only a century later. At Vienna University, Franz Exner (1849-1926) established from the middle of the eighties on a successful tradition of research in Luftelektrizität while in Germany the fundamental achievements and insights of the remarkable tandem formed by the Gymnasium teachers Julius Elster (1854-1920) and Hans Geitel (1855-1923) inspired the work of many local and foreign researchers 1.
At the beginning of the 20th century, the community of investigators in atmospheric electricity included scholars from most (Western) Europe countries and even beyond (Canada). One of the most noteworthy was Albert Gockel (1860-1927) from Freiburg (CH) University 2. The SPS-Communications featured recently a paper on the initial years of the Freiburg Institute of Physics and in particular the opposition of scientific styles between the Institute long-term director Joseph de Kowalski and his assistant and then colleague Gockel 3. Here, I want to take a closer look at Gockel’s life-long interest in atmospheric electricity phenomena, and in particular at his substantial contribution to the discovery of cosmic radiation.
In order to understand Gockel’s achievements one has first to recall what were at the time the research trends and the main issues in the field of atmospheric electricity. After Elster and Geitel concluded that the spontaneous discharge of electrometers was due to the presence of ions in the atmosphere (1900) came the question of the their origin. The recent discovery of radioactivity (1896) and of its ionizing properties on gases led Elster and Geitel to investigate its presence in the air (1901-1902): the radioactive gaseous emanations corresponding to decay products of active minerals they detected made them conclude that it was indeed the primary cause of the conductibility of air. It was the time when one investigated the natural radioactivity of soils, rocks and air: its effects were examined underground, at ground level and in mountain heights, in land and in seas. While one learned more and more about the radioactive substances and their decay products, hypotheses on the location of the sources responsible for atmospheric ionization were getting more precise. When it turned out that the radioactive decay products of the emanations (Radium A, Thorium A and Actinium A) in the atmosphere could hardly account alone for its total ionization, the direct outgoing radiation from the active substances in the Earth crust came to be considered as the next candidate. Contemporary experiments conducted by Canadian teams on ionization of air in sealed vessels showed on the other hand that, in spite of shielding, the enclosed air was still ionized by a very penetrating radiation different from the radiation originating from the vessels walls or nearby artificial or natural formations (1903). For some time it was commonly accepted that the ionizing radiation from substances in the Earth’s crust could explain it all. At the very end of the first decade it was realized however that this radiation, given its decreasing strength with distance, could not account for the ionization of the atmosphere at least in its higher layers. After a series of investigations in situations where the effects of the direct radiation from the ground could be ruled out, it became more and more evident that a new source of radiation of non-ground origin was involved. Most remarkably, instead of decreasing rapidly with altitude, the penetrating radiation was found, after an initial albeit slower than expected fall, to rise again. In 1912, Victor Hess brought finally indisputable evidence that the radiation had to come from outside the atmosphere, hence was genuinely of extraterrestrial origin. His series of celebrated balloon observations in 1911-1912 which enabled him to reach this conclusion were preceded by extensive attempts by some other investigators who, to various degrees, reached similar conclusions. Among the latter Albert Gockel deserves particular attention: with a series of balloon flights almost two years before Hess’, he preceded the Austrian on many key observations. However, due to reasons that will be discussed shortly, it was not given to Gockel to bring forth evidence as strong as Hess’. Lack of material means and support, including from his home institution, ruled him out of the game just when the investigations reached their climax and that first priority claims were issued. Let us have a closer look at Gockel’s research and his early conclusions about a penetrating radiation of possibly non-terrestrial origin.
Atmospheric conductivity and more generally atmospheric electric phenomena were Gockel’s life-long interest. He devoted to them most of his time from his initial research years till the end of his life with the exception of some circumstantial work he did to obtain his academic degrees or fulfill his institutional duties. Right after his Ph. D. in Heidelberg (1885) Gockel went to teach at the Ladenburg Gymnasium where he initiated a systematic study of atmospheric phenomena with special emphasis on thunderstorms 4. In 1896 he was hired as assistant in the newly created Physics Institute of the Freiburg (CH) University. Serving under the directorship of Joseph de Kowalski, the applied research oriented almighty head of the Institute, Gockel had certainly less time for his interests because of the work for his habilitation 5. However, according to his list of publications 6, he managed somehow to still devote time to his favorite investigations following closely the latest developments in atmospheric electricity and related phenomena. Gockel took early an active part in the most advanced research over the ionization of air and natural radioactivity. It was not easy given the adverse institutional conditions that he had to confront because of his non-compliance with the strong local preference for applied research dictated by Kowalski. In spite of his apparently successful career (he became extraordinary professor in 1903 and ordinary in 1910) Gockel worked practically without help and with a severely limited research budget 7. In spite of this, he managed nonetheless to produce original contributions extending and sometimes going beyond those of the best scholars in the field. Indeed, in the years when cosmic rays were finally discovered, Gockel was one of the most accomplished and among the top Luftelektrizität researchers and his contributions, often published in the Physikalische Zeitschrift which was then hosting the best publications in the field, were widely known and systematically quoted.
Restricted financial means could seriously hinder one’s research as, besides the expenses occasioned by field research at various geographical locations, the techniques and instruments used were sometimes of considerable sophistication. Today one remembers mostly the ionization cavities sitting on top of electrometers, instrumental in the study of the ionizing radiation, but Gockel used, as many others, a broader range of techniques and devices, many resulting from the pioneering attempts of Elster and Geitel 8 and perfected by their followers. To study the distribution of radioactive substances, Gockel used the capture of the positive active ions over extended negatively charged suspended wires: the activity of the deposit was estimated on the basis of the rate of discharge of special electrometers. To investigate the ion density of air, Gockel measured the intensity of the ion current between the plates of condensers: to secure a regular intake of ions, the latter were aspired into the condenser by engine or hand driven air pumps. His investigations of the intensity of the ionizing radiation were finally based on the rate of ion production in ionization cavities containing sealed air: here the issue was to secure as small electrical capacity as possible to make minute charge variations due to ionization as detectable as possible. All of these techniques required often electric batteries as sources of electric potential and, at the final stage of the experimental setups, very sensitive electrometers: Gockel followed the trend from the first electrometers of Elster and Geitel to the much improved devices designed on purpose by Theodor Wulf (1868-1946) for the study of the penetrating radiation (1909). Given the experimental setups involved, one can then better appreciate the challenge that air-borne balloon experimentation represented then: one had to solve problems of restricted space, varying pressure and of large temperature swings.
The dedication of Gockel to his research made him use any opportunity to extend and improve his observations. Gockel’s experiments were geographically broadly distributed, not only over a substantial part of Europe, but they covered also Turkey and some countries of North Africa 9. He did his investigations underground in caves and tunnels (Simplon), in the mountains and glaciers of the Alps (Briener Rothorn, Zermatt 10, Jungfraujoch), in land and water, in lakes (Bodensee) and seas (Mediterranean), etc. But his most important results were obtained studying the ionizing radiation at high altitudes using balloon flights, among the very first to be done with this purpose 11. Gockel’s first flight took place in December 11, 1909, in a time when the hypothesis of ionizing radiation coming from Earth was largely undisputed. The material conditions of the this flight are perfectly illustrative of Gockel’s limited research means: he benefited from the generosity of the East section of the Swiss Aeroclub that funded the flight of its balloon, the Gotthard, on the occasion of the International Balloon Week. Besides the pilot, another scholar, Alfred de Quervain, then research associate at the ETH and associate director of the Schweizerische Meteorologische Centralanstalt, took part in the flight. De Quervain, himself a balloon pilot, was familiar with the flight activities of the Aeroclub and, as we shall see, proved instrumental in making Gockel’s further flights possible 12.
Filled with coal-gas, the Gotthard reached a final altitude of 4500 meters before landing after more than four hours of flight. Gockel did measures of ion density but what caused real surprise were his results on the variation of the ionizing radiation with altitude obtained using the standard setup made of Wulf’s electrometer coupled to a ionization chamber. Gockel observed initially a decrease in accordance with the hypothesis of radiation originating from ground, but its rate with altitude was much smaller than expected; at higher altitudes Gockel even recorded back an increase. The main conclusion of the account Gockel published some months later in the Physikalische Zeitschrift 13, was accordingly that:
Das Resultat der Messungen ist demnach, dass in der freien Atmosphäre zwar eine Verminderung der durchdringenden Strahlung eintritt, aber lange nicht in dem Masse, wie man es erwarten könnte, wenn die Strahlung in der Hauptsache vom Boden ausgeht 14.