The early times of cosmic-ray studies have been partly examined in different books that are mainly concerned with either (the history of) particle physics or astrophysics. Due to a lack of more specific literature about the different stages of cosmic-ray studies or even astroparticle physics, the following paragraphs will give a brief survey of the disparate parts that form the current state of research on the topic. Of course, there is a huge amount of material from a scientific point of view, but it will be not listed here, as this would exceed the idea of a brief survey of the historical or at least historically-oriented literature by far.
As already indicated above, there are numerous aspects of (early) cosmic-ray research that have been examined so far, yet they form no uniform approach to the topic. Rather, those individual parts are dispersed over not only different works, but also over different disciplines. There are historical papers on technical aspects of particle physics, next to autobiographical notes from famous physicists involved in the field of cosmic rays. There are also physics textbooks that touch on some historical aspects and even philosophical works that take examples from cosmic-ray studies in order to illustrate problems of the philosophy of physics. Yet the problem remains that these books either treat cosmic-ray studies or astroparticle physics as one example, while the thematic emphasis lies on some different point or they have a mere scientific standpoint, sometimes lightened up by a few biographical anecdotes. But all this cannot substitute methodical historical work on the development of astroparticle physics. One of the major questions such a historical analysis would have to answer would be how the different fields developed and how they came to influence particle physics, astrophysics and of course astroparticle physics. The following paragraph will try to piece together the various smaller parts that have been written so far, in order to create a more profound overview of the state of research, no matter how narrow it might be. Maybe the one or the other contribution will be omitted, yet the overall image of a puzzle being by no means complete yet, will become clear.
One of the first to deal with a chronology of cosmic-ray studies is a paper by Ginzburg from 1967 . Giving a short overview of the major developments in the field of cosmic-ray studies, he stresses the importance cosmic-ray studies had and have on particle physics and astrophysics. A few years later, in 1972, Hillas’  work was to become one of the first authoritative books on the topic. Still, this survey of the progress of cosmic-ray research from Hess on to the discovery of new particles and their properties to the x-ray sources in our galaxy, which was one of the fundamental questions at that time, is mainly a scientific textbook with little emphasis on historical questions. The 1970s witnessed other such textbooks and sourcebooks that also touched on problems of cosmic-ray physics, like the comprehensive one by Gingrich and Lang  from 1979, that, though being concerned with astrophysics and astronomy, also cites many publications about cosmic rays.
In the 1980s, works that also mentioned aspects of cosmic-ray studies had a more articulate historical ambition. One of the first being Segrè’s  book, which deals with the most important milestones in the development of particle physics and the most prominent physicists involved; he gives a first broad view of the incidents of the years 1932 and 1936, which were decisive for the postulation and discovery of previously unknown elementary particles. The works edited by Brown and Hoddeson in 1983 and 1989 [34, 33] also stick to the field of the history of particle physics, cosmic rays being just mentioned as a sort of “toolbox”. Interestingly they clearly stress the scientific content, rather than the historical. In 1985 the book “Early History of Cosmic Ray Studies”  was the first and so far only one that intended to give a complete historical survey of the development of this field. This book covers a broad range of topics in time, as well as in scientific detail, though the majority of the articles in it have a clearly autobiographical viewpoint.
Beginning with the earliest history of the confirmation of the existence of cosmic rays, mainly the work on the different phenomena of radioactivity and electricity, which led to the discovery of the residual ionization effect , the book concentrates very much on the various contributions of cosmic-ray research to early particle physics. The first photographic evidence for the particle nature of cosmic rays in the form of a particle track in a cloud chamber, the postulation and discovery of new particles and the successful work on the properties of such particles [200, 15, 3, 53, 172] are just a few examples of the different stages of development covered. Another section is dedicated to the improvement of technical and experimental equipment, as in the case of measuring devices for unmanned balloon flights . These had to be optimized in order to reach the stratosphere with the balloon-borne experiments and laid the foundation for satellite-borne experiments. Though a rather complete overview of early cosmic-ray studies, the book also has its shortcomings. Primarily that it is mainly based on “personal reminiscences”, as the title indicates . Without discussing the problems of objectivity in historiography at this point, it seems to be quite obvious that a collection of autobiographical articles by the physicists that have been involved in most of the events being described is less objective than an analysis of the facts by historians of science. Another critical point is the strong emphasis on the major scientific events, that neglects to a certain degree the interrelations between the various stages of development. Besides, as the work was edited a few years before modern astroparticle physics was established , “Early History of Cosmic Ray Studies”  often conveys the feeling that the era of cosmic-ray studies is finally over, an impression enhanced by the biographical air of the articles.
Also written from a biographical point of view, Pais’ 1986 work “Inward Bound” , which deals with the history of particle physics, examines the role early cosmic-ray studies played and even takes into account the preceding fields of study, like the research on radioactivity and electricity. Lanius’ “Mikrokosmos, Makrokosmos”  is a textbook with a less scholarly tone, which not only gives a brief history of particle and astrophysics, touching on the topic of cosmic rays by and than, but also tries to disclose the links between the two. Cahn and Goldhaber in 1989  chose an approach similar to Hillas and Gingrich. Starting with the scientific publications as a source, they examine the development of the research of the phenomena of extraterrestrial radiation and its meaning for the progress of particle physics. In the 1990s, it was again Ginzburg  who brought the topic of early cosmic-ray studies back into discussion, giving a review of the topic as an introduction to conference proceedings. A little later Fraser edited “The Particle Century” , a history of particle physics that not only examines the importance of cosmic-ray research for the detection of new particles, but also casts an eye on the contexts of discovery and the technical improvement in the field of particle detection, as well as the role that accelerators have played in the dominance of particle physics after World War II. Unfortunately, the book goes into deeper scrutiny only when it comes to scientific questions, but not in the case of historical ones. The fact that historical analysis is needed is even expressed by physicists. Stanev emphasizes in his book “High Energy Cosmic Rays” , a summary of the current state of research in cosmic-ray studies, the importance of the history of cosmic-ray studies, when it comes to the question of definition: “A better definition than an outline of its history and its ever-changing priorities is hardly possible.” (, page 9)
Another book that gives a profound, though incomplete, survey of the discovery of cosmic rays, as well as the different scientific aspects that make up today’s astroparticle physics is Longair’s history of cosmology and astrophysics  from 2006. Though it is just one chapter in the book, the interesting point lies in the fact that here the connection between cosmic-ray studies and its neighboring fields is discussed.
General works on the history of physics, like the one by Schreier  or the very broad one by Simonyi  about the cultural history of physics, often neglect cosmic-ray studies, not to mention astroparticle physics, as a rather young field of study. Sometimes it is even reduced to the short mentioning of Hess’ discovery of cosmic rays as the basis for particle physics. The same holds true for those works that go deeply into the social or structural contexts of scientific research in the early 20th century, like the edition by Krige and Pestre  from 1997. Its articles touch on various aspects of nuclear and particle physics, still it does not deal with cosmic-ray studies or astroparticle physics. One should not forget to mention that there are a number of biographies that have been written about physicists involved in cosmic-ray studies [19, 119]. But as they deal mainly with one outstanding personality, they are not very fruitful sources for research on historical developments in cosmic-ray studies in more general terms. The biography of Millikan is to some degree an exception, as it does also try to have a look at the institutional frame of his work, i.e. the role Millikan played in the establishing of the Caltech Institute.
Of course there have also been written many different articles and books concerning the history of (experimental) physics, but to cite all of them would lead too far astray from the contents of this article. But one should mention that there are a number of works that have linked aspects of history and philosophy of science successfully . One example on which one might model future work about the history of astroparticle physics, Hentschel  has shown for astrophysics, more precisely the research on the phenomenon of the solar redshift, how rich such a topic can be when examined from an historical and philosophical view at the same time. There are also various other books and articles that analyze cosmic-ray studies and their role concerning early particle physics. Especially more profound historical studies on the discovery of electricity or the early times of nuclear physics and there meaning for the upcoming field of particle physics, like the collection on the history of the electron, edited by Buchwald  in 2001 or Dahl’s work on cathode rays , do merely touch on the topic of cosmic rays. One could go on mentioning those works that go into details about the philosophical implications of early particle physics and the role cosmic-ray studies played in its development , but the depth of some of those philosophical analyses seem only to emphasize the need for more fundamental historical research.
Working with cathode rays, Röntgen observed x-rays in 1895. He made his results accessible to the public shortly thereafter, putting special emphasis on the medical usage of his discovery. In a remarkably short time a number of further research projects involving x-rays – outside the field of medicine – were launched: For example, Becquerel, inspired by Röntgen’s findings, discovered radioactivity just one year later. And thus the founding stone had been laid for more and deeper experiments into the nature of radioactive materials , though of course we know today that there are other sources of x-rays as well.
Almost simultaneously the concept of the electron had been experimentally confirmed by Thomson [49, 136, 36]. The term itself being as old as the Greek philosophers’ study of the phenomena of natural electricity, in that case static charging, it had become a focus of interest for 19th century physicists. Even though x-rays were discovered in the course of investigating cathode rays , the phenomenon of electricity was to become an important scientific matter of its own. Yet, on the other hand, the question of the nature of the electron is of course strongly linked with the key questions that were later to become quantum mechanics – how do we describe properly the elementary components of matter – as well as those of particle physics. Research from the different fields of physics was very often all mingled into one, often depending on the biographical background of the physicists themselves. While the Curies were very determined to follow one line of research, working in a laboratory of their own , others, like Elster and Geitel, were still focusing their work on numerous phenomena from radioactivity to physics of the atmosphere to discharge in enclosed vessels . Today one would maybe praise their approach as interdisciplinary, yet, it makes it more difficult to classify the first steps that later led to the discovery of cosmic rays, according to disciplines or fields. That might be the reason that sometimes the various contributions that made Hess conduct his experiments, are more or less ignored.
From the initial points described above, experimental work led to the problem of ionization of air. Though air is a good isolator, experiments with a gold-leaf electroscope showed that beta-rays and other similar rays are able to ionize air [59, 75, 223]. Nevertheless, the discovery of radioactivity was not the only reason the phenomenon of ionization was being examined. It appears rather as if it was fostered by a field that had been popular at least since the 18th century, the study of discharge in gases. It seems to have formed the counterpart to experiments with vacuum tubes, like those Röntgen was occupied with when making his discovery . The ionization effect, Sekido  suggests the term “dark current”, had been recognized before by Coulomb, when he tried to observe and describe the phenomena of electricity and magnetism. Yet it is doubtful that he had already “discovered” this effect, as Sekido put it, for Coulomb considered the discharge of his instrument that he witnessed to be the measuring error of his set-up .
In 1900 Elster and Geitel [75, 59], as well as Wilson , found out independently from each other that even without a source of ionizing rays like radioactive dust in the sample of air or contamination of the material the electroscope or its casing, some residual effect can still be noticed: Electrified bodies lose their charge when exposed to the air. These findings gave way to investigations into the cause of the phenomenon, aiming at the understanding of the origin of those free ions in air. Very early the possibility of the ions being of cosmic origin was obviously heatedly debated, for in 1909 we find an article that mentions the cosmic aspect as being one alternative out of three. The author suggests that the ions might be coming from the sun. Finally, he turns the idea down in favor of the hypothesis that they are produced in the Earth’s mantle . The article also hints at various experiments with manned balloons, though they were of no significant outcome. The article is not very specific about the nature of the experiments conducted during those flights, but as it mentions experiments on the ground and in water that made use of electroscopes, it might be safe to assume that they were also taken on the flights.
When in 1911 Hess started several balloon flights, taking an electroscope on board, he expected that due to the balloon’s height, the effect of the “penetrating rays”, which he supposed to be a kind of gamma-ray, would be minimized because of the greater distance to probable sources of natural radiation on the ground. Some others, having had the same idea, had already conducted experiments with electroscopes at an even greater height, like Wulf and Gockel , but their findings had not brought about convincing evidence for this hypothesis, nor for its contradiction. So when Hess went on his first balloon experiment, he was looking for the final proof of a lower ionization effect at greater height. But instead he saw that the effect did not decrease significantly. Therefore he decided to carry out further flights to collect more data to analyze the phenomenon properly . So in 1912, Hess set out on several more flights, which astonishingly even showed an increase in the number of pairs of free ions after he had reached a certain height. From that Hess concluded that the penetrating rays might be of cosmic origin, but though a similar flight experiment by Kohlhörster in 1913  at a more extreme height seemed to prove the idea, doubts remained for a couple of years. With the outbreak of World War I research topics had changed their focus away from fundamental research, but after the war, interest turned again towards radiation phenomena, after a book by Nernst in 1921 had proposed the idea that penetrating rays could be understood as emissions from decaying stars . But, against that argument ran the notion to interpret the measured increase of ionization as a problem of electrostatic charging. Nevertheless, evidence grew that the cosmic rays not only existed, but also consisted of (charged) particles. Millikan especially devoted a great part of his work to this problem. By sinking sealed electroscopes into deep waters (a type of experiment that had been conducted by Kohlhörster already in 1913 , see Section 3.1) he found that the number of ions decreased in the water. Besides, he made the observation that at high altitude the number of pairs of ions, which were supposed to be produced by cosmic rays rose. So in 1926, he felt safe to say that he had “quite unambiguous evidence for the existence of very hard ethereal rays of cosmic origin entering the earth uniformly from all directions” . Finally, in 1927, Skobeltzyn happened to photograph a track of cosmic radiation when experimenting with tracks of gamma-rays in a cloud chamber. He recognized that on some of the photos he had taken there were tracks that could not be related in any way to his experimental set-up. He interpreted them as tracks of very fast beta particles relying on a paper by Bohr he had read : Tracks from cosmic rays had been photographed for the first time.
The cloud chamber, invented by Wilson, made the tracks of particles visible by means of condensing water vapor. Those tracks could be photographed and analyzed, which led to the aforementioned discovery of tracks from cosmic rays. Wilson invented the chamber when working on the condensation of super-saturated water vapor shortly before 1900 . Those experiments would later lead him to the discovery of the residual effect, mentioned before. That is most interesting, for it means that the first witnessing of this effect and thus, as a consequence, the first time tracks of cosmic rays – though at that point not yet being recognized as such – had been witnessed, is identical to the invention of the cloud chamber. This coincidence becomes important when considering the interdependence of experiment and theory as philosophers of science do (see Figure 2).
After the chamber was thoroughly revised in 1911 it became an important device for making atoms and their tracks visible . Wilson was awarded one half of the Nobel Prize for Physics in 1927 for the invention of the chamber – the other half being awarded to Compton, for the discovery of the effect named after him. From at least 1920 onwards the use of the chamber seems to have played a most decisive part in the further developments of cosmic-ray physics, especially when the chamber was placed in a magnetic field. For in 1927 this experimental set-up enabled Skobeltzyn  to take the first photographs of cosmic particle tracks in the Wilson cloud chamber. Even after World War II the cloud chamber remained important for detecting new particles, namely for the search for strange particles, also called V-particles, due to their V-shaped tracks, in the 1940s and 1950s . After the detection of the first strange particles by the means of conventional cloud chambers in 1947, two decisive refinements of the method fostered further progress in this field. The first was the penetrating-shower selection. Making use of coincidence-counters (see Section 3.2) and layers of lead, this method helped to tell showers produced in interactions of nuclei apart from more common showers like electron-induced ones . Another device being used was the counter-controlled cloud chamber, which enabled physicists to shut out the background when seeing an event, for the event itself would trigger the chamber. Counter control also provided time information and thus made it possible to observe the decay of particles with neutral current, which other experiments could not separate from nuclear interactions .
One of the most important findings in early cosmic-ray studies was the fact that not all the ionizing particles that had been found were the original – or primary as they are called today – particles of cosmic radiation, but often consisted of their products of decay. When entering the Earth’s atmosphere, cosmic rays interact with its atoms, mainly oxygen and nitrogen. The particles that are thus produced are various mesons, like pions or kaons, neutrons or protons. The charged mesons might then decay again into a number of different particles. For example, produced pions decay like this:. In 1939, he proved his hypothesis that the showers were not only produced locally in the lower atmosphere. By means of measuring long-distance coincidences at high altitude he demonstrated that there are interactions of primary particles in the high atmosphere. For his study of the showers, Auger also used the Wilson chamber, at sea level as well as in high altitude laboratories . The fact that, in a textbook by Geiger published in 1940 this idea of particle showers was already presented as commonly acknowledged by physicists might serve as good evidence for the immediate acceptance of Auger’s findings .
From that basis scientific work focused on the more complex phenomena of cosmic-ray physics. One of the most important aspects was the discovery of new elementary particles, for which cosmic radiation became a rather fruitful field. Though in the early 1930s there were already particle accelerators that did work, it was not until the 1950s that they managed to compete with the success of cosmic-ray studies in detecting new particles [165, 136]. In 1932, however, a great number of particles were discovered. The first half of 1932 saw Chadwick, basing his work on the earlier findings of the Joliots, discover the neutron; deuterium was also discovered. Later that year the positron was discovered via cosmic-ray physics. [165, 192]. Working with a cloud chamber in a magnetic field, Anderson came across strange tracks of particles that could not have been caused by his experimental setup and that resembled those of the electron, but were obviously of positive charge. The new particle was later called a positron. Dirac had already predicted such a particle theoretically, but though Anderson knew this theory, he discovered the particle by mere coincidence, not aiming to find evidence for the theory with his experiment . Several other physicists had already photographed positron tracks, but none of them concluded that these traces were related to the predicted new particle.
The case of the discovery of the positron is a good example for the sometimes complicated relationship of experiment and theory in cosmic-ray studies and later astroparticle physics. The general assumption that the experiment either proves or falsifies the theory or that the theory builds the basis for experimental work seems not to hold true in many cases of cosmic-ray studies. It appears rather as if different experimental and theoretical aspects taken together formed the crucial new information. Though investigations into the role of experiments in the natural sciences is not rare, further work needs to be done concerning the special case of cosmic-ray studies and its successor, astroparticle physics. Undoubtedly the study of cosmic radiation, in experiment and theory alike, can be understood as the basis of high-energy physics (HEP) and particle physics. This point of view is further backed by the fact that publications from the 1940s – like Fermi’s famous article  on the acceleration principle of nuclei in cosmic rays – show the immediate link between these two fields of research. Though scientists were already making use of particle accelerators , the 1940s and 1950s still saw a number of important results to do with new particles and the properties of known particles made by experiments with cosmic rays [183, 170]. The apparati being used were mainly cloud chambers, sometimes in conjunction with magnetic fields, the counter-control method, and the emulsion technique (see Section 3.5).
Another such entanglement of astroparticle and particle physics that mirrored the growing tendency towards favoring the latter was the first steps of the discovery of pions and muons. In 1935, Yukawa had speculated about the characteristics of quanta in the field of nuclear forces. In his calculations he found there must be an unknown particle with a mass of about 200 times the mass of an electron. In 1936, Anderson and other cosmic-ray physicists found new particles they called mesotrons, that shared many properties of the particle predicted by Yukawa, but as it turned out, were not identical . For in 1938, Heisenberg and Euler made a calculation that explained perfectly well the properties of the particle found by Anderson, yet the mass they calculated was not the same as that Yukawa had calculated . Most interestingly, long before Yukawa’s prediction“mesotrons” had been experimentally discovered by the German physicist, Kunze, in Rostock in 1933 . But his results did not arouse as much attention as the later ones. Since war broke out and stopped further cooperation between Germany, Japan and other nations, it took until after the war to establish experimentally that the mesotron, which Heisenberg and Euler had predicted, was the product of the particle decay of Yukawa’s particle. The latter were called pions by then, the former muons . At this juncture one should note that many early developments of cosmic-ray studies and particle physics have been achieved independently by Western and Eastern (mainly Japanese) scientists alike. This correlation has not been the topic of more detailed research so far.
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