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3 Technical Improvement and its Meaning

Röntgen may never have come across x-rays had he not been in the lucky position of having a very good vacuum tube at hand. As historians point out, the production of a vacuum was a very intricate matter and therefore the quality of the vacuum produced was one way to evaluate the quality of laboratory in the 19th century [49]. But in the case of cosmic-ray studies, numerous technical improvements played a decisive role in the progress of experimental know how. In some cases, the new developments mark the transition between cosmic-ray studies and new fields, like particle physics.

3.1 Wulf’s apparatus and its version by Kohlhörster

The first and probably most important instrument was invented by Wulf in 1909. It consisted of an airtight cylinder containing a two-fiber electrometer [227172Jump To The Next Citation Point]. Its greatest advantage was that it was easily portable and therefore could ideally be taken to great heights for the observation of penetrating rays. As a further development of the conventional ionization chamber, the high-pressure ionization chamber was able to multiply the effect of the ionization measured in the conventional apparatus about fifty times. Kohlhörster modified the apparatus, laying special emphasis on more reliable air-tightness. Another aspect he approached was the change of temperature; since his balloon-carried experiments reached up to 6300 m, the great cold endangered the reliability of the instruments. Finally he also dealt with the question of what effect different pressures would have on the results of the measurements. Due to the modifications he tried out, Kohlhörster was not only able to confirm Wulf’s and Hess’ findings of the rise of the number of ions per cubic-cm per second, but also to discover, by sinking the apparatus into the Saale river, that under water the number of ions declined [123Jump To The Next Citation Point].

3.2 The Geiger counter and the coincidence method

In 1928, Geiger and Müller invented the wire counter, which was able to count single events of an ionizing ray hitting the counter’s tube [74Jump To The Next Citation Point]. The wire counter was the basis for another important development in cosmic-ray studies, the coincidence method, which was for the first time described by Bothe and Kohlhörster in 1929 [30]. This method makes use of the Geiger counter by connecting at least two of them. Observing the results of both counters one finds many events that are not related, but several of them coincide and so give a good clue that those hits have been caused by strong penetrating rays, which are able to hit both tubes without losing all of their energy in between. So, after the idea of cosmic rays had started to settle in scientists’ understanding of the natural world, these inventions helped to widen the range of research. The fundamental work that had been done up to around 1932 and the knowledge that had been gained through it was summed up by Geiger in 1940 [74]. What had been acknowledged by then as a fact was the cosmic origin of the radiation. In addition, it had become clear that the intensity of the rays was independent from the time of day that they were measured as well as from the direction they appeared to come from. With about one pair of ions per second, per cubic centimeter, ionization at sea level was barely measurable; though the radiation, for lack of a better definition up to then simply called “ultra-radiation”, was extremely penetrating, even in water or solid rock.

3.3 Balloons for cosmic rays: the beginning of spaceflight?

Another technical device that played a decisive role in the early research of cosmic radiation has not been regarded here properly so far: the balloon. It had helped Hess to detect cosmic rays and through constant improvement would finally mark the transition from earth-bound studies to the beginnings of modern space travel. Hess had made a total of ten flights, all manned and equipped with Wulf’s electrometers and oxygen supply. A malfunction of the gas in the balloons prevented Hess on several occasions from reaching his desired height [107]. But finally he succeeded, as did Kohlhörster one year later [123]. After World War I the importance of manned flights lessened and work with unmanned balloons gained significance. Millikan had used them in 1923 to gain greater height in his flights [146], using a Wulf apparatus that weighed less than 200 g, thus being able to ascend higher than any manned balloon could have done. In Germany, Regener made use of unmanned “pilot-balloons” in order to scrutinize the characteristics of cosmic rays at ever greater heights. He had built a Wulf electrometer, the “balloon-electrometer”, that fit the special needs of high altitude, including being resistant to high air pressure and intensive coldness. This instrument, and the use of several balloons to carry it, allowed ascents up to 25 km, outdoing the most extreme manned balloon flights by almost 10 km in 1932. These high altitude ascents showed that the increase of ionization seemed to reach a constant level in the highest part of the atmosphere. The balloons would be put together in tandem carrying a gondola of up to 12 kg with the electrometer. The landing was initialized by the bursting of a rubber membrane on one of the balloons. That was the main disadvantage of unmanned balloons. The instruments that had been on board had to be collected after landing, which meant a great deal of extra work, as they first had to be located in sometimes rather impassable terrain and then had to be brought back to the institute at which the measurement data was to be analyzed. Regener is reported to have launched balloons that were found more than 200 km away from the starting point [172]. Therefore, the technical progress that had been made in the field of data transmission and data archiving helped a lot to simplify the balloon flights of cosmic-ray physicists. Knowing about the difficulties, Russian scientists tried to circumvent the problem by using radio signals to transmit their data. First the data were sent to a telephone, from which they had to be recorded by hand. Later oscilloscopes made visible the measured data so that they could be recorded on film [213Jump To The Next Citation Point]. Some [213159Jump To The Next Citation Point] consider these manned and unmanned balloon experiments to mark the beginning of space physics.

Already during World War II, Regener had invented a container in the shape of a barrel that would have been able to carry a great number of different experimental set-ups in a space rocket and protect these devices against the hostile conditions of such a flight, e.g. extreme temperatures and pressure. Unfortunately, Regener’s invention did not come into regular use, as the A4 rockets, now better known as the V2, were, after some test flights, not used for scientific purposes, but for military purposes only and so the prototype of that device got lost in the war. Still, it brought Regener the fame of becoming the founder of extra-terrestrial physics [164]. Another basis for later satellite-borne experiments conducted by American and Russian researchers was a mixture of both known devices, balloons and rockets, balloon-borne rockets called rockoons. Invented in 1949, rockoons were used by Van Allen during the 1950s to explore the magnetosphere of the Sun, as well as to learn about the properties of cosmic rays. He came to the conclusion that the Earth’s magnetic field shields us from the majority of cosmic radiation [47Jump To The Next Citation Point].

This interesting blending of different methods points at another important aspect concerning the historical analysis of early cosmic-ray studies. Because scientists at the beginning of the 20th century felt somewhat uncertain about the nature and characteristics of the many different kinds of radiation, there was a vast range of interrelated methods applied to this problem. On the one hand, different radiation phenomena were detected and analyzed with experimental gear that would all be carried at the same time in either a balloon or a plane. [220]. On the other hand, the same methods and technical devices could be used for very different purposes [178179]. This makes it sometimes a bit difficult, but also very interesting, for historians of science to find out which scientists worked in the field of cosmic-ray studies, as they sometimes came from very diverse fields of physics.

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Figure 3: Rockoon with Deacon rocket [47]

3.4 High altitude observatories

But balloon flights were not the only means of high altitude research; a number of observatories were erected and existing stations were extended for the special needs of cosmic-ray research. In Europe the Alps became the focus of interest in Earth-bound studies at great height, the Andes in South America. There were also several observatories in North America and in Asia. Though they were later to be important for the investigation into the nature of the eruptions from the Sun in the International Geophysical Year of 1957/58 and some of them are still in use today, further work needs to be done in order to learn what role exactly they played in the early stage of cosmic-ray studies [124]. The stunning thing is that they came into use for the purposes of cosmic-ray studies very soon after Hess’ “discovery”, at least as early as 1916 [108]. That means, that a – up to today not historically analyzed – scientific, financial, administrative and human effort of an enormous extent had been made in order to construct proper equipment for the experiments and take it to the stations on mountain tops, if these stations already existed at all. In addition a huge investment of man-power was required in the execution of the experiments, not to mention the necessary supplies for the experimenters. If one takes into account the time of these first experiments and the other epoch-making events of that day, like World War I, it is quite astonishing that there was still enough interest to mobilize such powers for a scientific purpose of this extent.

3.5 The photographic emulsion technique

The photographic emulsion technique is based on the improvement of the usual photo-plates, like those used by Röntgen and Becquerel. The emulsion is thicker and has a high concentration of silver bromide grains that condense along the tracks of particles that pass through the emulsion. Microscopic examination of the properties of the grains that form a particle track gives information about the mass of the particle. Yet the standard products available in the 1930s could only detect slow protons and alpha particles. After the emulsion had been greatly improved in the mid-1940s, the pions, long since theoretically predicted, could be traced experimentally [136Jump To The Next Citation Point]. From then on the method was successfully used for investigating the properties of muons and pions that were contained in cosmic rays and the detection of more as yet unknown particles, like the tauon, the kaon and others [170].

As the technique was rather cheap it was an attractive means of cosmic-ray studies after World War II. Numerous emulsion plates would be stacked and placed on unmanned balloons. Unmanned balloons having the main disadvantage of having to be traced on quite long distances, the emulsion technique was one of the first to require larger international collaborations than the rather small ones of the 1930s, benefiting from the fact that the physicists involved could cover a much greater terrain, literally, but also in a figurative sense [136].

3.6 Bubble chambers, spark-chamber detectors and drift chambers

Particle detectors, as we have already seen in the case of the cloud chamber, are important in particle physics and astroparticle physics alike. As the objects in these fields of physics cannot be directly encountered by human sensory perception, they have to be made “visible”. Philosophically speaking that makes them special objects when talking about the “reality” of such entities, yet from a scientific point of view there are various means to make them either visible or audible and then analyze their different properties with the help of detectors [185Jump To The Next Citation Point]. There are many different kinds of detectors, the following being just a very short overview of the most important ones. Their development through time mirrors the development of the relationship between early cosmic-ray studies and particle physics. From the 1950s onwards, detectors were optimized for use in connection with accelerators, while in principle they could have been used with cosmic rays as well. Detectors in modern astroparticle physics are even more complex than the detectors briefly described below; they have developed into huge experimental set-ups that make use of very different technical know-how (see Section 4.5).

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Figure 4: Charmed baryon and neutrino-proton collision in bubble chamber [32]

Invented by Glaser in 1952, for which he received the Nobel Prize in 1960 [84Jump To The Next Citation Point], the bubble chamber works with the same principle as the cloud chamber, being a volume of liquid that makes tracks of charged particles visible by condensation. But the bubble chamber makes use of a different mechanism: A cylinder filled with super-heated liquid, like diethyl ether in Glaser’s first set-up [84Jump To The Next Citation Point] or hydrogen, is placed in a magnetic field. Then – while the liquid is close to the boiling point – the pressure in the chamber is lowered. When particles now enter the chamber, their loss of energy in the liquid makes a bubbling effect, of which a photo is taken, showing characteristic particle tracks [185Jump To The Next Citation Point] (see Figure 4View Image). Usually bubble chambers are used for taking pictures of particles from accelerators, but they are also able to trace cosmic rays. When looking for the ideal liquid to build the detector he had in mind, Glaser came across the problem some chemists encountered while working with super-heated diethyl ether. At certain intervals the super-heated liquid started to erupt into bubbles. Glaser’s calculations showed that the disruptions the chemists had witnessed were in total agreement with the cosmic radiation and the general radioactive background at sea level [84].

Spark-chamber detectors are a good device for detecting weakly interacting particles. Several plates of light metal are arranged in a gaseous atmosphere such that there is no overlap between them, if a voltage is supplied to them. Incoming particles are then able to ionize the gas and produce a cascade of further electrons that can be traced by the sparks they release from the plates.

Drift chambers became the more commonly used as they deliver immediate electronic data, rather than relying on photographs, and thus are able to analyze much more information. Drift-chamber detectors, invented by Charpak around 1967, are a modified form of wire chambers [41]. The latter make use of the fact that incoming particles may produce a shower of electrons in the gas of the chamber. The freed electrons drift through the gas to the wire where high voltage gives enough energy to produce an avalanche of secondary electrons, which can then be detected. This method gives the opportunity to reconstruct the time of the primary event [185Jump To The Next Citation Point].

3.7 Computers

Today the usage of computers in general is so common among physicists that this method might be seen as self-evident, but the application of ever-better computer programs to various scientific problems must have been quite a relief for researchers in the 1950s and 1960s, both for the mathematical analysis of collected data and the calculation of theoretical predictions.

One good example might be the Monte Carlo simulation method. After being successfully applied to the problem of predicting the development of particle showers in 1958 [37], it became a standard method for calculating electromagnetic cascades [205Jump To The Next Citation Point]. One advantage of the method is the ability to track particles individually, thus providing reliable results concerning the lateral spread of cosmic-ray and other particle showers. Yet in the very early days of computers in cosmic-ray physics, the rather low standard of computers forced researchers to deal intensively with the scientific processes behind the application of technical equipment; e.g. the methods of sampling had to be considerably improved in order to get good results out of the calculations [205Jump To The Next Citation Point].

But as the history of computer science has its own peculiarities [140], I will not go into any detail on that point.

3.8 Particle accelerators

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Figure 5: Draft of the cyclotron in the patent specification from 1932 [135]

Particle accelerators do not, of course, belong to cosmic-ray physics. Yet, as this whole section tries to show the influence technical improvement may have on the development of science, a short overview of some of the most important kinds of accelerators seems to be appropriate at this point, as they are generally held responsible for the rise of particle physics after the 1950s. What is even more important, discoveries of numerous new particles enabled researchers to develop the Standard Model of particle physics [58], which was to become one important basis of modern astroparticle physics. Moreover, man-made accelerators taught researchers the basic mechanisms of particle acceleration and propagation, so that their findings could be applied to cosmic accelerators of particles, like the shockwaves of supernovae. Especially as the problem of the acceleration of particles by interstellar magnetic fields had been a topic of interest even before [64].

In 1919, the Norwegian electrical engineer Wideröe was the first person to speculate about the concept of circular accelerators, calling them “ray transformers”. Because he was interested in Rutherford’s scattering experiments and the problems of nuclear fission, he applied the theory of general relativity to describe particles moving close to the speed of light [225Jump To The Next Citation Point]. This concept of a ray transformer, using changing magnetic fields to accelerate electrons, was later to become a betatron, but as first experiments Wideröe conducted with this set-up failed, he turned to linear accelerators. Invented by Ising in 1924, the device was a success, as it was able to accelerate ions of sodium and mercury. In the 1940s, Alvarez improved the linear accelerator so that it became possible to work with proton beams [185Jump To The Next Citation Point].

The next step towards larger and more effective particle accelerators was marked by the invention of the cyclotron, usually ascribed to Lawrence (see Figure 5View Image). He had the idea of combining two of Wideröe’s drift tubes so that the particles would be accelerated by a magnetic field in a spiral-like circular movement. The principle worked out quite well, though early cyclotrons of smaller diameter faced some problems in surpassing the 30 MeV energy limit at which protons become relativistic. As cyclotrons turned out to be useful for a number of applications, like nuclear physics, they were used during World War II for producing fissile material. In the meantime Wideröe’s idea was taken up again, this time to be turned into the betatron. This electron accelerator was able to reach energies of about 300 MeV. With the increase in energy came an analogous increase in weight: even rather early betatrons in the 1940s weighed up to a few hundred tons [225Jump To The Next Citation Point].

It was this desire to accelerate particles at higher energies – an obstacle that in the opinion of some researchers could only be overcome by cosmic accelerators [80Jump To The Next Citation Point] – that paved the way for the synchrotron. After the principle of phase stability had been discovered, cyclotrons were first developed into synchro-cyclotrons until they were finally merged together with the betatron to form synchrotrons. These machines were not only able to accelerate electrons and protons alike, but also soon after their invention reached energies in the scope of a few GeV [185Jump To The Next Citation Point].

Finally, the concept of storage rings was born out of the conclusion that a head-on collision of two particles would give the opportunity to use all the acceleration energy for the collision and no longer waste a great deal of energy on the recoil. Thus, the first storage rings reached collision energies of a few GeV, and future colliders are heading for the TeV mark [185]. In comparison, cosmic rays are able to exceed even the TeV scope and reach up to a few PeV [163Jump To The Next Citation Point].

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