Spark chamber

Spark chamber

by Franco Laeri

The discovery of cosmic radiation

Karl Bergwitz (1875-1958), who held an assistant position in Darmstadt for one year after his doctorate in Rostock, registered on a balloon flight in 1908 that air ionization was dependent on altitude. His measurements initially showed a decrease in air ionization, but it increased again as the altitude continued to rise. This result, “strange” to him, made him doubt the proper functioning of his measuring device, and he decided not to publish the measurements. In 1912 the Austrian Viktor Hess (1883-1964) , who was in contact with Bergwitz and knew his balloon measurements, repeated the experiment. In several further balloon flights he proved with the help of two Wulf radiation apparatuses [1] that air ionization is substantially influenced by “penetrating” radiation from space. He summarizes the findings thus [2]:

The results of the present observations seem to be most readily explained by the assumption that radiation of very high penetrating power enters our atmosphere from above, and causes even in its lowest layers some of the ionization observed in closed vessels. The intensity of this radiation seems to be subject to temporal variations, which can still be detected at one-hour reading intervals.

Together with Carl David Anderson, he was awarded the Nobel Prize in Physics for the discovery of cosmic rays in 1936.

To carry out longer measurement campaigns at altitudes above 3000 m in balloons is not very practical. So it was a good thing that in August 1912 the railroad to the Jungfraujoch, located in the Bernese Oberland (Switzerland) at 3500 m, was opened. In 1926, from there, the later Nobel Prize winner Walter Nernst, his then assistant Werner Kolhörster together with Gubert von Salis, undertook an expedition to the neighboring summit of the Mönch at 4100 m, where they set up a measuring station in a snow cave and collected data during two weeks [3]. On the initiative of the Geneva astronomers R. Gautier and G. Tiercy, a small astronomical observatory was built on the Jungfraujoch in 1928. Then in 1931, after a year of construction, the “Jungfraujoch High-Alpine Research Station” was inaugurated and has been providing data on cosmic rays without interruption ever since. In 1950, under the direction of P. M. S. Blackett of the University of Manchester, a cloud chamber equipped with a 14-ton magnet was even installed in the “Sphinx Building”, which made it possible to accurately measure the energy of electrically charged particles of cosmic rays.

The cosmic radiation

The total amount of the energy flow of the irradiated cosmic rays corresponds approximately to the irradiated light flow of the stars at new moon. What makes cosmic rays physically interesting is that they consist of particles which individually have a very high energy. Through experiments on the Jungfraujoch, Pierre Auger showed in 1938 [5] that these particles can have an energy of 10 15 eV. Since then, particles as high as 10 21 eV have been observed, which is far beyond what can be achieved with the LHC. These primary particles (mostly protons) collide with the nuclei of nitrogen stoffs and oxygen stoffs in the upper layers of the atmosphere (about 20 km altitude), producing a shower of over a million secondary particles per collision, but only a small fraction of these reach the Earth's surface. The particles that reach the Earth's surface are mainly muons. Their flux density at sea level is about 100 particles per square meter per second and their average energy is around 4 GeV [4]. Many aspects of cosmic rays are still mysterious and are being studied in large experiments [6].

The muon in the secondary cosmic rays

Like electrons, muons belong to the lepton family, i.e. they are elementary particles which are not subject to the strong interaction, but only to the electroweak one. Like the electron, the muon has a negative elementary charge and a spin of 1/2, but it is about 200 times heavier and not stable. It has an average lifetime of about 2.2 µs. Because of its large mass of 106 MeV/c², its production requires a correspondingly high energy. This is beyond the energies achieved in radioactive decay or nuclear weapons. Particle accelerators like CERN are required, or astrophysical processes. In fact, they were identified in the study of cosmic rays in 1936 by C. D. Anderson and S. Neddermeyer. Since then, cosmic ray muons have been studied in numerous, large-scale experiments around the world. In recent years, observations at high energies of 10 19 eV have been accumulating of an excess of muons that cannot be explained by the standard models of high energy physics [7]. This “muon enigma” is currently the subject of much experimental effort and debate [8].

As mentioned above, the lifetime of a muon is 2.2 µs. Muons reaching the Earth's surface still have an average kinetic energy of 4 GeV, which is about 40 times their rest mass. That is, they travel practically at the speed of light. Thus, in classical terms, they would travel about 660 m, by which time half would have decayed. Since they originate in the upper layers of the atmosphere in heights over 10 km, none of them should be observable at the ground any more. BUT – according to the special relativity theory, moving muons have a longer lifetime than resting ones: Time dilation of moving particles . The resting observer on the earth notes for the muon observed from him, flying with the constant velocity, a longer lifetime by the Lorentz factor.

Thus, for a 4 GeV muon, the prolongation of the lifetime corresponds to about 40 times, i.e. 90 µs. Therefore, on the way from the upper atmospheric layers to the ground, only about half of the muons will decay. This leaves us with enough muons to observe in the spark chamber.

The spark chamber

Plate stack of the spark chamber (left and right corresponds to top and bottom during operation); Photo: F Laeri
Plate stack of the spark chamber (left and right corresponds to top and bottom during operation); Photo: F Laeri

In the spark chamber, the muons of cosmic rays are made visible by electric flashovers, sparks, in a noble gas. The high-energy muons (kinetic energy about 40 times the rest energy) constantly collide on their way with the atomic nuclei of the matter they pass through. During these collisions, gas atoms are ionized, i.e. a positively charged ion and a free electron are created at this point for a short time. Such an ion recombines in the gas within about one microsecond to a neutral atom by capturing an electron. So the muon leaves a trace marked by ions and electrons for a short time in the flyby, so also in the gas in the spark chamber. We actually only need to make this track glow. The muon has left electrons in the gas. We can give these additional kinetic energy so that they in turn collide with neighboring gas atoms and ionize them. Then we would have turned one free electron into two. Because this works well, we repeat it. This is how an electron avalanche is created. But how do we impart additional energy to the electrons? Through an electric field that accelerates the electrons. To do this, we introduce thin metal plates into the gas, to which we alternately apply a voltage. The avalanche of electrons moves toward the plates, creating a conductive channel between the charged metal plates. The current breaks through at these points. The ions present in the channel recombine. If we have selected the right gas, the energy released during recombination is emitted as a visible glow.

But there is a catch. In the volume of the spark chamber – cf. figure on the right – there are a lot of ions and electrons at any time. There would constantly be sparks somewhere between the plates. To detect a trace among all these sparks would not be possible. We therefore have to filter out the moment when a muon flies through the chamber and only then quickly apply the voltage to the plates. Hopefully, if all this happens fast enough, no other event will interfere, and the sparks will really only occur along the one muon track. But how do we find out that a muon flew in and out of the chamber?

Muons can not only ionize gas, but they can also cause certain materials to fluoresce/scintillate. This fluorescent light can be evaluated in so-called scintillators as a signal for the passage of a muon. We have equipped the spark chamber at the top and bottom with such scintillator plates; cf. figure on the entry page, Fig. 2 and [9]. The fluorescent light excited by the muon in these plates is then converted into an electric pulse by a fast photodetector (photomultiplier [10]). When the electronics simultaneously receives a pulse from the upper and lower scintillation detectors, it sends the coincidence signal to the high voltage switch (thyristor switch [11]), which then turns on the high voltage at the plates in the spark chamber. All this happens within a few 100 nanoseconds (cf. Fig. 3), so that in the chamber gas most of the ionized atoms have not yet been able to recombine and are thus still available for the sparking process.