December 12, 2008, South Pole Station – Hermann Kolanoski
IceTop Science
Cosmic Rays
The Earth's atmosphere is constantly bombarded by an abundant flux of particles from outer space--the cosmic rays. Despite the fact that we've known about this radiation for nearly 100 years its origin and composition are not fully understood and remain a major field of research. In particular, it is still puzzling where the most energetic particles, reaching energies of more than 1020 eV, are generated [physicists measure particle energies typically in electron volts (eV) where 1 eV is the energy an electron achieves when accelerated in a field of 1 Volt]. The observed energy range of cosmic particles exceeds the energies which can be reached with earth-bound accelerators by a factor of about 100 million.
Figure 1. The Crab Nebula is the remnant of a supernova explosion that happened in 1061. In the shock fronts that expand into the interstellar medium, particles are accelerated.
It is commonly believed that in our galaxy, the Milky Way, there exists no astronomical object which would be able to generate such high energies. On the other hand, we know that in violent star explosions, called supernovae, medium-high energies, up to about 10 to the 14th power eV, are generated in the outgoing shock front of the explosion (see Fgure 1). The rate of supernova explosions in our galaxy is such that it can explain the cosmic particle flux up to those energies. At higher energies, there must be a transition to radiation with extra-galactic origin when going to the highest observed energies. This regime, where the supernova explosions run out of steam and where we expect that this transition occurs, is the field of research with the IceTop Air Shower Array. For reasons I will explain below, it is of great advantage and a unique feature of our experiment that we can use the IceTop air shower array in combination with particle detection in the deep ice.
What kind of particles constitute the cosmic radiation? There are, in fact, a variety of particles reaching us from the cosmos: charged particles and also neutral particles like photons and neutrinos. The neutral particles have the advantage that they are not deviated by the galactic and intergalactic magnetic fields so that one is able to determine the direction where they are coming from and thus the astronomical source which generates them. High-energy photons can only be observed over a limited range (in cosmic dimensions) because they are absorbed by the cosmic medium. Therefore (and for other reasons), the IceCube detector is specialized to use high-energy neutrinos to detect distant sources of high-energy radiation because neutrinos have an extremely low probability to interact on their way through space. But this is another topic which is described elsewhere.
Figure 2. Sketch of an air shower.
Chemical Composition of Cosmic Rays
The domain of the IceTop detector is the charged particle component of the cosmic radiation (the term "cosmic rays" is mostly used just for this charged particle component). The detector is covering an energy range from about 3⋅10 to the 14th power eV to about 10 to the 18th power eV -- a region where the change from galactic to extra-galactic particle origin should occur. With this transition also a change of the particle composition is expected.
At least for the energy range below the IceTop range, it is well known that the dominant part of the charged radiation consists of protons--the nuclei of the hydrogen atom. This is not surprising because the visible matter in the universe is mainly hydrogen. Only in the energy-producing processes in stars are heavier nuclei generated (our sun, for example, burns mainly hydrogen to helium). For understanding these processes in stars and many other aspects, it is important to measure how the cosmic rays are composed of different nuclei ("chemical composition"). Below the energy range of IceTop, the cosmic rays are mostly contained in the Milky Way by the forces of the galactic magnetic field so that they stay within the galaxy for millions of years. The particles are bent by a magnetic field on a radius which is proportional to their energy and inversely proportional to their charge. That means they are less bent at higher energies and more bent for higher particle charges. The effect is finally that as you go to higher energies, it becomes more and more probable that particles are lost to the extra-galactic space, while heavier nuclei with higher charges have a better chance to be contained.
The main scientific goal of the IceTop detector as a part of IceCube is the study of the chemical composition of the primary cosmic rays and the transition to the dominance of extra-galactic radiation which is intrinsically related to the composition question.
The challenge of such an experiment is that at these energies the primary particles can only be observed indirectly. As was explained by Tom Gaisser in the previous blog the high-energy charged cosmic rays are observed by detecting the particle shower generated by the primary cosmic rays in the atmosphere (see Figure 2). At sufficiently high energies of the primaries, the shower detectors are hit by millions of particles. How can one conclude from this debris what the nature of the primary particles is? The answer is that indeed it is not possible to say for each event what the primary particle was, but on a statistical basis it is possible. The principle ideas for these analyses will be explained in the following.
As you see in the picture, an air shower develops continuously distributing the available energy into more and more particles. The particles are losing energy by traversing the air so that at some point there is not enough energy left to further generate particles and the shower fades out. As a consequence, the number of particles in the shower first increases and than decreases developing a ‘shower maximum’ in-between. The average position of the shower maximum depends on the energy of the primary particle: the higher the energy the closer the shower maximum approaches the surface. Since the South Pole glacier has a height of nearly 3000 m, IceTop is favorably close to the shower maximum for the considered energies.
Figure
3: Measurement of an air shower in IceTop in coincidence with
a muon
bundle in the deep detector (left). For a given energy measured by IceTop
(horizontal axis on the right) the number of muons measured in the deep ice
(vertical axis) is on average very different for hydrogen and iron.
A nucleus is composed of nucleons, protons and neutrons, which have both similar interactions with other nuclei. The mass of a nucleus is proportional to the number of nucleons it contains, for example, hydrogen has one, helium four and iron 56 nucleons. At high energies the interaction of a nucleus with the air molecules is so that each nucleon interacts independently, each nucleon generating its own cascade, but each cascade has only a fraction of the total energy. The resulting full shower has the shower maximum at the position corresponding to the energy per nucleon but the total shower energy sums up to the total energy of the nucleus. This is one of the characteristics for the mass of the primary nucleus which is exploited by IceTop: for the same energy the shower maximum of a heavier primary nucleus is shifted upwards relative to a lighter nucleus.
A second and for the IceTop-In Ice combination more important feature of the showers is explained in the following. In the previous blog the composition of particles in an air shower was explained: on the detector level there remain mainly an electromagnetic component (electrons and photons) and a ‘muonic’ component which is the penetrating component of the air showers. The number and energy of muons is very characteristic for the primary nucleus. This can be understood as follows: the muons originate from decays of charged pions (see the previous blog) pions have a lifetime and interaction probability which compete with each other. In the thinner upper atmosphere it is less likely that pions interact so more of them will decay while travelling through the air. The muons from these decays in the high atmosphere will on average have relatively high energy because the energy was not yet often split. For a heavy nucleus with many simultaneous primary interactions, more pions will be produced early on and more high energy muons will emerge. This is the second characteristic feature: the ratio of the energy deposited by muons in our detector and the electromagnetic component is larger for heavier primary nuclei. The separation of muonic and electromagnetic components is particularly distinct in the IceTop-InIce combination: the signal in the IceTop tanks is dominated by the electromagnetic component and only penetrating muons can make it through the ice to be detected by the deep detector. A special advantage of IceCube is also that only high energy muons make it through the ice, so that the detector filters out the muons from the early interactions in the high atmosphere which are most sensitive to the mass difference of the primary nuclei.
The methods for the composition determination are summarized in the figure above: compared are the electromagnetic (red) and the muonic (blue) components as a function of depth in the atmosphere. The electromagnetic component develops the characteristic shower maximum which can be scanned on its falling slope by observing showers at different angles corresponding to different effective depths as seen by the surface detector (middle plot). Most sensitive is the comparison of muon production in the upper atmosphere, as measured by the deep detector, to the signal in IceTop (left plot). With some restrictions also the muons can be counted in IceTop alone yielding another characteristic difference between light and heavy primary nuclei (right plot).
