COSMIC RAYS THE FUTURE OF COSMIC RAY RESEARCH

. 8 THE FUTURE OF COSMIC RAY RESEARCH As we have seen, a few cosmic rays can be detected with energies somewhat greater than eV. These high energies must be due to particle accelerations taking place within or beyond our own galaxy. Primordial source points must therefore be galactic or extra-galactic. During the acceleration process the particles must travel many interstellar distances and pass through many magnetic fields before reaching the earth. Details of this acceleration process are not known, nor do we know for certain the origins of the source points of cosmic rays. Current theories are that pulsars and supernovae explosions are likely sources of primary cosmic rays, at least up to eV.

Modern research (1978) is broadly in two directions: (i) to find a theory and account for the origin of cosmic rays and to evaluate them experimentally; and (ii) to use the high-energy particles in the cosmic ray spectrum to investigate nuclear collision products in the search for new particles and the structure of nucleons.

THE ORIGIN OF COSMIC RAYS In considering the origin of cosmic rays, I has been convenient to divide the spectrum into three bands, 1. the low-energy band of eV, 2. the intermediate band at about eV 3. the high-energy band above eV. Most work has been done on the low-energy band where the new subject of  -ray astronomy has indicated a galactic origin for these cosmic rays.

Next, the _10 17 eV band has been investigated in terms of anisotropies in the intensities and directions of these cosmic rays. The predicted anisotropies are small, but on balance point to a galactic origin for this band. For energies>10 18 eV there are difficulties in containing the particles within the galaxy because of their high energies. An extragalactic 'universal' model has therefore been proposed for these high- energy cosmic rays. On this model there should be a sharp energy cut-off in the spectrum at 6 x eV due to the attenuation of the primary protons by the 2.7 K black body radiation field, a relic of the big bang hypothesis of cosmological theory. This cut-off is not observed experimentally.

In fact, several particles with energies above eV have been recorded and the shape of the energy spectrum found by plotting the number of particles with a given energy against that energy above eV is quite different from that expected, showing a tendency to flatten out rather than to drop to zero. Furthermore, many particles are observed arriving from directions nearly perpendicular to the galactic plane indicating that some of the highest energy cosmic rays have an extragalactic origin.

The proponents of galactic origin above eV suggest that the particles are mainly heavy nuclei, such as iron, for which the galactic trapping mechanism is stronger than for lighter particles. Such measurements on mass as have been made indicate that the particles are mainly protons and the situation is at present unresolved. Many workers favour a compromise with most of the particles above eV or eV coming from 'local' extragalactic sources such as a local explosion galaxy.

Experimentally the problem is very difficult because at these high energies the cosmic ray flux is very small; for example, above eV the flux is only 1particle per square kilometer per year! At such a low flux the cosmic ray components are difficult to resolve so that the presence of iron and other heavy components is not easy to detect. For this reason the cosmic ray detectors used in this work cover very large areas. At Haverah Park, near Harrogate, England, the U.K. array is 12 km 2 in area.

The Pierre Auger Observatory Auger will detect the shower in two ways. Twenty four hours a day, an array of over 1600 particle detectors will measure shower particles as they hit the ground, which will allow a reconstruction of the shower providing measures of the original cosmic ray's energy, arrival direction, and mass.

The Fly’s Eye(s) located in the West Desert of Utah, within the United States Army Dugway Proving Ground (DPG). The detectors sit atop Little Granite Mountain. Dugway is located 160 km southwest of Salt Lake City.

The origin of the low-energy ( eV) band can be inferred from a study of cosmic ray  - photons, and it is here that most progress is being made. The  -rays arise from the interaction of primary particles with interstellar matter. Primary electrons will produce Bremsstrahlung radiation while protons will give  -rays from neutral pions as shown in Fig These  -rays travel in straight lines and it is found experimentally that there is a concentration of  - rays in the plane of the galaxy to such an extent as to support a galactic rather than a 'universal' model.

Some  -rays also come from the pulsars of the Crab and Vela supernovae remnants. This is known because the  -ray intensity pulses at the same rate as the radio pulses and several of these  -ray pulsar sources have now been detected. The very fact that some  -rays have been coming from pulsars and that  -rays are themselves part of the cosmic ray flux means that at least we know that some cosmic rays have their origin in pulsars.

Crab nebula the Crab Nebula is a pulsar wind nebula associated with the 1054 supernova. It is located about 6,500 light-years from the Earth 

Whether or not protons are accelerated in these sources as well is not known with certainty, but it seems likely that some, and perhaps most, are. The mid- energy band of _10 17 eV is also thought to be of galactic origin although the measurements, based on cosmic ray anisotropies (i.e. the dependence of intensity on direction in space), are not so conclusive as the  -ray evidence for the low-energy band.

Returning to the high-energy end of the spectrum, an alternative suggestion to explain the flattening of the spectrum above eV has been made. This involved the escape of neutrons from clusters of galaxies. Very energetic nuclei are probably produced in certain galaxies and these interact with the gas and the photons in the clusters.

The charged fragments are then trapped by the intergalactic magnetic fields but the neutrons escape and produce fast knock-on protons by striking gas nuclei or decay in flight into protons. Calculations show that at eV a neutron would have a mean free path of the same order as cluster dimensions and thus some neutrons could escape above e V and produce the relatively high flux at this energy

. All modern cosmic ray theories are speculative but experimental data are being collected at such a rate through large-scale laboratory measurements and satellite observations that these theories can be tested much more rapidly than before. A crucial factor in interpreting cosmic ray data is the mass composition of the rays in the various energy bands, and as the mass spectrum becomes more exactly known the problem of the 6 x eV cut-off will ultimately be solved -and with it the origin of these cosmic rays.

HIGH-ENERGY PARTICLE COLLISIONS We now turn to the second line of research using cosmic rays as high energy bombarding particles. Although cosmic rays have a low flux their high energies compensate for this in single particle collision investigations. In the never-ending search for the detailed structure of the nucleus and of the nucleon it is essential that the probe particles have energies as high as possible. Accelerating machines can now give' energies up to 1000 GeV (10 12 eV) with a very high flux compared with that of the cosmic rays. Thus nuclear plates used in cosmic ray research require relatively longer exposure times, but often lead to events not produced anywhere else.

Many of the sub-nuclear particles were discovered in cosmic rays, starting with the discovery of the positron, the muon and the pion. However, much of the current work on fundamental particles is largely confined to accelerators. The role of cosmic rays is now to be found in examining gross features of interactions at energies well above those available from accelerators and the search for exotic particles such as free quarks (Chapter28) still continues in cosmic rays. These particles are playing a large part in our search for the details of nucleon structure and form the subject of the next three chapters.