Transcript Cosmic rays 4 1435-1436

COSMIC RAYS 4
1435-1436
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 1020 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 1017eV.
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 109 -1010 eV ,
2. the intermediate band at about 1012-1017 eV
3. the high-energy band above 1018 eV .
Most work has been done on the low-energy band
where the new subject of g-ray astronomy has
indicated a galactic origin for these cosmic rays.
Next, the 1012_1017 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>1018 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 highenergy cosmic rays. On this model there should be a
sharp energy cut-off in the spectrum at 6 x 1019 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
1020 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 1019 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 1018
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
1018 eV or 1019 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 1019 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 (109 -1010 eV) band
can be inferred from a study of cosmic ray gphotons, and it is here that most progress is
being made. The g-rays arise from the interaction
of primary particles with interstellar matter.
Primary electrons will produce Bremsstrahlung
radiation while protons will give g-rays from
neutral pions as shown in Fig. 25.6. These g-rays
travel in straight lines and it is found
experimentally that there is a concentration of grays in the plane of the galaxy to such an extent
as to support a galactic rather than a 'universal'
model.
Some g-rays also come from the pulsars
of the Crab and Vela supernovae
remnants. This is known because the g-ray
intensity pulses at the same rate as the
radio pulses and several of these g-ray
pulsar sources have now been detected.
The very fact that some g-rays have been
coming from pulsars and that g-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
g
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 midenergy band of 1012 _1017 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 g-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 1019 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
1019 eV a neutron would have a mean
free path of the same order as cluster
dimensions and thus some neutrons
could escape above 1019 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 1019
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 (1012 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.