Page 311 - Radiochemistry and nuclear chemistry

294 Radiochemistry atut Nuclear Chemistry

steady exchange, J'e to J,~, to J'r etc, i.e. they oscillate between the various states. Since
most neutrino detectors are sensitive to only the i, e, detectors have registered only 1/3 of
the expected number of solar neutrinos. There has been time for the fusion neutrinos to
equilibrate as it takes the neutrinos several hundred thousand years to diffuse from the solar
core. The flux of neutrinos is copious at the earth's surface, about 1011 s-1 m-2.
A large international collaboration ("Gallex") is setting up a neutrino detection station in
a rock facility in the Mont Blanc. Some 71Ga atoms in 30 tons of gallium metal is expected
to react with solar neutrinos to form 71Ge (tt h 11.4 d) which is to be converted to the
gaseous hydride, GeH 4, and counted in a proportional detector. About 1 atom of 71Ge
formed per day is expected.
In 1987 a large underground neutrino detector near Fairport, Ohio, in a few seconds
registered a sudden burst of 8 events. Taking into account that the normal background rate
is about 2 events per day, which is believed to be causeA by neutrinos produced in the sun's
fusion reactions, this was an exceptional occurrence not only because of the event rate but
also because the source was located outside our solar system and was a bright new
supernova, SN1987A, appearing in the Large Magellanic Cloud. This was a lucky
observation because the previous "near by" supernova was observed in 1604 by Johannes
Kepler. The neutrino observation preceded the optical confirmation and it has been
calculated that about 1058 neutrinos were released in the explosion.
One of the most significant effects of the neutrino mass relates to the mass of the
Universe. According to the Big Bang Theory of the origin of the Universe (see Ch. 17)
there should be as many neutrinos as there are photons in the microwave background
radiation remaining from the Big Bang, or about 100 million times as many neutrinos as
other particles. If these neutrinos have a mass > 10 eV they would constitute the dominant
mass in the Universe. This would mean that there would be enough mass in our Universe
for gravitational attraction eventually to overcome the present expansion, and consequently
we would have a closed, or possibly pulsating universe, instead of a universe which will
continue to expand inf'mitely.

10.7. Quarks and the Standard Model

All particles are considered to be possible states in which matter can condense. These
states are related to the force that forms them. In this sense the solar system is a state of
gravitational force, an atom is a state of electromagnetic force, and a nucleon is a state of
the strong interaction force. A particle can represent a positive energy state of a system
while its analog antiparticle represents the negative state of the same system. Some regular
patterns have been formed for the elementary particles which indicate that many of them
in fact may only be exited states of the same particle, differing in quantum numbers such
as spin (or "hyper charge"); in fact hundreds of such states are now known. For example,
the neutron has a mass corresponding to 939 MeV and spin 1/2, and there is a baryon with
mass 1688 MeV and spin 5/2 with all its other properties like those of the neutron: the
heavier particle must be a highly excited state of the neutron.
Though many attempts have been made to unify all particles into one simple theory, this
has not succeeded until recently when the quark theory was developed. To explain this we
have to go back somewhat in time.