figure
1 the conservation of linear momentum from beta decaySpace Phenomenon
1.13
The Neutrino
Neutrinos are a type of fundamental particle with no electric charge, the property of mass is at present unresolved but tends towards zero. It is a lepton and therefore correspondingly has spin ½. As a lepton it also follows that the particle like the electron and other leptons is not subject to the strong nuclear force. There are three known neutrinos each associated with a given charged lepton. The three are the electron neutrino, the muon neutrino and the tau neutrino.
figure
1 the conservation of linear momentum from beta decay
From Classical physics we know that momentum is always
conserved, that is:
Mprevious = Mresultant more specifically for decay:
The energy of the proton and electron must balance. If
therefore the neutron were stationary, and we can define the neutron as
our frame of reference and assume it is then the momentums after the decay must
equal those before:
Momentum = Mass * Velocity
In this case it would be easy to
imagine the two particles moving away at 180°
from one another, the proton moving much more slowly than the electron:
The mass of the electron can be
expressed as:
9.1096 * 10-31Kg
Against the mass of the
significantly heavier proton:
1.67252 * 10-27Kg
therefore one would expect the
difference in velocity to be of the order of
1.67252 * 10-27 Kg /
9.1096 * 10-31 Kg = 1835.99718977781680864143321331343
1835.997 to three decimal places
However
in observations the proton and electron were not seen to move in opposite
directions in all experiments and the velocity equations did not balance. It
followed that there must be a more elusive particle that was neutral in charge,
so as not to influence the balanced charge equation but able to alter the
momentum. It was known that the mass of the proton and the electron were to
within measurable resolution the same as that of the neutron so this meant that
the neutrino must possess very high velocity to balance the equation.
Experimental data states that the mass of the electron-neutrino must be less
than 0.0004 that of the electron. There is, however, no compelling theoretical
reason for the mass of the neutrino to be exactly zero.
This relationship was first
proposed in 1930 by the Austrian physicist Wolfgang Pauli. The relationship was
further elaborated upon in 1934 by the Italian-born physicist Enrico Fermi It
was Fermi who first gave the particle its name that of the electron-neutrino, it
is noted that during negative beta decay an electron-antineutrino is emitted
Neutrinos
only interact with matter through the weak nuclear force and as such rarely
interact with other particles and since they are not charged do not ionise
materials. This gives them the ability to penetrate for great distances.
It is believed that only 1 in 10
billion, travelling through matter a distance equal to the Earth's diameter,
reacts with a proton or neutron.
Electron-neutrinos were first
experimentally observed in 1956 at the nuclear reactor at Savannah River, when a
beam of antineutrinos from a nuclear reactor produced neutrons and positrons by
reacting with protons. This therefore strictly created antineutrinos.
Another
type of neutrino, produced when pi mesons (pions) decay, was conclusively shown
(1962) to be a different species: the muon-neutrino. Although they are as
unreactive as the other neutrinos, muon-neutrinos were found to produce muons
but never electrons when they react with protons and neutrons. The American
physicists Leon Lederman, Melvin Schwartz, and Jack Steinberger received the
1988 Nobel Prize for Physics for having established the identity of muon
neutrinos. In the mid-1970s, particle physicists discovered yet another variety
of charged lepton, the tau. A tau-neutrino and tau-antineutrino are associated
with this third charged lepton.
Encyclopaedia
Britannica
The
masses of the various neutrinos would differ. It seems likely that though the
electron neutrino is constrained to very low levels the constraints on the mass
of other neutrinos is not as strict, though all would be significantly smaller
than that of the proton. This property led to the belief that neutrino mass
might be an uncounted part of the known mass of the Universe. This came at a
time when people were becoming aware that there seemed to be a missing part of
mass in the Universe. These particles are therefore theorised to add to the
mass, they would be dark matter in that they are
effectively undetectable, but could number in enormous quantities as relics of
the big-bang. These and other particles of a similar nature are referred to as
Weakly Interacting Particles or WIMPS.
Gribbin
1998
Unfortunately,
the heavier types of neutrino are significantly less abundant than the
electron-neutrino. The highest predicted mass so far is
discussed by cosmologist Joel Primack (University of California, Santa Cruz),
“The Super-Kamiokande results
imply that these elusive particles constitute at least 1/3 of 1 percent of the
mass in the universe - as much as all of the visible stars and galaxies. If
neutrinos weigh no more than that, they could not have played a significant role
in shaping the universe's large-scale structure.” But David O. Caldwell (University of California, Santa
Barbara), an investigator with the Liquid Scintillator Neutrino Detector
experiment at Los Alamos National Laboratory, says his team found that neutrino
mass could be much higher.
Sky
Telescope; September 1998
The most common way of detecting neutrinos is to use large vats filled with solutions rich in chlorine atoms, the neutrinos can convert a few of the chlorine-37 (37Cl) atoms to argon-37 (37Ar) atoms with a half-life of 35 days. These atoms can then be detected by nuclear decay counting to determine the flux of the high-energy neutrinos striking the Earth.
v + 3717 Cl > 3718 Ar + e-
These detectors are in the form of very large (of the order of 600 tonnes of tetrachloroethylene (Cl2Cl4). One in four of the chlorine atoms will be of the type 3717 Cl and hence, on average one atom in the molecule of chlorine will be of the type. The tank will contain something of the order of 2*1030 atoms of 3717 Cl. The detection involves the analysis of the electron type emitted in the decay of the argon, which is a 2.8 KeV event. The current detection is about one event every other day, whcih is only around a third the amount expected from models of solar neutrino modelling. (Roy and Clarke 1988 (a)). A similar experiment for detecting the much larger flux of the beryllium-7 (7Be) neutrinos of lower energy can now be done because of the ability to count a small number of krypton-81 atoms produced by neutrino capture in bromine-81 (81Br). Since the atoms are counted directly without waiting for radioactive decay, the 210,000-year half-life of krypton-81 is not an impediment.
Encyclopaedia Britannica
There
are many neutrino detectors but the largest was constructed by a group of
Japanese and American researchers led by Yoji Totsuka of the University of
Tokyo. They built a detector consisting of a 12-story tank filled with pure
water more than half a mile under a mountain in the Japanese Alps. Billions
of neutrinos passed through the detector, called Super-Kamiokande, each second;
every once in a while one would interact with a neutron or proton in the water,
creating a flash of light. The tank is lined with 13,400 detectors that monitor
flashes of light resulting from neutrinos interacting with the water molecules.
The photo tubes along the inside of the tank recorded each event. While
researchers detected pretty much what they thought they should, one type of
neutrino failed to show up in the expected numbers.
Magazine:
Astronomy, September 1998
return to homepage