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The definition of a Muon
A MUON is an elementary particle like the electron
but with 200 times the mass. Muons are naturally occurring particles
which were first observed coming from the sky
in particle showers (in 1937) but are now routinely
made in accelerator laboratory particle beams.
Muons are spin 1/2 particles known as Leptons (from the Greek word leptos meaning light) and are theoretically described by four-component spinor-fields whose dynamics satisfy the Dirac equation. Neutrinos are also spin 1/2 leptons and there is one each associated with electrons, muons and tauons. (Tauons are
1700 times the electron mass.)
Quarks are an additional kind of spin 1/2 "Dirac" particle but quarks carry an additional strong force "(color)-charge" which leptons do not carry at all.
Muons in cosmic rays
Particle showers originate
from outer space; usually they are started by protons
from the sun but they can also be started by very different
particles (and anti-particles) from
unknown origins which are all collectively called Cosmic Rays. When extra-planetary particles
hit the earth's upper atmosphere
they collide with air molecule nuclei. These naturally occuring
upper atmospheric nuclear collisions produce debris which
showers down on earth. The typical muon-producing
reaction is proton hits nucleus to make pions
and pions then decay to muons and neutrinos. These muons
from pions from upper atmospheric nuclear collisions
can thus be called "tertiary cosmic ray particles"
since the original protons are the "primary cosmic rays"
and the pions are "secondary cosmic rays"!
Muons can catalyze nuclear fusion
The 200 times greater mass of Muons over electrons
results in muonic atoms and muonic molecules that
are 200 times smaller in size than their electronic
counterparts. Smaller molecules mean that the nuclei
in muonic molecules are 200 times closer to each
other which leads to their nuclear fusion. This fusion
generates energetic outgoing neutrons which can boil
water, make steam, turn turbines and thus make electricity.
Muon catalyzed fusion (muCF) can thus be the basis of a new
nuclear reactor technology which happens to also be environmentally friendly.
Fusion is clean waste free nuclear energy ( Fission creates lethal waste as byproduct )
There are two types of nuclear reactors: Fission reactors
whose output is lethally radioactive material which remains radioactive for thousands of years, while Fusion reactors generate output which is only the chemically inert noble gas Helium that
makes ppl talk funny at parties. Clearly Fusion energy is the superior nuclear energy by its waste-free nature.
Fusion is not without its own comparitively low level
of radioactivity (from the input tritium) but by comparison tritium radioactivity is far smaller and therefore much more managable than the radioactivity of Fission waste.
It is emphasized that this type of radioactivity in fusion is input not output.
The tritium has a very low level of radioactivity and is consumed by the fusion process. That means there is less radioactivity after the fusion process than before;
and there will not be a radioactive waste problem with fusion after 50 yrs;
radioactive waste is replaced by the much easier, more reliable and far less dangerous tritium handling processes for the radioactive input fuel.
An additional source of radioactivity that must be managed in a fusion reactor is from the neutrons which boil the water; they also activate the chamber walls and other materials they pass through before and after they boil the water. But this radiation is much easier to manage than the piles of waste growing from all the fission reactors currently in operation. If there is ever a trade off between fission waste and neutron irradiated materials from a fusion reactor the latter are much easier to manage.
Media distortion of Radioactivity
Unfortunately the media whips up hysteria scares for just the
very mention of the word radioactivity when it should be known
that there are places on earth, such as Kerala, India,
where the phosphorous in the ground causes unreasonably high but completely natural background radioactivity in what are considered
unhealthy doses according to the safety standards of the US Dept of Energy. however, humans have evolved for millions of years with radioactivity (it is what makes the interior of the earth molten rock!) and it is not always a killer! At the same time, excesses and mistakes do need to be carefully monitored but that is what is meant by "managable" risk.
Muon Catalyzed Fusion to the rescue of the world energy crisis
The field of muon catalyzed fusion (muCF) is an exciting
field of interdisciplinary research which borders
on atomic, molecular and nuclear physics and has the potential
of creating a controlled thermonuclear reactor here on
earth to generate electricity with no radioactive output at all,
standard radioactive input technology (tritium handling),
standard irradiated-materials control systems,
and overall with an extremely low environmental impact.
Given that there is 20% less solar radiation making it
to the surface of the earth now than 30 yrs ago
we need to STOP polluting the atmosphere and the sooner
we go nuclear fusion the better. The risks of managing fusion radioactivity
are FAR easier to deal with than the risks of more
particulate pollution blocking solar radiation for plant
life on earth. Of course Fusion is preferable to Fission
but I would even venture to say that Fission is better
than killing all photosynthesis on the planet.
It is a pity that there are existing technical obstacles
to a fully functioning energy producing muCF reactor.
In fact, muCF is so good that once it does work
and the technical obstacles are surmounted the reaction can
potentially produce MORE energy than it consumes!
This is why there is an energetic community of
muCF researchers worldwide who are after this Holy Grail
of physics and Holy Grail of energy technology.
The politics of Fusion research
Unfortunately Congress has become sick and tired
of the Tokamak fusion researchers having promised
Fusion energy decades ago and never having overcome
their technical obstacles (which are very different from
the ones in the muCF field). Funding for Tokamak
research has been cut and even discontinued completely;
although I think they have revived somewhat the US
participation in the ITER (International Thermonuclear Experimental Reactor). Funding for muCF in this country, however, has dwindled
to almost nothing: just a handful of researchers
at Los Alamos and maybe a few isolated individuals at
universities. The bulk of the muCF community is in Japan,
Canada, UK, Switzerland, Sweden and other foreign laboratories.
The physics of Hydrogen Fusion
In order to understand a muon catalyzed fusion (muCF)
reaction, some hydrogen physics must be summarized.
A hydrogen atom is just a proton surrounded by an
orbiting electron. (There is a great story for the mathematics
which determine the precise shapes the electron orbits take around the proton in Hydrogen, which would make a really cool web page and maybe it is already on the web someplace. In any event, I hope it makes it onto this web space at some point.)
The proton can join with a single neutron
by the strong nuclear force; the combination remains positively charged since the neutron is chargeless
so a single electron is still captured in orbit. A proton-neutron
bound state is called a deuteron since there are two
"nucleons" in it. (The proton and neutron happen to be two different states, isospin up and isospin down, of a nucleon.)
A deuteron with an orbiting electron is the Deuterium atom.
Binding Energy
When free protons and neutrons get close enough it becomes energetically favourable for them to bind
by the strong force. Energetically favourable means the bound
state has less energy than the state of two free particles.
Since nature generally relaxes to lowest energy states,
the deuteron will form only if the energy difference between the
bound state and the free state can be released. This is
called Binding Energy and it must have a path for exit otherwise the bound state cannot form.
To repeat, if the free proton and neutron
do not have a way to release the binding energy the deuteron will not form! Practically, the binding
energy of their formation can be released in the velocity
of the new deuteron. Perhaps we can imagine
some free protons and neutrons at rest but very
close to each other; if two get close enough they'll
bind together into deuteron but then the deuteron formation process has to release the binding energy and so it shoots off at high speed, since velocity is kinetic energy.
The binding energy is identified as the mass difference between
the bound state deuteron and the total mass
of its constituent parts. Said another way, the free proton and free neutron
mass together are more than the mass of a bound deuteron.
This mass difference is an energy difference by Einstein's
E=mc^2 relation and so this mass difference actually is an energy (called
the Binding Energy).
Hydrogen Isotopes
A single proton can form a bound state with 2 neutrons
and when it does this it is called a triton, since there are three
nucleons. If a triton captures an electon in its orbit it is called the Tritium atom. Atoms with differing numbers of neutrons
for a fixed number of protons are called isotopes. Deuterium
and Tritium are isotopes of Hydrogen, in this case heavy isotopes
and when deuterium replaces hydrogen in water (D2O instead of H2O)
it is called heavy water. There is one D2O molecule for every 6000 H2O molecules in sea water which means deuterium can be mined from sea water. (This may in fact become the energy source of the future if
muon catalyzed or any other fusion reactors become the energy technology of the future!)
A proton is not stably bound with 3 neutrons,
this is just a fact of nature which physics theory just
does not yet understand; this is just a feature of the strong force, it just does not allow it. It may be a consequence of the fact
that the quarks which make up nucleons have electric charge
and when you get too many neutrons together you really have
a lot of quarks in a kind of like bag (there is an "MIT bag model")
but the theory of quarks and gluons does not yet explain
why a proton and 3 neutrons is not stable. That is someone's
PhD thesis of the future and it is not written yet!
(Interestingly, the theory of QCD is already written down
and fairly well exactly specified --it won the
Nobel prize in 2004-- but this particular consequence of the theory
has just not been worked out and derived from the theory
yet.)
The muon catalzyed fusion reaction
A typical muCF reaction takes place when a muon
enters a hydrogen gas. The hydrogen gas needs to include
the isotopes of deuterium and tritium. Since tritium
is the heaviest hydrogen isotope it is energetically
most favourable, i.e. it is the state with the lowest
energy, for the muon to be bound to a triton not a deuteron
or a proton. So ultimately you will have a bunch of t-mu atoms
flying around in the hydrogen mixture with various velocities.
The hydrogen mixture before, during and after the arrival
of muons and the formation of t-mu atoms in its mix,
will have good old-fashioned everyday electronic Deterium
and Tritium molecules floating around and these are
all diatomic which means there are two nuclei in each molecule
not just one (again it is energetically favourable to have
a diatomic molecule instead of two free mono-atomic atoms nearby each other, and again the binding energy of formation has to be released else they cannot form).
The Vesman Mechanism
The reason Binding Energy has been discussed above is that
it is crucial to the muCF reaction process. A critical leap
forward in the understanding of muCF reactions took place in
1967 when the Vesman mechanism explained the temperature
dependence of muCF experiments. This was the realization that
changing the temperature of the Hydrogen gas target changes the velocity of the
t-mu atoms in the hydrogen isotope mixture which is
the standard target in all muCF experiments.
tmu velocity + electronic D2 molecule --> excited electronic D2 molecule + dtmu bound state
The electronic Deuterium molecule thus "hosts" the formation
for a dtmu mini-molecule inside it, since the electronic Deuterium molecule can absorb the dtmu binding energy in one of its own excitations. It is precisely this electronic orbit excitation which is the avenue through which the binding energy for dtmu formation is released. Because there is a way to release the binding energy then the fusion reaction occurs, and as it turns out experimentally, it occurs resonantly at certain temperatures more than others. This is due to the fact that electronic orbit excitation energy is a very specific discrete amount of energy (not a wide continuum) and so only certain t-mu velocities can excite the host electronic Deuterium molecule. So it is that nature has found a natural way for binding energy release which allows the nuclear fusion reaction to proceed, and this can be engineered and exploited to make this reaction work when it might otherwise not.