Hi Julia,
You are right. A nucleus which decays by emitting an alpha
particle loses two neutrons and two protons, in order to change its
atomic number by -2 and mass number by -4. So a nucleus with an atomic
number of 2 or less or a mass number of 4 or less cannot decay by
emission of an alpha particle -- there is less than an alpha particle
there in the first place. Typically it is the very heavy nuclei or ones
which have a superabundance of neutrons which decay by emitting alpha
particles.
In nuclear decay, energy isn't really "created", it merely shifts
from one form to another. Some nuclear decays actually create some new
particles where there weren't any before. If a nucleus decays by beta
radiation, one of its neutrons turns into a proton and it emits an
electron and an electron antineutrino. The whole process is mediated by
the Weak Nuclear Force. Neutrons do this all by themselves if they are
not bound up inside a nucleus, and the half-life of this process is
about 11 minutes. The electron and neutrino are created during the
process, and the proton is just a little bit lighter than a neutron.
E=mc**2, and so the little bit of mass difference between the neutron
and proton is enought to not only make the electron and its
antineutrino, but to give them some energy to fly away.
The electron interacts with other charged particles because it is
charged itself. They will slow down and stop when passing through
materials, depositing their energy. If they have lots of energy, they
may knock another electron off of the atom or molecule they strike,
thus "ionizing" it. Ionizing radiation is a potential hazard because it
changes the chemical nature of the molecules which are ionized. These
may re-form into different molecules. If these molecules are part of
the body, burns and/or cancerous changes in the DNA material may
result.
Alpha particles have two units of charge and tend to stop more
easily than beta particles (which are electrons). Small amounts of
shielding are usually enough to keep alpha and beta particles from
straying. They become nasty however when the radioactive materials
which produce them are ingested. Then damage to tissues may occur even
for these very short-ranged particles.
Gamma rays (photons), like their lower-energy cousins X-rays, may
penetrate through solid objects to cause problems elsewhere. Lead and
tungsten are good shields against gamma rays. Free neutrons are also
sometimes produced in nuclear reactions. These may diffuse slowly
through materials and get captured by nuclei, which may then decay by
beta radiation, changing their atomic number and hence their chemistry.
It is good to shield well against free neutrons.
Energy is released in nuclear decay because the total energy of
the nucleus after decay is less than that before. If the nucleus falls
apart into two pieces (as in alpha decay or in the more dramatic
nuclear fission), the pieces are more tightly bound than the original
nucleus, and energy is given up in that way. If a nucleus has many more
neutrons than protons, the neutrons occupy filled orbitals in the
nucleus that fill independently of those for protons. If a neutron
decays into a proton, it may then drop into a lower shell and release
energy that way. In all cases, the total energy of the system
(including all the masses of the particles) is the same before and
after, but some of the binding energy (or difference between a proton
and neutron mass) may get converted into the kinetic energy of an alpha
particle or to produce an electron and antineutrino and to give them
kinetic energy. These fast-moving particles continue to fly until they
hit something, and that object may be damaged in some way when the
energy is deposited.
Tom
(published on 10/22/2007)
