Alpha Decay
Beta Decay
In 1899, Ernest Rutherford discovered that uranium compounds produce
three different kinds of radiation. He distinguished the radiations according
to their penetrating abilities and named them alpha, beta, and? gamma radiation,
after the first three letters of the Greek alphabet. Alpha radiation can
be stopped by a sheet of cardboard and was later found to be the nucleus
of a helium atom. Beta particles were later found to be high speed electrons.
The emission of an alpha particle from a substance is called alpha decay.
Since alpha particles contain protons and neutrons they must come from
the nucleus of an atom. The nucleus that comes from alpha decay will have
a different mass and charge from the original nucleus. Through radioactive
decay, the process of transmutation occurs and a substance can transform
into a different one.
Careful measurements show that the sum of the masses of the daughter
nucleus and the ? particle is a bit less than the mass of the parent isotope.
Einstein's famous equation, E=m•c2, which says that mass is proportional
to energy, explains this fact by saying that the mass that is lost in such
decay is converted into the kinetic energy carried away by the decay products.
Beta particles are negatively charged electrons emitted by the nucleus.
The mass of an electron is a fraction of an atomic mass unit so an nucleus
that undergoes beta decay is only changed by a small amount.
In a stable nucleus, the neutron does not decay. A free neutron, or
one bound in a nucleus that has an excess of neutrons, can decay by emitting
a ? particle. The source of the energy released in ? decay is explained
by the fact that the mass of the parent isotope is larger than the sum
of the masses of the decay products. Mass is converted into energy just
as Einstein predicted.
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