In 1896, Henri Becquerel was working with compounds containing the
element uranium. To his surprise, he found that photographic plates
covered to keep out light became fogged, or partially exposed, when
these uranium compounds were anywhere near the plates. This fogging
suggested that some kind of ray had passed through the plate coverings.
Several materials other than uranium were also found to emit these
penetrating rays. Materials that emit this kind of radiation are said
to be
radioactive and to undergo
radioactive decay.
In 1899, Ernest Rutherford discovered that uranium compounds
produce three different kinds of radiation. He separated the radiations
according to their penetrating abilities and named them
a alpha,
b beta, and
g gamma radiation, after the first three letters of the Greek alphabet. The
a radiation can be stopped by a sheet of paper. Rutherford later showed that an alpha particle is the nucleus of a He atom,
4He.
Beta particles were later identified as high speed electrons. Six millimeters of aluminum are needed to stop most
b particles. Several millimeters of lead are needed to stop
g rays , which proved to be high energy
photons. Alpha particles and
g
rays are emitted with a specific energy that depends on the radioactive
isotope. Beta particles, however, are emitted with a continuous range
of energies from zero up to the maximum allowed for by the particular
isotope.
α decay
The emission of an
a particle, or
4He nucleus, is a process called
a decay. Since
a particles contain protons and neutrons, they must come from the nucleus of an atom. The nucleus that results from
a
decay will have a mass and charge different from those of the original
nucleus. A change in nuclear charge means that the element has been
changed into a different element. Only through such radioactive decays
or nuclear reactions can
transmutation, the age-old dream of the alchemists, actually occur. The mass number,
A, of an
a particle is four, so the mass number,
A, of the decaying nucleus is reduced by four. The atomic number,
Z, of
4He
is two, and therefore the atomic number of the nucleus, the number of
protons, is reduced by two. This can be written as an equation
analogous to a chemical reaction. For example, for the decay of an
isotope of the element seaborgium,
263Sg:
263Sg ----> 259Rf + 4He
|
The atomic number of the nucleus changes from 106 to 104, giving rutherfordium an atomic mass of 263-4=
259.
a
decay typically occurs in heavy nuclei where the electrostatic
repulsion between the protons in the nucleus is large. Energy is
released in the process of
a decay. Careful measurements show that the sum of the masses of the
daughter nucleus and the
a particle is a bit less than the mass of the
parent isotope. Einstein's famous equation, E=mc
2,
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.
β Decay
Beta
particles are negatively charged electrons emitted by the nucleus.
Since the mass of an electron is a tiny fraction of an atomic mass
unit, the mass of a nucleus that undergoes
b decay is changed by only a tiny amount. The mass number is unchanged. The nucleus contains no electrons. Rather,
b decay occurs when a neutron is changed into a proton within the nucleus. An unseen neutrino,
, accompanies each
b decay. The number of protons, and thus the atomic number, is increased by one. For example, the isotope
14C is unstable and emits a β particle, becoming the stable isotope
14N:
14C ----> 14N + e- +
|
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 b particle. Sharing the energy with the b
particle is a neutrino. The neutrino has little or no mass and is
uncharged, but, like the photon, it carries momentum and energy. The
source of the energy released in b 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.
γ Decay
Gamma rays are a type of
electromagnetic radiation that results from a redistribution of electric charge within a nucleus. A
g ray is a high energy photon. The only thing which distinguishes a
g ray from the visible photons emitted by a light bulb is its wavelength; the
g
ray's wavelength is much shorter. For complex nuclei there are many
different possible ways in which the neutrons and protons can be
arranged within the nucleus. Gamma rays can be emitted when a nucleus
undergoes a transition from one such configuration to another. For
example, this can occur when the shape of the nucleus undergoes a
change. Neither the mass number nor the atomic number is changed when a
nucleus emits a
g ray in the reaction:
152Dy* ----> 152Dy + γ
Source:
http://www.lbl.gov/abc/Basic.html
Lamarsh, John. Introduction to Nuclear Engineering. 3rd ed. Englewood Cliffs, NJ: Prentice Hall, 2001. ISBN: 9780201824988.
