Radioactivity is the spontaneous disintegration of an unstable
atomic nucleus and the emission of particles or electromagnetic
radiation. All naturally occurring elements with atomic
numbers greater than 83, as well as some isotopes of lighter elements,
are radioactive. Three different types of radiation are identified.
Alpha particles (a)
are helium nuclei, containing two protons and two neutrons. They are
deflected slightly in an electric of magnetic field. Their penetrating
power is very low, being stoppable by a thin sheet of aluminum or
Beta particles (b)
are electrons capable of travelling at speeds approaching the speed of
light. Their low mass allows them to be deflected greatly in an
electric or magnetic field, in the opposite direction as the deflection
of alpha particles. Their high speed gives them greater penetrating
power than alpha particles. Some beta particles can penetrate several
centimetres of aluminum. (Some refer to beta particles as "beta
negative particles", to distinguish them from beta positive particles
-- positrons.) Alpha particle emissions and beta particle emissions
change the composition of the nucleus.
(g) are high energy
electromagnetic radiation with short wavelengths. Gamma rays, unlike
alpha and beta particles, do not change the composition of the nuclide.
They have the highest penetrating power, being able to penetrate at
least 30 centimetres of lead.
Background radiation comes from a variety of radioactive sources.
Cosmic rays penetrating the Earth's atmosphere from outer space
usually account for less than 25% of background radiation (but this
depends on altitude). Minute quantities of naturally occurring
radioactive sources in the surroundings (e.g., soil, air, drinking
water, building materials, food, etc.) also contribute to background
affect the emulsion of photographic film, ionize surrounding air
molecules, make certain compounds fluoresce, and have certain special
biological effects. They undergo radioactive decay.
Nuclide charts, with atomic number plotted against neutron number,
are used in nuclear physics to illustrate a decay series.
found in naturally occurring sources and in artificially produced ones.
People are constantly being exposed to radiation from a
variety of natural and human-created sources. Exposure should be
minimized, but it can never be reduced to zero.
Dosimetry is the measurement of radiation and the study of its effects
on living organisms.
There are several different units used to measure
The absorbed dose describes the amount of energy deposited
per kilogram of exposure time, measured in the gray
1 Gy = 1 J / kg = 100 rads
(rads are non-SI, but in general use.)
The biological damage produced on a given organism is called the dose
equivalent, measured in sieverts (Sv).
1 Sv = 100 rem = 105 mrem
Becquerel (Bq) is the
activity of a source produced when one disintegration per second occurs
from a radioactive source.
1 Bq = 1 disintegration per second
kBq and MBq are often used to express
the radioactivity of a source.
1 curie (Ci) = 3.7 × 1010 Bq