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Physical chemistry 208

radiation can be emitted during radioactive decay; α-particles, β-particles or γ-
radiation.
The activity of a radioactive source is measured in bequerel (Bq) which is defined as
10
one nuclear disintegration per second. (The older unit of curie (Ci) is equal to 3.7×10
disintegrations per second.) The unit of absorbed dose is the gray (Gy) which is defined
as one joule per kilogram.

α-decay
An α-particle consists of two protons and two neutrons and is effectively the nucleus of a
helium-4 atom, . Therefore, the daughter isotope resulting from emission of an α-
particle has atomic mass number, A, four units less than the parent isotope and atomic
number, Z, two units less, and is an isotope of the element two places to the left in the
periodic table. Decay by α-particle emission usually only occurs amongst the heaviest
elements which have mass numbers greater than 200. An example of α-decay is the
transmutation of the uranium-238 isotope to an isotope of thorium:

α-particles are the most massive and highly charged (+2 charge units) of the particles
emitted during spontaneous radioactive decay and travel relatively slowly, approximately
10% of the speed of light Since α-particles readily lose their energy and neutralize their
charge in collisions with surrounding elements, their effects are short-range (a few
millimeters) and, in general, α-emitters are not considered particularly hazardous since
they cannot penetrate through skin. However, they may cause burns to the outer layers of
skin and are dangerous if ingested.

β-decay
A β-particle is an electron. It therefore has a small mass, and a charge of −1 atomic
charge unit. β-decay occurs when a neutron spontaneously converts into a proton, which
remains in the nucleus, and an electron, which is emitted. Consequently, the parent and
daughter nuclides have identical atomic mass number, A, but the daughter nuclide is an
isotope of the element one atomic unit higher than the parent. An example is the β-decay
of the carbon-14 radioisotope into the equivalent mass number isotope of nitrogen:

The velocities of β-particles are greater than those of α-particles, because of the much
lighter mass, and it is possible for high-energy β-particles to penetrate skin and reach
internal organs.

γ-decay
Gamma (γ) rays are very high energy photons often released during α- and β-decay
processes when the daughter nuclide decays from an excited to a more stable state. Since
γ-rays are photons of the electromagnetic spectrum they have no mass and no charge
and travel at the speed of light. Photons of γ-rays are even more energetic than those of
X-rays and are thus extremely penetrating and highly damaging. The subsequent β-decay