Page 136 - Environmental Control in Petroleum Engineering

The Impact of Drilling and Production Operations 123

The decay rate of a group of radioactive nuclei can also be expressed
in terms of the total number of decay events per second, which is
called activity. Activity is the primary measure of the radioactivity of
a material. The units of activity are the Becquerel (Bq), which is equal
to 1 decay/sec. A more common unit of activity is the Curie, which
10
is equal to 3.7 x 10 decays/sec.
A related measure of activity is the specific activity. For the specific
activity, the concentration of radioactive nuclei is typically normalized
in terms of activity per unit mass (for solids), activity per unit volume
(for fluids), or activity per unit area (for surfaces).

3.8.2 Health Physics

The study of the effects of nuclear radiation on human health is
the science of health physics. The effects of radiation are measured
in terms of exposure or dose. Exposure is defined as the electrical
charge released from ionization per unit mass of air. Dose is defined
as the energy from the radiation absorbed per unit mass of material.
One of the most widely used measures of radiation dose is the radia-
tion absorbed dose (RAD), where
1 RAD = 100 erg/gram

The unit of RAD is not particularly useful for measuring human
exposure because it neglects the biological effects of radiation. Dif-
ferent types of radiation have different biological effects for the same
energy deposition. To account for these different biological effects, the
RAD is multiplied by an empirical quality factor. The resulting value
is called the dose equivalent and its most common unit is the REM
(roentgen equivalent man). The quality factor for gamma radiation is
1 (one). Virtually all environmental impacts of nuclear radiation from
the petroleum industry are from gamma radiation.
The impact of radiation exposure also depends on the type of
radiation and where the source is located. The dose from alpha par-
ticles from a source external to the body is zero, because alpha
particles cannot penetrate the skin and reach living cells. Beta particles
are able to penetrate the surface layers of the skin and can provide a
dose to living skin tissue. Any other exposure from alpha or beta
particles can come only from ingesting or inhaling the radionuclide
that emits the particle. Gamma rays, on the other hand, can penetrate