Environmental Engineering FOURTH EDITION

Radioactive Waste 3 19

A becquerel (Bq) is the SI measure of source strength or total radioactivity, and
is defined as one disintegration per second; the units of becquerels are second-l. The
decay rate dN/dt is measured in becquerels. The historical unit of source strength,
the curie (Ci), is the radioactivity of 1 g of the element radium, and is equal to
3.7 x 1O1O Bq.3 The source strength in becquerels is not sufficient for a complete
characterization of a source; the nature of the radionuclide (e.g., Pu-239, Sr-90) and
the energy and type of emission (e.g., 0.7 MeV, y) are also necessary.
The relationship between activity and mass of a radionuclide is given by

(ln2)(No) -

1112

Q= (’) = KbNo ($) ,

where

(16.4)

Q = number of becquerels,

Kb = the disintegration constant = 0.693/t1/2,

1112 = half-life of the radionuclide, in seconds,

M = mass of the radionuclide, in grams,

No = Avogadro’s number, 6.02 x id3 atomslgram-atom, and

W = atomic weight of the radionuclide, in gramdgram-atom.

The gray (Gy) is the SI measure of the quantity of ionizing radiation that results
in absorption of one joule of energy per kilogram of absorbing material. One gray is
the equivalent of 100 rad, the historical unit of “radiation absorbed dose” equal to
100 ergs/g of absorbing material. The gray is a unit of absorbed radiation dose.
The sievert (Sv) is the SI measure of absorbed radiation dose equivalent. That
is, 1 Sv of absorbed ionizing radiation does the same amount of biological damage to
living tissue as 1 Gy of absorbed X-ray or y radiation. The standard for comparison
is y radiation having a linear energy transfer in water of 3.5 keV/Fm and a dose rate
of 0.1 Gy/min. As previously stated, all ionizing radiation does not produce the same
biological effect, even for a given amount of energy delivered to human tissue. The
dose equivalent is the product of the absorbed dose in grays and a quality factor (QF)
(sometimes called the relative biological efectiveness, or RBE). The historical dose
equivalent unit is the rem, for “roentgen equivalent man.” One sievert is equivalent to
100 rem. These units are related as follows:

QF = Sv/Gy = rem/rad.

Quality factors take into account chronic low-level exposures as distinct from acute
high-level exposures and the pathway of the radionuclide into the human body (inhala-
tion, ingestion, dermal absorption, external radiation) as well as the nature of the

3Marie Curie is said to have rejected a unit of one disintegrationlsecond as too small and insignificant.
The much larger unit of the curie was then established.