Environmental Engineering FOURTH EDITION

322 ENVIRONMENTAL ENGINEERING

Table 16-5. Representative LET Values

Kinetic energy Average LET

Radiation (MeV) (keV/p,m) Dose equivalent

x-ray 0.01-0.2

Y 1.25

B 0.1

1 .o

a 0.1

5 .O

Neutrons Thermal

1 .o

Protons 2.0

5 .O

3.0

0.3

0.42

0.25

260

95

20

16

8

1 .mu

0.7

1 .o

1.4

10

4-5

2-10

2

2

=These equivalents are in terms of X-ray dose, which is defined as 1.00.

Molecules along the path of the ionizing radiation are damaged in the process, as
chemical bonds are broken and electrons are ejected (ionization). Resulting biological
effects are due mainly to the interactions of these electrons with molecules of tissue.
The energy transferred through these collisions and interactions per unit path length
through the tissue is called linear energy tmnsfer (LET) of the radiation. The more
ionization observed along the particle’s path, the more intense the biological damage.
LET can serve as a qualitative index for ranking ionizing radiation with respect to
biological effect. Table 16-5 gives some typical LET values.
Biological effects of ionizing radiation may be grouped as somatic and genetic.
Somatic effects are impacts on individuals who are directly exposed to the radiation.
Radiation sickness (circulatory system breakdown, nausea, hair loss, and sometimes
death) is an ucufe somatic effect occurring after very high exposure, as from a nuclear
bomb, intense radiation therapy, or a catastrophic nuclear accident. Such an accident
occurred at the Chernobyl nuclear electric generating plant (located near Kiev, Ukraine)
in April 1986. Forty-five people received wholebody doses between 4 and 16 Gy, and
died during the 50 days following the accident. An additional 158 people who received
doses between 0.8 and 4 Gy suffered acute radiation sickness; all but one of these
individuals recovered after treatment with red blood cell replacement.
A very tragic accident at a uranium processing facility at Tokaimura, Japan, in
September of 1999 provided an opportunity to study these effects in detail, as well
as developing treatment for acute radiation sickness. Three workers were transferring
a uranium solution from one container to another without proper precautions, when
the solution went critical, releasing a burst of neutrons. It is estimated that two of the
workers received about 12 Sv and a third about 7 Sv. The two most heavily exposed
individuals lived about three and nine months, respectively. The third exposed worker
recovered. Treatment with tissue transplants and a variety of enzymes, blood cell
replacement, and hormone treatment prolonged both the life and comfort of the exposed
workers.