PLUTONIUM
D-3
APPENDIX D
The activity of a radionuclide at time t may be calculated by:
-0.693t/Trad
A = A
o
e
where A is the activity in dps or curies or becquerels, A
o
is
the activity at time zero, t is the time at which
measured, and T
rad
is the radiological half-life of the radionuclide (T
rad
and t must be in the same units of
time). The time when the activity of a sample of radioactivity becomes one-half its original value is the
radioactive half-life and is expressed in any suitable unit of time.
Table D-1. Characteristics of Nuclear Radiations
Typical
Path length
b
Radiation
Rest mass
a
Charge
energy range
Air
Solid
Comments
Alpha (α)
4.00 amu
+2
4–10 MeV
5–10 cm 25–80 μm Identical to ionized
He nucleus
Negatron (β
–
)
5.48x10
-4
amu;
–1
0–4 MeV
0–10 m
0–1 cm Identical
to electron
0.51 MeV
Positron (β
+
)
5.48x10
-4
amu;
+1
0-4 MeV
0–10 m
0–1 cm Identical to electron
0.51 MeV
except for sign of
charge
Neutron
1.0086 amu;
0
0–15 MeV
b
b
Free half-life: 16 min
939.55 MeV
X ray
(e.m. photon)
–
0
5 keV–100 keV
b
b
Photon from
transition of an
electron between
atomic orbits
Gamma ()
(e.m. photon)
–
0
10 keV–3 MeV
b
b
Photon from nuclear
transformation
a
The rest mass (in amu) has an energy equivalent in MeV that is obtained using the equation E=mc
2
, where 1 amu = 932 MeV.
b
Path lengths are not applicable to x- and gamma rays since their intensities decrease exponentially; path lengths in solid tissue
are
variable, depending on particle energy, electron density of material, and other factors.
amu = atomic mass unit; e.m. = electromagnetic; MeV = MegaElectron Volts
The specific activity is
a measure of activity, and is defined as the activity per unit mass or per unit
volume. This activity is usually expressed in curies per gram and may be calculated by
curies/gram = 1.3x10
8
/ (T
rad
) (atomic weight)
or
[3.577 x 10
5
x mass(g)] / [T
rad
x atomic weight]
where T
rad
is the radiological half-life in days.
In the case of radioactive materials contained in living organisms, an additional consideration is made for
the reduction in observed activity due to regular processes of elimination of the respective
chemical or
biochemical substance from the organism. This introduces a rate constant called the biological half-life
(T
biol
) which is the time required for biological processes to eliminate one-half of the activity. This time
is virtually the same for both stable and radioactive isotopes of any given element.
PLUTONIUM
D-4
APPENDIX D
Under such conditions the time required for a radioactive element to be halved as a result of the combined
action of radioactive decay and biological elimination is the effective clearance half-time:
T
eff
= (T
biol
x T
rad
) / (T
biol
+ T
rad
).
Table D-2 presents representative effective half-lives of particular interest.
Table D-2. Half-Lives of Some Radionuclides in Adult Body Organs
Half-life
a
Radionuclide
Critical organ
Physical
Biological
Effective
Uranium 238
Kidney
4,460,000,000 y
4 d
4 d
Hydrogen 3
b
Whole body
12.3 y
10 d
10 d
(Tritium)
Iodine 131
Thyroid
8 d
80 d
7.3 d
Strontium 90
Bone
28 y
50 y
18 y
Plutonium 239
Bone
surface
24,400 y
50 y
50 y
Lung
24,400 y
500 d
500 d
Cobalt 60
Whole body
5.3 y
99.5 d
95 d
Iron 55
Spleen
2.7 y
600 d
388 d
Iron 59
Spleen
45.1 d
600 d
42 d
Manganese 54
Liver
303 d
25 d
23 d
Cesium 137
Whole body
30 y
70 d
70 d
a
d = days, y = years
b
Mixed in body water as tritiated water
D.2.3 Interaction of Radiation with Matter
Both ionizing and nonionizing radiation will interact with materials; that is, radiation will lose kinetic
energy to any solid, liquid or gas through which it passes by a variety of mechanisms. The transfer of
energy to a medium by either electromagnetic or particulate radiation may be
sufficient to cause
formation of ions. This process is called ionization. Compared to other types of radiation that may be
absorbed, such as ultraviolet radiation, ionizing radiation deposits a relatively large amount of energy into
a small volume.
The method by which incident radiation interacts with the medium to cause ionization may be direct or
indirect. Electromagnetic radiations (x rays and gamma photons) are indirectly ionizing; that is, they give
up their energy in various interactions with
cellular molecules, and the energy is then utilized to produce a
fast-moving charged particle such as an electron. It is the electron that then may react with a target
molecule. This particle is called a “primary ionizing particle. Charged particles, in contrast, strike the
tissue or medium and directly react with target molecules, such as oxygen or water.
These particulate
radiations are directly ionizing radiations. Examples of directly ionizing particles include alpha and beta
particles. Indirectly ionizing radiations are always more penetrating than directly ionizing particulate
radiations.