Since protons and neutrons are clustered together in the nucleus, the
mass of an atom is concentrated in the nucleus. The atom in Figure
A–1 has two protons and two neutrons; therefore, it is a helium
atom. The atomic weight of an atom is the sum of the protons and
neutrons.
RADIATION THEORY
Radioactivity is the spontaneous breakdown of unstable nuclei
(radioisotopes) with the resulting emission of radiation. The basic
unit of radiation used in the U.S. is the
curie
(
Ci
). The curie is
defined as 3.7
×
10
10
disintegrations of nuclei per second. In the
“Special Form” encapsulated sealed sources used in the 4640-B, the
unit of measure is the
millicurie
(1/1,000 of a curie). The SI unit of
radiation is the
becquerel
(
Bq
). A becquerel equals one
disintegration per second. Therefore, one curie equals 3.7
×
10
10
becquerels.
The strength of radioactive material is measured by its activity, or
rate of decay. This activity decreases with time. The length of time
it takes a given amount of radioactive material to decay to half of its
original strength is referred to as the
half-life
. The half-life of the
cesium-137 (Cs-137) source is 30 years.
A
. R
A
D
IA
T
IO
N
T
H
E
O
R
Y
RADIATION TERMINOLOGY
The
rad
or
radiation absorbed dose
, is the unit of absorbed dose
equal to 0.01 joules/kg in any medium. To account for the effect of
various types of radiation on biological tissue, the
roentgen
equivalent man
(
rem
), or more appropriate for Troxler users, the
millirem
(mrem), is used when measuring radiation dose. The rem is
derived from scaling the radiation absorbed dose (rad) by a quality
factor (QF). One
rad
is equal to the exposure of one
rem
of gamma
radiation.
For example, the quality factor (QF) for the cesium-137 gamma
source in the 4640-B is one. Therefore, one rad of absorbed dose
produces a dose equivalent (absorbed dose
×
QF) of 1 rem. Note
that other types of radiation may have quality factors greater than 1.
Model 4640-B
Appendix
A–3
Содержание 4640-B
Страница 6: ...NOTES vi...
Страница 12: ...NOTES xii...
Страница 17: ...1 GENERAL INFORMATION Figure 1 1 Model 4640 B Parts and Accessories Model 4640 B 1 5...
Страница 22: ...NOTES 1 10...
Страница 26: ...NOTES 2 4...
Страница 84: ...NOTES Appendix A 12...
Страница 108: ...NOTES Appendix C 18...
Страница 130: ...NOTES Appendix F 8...
