APPENDIX E: Roentgens, RADs,
REMs, and other Units
Relationship Between the Roentgen and the Rad
Roentgen (R) (top)
The Roentgen (R) is the special unit of exposure, which is the measure
of the ionization produced in air by x or gamma radiation. Exposure
is the sum of the electrical charges on all ions of one sign produced
in air when all electron liberated by photons in a volume element of
air are completely stopped in air, divided by the mass of the air in
the volume element. Specifically, the Roentgen is defined as 2.58 x
10-4 Coulombs of charge produced by x or gamma rays per kilogram of
air. Thus, the Roentgen characterizes a radiation field by an indirect
measurement of an effect, namely, the ionization produced in air.
The Roentgen has several limitations. For example, it is limited to
x or gamma radiation; it is impractical above several MeV; and more
importantly, it is not a measure of absorbed dose. It is retained and
used because it is satisfactory for most gamma energies encountered
and because measurement of air ionization is still widely used.
The energy actually absorbed by a sample or a biological system is
obviously more important than the effect the incident energy has on
air, especially when attempting to relate dose and effect. For this
reason the concept of absorbed dose is used; i.e., the energy absorbed
per unit mass. An absorbed dose applies to the energy deposited by any
kind of radiation in any kind of material. The special unit of absorbed
dose, the rad, is equivalent to the absorption of 100 ergs of energy
per gram of material.
Relationship Between the Roentgen and the Rad
It can be shown that one gram of air will absorb 87 ergs of energy
and that one gram of soft tissue will absorb 96 ergs of energy when
exposed to a radiation field which produces an exposure of one Roentgen.
This is true to within two percent for gamma energies from 0.1 MeV to
3 MeV. Thus, for many practical health physics problems, over the range
of energies usually encountered, the rad and Roentgen are often used
For radiation protection purposes it is useful to define a quantity,
the dose equivalent, which describes the effect of radiation on tissue.
Equal absorbed doses of radiation may not always give rise to equal
risks of a given biological effect, since the biological effectiveness
may be affected by differences in the type of radiation or irradiation
conditions. Thus, the dose equivalent is defined to be the product of
the absorbed dose and a modifying factor or factors:
Dose Equivalent = Absorbed Dose (rads) x Quality Factor,
where the quality factor, the most common modifying factor, takes into
account the relative effectiveness of the radiation in producing a biological
effect. The special unit of dose equivalent is the rem.
Quality Factor (top)
The values for quality factor given in the table below are those recommended
by the International Commission on Radiological
Protection in ICRP Publication 26:
|Types of Radiation
||Quality Factor (QF)
|x or gamma rays
|*neutrons and protons of unknown energy
|singly charged particles of unknown energy with rest mass greater
then 1 amu
|particles of multiple or unknown charge of unknown energy
|*QF is a function of energy. The values of QF for
neutrons and protons of unknown energy are found in ICRP Publication
21. The QF for neutrons is tabulated in Appendix G.
The value of the quality factor for each type of radiation depends
on the distribution of the absorbed energy in a mass of tissue. For
example, the increased effectiveness of neutrons relative to gamma rays
is believed to be related to the higher specific ionization of the recoil
protons liberated by neutron bombardment as compared to the specific
ionization of the secondary electrons arising from gamma ray irradiation.
The values of quality factor are known to vary with the biological effect
being observed, and in some cases are still a matter of controversy
for the same biological effect.
Curie (Ci) (top)
The Curie is the special unit of activity, which is a measure of the
amount of radioactivity present in a substance. One Curie is that amount
of radioactive material which will produce 3.70 x 1010 nuclear transformations
(disintegrations) per second.
New Units (top)
The International Commission on Radiation Units has called for a conversion
from the present radiation units to the System International system
of measurements. These new units will be phased into use and will replace
the old units over the next few years. They are:
1 Bequerel (Bq) = 1 disintegration per second; replaces the Curie.
No special unit; Coulombs per kilogram will be used instead of the
1 Gray (Gy) = 104 ergs per gram; replaces the rad as a unit.
1 Sievert (Sv) is equivalent to 100 rems, but replaces the rem as