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Radiation Safety Guide


Radiation Safety Guide

APPENDIX E: Roentgens, RADs, REMs, and other Units

Roentgen (R)


Relationship Between the Roentgen and the Rad


Quality Factor

Curie (Ci)

New Units

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.

RAD (top)

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 (top)

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 interchangeably.

REM (top)

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 1
beta particles 1
*neutrons and protons of unknown energy 10
singly charged particles of unknown energy with rest mass greater then 1 amu 10
alpha particles 20
particles of multiple or unknown charge of unknown energy 20
*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 Roentgen.

Absorbed Dose:

1 Gray (Gy) = 104 ergs per gram; replaces the rad as a unit.

Dose Equivalent:

1 Sievert (Sv) is equivalent to 100 rems, but replaces the rem as a unit.



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