Radiation Dosages

Ionizing radiation comes from natural and artificial sources. The energy absorbed from exposure to radiation is called a dose. Absorption of a dose changes the state of a device, and the changes provide measures of the dosages received. Such devices are called dosimeters. Physical, chemical, and biological changes are used as the bases for dosimeters.

Units in Source, Absorption, and Dosage
Process Type Unit
§ ¨ © ª
¯ ¯ ¯ ¯
a, b, g
neutron (n)
becquerel, (Bq)
curie (Ci)
¯ ¯ ¯ ¯
joul (J)
¯ ¯ ¯ ¯
Gray (Gy)
roetgen (R)
or rad
§ ¨ © ª
¯ ¯ ¯ ¯
X-, b-, g- rays
thermal n
Fast n, a, p
Recoil nuclei
rbe = 1
rbe = 3,
rbe = 10
rbe = 20
in human
Sievert (Sv)
  Sv = Gy*rbe
Rem = rad*rbe
However, radiation effects depend on the type of radiation, and various units are used for dosages.

Radiation Sources

Radioactive sources emit alpha, beta, or gamma rays. Each type has a unique effect on health of living beings. Strengths of sources are measured in the SI unit Bq (becquerel), which is the number of disintegration per second, disintegration or decay rate. However, the cgs unit curie (=3.700x1010 Bq), is still used in medical and technical practices. For convenience, modifiers have been used for the unit Ci. Decay rates say nothing about energies or type of particles emitted.

When neutron and other particles are the sources, the intensity is either expressed as the total number of particles per unit time or the number of particles per unit time per unit area. However, these numbers do not contain information on energy of the beam.

For electromagnetic radiation such as laser, the rate of energy emission (watt) of the beam is often specified. No particular unit is used for intensity of X-rays, but the rate of photon emission is similar to the rate of gamma ray emission.

Energy Exposure and Absorbed Dose

The radiation effect depends on the amount of energy and the type of radiation a person is exposed to. The amount of energy a subject exposed to differ from that absorbed. However, to tell them apart is very difficult. In practice, the reading from a dosimeter represents the amount of energy exposure and absorption.

The amount of radiation energy exposed to or absorbed by a subject is called a dose.

A roentgen (R) is the dose of X- or gamma-rays that produce 1 esu (negative and positive each) charge in 1.0 L (at standard temperature and pressure, STP, 298 K and 1 atmosphere) of air. This dose is equivalent to 0.12 erg absorbed by 0.00123 g of mass, or approximately 100 erg in 1.0 g.

For other particles, absorption of 100 erg per gram is called a rad. Rad and reontgen are equivalent, (1 R = 1 rad).

The SI dose unit is gray (Gy), which is the absorption of 1.0 J per kg of mass. Thus,

1 Gy = 100 R or rad

The Quality Factor and Roetgen Equivalent Man (rem)

Radiations Q or rbe
b, g and X-rays 1
Thermal neutrons (n) 3
Fast n, a, and protons 10
Heavy and recoil nuclei20
The absorbed dosage contains neither information on the type of particles nor the degree of damage.

The study of radiation victims indicated that receiving the same dosage from gamma radiation suffered much less harm than from alpha particles. Alpha particles cause 3 times more damage than do gamma rays or beta particles.

The study of radiation victims indicated that receiving the same dosage from gamma radiation suffered much less harm than from alpha particles. Alpha particles cause 3 times more damage than gamma rays or beta particles. Thus, quality factor (Q) or relative biological effectiveness is introduced to reflect the relative harmfulness of the four types of radiation.

Multiplication of the absorbed dose (in rad or Gy) by the Q factor converted it to a effective dosage equivalent to that of X-ray or gama-ray. The cgs unit used for the dose equivalent is roentgen equivalent man (rem) whereas the SI unit for it is the sievert, (Sv). The relationship between Sv and rem is:

1 Sv = 100 rem
Thus, the rem is the dose from any radiation that produces biological effects in man equivalent to one rad or R of X-ray, gamma- or beta rays.

Multiplying the absorbed dose in Gy or rad by the Q factor converted it to a radiation effective dose in sievert (Sv) and roentgen equivalent man (rem) respectively. Thus, the dose in Gy is also the effective dose in Sv for of X-ray, gamma or beta rays; similarly, the value remains the same in rad, R or rem for this class of radiation. For other types of radiation, their values differ by a factor Q. Dosages are usually given in rem or Sv.

A unit for total energy absorbed by a particular organ called integral dose is gram-rad or g-rad.

Similar to g-rad, the unit g-rem is used for the integral dose of an organ (used in medicine) when other radiation than X-rays, gamma rays, or beta particles are in question.


A dosimeter is a device for the measurement of exposed dose. There are many different types of dosimeters: filmbadges, electroscopes, ionization chambers, chemical dosimeters, biological dosimeters, and sophisticated computer-controlled instruments.

Dosimeters are commercially available from various manufactures. Their choice depends on the need of the work place. Shelf life, lineality of dose response, stability, simplicity in their handling procedure, easy-to-read meters, independence of dose rate and equal responses to X-rays, gamma rays and beta particles are some of factors on the quality of dosimeters.

Radiation causes chemical and biological changes. If the changes are quantitatively related to the absorbed dose, they are dosage indicators. Their applications lead to chemical and biological dosimeters.

Radiation Exposure

Exposure Levels: Effects on Human systems: