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Units of Radiation Measurement

Activity (Unit: Curie)
Since the discovery by William Roentgen in 1895 that energetic electrons impinging on a target of high atomic number produce rays that easily penetrate matter and can expose photographic film (X rays), the scientific community has adopted special units to describe the amount and nature of ionizing radiation.

The International Commission on Radiological Units (ICRU) was formed to develop a system of units and nomenclature specific to the needs of physicians and other persons working with not only X rays, but other types of radiation found in nature or produced by man. The units that have been developed were named after pioneers in the field (Roentgen, Curie) or began as descriptive terms that turned into acronyms, then into units (rem-"roentgen equivalent man"). The ICRU designated units on the basis of observed quantities. Thus the special unit of activity, the curie, was equal to the number of disintegrations taking place per unit time from 1 gram of radium. The curie (Ci) was later redefined as the activity of that quantity of radioactive material in which the number of disintegrations per second is 3.7E10 (a number nearly the same as the number of disintegrations per second from 1 gram of radium).

We have since learned that a Curie of any radioisotope is a very appreciable amount, too great for most laboratory applications, so we commonly find activity expressed as millicurie (mCi, 1E-3 Curie) or microcurie (µCi, 1E-6 Curie). It is essential that one not confuse the symbol for micro with that for milli. The 1,000-fold error that results may mean the difference between an almost inconsequential radiation problem and a major radiation hazard. A useful number to remember is 2.22E6 disintegrations per minute per microcurie. Most tracer applications require microcurie quantities, although it is not unusual to find millicurie quantities of 3H, 32P, and 125I in many laboratories.

Exposure ( Unit: Roentgen)
The ICRU defined the special unit of exposure in air to be the Roentgen (symbolized by R). R = 2.58E-4 coulomb kg air This unit is special in that it is defined only for X or gamma radiation in air. Thus, the Roentgen is not applicable to alphas, betas, or neutrons. Many survey instruments provide output data in terms of mR/hr (mR, 1E-3R). The Roentgen is not always useful for making accurate evaluations of energy absorbed due to radiation impinging on material. It is the absorbed energy that is a true index of biological damage. If one knows how well a certain material can absorb radiation as compared with air, the energy absorbed by that material when exposed to 1 R can be calculated. It is very easy to measure ionization in air with inexpensive equipment, so that the Roentgen can be measured directly. It is not so easy to measure the energy absorbed in material directly.

Absorbed Dose (Unit: rad)
The rad is the special unit of absorbed energy. It is defined as that amount of ionizing radiation that deposits 100 ergs/gram of material. The rad is applicable to all types of ionizing radiation, yet it is difficult to measure directly. Normally ionization in air or another gas is measured and the absorbed dose in a particular material calculated. One Roentgen results in 87.7 ergs being absorbed in 1 gram of air; if muscle tissue is placed in the same radiation beam, 1 R in air corresponds to about 95 ergs/gram. For most applications of x rays and gamma rays, it is reasonable to assume that 1 R = 1 rad. One Roentgen is a large exposure, therefore, we more often see the term millirad (mrad, 1E-3 rad).

Dose Equivalent (Unit: rem)
The rem is the unit of dose equivalent. The dose equivalent accounts for the difference in biological effectiveness of different types of radiation. It is the product of the absorbed dose (rad) times the quality factor (QF) of the radiation. The quality factor for x, gamma, and beta radiation is 1, therefore for these radiations 1 rad = 1 rem. The quality factor for alpha radiation is 20 and the quality factor for neutron radiation varies with energy from 2-11.