|Units in Source, Absorption, and Dosage|
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|a, b, g|
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|X-, b-, g- rays |
Fast n, a, p
|rbe = 1|
rbe = 3,
rbe = 10
rbe = 20
Sv = Gy*rbe
Rem = rad*rbe
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.
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|
|Radiations||Q or rbe|
|b, g and X-rays||1|
|Thermal neutrons (n)||3|
|Fast n, a, and protons||10|
|Heavy and recoil nuclei||20|
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|
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.
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.