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This publication is no longer validPlease see http://www-ns.iaea.org/standards/"The roentgen shall be the quantity of X- or gam m a radiationsuch that the associated corpuscular emission per 0.001293 gof air produces, in air, ions carrying 1 esu of quantity ofelectricity of either sign. "The roentgen, which is the unit of exposure, is not a radiationunit in the strict sense of the term. It describes neither the numberof photons in the beam, nor their energy; it merely gives the effectof that radiation in 1 cm 3 of air at N . T . P . W hen the charge carriedby all ions of either sign produced by the charged particles arisingin each cm 3 of air is found to be 1 esu, then the original volumeof air is said to have been exposed to one roentgen of X- or gam m aradiation. It can easily be shown that one roentgen of X- or gam m aradiation results in the absorption of 86. 9 erg/g of air, but in substancesof different atomic number and different density the amountof energy per unit mass for the same quantity of radiation will bedifferent — for instance, in soft tissue the energy absorbed per gramis about 98 erg.5. 1. 2. The rad, DThe term "roentgen" applies only to measurements in air ofX-rays and gam m a energies up to 3 M e V . However, ionization intissue can also be produced by radiations other than photons, suchas alpha particles, beta radiation, neutrons and protons. As radiationhas to be measured in terms of the energy absorbed in themedium of interest, it was considered necessary to introduce aunit which would depend only on the amount of energy absorbed perunit mass of irradiated material and not on the energy or type ofradiation, or on the nature of the absorber. Hence the <strong>International</strong>Commission on Radiological Units and Measurements introduced theconcept of "absorbed dose" (now called "dose") in 1953 and this isdefined as the amount of energy imparted to matter by ionizingparticles per unit mass of irradiated material at the place of interest.The rad is the unit of dose and is 100 erg/g. Thus the term radiationdose means one quantity only, i.e . the amount of energyabsorbed. It may be noted that in soft tissue one rad is approximatelyequal to the absorbed dose delivered when such tissue isexposed to one roentgen of medium-energy X-rays.5. 1.3 . The rem, H (dose equivalent)From the biological point of view, evidence has accumulatedthat the effects of the various types of ionizing radiations are not30
This publication is no longer validPlease see http://www-ns.iaea.org/standards/the sam e. One can assume that radiation can bring about a changein a system only by virtue of the energy actually absorbed. Abiological effect, however, may also depend upon the spatial distributionof the energy released along the track of the ionizing particle.It will depend therefore on the type and quality of the radiation,and equal energy absorptions of different types of radiations maynot produce the same biological effects. Generally, the effect ofradiation on cell structures increases with increasing L E T , althoughcertain "all-or-none" effects, like the inactivation of bacteria andviruses, become less efficient per unit energy absorption as theL E T of the radiation increases.The term R B E (relative biological effectiveness) is used tospecify the variation in the degrees of effectiveness of differenttypes of radiation and is defined as follows:_ Dose of 250-kV X-rays to produce a certain biological effectDose of a given radiation to produce the same biological effectThe biological effect of a particular type of radiation depends thereforenot only on the absorbed dose but also on the R B E of the radiation.To take these factors into account, a new unit, the "rem "(roentgen equivalent man) was defined as that dose of any ionizingradiation which, when delivered to m an or m am m al, is biologicallyequivalent to the dose of one rad of X- or gam m a radiation. R e mis here taken as the unit of R B E dose.One thus has the relationship:R B E dose in rems = Dose in rads X R B EThe use of the term R B E both in radiobiology and in radiation protectionpresents certain problems. Hence the <strong>International</strong> C o m mission on Radiological Units and Measurements (ICRU) recom mended that the term R B E be used only in radiobiology and that forprotection purposes a separate name, quality factor, Q , be usedfor the linear-energy-transfer-dependent factor. Therefore, thedose equivalent, H, as defined by IC R U in its Report No. 19 isgiven by: 1H = D Q Nwhere D is the absorbed dose, Q is the quality factor and N is theproduct of any modifying factors such as distribution factors, etc.31
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