应急人员防护基本知识.pptVIP

* To account for differences in LET when measuring the effect of radiation, each type of radiation has been assigned a radiation weighting factor (WR). This was done by measuring how much of each radiation type it took to produce the same biological effect as 200-keV X rays. As shown, all photons, beta particles, and electrons do the same amount of damage. Thermal neutrons do somewhat more damage, and fast neutrons and alpha particles are extremely damaging. Indeed, of the radiation types that we deal with, alpha particles and high-energy neutrons are the most damaging. * The unit of equivalent dose is the sievert (Sv) named after Rolf M Sievert, the Swedish physicist who laid the foundations of modern radiation physics. The old unit of equivalent dose is the rem, which is equal to the absorbed dose, rad, multiplied by the radiation-weighting factor. Also, just as 1 Gy is 100 rads, 1 Sv is 100 rems. * It is now understood that different organs of the body vary in their sensitivity to absorbed doses of radiation. The ICRP has introduced a quantity, the effective dose equivalent, which reflects not only specific organ doses but also the relative radiosensitivity of the organs. The calculation of effective dose equivalent required knowledge of the radiation doses to individual organs. These were then multiplied by “tissue weighting factors”(WT) to take account of the relative sensitivity of each type of tissue to radiation. This is a weighted sum of doses to individual organs where the value of the tissue weighting factors is based upon the estimates of the relative risk of stochastic effects from the irradiation of the different tissues. E(Sv)= ΣT WT x HT WT :Tissue weighting factor for organ T HT :Equivalent dose received by organ or tissue T Effective dose is expressed in sieverts (Sv). Use of this concept enables comparison between situations where the whole body is irradiated uniformly and where individual organs receive relatively higher doses. * These