Effective Dose

Date: 2015-10-07; view: 726.

Equivalent Dose Rate

The equivalent dose rate is the equivalent dose per unit time. Dose rates are expressed as sievert per unit time, i.e. Sv/h or or more usually as mSv/h or and or .

If the exposure to different organs or tissue is not uniform, as is the case when radionuclides are deposited in the body, the concept of effective dose is used. The basic idea is to express the risk from an exposure of a single organ or tissue in terms of the equivalent risk from an exposure of the whole body. The effective dose is calculated from the expression:

where:

The way this works can be illustrated by the following example. Suppose a worker has received a tissue equivalent dose, , of to the thyroid gland. How can this be compared to an exposure of the whole body?

The tissue weighting factor for the thyroid is 0.05, and so,

This means that the risk from exposing the thyroid to an equivalent dose of is the same as exposing the whole body to .

If more than one tissue is exposed, the 's for all the tissues are added to to give the :

the sum of all

The recommended values of for the various tissues are shown in Table 3.2 .

For purposes of calculation, the remainder is composed of the following tissues and organs: adrenals, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, and uterus.

In practice, most exposure to radiation at TRIUMF is whole body exposure. A few people may get thyroid or extremity doses .

Table 3.2: Tissue weighting factors