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Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

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248 CHAPTER 6<br />

If the fraction of this emitted energy that is absorbed by the target is called ϕi,<br />

then the amount of energy absorbed by the target due <strong>to</strong> emission from the source<br />

is given by<br />

χai = χei × ϕi = 1.6 × 10 −13 × AS × Ēi × ni × ϕi J/s. (6.79)<br />

Since 1 Gy corresponds <strong>to</strong> the absorption of 1 J/kg the dose rate from the ith particle<br />

<strong>to</strong> a target that weighs m kg is given by<br />

˙D =<br />

<br />

−13 J<br />

1.6 × 10 × As × Ē i × ni × ϕi<br />

1 J/kg<br />

Gy<br />

× m kg<br />

If we let<br />

i = 1.6 × 10 −13 kg · Gy<br />

× ni × Ē i<br />

Bq · s<br />

then Eq. (6.80) can be written as<br />

˙Di = AS<br />

m × ϕi × i<br />

s<br />

. (6.80)<br />

(6.81)<br />

Gy<br />

. (6.82)<br />

s<br />

i is the dose rate in an infinitely large homogeneous mass of tissue containing<br />

a uniformly distributed radionuclide at a concentration of 1 Bq/kg. Numerical<br />

values for i for each of the radiations generated by radioiso<strong>to</strong>pes in infinitely<br />

large masses of tissue are included in the output data section of the decay schemes<br />

and nuclear parameters for use in radiation dose estimation that have been published<br />

by the MIRD Committee of the Society of Nuclear Medicine. Considering<br />

all types of the particles emitted from the source, the dose rate <strong>to</strong> the target<br />

organ is<br />

˙D = As<br />

m<br />

ϕi i. (6.83)<br />

Since ˙D is a function of As, which is a function of time, ˙D <strong>to</strong>o is a function of<br />

time. The dose commitment, that is, the <strong>to</strong>tal dose due <strong>to</strong> the complete decay of<br />

the deposited radionuclide, is given by integrating the dose rate with respect <strong>to</strong><br />

time:<br />

D =<br />

∞<br />

0<br />

˙D(t)dt =<br />

ϕi i<br />

m<br />

∞<br />

0<br />

As(t)dt. (6.84)<br />

If we call the time integral of the deposited radioactivity the cumulated activity, Ã,<br />

à =<br />

∞<br />

0<br />

As(t)dt, (6.85)

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