30.06.2013 Views

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

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

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

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

EXTERNAL RADIATION SAFETY 515<br />

By the inverse square law, this distance is found <strong>to</strong> be 2.3 m. Because of the inverse<br />

square effect, the dose rate increases rapidly as a person approaches a point source,<br />

and it decreases rapidly while the person moves away from the source. At any distance<br />

d from a point source of activity A and specific gamma-ray constant Ɣ, the doseequivalent<br />

rate is given by<br />

H ˙<br />

Ɣ × A<br />

=<br />

d2 × w R. (10.1)<br />

The <strong>to</strong>tal dose <strong>to</strong> a person approaching a source during the time t is given by<br />

H =<br />

t<br />

0<br />

˙H dt. (10.2)<br />

Equation (10.2) can be solved in terms of the velocity of approach v and the closest<br />

distance d0 <strong>to</strong> which the source is approached. If we let<br />

then<br />

d = d0 + v × t, (10.3)<br />

dd = v dt, (10.4a)<br />

dt = dd<br />

. (10.4b)<br />

v<br />

If we now substitute Eqs. (10.1) and (10.4b) in<strong>to</strong> Eq. (10.2), we have<br />

H =<br />

∞<br />

d0<br />

Therefore,<br />

Ɣ × A × w R<br />

d 2<br />

× dd<br />

v = Ɣ × A × w R<br />

v<br />

∞<br />

d0<br />

dd<br />

. (10.5)<br />

d2 H = Ɣ × A × w R<br />

. (10.6)<br />

v × d0<br />

W Example 10.1<br />

A 10-Ci (3.7 × 10 5 MBq) 60 Co source failed <strong>to</strong> be retracted in<strong>to</strong> its shield. The<br />

opera<strong>to</strong>r walks <strong>to</strong>ward the source at a rate of 1 m/s, and, at a distance of 1 m from<br />

the source, he s<strong>to</strong>ps, looks at the exposed source for 15 s, realizes that the source is<br />

not in the shield, and leaves at a rate of 2 m/s. What is his dose commitment from<br />

this exposure?

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!