Homework Help: calculus -- PLEASE HELP!

Posted by anonymous on Tuesday, February 5, 2008 at 9:46pm.

Iodine-131 is used to treat hypo-thyroidism since it is preferentially absorbed by the thyroid and typically involves a total radiation dosage of 10,000,000 millirem. Iodine-131 has a half-life of eight days.

1. Set up the appropriate integral to represent the radiation dosage delivered by the absorbed iodine in eight days.

2. Use the fact that eight days is the half-life of the isotope to find the initial radiation intensity in millrems/hour.

3. To the nearest 10 millirems, how much of the total radiation is delivered in six weeks?

I got this:

D = int dt Io e^-.0866 t from t=0 to t = 8
but what do I do after that to get B and C?

calculus -- PLEASE HELP! - Damon, Tuesday, February 5, 2008 at 10:20pm
Now I did this yesterday :)
However I did it in days and for part 2 you need it in hours.

To do it in hours, get 8 days in hours
8*24 = 192 hours half life
so
I = Io e^-kt now t in hours
.5 = e^-192 t
ln .5 = -.693 = -192 k
k = .00361
so
I = Io e^-.00361 t
Now do your integral from t = 0 to t = 8 days
D = int I dt = int Io e^-.00361 t from t = 0 to t = 192
or
D = (Io/.00361)(1/2)= 138 Io
so
10^7 millirem = 10^4 rem = 138 Io
so
Io = 10,000/138 = 72.5 rem/hr = 72.5*10^3 millirem/hr

now in 6 weeks
6 weeks = 7*24*6 = 1008 hr
D = (Io /.00361)(1-e^-(.00631*1008))
D = (72.5*10^3/.00361)(1 essentially)
D = 20083*10^3 millirems = 20*10^6 millirems essentially or about twice the total dose we got in one half life logically enough

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