Answer
For^201 Tl,
A= 10 ×3.7 × 107 =3.7 × 108 dps
Using Eq. (3.2),
Because 1 g · atom^201 Tl =201 g^201 Tl =6.02 × 1023 atoms of^201 Tl (Avogadro’s
number),
=46.7 × 10 −^9 g
=46.7 ng
Therefore, 10 mCi of^201 Tl contains 1.4 × 1014 atoms and 46.7 ng.
Problem 3.2
At 10:00 a.m., the 99mTc radioactivity was measured as 150 mCi (5.55 GBq)
on Wednesday. What was the activity at 6 a.m. and 3 p.m. on the same day
(t1/2of 99mTc =6 hr)?
Answer
Time from 6 a.m. to 10 a.m. is 4 hr:
At=150 mCi (5.55 GBq)
A 0 =?
Using Eq. (3.4)
150 =A 0 e+0.1155×^4
A 0 = 150 ×e0.462
= 150 ×1.5872
=238.1 mCi (8.81 GBq) at 6 a.m.
Time from 10 a.m. to 3 p.m. is 5 hr:
A 0 =150 mCi
At=?
lfor m99 Tc hr^1
0 693
6
==0 1155 −
.
.
Mass of Tl in mCi MBq
201 1.40 10^14
(^1037023)
201
602 10
()=
××
. ×
N
A
==
×
×
− =×
l
37 10
2 638 10
140 10
8
6
. 14
.
. atoms
l=
×××
=×−−
0 693
304246060
2 638 10^61
.
.
. sec
28 3. Kinetics of Radioactive Decay