# Homework Help: Activity of I_131 after 2 days

1. Apr 15, 2009

### soopo

1. The problem statement, all variables and given/known data
The initial activity of I_131 is 0.74MBq.
The half time of I_131 is 8 days.

How large is the activity after two days?

2. Relevant equations

$$A = A_0 e^{-\lambda t}$$

3. The attempt at a solution

We know
t = 2 days
A_0 = 0.74 MBq
T_0.5 = 8 days

1. Solve the activity constant
$$\lambda = ln2 / T_0.5$$

2. Plug it to the equation
$$A = A_0 e^{(-ln2 / T_0.5) * t}$$

I standardise the units to SI and then omit/cancel them
$$A = 0.74E6 * e^{-ln2 / 4}$$
$$= 6.22E5 Bq$$

---

The right answer is 0.4 times what I get
$$A = 0.4 * 6.22E5 Bq$$
$$= 250 kBq$$

I am not sure where the 0.4 is got.

2. Apr 15, 2009

### soopo

The problem is now solved.

There is the following sentence two pages before the exercise
"Only 40% of the activity in a human is of the form I_131"