First slide

Radio activity

Question

A sample radio active subsistence has 10⁶ radio active nuclei. It's half-life is 20 sec. How many nuclei will remain after 10 seconds?

Moderate

A

7 × 10⁵
B

8.5 × 10⁵
C

8 × 10⁵
D

7.5 × 10⁵

Solution

$N = \frac{N_{0}}{2^{t / T}} = \frac{10^{6}}{2^{10 / 20}} = \frac{10^{6}}{2^{1 / 2}} ∴ N = \frac{10^{6}}{\sqrt{2}} = 0.707 x 10^{6} = 7 x 10^{5}$

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