Question

The count rate of a Geiger-Muller counter for the radiation of radioactive material of half life of 30 minutes decreases to 5 s^–1 after 2 hours. The initial count rate was –

• A. 25 s^–1
• B. 80 s^–1
• C. 625 s^–1
• D. 20 s^–1

Answer 1

Answer: (B)

80 s^–1

Answer 2

GivenT_1/2 = 30 min,

t = 2hr R = 5 sec^–1 R₀ = ?

n = $\frac { \mathrm { t } } { \mathrm { T } _ { 1 / 2 } } = \frac { 2 \times 80 } { 30 }$ = 4

A.P = $\frac { \mathrm { R } } { \mathrm { R } _ { 0 } } = \frac { 1 } { 2 ^ { \mathrm { n } } }$

R₀ = 2ⁿ R = 2⁴ × 5

\R₀ = 16 × 5 = 80 sec^–1