Question

A freshly prepared radioactive source of half-life

2 hours emits radiation of intensity which is 64 times the permissible safe level. The minimum time after which it would be possible to work safely with the source is –

• A. 6 hours
• B. 12 hours
• C. 24 hours
• D. 128 hours

Answer 1

Answer: (B)

12 hours

Answer 2

N = $\frac { N _ { 0 } } { 2 ^ { \mathrm { n } } }$ Ž 1 = Ž $\frac { 1 } { 64 }$ = $\frac { 1 } { 2 ^ { n } }$

$\frac { 1 } { 2 ^ { 6 } }$= $\frac { 1 } { 2 ^ { n } }$ Ž n = 6

$\frac { \mathrm { t } } { \mathrm { T } }$= 6 Ž t = 6 × 2 = 12 hours.