Question

A freshly prepared radioactive source of half-life 2 hr 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 this source is –

• A. 6 h
• B. 12h
• C. 24 h
• D. 128h

Answer 1

Answer: (B)

12h

Answer 2

$\frac { \mathrm { N } } { \mathrm { N } _ { 0 } } = \frac { 1 } { 64 } = \left( \frac { 1 } { 2 } \right) ^ { 6 } = \left( \frac { 1 } { 2 } \right) ^ { \frac { \mathrm { t } } { \mathrm { T } } }$ t = 6T = 12 hr