Question

A freshly prepared radioactive source of half life $2hr$ 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. $6hr$
• B. $12hr$
• C. $24hr$
• D. $128hr$

Answer 1

Answer: (B)

$12hr$

To work safely, intensity must reduce by $1/64$
$\frac{N}{N_0}=\frac{1}{64}={\left(\frac{1}{2}\right)}^{t/T}$ i.e. ${\left(\frac{1}{2}\right)}^6={\left(\frac{1}{2}\right)}^{1/T}$

$\frac{t}{T}=6$

$t=6T=12$ hrs