A freshly prepared radioactive source of half-life 2 hrs. emits radiations 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 hrs.
b. 12 hrs.
c. 24 hrs.
d. 128 hrs.
A thorium nucleus is formed when a uranium nucleus emits an . The atomic number of thorium is
The activity of the radioactivity element decreases to one-third of the original activity in a period of 7 years. After a further lapse of 7 years, its activity will be
A radioactivity sample has nuclei at a certain time. The number of active nuclei still remaining after 4 half-lives is