A freshly prepared radioactive source of half life 2 h emits radiation of intensity which is 64 times the permissible safe level. The minimum time after which it would be possible to work safety with this source is  


Correct option is

12 h

The radioactivity of a sample decreases to  in n half lives. Since the initial activity is 64 times the permissible level, it must become  of its initial value. 


In other words in six half-lives the activity will reduce itself to the permissible level. Since half life is 2 hours, the total time taken


                = 12 hours.



How much energy is released when one amu is wholly converted into energy?


In the equation     


The correct symbol for X is


The kinetic energy of a 300 K thermal neutron is


The energy released by the fission of one uranium atom is 200 MeV. The number of fissions per second required to produce 3.2 W of power is


The binding energy of deuteron  is 1.15 MeV per nucleon and an alpha particle  has a binding energy of 7.1 MeV per nucleon. Then in the reaction 


the energy Q released is 


What is the rest mass energy of an electron?


After two hours  of the initial amount of a certain radioactive isotope remains undecayed. The half life of the isotope is  


The half life of a certain radio isotope is 10 minutes. The number of radioactive nuclei at a given instant of time is 108. Then the number of radioactive nuclei left 5 minutes later would be


The half life of Pa – 218 is 3 minutes. What mass of a 16 g sample of Pa – 218 will remain after 15 minutes?


The decay constant of a radioactive sample is . The half-life and mean-life of the sample are (respectively) given by :