At a given instant are 25% undecayed radioactive nuclei in a sample. After 10 seconds, the number of undecayed nuclei reduces to 12.5%. Calculate the time in which the number of undecayed nuclei will further reduce to 6.25% of the reduced number.


Correct option is

40 seconds

The 6.25% of the reduced number (12.5%) is


Suppose, after 10 seconds, the number of undecayed nuclei reduces from 12.5% to 0.78125% in an interval of n half-lives. Then  



or        n = 4. 


                                                        = 40 seconds.



The activity of a radioactive element reduces to th of its original value in 30 years. Find the half-life and the decay constant of the element.


The half-life of thorium-X is 3.64 days. After how many days will 0.1 of the mass of a sample of the substance remain undecayed?


How much of 5.00 gram of polonium will decay in one year? The half-life of polonium is 138 days.



The half-life of a radioactive substance is  against . Calculate the decay rate for  atoms of the substance.


The half-life of  against . Calculate the activity of 1 g sample of . Avogadro’s number is .


The half-life of  is 28 years. How much is the disintegration rate of 15 mg of this isotope? The Avogadro’s number is .


1 g of particles per second. Find its half-life and average life. Avogadro’s number is .


Determine the amount of  required to provide a source of -particles of 5 millicurie strength. The half-life of Po is 138 days. The Avogadro’s number is 


The disintegration rate of a certain radioactive sample at any instant is 4750 disintegrations per minute. 5 minute after, the rate becomes 2700 disintegrations per minute. Calculate the half-life of the sample. . 


The normal activity of a living mater containing radioactive carbon C14 is found to be 15 decays per minute per gram of carbon. An archaeological specimen shows an activity of 9 decays per minute per gram of carbon. Estimate the age of the specimen. The half-life of C14 is 5730 years.