Physics (Essay) Paper 2 May/June 2015

Question 12

(a) Define nucleon number
(b) A radioactive isotope of Americium (Am-241) decays into a nucleus of
            Neptunium (Np-237) and an alpha (α) particle as shown in the nuclear equation below 

  •  State the number of neutrons in the nucleus of Americium  -  241
  •  Determine the values of a, b and c.
  • .
    (c)    (i)    Why are y – rays not deflected by electromagnetic fields?
    (ii)     State two properties of gamma rays that make them suitable for sterilizing medical
    (d)      A sample of radioactive substance was found to be left with of its initial count rate after 110 years.  Calculate its decay constant.   


    Part (a):  Nucleon number was well defined except few that did not add ‘total’ as well as in the nucleus.  Generally performance was average.

    Part (b):  Candidates performed above average.  The value of a, b and c were correctly evaluated by candidates thus showing that they have a fair knowledge of radioactive isotope.
    Part (c):  Few candidates that attempted this, part  had a fair knowledge of gamma rays.  In the (ii), candidates were able to mention only one out of the two demanded correctly.

    Part (d):  The numerical was poorly attempted.  The formula N  =  No  –λt  eluded many candidates. Some candidates even lack the computational skills involved to solve the problem.  Performance was poor.

    The expected answers are:

    12.  (a)  Nucleon number
    This is the total number of protons and neutrons in the nucleus of an atom                                       

    (b)(i)   Number of neutrons  =  (241- 95 )
    =  146          

                (ii)   a = 2                                                                                                                        
    b = 4                                                                                                                      
    c  =  95-2 =93                                                                                                        

    (c) (i)   - rays carry no charge/ are electrically neutral                                                                           

    (ii)   -   Possess low ionising power                                                                                

    - Possess strong penetrating power                                                                  

    (d)      N  =  No  –λt                               OR                = (  ) n              
    5  =  ()n  = (  ) 5    
    =    –λt                                                      n = 5                          
    =    –110λ                                                    T ½   =                        
          –  =  In  0.03125      =  -3.46574                                       =                    
    =   0.0315 year -1  = 3.2 x 10 -2 per year                               =     22 years  
    =  3.2 x 10-2 year-1