### 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
equipment.
(d)      A sample of radioactive substance was found to be left with of its initial count rate after 110 years.  Calculate its decay constant.

### Observation

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.

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
OR
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