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
+
.
(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.
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
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