Question: Q. 13. Heavy water is a suitable moderator in a nuclear reactor. Explain briefly why?
U [2008, 2011]
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Solution:
Ans. The moderator is used in nuclear reactor to reduce the velocity of neutrons so that it can break the uranium nucleus. Heavy water which has heavy hydrogen is very suitable moderator. The moderator is most effective in slowing down the speed of neutrons when its nuclear mass is of the order of mass of neutron.
This is because if nucleus is too heavy then neutron after collision with it returns back with the same speed. If nucleus is too light then neutron will move forward with almost same speed. If their masses are of the same order (which is in case of heavy water) then they will exchange their speed and neutron will get the thermal speed of deuterons.
Q, 14. The activity $R$ of an unknown nuclide is measured at hourly intervals. The results found are tabulated as follows :
| $\boldsymbol{t}(\boldsymbol{h})$ | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|
| $\boldsymbol{R}(\mathbf{M B q})$ | 100 | 35.36 | 12.51 | 1.56 | 1.56 |
(i) Plot the graph of $R$ versus $t$ and calculate half life from the graph.
(ii) Plot the graph of $\ln \left(R / R_{0}\right)$ versus $t$ and obtain the value of half life from the graph.
[SQP 2016]
Ans.
From the graph we can calculate half life $\approx 40$ minutes
(ii)
$$ \begin{align*} R & =R_{0} e^{-\lambda t} \ \ln R & =\ln R_{0}-\lambda t \ \ln \frac{R}{R_{0}} & =-\lambda t \ \lambda & =-\frac{\ln \frac{R}{R_{0}}}{t} \ & =-\frac{y}{x} \tag{1} \end{align*} $$
From the graph
