A-level Physics (Advancing Physics)/Fission/Worked Solutions

1. A neutron is fired at some Uranium-235. Barium-141 and Krypton-92 are produced:

$\underset{1}{\overset{}{0}}n{+}_{92}^{235}U\underset{56}{\overset{141}{\to }}Ba{+}_{36}^{92}Kr+N_0^{1}n$

How many neutrons are produced (i.e. what is the value of N)?

Consider the mass numbers:

$1+235=141+92+N$

$N+233=236$

$N=3$

2. What proportion of the neutrons produced must be absorbed in order to make the reaction stable?

Two thirds, assuming that Uranium-235 always produces three neutrons when split, which it doesn't.

3. What would happen if too many neutrons were absorbed?

The reaction would stop.

4. Alternatively, Uranium-235 can split into Xenon-140, two neutrons and another element. What is this element? (You will need to use a periodic table.)

Consider the mass numbers:

$1+235=140+N+2$

$N+142=236$

$N=94$

Then consider the atomic numbers:

$0+92=54+n+2(0)$

$n=38$

So, the element is strontium-94:

$\underset{1}{\overset{}{0}}n{+}_{92}^{235}U\underset{54}{\overset{140}{\to }}Xe{+}_{38}^{94}Sr+2_0^{1}n$