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

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

$_0^1n + _{92}^{235}U \to _{56}^{141}Ba + _{36}^{92}Kr + N_0^1n$

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:

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