A deuterium nucleus and a tritium nucleus fuse together to produce a helium nucleus and particle X - AQA - A-Level Physics - Question 9 - 2020 - Paper 1

To determine what particle X is, we should analyze the reaction of the fusion of deuterium ( $^2_1H$) and tritium ( $^3_1H$).

1. Identify the Products: The fusion process yields a helium nucleus ( $^4_2He$) and an additional particle X.

2. Balance the Nucleons: We need to ensure the equation balances in terms of nucleons (protons and neutrons).

   • The total initial nucleon count: Deuterium has 2 (1 proton + 1 neutron) and Tritium has 3 (1 proton + 2 neutrons), totalling 5 nucleons.
   • The helium nucleus consists of 4 nucleons (2 protons and 2 neutrons).

3. Deduce the Remaining Particle: Since we started with 5 nucleons and produced 4 in the helium nucleus, 1 nucleon must be left over. This can be a neutron.

4. Confirm by Elimination: The other options (electron, positron, and proton) either do not fit the nucleon count or their respective charges will not balance.

Thus, particle X is most reasonably identified as a neutron (Answer B).