| Clue | Source | Date |
|---|---|---|
| Helium nucleus | The Times Concise | 05 Dec 2022 |
| Helium nucleus given off by radioactive substances | The Times Concise | 15 Mar 2022 |
| Helium nucleus emitted by unstable isotopes | The Times Specialist Sunday | 27 Feb 2022 |
| Helium nucleus | The Times Concise | 09 Nov 2020 |