A radioactive source is positioned $20\,\text{mm}$ in front of a detector. The count rate from the source is $2000\,\text{counts/minute}$. When a thin sheet of paper is placed between the source and the detector, the count rate remains at $2000\,\text{counts/minute}$. When a sheet of aluminium $5\,\text{mm}$ thick is placed between the source and the detector, the count rate falls to $1200\,\text{counts/minute}$. Which radioactive emissions are being emitted by the source?
- Aalpha, beta and gamma
- Balpha only
- Cbeta and gamma only
- Dbeta only