A radioactive source is positioned close to a detector. The detector gives a count rate of 28 000 counts/min. If a piece of card is inserted between the source and the counter, the reading falls to 25 000 counts/min. If an aluminium sheet is inserted between the source and the counter, the reading stays at 25 000 counts/min. If a sheet of lead is inserted between the source and the counter, the reading falls to 19 000 counts/min. What is emitted by the radioactive source?
- A$\alpha$, $\beta$ and $\gamma$-radiation
- B$\alpha$ and $\beta$-radiation only
- C$\alpha$ and $\gamma$-radiation only
- D$\beta$ and $\gamma$-radiation only