A student determines the rate at which ionising radiation is emitted by a radioactive substance. He positions a detector at several different distances from the radioactive source. The table shows how the counter reading changes with distance $d$. The counter readings have been corrected to allow for background radiation. Which type of ionising radiation is being emitted by the substance?
- A$\alpha$-particles
- B$\beta$-particles
- C$\gamma$-rays
- DX-rays