A student measures the radiation level produced by a radioactive substance. He positions a detector very near to the substance. He places various absorbers between the radioactive substance and the detector. The student’s results are shown below. These results have been corrected to allow for background radiation. Absorber and counter reading (counts per minute): - none: $95$ - thin paper: $52$ - few mm of aluminium: $52$ - several cm of lead: $12$ Which types of radiation are being emitted by the substance?
- A$\alpha$-particles and $\beta$-particles only
- B$\alpha$-particles and $\gamma$-rays only
- C$\beta$-particles and $\gamma$-rays only
- D$\alpha$-particles, $\beta$-particles and $\gamma$-rays