A student measures the activity of a radioactive source by placing different materials between the source and the detector. The recorded activities are as follows: with air only, the activity is 820 counts per minute820\text{ counts per minute}820 counts per minute; with a sheet of paper, the activity is 815 counts per minute815\text{ counts per minute}815 counts per minute; with 5 mm thick aluminium, it is 230 counts per minute230\text{ counts per minute}230 counts per minute; and with 20 mm thick lead, it is 25 counts per minute25\text{ counts per minute}25 counts per minute.
Which type of radiation is emitted by the radioactive source?
Alpha particles only
Beta particles and gamma rays only
Alpha and beta particles only
Gamma rays only