A radioactive source is placed near to a detector, as shown in the figure below. The meter shows a reading of 
.
A sheet of paper is placed between the detector and the radioactive source. The meter shows a reading of .
The sheet of paper is removed and a thin sheet of aluminium is placed between the detector and the radioactive source. The meter shows a reading of 
.
Deduce the type of radiation emitted by the radioactive source.
The radioactive source is removed. The meter shows a reading of .
State why the meter does not show a reading of zero counts per second.
