A certain radioactive source emits different types of radiation. The sample is tested using a Geiger counter. When a pie

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A certain radioactive source emits different types of
radiation. The sample is tested using a Geiger counter. When a
piece of card is placed between the source and the counter, there
is a noticable drop in the radiation. When a thin sheet of
aluminium is added to the card between the source and the counter,
the count rate is unchanged. A thick block of lead, however, causes
the count to fall to the background level. What type (or types) of
ionising radiation is the source emitting? Explain your answer
carefully.