Radiation Q: C Answr w/1cm Alum Plate?

[User1265]

Homework Statement

12. A radioactive source is placed 6 cm from a radiation detector sensitive to all
forms of ionizing radiation, and records 74 counts/minute. When a 1 cm
thick aluminium plate is placed in the gap then the count rate falls to
45 counts/minute. If the source is removed entirely then the count rate
remains the same. If the aluminium plate is removed and the source is
placed 2 cm from the detector, the count rate rises to 5000 counts/minute.
What forms of radiation does the source emit?
A alpha only B beta only
C alpha and beta D alpha and gamma


Why is the Answer A and Not C?

Relevant Equations

Alpha, beta gamma

I thought the answer was C because what's not to say when surface is placed 6 cm from radiation detector beta particles are also present, and that surely all beta particles would be stopped with a 1cm thick aluminium plate?

 

[kuruman]

You need to take into account the observation that when the source is removed entirely, the count rate remains the same.