can someone explain me question 11 (the last one) in oct/nov paper 3 physics year 2007? pls save my life cuz tommorow is my exam 
For part a
First you take counter readings without any source or absorber only detector...This is the background
Then you take reading with Source and detector
Then take reading with source , detector Plus absorber which is aluminium in this case (one sheet at a time..u keep adding and take reading)
Finally subtract the background reading from the result
Part b:
The counter readind decreases as the thickness of aluminium is increased