If 1 million of alpha particles of 2.5 Mev hits a target of aluminium ?

Question

How many react producing a neutron for each, you know the target is high 10 mm.

Answer 1

You did not give enough information

perhaps

Al(27,13) + alpha (4,2)--> P(30,15) +(1,0)n

You arze sure that all the alpha particle are stopped

Answer 2

Not enough information.  What is the reaction cross-section and the target thickness?