Decay Constant?

1. The problem statement, all variables and given/known data
A small quantity of the thorium isotope Th(A=228, Z=90) (half-life 1.913 y) is prepared and electroplated onto a thick sheet of copper and overcoated with a thin film of gas-tight polymer. The plating and coating are both thin enough so that none of the alpha particles resulting from the decay are absorbed, and all that exit on the film side of the copper sheet are counted. Immediately after preparation, the counter registers 7340 alpha particles per second.

A)What is the decay constant of Th(A=228, Z=90)?

B)How much thorium has been electroplated onto the copper?

2. Relevant equations

At=Ao*e^[-λ*t]
λ=0.693/T

3. The attempt at a solution

A)
T=1.913y

λ=(0.693/1.913)
λ= 0.181 y^-1

This is incorrect.
Any guidance or hints would be appreciated!

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