Measuring Decay Rate with Geiger Counter: Calculating Error and Seeking Input

[nrm]

I was hoping that somebody would check my work. Any input is appreciated.
A decay rate of a radioactive sample is being measured using a Geiger counter. During a 10 second measurement 100 decays are registered.
what is the error associated with the result?
$$\sigma_{y} = \frac{\sigma}{\sqrt{N}}$$
My work:
$$\sigma^2 = np = (100)(1/10) = 10$$
so $$\sigma = \sqrt{10}$$
putting this into the previous equation where I make N=100 I get an error around 0.316. Did I do that right?

 

[daveb]

If you are loking for the standard deviation for the count rate, then your correct equation should be sqrt(n)/t where n is your count and t is the count time. If you are looking for the standard deviation for the count, then it's just sqrt (n).