Getting a periodogram for discrete data

I have a dataset that goes like: dataset =
t   |x
0   |x1
1   |x2
2   |0
3   |0
4   |0
5   |0
6   |x3
7   |0
8   |0
9   |0
10  |x4

and so on. I wish to detect the periodicity of occurrences. t is in
seconds and x are arbitrary, whose magnitude i am not interested in. I
just wish to get a best guess for the period of mere occurrences of x.
Tried using spec.pgram(dataset) but the periods I get are too small - of
the order of 1e-5. What am I doing wrong?