Anybody has a clue?

Than you in advance.
Mircea

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Before you buy.

You are correct in using a counter.  However, there is a way to do what
you want--buffer the counter.  The counter uses a 20 MHz clock for its
timing.  What you can program the counter to do is every time it
receives a rising edge from the photomultiplier, it will put
the "count" of 20 MHz pulses in a buffer.  Therefore, knowing that the
buffer counts are counts of a 20 MHz pulse, you can reconstruct when a
pulse occurred and the amount of time between pulses.  From this, you
can generate the data you need.

Mark