counting edges from a photon counting pmt

Hi, 

I am trying to use my DAQ (PCI 6115) to count photon signals from a photon counting PMT. 

What I have set up currently is 

1. creating a gate pulse with a width of integration time with counter 1 on DAQ 

2. creating a sencond counter to count the number of edges from the PMT within the given amount of time from #1. 

as I change integration time the width of the gate pulse should change therfore giving me more or less counts from my PMT, but this does not seem to be working correctly. Is there a better way to do this with my daq or am I performing something wrong? 

Thanks for any help.