I am in a lab class and we are currently measuring count rates for a specific source. Yesterday we ran a program that runs 3 second gate times on a CS-147 source with a voltage of 500v running through the detector, and a threshold of .5v on the counter. This 3 second gate time was to be repeated 135,000 times to be done by friday. In the gate time of 3 seconds we were getting an average count rate of around 250 counts per 3 seconds. However overnight this shifted unexpectedly to around 1500 counts prt 3 seconds giving us 2 very defined gaussian peaks at 250 and 1500 in our "gaussian curve". The setup was not touched or changed at all overnight as the room was locked, and upon inspection nothing had changed except now for some reason we get much increased count rates for our same exact source. Does anyone have any idea what could possibly cause this??