Peak pile-up is the consequence of two or more gamma ray photons being detected almost simultaneously and not being recognised as separate events.

• One method of avoiding the pulse height spectrum effects arising from a peak pile-up is to use some means of pile-up rejection.
• Commercial linear amplifiers are provided with active circuitry that can inspect the shape of the input pulses and discard those in which distortion is evident.
• The visual confirmation of the pile-up rejection is a green light on the front of the NIM unit and if pile up is too bad, this light turns red.
• Pile-up occurs if the source is too active or too near the detector and results in a large dead time leading to a non-recognisable spectrum.

Use the scrollbar on the left to bring the source closer to the detector.  Notice the increase in the number of pulses, and the level of pile-up rejection, as indicated by the red LED.  Change to Unipolar output and observe the effect.