This portion of the circuit sends signals back to a Raspberry Pi every time beta or gamma
radiation passes through the Geiger tube. The parallel RC circuit functions as an envelope detector, which registers the peak voltage of the incoming pulses from the Geiger tube.
A single Geiger event will consist of multiple high frequency peaks as the passing radiation initiates multiple Townsend discharges inside the tube.
Since we wish to count all these pulses as a single event, an envelope detector is used. It is a parallel RC circuit with a
chosen time constant that acts to combine all the little peaks into a single event pulse.
When an event pulse is sent to the base of transistor Q2, it conducts and briefly shunts VCC to ground. This low pulse can then be counted by the Raspberry Pi as one event.
This is how the circuit looks when placed on a breadboard for prototyping. A voltage regulator to convert 5 V DC to 3.3 V DC has been added. A power MOSFET receives a 3 V signal from a Raspberry Pi, instructing it to permit power to flow from the regulator to the circuit. This is an added safety feature to permit the Raspberry Pi to shut off the circuit when necessary. Another safety feature is the addition of a fuse on the 5V input to protect against short circuits. The 400 V components are located in the center, as far as possible from the 3 V components.
Geiger tubes and connected circuitry have some inherent “dead time”, meaning a time period 𝜏 after an event in which the detector cannot detect another event. The counts acquired will need to be corrected for this dead time. We do this by presuming that we could not count some events because the detector was busy and use the number of events we did count to estimate the true number of events we would have counted if the dead time of the detector was zero.
If N_m counts are recorded during a particular time interval T and the dead time 𝜏 is
known, the actual number of events (N) may be estimated by:
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