This article refers to
Vernier Radiation Monitor (VRM-BTD)
Go Direct® Radiation Monitor (GDX-RAD)
The dead time, tdead, is the duration of time, beginning at the start of a detection event, during which a detection system is incapable of producing an output electrical signal in response to additional radioactive particle detections.
The Geiger tube we use in both of our radiation monitors has a minimum dead time of 90 μs, according to the manufacturer (LND model 712).
When using the Vernier Radiation Monitor with a Vernier interface such as any LabQuest model or the (discontinued) LabPro, the radiation monitor creates a pulse for the interface to detect, which is on the order of 20 μs and which overlaps with the dead time window of the Geiger tube.
So how fast can the Geiger tube and pulse circuit detect events? Given that the 90 μs dead time is the limiting factor, the Geiger tube and circuit can theoretically produce up to 11111 events per second
(1/90 μs).
The circuit is connected to a microcontroller. The microcontroller uses the event pulse as a clock that increments a hardware counter. The hardware counter is very fast — it can be clocked in excess of 1 MHz so 11 kHz is no problem. But the counter is only 16 bits. So it must be read often enough so it doesn’t roll over/alias. That is where the sample rate becomes important. If you have a really hot specimen, and try to examine it at a really slow sample rate you might overflow the counter. The opposite is also true, if you have a really cold specimen and you examine it at a really fast sample rate, you will get a lot of 0 readings.
The default sample rate for the Go Direct Radiation Monitor is 0.1 samples per second — or 10 seconds per sample. This means that Graphical Analysis or LabQuest will read and reset the microcontroller counter once every 10 seconds. So the specimen under investigation shouldn’t be emitting faster than ~6500 counts per second to keep the counter from rolling over.
Working backwards from the maximum response of the Geiger tube (11111 events per second), the sample rate for Graphical Analysis or LabQuest really never needs to be faster than 5.9 seconds per sample (or 0.17 samples per second). Just to make nice round numbers, it should never be necessary to sample any faster than 1 Hz when using the Go Direct Radiation Monitor.
Typical use cases for the radiation monitors (for example, measuring background radiation or measuring radiation attenuation due to shielding) generally can be done using the default settings in software without concern about the dead time.
Vernier Radiation Monitor Troubleshooting and FAQs
Go Direct Radiation Monitor Troubleshooting and FAQs