Base level - radiation or electronics?

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4 years 11 months ago #3524 by hotblack43
Base level - radiation or electronics? was created by hotblack43
Hi. Looking at my own readings from a GC10 counter, and looking at everybody else's online, I notice that most of us have CPM rates near 20 or 30. Does anyone know if that is all counts from radiation of some sort, or if there is a sort of 'base level' that is there simply because of the electronics? Could I in theory put the counter inside a room with 10km thick lead walls and the readings would drop to zero?

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4 years 11 months ago #3532 by ChrisLX200
The average background count is made up of a combination of spontaneous tube discharges (a low number) plus normal radiation sources in your location (result of cosmic ray hits and presence of radioactive substances which vary a lot from area to area). So you will never see a zero reading but of course you can eliminate several of those sources with better shielding.

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4 years 11 months ago #3533 by hotblack43
Replied by hotblack43 on topic Base level - radiation or electronics?
Thanks a lot for the reply! Do you have a reference to what number of counts I should expect that are not due to radiation, for the GC10 machine?

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4 years 11 months ago #3534 by ChrisLX200
Not sure what tube is in that - is it 4011?

Anyway, by example, the data provided by GM for the 4011 in my GMC-300Eplus looks like this:

Both beta and gamma radiation detetion.
Working Voltage: 380-450V
Working Current: 0,015-0,02 mA
Sensivity to Gamma Radiation: 0.1 MeV
Own Background: 0,2 Pulses/s
Working Temperature Range: -50 +60 ะก
Length: 88mm
Diameter: 10mm

So 0.2 Pulses/sec would equate to 12 CPM (seems high?), but you also have to account for the electronics of the detector itself and not all pulses will be counted/reported.

ChrisH

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4 years 11 months ago #3535 by ChrisLX200
OK, looked it up and the GC10 is commonly fitted with an SBM-20 which has different characteristics:

Minimum Anode Resistor (meg ohm) 1.0
Recommended Anode Resistor (meg ohm) circuit diagram 5.1
Recommended Operating Voltage (volts) 400
Operating Voltage Range (volts) 350 - 475
Initial voltage (volts) 260 - 320
Plateau length (volts) at least 100
Maximum Plateau Slope (%/100 volts) 10
Minimum Dead Time (at U=400V, micro sec) 190
Working range (mkR/s) 0.004 - 40
Working range (mR/h) 0.014 - 144
Gamma Sensitivity Ra226 (cps/mR/hr) 29
Gamma Sensitivity Co60 (cps/mR/hr) 22
Inherent counter background (cps) 1
Tube Capacitance (pf) 4.2
Life (pulses) at least 2*1010
Weight (grams) 10 / 9

Which suggests the inherent background is 1 cps (or 60 CPM) which again, seems high but the counter itself is not going to detect every single pulse.

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