Calibration Question for Building an Instrument

[Higgie Smalls]

Hello all,

I am a long time lurker, very infrequent poster to the board, however enjoy most of the questions and post that are published here. Thanks to all the contributors.

My question is in regards to the calibration of a personal radiation detector. Lets say I put a GM tube accompanied by a scintillator into housing so that I can measure dose rate as well as identify isotopes. Now the housing of the detector has a much larger active area then the GM tube within the housing, and the GM tube is not centered in the housing (the scintillator is). Picture attached:

GamSpec.png (13.34 KiB) Viewed 1098 times

Now, when calibrating the GM tube, would it make more sense to calibrate with the source centered relative to the GM tube, or the source centered relative to the housing?

I feel it logically makes sense that when trying to calibrate the GM tube it should be planar to the tube, however, if somebody other then I was using the instrument, they really would not know where the GM tube is, and I believe operationally they would place items of interest in the center of the housing... I am curious what would be best practice?

Thanks
Tom

[GigaBecquerel]

What do you mean by centered?
Your drawing is not very clear, is this supposed to be a handheld device, is it suppose to be a lead castle sitting somewhere or what?
GM tubes are made to read dose rates in homogeneous fields.
Also, why not use the scintillator to determine dose rate as well?

[Higgie Smalls]

GBq,

my picture would correspond to the front facing side of a rectangular hand held detector housing.- so by centered, I mean centered relative to the volume of the housing.

I am using the GM with the Scint to extend my dose rate range.

Thanks
Tom

[isoenzyme]

GMtubesNsample.png (69.67 KiB) Viewed 1077 times

The easiest answer is that the source should be located (relative to the GM tube) where you expect to be taking measurements with the fully calibrated instrument. Remember that distance matters in radiation measurements. Attached are drawings of an instrument that I'm assembling and characterizing (not quite ready for a post yet) using 4 GM tubes (integrating signals) - the sample tube is kept equidistant from each tube but the tubes are crowded as close as I can reasonably get to maximize detection of lower levels of gamma rays. What makes it work is that the sample is presented to the tubes exactly the same way each time and that the "active" portion of the GM tubes are close to where the sample is located. (In the provided drawing the GM tubes are labeled "3.")