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Thanks for your reply about Coincidence Box. I have been using the RM-60 for some time now & only recently tried it on 14-C. It's not proving that sensitive to 14-C beta emission. How much better, if at all, are the RM70 & RM80 or your other detectors with this isotope? I find that I must use a lot of tracer and long count times to get significant numbers out of the RM60.
Since beta energy from 14-C is approx. 50KEV average, 150KEV Max., approx 35% will make it through the window and create a count, but absorption by air between sample and window must be considered.
The RM-70 window has ten times the surface area as compared to the RM-60, RM-80: 24 times the surface area, so they should provide 10 and 24 times the sensitivity to 14-C beta, provided you can spread your sample out on say a piece of filter paper, to match the increased diameter of the RM-70 or RM-80 window. Diameter of window: RM-60: 9.14 mm; RM-70: 28.57 mm; RM-80: 44.45 mm.
The background count rate will not increase by 10 and 24 since background is composed mostly of photons and muons which can pass through side of tube. Background, as compared to RM-60, should increase by: RM-70 1.4; RM-80 3.8.
In addition, the stainless steel screen on the RM-70 and RM-80 is fused to a screw-on cap which can be removed, thereby exposing the entire window. (RM-60 screen is fused to case and is not removable). The screen on all the units has approx. a 71% open area. By unscrewing the screen from the RM-70 or RM-80, one not only removes the low energy beta shielding effect of the screen but one can also place the sample closer to the window.