Hacker News Reader

Comment by iterance | Hacker News Reader

Your fact checking was not successful: there is no typical background expressed in counts. Background count rates vary globally by significant amounts, but importantly, they vary by device. Some devices may get a measly 10-20 counts per minute in a background, next to another device that could get 500. It also matters what kind of radiation the device is configured to detect.

At any rate 300 is widely recognizable as not an alarming value for a typical contamination detector in a typical configuration, but the report is likely slightly deficient because it does not specify how the measurement was taken. However, even if we accept 100 as the background CPM value, 300 on 100 does not represent significant contamination in a typical environment (but does imply some occurred).

View on HN

>but importantly, they vary by device.

Of course it varies for "civilian" devices from EBay.

They put that count into NRC report. It means that it has pretty specific calibrated meaning for that regulated environment.

>However, even if we accept 100 as the background CPM value, 300 on 100 does not represent significant contamination in a typical environment (but does imply some occurred).

report mentions 300 clearly as something above normal, whatever normal is there. And that is after decontamination. Clearly the source of contamination - the pool - is much higher than 300.

CPM has no calibrated amount by itself.

CPM is pretty well defined measure of flux.

Anyway feel free to explain why supposedly not calibrated value was put into an official NRC report instead of some calibrated value.

CPM is literally not.

It is counts per minute in an undefined, arbitrary sensor. Which could have a alpha radiation transparent sensor (and hence show alpha particles), or be from a low sensitivity geiger counter which can only detect high energy gamma radiation (for say fallout/emergency use). https://en.wikipedia.org/wiki/Geiger_counter

It could have a small sensor, and hence require high flux for a given CPM, or a physically large one - and catch more disintegrations per minute/CPM for the exact same actual amount of radiation.

As to why it is in a government document is why we’re all wondering what is going on. It certainly isn’t the only WTF thing the government is doing right now, is it?

you're just don't know what you're talking about. Google "nrc calibration cpm geiger". In short - they are calibrated either on dose or cpm with conversion factor.

I found nothing in the Google search results for that, or follow up search results, that indicate what you are saying is correct.

Mind linking to something concrete?

What I did find was numerous documents noting that Geiger counters needed to be calibrated to generate useful dose rates because CPM by itself is useless without a bunch of other work to characterize the sensor and radiation type.

Reply on news.ycombinator.com