MDA vs. MDC

[Bear3705]

What is the rational for using minimum detectable concentration over minimum detectable activity for portable friskers?

[Rennhack]

A frisker should be expressed as L_c (Critical detection level: the response level at which the detector output can be considered "above background", in net CPM), or MDCR (Minimum detectable count rate).
Neither of those items take into account detector efficiency, or unit conversions.  It is a 'pure' measure of an instruments ability to make that clicking sound with consistency. Independent of WHAT it is looking at.

MDA (Minimum detectable activity) adds those efficiencies.  But you would need to know the detector efficiency for your specific radionuclide(s) of interest.


Then there is LLD (Lower limit of detection), which factors in mass unit, area or volume.
or L_D (Minimum significant activity level, i.e., the activity level that can be seen with a detector with a fixed level of certainty)

I guess I CAN remember things from 20 years ago.

[GLW]

A frisker should be expressed as L_c (Critical detection level: the response level at which the detector output can be considered "above background", in net CPM), or MDCR (Minimum detectable count rate).
Neither of those items take into account detector efficiency, or unit conversions.  It is a 'pure' measure of an instruments ability to make that clicking sound with consistency. Independent of WHAT it is looking at.

MDA (Minimum detectable activity) adds those efficiencies.  But you would need to know the detector efficiency for your specific radionuclide(s) of interest.


Then there is LLD (Lower limit of detection), which factors in mass unit, area or volume.
or L_D (Minimum significant activity level, i.e., the activity level that can be seen with a detector with a fixed level of certainty)

I guess I CAN remember things from 20 years ago.

[youtube height=200 width=400]https://www.youtube.com/watch?v=j1wjvP-raOI[/youtube]

[peteshonkwiler]

A frisker will detect alpha, beta, and gamma radiation.
MDC is more applicable for the different radiations and mix of nuclides present while MDA is more applicable when the isotopes are narrowed to those known to be present and/or for which the instrument is source checked.

[Bear3705]

Thank you both. That makes more sense to me. It was eating me alive.

[Rennhack]

Self consumption can be a slippery slope indeed. For an additional apertif, consider the fact that MDC is apriori and confident at a level of 0.95. Could come in handy in China. Raising local population dose limits will not affect the statistical variation(statistics) of counting instrumentation. ALARA applies to all humans, regardless if they have been trained or not.

[SpyCat]

I see 2 different things being compared here.  You mention
MDC minimum detectable concentration
but since this is relative to a frisker, maybe you mean
MDCR minimum detectable count rate?

Generally MDA compared to MDCR would be Activity DPM vs Count rate CPM.  If we accept that a frisker has a basic 10% efficiency (100 CPM/1000 DPM), the comparison is math.
But if Activity is compared with Concentration, different units are involved (DPM or uCi, versus uCi/cc).
In either event, your HP staff should be able to provide the rationale for using their particular method; it's likely based on the site's energy mix.