[ RadSafe ] Low Level Readings using probes

[Thompson, Dewey L]

Wow.  Where to start.  I suggest you get a copy of Glen Knolls book, or possibly the Greek from University of Missouri Rolla, Tsoulfanidis.  

The word "linear" in a question about radiation detectors gives me the willies.    Radiation detectors have a number of parameters, in general they are not "linear" in any way.  

OK.......here goes my two cents worth.  For photon radiation:

Energy response.  
This is linear only with well designed ion chambers.  IF you have electronic equilibrium in the chamber walls that is......The problem with ion chambers is that they are rather insensitive due to the extremely low current produced inside the chamber.  This also assumes severe limitations on the energy range measured.

Geiger Mueller detectors are quite energy dependent, primarily because they are an event rate counter.  Properly designed filtration on the tube can "flatten" the energy response such that the dose is "somewhat" linear (probably good enough for most uses).

Solid state detectors are generally energy dependent, this is a function of the probability of photon interaction.  "in general", photoelectric effect, Compton Scatter, and Pair Production are dependent upon both the Z of the absorber and the energy of the photon. Thus, the detector will be  energy dependent as well.

Rate response.  
In general linear for ion chambers and solid state detectors, within the ability of the electrical circuit to carry signals.

Geiger tubes have longer dead time, (dependent upon the tube design), and generally will under respond at higher rates due to swamping.

The whole point of all this is that "there really isn't a perfect radiation detector".  They all have strengths, weaknesses, and idiosyncrasies.  Generally, you need to know the characteristics and rate limits of the radiation you want to detect, and equip as indicated.

You appear to be asking about linear rate response.  Not bad with a scintillator (generally), likely terrible with a GM tube.    

For radiation effects upon components and is the new digital stuff any better, well, that is a mixed bag.  Generally dirt and grime is a much larger enemy to electronic components than radiation damage.  The digital stuff just allows the bright boy designers to design new, complicated "wow" features.

FWIW

Dewey
 



-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu [mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Matt Sargent
Sent: Tuesday, May 15, 2012 11:50 AM
To: radsafe at health.phys.iit.edu
Subject: [ RadSafe ] Low Level Readings using probes

Low Level Readings using probes

One thing I've read that I am trying to understand, is it sounds like the response output is only linear up to a certain point on certain detectors, after that readings become non-linear, this leads me into wondering about how well various products deal with this, from what I can tell any given meter/software solution had best be tied to a specific probe's characteristic, not only in the high voltage power supply requirement, but also to linearize the respond at various source levels which could potentially confront the detector. Is this something worth worrying about? I've yet to see any plots of how non-linear the response gets.

Up to this point in time, I have been comparing relatively small counts, if that was all I cared about, why spend a great deal more, get a really good detector and hook it up to just about any old meter/power supply and take the readings. Kind of what I've been thinking. However, if I want to have accurate comparable numbers from higher intensity levels, I have better make sure my detector probe model is supported (and it's non-linearity corrected) by whatever measurement meter/software solution we're using.

If that makes sense?

I have an old CDV-700 model meter and I am trying to decide if it is worthwhile to get an alpha, beta, gamma probe for it or upgrade completely to something new?  I am looking at the RDS-31 with the GMP-11 probe.  As I a very familiar with the RDS family I have a bunch of the RDS-30's on the Job Site.

One other question I have pondered is the degrading effect that Radiation has on electronics how does the new digital equipment shield itself from this?  Not that one is high rate areas all the time but how will they last over time?

Can any one point me to some papers or studies that can possibly give me some answers or make some recommendations?

Kindly,

Matt Sargent
Safety Compliance Officer

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