[ RadSafe ] Radiotracer dose at a distance.

[Geo>K0FF]

Energy response correction is a factor easily overlooked. A typical 
scintillator will report a
Gamma dose-rate range of up
 to 14X depending on the energy of the radiation. In other words a true 1 
mR/H of Co-60 will
show only 1/14th the same 1 mR/H of
Co-57. Most energy curves are "normalized" to 662 keV or Cs-137. All 
calibration and
dose-rate calculations pretend that every radiation detected is at 662 keV. 
Naturally they are not.

To rephrase, a properly calibrated 1" X 1" NaI(Tl) scintillator when 
presented with a true
flux of 1 mR/H of Cs-137 will read 1 mR/H. When presented with a true flux 
of 1 mR/H
of Co-57, the same meter/probe will read 7 mR/H. When the source is changed 
to Co-60, the
reading will be only 1/2 mR/H.

Lower energy reports higher than actual to the point where it stops reading 
altogether
due the self shielding of the housing material, typically about 25 keV. 
Higher energy reports lower than actual due to the more penetrating nature 
of the Gamma radiation having a tendency to pass through.

For these reasons, NaI(Tl) and other similar probes that are intended 
primarily to detect
LOW ENERGY GAMMA (LEG) have a thin
crystal and thin entrance windows ( low Z). The logic is that all the low 
energies will be absorbed in a few tenths of an inch, so why use a thicker 
probe which will only better pick up the higher energies and mask the area 
of interest. A typical crystal will be 1 or 2 inches in diameter and only 1 
mm thick.

It follows that a High Energy probe will be thicker, usually as thick as it 
is wide.
Typical probes use 1" X 1", 2" X 2" and 3" X 3". Specialty probe come in 
much larger sizes, with multiple 4" X 4" X 16"
units being used from aircraft.

Scintillators are not the only victims of the energy response dilemma, 
Geiger Mueller tubes have similar characteristics.
GM tubes are sometimes "equalized" to counteract this undesirable feature 
with the Ludlum 44-38 and Eberline HP-270 being just two examples. Ludlum is 
very accommodating and supplies all the correction curves in their webpages 
and in their manuals.

Most of us in field work prefer to leave the probes energy dependant for the 
extra sensitivity in detection. The only time you need energy equalized is 
when you are trying to do a true dose-rate quantification. Personally I like 
to use different instruments for different needs. For true dose-rate I 
usually select a Bicron Microrem or an ion-chamber.

Quantification can be tedious and for HP professionals I guess it is their 
life, but my interest lies mostly in discovering radiation in unlikely 
places and identifying it's source, so different equipment is appropriate, 
leaning heavily on rate not dose.

George Dowell




----- Original Message ----- 
From: "Brennan, Mike (DOH)" <Mike.Brennan at DOH.WA.GOV>
To: <radsafe at radlab.nl>
Sent: Friday, September 07, 2007 3:48 PM
Subject: RE: [ RadSafe ] Radiotracer dose at a distance.


Nicely done.  I am impressed by how well the measured (and corrected)
dose rate and the RADPRO projection matched.  There is certainly food
for thought here.

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of Geo>K0FF
Sent: Thursday, September 06, 2007 6:51 PM
To: radsafe at radlab.nl
Subject: [ RadSafe ] Radiotracer dose at a distance.

Interesting experiment.
Goal was to determine the second-hand dose at a distance from a patient
injected with a radiotracer.
This is the dose delivered to a bystander in the general area of a
patient who has had a radioactive isotope injection.