[ RadSafe ] Question re survey meters

Robert J Gunter rjgunter at chpconsultants.com
Thu Mar 17 10:44:12 CDT 2011

Greetings Ivor,

There are two ways of doing this.  Integrating the charge over a period of
time (yielding dose), or measuring the continuous current (Dose Rate).  Most
hand held ion chamber meters read dose rate.  Much of what you see as a hand
held meter measuring dose rate is typically a GM probe that has been
calibrated.  So be sure to figure out what type of meter you are using.
Something like an Eberline RO-2 or RO-20 are ion chambers measuring dose
rate.  A Bicron uRem is a scintillator (yet a different animal) that
measures dose rate using a plastic scintillator, or other models use NaI
detectors (with all the inherent energy dependency).  Mainly laboratory
instrument are used to measure accumulated dose with an electrometer (though
some read dose rate too).

Dose rate is often measured in R (or Sv) per hr regardless of how long the
integration period.

Robert J. Gunter, MSc, CHP
CHP Consultants/CHP Dosimetry
Toll Free: (888) 766-4833
Fax:  (866) 491-9913 
Cel:  (865) 387-0028
rjgunter at chpconsultants.com

-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Ivor Surveyor
Sent: Wednesday, March 16, 2011 10:40 PM
To: radsafe at health.phys.iit.edu
Subject: [ RadSafe ] Question re survey meters

I have a question about radiation survey 
instruments.   My understanding is that an air 
ionisation chamber for instance collects the 
charge released by an ionisation event (about 34 
ev/ion pair in air or air like material lining 
the chamber); the resulting current is measured 
by a sensitive electrometer and converted into 
Roentgen.  Now I understand the Roentgen is 
defined as 2.58E-4 Coulomb/ kg dry air at 0° C and 760 mm atmospheric

My question is that instruments read as R/ h or 
rem/h on the assumption that there is a rough 
equivalent between an exposure of IR and an 
absorbed dose of I rad (10mSV) to tissue.  Am I 
correct that the instruments electronics sample 
the ionisation currents over a time interval?  If 
this is correct is there a “standard time” 
sampling interval?  Can the operator vary the time sampling interval?

Ivor Surveyor
isurveyor at vianet.net.au 

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