Re: Low energy pocket dosimeters

[tdc@ehssun.lbl.gov (Ted M. de Castro)]

>> Does anyone know of a source for low energy pocket ion chambers
>> (self-reading)? Our purchasing department claims they are no longer 
>>available.

Isn't XETEX or DCA still selling them - or surely Nuc Assoc??


>> Alternatively, we need a method for getting real time measurements of the
>> total dose from exposure to a pulsed x-ray system with the following
>> characteristics: 50 nanosecond pulse, 25 pulses per second, 150 kVp and 1
>> microseivert per pulse at 1 meter (according to the manufacturer).

CAREFUL!!!  I've been there!!  200 mR low energy dosimeters are about a 
factor of 2 out of saturation at 10^4 R/hr!!!  With that duty cycle you may 
be surprised to find how easily you can drive such a dosimeter out of 
saturation.  Do the quick calculation!!!!

I have actually seen this with pulses about twice that long once a minute or 
so.  You should check this out, once you get the dosimeters, by comparison 
to TLD or film dosimeter - neither of which seem to have a rate problem with 
radiation.  (yes I know film has a reciprocity problem - but that is only 
with light NOT ionizing radiation).

I recomend using the HPI 1030 ion chamber survey meter to measure the dose 
rates - it IS expensive - but the only survey instrument I know of that can 
maintain saturation at high rates (unfortunately the trade off is that it is 
geotropic).  Alternative you could calibrate a silicon diode or FAST PM tube 
as a detector - but both with me VERY energy sensitive.

>>The dosimeters are to be used by nontechnical personnel (security guards) 
>>and are not intended to be record dosimetry. Our primary concerns are 
>>expense and ease of use.

LANL developed a CdTe real time dosimeter that MAY be fast enough - but I 
don't know if it ever became commercial - AND the room temperature noise of 
CdTe is about equivalent to 30 keV photons - thus limiting the low energy 
response.

You MAY be between a rock and a hard place here.

>>In a related issue, virtually the only instruments we have found that 
>>could detect the radiation emitted by this unit are integrating ion 
>>chambers.

I doubt that any of these in any useful range are in saturation either!




Ted de Castro
tdc@ehssun.lbl.gov
University of California Lawrence Berkeley Laboratory
Bldg B75B Rm 101
Berkeley, CA 94720
(510) 486-5256
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