Re: Measuring pulsed fields

[John Andrews <andrewsjp@chartertn.net>]

Ted, if the pulses occur closer together than the dead time of the 

detector, then no additional pulses occur.  If the pulses are spaced 

apart, say 60 Hz, then the GM detector and electronics reads the pulse 

rate, not the intensity.  Our GM detectors at the accelerator facility 

would read the pulse rate, not the dose rate.  Big problem!  We 

developed special ionization chambers to read the integrated dose.



Unless you are in the beam, then the pulse dose is much less than the 

dose required to cause non-linearity in the response of the ion 

chamber.  Much depends on the specific condition, as you say.   ...This 

was so long ago, I tend to forget...



John Andrews

Knoxville, Tennessee



Ted de Castro wrote:



>But if it is a pulse train of significant duration then under very

>specific conditions a GM can be used (its a matter of playing the

>statistics).  It most certainly is not the best choice - its just that

>in some circumstances it can be used to derive some information - in a

>pinch.  The question as posed does not contain enough information to

>answer.

>

>As for the IC - you do need to be very concerned about the peak exposure

>rate to be sure that saturation is maintained - 200 mR pencil dosimeters

>do seem to go out of saturation somewhere around 10^4 r/hr.

>

>The best choice is the HPI 1030 - its is made for this purpose!  As with

>any instrument we use for specific purposes - there are tradeoffs and

>that is certainly the case here - but no other portable survey

>instrument even comes close for the very low duty cycle or even single

>pulse.  Its even better if ordered with an output jack so that the pulse

>may be observed and/or recorded on a scope.

>

>It gets even more interesting when you consider very very short pulses

>like 50 femto seconds!  Then to think about saturation peak exposure

>rate is not so important as considering the actual number of photons

>involved - you very quickly see that the ion density is more like a much

>lower rate and cannot lead to significant recombination.  After all - an

>exposure to a single photon would be extremely short and a very high

>peak rate - but still only the ions from the reaction with a single

>photon in the chamber.

>

>John Andrews wrote:

>  

>

>>Kim, you cannot measure short duration pulses with a GM detector.  The

>>instrument responds with one count regardless of the size of the

>>radiation event.  You can use dosimeters (quartz fiber ion chambers, not

>>GM electronic) to measure the integrated dose.  Be sure the wall

>>thickness is sufficient to ensure electronic equilibrium for the energy

>>of the incident radiation.

>>

>>You didn't say what you wanted to measure.  I assume you want to measure

>>the total dose over some period of time.   My experince with flash x-ray

>>equipment is that the total doses are pretty small per flash because of

>>the very short duration.  Given that you may find that you need to

>>integrate over a number of events to get a reasonable reading.  Good luck.

>>

>>John Andrews

>>Knoxville, Tennessee

>>

>>Kim D. Merritt wrote:

>>

>>    

>>

>>>Is the Bicron 2000 w/ HP270 or the RO2 adequate for measuring pulsed

>>>x-ray fields with microsecond pulsewidths?  I don't have the manuals

>>>for these instruments.  Alternately, can anyone recommend an

>>>instrument that is good for measuring these kinds of fields.

>>>

>>>Kim Merritt

>>>Radiation/Laser Safety Officer

>>>HazMed, Inc.

>>>NASA Langley Research Center

>>>      

>>>

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>

>  

>





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