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Re: Measuring pulsed fields
Of course - OBVIOUSLY if the GM is counting the rep rate - its not a
valid reading - and doesn't sound right either!! (just like when an ion
chamber instrument reading around an accelerator is just too steady -
you are probably seeing RF and need to try orthogonal orientations to
confirm this)
BUT - for low fields when the response rate is much less than the rep
rate - say the probability of a count within a dead time interval is
less than 1% - the reading can be useful. 10% - kinda useful - 30% -
forget it!
For higher intensities it would be swamped anyhow.
Like I said - under very specific conditions ......
If this is a recurring measurement need - then get an HPI 1030.
Direct beam is not necessary for loss of saturation in a pencil IC - all
that is needed is a really low duty cycle! We encountered this for
leakage fields from an accelerator like device powered by a Marx
generator that produced a microsecond pulse every 4 minutes. When we
compared TLD integrations with pencil ICs we found the ICs more than a
factor of 2 low and then did the math and determined that lack of
saturation was the problem and later verified this in lower fields and
thus determined that 10^4 R/hr was about the level where the losses were
this factor of 2. These measurements where taken in low occupancy areas
in other areas where people were we of course had shielding - but we
needed to make these measurements to assess the usefulness of pencil ICs
for this project.
Integrated exposures were of course low and the pencil ICs were still
useful - once understood.
In our business instruments can never be used without thinking and
sometimes with considerable thought useful information can be derived
from less that optimal conditions.
John Andrews wrote:
>
> 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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