Re: Low energy x-ray (8 kev) survey meter

[steve hand <hand@wam.umd.edu>]

 Back in August of 1996 one of the radsafe subscribers posted the following:

"On the subject of calibrating survey equipment for analytical x-ray
>machines:
>
>>  could you specify what type of analytical equipment, such an electron
>> microscope?
>
>Unfortunately, both electron microscopes and x-ray diffraction units.
>Double-unfortunately, as we are a research university, current/voltage/target
>material in the XRDs is quite varied.  Triple-unfortunately, I also have
>amusing things such as accelerators, experimental fusion devices and the
>like to survey around. :-)  However, for the purposes of the question of
>calibration appropriateness, let's say the equipment consists of EMs and
>XRDs.  The XRDs are for the most part in interlocked, shielded enclosures.

A committee of x-ray "experts" met at Livermore last month and concluded:

A thin walled GM detector should be used to detect leakage.

An ion chamber should be used to measure dose rates.

At Los Alamos, we use ion chambers calibrated with Cs-137 to measure
leakage dose rates, and integrating ion chambers calibrated according to
NIST x-ray protocol to measure the main beam.  But I wonder if we are
making this more complicated than we need to.

As I understand it, a concern is that ion chambers show a decreased
response to lower energy photons (below 10 keV) and so calibration with low
energy photons might be appropriate."


I also use an end window GM and occasionally a pancake, along with an ion
chamber to survey the XRDs on our campus.   Ludlum advertises their low energy
gamma scintillator indicated use as X-ray survey, 10-60 kev range.  I have used
the model 44-3 as well. Both of the instruments have been capable of finding
leaks and/or scatter from a few XRDs on our campus.

Has anyone used a scintillator like the 44-3 or an end window to look for
x-rays coming from a plasma formed from ultra fast laser pulses, we'll be
looking for some in the next couple of days.

steve
university of maryland
radiation safety
hand@wam.umd.edu



TDC@ehssun.lbl.gov wrote:

> >At 8 kev, you have "LESS" than 0.01% efficiency with a G.M. (pancake, even
> >less with a end-window), but are exceeding 10% efficiency with a low
> >energy scintillation probe!
>
> 8 keV is the characteristic - but there are photons all the way up to the
> excitation potential.
>
> You're right - I have NOT compared a GM and scintillator side by side - BUT
> I have never found indications from the GM that weren't overwhelmingly
> clear!  A LONG ways from "fluorescent paper in a darkened room".  And -
> area monitors and dosimetry have yet to even suggest that I missed
> anything.  I feel the GM is sensitive enough for any level that matters and
> therefore will opt for the simpler more robust instrument.
>
> I don't doubt a scintillator is more sensitive.  I just don't think I need
> that much sensitivity.
>
> Since our analytical machines are all enclosed - we usually get to measure
> wider fields that in close.  But then when surveying in close (enclosure
> overridden) there is ample intensity even in a narrow stray beam.
>
> Have you often found leaks with your scintillator that were missed with the
> GM?  How signficant were they?  Were they at all significant at a distance
> of a few feet (where an enclosure would be)?
>
> > Another 2 cents,
>
> I believe that's the purpose of this forum - to discuss these kinds of
> issues.  Thanks for the input.