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

[Joel Baumbaugh <baumbaug@nosc.mil>]

	Steve,

	I have almost exactly the same "mix" of radiation producing machines that you do.  Ludlum gave you GOOD advice - I'd follow it!  I have my pick of almost any instrument you can think of to do my surveys of TEMs, SEMs, analytical x-ray machines, etc., and I use my Ludlum-3 with a 44-3, 44-2 or 44-10 scintillator (depending on the energy I'm looking at) and my 44-9 pancake G.M. probe for quick and dirty preliminary surveys.  Yes I even have a 42-9 for fast and thermal neutrons (doesn't everyone? {grin}) - no I do NOT have stock in the company.  BTW the Navy has some excellent instrumentation (like Snoopys and ion chambers, etc., and of course TLDs) when I have to "quantify" my dose rate.  

	For your real fasted-pulsed x-ray units (flash x-rays) I have found that I get a good agreement between a integrating survey meter (no matter the manufacture) and TLDs.  Of course one has to be careful with geometry factors, etc.... The scintillation probe or G.M. (I'd use a pancake due to the length of the handle - especially if its a high-intensity pencil beam.) will both respond but will just tell you that there's something there.  Still - that's often all you want to know - where's the beam and where is it safe....

	The list may be getting tired of "my opinion" on this so I'm going to shut up and get back to work...

	Hope this helps,

	Joel Baumbaugh
	baumbaug@nosc.mil

At 08:08 AM 6/11/98 -0500, you wrote:
> 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.
>
>
>
>
>

Joel T. Baumbaugh, MPH, MHP
baumbaug@nosc.mil
Radiation Safety Officer
SSC San Diego, CA


	NOTE:	The contents of this message have not been reviewed, nor approved by the Federal Government, the U.S. Navy, my bosses or my wife... My wife keeps complaining I never listen to her...or something like that.



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