Re: Ion chamber versus GM

[Chris Alston <alstonc@odrge.odr.georgetown.edu>]

If the 450P isn't a "radiation detection survey instrument", what, in
heaven's name, is it? The only real problem I can see with using it as your
HDR survey meter is that, if you have a very active HDR program, one in
which treatments may be done in real time, i.e., unscheduled, you really
should have an instrument dedicated to the purpose. The 450P is so useful
that it could wander off to other applications, e.g., post LDR implant
surveys, or xrays, it might then be unavailable when needed at the HDR lab.
If that were the case, a good, relatively inexpensive, substitute would be a
G-M counter of the type designed for industrial-radiographic surveys, e.g.,
an Eberline E-130A. The G-M meter from Nuclear Medicine is inappropriate,
because it doesn't range high enough. Civil Defense meters aren't anywhere
near this class.

You simply need to explain to the inspector what the 450P is, and how it
works. He should understand that the chamber is sealed and pressurized to
six (6) atmospheres with an 80/20 N/Ne mixture. This, plus the fact that it
is an entirely electronic device, as opposed to meters with
electro-mechanical movements, means that it is exceedingly fast. That's why
it's useful for machine surveys. Tell him that the response time on even the
lowest range (0-500uRh^-1) is <5 seconds to 90% max reading, and on the
highest range (0-5Rh^-1) it's <2 seconds. In integrating mode, it's response
time is <100 milliseconds. Remind him that the instrument is auto-ranging,
so that one needn't fool clumsily with a switch in order to track rapid
changes in radiation fields. Also let him know that the 450P has the rough
equivalent of anti-saturation circuitry: there is a programmable alarm that
causes the display to flash off/on every second when the exposure rate
exceeds the limit set (the default factory setting is the top of the highest
range, 5Rh^-1, I think).

I, too, have used the 450P for post-implant surveys of patients treated with
radiocesium tubes. As you say, the smallest such source would be nominally
12 mCi (ignoring the question of age). A 450P that typically reads 5 - 25
uRh^-1, absent a source other than background, would easily see one, if it
were anywhere in the patient's room. In fact, I have it on good authority
(from the supervisor of a very well-known calibration/service lab) that he
once located a missing SrY-90 source, one of no more than a few dozen
microcuries, with a 450P. It was behind a metal filing cabinet, or under
metallic shelving, or something (he told me the story several years ago).
Actually, if you'd really lost a gamma source, a scintillometer would be a
better choice than either the 450P or a G-M counter: more sensitive than a
G-M, and it would have a speaker, and/or headphone jack.

There should be no need to amend your license. It's just a matter of
convincing the inspector that the 450P meets, or soundly beats, his
definition of a "radiation detection survey instrument"; which it does.
Explain to him how you use it. I imagine that you wait until the control
console tells you that the treatment is over and the source has retracted,
and until the area monitor has ceased alarming (of course, it might have
decided to fail just at that moment). You probably then open the door and
walk down the maze with the 450P held well out in front of you (and the rad
RN following very closely behind you). Even if only a part of a 90-day old
HDR source were outside the shield, you'd likely read it as soon as you
opened the door.

You could invite the inspector to observe a demonstration. Set the 450P in
the room, where it can be read via the CCTV monitor, and run the source out.
I think he'd become a believer. Come to think of it, what are the exposure
rates at the door, when the source is in use? You could probably do the demo
without leaving the meter in the room.

If you think that Victoreen's promo/spec sheet would be helpful, I'd be glad
to fax you one. Good luck.

chris alston
alstonc@odrge.odr.georgetown.edu
My employer takes no responsibility for my pronouncements in this forum.


At 01:33  01/29/98 EST, you wrote:
>
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>My radioactive material license has a condition that says that immediately
>after retracting the HDR Ir-192 source from a patient into its shielded
>position  a radiation survey shall be made of the patient and the HDR device
>with a portable radiation detection survey instrument to confirm that the
>source has been removed from the patient.  I have been using a Victoreen 450P
>ion chamber to perform this survey.
>
>We recently had a state inspection. The inspector has threatened to issue an
>item of non-compliance because he claims the ion chamber is not a detection
>survey instument.  He says the regulations and license condition specifically
>call for a detection instrument.  He says I should be using a G-M meter
>because it has a faster response time.  But in his defense, he is offering me
>a chance to prove to him why I think the ion chamber is a better instrument to
>use in this situation.
>
>I verbally argued that the Victoreen 450P is superior to a G-M because of
>linearity over a wide energy range and its ability to operate in over wide
>range from microR up to 5 R/hr.  A GM meter used in Nuc Med is available but
>it only goes up to 50 mR/hr.  I said the response time is not important
>because I just have to survey for a couple more seconds.  I said that I wasn't
>looking for a "lost" source and didn't require the detection abilities of a
>GM.  What I am really trying to do is compare the exposure rate at the patient
>to the exposure rate next to the shielded container in the HDR.  If the
>reading is higher at the shielded container than I have shown myself that the
>source has properly retracted out of the patient.  Plus, I told him that if
>the source had not properly retracted into the HDR unit I would be reading
>something on the order of 1-5 R/hr at a meter from the source.
>
>To save myself this from this possible item of non-compliance, I have to write
>a convincing argument and submit it to him. He would then amend my license to
>allow the use of the ion chamber.  Can anyone offer any other arguments that
>would help.  The inspector is also unhappy that I use the ion chamber to
>survey the patient after Cs-137 brachytherapy sources are removed form the
>patient.  I use the same argument here.  Even a 5 mgRaeq source will read 4
>mR/hr at a meter and a misplaced source would easily be found with the ion
>chamber.  This is really a formality because I do the survey after I
>personally take the sources to the vault and physically account for them even
>before the room survey is done.  Of course, if I found a source to be missing,
>I would grab the GM to help in the search.   I find it hard to believe that it
>would be better to use even a civil defense GM meter than the excellent 450P
>ion chamber, but according to the inspector I would be better off doing that.
>I welcome any comments.  Thanks
>
>Leo Jablonski, M.S.
>DABR  Lincoln, NE
>