[ RadSafe ] Any experience in detection of polonium on large scale contamination ?
andrewsjp at chartertn.net
Mon Dec 11 20:45:07 CST 2006
parthasarathy k s wrote:
> Detection of polonium contamination by area monitoring has certain difficulties. Since the range of alpha particle from Po-210 is only a few cm in air and the gamma emision accompanying Po decay is very feeble, how do you carry out large scale area monitoring?
> If you are using large area alpha scintillation counters, based on Zn S(Ag), one of the problems is to maintain the background counting rate at a low value.The thin (0.1 mg/square cm)aluminium covered window develops pinholes leading to light leaks. This results in stray counting. We had difficulty in less trying circumstances. we had to replace the thin alunmnium foil very frequently
> This story is decades old! Now there may be better instruments availble commercially.I am an old hand and used to make large area scintillation counters by making alpha detector foils by sprayng fine ZnS(Ag) powder on transparent plastic sheets coated with silicone oil. Can any one in the list throw some light on newer methods if any? I was told that large area air proportional counters are available commercially. During the seventies we tried to develop spark counters with limited success.
> Urine sampling etc is well developed and relatively easy though time consuming and is not applicable in area monitoring!The UK Health Protection Agency must be combing large areas
> Not much is known about the methods used by them.
> The infamous polonium poisoning incident is worrying for those who have to carry out area minitoring in case some rogue elements try to spray it in busy malls, shoping centres etc.
Large area gas flow counters are available from Ludlum Measurements
(www.ludlums.com). here is a snip from their web page. I have used many
of their products over the years. I do not work for or represent Ludlum.
*LUDLUM MEASUREMENTS, INC.*
501 Oak Street / P.O. Box 810
Sweetwater, Texas 79556
800-622-0828 / 325-235-5494
E-Mail: ludlum at ludlums.com <mailto:ludlum at ludlums.com>
Ludlum Measurements Japan Office
Ginza Fugetsudo Bldg. 5F 6-6-1 Ginza
Chuo-ku Tokyo 104-0061 Japan
E-Mail: spickett at ludlums.com <mailto:ludlum at ludlums.com>
If using gas flow thin window detectors, small punctures do not cause
light leaks as with ZnS(Ag) detectors.
Seems to me that the use of floor monitors would be ideal for (after the
fact) detection of alpha contamination in a mall or other shopping venue
with clean smooth surfaces.
Also, for quick detection consider an alpha air monitor. These will
detect larger concentrations of alpha activity above the natural radon
background quite nicely. They also can be set to detect air in the
return intake to the air conditioners and thus monitor quite promptly
for major releases. Of course, no large release will ever occur! So,
the cost of equipment, training and operation is not especially well spent.
By the way, I believe that the Po-210 was expected to never be
detected. That is why it was used. Normal monitoring for radioactivity
would not detect it at all since it is normally sensitive to beta and
gamma or x-ray radiation. The perpetrators clearly (to me) thought the
victim would just get sick and die and no one would ever know what
caused it. My theory, anyway.
Second thought, now that the Po-210 is found to be a contaminant,
monitoring naturally reverts to the low limit detection processes that
are used, particularly for decommissioning or release for unrestricted
use. These levels are much to low to be appropriate for detecting
dangerous levels of alpha emitters. Any system used for alert to
dangerous conditions should be sized and designed not for low level
detection, but for detection at levels well below the danger level (I
don't know what that is...) but not necessarily at just above background
John Andrews, Knoxville, Tennessee
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