[ RadSafe ] Any experience in detection of polonium on large scalecontamination ?

[Kai Kaletsch]

You get some Po-210 on ventilation ducting used in uranium mills. (30 years 
of Rn-222 decay and Pb-210 buildup.) The issue for those working on the 
ducting, is inhalation and spreading the contamination around.

Airborne long-lived alpha is collected on a filter (Pall 61630 or similar) 
with a 6 - 10 hour sample pump (SKC-224 or similar) running at 2 - 3 l/min 
and then allowed to decay for days to get rid of the short lived Rn-222 and 
Rn-220 progeny. (If you were looking for a lot of activity, you could get by 
with a shorter sample and no decay time.)

Removable contamination is collected on a swipe (DefensAP WSD-8523 or 
similar), no decay period needed. Samples (filters or swipes) are counted in 
a portable alpha counter (TM372 made by my company, or similar). An alpha 
spectrum (sorry, I don't recall a brand name. Most mines send these samples 
to an off-site lab.) is taken on a few of the samples to confirm that you 
have Po-210.

If you want to detect fixed contamination,  you probably would still use a 
probe covered in aluminized Mylar. And, yes, the average life of those 
windows is approximately equal to the 1/2 life of Po-210.

Could you send me more detail on your method of depositing ZnS(Ag) with 
silicone oil? Do you remember a brand name of the silicone oil...? We have 
tried a few methods for depositing ZnS(Ag) and are not 100% happy with any 
of them.

Thanks,
Kai Kaletsch
Environmental Instruments Canada Inc.

----- Original Message ----- 
From: "parthasarathy k s" <ksparth at yahoo.co.uk>
To: "RadSafeInst" <RadSafeInst at cableone.net>; "John R Johnson" 
<idias at interchange.ubc.ca>; "David Schauer" <dschauer at usuhs.mil>; 
<radsafe at radlab.nl>
Sent: Sunday, December 10, 2006 10:54 PM
Subject: [ RadSafe ] Any experience in detection of polonium on large 
scalecontamination ?


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.


Regards

K.S.Parthasarathy





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