Re: PCs and Tritium

[gascione@ss_031.pppl.GOV (George Ascione)]

>
>Greetings from sunny California.
>
>We are currently investigating the ability of our Tennelec (now Oxford)
>LB 5100 proportional counter to detect tritium. The purpose is to
>demonstrate compliance with the external contamination limits in 49 CFR
>173.443(a) for our out going tritium shipments. We don't ship tritium
>very often (2 to 5 times a month) so justifying a LSC purchase is
>difficult. Do any of you have experience with this? We know the PC sees
>something when H-3 on a wipe is introduced into the chamber (with
>window), but have no idea as to the efficiency. We are working with
>Oxford, but REAL WORLD experience is always helpful. Any insights?
>
>Thanks in advance.
>
>Donald P. Mercado, O/47-20, B/106      PROFS Nickname: DMERCADO
>Radiation Safety Officer        Internet: Don@LMSC.Lockheed.com
>Lockheed Missiles & Space Co., Inc.         Tel. (408) 742-0759
>Sunnyvale, Ca 94089                         Fax. (408) 742-0611
>
>

Greetings from the Princeton Plasma Physics Laboratory, Home Of the Tokamak
Fusion Test Reactor, Tritium Heaven

We have extensive experience with tritium on a swipe using PC's.  We 
currently have NMC PC-5's (which are not worth anything) and a Protean 
Instruments Model MPC 9300 which works great.  The efficiency is very 
difficult to determine.  The effects of conductivity, self absorption, 
chamber geometry, gas flow, and count time all play a role in determining 
the efficiency.  In order to perform the efficiency, we were required to 
develop a procedure which involved placing tritium oxide on a smear, 
counting it in the PC, oxidizing the sample, then performing LS.  We are 
able to achieve a 100% recovery of the tritium utilizing the oxidizer after 
reading the sample in the PC.  This procedure eliminated the effects of self 
absorption which is the most important effect when dealing with tritium on a 
smear.  We also used a tritium surface emission standard to determine the 
accuracy of the instrument.  When using the actual surface emission of the 
standard, we are able to achieve a 95 - 100% efficiency.  One important 
thing to keep in mind is conductivity.  You MUST !! use conductive smears 
when using a PC with tritium to achieve a respectable efficiency, 
reproducibility, and linearity.  In short, the beta decay will build up a 
charge on the surface of the smear over time (seconds) and will reduce the 
detection efficiency very rapidly,  You must eliminate the charge using 
conductive smears and a well grounded sample holder. If you do not, you will 
never be able to reproduce your efficiency and your linearity will be all 
over the place.  We are able to achieve a 15% efficiency with excellent 
reproducibility over the full range of the instrument.  This is verified 
almost daily through the use of operational checks.  We currently have an 
on-site inventory of 122,000 Curies of tritium in various forms for use as 
fuel in the test reactor.  On a normal operational day, we move 
approximately 12,000 curies of tritium through various
delivery systems.  We have a lot of experience.  If you need any help in 
this matter give me a call.


George Ascione
Deputy Manager, Health Physics
Princeton Plasma Physics Laboratory
Princeton N.J.     609-243-2513
E-Mail gascione@pppl.gov