AW: [ RadSafe ] Scintillation Counting

Wed Nov 5 05:46:51 CST 2008

Elaine,

Though there is a lot of information lacking in your description the reason
of this counting behaviour might be a result of the effect as Peter Bailey
has described it already, but I do not really believe that the effect should
be so extremely pronounced.

Information missing: What is the chemical form of your tritium? Is it
dissolved in an organic solvent or in aqueous solution? What is the
cocktail? What kind of instrument do you use? 

You use the expression dpm obviously wrongly (also?) for cpm. You should
clarify this and clearly distinguish between them. Did you use a tritium
standard? What about quench? 

The mirror systems in an LS counter are optimized for a geometry with a full
vial and I know, that in a technique which is used in ultra low-level C-14
counting and employing very small volumes of benzene vials from quartz were
constructed to hold the counting solution just in the middle of the space
provided for a conventional vial in the measuring chamber. 

I personally never dealt with other volumes than the 20 or 22 ml standard
vials, simply because I was doing ultra low-level measurements of tritium in
water. However I have a feeling, that this geometry factor can by far not
explain a loss of 90 % of the counts. 

I have used literally for decades only polyethylen vials. Glass vials for
low-level tritium have to be categorically refused, because they have much
higher background values in the tritium region due to the Cerenkov
contribution from K-40.

Best regards,

Franz

Franz Schoenhofer, PhD
MinRat i.R.
Habicherg. 31/7
A-1160 Wien/Vienna
AUSTRIA

-----Ursprüngliche Nachricht-----
Von: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] Im Auftrag
von Marshall, Elaine
Gesendet: Montag, 03. November 2008 22:36
An: radsafe at radlab.nl
Betreff: [ RadSafe ] Scintillation Counting

Does anyone have any insight so that I can get back to my researcher?

The contract specified a "6 ml polypropylene vials" for counting the 3H
samples. When I couldn't find those, and enquired, it turned out the
ones recommended were actually high density polyethylene.  There was a
specific recommendation from a particular vendor.  However, since we got
a much better deal on Vendor #2 HDPE 6 ml scintillation vials, we bought
those. 

The first samples we tried to count in those vials, with 1 ml of Scint
fluid (as recommended in the protocol), gave us < 10% of the expected
counts (925 instead of 12,000 DPM).  We were using the racks for the
mini-vials.  The technician then tried counting these in another
scintillation counter (the one used by Rad Safety), and they wouldn't
count at all:  this instrument gave error messages.  After determining
it wasn't a matter of bad pipetting or something, I had the technician
transfer that sample and its scintillation fluid to a HDPE 20 ml vial,
add 9 ml more scint. fluid, and he got the expected counts (just under
12,000).  I then had him take 5 ml of that mixture and put it back in a
clean 6 ml vial, and again, he got less than 10% of the counts expected
for the half of the volume counted (~500, instead of ~6000), so it
wasn't just the low volume of scintillation fluid suggested in the
protocol.  So it appears these vials don't count tritium well.   Do you
have any insight into this bizarre problem?  Vendor #2 has none, and
apparently has no data to actually support the use of these vials for
scintillation counting (at least for tritium, I didn't ask about other
isotopes).  They have offered to send me another type of vial, but that
doesn't entirely easy my mind.  

Any comments/experience with this sort of issue would be greatly
appreciated!!  Like, is it something that "everybody knows" (except me
and apparently the people that developed the protocol for the contract)
that one always has to count 3H in glass???   Have you ever heard of
HDPE vials contaminated with something that quenches the heck out of
tritium? 

Elaine T. Marshall, CHP

Radiation Safety Officer

Lovelace Respiratory Research Institute

2425 Ridgecrest Drive SE

Albuquerque, NM  87108

(505) 348-9578

emarshall at lrri.org

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