Tritium detection
Bruce Dunn
> Org. : Naval Ocean Systems Center, San Diego
> Person: Mark H. North
>
>
> You have it backwards. Tritium in solution is notoriously difficult due
> to possible contamination and chemiluminescence. I submit this is why
> there is still debate about the presence of Tritium and relatively
> little about the absence of Helium.
>
For general information, I can give some details about tritium detection.
I am a biochemist, and detection and measurement of tritium by scintillation
counting is a common task in labs that I have worked in. It is possible that
there are some wrinkles in measuring tritium from cold fusion cells that don't
apply to the type of samples that I have dealt with, but the principles are the
same.
To measure tritum in an aqueous solution, up to 5 ml or more of water is
mixed with emulsifying agents and an organic scintillation cocktail. The
cocktail is usually based on xylene or related aromatic hydrocarbons, and
contains dissolved organic scintillators. The mixing is done in a sample vial
with an approximate capacity of 20 ml. Beta particles released by the tritium
dump their energy into the aromatic hydrocarbon, which in turn excites the
organic scintillators. These emit photons, with the number of photons released
per beta particle being dependant on the beta energy. The sample vial is
counted in a dark chamber viewed by two photomultiplier tubes. Each tube has
random noise, so to lower the background electronic circuitry is set for
coincidence counting, in which a count is recorded only if both tubes see an
event essentially at the same time. Most beta-induced events release enough
photons to trigger both tubes, while noise is largely eliminated by the
coincidence requirement.
Counting background is about 20 counts/minute - given a 100 minute
counting period, it is quite easy to measure 10 counts/minute over background.
Counting efficiency is about 40%, meaning that 10 counts/minute is equivalent
to 25 disintegrations/minute. This corresponds to about 11 pCuries, or 0.39
femtomoles of tritium.
Chemiluminescence in the scintillation cocktail generates single photon
events, which if present in large enough numbers will get past the coincidence
circuitry by occasionally triggering both photomultiplier tubes simultaneously.
Chemiluminescence is particularly a problem with alkaline samples, however it
can be dealt with. For alkaline samples, the simplest thing is to put some
acetic acid in the scintillation fluid to neutralize the mix. Modern
scintillation counters can detect and correct for small amounts of
chemiluminescence, while simultaneously warning the user what it is doing and
flagging the sample as containing chemiluminescence. Finally,
chemiluminescence is dependant on reactive materials, which are depleted as the
reaction proceeds. The simplest cure for chemiluminescence is to put the
samples in a warm dark place over the weekend before counting them.
Any counts that are thought to be tritium can be subjected to a couple of
checks. The scintillation counter produces an energy spectrum of the
scintillations in the sample, in terms of the number of photons seen by the
photomultipliers. The energy spectrum of the suspected tritium should match
that of real tritium in the same scintillation cocktail. The counts should not
decay away if the sample is left for a week or so - this will rule out
chemiluminescence and possible contamination of the sample by short lived
radio-isotopes from outside the experimental setup. Finally, if the tritium is
thought to be in the form of water it should be possible to distill the
original sample without losing the tritium.
If anyone has any direct experience in trying to find tritium from cold
fusion cells, I would welcome the information.
--
Bruce Dunn Vancouver, Canada a7...@mindlink.UUCP