From: Dick Hahn (firstname.lastname@example.org)
Date: Tue Apr 19 2005 - 16:23:54 CDT
Hi to all from the BNL Group,
During Thursday's phone call, there was a bit of discussion about which Gd
concentration we should use. We all know the usual physics points: the
decrease of the delay time (between the e+ event and the n-capture) with
increasing Gd concentration, the increase of the size of the resulting
signal with increasing Gd concentration, and the (probable) increase in
radioactive contaminants with increasing Gd concentration.
After the phone call, Maury sent me an e-mail asking if there is another
factor to be concerned about, namely is there any dependence of the
stability of the Gd-LS (what he called the lifetime) on Gd concentration? In
our experience, especially at the low Gd concentrations that we are
considering, there is no obvious dependence on Gd concentration. Factors
that MAY cause deterioration of the Gd-LS, such as introduction of air that
might oxidize some components in the organic liquid and produce yellowing,
or formation of insoluble Gd hydroxides, oxides, or organic polymers caused
by elevated pH during our preparation steps, are not caused by increases in
the Gd concentration. If one is careful to control the conditions of the
synthesis of the Gd-LS, these problems should in principle be avoidable.
Note that we have samples of indium-loaded scintillator in the concentration
range >5% by weight that have been stable for more than 15 months, and still
counting. This concentration is >50 X higher than what we are interested in
for the Gd.
However, for completeness, we are beginning a series of experiments where we
have prepared samples of Gd-LS in PC at Gd concentrations of 1%, 0.8%, 0.5%,
0.2%, and 0.1%. We will monitor the absorption spectra and the light yields
of these samples over a period of some months, to see if any degradation
does occur, and if so, to see if the deterioration shows any obvious
dependence upon the Gd concentration. As we collect the data, we will send
you results periodically.
Minfang, Alex, and Dick
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