6.2. Organic Scintillators 347
(^400500)
50
100
Wavelength (nm)
Transm
iss
ion
(%)
Figure 6.2.9: Typical light transmission ca-
pability of base liquids commonly used for liq-
uid scintillation detectors.tration of dye in the liquid. It has therefore become a general practice to determine
the optimum value of the dye concentration based on the system requirements.
Organic liquid scintillators have been quite successfully used in large volume
detectors. for example in experiments involving neutrino detection the weight of the
liquid can be several hundred tons.
We mentioned earlier that one of the major problems with liquid scintillators is
their contamination from foreign elements. Some of these contaminants can deterio-
rate the scintillation properties of the liquid while others can affect the transmission
properties of the scintillation light. The most commonly encountered contaminant
of the second kind is water. Small amounts of dissolved water in a liquid scintillator
can significantly deteriorate its light emission properties. Fortunately water does
not dissolve easily in most liquid scintillators, specially the ones that are based on
mineral oil. However, since the effect of water contamination is significant, there-
fore every effort should be made to avoid any contact of the liquid with water. A
common problem encountered during handling of a liquid scintillator is the moisture
present in the inside of the container. If the container is not properly cleaned and
dried, water and other contaminants can slowly dissolve in the liquid and decrease
its efficiency.
Oxygen in air can also contaminate liquid scintillators and decrease its light out-
put. Even small amounts of air contamination has been seen to produce a decrease
in light output of 20-30%. Since it is extremely difficult, if not impossible, to avoid
air or oxygen exposure to scintillators, therefore a normal practice is to de-oxygenate
the liquid at least once before its use in a detector. The process is quite simple and
involves slowly passing dry nitrogen through the liquid, which bubbles out any oxy-
gen in the liquid. After this process is complete, the liquid should be kept in an
atmosphere of highly purified nitrogen, argon, or any other inert gas to avoid any
subsequent contact with atmospheric oxygen.
Some of the liquid scintillators can not only be highly flammable but can also
damage skin. Therefore extra care must be exercised while handling such liquids.
These problems are however do not make liquid scintillators any less desirable for
detectors than their solid counterparts. In fact a number of extremely sensitive