346 Chapter 6. Scintillation Detectors and Photodetectors
must be used in highly purified forms. Even addition of small amounts of water
can contaminate the scintillator and significantly deteriorate its light transmission
properties.
A large number of pure liquids and liquid mixtures have been identified as good
scintillators having properties relevant to their use for radiation detection. These
liquids have not been assigned any generic names but most of them are available
commercially from different manufacturers under their own names.
Most liquid scintillators emit light with a peak somewhere between the wave-
lengths of 400nmand 500nm. The spectrum is, however, somewhat skewed at
higher wavelengths as shown in Fig.6.2.8.
400 450 500
Wavelength (nm)0.51.0R
el
ativeLight YieldFigure 6.2.8: Light yield of a
typical liquid scintillator as a
function of wavelength.Emission spectra of the kind shown in Fig.6.2.8 are very favorable to develop liq-
uid scintillator detectors because most base liquids used in such liquids have optimal
transmission only above a wavelength of 400nm. This is illustrated in Fig.6.2.9,
which is a plot of some suitable transmission property with respect to wavelength of
photons (see also (20)). The reader should bear in mind that light transmission is
not the only criterion for the usability of a scintillator in a specific application. The
scintillation photons must also be efficiently detected by another detector such as a
photomultiplier tube. Since photodetectors have their own limitations with respect
to their sensitivity to different wavelengths, therefore the liquid scintillator should be
chosen such that it not only has high transmission capability for the photons it emits
but the photodetector also has high efficiency to detect them. Most photomultiplier
tubes work best for wavelengths in the green region of the electromagnetic spectrum
and therefore scintillators that have peak in this region can be used very efficiently.
As a reminder the middle of the green region lies at approximately 510nm.Some
scintillators emit light in the blue region and therefore require a wavelength shifter
to produce green light. The wavelength shifter can be a liquid dissolved in the base
along with the scintillator or an external solid. The external wavelength shifters will
be discussed later in the Chapter.
Most liquid scintillators show remarkable stability with respect to small temper-
ature variations. Less than 1% variation per decade in temperature in Celcius is
not uncommon (20). The light yield of liquid scintillators covers a broad range but
mot manufacturers produce products that have light output from 50% to 80% of
anthracene. Such a varied light yield is due to the strong dependence of the concen-