322 Chapter 6. Scintillation Detectors and Photodetectors
6.1.B LightYield
This is perhaps the most important parameter for any scintillation material. The
reason is that if the light output is very low, the overall signal-to-noise ratio of the
subsequent photodetector may not be acceptable. This is specially true for detectors
used for spectroscopic purposes where good photon statistics is necessary to yield
results well above systematic uncertainties of the system.
The light yield is usually measured in number of photons perMeVof absorbed
radiation. For commonly used scintillators a light yield of 20,000-30,000 photons per
MeVis not uncommon. In high energy physics it is also customary to characterize
light yield in units of number of photoelectrons per minimum ionizing particle. The
photoelectrons are produced in the photomultiplier tubes that are used to detect
photons produced by the scintillators. The photons from the scintillator are guided
to the photomultiplier tube where they are converted into electrons through the
process of photoelectric emission in the photocathode material. The combined ef-
ficiency of the scintillator and the photocathode is what is generally used in high
energy physics to define the light yield. Based on the physics considerations, system
electronics, and calibration requirements, a threshold light yield is set to decide on
the type of scintillator and the photocathode. 5-10 photoelectrons per minimum
ionizing particles is the usual threshold set by experimenters. We will learn more
about photomultiplier tubes and other types of photodetectors later in the Chapter.
The three main factors on which the light yield of a scintillator depends are
the scintillation material,the type of incident particles,the energy of particles, andtemperature.The dependence of light yield on the material type can be appreciated from
Fig.6.1.2, which shows the relative light output for various commonly used scin-
tillators with respect to electron energy. Note the considerable difference between
the qualitative and quantitative energy dependence of the materials shown. The
dependence of light output on energy can also change considerably if the impurity
concentration in the material is changed.
A troubling aspect of energy dependence of light yield as seen in Fig.6.1.2 is that
it degrades the energy resolution of the system. To understand this, let us assume
that an incident particle produces electron hole pairs along its track. The electrons
move around in the bulk of the material and in the process produce more electrons
through excitations. This results in broadening of the electron energy spectrum.
Since light yield depends on the particle energy therefore the measured energy will
also have a broad spectrum and consequently the energy resolution of the system
will degrade. An important point to note here is that even though we assumed the
energy of the original electrons to be single valued, the resulting energy spectrum
broadened due to the energy dependence of light yield. In practice the electrons
created by the incident radiation have their own energy spectrum and therefore this
effect is even more pronounced. The degradation in energy resolution is intrinsic to
the scintillation materials and has actually been observed. Fig.??shows the energy