6.4. Transfer of Scintillation Photons 365
Determine the minimum tapering angle of the light guide if the maximum
angle of light incident on photomultiplier surface should not exceed 60^0.Solution:
Forn=1.5, equation 6.4.5 reduces to
π
2+θt−θp ≥ 41. 80
⇒θt ≥ θp− 48. 2.Substitutingθp=60^0 in the above equation givesθt≥ 18. 20. (6.4.6)Hence the light guide must be tapered at least at an angle of 18. 20.The fish-tail is not the only geometry used in practical systems. The geometry
of a light guide is in fact application dependent. for example if the photodetector
can not be placed in line of sight of the scintillator then the light guide must have
one or more bends. In some applications the distance between the scintillator and
the photodetector is so large and the path is so complicated that it becomes ex-
tremely difficult to design and construct a stiff structure to guide the photons. In
such cases, optical fibers offer the required flexibility. Optical fibers have become a
standard in telecommunication where it is desired to transmit electrical signals at
large distances with extremely low attenuation and high bandwidth. Optical fibers
fulfill both of these requirements but for transmission of light signals. Therefore the
electrical signal is first transformed into an optical signal and then transmitted. At
the receiving end the signal is transformed back into the electrical signal and further
processed. Optical fibers work on the principle of total internal reflection and suffer
from very low light leakage.
A.3 HybridLightGuides......................
It is extremely difficult, if not impossible, to design and construct a light guide based
on total internal reflection that ensures perfect transmission of photons without any
loss. That is, there are always photons that impact the light guide at angles that
do not favor total internal reflection and therefore refract out of the light guide. for
example in the previous section we looked at the fish-tail type light guide and derived
an expression to determine the tapering angle for total internal reflection. However
the practical considerations of the system, such as surface areas of the scintillator
and the photodetector and distance between them, may not favor implementation
of a light guide that would ensure perfect total internal reflection and all angles of
incidence. Depending on the application, the loss of light intensity in the light guide
can be significant and even intolerable. Since it is extremely difficult to retain each
and every scintillation photon inside a light guide therefore the only option left is to
use some other means to decrease the loss. A simple reflecting surface around the
light guide can serve the purpose. Fig.6.4.3 shows two types of hybrid structures
with fish-tail light guide as one element and a reflecting surface as the other. The
easiest way to construct the reflecting surface is to paint a highly reflecting paint on