Physics and Engineering of Radiation Detection

336 Chapter 6. Scintillation Detectors and Photodetectors

Solution:

The maximum efficiencies of the two crystals can be obtained from substituting

the given parameters into equation 6.1.18. Hence forNaI:Tlcrystal we get

ηmax[NaI:Tl]=

hc

γEgλmax

=

(

6. 63 × 10 −^34

)(

2. 99 × 108

)

(3) (5. 9 × 1. 6 × 10 −^19 ) (410× 10 −^9 )

=0. 17

= 17%.

Similarly forCsI:Tlwe get

ηmax[CsI:Tl]=

hc

γEgλmax

=

(

6. 63 × 10 −^34

)(

2. 99 × 108

)

(3) (6. 2 × 1. 6 × 10 −^19 ) (550× 10 −^9 )

=0. 12

= 12%.

It should be noted that these are not the efficiencies one should expect the

scintillators to exhibit in a practical system since the computations were based

on the assumption that the materials had 100% luminous center emission

and quantum efficiencies. Of course in a practical system this should not be

expected due to the possibility of energy loss through other non-scintillation

transitions and transfers. Another point is that the scintillation efficiency

also has temperature and energy dependence, which we have neglected. An

actual crystal can have a scintillation efficiency that is up to 50% of the value

obtained from equation 6.1.18.

6.2 Organic Scintillators

Organic scintillators are extensively used in radiation detectors. They are found in
solid, liquid, and gaseous states. One of the biggest advantage of organic scintilla-
tors is that they can be produced in virtually any geometry and therefore can be
customized to specific applications.

6.2.A Scintillation Mechanism

The basic scintillation mechanism of organic scintillators is thefluorescenceor prompt
emission of light following the S-state electronic transitions. To better understand
this process, let us have a look at typical energy level diagram of organic scintillators
as shown in Fig.6.2.1. The incident radiation transfers energy to the atoms causing
electronic transitions fromS 0 ground levels to different vibrational levels in theS 1
band (higher band transition are of course also possible depending on the energy of
the incident radiation). The induced instability in the system forces the electrons