Physics and Engineering of Radiation Detection

6.1. Scintillation Mechanism and Scintillator Properties 325

Hence the efficiency of anthracene is given by

η =

(20,000)(2.63)

1 × 106

× 100

=5%. (6.1.1)

6.1.C RiseandDecayTimes......................

The time profile of a typical scintillation light pulse is shown in Fig.6.1.4. As shown,
the pulse rises very quickly with a typical rise time of less than 1 nanosecond. The
decay of the pulse is rather slow. In fact, the slow pulse decay in scintillators poses
a major problem for their use in detectors since it can decrease the overall efficiency
in high rate situations. The decay time of typical scintillation pulse can be as low
as a few nanoseconds and as high as several milliseconds.

Light Output

Time

Figure 6.1.4: Typical time profile of scintillator

output. Most scintillators produce pulses with

very fast rise times (less than 1 nanosecond).

To model this pulse we first split the pulse into rising and decaying edges, both
of which can be characterized by exponential functions. The time dependence of the
rise of the pulse rise can be written as

L∝ 1 −e−t/τr, (6.1.2)

whereLis the intensity of light in any convenient units andτris a constant we will
describe later. Similarly the decay of the pulse can be written as

L∝e−t/τd, (6.1.3)

withτdis another constant to be described shortly. Combining the two profiles we
get

L=L 0

(

e−t/τd−e−t/τr

)

, (6.1.4)

whereL 0 is the maximum height of the pulse.