7 Particle Detectors and Detector Systems
285
scintillator. Let N D be the total number of detected photons. The time resolution is
roughly proportional to 1/
√
N D . ToF detectors with high resolution, ∼ 100 ps,
therefore use scintillator thickness of 2–3 cm. The material budget then becomes
important.
The connection between the scintillator and the photon detector is a very
important step in order to maximise the light collection efficiency of the system.
These light concentrators are normally built around a Winston Cone [5] or a fishtail
as in Fig. 7.3b. A Winston Cone is a non-imaging off-axis parabola of revolution
which will maximise the collection of incoming rays. The ideal concentrator will
achieve the highest possible concentration of radiant energy permitted by the second
law of thermodynamics. This is equivalent to the general theorem of Liouville [6].
More specific in a case of a light guide, one can write:
n
2
− 1 ≥
d
2r
+ 1
2
(7.6)
where d is the light guide diameter and r is the bending radius. n is the refractive
index relative to air. See Fig. 7.3c. Charged particles going through the light guides
will give signal due to Cherenkov radiation and thereby give rise to an event
correlated background.
A well designed scintillator for ToF must provide a good photon collection
efficiency and a small time jitter. For fast timing one would normally rely on the
first direct photon impact. This puts further constrains on the photon detector.
The classic photon detector is the photomultiplier tube (PMT). Depending on the
window geometry, dynode chain and HV configuration, the transient time spread
is in the range of 1 ns. This can be reduced by instrumenting both ends of the
scintillator and then use mean timing. This will also take care of the after-pulsing
in the PMT. These are normally either due to ions in the residual gas in the PMT
which drift back, strike the photo cathode and liberate new photoelectrons or light
from the dynodes which hit the photo cathode. The first will give a signal about
100 ns after the event, while the latter signal comes after 30–60 ns. See Chap. 3 for
more information. However, still to overcome the path length and transient time
variation, the detector has to output a large amount of primary photons to achieve
total time resolution in the range of 100 ps.
An example can be found in [7]. Mean timing and time slewing corrections are
performed. Slew-correction time, T cor , is defined as:
T
cor
= T +
A 0
√
ADC
(7.7)
where the constant A 0 is normally evaluated for each PMT and ADC is the signal
pulse height. They report a nearly constant time resolution of σ ∼ 55 ps across a
detector length of 15 cm.
285
scintillator. Let N D be the total number of detected photons. The time resolution is
roughly proportional to 1/
√
N D . ToF detectors with high resolution, ∼ 100 ps,
therefore use scintillator thickness of 2–3 cm. The material budget then becomes
important.
The connection between the scintillator and the photon detector is a very
important step in order to maximise the light collection efficiency of the system.
These light concentrators are normally built around a Winston Cone [5] or a fishtail
as in Fig. 7.3b. A Winston Cone is a non-imaging off-axis parabola of revolution
which will maximise the collection of incoming rays. The ideal concentrator will
achieve the highest possible concentration of radiant energy permitted by the second
law of thermodynamics. This is equivalent to the general theorem of Liouville [6].
More specific in a case of a light guide, one can write:
n
2
− 1 ≥
d
2r
+ 1
2
(7.6)
where d is the light guide diameter and r is the bending radius. n is the refractive
index relative to air. See Fig. 7.3c. Charged particles going through the light guides
will give signal due to Cherenkov radiation and thereby give rise to an event
correlated background.
A well designed scintillator for ToF must provide a good photon collection
efficiency and a small time jitter. For fast timing one would normally rely on the
first direct photon impact. This puts further constrains on the photon detector.
The classic photon detector is the photomultiplier tube (PMT). Depending on the
window geometry, dynode chain and HV configuration, the transient time spread
is in the range of 1 ns. This can be reduced by instrumenting both ends of the
scintillator and then use mean timing. This will also take care of the after-pulsing
in the PMT. These are normally either due to ions in the residual gas in the PMT
which drift back, strike the photo cathode and liberate new photoelectrons or light
from the dynodes which hit the photo cathode. The first will give a signal about
100 ns after the event, while the latter signal comes after 30–60 ns. See Chap. 3 for
more information. However, still to overcome the path length and transient time
variation, the detector has to output a large amount of primary photons to achieve
total time resolution in the range of 100 ps.
An example can be found in [7]. Mean timing and time slewing corrections are
performed. Slew-correction time, T cor , is defined as:
T
cor
= T +
A 0
√
ADC
(7.7)
where the constant A 0 is normally evaluated for each PMT and ADC is the signal
pulse height. They report a nearly constant time resolution of σ ∼ 55 ps across a
detector length of 15 cm.