Service manual

9
6.2. TIMING WITH Nal(TI) SCINTILLATORS
Timing with Nal(Tl)) scintillators is similar to timing
measurements with fast scintillators. However, one
additional problem must be considered. The
photoelectron statistics for low-energy gamma-ray
applications are so poor that individual events near
the trailing edge of Nal(TI) pulses can trigger the
584. Thus a single scintillation event can produce
two or more discriminator output pulses. In the 584,
this problem is overcome by setting the Blocking
Width adjustment to 1000 ns. The 584 can also
be used successfully on longer decay scintillators,
but the maximum blocking width time may have to
be increased to prevent multiple triggering.
The timing resolution obtainable with an Na I (TI) in
a given application depends on many variables.
Typical timing resolution for a properly adjusted
system using a 1 in. x 1 in. KL-236 and an RCA
8575 PMT in the start channel, a 1 in. x 1 in. Nal(TI)
and an RCA 8575 PMT in the stop channel, and a
50:1 dynamic range with Co is 900 ps FWHM
60
and 1.9 ns FWTM.
6.3. TIMING WITH LARGE VOLUME HPGE
DETECTORS
Figure 6.2. shows a typical timing coincidence
system using a large volume HPGe detector in the
stop channel. In this application, the timing
resolution is dominated by the charge collection
characteristics of the HPGe detector. The output
signal from the HPGe detector preamplifier must be
shaped prior to processing by the 584. Either a Fast
Filter Amplifier (ORTEC Model 579) or a Timing
Filter Amplifier (ORTEC Model 474) can perform
the necessary pulse shaping. In typical applications,
the 579 is set at
200 ns differentiation time
constant and the integral time constant is set at
Out.
Figure 6.2. Gamma-Gamma Coincidence System Using a Plastic
Scintillator and a Large HPGe Coaxial Detector