ORTEC 473 - Power Supplies; Applications; Sj.2. Timing with Nal(TI) Scintillators; Timing with Large Ge(Li) Detectors

The remaining portion of the Fig. 5.2 circuit that is affected by the selection of Ext is the duration of the dead time, which is
in the feedback loop from the Pos Out circuit to gate G4. Section E (the last wafer) of switch 81, the detector selector,
completes a circuit from the switch wiper to ground for the Ge(Li), Scint, and Ext settings, and the resulting controlled dead
time is approximately 50 ns. If a longer dead time is desired for Ext operation, the grounded connection for the 4th switch
pciition can be clipped and the dead time will then be approximately 1 iis, the same as for Nal operation.
5.11. POWER SUPPLIES
Two power supply levels are generated in the 473 for use in its integrated circuits. One is at —5.2 V and the other is at —2 V.
The ac input power line is connected through the bin and power supply distribution circuits to each module location. It is
accepted from this circuit into the primary of transformer T1 on the 473 chassis. The output of T1 is full-wave rectified by
D5, regulated by Q20 through 023, and filtered by C81. T1, D5, and C81 are all chassis-mounted and the remaining
components are located on the printed circuit board. Using the —12 V bin power dc level as a reference, this circuit furnishes
the —5.2 V level that is required in most of the integrated circuits.
Another circuit uses the —12 V reference and reduces the level from —5.2 V down to —2 V for other operating requirements.
025 is the series pass transistor in this dropping circuit and 024 furnishes the reference level from a bleeder in the —12 V
source.
Each of the four standard dc levels (+12 V and ±24 V) are accepted from the bin and power supply through four separate
filter networks that involve LI 2 through LI 5 and C73 through C80.
6. APPLICATIONS
6.1. TIMING WITH FAST SCINTILLATORS
Figure 6.1 shows a typical system for timing with fast scintillation detectors such as Naton-136, Pilot B, KL236, NE-102,
NE-111, NE-213, etc. A 473 Constant Fraction Discriminator is used in each of the two channels of the time to pulse height
converter. Figure 6.2 is a typical timing spectrum that was taken with this system.
A plot of time resolution versus dynamic range is shown in Fig. 6.3 for a system using an RCA 8850 photomultiplier tube. A
similar plo' for the RCA 8575 PMT is shown in Fig. 6.4. The output pulse from the RCA 8575 is slightly slower than that
from the h. A 8850. Because of this risetime difference, the best timing resolution is obtained with the 473 set for Scint
when using the RCA 8850 and set for Nal when using the RCA 8575.
The input to the 473 should have approximately 5-V pulses for the Compton edge of the 511-keV gamma from ^''Co so that
the dynam'-: i nnge can be 100:1. The lower level discriminator should be set at ~50 mV. For some tubes, the 50i2 back
terminatioi tI to the tube base must be removed to accomplish the maximum dynamic range.
6.2. TIMING WITH Nal(TI) SCINTILLATORS
This type of measurement is similar to timing with fast scintillators except for one additional problem that must be
considered. The photoelec'ron statistics are so poor for low-energy gamma-ray work that individual photoelectron events near
the trailing edge of the Nal(TI) pulses will trigger the 473. Thus a single scintillation event can produce two or more
discriminator output pulses. In the 473 this problem is overcome by using the Nal mode in which an internal dead time of ~1
IIS is generated. The 473 can then be operated in the Nal mode and be used successfully on even longer decay scintillators,
but the internal dead time may have to be increased to prevent multiple triggering.
Figure 6.5 is a typical timing spectrum that was taken with a Nal(TI) detector, based on triggers from a KL236 detector.
Figure 6.6 is a plot of time resolution versus dynamic range for the Nal(TI) detector, using an RCA 8575 PMT.
6.3. TIMING WITH LARGE Ge(Li) DETECTORS
Figure 6.7 is a block diagram of another gamma-gamma coincidence system. In this system the start channel uses a fast
scintillator and the stop channel employs a large coaxial Ge(Li) detector. A typical timing spectrum for a narrow dynamic