Higher scope sensitivities can also be used, but result in a less precise pole
zero adjustment. However, most scopes will overload for a 10 volt input sig
-
nal when the vertical sensitivity is set for 50 mV/cm. Scope overload will
distort the signals recovery to the baseline. Thus the pole/zero will be incor
-
rectly adjusted resulting in a loss of resolution at high count rates. To prevent
scope overloading a clamping circuit, such as the one illustrated in Figure 12
can be used at the scope input.
Note 2: When adjusting the pole/zero using the square wave technique, the calibra-
tion square wave generated by the oscilloscope can be used Most scopes gen-
erate a 1 kHz square wave used to calibrate the vertical gain and probe
compensation. Connect the scope CALIBRATION output through an attenu-
ator to the preamp test input and repeat Performance Adjustments step 2
(pole/zero adjustment).
4.
The AMP OUTPUT DC level is factory calibrated to 0 ± 5 millivolt.
5. To get optimum resolution. the Lower Level Discriminator on the MCA
ADO should be set just above the noise so that the effects of pileup are
minimized.
Resolution Versus Count Rate and Shaping
A 2 µs shaping is optimum for Ge(Li) detector systems over a wide range of incoming
count rates. For high resolution larger shaping time constants offer a better signal to
noise ratio, resulting in better resolution. However, as the count rate increases, the ef
-
fects of pileup degrade the resolution much sooner. The optimum shaping time con
-
stant depends on the detector (such as its size, configuration and collection
characteristics), preamplifier and incoming count rate. Below is a list of optimum
shaping time constants for some other common detectors.
12
Operating Instructions
Figure 12 Scope Input Clamp
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