ORTEC 420 - Foreword

MODEL 420
TIMING SINGLE CHANNEL ANALYZER
Foreword
Introduction to Fast Timing With Linear Signals
Precise determination of the time of occurrence of nuclear events is often compli
cated. Detection of these events usually results in a voltage pulse whose amplitude
is a function of the energy loss in the detector, and whose rise time is a function of
the conversion process from detector energy to charge. When time determination is
derived from the leading edge of such a pulse (by means of a discriminator, e.g., a
Schmitt trigger), there is a time shift of the discriminator output that depends on the
pulse rise time and the range of pulse amplitudes involved. (See Figure 1(a).) The
rise time of these pulses is usual ly limited by the linear amplifier which is necessary
for pulse height analysis; such amplifiers are the Models 410, and the older versions
OS the Model 203, A-8 and DD2, etc. For those amplifiers which have variable RC
shaping, e.g., the Model 410, the rise time and fall time will vary directly with
the integration and differentiation time constants chosen; therefore, the zero crossing
time will vary in real time. The time of zero crossing in real time is approximately
2 time constants after the detector event when the double RC differentiation is used.
Figure 1(b) shows a series of idealized output pulses from a double delay line clipped
amplifier, superimposed upon each other. All of the pulses pass through the baseline
at the same time; this is true because in the amplifier clipping process, the voltage
which is forming the pulse shape goes from a positive value (+E) to on equal negative
value (—E) in the same time that wasrequired forthe voltage to go from 0 to +E, i.e.,
one rise time. The pulses, therefore, pass through the baseline (zero volts)at a time
Tr r
-01 DISC. TRIGGER LEVEL
Tr/i
— DISC. TRIGGER LEVEL
DISC. \i,
OUTPUTS h J
TIME SHIFT WITH LEADING EDGE TRIGGER
(6)
TIME SHIFT WITH CROSSOVER PICKOFF
Figure 1. Timing From Linear Amplifier Output Pulses