A patented trailing-edge constant-fraction (CF) timing technique is used, providing excellent timing on either unipolar or bipolar signals and giving better results than are possible with conventional leading-edge discriminators. Two independent CF time-derivation channels are used to evaluate the shape of the input waveform. When, for example, these discriminators are set to 10 and 90% fractions, the time interval between the two outputs thus generated will be a measure of the input signal fall time.
The versatility of the Model 552 is evidenced by the three functions it can be used for: time derivation, pulse-height analysis, and adjustable output delay.
Time derivation is an important parameter in many experiments. With SCAs that utilize leading-edge timing, the rise time of the input pulses causes degradation of time resolution due to their amplitude variations. Trailing-edge constant-fraction timing, on the other hand, compensates for varying amplitudes and essentially eliminates this time shift, giving consistently better results. Figure 1 shows that walk with the Model 552 is <±250 ps for a 10:1 dynamic range when the output of the main amplifier is being directly analyzed. A built-in input attenuator is provided so that the front-panel walk controls can be adjusted rapidly and precisely, to achieve excellent timing performance.
Pulse-height analysis with the Model 552 can be done in three different basic operating modes: as a high-resolution, narrow (0 to 10%) window, single-channel analyzer; as a wide-window SCA in which the upper-level and lower-level controls are independently variable from 0 to 10 V and an output is generated for pulses analyzed between the levels; and as a wide-dynamic-range integral discriminator for leading-edge timing or pulse routing via the separate rear-panel LL Out and UL Out outputs.
Another feature that makes the Model 552 a versatile instrument is a continuously adjustable output delay, which allows output signals with actual time differences to be aligned without the need for additional delay devices or modules. Alternatively, an external strobe input can be used to produce an SCA output at the desired time.
When it is desirable to scan an entire spectrum, a rear-panel connector can be used to provide an external baseline sweep input. With the lower-level Ref switch in the Ext position, the baseline (lower-level threshold) on which a window is riding can be swept through an energy range and the count rate recorded as a function of energy.
Analysis of the amplifier pulse shape can be useful in separating the detected events from different types of radiation. The best known example is the difference in the neutron and gamma-ray response in some scintillators. In these cases, stringent conditions are imposed on the electronics because of the nonlinear response of the scintillator as a function of neutron energy. For example, for neutrons with energies from 200 keV to 10 MeV the response of an NE-213 scintillator can vary over a 500:1 range. The Model 552, in conjunction with the ORTEC Model
457, 566, or 567 Time-to-Amplitude Converter, will satisfactorily resolve shape variations over a 200:1 dynamic range in such neutron-gamma applications.
Other applications for this feature of the Model 552 are its use with gaseous detectors for particle identification, with large germanium detectors to help optimize their energy resolution, and for determining the position of interaction in a position-sensitive proportional counter.
In all cases, the Model 552 provides a measurement of the input signal shape by evaluating its timing at two different fractions.