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Detailed introduction
The Type 672 input accepts a positive or negative signal from the radiation detector preamplifier and provides a positive 0 to 10 V output signal suitable for single or multi-channel pulse height analyzers. Its gain varies continuously between 2.5 and 1500.
Automation of all key adjustments makes the Model 672 easy to set up for any radiation detector, enabling optimal performance even if the operator has no expertise.
An automatic pole-zero adjustment feature significantly simplifies amplifier tuning, compensating for the decay time of the preamplifier pulses. This minimizes operator skill requirements and effort to achieve good energy resolution and peak position stability at moderate to high count rates. After changing the time constant or replacing detectors, simply press the AUTO PZ button and wait a few seconds for the AUTO PZ BUSY LED to turn off for precise pole zero (PZ) adjustment. This procedure does not require an oscilloscope. AUTO PZ memory is protected against power failure.
In cases where, for example, the preamplifier pulse shape starts to deform from the normal exponential decay, then PZ cancellation is not possible. In this case, using manual PZ adjustment mode can sometimes slightly improve high count rate performance, resulting in a compromise solution.
A front panel switch on the Model 672 selects triangular or Gaussian pulse shape for the UNIPOLAR (unipolar) output connector. The noise performance of the triangular pulse shape is equivalent to the Gaussian pulse shape with a 17% longer shaping time constant. In applications where the series noise component is dominant (short shaping time constant) and a stacking rejector is used, the triangular shape usually has the same dead time and slightly lower noise than the Gaussian pulse shape. Front panel switch selectable shaping time constant for each detector and application. Six time constants range from 0.5 to 10 μs, and the TRIANGULAR/GAUSSIAN switch combination provides 12 different shaping times. Bipolar outputs are also provided for measurements requiring zero-crossing timing.
To minimize spectral distortion at medium and high count rates, the unipolar output uses a high-performance, gated, baseline restorer with multiple automation levels. An automatic positive and negative noise discriminator ensures that the baseline restorer operates only on the true baseline between pulses and is not affected by noise level changes. It does not require the operator to adjust the baseline restorer when gain, shaping time constant, or detector characteristics change. Negative overload recovery from reset pulses generated by transistor reset preamplifiers and pulsed optical feedback preamplifiers is also handled automatically. The monitoring circuit will shut down the baseline restorer and provide a reject signal to the multi-channel analyzer until the baseline has safely recovered from the overload.
Several operating modes can be selected for the baseline restorer. To make PZ adjustment manually or automatically, select the PZ position. This location may also be used in applications requiring the slowest baseline recovery rate. For situations where low-frequency noise interference is a problem, high rates can be selected. On detectors where PZ cancellation is not possible, the automatic baseline restorer rate provides performance at low and high count rates.
An efficient stacking rejector is integrated into the Type 672 Spectral Amplifier. It provides output logic pulses to associated multi-channel analyzers to suppress spectral distortion caused by pulses stacking on top of each other at high count rates (Figure 3). The fast amplifier in the stack rejector includes a gated baseline restorer with automatic noise discriminator. A multi-colored stack-up indicator LED on the front panel indicates the amplifier's processing efficiency. At low count rates, the LED flashes green. The LED will turn yellow at moderate count rates and red when pulse stacking loss >70%.
If long connecting cables are used between the detector preamplifier output and the amplifier input, cable noise caused by the environment may have an impact. The Type 672 offers two solutions. For low to moderate interference frequencies, the differential input mode can be used with paired cables from the preamplifier to suppress noise. At high frequencies, a common-mode suppression transformer built into the Type 672 input reduces noise pickup. Transformers effectively eliminate interference from display raster generators in personal computers.
All toggle switches on the front panel can be locked to prevent accidental changes to desired settings.
