ORTEC 672 Spectroscopy Amplifier



This is an ORTEC 672 with Operating and Service Manual. The ORTEC Model 672 high-performance, energy spectroscopy amplifier is ideally suited for use with germanium, Si(Li), and silicon charged-particle detectors. It can also be used with scintillation detectors and proportional counters. The Model 672 input accepts either positive or negative polarity signals from a detector preamplifier and provides a positive 0 to 10-V output signal suitable for use with single or multichannel pulse-height analyzers. Its gain is continuously variable from 2.5 to 1500. Automation of all the critical adjustments makes the Model 672 easy to set up with any detector, and provides a performance that is nearly independent of operator expertise. The Automatic Pole-Zero Adjustment feature significantly simplifies the tuning of the amplifier to compensate for the decay time of the preamplifier pulse. This minimizes the operator skill and effort needed to achieve good energy resolution and peak position stability at moderate to high counting rates (Fig. 1). When changing time constants or detectors, an accurate pole-zero (PZ) adjustment is achieved by simply pushing the AUTO PZ button and waiting a few seconds for the AUTO PZ BUSY LED to turn off. No oscilloscope is required for this procedure. The AUTO PZ memory is protected against power failures. In extreme situations, where the preamplifier pulse shape is deformed from the normal exponential decay, complete PZ cancellation is not possible. In such cases, a slight improvement in the high counting rate performance can sometimes be achieved using the manual PZ adjustment mode to arrive at a compromise solution. A front-panel switch on the Model 672 provides the choice of either a triangular or a Gaussian pulse shape on the UNIPOLAR output connector. The noise performance of the triangular pulse shape is equivalent to a Gaussian pulse shape having a 17% longer shaping time constant. In applications where the series noise component is dominant (short shaping time constants), and the pile-up rejector is utilized, the triangular shape will generally offer the same dead time and slightly lower noise than the Gaussian pulse shape. A front-panel switch permits selection of the optimum shaping time constant for each detector and application. Six time constants in the range from 0.5 to 10 μs, and the TRIANGULAR/GAUSSIAN switch combine to offer 12 different shaping times. A bipolar output is also provided for measurements requiring zero crossover timing. To minimize spectrum distortion at medium and high counting rates, the unipolar output incorporates a high-performance, gated, baseline restorer with several levels of automation. Automatic positive and negative noise discriminators ensure that the baseline restorer operates only on the true baseline between pulses in spite of changes in the noise level. No operator adjustment of the baseline restorer is needed when changes are made in the gain, the shaping time constant, or the detector characteristics. Negative overload recovery from the reset pulses generated by transistor-reset preamplifiers and pulsed optical feedback preamplifiers is also handled automatically. A monitor circuit gates off the baseline restorer and provides a reject signal for a multichannel analyzer until the baseline has safely recovered from the overload.
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