673 Spectroscopy Amplifier and Gated Integrator

• Dual purpose: High-rate and low-rate energy spectroscopy with Ge detectors
• Both Semi-Gaussian and Gated Integrator outputs
• Semi-Gaussian output offers optimum resolution at low counting rates
• Shaping time constants from 0.25 to 6 µs
• Built-in gated BLR and pile-up rejector
• Gated Integrator compensates for charge collection time variations in Ge detectors for improved energy resolution and throughput at high counting rates

The ORTEC Model 673 Spectroscopy Amplifier and Gated Integrator is a dual purpose amplifier for high-resolution energy spectroscopy with germanium detectors at both low and high counting rates. In addition to a conventional, semi-Gaussian shaping amplifier, the Model 673 includes a Gated Integrator to achieve excellent energy resolution at high throughputs.¹ The UNIPOLAR output of the semi-Gaussian amplifier is identical to the ORTEC Model 572, except that the shaping time constants range from 0.25 to 6 µs. The longer shaping time constants on this output provide the best energy resolution at low counting rates.

At high counting rates, short shaping time constants are necessary to achieve high throughput. Normally, the charge collection time variations in the Ge detector would severely degrade the energy resolution at such short time constants (Fig. 1a). The Gated Integrator solves this problem (Fig. 1b) by integrating the area under the unipolar pulse and by setting an integration period that ensures complete integration of the longer pulses that result from slower charge collection in the Ge detector (Fig. 2 and Fig. 3). The result is significantly improved energy resolution (Fig. 1b and Fig. 4) at a throughput that is approximately four times the maximum counting rate achievable with conventional semi-Gaussian shaping (Fig. 5). The Model 673 Gated Integrator output can maintain excellent resolution and peak position stability to a much higher counting rate than is feasible with semi-Gaussian shaping (Fig. 6 and Fig. 7).

A pile-up rejector is included to minimize the spectral distortion caused by two or more photons arriving at the detector within one amplifier pulse width. The pile-up rejector connects to the anti-coincidence gate of a multichannel analyzer, and provides protection for either the UNIPOLAR or the Gated Integrator output. A front-panel switch allows either manual or automatic adjustment of the noise threshold for the pile-up rejector and the baseline restorer. The manual mode is useful for transistor reset preamplifiers.

The Model 673 accommodates both resistive feedback preamplifiers and transistor reset preamplifiers (TRP). With transistor reset preamplifiers a logic pulse derived from the preamplifier reset signal can be provided to the GATE INPUT of the Model 673 for the duration of the overload caused by the preamplifier reset. The GATE INPUT is "ORed" with the pile-up rejector signal at the GI INH output and is used by the multichannel analyzer to prevent the analysis of pulses distorted by the reset.

The UNIPOLAR output also functions as a high-performance semi-Gaussian shaping amplifier that can be used with a variety of detector types, including germanium detectors, silicon charged-particle detectors, Si(Li) detectors, proportional counters, and scintillation detectors.

¹T.H. Becker, E.E. Gross, R.C. Trammell, "Characteristics of High-Rate Energy Spectroscopy Systems with Time-Invariant Filters," IEEE Trans. Nucl. Sci., NS-28, 598 (1981).

Fig. 1a.  Energy resolution with Semi-Gaussian Shaping and a 0.5 µs Shaping Time Constant. Maximum throughput capability is the same as for Fig. 4.

Fig. 1b.  Energy resolution at the Gated Integrator Output with a 0.25 µs Shaping Time Constant.

Fig. 2 Gated Integrator (GI) Output and Unipolar Output.

Fig. 3  Simplified block diagram of the Model 673 Spectroscopy Amplifier and Gated Integrator.

Fig. 4 Resolution as a function of Shaping Time Constant for Semi-Gaussian and Gated Integrator Pulse Shaping.

Fig. 5.  Example of the throughput improvement using the Gated Integrator technique.
 

Fig. 6.  Resolution and baseline stability vs counting rate for the GI Output of the Model 673 using a 0.25-µs shaping time. Measured on a 10% realtive efficiency GMX detector.

Fig. 7.  Resolution and baseline stability vs counting rate for the Unipolar (Semi-Gaussian) Output of the Model 673 using 2-µs shaping time. Measured on a 10% relative efficiency GMX detector.

Fig. 8  Background reduction obtained from pile-up rejection.

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