State-of-the-art manufacturing techniques have virtually eliminated exceptionally slow rise times in germanium detectors. However, one may still encounter older detectors that produce pulses with rise times much longer than those described above. Usually, these pulses are caused by gamma rays interacting in regions of the detector that have a weak field and slow charge collection. When the ARC timing method is applied to these pulses, the zero-crossing detector can trigger before the leading-edge arming discriminator (Fig. 4). As a result, the timing output of the constant-fraction discriminator will correspond to the leading-edge trigger instead of the zero-crossing detection. These events have excessive "walk" associated with them, and cause a long tail on the timing peak (Fig. 10). Pulses with normal rise times, but with amplitudes close to the leading-edge arming discriminator threshold, cause similar behavior.

Some constant-fraction discriminators have a Slow Rise Time rejection mode (SRT) that can be used to reject these errant events, thus improving the symmetry of the peak in the time spectrum¹⁰ (Fig. 10). The SRT mode blocks the timing output when the zero-crossing detector triggers before the leading-edge arming discriminator. Although the SRT mode improves the shape of the timing peak, it does so by rejecting events that would appear in the full-energy peak of the analyzed energy spectrum. Thus, detection efficiency can be compromised in favor of time resolution, or vice versa. The SRT mode can be enabled or disabled on the ORTEC modules that offer this feature.

For practical examples of the time resolution obtained with ARC timing on germanium detectors, see the data sheets on the Models 474 and 579.

¹⁰M. Bedwell and T. J. Paulus, IEEE Trans. Nucl. Sci. NS-23 (1), 234 (1976).

Figure 10. The Effect of Slow Rise Time Rejection on a Timing Spectrum from a Germanium Detector.