Energy Resolution of Ge Detectors

The energy resolution of a Ge detector is given in terms of the FWHM (F). The width r consists of the following two components  

Since these two components are uncorrelated, they are added in quadrature to give the total width, F, 

As shown in Chap. 9, the width F is energy dependent and is given by 

The width F increases when the detector capacitance increases. The detector capacitance, in turn, generally increases with detector size and may change with detector bias. Good Ge detectors have a flat capacitance-bias relationship over most of the range of bias voltage applied. 

The capacitance of the detector has an effect on the energy resolution because it influences the performance of the charge-sensitive preamplifier that accepts the detector signal. The contribution of the preamplifier to the value of Fg increases with the input capacitance. One of the manufacturers, Canberra, reports a 0.570-eV F with zero input capacitance and a slow increase with higher values as shown in Fig. 12.32. Clearly, the resolution improves if the capacitance is kept low. The other component of the input capacitance comes from items like connectors and cables. Reduction of the length of input cable and of connectors capacitance is helpful. For the best resolution with a given system, the preamplifier should be located as close to the detector as possible. 

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For a good measurement, the amplifier should satisfy many requirements. Not all types of measurements, however, require the same level of performance. For example, if one measures only the number of particles and not their energy, the precision and stability of the amplification process can be relatively poor. It is in spectroscopy measurements, particularly measurements using semiconductor detectors, that the requirements for precision and stability are extremely stringent. Since the energy resolution of Ge detectors is of the order of 0.1 percent, the dispersion of the pulses due to the amplification process should be much less, about 0.01 percent. 

Despite the superb energy resolution of Ge detectors compared to that of NaI(Tl) scintillators (see Fig. 12.32), analysis of complex gamma spectra is... 

Both types give excellent resolution, for a good Si(Li) X-ray detector about 160 eV at S.9 keV and for a good Ge(Li) detector about 1.75 keV FWHM for 1.33 MeV 7. Energy resolution of Ge detectors varies with energy below l.S MeV according to the enq>irical... 

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