ORTEC DSPEC 50, Руководство пользователя по оборудованию

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932502G / 0618 1. INTRODUCTION
Note that the selection of shorter pulse widths inevitably degrades the signal-to-noise ratio com-
pared to the optimum achieved at longer pulse widths. Thus, operating at shorter pulse widths
will compromise the energy resolution of low-energy gamma rays, for which the preamplifier
noise makes a significant contribution to the energy resolution. The signal-to-noise degradation
is independent of the ballistic deficit effect.
The flattop and other shaping controls are on the Amplifier 2 tab under Acquire/MCB
Properties... ; see Section 4.2 for a more detailed discussion.
1.3. Resolution Enhancer Corrects for Charge Trapping
When a gamma ray interacts with a germanium detector, mobile electrons and holes are gene-
rated. The electrons and holes are swept to the detector electrodes by the electric field inside the
detector. If all the electrons and holes travel the complete path to the detector electrodes, a signal
is produced proportional to the energy deposited in the detector by the gamma ray. If some of the
electrons or holes do not reach the electrodes, the signal produced will be smaller than expected.
The process of interrupting the movement of an electron or hole is referred to as “charge
trapping.”
If charge trapping did not occur, the resolution of a reasonably low-noise germanium detector
for the 1.33-MeV gamma ray from
60
Co would be about 1.5 keV FWHM. Real detectors typi-
cally have a resolution of 1.8 to 2.2 keV FWHM. ORTEC’s Charge Trapping Corrector (CTC)
helps reduce this energy resolution loss caused by charge trapping, yielding the DSPEC 50's
Resolution Enhancer Mode . The controls are accessed via the Acquire/MCB Properties... com-
mand in the accompanying MAESTRO
®
Multichannel Analyzer Emulation Software (A65-BW)
and other ORTEC C
ONNECTIONS
compliant software packages.
Charge trapping is caused by various defects and impurities in the germanium crystal, and thus
varies from detector to detector. The amount of charge lost due to trapping also depends on the
distance the charge carrier (electron or hole) has to move to reach the collecting electrode. If the
carrier must travel a long distance, it is more likely to be trapped. If some of the charge is
trapped, it does not contribute to the signal. The reduced signals have a different rise time than
the full signals. The relationship between rise time of the signal and charge trapping is the basis
of the Charge Trapping Corrector.
The digital filter in the DSPEC 50 measures the amount of charge collected for each event and
uses the result to increment the spectrum memory location associated with that charge measure-
ment. The Charge Trapping Corrector also measures the pulse rise time for the event. The pulse
rise time is used to adjust the very fine gain. For each pulse, the measured charge is increased by
the appropriate fine gain factor to correct for the signal lost due to carrier trapping.
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