ORTEC DSPEC Pro, Руководство пользователя по оборудованию

Страница: 14 / 108

DSPEC Pro
®
Digital Gamma-Ray Spectrometer User’s Manual 794380D / 0914
4
subsequent peak-detects until the protection time has elapsed. Note that the conventional dead
time illustrated in Fig. 1 is the same as the maximum protection time in Enhanced
Throughput Mode.
The protection-time settings with the low-frequency rejector (LFR) filter off range between:
!
Minimum PT (highest throughput) = (1 × rise time) + (1 × flattop)
!
Maximum PT = (2 × rise time) + (1 × flattop)
With LFR on, the protection-time settings range between:
!
Minimum PT = (3 × rise time) + (2 × flattop)
!
Maximum PT = (6 × rise time) + (3 × flattop)
1.3. Resolution Enhancement Mode Corrects for
Charge Trapping
When a gamma ray interacts with a germanium detector, mobile electrons and holes are gen-
erated. 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 Pro’s
Resolution Enhancer Mode. The controls are accessed via the Acquire/MCB Properties...
command in the accompanying MAESTRO
®
Multichannel Analyzer Emulation Software (A65-
BW) and other ORTEC C
ONNECTIONS
applications.
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.