783620H / 1013 5. MENU COMMANDS
IMPORTANT
The MCB Properties dialogs are part of the standard C
ONNECTIONS
tools available in all ORTEC application software. As such, this interface is available only in English, although our higher-level applications such as GammaVision support other languages. If you use comma delimiters instead of decimal points in these dialogs, you may encounter unexpected results or other problems.
If the Detector is password-locked (see Section 5.7.3), you must know the password before you can modify its MCB properties. To view a locked Detector’s properties in read-only mode, click
Cancel when the Unlock Password dialog opens.
5.2.11.1. DSPEC-50
Amplifier
Figure 77 shows the Amplifier tab.
This tab contains the controls for
Gain, Baseline Restore, Preamp-
lifier Type, Input Polarity, and
Optimize.
NOTE
Be sure that all of the controls on the tabs have been set before clicking the Start Auto (optimize) button. The changes you make on most property tabs take place immediately.
There is no cancel or undo for these dialogs.
Fig. 77. DSPEC-50 Amplifier Tab.
Gain — Set the amplifier coarse gain by selecting from the Coarse droplist, then adjust the Fine gain with the horizontal slider bar or the edit box, in the range of 0.5 to 1.1. The resulting effective gain is shown at the top of the Gain section. The two controls used together cover the entire range of amplification from 0.5 to 140.8.
Input Polarity — These buttons select the preamplifier input signal polarity for the signal from the detector. Normally, GEM (p-type) detectors have a positive signal and GMX (n-type) have a negative signal.
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Baseline Restore — This is used to return the baseline of the pulses to the true zero between incoming pulses. This improves the resolution by removing low frequency noise from dc shifts or mains power ac pickup. The baseline settings control the time constant of the circuit that returns the baseline to zero. There are three fixed choices (Auto,
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Fast, and Slow). The fast setting is used for high count rates, the slow for low count rates. Auto adjusts the time constant as appropriate for the input count rate. The settings (Auto, Fast, or Slow) are saved in the
DSPEC-50 even when the power is off. The time constant can be manually set on the InSight display (see the discussion beginning on page 98).
You can view the time when the baseline restorer is active on the InSight display as a Mark region (see the Marks discussion, p. 100). In the automatic mode, the current value is shown on the InSight sidebar. For a low-count-rate system, the value will remain at about 90.
Preamplifier Type — Choose Transistor Reset or Resistive Feedback preamplifier operation.
Your choice will depend on the preamplifier supplied with the germanium detector being used.
Optimize
The DSPEC-50 is equipped with both automatic pole-zero logic
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and automatic flattop logic.
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The Start Auto (optimize) button uses these features to automatically choose the best pole-zero and flattop tilt settings. Note that if you selected Transistor Reset as the Preamplifier Type for this DSPEC-50, optimization does not perform the pole zero.
NOTE
You cannot optimize with LFR mode enabled; see Section 5.2.11.1.
As with any system, the DSPEC-50 should be optimized any time the detector is replaced or if the flattop width is changed. For optimization to take place, the DSPEC-50 must be processing pulses. The detector should be connected in its final configuration before optimizing. A count rate guidance message on the lower-left of the Amplifier page will help you position a radio active source to deliver the correct count rate for optimization. The Start Auto optimization button will be disabled (gray) until the count rate is suitable.
Select either the Resistive Feedback or Transistor Reset option and click on Start Auto. The optimize command is sent to the DSPEC-50 at this time and, if the DSPEC-50 is able to start the operation, a series of short beeps sounds to indicate that optimization is in progress. When optimizing is complete, the beeping stops.
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U.S. Patent 5,912,825.
16
U.S. Patent 5,872,363.
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U.S. Patent 5,821,533.
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783620H / 1013 5. MENU COMMANDS
During optimization, pole zeroes are performed for several rise-time values and the DSPEC-50 is cycled through all the rise time values for the determination of the optimum tilt values. All values for all the combinations are then saved in the DSPEC-50, so you do not need to optimize for each possible rise time. Optimization can take from 1 to 10 minutes depending on count rate, but typically takes 5 minutes.
NOTE
Be sure to repeat the optimization if you change the flattop width.
The effect of optimization on the pulse can be seen in the InSight mode, on the Amplifier 2 tab.
Note, however, that if the settings were close to proper adjustment before starting optimization, the pulse shape may not change enough for you to see. (In this situation, you also may not notice a change in the shape of the spectrum peaks.) The most visible effect of incorrect settings is high- or low-side peak tailing or poor resolution.
Amplifier 2
Figure 78 shows the Amplifier 2 tab, which accesses the advanced shaping controls including the
InSight Virtual Oscilloscope mode.
The many choices of Rise Time allow you to precisely control the tradeoff between resolution and throughput. The value of the rise time parameter in the DSPEC-50 is roughly equivalent to twice the integration time set on a conventional analog spectroscopy amplifier. Thus, a DSPEC-50 value of 12
μs corresponds to 6 μs in a conventional amplifier. Starting
Fig. 78. DSPEC-50 Amplifier 2 Tab.
with the nominal value of 12.0, you should increase values of the rise time for better resolution for expected lower count rates, or when unusually high count rates are anticipated, reduce the rise time for higher throughput with somewhat worse resolution.
The DSPEC-50 Rise Time ranges from 0.8
μs to 23.0 μs. Once the unit has been optimized according to Section 5.2.11.1, you can use any Rise Time without having to re-optimize. The most recent settings are saved in the DSPEC-50 firmware even when the power is turned off.
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