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For the more advanced user, the InSight mode allows you to directly view all the parameters and adjust them interactively while collecting live data. To access the InSight mode, go to the
InSight section on the Amplifier 2 tab and click on Start. The InSight mode is discussed in more detail in Section 5.2.11.5.
The Rise Time value is for both the rise and fall times; thus, changing the rise time has the effect of spreading or narrowing the quasi-trapezoid symmetrically.
The Flattop controls adjust the top of the quasi-trapezoid. The Width adjusts the extent of the flattop (from 0.3 to 2.4
μs). The Tilt adjustment varies the “flatness” of this section slightly. The
Tilt can be positive or negative. Choosing a positive value results in a flattop that slopes downward; choosing a negative value gives an upward slope. Alternatively, the optimize feature on the Amplifier tab can set the tilt value automatically. This automatic value is normally the best for resolution, but it can be changed on this dialog and in the InSight mode to accommodate particular throughput/resolution tradeoffs. The optimize feature also automatically adjusts the pole-zero setting.
The dead time per pulse is approximately
In the Pole Zero section, the Start button performs a pole zero at the specified rise time and other shaping values. Unlike the optimize feature, it performs a pole zero for only the one rise time. The pole-zero Stop button aborts the pole zero, and is normally not used.
When you are satisfied with the settings, Close the Properties dialog and prepare to acquire data.
Once data acquisition is underway, the advanced user may wish to return to MCB Properties... and click on the Insight section’s Start button to adjust the shaping parameters interactively with a “live” waveform showing the actual pulse shape, or just to verify that all is well.
Amplifier PRO
This tab (Fig. 79) contains the controls for the Low Frequency Rejector (LFR) filter, high-frequency Noise Rejection Level, Resolution Enhancer, and Enhanced Through-put Mode. To enable a particular feature, mark the corresponding checkbox. Any or all of these features can be used at one time, however, the LFR and Enhanced Throughput modes must be set up before the Resolution Enhancer is configured, as discussed below. Note that once an MCB is “trained” for the Resolution Enhancer (see the following section), it must be “retrained” if any settings are changed that can affect peak shape or position (e.g., bias, gain, rise time, flattop, pole-zero).
Low Frequency Rejector — This filter is designed to minimize low-frequency noise, and is discussed in detail in the hardware manual. You cannot optimize or pole-zero the DSPEC-50
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while in LFR mode. The Optimize feature must be used with the LFR filter off. Subsequent measurements can then be taken with the LFR filter on. Also, LFR mode affects the available range of protection times in Enhanced Throughput Mode, as discussed in the next paragraph.
Noise Rejection Level — This setting adjusts a filter that rejects high-frequency noise from the ambient environment. It ranges from 0 to 4. The default setting,
2, will be suitable for most applications.
If the system is exhibiting high dead time with no source on the detector, the noise may be induced by nearby RF interference or a result of a ground loop. If possible, resolve the source of
Fig. 79. DSPEC-50 Amplifier PRO Tab.
the noise by physical means such as common grounding between detectors and instruments, shielding cables, removing nearby motors/generators, etc. If you cannot eliminate the noise, increase the rejection level setting until the dead time returns to the expected low value.
! Note that higher values may reduce the effectiveness of the pile-up rejector when processing very low-energy pulses.
! On systems for which very high dead times are expected (i.e., >60%), especially with verylow-energy sources (e.g.,
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Am), decreasing this setting can improve the performance of the spectrometer with respect to live-time correction and the ability to process signals at higher input rates.
Enhanced Throughput Mode — This feature can help reduce the low-side peak tailing that results from increased charge trapping; see the discussion in the hardware manual for more details. This function will not improve poor resolution due to other causes. The valid Protection
Time settings, in 25-ns increments, range as follows:
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LFR Mode
Off
On
Highest Throughput
(minimum Protection Time)
Lowest Throughput
(maximum Protection Time)
(Rise Time + flattop) (2 × Rise Time + Flattop)
(3 × Rise Time + 2 × Flattop) (6 × Rise Time + 3 × Flattop)
Turning on this feature automatically sets the minimum protection time (highest throughput rate) based on your current Rise Time and Flattop settings, however, you can adjust this value at any time. Each time you change the rise time or flattop, the DSPEC-50 will automatically set itself to the new minimum protection time.
“ Training” the Resolution Enhancer
The Resolution Enhancer operates by measuring the rise time (collection time) of the pulses and adjusting the gain based on the rise time. This is done on each pulse. The gain adjustment value for the rise time is stored in a table. The values in the table must be set by “training” the Resolution Enhancer. Marking the Resolution Enhancer checkbox enables/disables the “learning” mode. Note that, once trained, the enhancer operates continuously until disabled as discussed in
Step 13 below. To train the enhancer:
1) Set the bias, gain, rise time, flattop, and PZ as you would for data collection.
2) If you wish to use LFR Mode, turn it on.
3) If you wish to use Enhanced Throughput Mode, turn it on and either accept the automatically calculated, highest-throughput protection time, based on the current rise time and flattop; or enter the desired setting. (The latter might require one or more data acquisitions.
When finished, proceed to Step 4).
4) Clear the MCB and acquire a well-isolated peak.
5) Mark the Resolution Enhancer checkbox to turn on the learning mode.
6) You will now use the gain stabilization section of the Stabilizer tab to configure the Resolution Enhancer (the gain stabilizer is disabled in the learning mode). Enter the Center channel and Width of the peak acquired in Step 4; the maximum Width is 255 channels. If you wish, use the Suggest button. The selected region should be as narrow as possible but should completely include the peak.
7) Click on Initialize to clear all the Resolution Enhancer settings. Initialization does not change the current Center channel and Width.
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8) Clear the MCB, re-start acquisition, and monitor the FWHM of the target peak, using the
Peak Info command (available by right-clicking in the spectrum window) to show the
FWHM and peak counts. Collect about 5000 counts in the peak and record the FWHM.
Clear the data and collect another 5000 counts, recording the FWHM. Repeat until the
FWHM no longer changes. Typically, the more charge trapping exhibited by the detector, the longer the data collection time.
9) When you are satisfied that the FWHM has reached the best possible value, clear the MCB and collect another spectrum for confirmation.
10) At this point, the Resolution Enhancer is now “trained” for the current peak shape parameters and the learning mode should be turned off by returning to the Amplifier PRO tab and unmarking the Resolution Enhancer checkbox. The table of adjustments will be stored in the DSPEC-50's memory.
11) If you change any parameters that affect peak shape, you must repeat this “training” procedure.
12) Once the Resolution Enhancer has been trained and its checkbox has been unmarked, the
Stabilizer tab once again operates on the gain stabilizer (that is, it no longer adds values to the table of adjustments).
NOTE
The peak selected for the gain stabilizer can be different from the peak used for training the Resolution Enhancer.
13) To turn off the Resolution Enhancer, mark its checkbox to turn on the learning mode, go to the Stabilizer tab and click on the Initialize button for the gain stabilizer. This will set the adjustment to zero. Now return to the Amplifier PRO tab and unmark the Resolution
Enhancer box.
ADC
This tab (Fig. 80) contains the Gate, ZDT Mode, Conversion Gain, Lower Level Discriminator, and Upper Level Discriminator controls. In addition, the current real time, live time, and count rate are monitored at the bottom of the dialog.
Gate — This allows you to select a logic gating function. With this function Off, no gating is performed (that is, all detector signals are processed); with the function in Coincidence, a gating input signal must be present at the proper time for the conversion of the event; in Anti
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