Improvements in SDD Effi ciency
Figure 5 : Spectra from a carbon-coated SiO 2 sample illustrating energy resolution and low-energy performance at fast processing times for two detectors. The spectra are normalized to the Si K peak. Although these detectors have comparable performance at the slow pulse processing times, the reduced overall noise of the CUBE pre-amplifi er results in signifi cantly better energy resolution and low-energy cut-off at fast pulse processing times. Note that C is not detectable at the 0.48 μ s processing time in the older generation Octane detector. The increased O signal for the Octane Elite detector is attributable to the silicon nitride window.
X-rays that the pulse processor can analyze in a given amount of time at diff erent pulse processor settings. Unfortunately, the terminology is not as rigorous as could be desired and oſt en the terms amplifi cation time, processing time, time constant, and peaking time are used interchangeably to either describe the rise/fall time or the entire pulse time. In Si(Li) detectors the charge cloud had to travel through the bulk of the Si crystal, while the SDD detector utilizes a driſt fi eld closer to the surface of the detector, resulting in faster charge collection and thus higher throughput. For SDDs the holding time is typically on the order of a few hundred nanoseconds while the rise/fall time varies from hundreds of ns to tens of μ s, resulting in total pulse
Figure 6 : Theoretical SDD output count rate as a function of input count rate for different amplifi er processing times. Straight blue lines show that an input count rate of 884 kcps yields 585 kcps output count rate into the spectrum.
2017 March •
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processing times ranging from roughly 0.5 μ s to several tens of μ s. T e lower limit for the total pulse processing time a given detector can run with, and consequently the upper limit for the throughput, is defi ned by the amount of time it takes for an electron cloud generated in the detector chip by an X-ray event to move to the detector anode for collection. In order to measure the generated charge correctly, the entire charge cloud has to be collected within the pulse time. If the pulse processing time is shorter than the charge collection time, only part of the energy will be collected within a pulse, resulting in a low-energy tail on the primary peak. Processing time . An important factor related to the final OCPS is the detector busy time, when the detector system is doing the following: (a) analyzing an incoming pulse, (b) resetting the accumulated charge (reset times are typically less than 1 μ s), and transferring data (on newer systems this is done continuously and the detector does not need to stop acquisition to transfer data). When the detector is busy, it should not accept a second X-ray pulse because the system can only process one X-ray event at a time. However, when two X-ray pulses fall within the same processing time window, the energies of the individual X-rays cannot be resolved and both pulses must be ignored. The time elapsed for measuring the discarded pulses is called “dead time.” As the ICPS increases, the probability of these near-coincident double pulses also increases and so does the dead time. For a given amp time, a low ICPS registers a low dead time, and a high ICPS yields a high dead time. Thus, the dead time can be used as a relative rate meter. T roughput versus energy resolution . Many electronics processing architectures provide user selectable “Processing Time” or “Amp Time” settings: short times for high throughput or long times for improved energy resolution to resolve peaks with energies that are closely spaced. T us, traditionally there was a known trade-off in deciding settings for speed and counting statistics versus energy resolution. T e energy resolution of a peak (peak width) is dependent on two factors: the statistical distribution of the charge carrier conversion in the sensor and the thermal noise of the amplifi cation process, primarily from the FET, the fi rst stage of amplifi cation [ 5 ]. At short amplifi er times there would typically be more noise, leading to peak broadening and a low-energy cut-off as the noise became comparable to the charge contribution of low-energy X-rays. However, recent processing routines using the external CMOS “CUBE” preamplifi er signifi cantly reduces the noise at short pulse processing times compared to that of JFET devices [ 7 ]. T us, the trade-off between count throughput versus energy resolution is no longer a factor for EDS systems using CUBE technology. Figure 4 shows representative Mn Kα energy resolution versus amp time for a selection of detectors, and the improvement at short times for the Octane Elite detectors is evident. Figure 5 shows the improvements in energy resolution and low-energy response for the Octane Elite detector at a short processing time.
Short amplifier processing time is the key to the SDD system’s ability to efficiently collect and analyze X-rays at
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