Count rate limitation

Only few small angle scattering experiments have been performed with this techniquehowever, which is due to the potential deterioration of organic material in the polychromatic beam and the count rate limitation of the detection system. Current applications of this method ly mainly in the area of special environment experiments, e.g. under high pressure and for small structures. 

Although the overall count rate limitation with negligible deadtime correction losses is about 10 counts s , experimental expefience shows that it is rather easy to saturate the system, especially for highly localized scattering events (e.g. strong Bragg reflections). This shows the interest in the development of new detection systems. 

The only disadvantage in synchrotron radiation applications is the apparent counting rate limitation due to the propagation delay in the line. But, a comparison of this propagation delay with charge collection times in other methods is still advantageous for the delay-line method. 

These operations are quite time consuming, even with modern components resulting in counting rate limitations which are below those of the delay-line method at equal spatial resolution. 

The same count rate limitations as with the previous method apply due to the charge collection time, the determination of the ratio of the signals and the analog-to-digital conversion. 

Because of the counting rate capability of single photon counting PSD s, the access time of memory systems has to be short. Otherwise the counting rate limitation could well be determined by the data acquisition system. However, a large number of experiments do not require the ultimate speed. A data acquisition system with a one microsecond access time is perfectly feasible for many meaningful experiments, even with synchrotron radiation. 

Detector System Data Type SAXSIWAXS Dynamic Studies Count Rates Local Count Rate Limits... 

Depending on Local Count rates. Limited by the speed of the readout electronics. 

However in the GI case, there is no count rate limitation, and the sensitivity extends easily into the tenth pM regime, and usefully into the hundredth pM regime. This translates in our case to 1% ML surface coverages and below. Of course the tradeoff here is in the numbers of photons deliverable to the sample surface. At SPEAR II this would only be a few percent of a full beam, thus largely mitigating the advantages of the GI geometry. At SPEAR 1 and at the APS all of the beam from insertion devices can be placed onto suitably sized crystals, so that the comparison would be strictly valid. 

Restriction of the method is that the sample must have mass between 1 and 100 p.g and a thickness of 10 nm-10 p.m to prevent matrix effect and compensate for count rate limitations. 

The eount rate of the event recording principle depends on the memory read and write rates. The peak count rate is limited by the write rate, the average count rate limited by the rate the FIFO can be read at the output. Currently, the count rates for deviees based on event recording techniques are substantially lower than for the multisealers employing direct accumulation techniques. 

The ability to simultaneously measure a wide range of elements is one of the greatest advantages of EDXRF. This advantage is strongly reduced when the count rate limitation of the ED detection electronics is taken into consideration. 

Th e are many subtleties in TCSPC which are not obvious at fust examination. Why is the photem counting rate limited to one photon per 100 las pulses Present electronics feu TCSPC only allow detection of the first arriving photon. Once the first photon is det ted, the dead time in the electronics prevents detection of anoth photon resulting from the same excitation pulse. Recall that onis-slon is a random event. Following the excitation pulse, mc e photons are emitted at early times than at late times. If all could be measured, then the histogram of arrival times would represent the intensity decay. However, if many arrive, and only the first is counted, then the intensity decay is distorted to shorter times. This effect is described in more detail in Section 4.5.F. 

In addition to data manipulation and display, the software conducts control functions for the sample delivery system. The acceleration control of the sample injection pump is determined from the particle count rate and the count rate limit of the sensor and directions are sent to the syimge pump controller limiting the count rate to a value below the maximum count rate of the sensor. PDAS does the actual conversion from measured signal voltages to particle size using the sensor calibration curve. 

A number of methods have been employed to increase the Unear output current limit of a detector. One method is simply to increase the bias current of the multiplier by using smaller resistor values in the voltage divider network connected to the dynodes. Special high-current multiplier models are in common use that have a total resistance across their dynodes of only 3 MS2, giving a bias current of > 600 ptA. These multipliers have Unear count rate limits of 20 X 10 cps (when operated at a gain of 10 ). Alternate approaches use a variety of specialized voltage divider networks to increase the linear limit of the detector. These networks incorporate zener diodes and additional capacitance to stabilize dynode voltages. ... 

There are three principal types of position-sensitive X-ray detectors of use for crystallography—the vidicon, solid-state devices (largely charge-coupled devices), and proportional counters. Vidicons are area detectors which use image intensifier techniques to produce TV-type images. They are not count rate limited and are therefore well suited for use with high-flux sources... 

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