Detector characteristics count rate

The spectrometer is set to the appropriate Bragg angle 0 of the requisite characteristic wavelength, and only these X-rays will reach the detector and be counted. The detector employed is the gas proportional counter, which can operate at much faster count rates than the EDS crystal detector. 

Quality assurance (QA) measurements also are performed with a set of QA gamma-ray sources to confirm that the radiation detection instrument is functioning normally. The measurements are performed at regular intervals and the results are plotted to show the mean value and random deviations by 1 and 2 standard deviations (cr and 2cr). The factors that are considered include the count rate at characteristic control source peaks, the resolution of these peaks, and the background radiation shown by the detector. Any significant deviations beyond the 2-sigma values on the control charts require a repeated measurement and - if confirmed - corrective actions before further measurements are performed. 

Count the sample for a sufficient time period and frequency to collect reliable data for determining the gamma-ray energy, the count rate for characteristic gamma-ray energy peaks, and the rate of decay of that peak. Also count the empty container with the Ge detector and spectrometer to determine whether any radionuclides remain sorbed on container walls and to estimate the fractional loss by sorption for each radionuclide of interest. Note that the counting efficiency for radionuclides retained on the container must be estimated. 

Step 7. Place counting container on Ge detector and immediately check that spectrometer live-time dial reads near 100% to assure that count rate is not excessive. If dead time is significant (live time <90%), remove sample and prepare a more dilute sample, as in Step 5. If live time in near 100%, set the count time to accumulate sufficient gamma ray data (typically 600 s) to obtain at least 1,000 counts in each of the characteristic gamma ray peaks of the shorter-lived radionuclides that are readily observable and of interest in the first count. 

The counting efficiency of a GM counter for detecting p-rays is defined as the ratio of the count rate to P-activity of the source. Thus, the counting efficiency is one of the most important factors to determine the characteristics of the detector. 

All the early EDS systems were operated in a primary mode in which X-rays emitted from the X-ray tube irradiate the specimen direcdy. This had drawbacks because the X-rays could excite more photons than could be counted by the detector, because there is a maximum counting rate of photons for an energy detector. If the number of characteristic photons excited in the specimen is too great, a portion of photons may not be counted. Thus, the measured intensities may be lower than the actual emitted intensity of X-ray photons. The period during which the detector cannot respond to the number of photons correctly is called the dead time of the detector. The dead time is in the order of 0.5 x 10-7 seconds. 

The most easily applied technique for measuring the effective deadtime for a wavelength-dispersive x-ray fluorescence spectrometer assumes that the expected mean counting rate of x-ray photons at the detector is proportional to the x-ray tube current i. A stable specimen that will generate the characteristic lines and the pulse height spectrum of interest is placed in the fluorescence spectrometer. All... 

Because of the electronic noise characteristics of semiconductor detectors, the maximum count rate of an ED system is usually no more than 30 000-50 000 counts per second, compared with in excess of 10 counts for a WD spectrometer, acting as a single channel analyzer. The reason is the relatively long count time constant that must be selected for ED preamplifier circuits (typically 6 ps), compared with < 1 ps for counter preamplifiers used on WD spectrometers. As a consequence, equivalent analytical precision can only be achieved by extending ED spectrum count times to a significant extent compared to WD. 

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