Multichannel pulse height analyzer

Resolution in a semiconductor detector EDXRF system is a function of both the detector characteristics and the electronic pulse processing. The energy resolution of semiconductor detectors is much better than either proportional counters or scintillation counters (SCs). Their excellent resolution is what makes it possible to eliminate the physical dispersion of the X-ray beam without the energy resolution of semiconductor detectors, EDXRF would not be possible. Resolution is generally defined as the smallest energy difference observable between peaks. In EDXRF, the energy resolution is defined 

The most common detectors in benchtop EDXRF units are SDDs of 10 mm area with a resolution of 150 eV for the Mn K line at 100,000 counts/s. 

In handheld or portable instrumentation, Si-PIN detectors of 10 mm area are common, with a resolntion of 170 eV at 40,000 cps. For the detection of light elements (Mg to S), SDDs are more snitable and achieve a resolution of 190 eV at 90,000 cps with detector areas of 25 mm.  

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Figure 1. Block diagram of single-photon time-correlation apparatus from Barker and Weston 11 HV, high-voltage supplies L, lamp PI, photomultiplier M, monochromator FURN, furnace C, sample cell LP, light pipe F, interference filter P2, photomultiplier AMP, amplifier DISCI, discriminator D1SC2, discriminator T-S, timer scaler DL, delay line TAC, time-to-amplitude converter BA, biased amplifier MCPHA, multichannel pulse-height analyzer TTY, teletype printer and paper-tape punch REC, strip-chart recorder.

The output from the coincidence electronics passes to a multichannel pulse height analyzer and thence to a computer. The computer can also take data from other detectors that are in line with the column eluent in addition to instrument control. Data is processed and then presented on a monitor or printer. Cells for both homogeneous and heterogeneous counting are available. Homogeneous counting cells provide efficiencies of 55% for tritium and at least 95% for " carbon, whereas the heterogeneous cells provide about 10% for tritium and about 90% for " carbon. 

The counting rate varies linearly with x-ray intensity up to rates of about 5,000-10,000 cps. Counting losses in the counter-electronics system occur in the electronics rather than the counter. The electronics are more complex than usual and include, besides the usual pulse amplifiers and shapers, a multichannel pulse-height analyzer (Sec. 7-9). 

Figure 4.6 Pulse shapes encountered with a NaI(Tl) detector and spectrometer electronics. The detector is responding to the 22-keV silver K lines from a radioactive ° Cd source and the 6-keV Mn K lines from a radioactive Fe source, (a) The preamplifier output pulses, (b) Output pulses from a delay-line clipped pulse-shaping amplifier with a 1-/Ltsec delay line clip and a l/4-)US integration time constant, (c) Output pulses from the alternative semigaussian shaping amplifier with a 0.5-fxs time constant, (a), (b), and (c) are multiple traces on an oscilloscope, (d) The energy (pulse height) spectrum obtained by analyzing the delay line amplifier output on a multichannel pulse height analyzer. (Reprinted by courtesy of EG G ORTEC.)...

Figure 4.23 An analogy representing the pulse-height sorting function in the multichannel pulse-height analyzer. (Reprinted by courtesy of EG G ORTEC.)...

If -emitting substances are thought to be present in water, then these substances are to be analyzed with a scintillation crystal, for example in the geometrical configuration of a well-type crystal, and using a multichannel pulse height analyzer. Concentration of the radionuclides in the water is usually necessary for this measurement as well if a sensitivity level of, for example, 1 Bq/1 is to be attained. 

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