Background counts

With modern detectors and electronics most Enei -Dispersive X-Ray Spectroscopy (EDS) systems can detect X rays from all the elements in the periodic table above beryllium, Z= 4, if present in sufficient quantity. The minimum detection limit (MDL) for elements with atomic numbers greater than Z = 11 is as low as 0.02% wt., if the peaks are isolated and the spectrum has a total of at least 2.5 X 10 counts. In practice, however, with EDS on an electron microscope, the MDL is about 0.1% wt. because of a high background count and broad peaks. Under conditions in which the peaks are severely overlapped, the MDL may be only 1—2% wt. For elements with Z < 10, the MDL is usually around 1—2% wt. under the best conditions, especially in electron-beam instruments. 

A modem variant is to count the number of atoms directly in a mass spectrometer.) The practical limit is about 50000 y since by this time the activity has fallen to about 0.2% of its original valuable and becomes submerged in the background counts. is also extremely valuable as a radioactive tracer for mechanistic studies using labelled compounds, and many such compounds, particularly organic ones, are commercially available (p. 310). 

In most ordinary cases, the disadvantages of x-ray excitation are more than compensated by the absence of the disadvantages peculiar to electron excitation, by the great convenience of Coolidge tubes (1.3), and by the absence of the large background count to which the continuous x-ray spectrum excited by electrons gives rise (1.5). 

Figure 8-9 shows how the calibration for niobium varies with the background counting rate at a single Nb2Os concentration. Similar curves have been proved useful in a large number of uranium determinations on shipping pulp and ores. 

It was found essential to maintain cleanliness around the Coulter Counter at all times. Dust and moisture must be avoided or high background counts and instability will result. The Coulter Counter analysis was found to be more rapid than the Micromerograph, requiring approximately 15 minutes per analysis as compared to 90 minutes for high surface material on the Micromerograph. The use of a multichannel Coulter Counter would reduce the time per analysis to less than five minutes... 

Marshall, R. A. G., Cumulative Sum Charts for Monitoring of Radioactivity Background Count Rates, Anal. Chem. 49, 1977, 2193-2196. 

The difference in rates of release of BCNU from wafers produced by the trituration or solution methods is also seen in vivo (11,14), as is shown in Fig. 6. Wafers of PCPP-SA 20 80 were prepared by either the solution or trituration methods, as described above, and were implanted into the brains of rabbits. The animals were sacrificed at various times after implantation and the brains were removed, fixed, and processed for quantitative autoradiography. To quantitate the percentage of the brain exposed to BCNU released from these wafers, the following calculation was performed. The percentage of the brain in which the radioactivity from the tritiated BCNU released from the wafers exceeded the background counts by at least two standard deviation units was plotted as a function of time following implantation in Fig. 6. A control set of rabbits had a solution of BCNU injected directly into the same location in the... 

The derivation of the expressions (3.3)-(3.6) is found in Appendix A (cf. CD-ROM). Since for most Fe-spectra the level of nonresonant background counts, Ab, may be in the range of 10-30% of the total counts, the absorber thickness is usually best adjusted to a value between the limits given above. The maximum of SNR( ) is naturally rather broad, such that deviations from / opt of even dz50% are fairly immaterial. 

The 99 keV y-quanta are usually counted with Nal(Tl) scintillation counters or Ge(Li) diodes in transmission geometry. A Cd absorber should be used to reduce the background counting rate of the K X-rays and to avoid pile-up of the different X- and y-rays (cf. Fig. 4 in [325]). 

Marshall AG (1977) Cumulative sum charts for monitoring of radioactivity background count rates. Anal Chem 49 2193... 

Despite its smaller cross section, the other nuclear reaction used for deuterium analysis, the (2H,p) reaction, can be made extremely sensitive, as Myers (1987) has shown. The key is that the proton is emitted with such high energy, 15 MeV, that the detector can be mounted behind the sample wafer (Fig. 1) ensuring a very low background count rate and hence good sensitivity. Myers measured deuterium amounts as low as 1012 atoms/cm2 in his study of deuterium uptake by Si02. However, the depth resolution of this measurement was poor, l jum. 

The possibility of centring the photopeak into the counting window for each individual sample allows the selection of a relatively narrow window, whereas in the conventional technique the windows have to be set much broader in order to compensate for drifts. The result is a substantial reduction in background count rates. 

Traditionally thermal ionization mass spectrometry was the instrument of choice for the isotopic analysis of metals because thermal ionization produced an ion beam with a very small kinetic energy spread ( 0.5 eV). Therefore only a magnetic mass analyzer is needed to resolve one isotope from another. Moreover, ionization of unwanted material, such as atmospheric contaminates, hydrocarbons from pump oil, or production of doubly ionized particles is almost non existent, thus background counts are minimized and signal-to-noise ratio is maximized. 

A significant increase in the number of his + colonies above the background count (the number of colonies on Control Plate C when no sample is... 

In INAA, a rock or mineral sample is irradiated in the reactor. The irradiated sample is removed from the reactor, and the dangerous radioactivities are allowed to decay. Then the sample is placed into a counter and the y-rays emitted by each element in the sample are counted. A variety of counters are used, including scintillation counters, gas ionization counters, or semi-conductor counters. For the most precise results, background counts in the detectors produced by electronic noise, cosmic rays, and other radioactive decays must be eliminated. The technique is very sensitive, and samples as small as a few tens of milligrams can be measured. 

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