Background count rate

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. 

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

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. 

It is desirable to count tritium as methane since four atoms of hydrogen per mole of gas are present. At 10 atm. using the II counter, approximately one mole of water can be counted. The background count rate would be less than 5 c.p.m. in the tritium window. The sensitivity of this method would be about 1 TU without enrichment. 

The ratio r/ r-min is plotted for various ratios of the background count rate to the initial signal count rate, b/S0. It can be seen that even relatively small amounts of background greatly increase the relative error, especially for short observation times. The data shown use MLE estimates for the error, and these approach the minimum possible error in the limit of large n. 

The standard mass analyzer of ICP-MS is still the quadrupole. He allows the resolution of nominal mass units clown to 0.2-0.5 mass units and is therefore a low-resolution device. The performance of all ICP-MS instruments is limited by the transmission of the interface and mass analyzer unit, the background count rate clue to photons and the remaining gas pressure and the background count rate caused by molecular ions or doubly charged ions. Typical quadrupole instruments offer instrumental background count rates of 10 cps, newer instruments with an off-axis quadrupole show less than 1 cps like high-resolution instruments. 

The ability to observe added radioactivity in a food sample will depend on the inherent background counting rate of the sample and the ability to reduce the background activity relative to the added activity under ob-... 

The background count rate of the modified coincidence detector used in this work was approximately 30 counts per minute. Data from this detector was acquired and analyzed using the Chromatographies... 

When sector-based mass spectrometers are used in a low-resolution mode the sensitivity can be higher than attainable with current quadrupole mass spectrometers. Random background count rates are also typically lower for sector-based mass spectrometers. As a result, detection limits are one to two orders of magnitude better than for typical quadrupole ICP-MS instruments. 

The blank shows whether the reagents, glassware, and work environment contribute any radioactivity to samples. A blank will have a zero net count rate - that is, the measured or gross count rate minus the detector background count rate - if no radioactivity is observed. If the net count rate is above zero, it should be low compared to the net count rate of the analyte. Efforts should be made to find and reduce the source of contamination. 

Figure 2A.1 Cross-sectional view of a low-level anti-coincidence beta-particle counter A. Sample on a planchet. B. Thin window detector. C. Guard detector. Lead shielding surrounds the entire detector system. Typical background count rates are about 1 count per minute for beta particles and 0.1 count per minute for alpha particles. A sample mounted on a planchet (A) is placed below the thin window. When the guard detector (C) is triggered by an extraneous radiation that penetrates the lead shield, the sample detector (B) is inactivated. Immediately following, the detector (B) responds to beta particles from the sample. For low-activity samples, the probability is low that a particle from the sample registers a pulse at the same time that the counter is inactivated.

The counting efficiency (e) of the proportional detector is calculated as the ratio of the net count rate, in s, to the activity (A), in Bq, of this standard radionuclide solution. The net count rate is the standard s gross count rate (RG) minus the detector s background count rate (RB). The reported disintegration rate (A) is the product of the radionuclide concentration, in Bq L 1, and the amount of counted sample, in L, adjusted for the radioactive decay of the radionuclide between standardization and measurement. Equation 2A.1 is the general form of this equation. 

Step 1. Place 10 blank planchets in the proportional counter system and count each for 50,000 s at settings (a) and (b) to determine the beta-particle background count rate. 

Step 4. Compare the background count rate values at specified energy regions to those reported in the count room QA data. 

If computation is by hand, calculate the net counts per second (c/s) of the 40K gamma-ray peak at 1.461 MeV. Examine the background count rate and subtract the background contribution from the 1.461 MeV peak count rate. Note that the background count rate includes a peak at that energy (due to potassium salt in the environment). Enter data into Data Table 3.2. 

Net background count rate calculated for K-40 channels from water solution c/s... 

Count each planchet for 5,000 s at the operating voltage for beta particles. Obtain the background count rate with a blank planchet. Calculate the net cps with Eq. 4.4.and record in Data Table 4.2... 

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