Counting rate gross

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. 

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. 

Carefully swirl to mix. Be certain to cover the bottom evenly, but do not swirl the solution up onto the container walls. The total initial volume is 5.0 mL. Screw the lid on the container and carefully place in position in the germanium gamma-ray detector counting chamber it should be centered and level. Count twice for a sufficient time period to accumulate 2000 counts (typically 100 s). Check to confirm that at least 2,000 counts have been accumulated at each of the peaks used for calibration. Collect the gross gamma-ray count rates for the full-energy peaks in Data Table 2B.1. 

To calculate the tritium activity in an analyzed sample, the count rate in the blank sample - the background - first is subtracted from the gross... 

Date Time Gross Count Rate Net Count Rate... 

Calculate the net count rate from the gross count rate and the background. 

Pipette 5 mL 131I tracer into the same type of vial used to collect the eluted samples and count for a sufficient time period to accumulate at least 5,000 net counts. Calculate the net count rate by dividing the gross count by the counting time and subtracting the background count rate. Compare all other results to this net count rate. 

Gross count rate,cps Net count rate, cps Activity, dps Counting efficiency,89 Sr Counting efficiency,90 Sr Counting efficiency, 90Y ... 

Step 6. Place each filter in a filter holder, identical to that used in the procedure. Count as soon as possible (within several hours) after the precipitation. Record gross count rates and background count rate in Data Table 13.3. 

Ingrowth/decay interval, d Gross count rate, cps Net count rate, cps Counting efficiency... 

Sr on the basis of the 90Y ingrowth. Count the two filters with 89Sr to calculate the counting efficiency and observe the radioactive decay of the radionuclide. Record the gross count rates and background count rate in Data Table 13.4. 

RSr = gross count rate of the sample, in cps Rstd = gross count rate of the standard, in cps Rb = background count rate, in cps... 

The net count rate (R ) is calculated as the gross count rate (Rg) less the background count rate (Rb) ... 

The gross counting rate of the sample in W1 (i i) contains counts due to Sr and two contributions firstly, background measured in the blank vial (Rhu, i) and secondly, counts due to Sr and (C9o i), which have to be deduced. will be established by means of the counting efficiencies of couple Sr/ V in the two windows (t79o i nd 7790-2) of the sample net counting rate in W2 (R2 -Rss-i) (without the Sr contribution). 

A special case. Sometimes the background rate is negligible compared to the gross counting rate. Then, Eq. 2.98 becomes... 

In this case, the question is. What is the optimum time to be used for gross and background counting Optimum time results in minimum statistical error for the net counting rate. The optimum time is determined as follows. 

It is assumed that an approximate gross counting rate is known. 

Answer The first step is to obtain an approximate gross counting rate. Assume that the sample gave 800 counts in 2 min. Then g = 800/2 = 400 counts/ min and, using Eq. 2.102,... 

Equation 2.108 corrects the observed gross counting rate g for the loss of counts due to the dead time of the counter. 

Notice that the product gr = 0.10, i.e., the product of the dead time and the gross counting rate, is a good indicator of the fraction of counts lost because of dead time. 

The dead time is measured with the two-source method as follows. Let i,/i2> i2 be the true gross counting rates from the first source only, from the second source only, and from both sources, respectively, and let be the true background rate. Let the corresponding observed counting rates be gj, g2, g,2, b. 

A counting experiment has to be performed in 5 min. The approximate gross and background counting rates are 200 counts/min and 50 counts/min, respectively. 

The strength of radioactive source is to be measured with a counter that has a background of 120 8 counts/min. The approximate gross counting rate is 360 counts/min. How long should one count if the net counting rate is to be measured with an error of 2 percent ... 

If the dead time of a detection system using a scintillator is 1 pts, what is the gross counting rate that will result in a loss of 2 percent of the counts ... 

A typical dead time for a scintillation detector is 5 fis. For a gas counter, the corresponding number is 200 /ns. If a sample counted with a gas counter results in 8 percent loss of gross counts due to dead time, what is the corresponding loss in a scintillation counter that records the same gross counting rate ... 

Establishing the absence of a radionuclide becomes important when radioan-alytical chemistry is used to demonstrate that a radiological incident, such as a threatened terrorist act, did not occur. Absence of a substance in a sample is a relative conclusion that could be altered by a more sensitive measurement. A radionuclide is reported as measured or as less than the lower limit of detection. A more sensitive detection method may replace the less than description with an actual value, or continue to report less than, but at a lower value. Such less than values are based on net count rates that may be zero or a positive value sufficiently near zero that is too uncertain, as discussed in Section 10.4, to be reported. This net count rate can be the difference between the gross count rate and (1) the detector background count rate or (2) the count rate in the sample attributed to background from various sources, as defined above. 

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