Example CPAM Response Plots
These plots show the predicted CPAM countrate responses for these parameter settings: Detection efficiency, 0.2; Flowrate, 5 cubic feet per minute (cfm); Collection efficiency, 0.7; Constant concentration, 1E-09 Ci/cc; Rectangular window length, 2 inches; Circular window radius, 1 inch; Media (tape) speed, 1 inch/hour. The concentration instantly steps up to its constant value when the time reaches 30 minutes, and there is a 100 count per minute (cpm) constant background. Note: A microcurie (Ci) is a measure of the disintegration rate, or activity, of a radioactive source; it is 2.22E06 disintegrations per minute.
In the LL plot, note that the FF countrate continues to increase. This is because there is no significant loss of radioactivity from the filter medium. The RW and CW monitors, on the other hand, approach a limiting countrate and the monitor response remains constant as long as the input concentration remains constant.
For the SL plot, all three monitor responses approach a constant level. For the FF monitor, this is due to the source and loss terms becoming equal; since 88Rb has a half-life of about 18 minutes, the loss of radioactive material from the filter medium is significant. This loss also happens on the RW and CW monitors, but there, the loss due to the filter movement also plays a role.
In both plots, Poisson "noise" is added and a constant-gain digital filter is applied, emulating the countrate responses as they would be observed on a modern CPAM. The horizontal dotted lines are the limiting countrates calculated from the equations given in the previous section.
Also in both plots the transit times are indicated; note that these times are measured from the start of the concentration, at time 30 minutes, not from the arbitrary time zero of the plots. In these example graphs, the length of the RW and the diameter of the CW are equal; if they were not equal, then the transit times would not be equal.
Read more about this topic: Airborne Particulate Radioactivity Monitoring
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