Sample throughput in a counting system is most affected by efficiency and background. Together these determine the sensitivity of the system. Low-background systems are often specified using system performance data and sample performance data. It is important to understand both to compare systems. System performance is the best possible performance available for the low-background system. System performance is measured by minimizing sample effects as much as possible. In most cases system performance cannot be changed, so selecting a model with the best system performance is critical to sample throughput.

Sample background and efficiency depend on preparation techniques, geometry, and content. All are variable and are controlled to some extent by the user. For example, a sample can be placed close to the detector to improve efficiency, or further away from the detector to reduce the chance of detector window breakage. When you compare systems, it is important to compare system performance to system performance, and compare sample performance to sample performance. This is relatively easy if you calculate a “Figure of Merit,” using equivalent performance specifications for each system. Figure of Merit is calculated by the equation: E^2/B where E is the efficiency (in percent) for the sample type of interest, and B is the background (in counts per minute - CPM) for the system. Given the same sample and counting sensitivity requirements, a system with twice the Figure of Merit will have twice the sample throughput.

Using the table below, you can see that for Sr90, a Protean system is 2.6 times as sensitive as the nearest competitor and 1.7 times as sensitive for Ni63. These dramatic increases in sensitivity translate directly into shorter count times in the laboratory.

picfom

 
*All comparisons are based on the latest available published specifications. “Typical” efficiency and background specifications are used. The same comparison can be made using the “warranted” specifications. All specifications are based on a gas flow detector with 80 µgm/cm² window.

Notes regarding the results in this table:
(1) Only one vendor publishes C14 efficiency. That value (30%) was used for all vendors. Individual beta efficiency values were used.
(2) One vendor does not publish separate alpha and beta background values. The published total alpha+beta value was used. In fairness to that vendor the Figures of Merit would be better than shown here. However, they are highly unlikely to be better than Brand X.
(3) PIC is the only vendor that publishes Tc99 efficiency. That value was used for Brands X and Y.
(4) PIC factor is the ratio of Protean’s Figure of Merit to the next highest competitor’s value.

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