1. The cross section of the target element. 2. The half-life of the radioactive isotope produced. 3. The time available for irradiation. 4. The flux of neutrons available for irradiation. 5. The promptness with which we can begin measuring radioactivity and the efficiency of this measurement. 6. Possible interferences due to the presence of elements yielding the same radioactive elements or those yielding very similar radiations. In the next section of this booklet, there are several examples that will show you how all this works in practice. But to summarize what these factors mean in terms of sensitivity let us look at the chart in the figure on page 18. Here all the elements are arranged in a periodic table. The sensitivities are shaded in coded ranges representing measurable quantities. They are calculated on the basis that there are no interferences, that the neutron flux is 10¹⁴ neutrons per square centimeter per second, and that we can measure 100 gamma rays per minute without much difficulty assuming a gamma-ray detector efficiency[7] of 10%. The elements labeled β yield radioisotopes that emit few or no gamma rays and can only be analyzed by neutron activation using appropriate chemical separation procedures followed by beta radioactivity measurements. Such chemical separation procedures (to remove unwanted radioactive isotopes of other elements) are also sometimes useful to improve the sensitivity of the analysis of gamma-ray emitters if necessary. [Illustration: Graph: “Fraction of Sodium-24 remaining” vs. “Time of decay (hours)”] The radioactive decay curve of sodium-24. The vertical scale is not _linear_ but _logarithmic_. Thus, each factor of two in radioactivity occupies the same distance along the vertical axis. When two samples are being analyzed for sodium by activation analysis, they must be compared at the same time after they have been removed from the neutron flux. If this period of time is different, then a correction must be applied to one of them, based on the decay curve shown here, to allow for the difference in decay time for the two. Waiting too long after the irradiation is completed results in much poorer sensitivity for the analysis depending on the half-life of the activation product. In this case, after 2 days it takes approximately ten times as much sodium to yield the same radioactivity as it would if the sample were measured when it was fresh out of the reactor.