activated n ith high sensitivity a t different irradiation tinies. Furthermore, the graphs should prove helpful in estimating the activities of one isotope relative to another for specific conditions of irradiation. It is interesting to note how poorly the absolute cross sections for most of the isotopes are knonn. Values for a few isotopes such as manganese-55 have been determined quite accurately. However, most of the cross sections for the remaining isotopes are knon n only to within 10, 20, or even 40%. Thus, it is of utmost importance when performing analyses by activation to utilize the standard analytical practice of comparison against a standard. This practice eliminates the necessity for knoiving accurate cross-section values as \\ell :is absolute values for the geometry. counting efficiency, and the like of the experimental arrangement. Whenever absolute values of the cross section must be used, it can lie expected that the work \vi11 be considerably less accurate. Comparison of Figures 1 and 2 indicates that isotopes not found in nature generally have high cross sections, very few of which are less than 1.0 barn. As more of these cross sections are determined it n-ill be interesting to note hether this is a fundamental relationship or a factor dependent a t present upon technical difficulties.
Table I.
Thermal Neutron Cross-Section Values Not Included in Figure 1
Log Half I d c oi Daughter, AZinutes 5 81 2 15
I’aieiit
Log Atomic Cross Section for Thermal S e i i t m i Activation, Balm
Standard Eiioi of Cross Section,
8 93 2 48 05
