Sensitivity Charts for Neutron Activation Analysis W. WAYNE MEINKE Department o f Chemistry, University of Michigan, Ann Arbor, Mich.
b Sensitivity values for elements activated by thermal neutrons have been plotted in three charts for irradiation periods of 6 minutes, 10 hours, and 1000 hours. These calculated values are based on a neutron flux of 10l2 neutrons cm.-z set.-' Counting efficiencies (mostly for Geiger counting) were considered but decay time required for radiochemical separations was not included.
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concise summaries of sensitivity of the elements for thermal neutron activation analysis have been published (1-3, 6). Leddicotte and Reynolds (9) computed the sensitivity of activation analysis for each element, assuming a neutron flux of 5 X 10" neutrons cm.-2 second-' and an irradiation period of 10 half lives (saturation) or 30 days, whichever was shorter, but did not take into account the counting efficiency of the products. These sensitivity values have been used (3) in comparing the sensitivities of activation analysis with other conventional analytical methods. Jenkins and Smales (1) assumed a similar irradiation period in their computations, but they assumed a neutron flux of 10l2 neutrons cm.-2 second-', introduced a time of 2 hours for radiochemical separation, and also considered the detector counting efficiencies for the beta- or gamma-emitting isotopes. Recently in this laboratory Schindewolf (6) computed sensitivities based on a neutron flux of 1012 neutrons cm,-2 second-' and irradiation times of 6 and 600 minutes (10 hours). He considered the counting efficiency (using a Geiger counter in most cases) but did not include the time required for radiochemical separations. These very useful EVERAL
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comparisons have been transposed into the graphical form used in a previous publication (4). The values given by Schindewolf for activations of 6 minutes and 10 hours have been extended to 1000 hours and plotted to give three graphs, showing how the sensitivity for activation analysis can be varied, and discrimination obtained, by using different times of irradiation. If the graph for 1000-hour activation is compared to a plot of the it is apparent that a cross sections (4, number of the isotopes in the lower lefthand corner with low cross section and long half life have been omitted. This is the result of Schindewolf's original interest in short-lived activities. All these isotopes fall below the detection limit for a 6-minute irradiation and no attempt has been made to include them. I n the graphs the negative logarithm of the sensitivity, pS, has been plotted. This is defined as the number of grams of an element required to give about 60 counts a minute on a Geiger counter. This pS is similar to the p H used in acid-base work and is used as a convenience in plotting these sensitivities. For example, the sensitivity for vanadium-51 for a 6-minute irradiation is 1.7 X 10-lo gram, while that for cobalt-59 is 4.5 x 10-8gram. It is possible to change the sensitivities by using more sensitive detectors, etc., but these plots are intended to give a general impression of the relative sensitivities of these isotopes An isotope that is underlined on the graph indicates (4) that neutron activation produces a metastable daughter activity. A wavy line, on the other hand, indicates activation to the ground state of an isomeric pair. I n most cases the metastable state decays independently by beta emission or con-
tributes only a small amount to the activity of the ground state. I n some cases, however, the metastable state decays completely into the ground state both directly and by decay of the shortlived metastable state. Further information on these sensitivities is given in the original article by Schindewolf (6). These sensitivity values have been based primarily on Geiger counter detection. Scintillation counters were considered only when no beta emission was reported for the isotope. Similar graphs of this sort could be made for a scintillation well counter utilizing the gamma radiations given off by the isotope. I n some cases these would juxtapose the sensitivities for a particular isotope. ACKNOWLEDGMENT
This work has received the combined support of the U. S. iltomic Energy Commission and the Michigan Memorial Phoenix Project. Thanks are due U. L. Schindewolf for extending his published sensitivity values to 1000 hours. The help of M. L. Sargent and R. S. Maddock in preparing the graphs is also greatly appreciated. LITERATURE CITED
(1) Jenkins, E. N., Smales, A. A., Quart. Rev. Chem. SOC.London 10, 83-107 IIS.56i. \----,.
(2) Leddicotte, G. TV., Reynolds, S. A.,
U. S. Atomic Energy Comm. AECD-
3489 (1953).
(3) . , Meinke, W. W., Science 121, 177 (1955). (4) Meinke, W, W., Naddock, R. S., ANAL.CHEM.29, 1171 (1957). (5) Schindewolf, U., Angew. Chem. 70, 181-7 (1958).
RECEIVEDfor review October 6, 1958. Accepted January 20, 1959.
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