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LETTERS TO THE EDITORtion for Sailor's method, but a more accurate Doppler correction would reduce T below 0.030 ev by a verysmall amount. 3 The energy of the peak is quite high for the peak to be a scattering cross section discontinuity caused by a crystal edge in the sample. In addition, such an effect would result in a sawtooth peak rather than a symmetrical one.A search of existing cross-section data 4 indicates that the 0.43-ev peak is probably not due to a contaminant. If it is assumed that any element having such a narrow resonance (0.030 ev) must have an atomic weight ^4>150, 5 most of the remaining elements can be ruled out by their cross section data. Gd, Tb, and the radioactive elements Ra, Ac, and Pa are the only elements for which measurements for the 0.43-ev region have not been published. The radioactive elements can be ruled out as possible contaminants since the sample shows no alpha activity. Effects of a high resonance at 0.43 ev in Gd would be expected to extend into regions where measurements exist, even though there is a gap in existing data from 0.2 to 1 ev. If the resonance is assumed to be caused by a Tb impurity, and its width and atomic weight are compared with the data from Fig. 1 of the paper of Hughes and Harvey, 5 then the point for this resonance is seen to be farther below the solid curve than any other point plotted there. Thus a Tb impurity is probably not the cause of the 0.43-ev peak. If the resonance is due to Ta 181 , the value of gT n would be expected to be of the order of 10~~4 ev instead of 0.8XlO-8 ev.The above considerations led to the hypothesis that another stable (or long-lived) isotope of tantalum exists. Earlier measurements 6 placed the upper abundance limit of a second isotope at 100 ppm (parts per million). If such an isotope exists and has a resonance at 0.43 ev, it would be expected to have a peak height of the order of 10 4 to 10 5 barns. Considering the observed peak, this would correspond to a relative isotopic abundance of the order of 20 to 200 ppm. The other isotope would probably be within two mass numbers of 181 since there are no known cases of odd-Z elements with Z> 19 having more than two naturally occurring isotopes. When such elements do have more than one isotope, not more than two mass numbers separate them. Thus the atomic weight of a second Ta isotope should be 181d=2. It is quite probable that the second isotope is radioactive since the second naturally occurring isotopes of other odd-Z elements with Z between 64 and 76 are radioactive.The authors suggested the hypothesis, that a second isotope of Ta exists, as one which could be proved or disproved by use of the recently completed high intensity mass spectrograph at Knolls Atomic Power Laboratory, Schenectady, New York. The KAPL group accepted the suggestion and obtained the results given in an accompanying "Letter to the Editor." 7 If one uses their measured value of 123±3 ppm for the relative abundance of Ta 180 , the present total cross section data give the following isotopic single-l...