Sensitivities for Activation Analysis of 15 Light Elements with 18-MeV Helium-3 Particles Enzo R i c c i a n d R. L. Hahn Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. An experimental survey was made to appraise the possibilities of 3Heactivation analysis in the determination of elements from beryllium to calcium. Thick targets of these elements were bombarded with 18MeV SHe particles from the ORNL Tandem Van de Graaff accelerator. A simplification was introduced in the mathematical treatment of charged particle activation analysis that makes it possible to use range data directly read from range-energy tables or curves without any further elaboration. Instrumental sensitivities for gamma spectrometry were obtained for the above elements from duplicate experiments and absolute sensitivities were calculated from the instrumental values. The sensitivities are all high; in particular, those for beryllium, boron, carbon, nitrogen, oxygen, fluorine, and sulfur range from 0.123 to 1.U cpm/ppb FA. This group of elements cannot be easily determined at the ppb level by other techniques; thus, 3He-activation promises to fill the gap left by other methods.
SINCEMARKOWITZ AND MAHONY ( I ) suggested the method of 3He activation analysis in 1962, the same authors ( 2 3 ) and other workers (4-8) have studied different aspects and problems inherent in this technique and also have applied it to a number of matrices. Although all these authors observed the great potential of this method in the analysis of light elements, no systematic, comparative survey of analytical sensitivities has been attempted to date. The main purpose of this paper is to discuss the results of a survey of this type, which involves the bombardment of elements from Z = 4 to Z = 20 with 18-MeV 3He particles. In our previous publication (5) we developed a mathematical treatment for charged-particle activation analysis that drastically simplifies the handling of the data obtained by this technique. During the present work, which involved a great number of thick targets, it became apparent that the definition of thick-target average cross section, ii, used in our former paper, although theoretically correct, was rather impractical. It was therefore necessary to choose a more suitable definition of 5, and to prove its consistency with the basic equations of charged-particle activation analysis.
PR
Jo where u f (in sq cm) represents the value of the nuclear reaction cross section at depth t (mg/sq cm) in the target as before (3, but R (in mg/sq cm) is now the total range-Le., the total depth penetrated by the particles in the target. In the old definition an effective range was used; it was defined as the target thickness necessary to reduce the particle energy to a value below which the nuclear reaction could no longer take place. Let us call R' this effective range now, and 5' the corresponding (old) average cross section. Clearly, the particle reaches R' when its kinetic energy has been degraded to a value equal either to the Coulomb barrier or to the threshold energy of the nuclear reaction. Practical reasons made the redefinition of average cross section desirable. Due to the quantum-mechanical tunnel effect, the Coulomb barrier is not precisely defined. Also, it is much easier to read the total ranges, R , from tables (9,10) than to calculate the values of R' from them, using reactionthreshold or Coulomb-barrier energies. In the new definition of a, the value of u t vanishes in the interval R' 6 t R by definition of R'. Thus,
