Novel studies in x-ray emission analysis using radioisotopes

Abstract

Two novel techniques employing radioisotopes for x-ray emission were studied: internal x-ray emission analysis and low energy proton activation with counting of radioactive x-ray emitters. An inexpensive, sensitive technique for the analysis of a broad range of elements (Z 16 through 47 were studied) was developed by implanting radioisotopes (('3)H, ('35)S, ('63)Ni, ('75)Se, ('91)Tc, ('125)I, ('195)Au) into a sample. These radioisotopes decay by x-ray or (beta)('-) emission which in turn excites characteristic x-rays from elements in the sample. The application of this technique to trace analysis was demonstrated with a multielement standard sample containing 0.1% of Ag, Ni, Cr, Zn, Co, Rb, Sc, Br, Mo, and As. Limits of detection varied with the radioisotope and its specific activity, ranging from 470 ppm Br with 0.74 MBq (.) g('-1) (20 (mu)Ci) ('35)S to 20 ppm Ag with 74 kBq (.) g('-1) (2 (mu)Ci) ('125)I. These limits are less than with conventional external radioisotope excited x-ray systems. For example, with a 74 kBq internal ('125)I source a limit of detection of 52 ppm is obtained for Ge vs. a 70 ppm value obtained with a 4 MBq (100 (mu)Ci) external ('109)Cd source. Low energy proton radioactivation (3.6-6 MeV) with subsequent nondispersive x-ray counting was evaluated for trace analysis. Thick target yields and interference-free detection limits were measured for 15 isotopes (('37)Cl, ('49)Ti, ('51)V, ('57)Fe, ('65)Cu, ('67)Zn, ('71)Ga, ('73)Ge, ('75)As, ('85)Rb, ('88)Sr, ('89)Y, ('92)Zr, ('95)Mo, and ('105)Pd) with 5 MeV yields ranging from 1.26 E-4 cpm/(mu)g for Fe to 1.86 E-1 cpm/(mu)g for Ga and limits of detection ranging from 1.33 E-1 (mu)g Fe and 9.18 E-3 (mu)g Ga. This technique was applied for the determination of V and Ga in NBS glasses doped at the 500 and 50 ppm level (measured values were 431 (+OR-) 21 and 45 (+OR-) 11 ppm for Ga, and 494 (+OR-) 54 and 42 (+OR-) 4 ppm for V, respectively). Elemental mapping was accomplished by autoradiography. Data from a Ga doped Si semiconductor sample irradiated with 5 MeV protons showed a spatial resolution of 50-100 (mu)m, with an ultimate limit of 3-10 (mu)m.