The Delayed X-Ray Technique Principles, Comparison with Other Techniques, Practical Applications A. E. Pillay University of the Witwatersrand, P.O. WITS. 2050, South Africa
A novel aspect of neutron and charged-particle activation has- recentlv ~ ~-~ ~ ~ come to the fore. The technique involves the use (for analy&) of delayed X-rays emitted from artificially produced radionuclides. The principles of the method are outlined, and its practical utility in comparison with other contemnorarv. techniques are discussed. X-ray spectrometry preceded by thermal neutron activation was first reoorted by Shenberp, et al. ( 1 ) in 1967. In I972 Mantel and mie el (2) e"aluated the sensitivity of the technique over a wide range of elements. Since then there have been various practical applications of the method with thermal neutrons ( 3 4 ) .In 1988 Pillay and co-workers (6) were the first to demonstrate the viability of the technique with protons and more recently in 1989 (7) with fast-reactor neutrons neutrons from an isotope source. This .---and thermal ~-~~ paper discusses the potential of the technique as an analytical tool and its ability to compete with rival contemporary methods. -
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Princlpies The principles of the technique are straightforward. The method strongly resembles the delayed gamma-ray technique. Proton and neutron activation of stable isotopes leads to the production of radionuclides that decay by emitting characteristic L and K X-rays. The emitted X-rays proceed from three commonly known decay processes: (1) internal conversion of nuclear isomers that gives X-rays of the product element Z; (2) orbital electron capture that yields X-rays of the daugher element, (Z - 1);and (3) beta decay followed by internal conversion that produces Xrays of the daughter element (2+ 1). The sensitivity of the technique is controlled largely by the cross section of the nuclear reaction in question, the half-life of the radioactive product, and the isotopic abundance of the target isotope. Very long half-lives and very low isotopic abundances make the method impractical. Generally speaking the cross sections for nuclear reactions are a few orders of magnitude lower than those for the production of X-rays by XRF and PIXE. This factor looks discouraging hut can be offset if the background is low enough to accentuate the sienal. In this reeard the oerformance of a low-enerrr sensia vital role in producing practicatiie photon d e t e k r ble signal-to-noise ratios. Commrlson wlth Other Tachnlaues The method compares favorably with other current methods of analysis such as XRF, PIXE and prompt gamma-ray spectroscopy. Although these contemporary techniques are on-line aoolications, delayed X-ray spectrometry offers certain distgct advantages over thesd methods: (1) counting is not accompanied by the undesirable presence of bremsstrahlung radiation, (2) the absence of high Compton edges facilitates peak integration;
(3) extended off-line counting for the attainment of better deteetion Limits can be undertaken; (41 the energy of the delayed X-rays (following the same decay made) element: hence. in ~. is identical for all isotones of a eiven " the rRse where an element has several rrable isotopes, the X ray intensity is the accumulated activity of all the radioisotopes: this factor enhances the sensitivity; (5) forelementswithZ t 5: theKn,, KO?,K&,andKA! X-raysere generally well resolved with a high-rearlutiun detPcror, and this provider alternative lines on which the analysis can he based in the case of interferences; (6) consequently a wide range of elements can be determined (interference-free)in a single irradiation; (7) the use of s high-resolutionlow-enerm photon detector that iu particularly effective at energies helow 150 keV eliminates interferences from energetic gamma rays and consequently optimizes signal-to-noise ratios; (8) the measurement of relatively long-lived products offers the merit of eliminatingshort-livedcomponents which cause interferences and raise background levels; (9) the background, due largely to heta radiation, can be suppressed hy magnetic deflection (8); (10) the possibility of more than one nuclear reaction, e.g., (n,p), (n,ol) or (p,n), (p,n) provides an excellent means of obviating interferencesfrom adjacent elements and impurities; (11) the technique is capable of extending down to trace and ultratrace elemental levels; (12) small and hulk samples can be analyzed when the technique is used in association with neutron activation. These factors provide the analyst with a host of attractive features that denote that the method holds promise and is potentially useful for low-level chemical analysis. The versatilitv of the method makes it particularly suitable for wide and-varied applications and istrong rivd t o present trace methods of analysis. Pracllcal Applications. The possible practical applications of the technique are manifold. Some useful areas of application are: geology, mineralogy, biology, and environmental studies. Figure 1 (6) represents a portion of a delayed X-ray spectrum of a proton-activated preconcentrated platinum ore sample. The actual levels of the platinum group elements (PGE's) in the ranee. A visual examinasamole were in the art-oer-billion tionof this spectrum reveals that all Gx of the PGE's were detected and that the method is c a ~ a b l eof comfortablv extending down to ultratrace levels. u he analysis of some o> the PGE's is considered somewhat difficult by conventional means. The determination of 0 s and Ir by PIXE and Ru hy conventional neutron activation analysis is generally prohlematic. The delayed X-ray method, therefore, asserts its superiority in this regard. Figure 7 is a cut from a spectrum of a typical monazite sample activated with thermal neutrons from a l-mg 252Cf source. This clip from the spectrum represents an example of the capability of the technique to produce a practicable Xray yield even with a limited thermal neutron flux. The Volume 68 Number 2
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PGE COnlent ol SamPC 255 pd: 20
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Figure 1. A portion of a fire-assaypreconcentrated PGE sample activated wim 10-MBV protons. Inadlation time, 2 h, decay time, 12 h; count time, 20 h. experimentally determined level of Dy in this monazite was 0.13%. Factors that generally affect the linearity of the method are: second-order interferences, branching decay, internal conversion coefficient, sample self-ahsorption, and flux inhomogeneities. In particular, errors arising from second-order interferences can seriously affect the analysis. For example the Dy K lines in Figure 2 arose from the reaction 164Dy(n,y)L65Dy. Radioactive 165Dypossesses two components that have thermal absorption cross sections of 2100 and 3900 barns. With such extraordinarilv " hieh cross section values appreciable error in the analysis is possible. Such factors tend to diminish the analvtical utiltv of the method. However, when the linearity for Dy in particular was examined (Fie. 3) the results showed that adverse factors affecting the &&lysis were minimal and that a suitable linear relationship between the measured signal and elemental concentration could he obtained. This ultimately demonstrates that the techniaue is favorable for use as an analytical tool and suitable for general routine applications in t h e same leame as PIXE, XRF, and instrumental neutron activation d y s i s .
Figure 2. A cut hom a specbum of a monazite sample activated with thermal neutrons hom a l-mg 2526fsource. lnadiation time. 24 h; decay time, 35 min: count time, 1800 a.
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Conclurlon The principles and general practickil applications of the relatively novel delayed X-ray technique were demonstrated. The method is capable of multielemental trace-level analysis and compares favorably with rival techniques. Its greatest practical utility would he in the earth sciences. Lnerature Clted 1. Shenberg,C.;Gilat,J.:Fiatoh, H.L.Ana1. Chem. 1967,39,780-785.
S. Anal. Chom. 1972.44,548-553. 3. Hsrfagen, G.;Gilbei8.R.Anol.Chim.Ado 1571.66,61-82. 4, Roeenberg, J.; Wiik, H. 8. Rodioehem. Radioanal. Left. 1971,6,4565. 5. Piiisy. K. K, S.;Miller. N. W. J. Rodioond. Chem. 1969.2.91-107. 6. Pillay,A.E.:Era~mus,C.S.;Andeweg,A.H.;Selischop,J.P.F.:Annegarn.H,J.;Dunn. J. Nurl. Insf. m d Methods I988,835,55b560. 7. Mbowmi, C.: Msahilo, N. Univcnify of the Witwatmrand, personal mmmunication. 8. Mantel, M.; Amiel, S. Non-Desfrueliur Acfiuotion Anoly8is: Elsevier: Amterdam. 1981:Vol. 3, P 25. 2. Mantel, M.; Amiel,
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Figure 3. A callbration plot of Dy demonshatlng the linearity of Uw delayed Xmy technique.
