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Neutron-Capture Prompt y-Ray Activation Analysis for Multielement Determination in Complex Samples M. P. Failey,’ D. L. Anderson, W. H. Zoller, and G. E. Gordon” Department of Chemistry, University of Maryland, College Park, Maryland 2 0 7 4 2
R. M. Lindstrom Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234
Gamma-ray spectra were taken up to 11 MeV from a wide range of samples and elemental standards while under neutron Irradiation to determine the elements whose prompt y rays are observable and can be used for analytlcal measurements. Up to 17 elements from among the set H, B, C, N, Na, Mg, AI, SI, P, S, CI, K, Ca, TI, V, Mn, Fe, Cd, Nd, Sm, and Gd are measurable in samples of coal, fly ash, orchard leaves, and bovine liver by neutron-capture prompt y-ray activation analysis (PGAA). The combination of PGAA and instrumental neutron activation analysis (which uses the same equipment) can be used to measure concentrations of 40 to 50 elements In lndlvklual samples of many types of material. Concentrations are reported for the elements measurable by PGAA In National Bureau of Standards Standard Reference Materials: coals (SRMs 1632, 1632a, 1635), fly ashes (1633, 1633a), orchard leaves (1571), and bovine liver (1577).
There is continuing need for improved methods for nondestructive multielement analyses of complex samples encountered in the study of environmental, geochemical, and biomedical problems. Instrumental nuclear methods of analysis have been very useful in these applications, especially instrumental neutron and photon activation analysis (INAA and IPAA, respectively) (1-3). For these techniques, both the nuclear projectiles and the emitted y rays have such long ranges in materials that there are rarely significant problems of self-shielding or -absorption by samples. As the methods are instrumental, there is no need for chemical manipulation of the samples, which could allow coprecipitation of trace elements on insoluble residues or container walls, or contamination of samples by impurities in the reagents. Capabilities of INAA and IPAA were demonstrated in a fourlaboratory analysis of National Bureau of Standards (NBS) Standard Reference Materials (SRMs) coal and fly ash ( 4 ) . Concentrations of about 40 elements were determined in the SRMs, with results in better agreement between laboratories and with NBS certified values than achieved by laboratories using other methods in a blind, round-robin analysis of the standards. Despite the strengths of INAA and IPAA in the analysis of complex samples, further improvements are needed. First, measurements of some key elements (e.g., B, Cd, S) are impossible or marginal in many samples. Second, electron accelerators needed for IPAA are not widely available. Third, INAA studies of biological and marine samples are not as successful as for more “crustal” samples (coal, fly ash, rocks) because of interference from high levels of Na, K, and C1. The interference can be partly overcome by chemical removal of these elements, but this eliminates many advantages of nuclear methods. Fourth, some samples have restrictions that make Present address: Babcock and Wilcox, Lynchburg, Va. 0003-2700/79/0351-2209$01 .OO/O
it impossible to insert them into reactors, e.g., they might decompose or explode, they are physically too large, or the heating, radiation damage, and residual activity are undesirable. Fifth, both INAA and IPAA have two-week or greater turn-around times for complete analyses. T o overcome some of these problems, yet take advantage of the beneficial aspects of instrumental nuclear methods, we investigated the use of neutron-capture prompt y-ray activation analysis (PGAA). In PGAA, one observes y rays emitted while the sample is being irradiated with neutrons. Nuclei formed in capture have excitation energies equal to the binding energy of the added neutron, from 5 to 11 MeV. The excitation energy is released by emission of one or several s. Thus, nearly every “prompt” y rays over times neutron capture yields y rays that are potentially usable for analysis for the capturing element. By contrast, neutron capture does not necessarily form a radioactive species that can be used in INAA. The resulting product may be stable, have a very short or long half-life, or emit no intense y radiation. For example, Cd has an enormous neutron-capture cross section, but most of the cross section is for the li3Cd(n,y)reaction, which forms stable ll*Cd, which is of no help in INAA. [Cadmium can sometimes be observed by the reaction l14Cd(n,y), which produces 2.3-day llSrnCd.] But in PGAA, one may observe prompt y rays from capture by W d . Several papers on PGAA appeared over the past decade (5-10). These studies demonstrated the potential of PGAA by calculations or by analyses for one or two elements in several samples. The only multielement applications to complex samples are recent reports of concentrations of nine elements in several NBS and U.S. Geological Survey standards (11) and of three elements in several standards (12). In order fully to exploit PGAA, one must identify the species responsible for each of the hundreds of lines in prompt y-ray spectra (13-15) and irradiate pure standards of each observable element to determine inter-element interferences of y-ray lines in the spectra. We have constructed a facility a t the NBS Reactor to irradiate samples in an external thermal neutron beam and observe prompt y rays with a high resolution y-ray detection system. This setup was used to identify species observable by PGAA of various types of samples, to determine the best lines of each element, and to identify significant interferences between lines of similar energies of different elements. Applications of the method have been demonstrated by analysis of several classes of NBS Standard Reference Materials.
EXPERIMENTAL Irradiation Facility. One can either place samples inside the reactor, where the flux is large, and observe the 7 rays outside of the reactor, or bring a neutron beam outside to a sample mounted close to the detector. The former has a high neutron flux, but poor counting geometry and the latter is just the opposite. 0 1979 American Chemical Society
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Gladney et al. (9)chose the former and we have chosen the latter. Flux and geometry factors balance out to about the same overall efficiency, but the external setup has several advantages: (1) samples are subjected to no heating, little radiation damage, and have very little residual activity; (2) one can use samples that are too large or fragile to be inserted into a reactor; (3) one can put fiters in the neutron beam to remove the y-ray flux from the beam or neutrons of certain energy ranges; and (4) one has greater flexibility in the sample and detector setup, e.g., one can do coincidence counting with two or more detectors. A vertical beam port was installed on top of the NBS research reactor, a 10-MW DzO-cooledand -moderated reactor with internal fluxes up to 1014n/cmz-s. The internal beam tube extends 5.8 m into the reactor. It is made of A1 in the lower portion and steel near the top and contains neutron and y-ray collimators of decreasing diameter up the tube. The external beam tube, which provides shielding around the beam between the top of the reactor vessel and a beam stop, is designed to be purged with helium if desired to prevent neutron scattering by air molecules and neutron capture in Nz near the target. Boron carbide (B4C)in paraffin and natural LizC03in paraffin are used to thermalize and absorb scattered neutrons to reduce the background in the detectors. A Plexiglas sample box, 1.1 m above the floor, is surrounded by plates of B4C in polystyrene to absorb neutrons scattered by the sample. A beam stop, consisting of 6Li2C03in polystyrene surrounded by natural Li2C03in polystyrene with P b shielding surrounding the latter, is mounted at the top of the beam tube. The mass of material placed near the sample must be kept small to minimize neutron and y-ray scattering into the detector and must have small capture cross sections to keep the capture y-ray background low. We seal most samples in 0.0025-cm Teflon film which is suspended in the beam with nylon fish line. The carbon and fluorine of Teflon have low capture cross sections. At the sample position, the beam has a 4.5-cm diameter, with a flux constant to about 3% over the central 3 cm. The thermal neutron flux is about 2 X lo8 n/cm2-s, with an integrated flux over the entire beam of 3 x log n/s and a gold-cadmium ratio of thermal/fast neutrons of about 55 (16). Additional details on the beam tubes are given elsewhere (16, 17). Detection System. The detection system must span energies from 100 keV to nearly 11 MeV with good resolution, especially up to about 2 MeV, as the spectra have a high density of lines below that point. The detector is a true coaxial Ge(Li) detector of 24% efficiency (relative to a 7.6 X 7.6 cm NaI crystal) yielding a peak with full-width at half-maximum of 1.9 keV at 1332 keV. The Ge(Li) detector is mounted inside a large NaI crystal that will be used for Compton suppression and pair spectra. The detection system must be well shielded from y rays and neutrons, the latter to reduce the prompt y-ray background from capture in the detector during experiments and to prevent long-term activation and radiation degradation of the detector. The detection system is surrounded by 5 to 10 cm of Pb, and a 2.5-cm thick layer of B4C in polystyrene, all sealed in an A1 shell. A P b collimator, 15-cm long with a 2.5-cm i.d. is placed between the sample position and the Ge(Li) detector. A 1.3-cm thick, fused %i2C03 plug is placed in front of the collimator to absorb neutrons. These measures have not entirely removed backgrounds, but have reduced them to acceptable levels (see below). To take full advantage of the resolution of the Ge(Li) detector, events must be sorted into a large number of channels. The data-handling system is a Tennecomp TP-5000 analyzer whose central component is a Digital Equipment Corporation PDP-11/34 computer with a 128K-wordmemory. The system has inputs from six 8192-channel analog-to-digital converters (ADCs). For spectra reported here, we used two ADCs, one covering the 0- to 4-MeV range and the second, with input from a biased amplifier, covering the region from 3.2 to 11 MeV. Irradiation Procedures. For most samples and standards, we use about 1g of material, formed into a 1.3-cm diameter pellet in a press, sealed in 0.0025-cm Teflon film and suspended in the central, uniform portion of the beam, oriented at 45O to the beam. For elements with very high cross sections (B, Cd, Sm, and Gd), calculations and experiments show that much smaller standards must be used to avoid self-shielding of the neutron beam. For Gd, for example, surface densities of C150 bg/cm2 must be used to keep self-shielding to
