Study of the Chemical Etching Procedure Used to Remove Surface Oxygen Contamination in Charged Particle Activation Ana lysis Harry L. Rook, Emile A. Schweikert, and Richard E. Wainerdi Actiaation Analysis Research Laboratory, Texas A&M University, College Station, Tex. 77843 In charged particle activation analysis, the procedure used to eliminate surface oxygen contamination is to use a single post-irradiation chemical etch deep enough to remove the activated surface layer including recoil activity. Fluorine-18 is the radioisotope produced by most analytically useful charged particle reactions with oxygen. It is assumed with this procedure that adsorbed activity from the etching solution onto the sample surface can be neglected. However, the results of this experimental study show that adsorbed fluorine-18 activity can become a significant source of error in the determination of oxygen in aluminum matrices. For example in the case of a helium-3 activation, the determination of 10 ppb oxygen would be affected by an error of approximately 500%. It was found that this potential interference can be eliminated by using a post-irradiation etching procedure which employed two successive etches, the first to remove the activated oxide layer and recoil activity, and the second, of less than 5 microns per face, to remove adsorbed activity.
RECENTLY, there has been considerable interest in activation analysis using charged particles as a means of extending the sensitivity for many light element analyses from the part per million (ppm) to the part per billion (ppb) range. By using helium-3 activation for the determination of oxygen, for instance, several authors have estimated that sensitivities as low as one to ten ppb should be feasible (1-4). However, charged particle activation analysis has not yet realized its full potential. Many problems, both experimental and theoretical have not been satisfactorily resolved. The number and complexity of these yet unsolved problems, as well as the limited cyclotron irradiation time presently available to the analyst, is illustrated by the relatively few analytical results reported in the literature on actual samples. Again considering the determination of oxygen, no actual analyses have yet been reported in the literature in which the one to ten ppb estimated sensitivity has been approached. This report concerns the results of a study on one such problem, that of errors arising from fluorine-1 8 activity adsorbed onto the sample matrix during a chemical etching procedure used to remove surface contamination. Fluorine-18 is the radioisotope produced by most of the useful charged particle reactions with oxygen, namely: I6O(3He,p)18F, W(cU,d) l8F, 180(p,n)18F, lGO(d,y)l8F, and 180(d,2n)l8F. Because all materials have a surface oxygen contamination resulting from atmospheric contact, it is a common practice (1) S. S . Markowitz and J. D. Mahony, “Radiochemical Methods of Analysis,” Vol. 1, International Atomic Energy Agency, Vienna, 1964, p 419. (2) E. Ricci and R. L. Hahn, ANAL.CHEM.,37, 743 (1965). (3) [bid., 39, 794 (1967). (4) Ph. Albert, Chimia (Aarau), 21, 32 (1967).
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to etch the surface after irradiation to eliminate the surface activity. It has been generally assumed that a single 10-30 micron per face etch, carried out with fluorine-19 as carrier, was sufficient to eliminate any interference. In this study, aluminum and silicon samples were etched, using solutions containing known amounts of fluorine-1 8, to see if adsorbed activity could become a limiting factor in the ultra trace analysis. Simultaneous experiments were conducted using chlorine-38 as the adsorbed radioisotope in a n attempt to elucidate the nature of the adsorption effect with the assumption that adsorption of chlorine would be a physical effect, EXPERIMENTAL
The fluorine-18 was produced in the Texas A&M Research Reactor by irradiating 0.5 gram of lithium hydroxide in a neutron flux of approximately 5 x 10“ n/cmZ/sec for one hour using the following nuclear reaction sequence: 6Li(n,a)t, ‘60(t,n) 18F. Chlorine-38 was produced by irradiating 50 mg of CaCl2-2H20at a similar neutron flux for 1 hour. The pf annihilation radiation of the fluorine-18 was counted with a gamma-gamma coincidence system of modular design using RIDL Designer Series equipment and two 3” X 3 ” NaI(T1) crystals. The 1.6 MeV gamma rays from chlorine-38 were counted on a 3” x 3” NaI(T1) crystal coupled to an RIDL Model 34-12b 400 channel analyzer. The activities were then compared under standard conditions. The aluminum etching solutions were composed of 70% concentrated phosphoric acid, 20 % concentrated sulfuric acid, and 10% concentrated nitric acid by volume. The etching was carried out a t 80 “C with a resulting etching rate of approximately 5 microns per face per minute. The silicon etching solution was composed of 68% concentrated nitric acid, 23% glacial acetic acid, and 9 % concentrated hydrofluoric acid (50%) acid by volume. The silicon etch was carried out at room temperature with a n etching rate of approximately 0.5 micron per face per minute. The samples were of high purity aluminum of 99.99% ( 5 ) and zone refined silicon of 99.999 % purity (6). The procedure for etching was as follows: 15 ml of the etching solution were measured into a 25-1111 polyethylene vial. The irradiated lithium hydroxide was added and the solution heated in a water bath to 80 “C, while stirring, until all of the lithium hydroxide dissolved. A maximum of 0.8 gram of lithium hydroxide could be irradiated because higher concentrations in the etching solution exceeded the solubility of the lithium ion. When silicon was being ( 5 ) Ph. Albert, Centre $Etudes de Chimie Metallurgique, Wry,
France, personal communication, 1967. (6) D. L. Kendall, Texas Instruments Inc., Dallas, Texas, personal communication, 1967.
etched, the lithium hydroxide was dissolved at 80 “C and then the solution cooled to room temperature. The metal samples, with normal dimensions of 2 cm x 2 cm X 0.2 cm, were placed on edge in the radioactive solution and etched for the desired length of time. Nitrogen dioxide gas evolution stirred the solutions sufficiently to ensure a uniform etch. The contact times were varied to give etches of from 5 to 150 microns per face. After etching, the samples were removed, washed in three successive 50-ml aliquots of distilled water, and finally flushed under a stream of water. The samples were blotted dry with filter paper and counted for two half lives of fluorine-18 or chlorine-38 decay. Individual decay curves were compared under standard conditions. After the etch was completed, the etching solutions were counted in their vials, with a correction made for geometry, and the count rates of the etched samples were normalized to the count rate of the respective etching solution. The resultant normalized count was then the fraction of activity, from solution, which had adsorbed on the metal surface. This fraction was divided by the surface area of the sample and the volume of etching solution to give a fractional adsorption coefficient which could then be used for other sample geometries and etching solution volumes. RESULTS AND DISCUSSION
The post-irradiation procedure previously used to eliminate surface oxygen contamination was to remove this highly radioactive oxide layer with a suitable chemical etch. This etching procedure assumed that the amount of adsorbed etching solution, containing fluorine-18, was negligible. Calculations with aluminum as the sample matrix show that this assumption is valid, providing the adsorption is considered to be solution physisorption. This requires that the mechanism is one where the etched aluminum surface continually forms a hydrate layer of the etching solution, which when the etching is stopped and the sample washed, is not removed but incorporated into the new aluminum oxide layer. Radioactive fluorine-18 is carried along as solvated ions and incorporated into the Al2O3layer in the same concentration as in the etch solution. Considering this, an equation was derived which gives the activity of fluorine-18 adsorbed on an aluminum surface:
where SiTr = surface area irradiated, S, = surface area of the sample, t = thickness of A1203 layer, d A 1 2 0 1 = density A1203,daze = density H,O, A = specific activity of ISF per gram of oxygen, Aads = activity adsorbed, and V = volume of etching solution. From the above expression, a fractional adsorption coefficient (Kads)was derived which is the fraction of the activity in the etching solution adsorbed on the sample surface. Considering the mechanism previously discussed, the fractional adsorption coefficient (Kads) is given by the ratio of the volume of etching solution adsorbed to the unit volume of etching solution:
Table I. Experimentally Determined Fractional Adsorption Coefficient Lithium Fractional ion adsorption concenEtch coefficient, Fluoride ion tration, thickness, X cm concentration, M M ,a/face z 30 2.48
