Precision absolute thin film standard reference targets for nuclear reaction microanalysis of oxygen isotopes

N U C L E A R I N S T R U M E N T S AND METHODS 149 (1978)

713-720 ; ©

N O R T H - H O L L A N D P U B L I S H I N G CO.

PRECISION ABSOLUTE THIN FILM STANDARD REFERENCE TARGETS FOR NUCLEAR REACTION MICROANALYSIS OF OXYGEN ISOTOPES* Part II: ~80 and ~70 standards G. AMSEL, J. P. NADAI ?, C. ORTEGA and J. SIEJKA Groupe de Physique des Solides de I'Eeole Normale Sup&ieure, Universitk Paris VII, Tout" 23, 2 Place Jussieu, 75221 Paris Cedex 05, France

The methods for constructing absolute high precision thin film 160 standards presented in part I are extended to 180 and 170. The special techniques required to handle separated isotopes in small quantities and the problems which arise when using enriched compounds are discussed, both for electrochemical (anodic) and thermal oxidation methods, the 180 contents may be deduced approximately from the known enrichment of the oxidizing medium and further determines with high precision by the "160 defect method", in which the 160 content is measured using the 160(d, p)170~ reaction, the 160 defect, as compared to an identically prepared pure 160 target, being equal to the 170 plus 180 content. For 180 enrichments above 75% the overall precision is again _+_3%, as for 160. Results for primary anodic 180 standards are presented. Simplified methods, allowing one to operate in very small cells (0.5-1 cm 3) are described for constructing easily, by anodization, secondary isotopic standards in an economic way. Typical isotopic concentrations are around 80% for ISO and 40% for 170, with optimal thicknesses around 1000 A Ta205. Error estimations and possible artefacts are discussed in detail.

1. Introduction Isotopic tracing of oxygen is a basic method to study oxygen transport processes in fields like thin films, dielectrics, solid state electrochemistry, catalysis, surface physics, metallurgy, chemistry, biology, etc. As no long lifetime radioactive isotopes of oxygen exist, tracing is carried out with stable isotopes, mainly ~80(0.204% natural abundance), but in some cases the r a r e r 1 7 0 ( 0 . 0 3 7 % ) . lsotopically enriched or "labelled" compounds are used: water, oxygen gas, various salts and acids, etc. Nuclear reaction microanalysis is among the best techniques to determine absolute amounts of 180 or 170 near the surface of solids, or 180/~60 ratios, etc. ~80 is measured with ~80(p, cz)lSN 1,2)(Q= 3.97MeV), 170 with 170(d, cz)lSN(Q=9.81MeV). ~80 enriched compounds may also be used to reduce, up to N400 times, background effects due to oxygen already present in the near surface region of the samples or to spontaneous oxidation of the samples after their treatment, during their handling in atmosphere. ~70 tracing is useful when ~80 detection is difficult (for example due to interference with (p, ~z) reactions on liB, in the presence of boron) or if deep oxygen profiles are studied, J70(d, ~z)lSN yielding higher energy alphas Work supported by the C.N.R.S. (R.C.P. no. 157) and the D.G.R.S.T. + Present address: Universit6 des Sciences et Techniques du Languedoc, Centre d'l~lectronique des Solides, 34000 Montpellier, France.

than 180(p,~z)~SN); ~70 is however generally avoided due to its very high cost and lower available enrichments. Standards of both isotopes are required for isotope effect studies or to calibrate isotopic targets in nuclear physics for high precision cross section measurements. General papers on various uses of ~80 may be found in refs. 3-6. The application of these standards and of anodic oxidation of tantalum to the micro determination of 180/~60 ratios in various aqueous compounds (chemical or biological) will be the subject of a forthcoming paper (see ref.5). The present paper is devoted to high precision isotopic oxygen standards required to apply nuclear microanalysis. 160 standards were described in part I of this paper7), referred to as "part I" in what follows. Precautions to be taken and sources of error are discussed in great detail for the various methods presented. A simple method to prepare secondary isotopic standards is described: this operation is not as difficult as a first glance to this paper could suggest, especially if maximal accuracy is not aimed at. 2. Principles 2.1. GENERAL Any method outlined in part I for the construction of 160 standards may be used for 180 or 170 enriched standards, the usual oxidizing medium being prepared with isotopically enriched materials, in practice water or oxygen gas. The problem X. A P P A R A T U S AND T A R G E T S T A N D A R D S

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is then to know exactly the overall oxygen content X of the film, i.e. its content in oxygen atoms/cm 2 irrespective of their isotopic nature (elemental oxygen content) and further their isotopic contents X 16, X ~8 and X 17, where X=X16-~-XIS-~-X 17. The first basic assumption required for the determination of the isotopic contents is that one can control the oxidation process in the enriched medium so that the overall content X is as precisely determined as in the usual case, where compounds of natural isotopic composition are used. We shall call this assumption the principle of atomic thickness identity summarized by the statement that: any operation carried out in an enriched medium yields a film identical from atomic point of view to that obtained by the same operation in the usual medium. The following conditions should be satisfied for this assumption to hold: 1) Identical experimental conditions for the natural and enriched cases; in particular: - Same type of cell (geometry of cell and electrodes) or furnace. As enriched solutions imply the use of miniature cells due to the high cost of the material and similarly enriched gases imply the use of furnaces with closed circuit gas circulation both the natural and the enriched samples should be prepared in the same type of set up for maximum accuracy (see below). If water is used (either to anodize or to oxidize thermally) the isotopically enriched water purchased should be of the so-called "normalized" type, i.e. having a hydrogen isotopic concentration near to natural. In fact water enriched in JsO or 170 is produced usually by distillation procedures which concentrate the heaviest isotopes and are, at high enrichments, obtained at the plant as nearly pure D20. This may then be "normalized" by various hydrogen exchange procedures. If the water is obtained from enriched 02 burnt in H2, it is automatically "normalized", This condition is fundamental as the isotope effects for hydrogen (and possibly for OH-) are large and may influence the processes considered: thus aluminum presents completely different behaviours when anodized at low current densities in H20 or D20 s). Moreover the definition of pH in D20 is not straightforward. For water again, use as far as possible similar purification procedures of at least similar final purities; use the same salts and same pH for both solutions. For thermal oxidation use a single silicon wafer -

-

-

et al.

for tantalum deposition and cut it in two parts, one for natural the other for enriched treatment, so that the Ta/cm 2 contents of both samples are equal to within uniformity. 2) No isotope effect on atomic thickness should exist for oxygen in the processes considered. As for the latter condition we shall assume that it is satisfied for the coulometric behaviour of anodic oxides or for the stoichiometry of thermal oxides like that of fully oxidized thin film tantalum. As will be shown below in the anodic case the results are consistent with these assumptions. Note that no difference of colour (i.e. optical index) was ever observed between anodic tantalum oxides enriched up to 90% ~sO and films of natural oxygen isotopic composition. Note that we do not use procedures where Idnetics are involved: thermal oxides for example are obtained at thermodynamic equilibrium. Thus in our case isotope effects on oxygen content are most likely to be negligible. Let us hence assume that for two samples, one natural (Sn), the other enriched (Se), the atomic thickness identity is granted, to within experimental reproducibility, the common thickness, in oxygen atom/cm 2, being X. Let us further assume that the amount of oxygen present on the sample surface before oxidation is negligible. The oxide obtained in the enriched medium will then have the isotopic composition of the latter provided isotope effects are here again negligible; writing the isotopic concentrations of the medium as c~6, c17, c18, with c16+c~7+cls= 1, we have X Is =c~sX, etc. X is measured by comparison of Sn to a standard, using nuclear microanalysis; alternately Sn may be a primary or a secondary standard. The precision is set by the knowledge of X and by the accuracy on c~s, etc. The latter information is generally obtained by mass spectrometry and provided by the supplier. The above ideally simple situation is generally not satisfied as: 1) there is a non-negligible amount of spontaneous oxide on the sample before oxidation, which is pure ~60 to within 0.2%; 2) the initial measurement of cl, etc, may be inaccurate and even if it is correct, considerable degradation of the enrichment may occur between the opening of the container and the time when the actual oxidation process takes place; 3) no precise proof seems to exist as to the absence of isotope effects for oxygen on isotopic concentration in the films in anodic oxidation or

THIN FILM S T A N D A R D R E F E R E N C E TARGETS II

at thermodynamic equilibrium at high temeperature. Note that such isotope effects are the very basis of the enrichment procedures: differences between the isotopic composition of water in equilibrium with its vapor may be of the order of 1% in relative terms. For anodic oxidation (1) will be discussed below in detail. For (2) it was our experience that even if handling H~80 using vacuum lines, except when introducing the samples, after many operations a strong decrease of c18 may be observed for c18>80%, although relatively large amounts of water were used (5 cm3). The decrease is due to exchange with atmosphere C20 or dilution with ambient humidity, etc. No exchange with atmosphere 02 occurs. The higher the enrichment the faster is the degradation. It should be noted that we chose to use an oxygenated salt (unlabelled) in our electrolytes to insure a well buffered pH: as will be shown below, this has a negligible influence on the ~80 content of the films. Other oxygenated natural compounds might be a direct source of oxygen and should be avoided. For thermal oxidation (1) must be carefully taken into account. Both the underlying SiO2 layer and the spontaneous oxide formed on the surface of the tantalum deposit may or may not be converted into enriched oxide, according to experimental conditions: H20 or 02 as oxidizing agent, temperature and duration of the equilibration. The isotopic exchange of oxygen in Ta205 with the gas is by orders of magnitude slower than the growth rate9). For (2) the degradation of the labelled gas (H20 or 02) might be very fast if special precautions are not taken to avoid dilution by leakages (both from outside and from wall desorption) and isotopic exchange with the walls of the furnace (generally oxides like silica or alumina; even metallic walls are oxidized and present memory effects). The lower the pressure (and thus the cheaper the procedure), the faster is the degradation. A very thorough knowledge of separated isotope handling is required to minimize these effects. In any case only an in situ mass spectrometric analysis might give c18 etc. with sufficient accuracy, unless high pressures are used. It appears hence that using mass spectrometric data on the oxidizing medium is not a reliable method for constructing accurate 180 or 170 standards. We shall now present a technique which overcomes these difficulties.

715

2.2. THE " 160 DEFECT" METHOD

This method was first proposed in ref. 10 and investigated in ref. 11. It was since used systematically in our laboratory and is the basis of the techniques presented here. Let Sn and Se have the same oxygen content X. Then X18-~-X 17 = X - - X 16,

(1)

i.e. the rare isotope content is equal to the "missing" 160 content of Se. Now, both X and X t6 can be directly measured on Sn and Se restrictively using the reaction 160(0, p)170~. If the X17/X ~8 ratio is known, both X 18 and X 17 may be deduced from eq. (1). Two cases arise: 180 standards." materials highly enriched in ~80 are usedC The ~70 content is then low, c17 ranging from 0.5% to 1% for c~8>80%. The actual values are very scattered from one batch to another, the c17/c~8 ratios ranging randomly from 4× 10 -3 to 14×10 -3 for 10%
neglecting the ~80 and ~70 content of the spontaneous natural oxide layer. Hence:

x~8_

1 (x_x~6).

(2)

l+a

This correction for JTO is anyhow small and a crude knowledge of Clv is enough to get a high accuracy for X 18. Let us make two remarks: - Even if the enrichment is degraded after the initial isotopic analysis, the c~z/c18 ratio remains unchanged, as practically only 160 is incorporated into the medium, whether by dilution or by isotopic exchange. This is not true if degradation would occur by contract with walls which would be labelled previously by exchange with a different, non-natural material (see below). - The spontaneous natural oxide may remain partially unlabelled (see below the case of anodic oxidation). In this case the 170 correction should be applied only to the oxide grown in the enriched medium. For thick enough films this correction is however small, due to the small values of c~7. 170 standards: the same principles hold, with the important difference that in ~70 enriched water c18 is usually ½c17 and c17 is rarely higher than 50%. All data given here are relative to enriched water produced at the Weizmann Institute, Rehovot, Israel, which was used in this work.

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The corrections discussed above are hence most critical. In particular the spontaneous oxide must be taken into account carefully (see below for the anodic case). Alternately, a "double defect" method can be used, measuring both X ~6 and X 18 with nuclear reactions with respect to corresponding standards. The precision on 170 standards will be set by the accuracy of these corrections and cannot match that of the 160 and 180 standards, at least as long as high ~70 concentration water is not available. Oxygen gas enriched up to 90% 170 is available. Standards prepared by thermal oxidation in such gases may be calibrated by measuring both X 16 and X 18 in Se, the precision being good, c17 being high in this case. The great advantage of the 160 defect technique is that, requiring no a priori knowledge whatsoever of the enrichment of the medium and not even a uniform depth distribution of isotopes (insofar that the nuclear reaction technique is used correctly for a not too thick oxide film, see below), the various enrichment degradations or isotope effects mentioned above have no effect on the result; samples made successively in the same way may have of course different isotopic contents if the isotopic composition of the medium is ill-controlled; this has however no bearing on the individual precision for each standard, the method being in fact self-calibrating. The only practical disadvantage of the method is that it requires a - 0 . 9 M e V deuteron beam which also produces an appreciable neutron flux. Some laboratories lack the shielding around their accelerator required from safety point of view and while being able to use (p, ~z) reactions cannot use (d, p) reactions. Standards may then still b e obtained from the isotopic data on the oxidizing medium, minimizing all the sources of error mentioned above; the precision will of course be inferior. 2.3. ERROR CALCULATION FOR THE 160 DEFECT METHOD Let Sn have oxygen content X and let us neglect for clarity the 0.2% correction for the natural abundance of 160. Let X~ = X + A X he the overall oxygen content of Se, allowing for a deviation ++_AX from the atomic identity of the two oxide films. Putting c17 = 0 for clarity, we have: X¢ = X + A X = X~6+X~ 8 (3) Let us assume that at the point of Sn hit by the beam the oxygen content X is exactly known. The

corresponding counting N, may be written: (4)

N n = KX.

K has mean value K0 and relative variance (a/,/N.). X~6 is hence deduced from the corresponding 160 counts from Se, Are, by X~ 6 = J~e//~ ,

(S)

with a corresponding relative statistical standard deviation: X~ 6 -

+

(6)

.

If the counting time (beam charge) for Sn and Se is the same: Ne ~-- c l 6 N n = (1 -c18 ) N~, (6a) and rTx°l~-1 (2-clst~ (6b) Xle 6 x/'Nn \1 - c 1 8 / " Hence from eqs. (3), (5), (6a) and (6b): X~ s = X - N e / K o ,

with a systematic error A X and a statistical error ax°" =

(l --c~s) Nn (2--c~8~ 4 Ko \1--c18 / "

The relative error on )t'~8 is then, summing the two errors and writing: X ~ 8 = c ~ s X = c18N,/Ko: AX~ s j~8

1

[ ( 1 - c l s )(2-c18)] {

%,/Nn

c18

AX -

+

+ -

C18 X

(7)

The second term vanishes for c~8 = 1; this is consistent with the fact that in pure 180 n o 160 measurement is required at all, the overall 180 content being X+_AX. This is not exactly true, as some ~60 may still be present from the spontaneous natural oxide layer, but it illustrates well the calculation. For c18 not too small, the second term may be easily kept negligible (for c~8 = 40% it is 2.3/~/Nn, requiring N n > 5 X 1 0 4 for being below 1%, most easy to satisfy). The first term is inherent in the method and may be kept reasonably small only by taking c18>75%. Thus high enrichments are necessary to preserve good accuracy. As A X is practically equal to the error of reproducibility of Sn (around + 1% for the standards described in part I; see below) we see that the corresponding error is barely larger for Cls>75%. In conclusion, in relative terms the precision of the 180 standards may be kept near 1%, as for the 160 standards. The overall precision is then again around 3%, the absolute calibration

THIN FILM STANDARD REFERENCE TARGETS I1

error of the ~60 standard ( + 2 % ) entering in the overall error through the choice of the absolute scale. For 170 standards the precision will be generally inferior; it might be calculated using the above formula, the corrections for 180 content further increasing the error. 2.4. EXPERIMENTAL DETAILS

Anodic standards will be discussed below. For thermal standards the largest source of artefact may arise during the preparation of Sn. If the same furnace is used both for natural and enriched media, memory effects (exchange with walls, etc.) may label the natural medium, ruining the basic hypothesis that Sn is pure 160 to within 0.2%. Thus two similar furnaces must be used; if this is not possible it is better to use an ordinary furnace for Sn, taking the risk of some increase of AX. Even at room temperature and when handling cm 3 amounts of water, we had to use two different vacuum lines for highly enriched and natural water, to avoid memory effects. It should be noted that thermal oxidation is the most economic method, requiring small amounts of material. Thus in spite of their difficulties they might be useful; they must be used for 170 above 50% enrichments. Let us remark that thermal oxidation of Ta films on silicon may be carried out in water vapor, at pressures if possible above 1 tom The oxide films so obtained seem to have the same properties as those grown in 02 9). This procedure is much easier to apply than oxidation in oxygen gas, as the enriched water may be most easily handled by vacuum distillation or freezing and as the pressure may be controlled by the temperature of the water reserve in equilibrium with its vaporl3). Of course in this case Sn should also be prepared in water vapor and calibrated with respect to an absolute 160 standard. 02 gas (for 90% 170 for example) is usually handled with molecular sieves: this procedure cannot be described here in detail for lack of space. Oxygen pressures around 1 torr are recommended for the treatments. Let us notice that atomic identity may be checked using other methods, like spectrophotometry12), if the identity of the indices is assumed. The measurement, using the 160(d, p)170* reaction, of 160 contents in the presence of large

717

amounts of asO or 170 presents some problems which should be handled with care. The proton peaks from 180(d, p)190 and 170(~, p)lSO are well separated from 160(d, p)170* (this is not true for the protons corresponding to the ground state!). However fi- radioactivity is induced on 180 through. 180(d, a)16N (Tl/2 = 7.4 s, Emax = 10.4 MeV) and 180(d, p)190 (TI/2 = 29 s, Ema×= 4.8 MeV). Similarly /3 + activity is induced on 170 through 170(d, n)18F (TI/2 = 112 ram, Emax = 0.64 MeV). These betas may give background counts, the more so because rather thick mylar absorbers are needed to stop the alphas from J60 and 180 (those from 170 are too energetic to be stopped), thus reducing the proton energy at detection below 1 MeV 2). Reducing the sensitive zone of the detectors to a minimum practically eliminates this effecff); it is advised to hide activated samples which are not being bombarded by a convenient absorber ( N 6 g / c m 2 for 10 MeV f i - , typically 5 mm lead) so that they are not seen by the detector. Care should be taken with very thick targets (above 1500/~Ta205) to avoid cross section variations with beam particles slowing down. The bombarding energy should be carefully chosen so as to remain on the plateau of the cross section between 800 keV and 900 keV 1).

3 Primary electrochemical 180 standards 3.1. EXPERIMENTAL Samples were prepared in the same way as in part I. The JsO enriched normalized water (around 80%) was, as received, highly contaminated with various impurities: it was purified on sodium by distillation under vacuum, as described in ref. 14. A pH 6 buffered solution was prepared with 5% by weight of natural ammonium citrate as in ref. 14. It has been shown using ammonium citrate enriched in 180 to 70% that no isotopic exchange occurs between this salt and water ~5) (the salt must be carefully dried in vacuo to avoid dilution effects) and moreover that only 0.18% of the oxygen in anodic Ta2Os originates in the salt15). Thus the oxygen content of this salt may be neglected. Note that similar results may be anticipated for dilute H2SO4 as this molecule is known to be highly stable against isotopic exchange at low molar concentrations. A special 5 c m 3 cell 16) and sample holder Iv) designed for isotopic solutions (minimizing consumption desirX. APPARATUS AND TARGET STANDARDS

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G. AMSEL et al.

able due to the high cost, while still maintaining good electrochemical conditions) were used; note that the same type of cell was used for S~ samples in part I. 3.2. RESULTS The coulometric measurements were carried out again fully, as in part I, so as to check reproducibility and atomic identity, with the highest possible precision. The ]80 standards described here are hence termed "primary". Fig. 1 shows the results of the coulometric measurements. As in part I for S, most of the points are within +0.5% of the estimated mean value 2.765× 10 -3 C / V . c m 2. This value is, however, slightly lower, by 0.4%, than the value found in natural solution in part I: 2 . 7 7 5 x l O - 3 C / V . c m 2. Moreover the scatter is also higher for the points outside the +__0.5% range, which are systematically below the mean value. These slight differences, which seem to be significant, are unexplained. They might be due to the different purification procedures. Anyhow, considering the different origins of the water, the results still show a satisfactory agreement between two completely different experimental series and hint to a good validity of atomic identity. They also show that for ]60 standards triply distilled water is to be preferred. Fig. 2 shows the number of cdunts from 1 6 0 ( d , p ) 1 7 0 ~ in the Se samples vs voltage, measured exactly in the same conditions as for Sn in part I. The inset shows the differences between the best linear fit and the actual countings. The scatter of the points is here much larger, reaching r

~

~

r

r

t

t

1

I

i

I

I

I

N=415 x VO+ 6000

(9

2.80

| !

|

_+0.5%

,IX

4.

1

U

tO ¢D Z

_+5% in relative terms, as compared to _+ 1.5% for S, in part I. It appears clearly that the reasons for this spread are here quite different, i.e. not related to non-uniformity. The latter should yield + 1.5% here again, and should increase in absolute value with V0, as it does in fig. 1 of part I. The phenomenon is related to the behaviour of the spontaneous natural oxide layer present on the metal before anodization, of oxygen content X0. It has been shown in ref. 14 that most of this layer retains its natural isotopic composition after anodization in an enriched solution. Hence X0 contributes fully to the t60 content, while in the thickness ~ ' - - X - X o , added by anodization [and given by the coulometry, according to eq. (4) in part I] the ]60 content is reduced by the factor g6. A good estimation of g6 is the ratio of the slopes of best fits in fig. 1 of part I and fig. 2 in part II, i.e. q 6 - - 2 2 % . As I"0 is equivalent to about 3 V (see fig. 3, part I), i.e. + 2 V on the V0 scale, the smaller slope explains the larger extrapolation to zero count

|

,_

ou

I

2.75

,~-lo

~L

i

O

.-I :D 2.70

0

10

20

30

40

50

60

70

80

VOLTS

O u

-10

t

J

i

0

10

20

i

30

I

[

i

I

I

40

50

60

70

80

VOLTS 0

10

20

30

40

50

60

70

80

VOLTS Fig. 1. C h a r g e / c m 2 divided by ( V o - 2 V) vs Vo , as in part I, fig. 3, but for an 180 enriched solution.

Fig. 2. Counts from the ]60(d,p)lTO e reaction vs V0 for a 78% 180 enriched solution. The inset shows the differences between the counts and the best linear fit. Counting conditions were identical to those in fig. 1, part I.

THIN

FILM

STANDARD

(VoN--14V) than in fig. 1, part I, as the line rotates with decreasing c16 around the fixed point at V0 = 2 V. It may be seen in fig. 2 that the scatter of the points does not increase with V0. It seems hence to be due to a voltage independent 160 quantity. This might have three origins: - a spread +-AXo of X0, - the transformation of a fluctuating fraction of X0 into tsO labelled oxide through isotopic exchange, - the random presence of an oxigenated contaminant which does not take part in the oxidation processes (varying oxygen content of the metal or a deposit on the surface). At V0 = 40 V the extra scatter in fig. 2 with respect to So, taking into account statistics (__+150), and the ~___600 observed for S~ (see part I) reduced by c16, i.e. N ___120, is around ___800, which corresponds to an equivalent thickness of about _+0.4V. Now the spread in coulometry, _+0.5% around 40 V for S~ allows only for a maximum AXo of ___0.2V. Hence most probably the scatter in fig. 2 is due to all three reasons put forward above. We may conclude that this effect could be observed here only because all the 160 counts from anodization were reduced by 0.22, but that most of the samples are good for the application of the 160 defect method. Those samples lying outside the -+0.5% range in coulometry or the -+ 1200 range in 160 counts were rejected, as in part I, being considered artefacts. These results strengthen our confidence in our samples and allow us to apply the 160 defect method to each individual sample. Thus although the 160 and hence the 180 contents are scattered, the 180 content of the individual samples may be calculated with good accuracy. The only difference with the 160 case is that it is not possible to construct standards as accurate for the reproducibility of their 180 content, although each sample is known for JsO content individually to within about --3% absolute and +1% relative error, provided each is measured for ~60 content by the 1 6 0 ( d , p ) 1 7 0 ~ reaction. Note that due to the possible presence of a slight random ~60 contamination, thicker standards seem more reliable, a good choice being 80 V, i.e. around 1300 A. No variation of the 180 content of these standards was detected even after long bombardment, in contrast to 160 in S~ which was observed to increase slowly (see part I). This confirms that the

REFERENCE

TARGETS

719

II

latter effect is due to radiation induced oxidation, without isotopic exchange, leaving the 180 content practically invariant. The same impact point should not be used for too long times however, so as to avoid energy loss effects due to a too heavy beam induced carbon contamination from the residual vacuum. 4. Secondary

electrochemical

isotopic standards

Good secondary isotopic standards may be obtained with simpler techniques, requiring smaller amounts of enriched water. The principle is to prepare Sn and Se in exactly the same set-ups and to calibrate Sn with respect to an absolute 160 standard prepared as described in part I. Atomic identity is obtained by oxidizing Sn and S~ to the same voltage, while controlling current density, current uniformity, temperature, etc., as well as possible, so as to keep g(i, T) as reproducible as possible for Sn and S~ [see eqs. (2) and 7a in part I]. Any salt may be used here (in particular oxygen-free salts), like KI, KCN, etc., with which tantalum oxidizes well as the absolute value of g(i, T), is not relevant, provided it is the same for Sn and Se. The concentration should be high enough to insure a low resistivity of the bath, and hence good current flow uniformity. Salts must be carefully dried before dissolution in enriched water. Considerable economy of enriched water may be reached by building miniature cells. Such a celt was described recently by Phillips and Pringle~8), requiring only 0.5 cm 3 of water. In our laboratory an O-ring type cell is used, the bottom of the cell being the tantalum foil on which the O-ring is pressed by a nylon cylinder. Enriched water is injected with a syringe, taking care not to form bubbles (bubble formation is readily seen on the oxized area, which should be quite circular, provided the voltage chosen yields a bright color). The cathode is a horizontal platinum foil touching the solution. The content of the cell is N 1 c m 3, the diameter of the oxidized area ~ 10 mm. The main source of error is here the ill-defined boundary of the oxidized area, which may reach a width r / = 0 . 4 m m with the Impervia W185 Orings used (as is seen easily under a microscope). For a circle of radius r the relative error on area, hence on i, being 2~l/r, to keep the latter below 15% [i.e. AX/X below 0.5% ; see eq. (7a), part I] we need r>~5 mm. Thus 10 mm is the minimum diameter; masking techniques may be used to reduce this figure but this complication does not X. A P P A R A T U S

AND TARGET

STANDARDS

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G. AMSEL et al.

seem wortflwhile. Another source of error could arise from high irreproducible electronic losses at the boundaries, reducing the real ionic current density i. The overall reproducibility of i is easily checked by monitoring the time needed to reach a given voltage, for fixed input current. We have thus observed that for unpolished tantalum and i = 1 m A / c m 2 the electronic losses may reach 50% in such a cell, while they are reasonably small and constant for polished samples. Finally temperature should be controlled to within - 2 ° C ( _ 0 . 2 5 V equivalent thickness variation around 45 V; see part I). To keep highly enriched solutions undegraded the operations must be carried out in a glove box with very dry atmosphere. If the same cell is used for S~ and Se, it should be very carefully rinsed and dried before passing from enriched to natural solutions, to avoid ~80 pollution of the latter. This simple method may give secondary 180 or 170 standards with still a good accuracy. For 180 it is reasonable to assume that the accuracy will be better than 4%. 5. Discussion

The standards described may be used as well for overall Oxygen isotopic content measurements as for average isotopic concentration, X17/Stota I or Xls/Xtota~, measurements. Due to the presence of natural oxide before enriched oxidation, the isotopic concentration in the external layer of the standards may be somewhat different from the average concentrations deduced from the overall isotopic contents. This may have an influence on SIMS measurements for example. This point will be discussed in our forthcoming paper on isotopic analysis using tantalum oxidation. Thin secondary isotopic standards, or films on

other backings may be obtained from those described here by recalibration using the 180(p, ~z)~SN or 170(d, ~)lSN reactions. The authors are indebted to A. Laurent for his assistance in the electrochemical experiments and his invaluable help in handling the enriched isotopes; he also constructed the O-ring cell described. Without the constant advice and support for 15 years of the staff of the Isotope Department of the Weizmann Institute this work could not have been carried out; we thank in particular Prof. D. Samuel and I. Wassermann for their help and patience. References t) G. Amsel and 13. Samuel, Anal. Chem. 39 (1967) 1689. 2) G. Amsel, J. P. Nadai, E. D'Artemare, D. David, E. Girard and J. Moulin, Nucl. Instr. and Meth. 92 (1971) 481. 3) G. Amsel, C. Cherki, M. Croset, G. Feuillade, J. P. Nadai, C. Ortega, S. Rigo and J. Siejka, Czech. Chem. Comm. 36 (1971) 883. 4) G. Amsel, in Physics of electrolytes, vol. i, (ed. J. Hladik; Academic Press. London, 1972) pp. 127-149. 5) G. Amsel, in Les isotopes de I'Oxyg~ne (ed. French Atomic Energy Commission, Cadarache, 1972) pp. 138-149. 6) B. Agius and J. Siejka, J. Electrochem. Soc. 120 (1973) 1019. 7) Preceding article. 8) j. Siejka and C. Cherki, Extended Abstracts Electrochem. Soc. Meeting, Cleveland (October 1971) p. 231. 9) S. Rigo, to be published. 10) G. Amsel, Thesis (Orsay, 1963). 11) j. p. Nadai, Thesis (Orsay 1967). 12) j. p. S. Pringle, J. Electrochem. Soc. 119 (1972) 482. 13) S. Rigo, Thesis (Paris, 1977). 14) j. Siejka, J. P. Nadai and G. Amsel, J. Electrochem. Soc. 118 (1971)727. 15) C. Ortega, to be published. 16) G. Amsel, Ann. Phys. 9 (1964) 297. 17) G. Feuillade and R. Jacoud, Electrochim. Acta 14 (1969) 1297. 18) j. p. S. Pringle, Nucl. Instr. and Meth. 135 (1976) 389.