Microanalysis of the Stable Isotopes of Oxygen by Means of Nuclear Reactions Georges Amsel Ecole Normale SupPrieure, Laboratoire de Physique, Paris, France
David Samuel Isotope Department, Weizmann Institute of Science, Rehouoth, Israel
A method has been developed for the determination of very small quantities of the stable isotopes of oxygen. The method is based on the use of the nuclear reaction s O18( p,a)N 15; 0 18(d,a)N 16; O16(d ,a)N 14; 018(d, p)0lg; and Ole(d,p)017. Either the absolute amount of each can be determined isotope of oxygen or the ratio 01*/016 by counting the charged particles emitted on bombardment with protons or deuterons. The reactions used are highly specific. Relatively low bombarding energies (up to 600 keV) can be used. The method is particularly useful for the determination of isotopic oxygen on or near the surface of solids-such as oxide films, silicates, etc.-and for the isotopic analysis of very small quantities of organic materials. Amounts gram of 0 1 8 can be detected. This of the order of method of analysis is rapid and nondestructive. The many advantages of this method, over all other methods of both nuclear and conventional isotopic analysis, are discussed. Methods of correcting for targets of finite thickness are given.
OWING TO their short half lives, the radioisotopes of oxygen are of very limited use as tracers. Even oxygen-15, the longestlived radiosotope, has a half life of only 2 minutes. Tracer studies with the stable isotopes 0 ’ 7 and 0 ’ 8 have been restricted to relatively simple chemical and biochemical studies because of the difficulties of converting the products of such reactions to a form suitable for mass spectrometric analysis ( I ) . Except for simple molecules, the direct mass spectrometric analysis of volatile products was, until recently, only rarely undertaken, owing to the fact that fragmentation of the original compound usually yields a complex mass spectrum which is not easily interpreted. Therefore, in most studies involving the isotopes of oxygen, the oxygen in the compounds to be analyzed must first be converted to a simple gas such as oxygen, carbon monoxide, or carbon dioxide for isotopic analysis. Quantities of the order of 0.1 ml of these gases at usually required for routine N.T.P.-i.e., 0.005 mole-are mass spectrometry, which sets the limit of sensitivity at about 1 pg of oxygen-18. This limit is difficult to attain in practice owing to the losses involved in the conversion of the compound to a suitable gas and in the purification of the gas. The stable isotopes of oxygen are being increasingly used as tracers in studies of surface reactions and solid-state phenomena. Such studies include the anodic and thermal oxidation of metals and semiconductors, various absorption phenomena, diffusion in solids, and the analysis of ultrapure materials such as beryllium and high resistivity silicon. Many of these projects deal with thin oxide films which involve very small quantities of very stable material, usually in the submicrogram range, which are extremely difficult to convert to a gas for mass spectrometry. In some cases such surfaces can be examined using an ion(1) D. Samuel, “Oxygenases,” Academic Press, New York, 1962, Chap. 2.
beam probe (2), which directly volatilizes the material at the surface for analysis by mass spectrometry. The yield of sputtered atoms emitted in a charged state largely depends on the structure of the material to be analyzed, but this difficulty is less serious if only relative isotopic compositions are measured, This method has not yet been used for the isotopic analysis of oxygen. In certain specific cases infrared spectrometry can be used for the isotopic analysis of 0 ’ 8 in surface layers, as was shown in the study of oxidation of silicon (3). However, this method is limited in sensitivity and cannot be generally applied unless accurate relative absorption intensities can be determined. Although fairly high enrichments of 0 ’ 7 are now available, the low sensitivity of 0 1 7 NMR spectrometry precludes its use for studying surface phenomena at the present time. Activation analysis has frequently been used as a practical alternative method of analysis to mass spectrometry. Here the nuclide to be estimated undergoes a suitable nuclear reaction and the activity of the product nuclide is measured. This method of analysis requires that the product nuclide be of sufficiently long half life to be counted accurately and also be readily distinguishable from all the nuclear products of the nuclides originally present in the sample. Thermal neutron capture-Le., 0 ’ 8 (n,y)OIO-could in principle be used for the analysis of 0 1 8 but is limited by the short life time of the product nuclide 0 1 9 (29 seconds), by a low capture cross section, and by the high background activity produced in the sample by competing reactions. This method has, however, been suggested ( 4 ) for the determination of oxygen in water and various compounds. The lower limit of detection was set at 0.1 gram of oxygen which corresponds to about 2 mg of 0l8.Owing to the experimental limitations, the precision is not very good. A more practical nuclear reaction for activation analysis of 0 1 8 is the 0 ’ 8 (p,n)F18 reaction in which the annihilation y-rays from the product nuclide fluorine-18 (a positron emitter with a half life of 112 minutes) are measured (5-7): the threshold energy and cross section determined by Hill and Blair (8) for this reaction are given in Table I. The use of the 0 1 8 (p,n)F18 reaction in practice, however, requires a bombarding energy above 3 MeV which means that either a cyclotron or a rather large van de Graaff accelerator must be used (5-7, 9). A (2) R. Castaing and G. Slodzian, Compt. Rend., 255, 1893 (1962). (3) J. R. Ligenza and W. 6. Spitzer, Phys. Chem. Solids, 14, 131 (1960). (4) T. Kamemoto, Nature, 203, 513 (1964). (5) I. Fogelstroem-Fineman, 0. Holm-Hansen, G, M. Tolbert, and M. Calvin, J. Appl. Radiation Isotopes, 2, 280 (1957). (6) A. Fleckenstein, E. Gerlach, R. J. Janke, and P. Marmier, Pfuegers Arch. Ges. Physiol., 271, 15 (1960). 33,583 (1961). (7) B. A. Thompson, ANAL.CHEM., (8) A. Hill and S. M. Blair, Phys. Rev., 104, 198 (1956). (9) R. H. Condit and J. B. Holt, J. Electrochem. SOC.,111, 1192 (1964). VOL. 39, NO. 14, DECEMBER 1967
1689
number of competing reactions also producing positron emitters are liable to occur at these energies, of which the most serious are the N14 (p,a)C" and C13 (p,n)N13 reactions, because carbon-l l has a half life of 20.5 minutes and N13 has one of 10 minutes. Inasmuch as traces of carbon and nitrogen occur in nearly all target materials, it is necessary to delay counting for at least 2 hours in order to reduce the N13 and 1211 activities to acceptable limits. In addition, many middle Z elements have a low threshold for (p,n) reactions and high cross sections, owing to the absence of a coulomb barrier for the emitted neutrons. Thus, proton bombardment of several of these elements-including nickel, copper, and zinc-gives rise to very high long-lived activities. Such elements must not be present in the target in concentrations greater than 10 ppm; hence only high grade tantalum (6) and platinum (5) foils can be used as supporting materials for the samples to be analyzed, Alternatively, the fluorine can be separated chemically from other product nuclides for counting. However, this technique entails the destruction of the sample and involves a fairly complex chemical process. Taking all these factors into account, the estimated limit of sensitivity (5)of the (p, n) reaction is of the order of 10-3 pg of 0 1 8 , which is rarely attained in practice. This technique has, however, been used (9) for a study of oxygen diffusion and inclusion in metals, using autoradiography as a means of detection. However, even when using 2.7-MeV protons a large number of elements (including Li, B, Ca, Ti, Ni, Cu, Si, Zn, Cd, and Ag) can cause serious problems in analysis because of competing reactions. Fluorine-18 activity can also be induced in oxygen-16 by the 016(He3,p)F18reaction (10) and, using 25-MeV alpha particles, by the 016(a, pn)F1e reaction (11). Both of these methods of activation analysis which require special accelerators give the total amount of 0 ' 6 in the area bombarded. Work now in progress (10) has improved the applicability of We3 activation analysis by using both 016(He3,p)F18and Ole(He3,p)FZ0 reactions. Owing to the short 20-second half life of the F", the accuracy of this method is not very good and relatively large samples (1 mg) have to be used. This method may, however, become practical for bulk analysis when He3 accelerators are more commonly available. Owing to the long range of 25-MeV a-particles, the OIe(a,pn)F18 reaction is of practical value for the determination of oxygen in the balk of solids and has already been used (11) for the determination of oxygen in transistor grade silicon, with a sensitivity of 10-3 PPm. Amiel and coworkers (12) have used the 01*(t,a)N17 reaction in which the product N17 with a half life of 4.14 seconds decays by beta emission to an excited stale of 0 1 7 which in turn emits a neutron to decay to 0l6. This delayed neutron emission is highly specific for OIS but also requires a special accelerator for tritions which is not easily available. Pile-produced tritons from the Li6(n,a)T reactions can also be used (12),but the efficiencyof this method is low. An alternative to activation analysis is the direct detection of charged particles emitted in a suitable nuclear reaction. The simplest of these reactions are those in which alpha particles or protons are produced, which are readily detected by semiconductor detectors that are now in common use. In contrast to the background counts inevitably obtained in y-ray detection (10) S . S. Markowitz and J. D. Mahony, ANAL.GHEM.,34, 329
(1962).
(11) T. Nozaki, S . Tanaka, M. Furakawa, and K. Saito, Nature, 190, 39 (1961). (12) S . Amiel and M. Peisach, ANAL.CHEM.,35, 323 (1963).
1690
o
ANALYTICAL CHEMISTRY
in conventional activation analysis, charged particle detection is virtually background-free. The general principles of this method have already been discussed (13). Its application to specific problems of isotopic analysis of oxygen will be described here in more detail. The nuclear reactions used are 01*(p,a)N16, 018(d,a)N1e,0 1 6 (d,a)N14,0 ' 8 (d,p)OIB, and Ole (d,p)017. Because the 0 1 6 (p,a)NI3reaction has a large negative Q value, it does not produce a-particles at low energies. The characteristics of these reactions are summarized in Table I (page 1694). Other nuclear reactions-such as the 017(d, p)018 and OL8(Hea, (Y)O" also produce charged particles which could be used for isotopic analysis, but appear to be of less practical interest and will not be discussed here. As seen from Table I, these reactions require relatively low bombarding energies (of about 600 keV) for high counting rates. This enables isotopes on or near the surface to be determined very readily. The experimental set-up is also fairly simple and yields immediate results. This is particularly useful for routine analyses. The technique for the determination of oxygen isotopes on or near surfaces will be described in detail here. More refined applications, such as the isotopic analysis of oxygen as a function of depth in solids, will be the object of a forthcoming article. EXPERIMENTAL
Instrumentation. The experiments described here were conducted using the 2-MeV van de Graaff accelerator at the Ecole Normale SupCrieure, Paris. Similar results were obtained with a 600-keV electrostatic SAMES accelerator. In order to study this method of analysis, 018-enriched aluminum oxide and tantalum oxide targets were prepared at the Isotope Department of the Weizmann Institute of Science, Rehovoth, Israel, using the anodic oxidation processes described previously (14, 15). The thicknes? of the aluminum oxide targets ranged from 100 to02500 A, and that of the tantalum oxide targets up to 2000 A. The emitted charged particles were detected with different types of semiconductor detectors depending on the nuclear reaction used. The principles of alpha-particle or proton detection with such detectors have already been described (16, 17). The advantages and disadvantages of using each type of nuclear reaction are discussed in detail below, Conventional amplifiers are used for the counting of the pulses as neither high energy resolution nor high counting rates are required. The spectra were registered with an INTERTECHNIQUE multichannel analyzer using ORTEC detectors and amplifiers, as described in detail below. The energy of the beam was chosen so as to reduce the range of elastically-scattered particles from the incoming beam well below the range of the alpha-particles or protons produced. Back-scattered particles were stopped by Mylar films (see below). The residual energy of the observed particles and their energy spread is such that detection is straightforward. The detector is placed at an angle of between 150" and 165" to the axis of the beam (Figure I), as the angular distribution of the emitted alpha-particles is sharply peaked at backward angles (14). However, at the larger angle only small detectors can be used without touching the beam. The best compromise appears to be 150" to the axis of the beam. The target holder is used as a (13) G. Amsel, L. A. L. Orsay Rept. No. 1053 (May 1963). (14) G. Amsel, Ann. Phys., 9, 297 (1964). (15) G. Amsel and D. Samuel, J. Phys. Chem. Solids, 23, 1707 (1963). (16) G. Amsel, P. Baruch, and 0. Smulkowski, IRENucl. Sei. NS, 8, No. 1, 21 (1961). (17) G. Dearnaley and D. C. Northrup, "Semiconductor Counters for Nuclear Radiation," Wiley, New York, 1963.
Tarsel. holder-Faraday
Collimator
2rnrn diameter
CUP
-1
Collimator
3rnrn
Aoving. detector holder Main A m p l i r i F Biased Amplifier 3 RTEC
I
input Multi Channel Coincidence Input
Figure 1. Experimental set-up Target holder and counting equipment shown schematically (in practice a target holder for several targets is used). Detector can be moved back and forth to vary solid angle. Multichannel analyzer operating in coincidence with single channel analyzer permits ready adjustment of window settings for peaks being counted
Faraday cup. In order to prevent the escape of secondary electrons from the target holder, the set-up shown in Figure 1 was used. Two energy-defining collimators are separated from the target holder tube by a 20-cm-long borosilicate glass tube. After a short time of operation, the charge collected on the inside surface of the tube is sufficient to repel secondary electrons produced by both collimator and target. This ensures the complete electrical insulation on the Faraday cup. A third collimator, with a slightly larger hole in order to avoid contact with the beam, reduces the solid angle of escape of secondary electrons from the target, By this means the effect of secondary electrons is reduced to a negligible amount in comparison with the intensity of the incident beam. The semiconductor detector itself is insulated from the Faraday cup. The Mylar absorber (which is metalized) is stretched tightly in front of the detector, with the metalized side facing the target and connected to the cup, and prevents the escape of secondary electrons from the Faraday cup. The beam current is monitored by a highprecision current integrator. A good vacuum (of the order of mm Hg) must be maintained in the target area to avoid both carbon deposition and ionization of the residual gas. A diffusion pump and liquid air trap are connected directly to the target chamber which is separated, as regards pressure, from the test of the accelerator by the first collimator (see Figure 1). It should be noted that mercury vapor can poison goldcoated semiconductor detectors and care should be taken to have an efficient liquid nitrogen trap between pump and target area, if a mercury pump is used. The target should be in good electrical and thermal contact with the Faraday cup. Nonconducting targets should be coated by vacuum deposition with a very thin film of metal (of the order of micrograms per cmz) in order to ground the surface of the target and thereby avoid electrostatic effects, including breakdown phenomena which can induce spurious counts in the detecting system. It should be noted nevertheless that in some casesLe., when volatile materials are studied-a low gas pressure can be maintained in the target area. A differential pumping technique must then be used to isolate the accelerator vacuum from the latter. Counting Rates and Sensitivity. The counting rate, N-Le., the integral of the peak due to a given particle group-is given by N = 51 i n a(E) for an ideally thin target, where i is the current expressed as the number of particles per unit
time in the incident beam, Q is the solid angle of the detector presented at the target, c ( E ) the differential cross section at bombarding energy E, and n the number of reacting nuclei per cm2. In this expression the counting efficiency of the detector is ignored because it is essentially unity. Two quantities can be deduced from such a measurement, the number n of nuclei per cm2 or the total number of nuclei Sn in the area S of the beam (assuming that the beam has a constant current density). The first quantity is useful for uniform targets such as oxidized surfaces and the second for nonuniform targets such as those prepared from the products of chemical or biological reactions (see below). The sensitivity of the method is determined by the solid angle, the maximum permissible current, and the minimum acceptable counting rate. In principle, the solid angle could be increased to nearly 2n by using several detectors. In practice, a 2-cm2 detector can be placed at about 2-cm distance presenting a solid angle of 0.1 of 2n. The current is limited by the permissible temperature rise at the target, which depends on the target material. When necessary, the diameter of the beam can be increased so as to reduce the surface density of deposited power. The exact conditions which are used depend on the material to be analyzed. Owing to the low beam energies used (about 600 keV), in a typical experiment 1 FA was deposited on an area of 1 mm2. Such a narrow beam permits different points on a surface to be analyzed separately. If it is assumed that the lowest acceptable counting rate is 10 counts per minute, then in 1 hour-under the above conditions-a statistical precision well below 10% can be obtained, provided no changes in the target material occur during this time. Using these figures, under the conditions stated, the minimum quantity of oxygen detectable is about 10I2 atoms corresponding to l O I 4 atoms per cm2 for 1 mb per steradian differential cross section assuming that the sensitivity is not reduced by background from competing reactions. This result, together with the appropriate cross sections, was used to estimate the maximum sensitivities given in Table I. Experimental details of isotopic analysis of oxygen in some examples are given below. Analytical Techniques. Absolute quantities of oxygen isotopes in very thin targets (as defined below) can be determined by comparison with thin standards of known thickness and composition. For 0 1 8 analysis, foils of tantalum anodically oxidized in OI*-enriched water of known composition are used. To avoid problems of isotopic dilution and exchange, 1% KC1 solutions in 0 ' 8 enriched water were used as the electrolyte. The uniformity and reproducibility of these films make them ideal standards for this purpose. The thickness of th? oxide layer is proportional to the applied voltage (16 A/V of Ta20s) (18). Tantalum foils may, however, be a source of error when both 0 l 6 and Olaare to be determined, because an appreciable amount of oxygen may be absorbed in the metal (7). Therefore, high purity tantalum foils should always be used, Aluminum can also be oxidized anodically in labelled 0 1 8 enriched water containing 018-labelled ammoniu? citrate (14). This yields films with a thickness of 13.7 A/V-Le., 0.47 pg/cm3A1203 (19). Although aluminum does not absorb appreciable amounts 06 oxygen, the natural oxide layer (usually about 50-100 A thick) must be taken into account (14, 15). This problem can be overcome by calibrating the enriched oxide against an oxide layer of the same thickness with the natural abundance of oxygen isotopes. This procedure involves a long counting time and need be made only once for each set of standards. Alternatively, highpurity silicon wafers can be anodically oxidized in dry glycol (18) A. Charlesby and J. J. Polling, Proc. Roy. SOC. (London), 227A,434 (1955). (19) L.Harris, J. Opt. SOC.A m . , 45,27 (1955). VOL. 39,
NO. 14, DECEMBER 19 67
0
1691
In the preceding section, the counting rate for ideally thin targets was defined. However, real targets have a definite thickness and the bombarding particles are slowed down when traversing that part of the target material which contains oxygen. Hence the bombarding energy E, and therefore the reaction cross section u(E), are not uniquely determined. If AE is the energy lost by the particles in the oxygen-containing part of the target at a given bombarding energy Eo, it is usually referred to as the “thickness” of the target, expressed in keV energy units at a particular energy EO. The effect of the variations of cr(E) for such a thickness must be calculated-Le., for EO - AE < E < EO. A target is defined as “very thin” if the variations of u(E) over this interval are negligible. The bombarding energy Eo must therefore be chosen in such a way as to obtain a stationary cross section in its vicinity (within AE for energies below Eo). This condition is of prime importance (13); if it is fulfilled, even relatively thick targets can still be considered as “thin.” In practice, however, this cannot always be obtained (see discussion) and one has to assume that u(E) varies slowlyLe., there is no resonance in the vicinity of EO. This means also that the cross section can be written as a Taylor series limited to its first term-that is, for an arbitrary increment 6E(6E < 0):
,I
/
3
I
I
1
400
500
600
This sets the condition for a “very thin target” as
E KeV
Figure 2a. Cross section of Ol*(p,~r)N*j reaction as function of bombarding energy at 165
I n practice there are three methods of dealing with real targets. (a) I n the determination of oxygen isotopes in very small samples-such as biological material deposited on a suitable support-the deposit can be made thin enough to have a negligible effect on the energy loss of the beam in the material to be analyzed (see Appendices I and 11).
Near 610 keV the rate of increase is much greater as 628 keV resonance (see Figure 2b) is approached
containing 0.4zKNODand 0.4% O18-enrichedwater (20,21). The thickness of these films is 6.2 A/V (18). The 0 1 s content of these silicon oxide films can be determined by comparison (as in the case with aluminum oxide films) with an oxide layer formed electrochemically under the same conditions with the same materials containing the natural abundance of oxygen. These (Sios) films are, however, more difficult to prepare than other standards but require smaller quantities of isotopically enriched materials.
6000,
I
1
I
I
I
(20) P. F. Schmidt and W. Michel, J . Electrochem. Soc., 104, 230 (1957). (21) G. Amsel, E. D’Artemare, M. Croset, J. P. Nadai, and D. Samuel, unpublished work, Ecole Normale, Paris 1966. I
I
I
I
I
1
0 CHANNELS
Ep KeV
Figure 2b. Absolute cross section of 018(p,~)N1b reaction as function of bombarding energy Detector placed at 165” and special techniques described in (23) used.
1692
o
ANALYTICAL CHEMISTRY
Figure 3a. Typical alpha spectrum from 0 1 8 ( p , ~ r ) Nreaction ~~ at 500 keV after passing through a 6-micron Mylar absorber Window of single channel analyzer should be set between channels 150 and 170. Origin of energy scale is channel 0,12 keV per channel
(b) When identical samples of different isotopic composition are to be analyzed-such as oxides formed by thermal or anodic oxidation in different media-a comparison of counting rates at the same bombarding energy gives the relative isotopic composition directly. Because the stopping power of the various isotopes of oxygen are almost identical, oxygencontaining targets of similar thickness can be compared. (c) When neither 'of these procedures can be used, and if high precision is wanted, a correction factor has to be applied for finite target thickness (See Appendix I). Irregular and nonuniform targets, such as those obtained by evaporating a drop of solution of material from a chemical or biological reaction on a solid backing, can be analyzed by this technique. A gold backing is particularly suitable for this purpose because it is not oxidized, does not absorb oxygen appreciably, and has a high Z value. Gold can be obtained very pure, and is a good conductor of heat, which reduces the rise in temperature of the target on bombardment. The simplest way of evaluating the total amount of oxygen (or of 0 1 8 within the area of the sample) is to use a beam which covers the entire area. This can be achieved by using large-diameter collimators and defocusing the beam in order to have a uniform current density over a relatively large surface. The spread of the beam can be verified by observing the fluorescence induced when a quartz disk is placed in the target position. The total amount of oxygen can be calculated by comparing the yield for the material to be analyzed with that for the standards described above, taking into account the area of the beam on the target. It should be noted that a technique of transfer of organic materials from paper chromatograms to solid backing has been developed by Calvin and coworkers ( 5 ) . This extends the usefulness of these methods to many fields such as biology, where only small amounts of material can be separated from living systems.
z
RESULTS AND DISCUSSION
The total amount of oxygen-18 can be determined by using the 0 I 8 (p, a)N15reaction. The characteristics of this reaction are given in Table I. The cross section of this reaction below 600 keV measured at an angle of 165" with respect to the beam, by the methods deMeasurement
of
Oxygen-18.
018(p ,a 1 NI5 770 KeV
3 Figure 3b. Typical alpha spectrum from 0l8(p,a)NI6 reaction at 730 keV using a 13-micron Mylar absorber 9.6 keV per channel
scribed above on a T a z 0 5target enriched to 9 2 S z in 0'8, is shown in Figure 2a. The curve can be expressed as an exponential of the form u = uoexp EIEowhere EO= 58 keV and u0 is 3.38 x mb between 400 and 600 keV. This simple relationship readily enables a correction to be made for the finite thickness of the target (See Appendix 11). Such corrections are minimized when operating in the vicinity of the plateau at 730 keV, shown in Figure 2b, where the cross section i s even higher. However, these energies require the use of a comparatively powerful accelerator, such as a 2-MeV van de Graaff. Using a proton beam the only particles produced at these energies are a-particles. Back-scattered protons from beams of up to -600 keV can be absorbed by a Mylar film 6 microns thick. A typical spectrum registered under these conditions is shown in Figure 3a. In order to reach the plateau shown in Figure 2b at 730 keV a 13 micron-thick Mylar film must be used. The corresponding spectrum is shown in Figure 3b, where the absolute width is greater than in Figure 3a because of straggling effects in the thicker absorber. Because the resolution in this case need not be particularly good, this is not a disadvantage. The detection of a-particles, therefore, presents no problem and semiconductor detectors of any thickness above 30 microns can be used, coupled to a low noise amplifier. In the vicinity of the alpha peak, there are practically no background counts. The Q-values for Li7, Bl1, N16, and Fl9are considerably higher than that of OI8 and lead to groups of a-particles which are well separated from those that are being counted, Lie bombardment leads to a-particles or He3particles with energies below 3 MeV. AlZ7and C13' have much lower Q-values and the corresponding a-particles are almost completely absorbed in the Mylar film. The only reaction which could lead to a background is that due to K41present in a natural abundance of 7 with a Q value of 4 MeV. The effect of the coulomb barrier of this middle Z nuclide on the incoming protons makes interference by this reaction at these low energies negligible. On the other hand, because the 0l8cross section is exceptionally high, and as there is virtually no background, the sensitivity of this method is limited only by the time available for a single measurement and the stability of the target material to bombardment by the beam. At 600 keV-using the estimated limit of detection of 10l2atoms for 1 mb per steradian differential cross section given above-the limiting sensitivity of this method is about 2 X 10" atoms of 0l8. The technique has been used for the investigation of the source of oxygen in the anodic oxidation of aluminum (15), tantalum (IS), and silicon (21) using 01* labeling and of the diffusion of oxygen in quartz (22). The following experimental counting rates illustrate the potentialities of the method. A 3-cm2ORTEC surface barrier detector coupled to an ORTEc 109 preamplifier, was placed at a distance of 55 mm from the target and at an angle of 150' from the beam, On bombarding a 55-volt-thick standard TasOj foil (880 A; natural isotopic abundance) with a l p A 730-keV beam for 3.5 minutes-Le., 210 pcoulombs-2900 counts were pg/cm2 oxygen-18 at recorded. This represents 3.7 X pg oxygennatural abundance, or a total amount of 3 X 18 under the beam area. When a TazOs target of the same thickness, but containing 92 atom 0l8,was used, the count was 134,500 for a 23-pcoulomb beam. This gives a 1 precision in 2 seconds of beam, provided low dead time electronics
z
z
(22) A. Choudhury, D. W. Palmer, G . Amsel, H. Curien, and P. Baruch, Solidstate Cornmuns. 3, 119 (1965). VOL. 39, NO. 14, DECEMBER 1967
1693
~~
Table I. Nuclear Reactions Used to Measure Oxygen Isotopes 01*(p,n)F18 O1*(p,a)N16 0 lS(d,a)NlG -2.45 3.97 4.24 Total cross section 40 75 5 (mb) 0.6 (at useful energy in 2.7 0.6 MeV) 2 Energy of particles 2.5 at detector (MeV) (Exptl.) Li, Cia, N14,Cu, Zn None NI4,F19 Competing nuclei Methods of coping Purity of materials; ... High resolution; chemical sepn. of with competing Thin targets; F1* reactions Separation of peaks 2 x 10-12 3 x 10-10 Estimated sensitivity (grams isotope) Q Values (MeV)
are used at such high counting rates. Under the typical conditions outlined above, the counting rate was about 400 counts per pcoulomb per pg/cm2 of OI8. The highest energy which can be reached, where the backscattered protons can still be absorbed by a Mylar film, is about 750 keV. However, the maximum sensitivity is reached at 830 keV (see Figure 2b) where special techniques must be used in order to count a-particles in the presence of a high proton flux. If stripped films can be used, there is no limitation on the energy of the beam as the scattered particles do not have to be filtered, their number being sufficiently low as not to saturate the electronics of a conventional counting system (16). This technique has been applied in an investigation of the transport mechanism in the anodic oxidation of aluminum (15). Here a hole was made in the back of the target holder to hold a long tube in order to prevent the impact of the beam being picked up by the detectors. When the bulk materials or compounds deposited on bulk backings are studied, the a-particles can still be detected in the presence of a flux of scattered protons producing some hundred million counts per second, by using an ultrafast electronic amplifier. Preliminary results (23) have shown that such a technique can extend the potentialities of this method considerably. This will be the subject of a forthcoming paper. Measurement of 0lB.In many cases, it is more accurate to determine the isotopic composition of oxygen (particularly at high 0 1 8 enrichments) by measuring the amount of 0 ' 6 using the 016(d,p)01' reaction. In addition, this reaction can be used to measure the absolute thickness of oxide films of natural abundance, and also for detecting small quantities of oxygen near the surface of solids. In this technique semiconductor detectors more than 100 microns thick must be used in order to stop all the protons produced. As shown in Table I and Figure 4 the O16((d,p)Ol7reaction yields two groups of protons about 900 keV apart. The group at higher energy corresponds to the ground state of 0 1 7 and denoted by PO; that at lower energy corresponds to the excited state of 0'7 denoted by pl. The C12(d,p)Cla due to the carbon always present in trace quantities in target materials also leads to an intense peak which is, however, well separated from the p1 pro(23) G . Amsel, R. Bosshard, and C. Zajde, I.E.E.E. Trans. Nucl. Sci., NS14, 1 (February 1967).
1694
e
ANALYTICAL CHEMISTRY
oyd,p)019 1.731 and 1.635 ca. 10
016(d,a)N14 3.11 15
0 yd,p)017 1.919 and 1.048
40 (excited state)
0.9
0.6
0.9
1.8
I
1.1 and 2
D, Al, Si Large angle for detector (150 ") ; Lower energy (for Al, Si) 2 x 10-1'
N14,F19 High resolution; Thin targets; Separation of peaks 10-10
Bll, NI4, Al, Si Use Po peak (for Bll, NI4)Lower energy (for Al, Si) 5
x
10-12
CHANNELS
Figure 4. Typical proton spectrum from (d,p) reactions on self-supporting AIz03 target (1000 A thick; natural abundance 0 ' 8 ) showing separation of proton groups Detector at 90' to incident beam (1.06 MeV). Deuterons and CYparticles RItered out by 15-micron Mylar absorber. 65 keV per channel tons. In addition, a peak may also appear in these spectra from the D(d, p)T reaction occurring on stopped deuterons occluded in the target. When measured at an angle of about 150" and 600 keV this group does not overlap with the pl peak and can always be identified by its increase in intensity with bombarding time. There is no a priori limit, to the bombarding energy in this case as the scattered deuterons have a shorter range than the protons produced and can be filtered out. For instance with a beam of deuterons of 1.5 MeV, a 19-micron-thick Mylar absorber is required. Thus the bombarding energy can be chosen so that the cross section is on a plateau. A suitable range for 0 1 6 analysis is in the 1- to 1.3-MeV range for the po proton group. Narrower ranges from 0.80 to 0.90 MeV must be used, if the p1 group is taken as a measure of oxygen content (Figure 5). The advantage of the latter is that the cross section is much higher. The procedure used for oxygen analysis is to use 0.9 MeV for targets with high Z backing as the
1
I
I
1
1
1
I
00
1500 I
IOOOC
018(d,p10'9 1.82 MeV
500
400
500
600
I
I
!
700
800
900
I
1000
I
1100
Ed K e V
Figure 5. Absolute cross section of reaction Ol6(d,p.)l7* (excited state) plateau is very broad in this range. For low Z targets (such as A1 or Si) a beam of 0.83 MeV is used to reduce the background by increasing the coulomb barrier. It should be noted, however, that peaks from the El1(d,p)B12 and N14(d,p)N15 reactions overlap the pl peak. Hence the po should be used if these nuclei (BI1 or N 1 3 are present in appreciable quantities. The choice of the thickness of both the Mylar absorber and that of the sensitive zone of the detector is rather critical in this case. The Mylar film must be thick enough to stop the backscattered deuterons and also the a-particles from parallel (d,a) reactions. At a bombarding energy of about 1 MeV, 13micron Mylar absorbs the a-particles from the Ole(d,a)N14 reaction, but 19-micron Mylar is required for those produced by the 018(d,cr)N1ereaction which may overlap the p1 peak. On the other hand, pulses due to the electrons from the pactivity in the target [induced by (d,p) and (d,n) reactions] may lead to a background at the low energy side of the spectrum. This effect is very marked when the 0l8(or C12)content of the target is high. This problem may be overcome by using a 100-micron-thick sensitive zone in the detector. The high capacitance obtained causes no difficulties if a low noise preamplifier is used. It should be noted that the parallel reaction 01*(d,p)019 gives a peak only 130 keV below the POpeak from the 0 ' 6 (d,p)017 reaction. If the target contains an appreciable amount of 0 I 8 , one must use a high resolution in order to distinguish between the two peaks as shown in Figure 6. Alternatively the p1 group from Ole can be used which does not are present suffer from this disadvantage. If both 0 1 6 and 01* in the target, these facts can be put to good use by counting protons from Ole and Q18 simultaneously (see below). This method, however, suffers from the disadvantage that many competing reactions with similar Q values can occur. If target backing of low Z material is used (AI, Si, etc.) the bombarding energy should be kept low enough so that the effect of the coulomb barrier favors the cross section for the O1fl(d,p)0l7 reaction. It is therefore difficult to set the overall limits of sensitivity of this method without considering the exact details of each potential experiment. When backing of a high Z material (such as Au) is used, the limit of sensitivity at 0.9 MeV using the p1 proton group measured at about 150" is 2 X loll atoms under the experimental conditions described above.
Channel
Figure 6 . Typical proton spectrum from (d,p) reaction at 820 keV on 018-enriched Ta206target (900 A thick; containing 92 atom 0l8) CI2 peak not shown. Detector at 165" to incident beam; 8 keV per channel (if detector is at 150" to incident beam 01e(d,p)O17and D(d,p)T peaks separate even further because of more rapid increase in energy of protons from latter reactions). 13-micron Mylar absorber
The following figures are typical for routinely made measurements of the oxygen content of surface oxides. A 3-cm* 100-micron depleted region ORTEC detector is placed 55 mm from the target at an angle of 150' from the beam and coupled to an ORTEC preamplifier. Using a 19-micron Mylar film, a 55-volt TazQ5 target (880 A; natural isotopic abundance) gives 47,000 counts for a 23-pcouIomb beam at 900 keV. This count is usually obtained in about 20 seconds. It appears that the counting rate in this case is about three times less than for the 018(p,a)N15reaction under similar conditions-i.e., about 130 counts per pcoulomb per ,ug/cm2of oxygen-16, Measurement of the 0l8/OleRatio. The most accurate isotopic analyses of oxygen are based on the direct measurements of 01*/016 ratios. The analysis is simpler and is particularly useful for examining nonuniform or ill-defined targets such as those of biological origin. It also permits the analysis of targets which are not very stable during bombardment, This can be achieved by nuclear reactions which yield charged particles from both 0l6and 01* such as the (d,p) or (d,a) reactions (see Table I). It should be noted that in this case corrections for finite target thickness may be very small or negligible provided 0 1 6 and 0l8cross sections vary proportionally in the vicinity of the bombarding energy EO(see Appendix I). The OIB(d,p)Ol7 and O18(d,p)0l9 Reactions. As discussed above, the proton groups leading to the ground state of 0 1 7 and 0 1 9 produced by these reactions yield two peaks separated by 130 keV. Using the same experimental set-up as described in the preceding section, but with high resolution detection techniques (16), these two peaks can be separated, provided the target is not too thick. Targets of organic material, for instance, should not be thicker than about 100 pg/cm2 at a bombarding energy of 1 MeV. The ratio of the integral of the two peaks (Figure 6), (corrected for finite thickness as described in Appendix I) compared to the ratio from a standard target gives the 0 1 8 / 0 1 8 ratio in the sample. The limitations on the sensitivity of this method are those discussed in the preceding section. Because the D(d, p)T reaction may interfere with the po peak VOL. 39, NO. 14, DECEMBER 1967
0
3695
CHANNELS
Figure 7. Typical alpha spectrum from (d,a)reactions on Ol*-enriched self-supporting AIBOs target (1000 A thick containing 80 atom 0 1 8 ) Very thin detector used to eliminate counts due to protons and placed at 165 to axis of incident beam (2 MeV). 12 keV per chawel O
from 0 ' 6 (as indicated in Figure 6)) one can use the pl peak for Ole and the sum of the po and pl peaks for 0 1 s . The 0l6(d,a)O and 01*(d,a)N16Reactions. The advantage of using (d,a) reactions for the simultaneous analysis of 0l6and 0l8,is that because of the coulomb barrier for aparticle emission, the cross sections of competing reactions of low Z nuclei (such as Al, Si, etc.) are much lower than those for the (d,p) reactions. In addition, the peaks due to 0 1 6 and 0l8,as shown in Table I and Figure 7, are well separated by more than 1 MeV, and hence there is no limitation on target thickness and high resolution detection is not required. Although four peaks are obtained in the 0 ' 8 (d,a)N16 reaction (see TabIe I and Figure 7)) it is not necessary to resolve them, the sum of these peaks being a measure of the 0l8present. The same problems limiting the energy of the beam, discussed for the determination of 0 I 8 by the (p,a) reaction, also exist in this case. Similarly, the back-scattered deuterons have to be filtered by using a 6-micron-thick Mylar film (at energies up to 600 keV), and the same (but smaller) type of correction for target thickness is applied (see Appendix I). The plot of cross section as a function of energy has a similar exponential shape in this region to that of the 01*(p,a> reaction (14) but greatly reduced in magnitude. An. additional difficulty is that of distinguishing between overlapping protons and a-particles8 This can, however, be overcome by using a semiconductor detector with a thin sensitive volume of the order of 30 microns under these conditions (16). In the absence of background, due to the lower cross section, the sensitivity of this method is an order of magnitude less than that of the 01*(p,(r)N16reaction. CONCLUSION
The advantages of using the nuclear techniques described above over other nuclear methods for the isotopic analysis of oxygen can be briefly summarized as follows : (1) Low energy (of the order of 600 keV) accelerators can be used, and they are now relatively easily accessible. (2) The absence of natural background and the high yield of charged particles on bombardment lead to a high sensitivity. Oxygen-18 of the order of lo-'* gram near the surface of a given material can be detected (see Table I). 1696
0
ANALYTICAL CHEMISTRY
(3) Either the total amount of 0'6 or 0 l 8nuclei or the Ol*] Oi6 ratio can be measured. Although the latter method is about a hundred times less sensitive than the former, it i s still at least as sensitive as the 018(p,n)F1*reaction (which does not give a ratio) (see Table I). (4) The reactions used are highly specific and suffer from relatively little interference from competing reactions on other nuclei. (5) The results are immediately obtained (in many cases within minutes) and the accuracy may be increased by repeated analyses on the same sample. (6) Oxygen on or near (within microns) the surface of a solid material can be analyzed both by scanning the area and in depth. It should be emphasized that these methods are not as suitable as is conventional activation analysis for the determination of small concentrations of various nuclides in bulk material. However, for surface reactions, such as thermal and anodic oxidation of metals and diffusion, and exchange and adsorption of oxygen on solids, this method is almost the only one available. The ion probe mentioned above (2) can in some cases be used as an alternative method of analysis. Work is in progress to evaluate the relative merits of the two methods. The dependence of the ionization yield in the actual structure of the material being bombarded in the ion probe method is a drawback which can, however, be used to some advantage in certain cases. The ion probe method also requires a very high vacuum, whereas the nuclear method can work under relatively low vacuum if differential pumping or cooling of the target is used, thereby permitting relatively volatile materials to be examined, The nuclear method described in this paper is thus of great potential value in the analysis of submicrogram quantities of oxygen in both solids or in material of biological origin. APPENDIX I
The correction factor for a target of finite thickness is necessary only if a high precision is required. It can be evaluated in the following way: The counting rate N , is given by the expression
N
= ni
L*n'(x)u[E(x)ldx
where u[E(x)]is the cross section fer energy E at a depth x from the surface and n'(x) is the concentration of oxygen isotope also at a depth x. Xis the total thickness. In this expression, the energy straggling of the beam in the target material is neglected. In targets of uniform distribution n' is a constant. Cases where the isotopic content varies markedly as a function of depth are discussed elsewhere (13). The total number of reacting nuclei per cm2, is n = Xn' and hence
N
Qina
=
where B is the mean value of the cross section with respect to depth
,
rx
The correction factor is therefore, f=-
a
'J(Eo)
Eobeing the bombarding energy. EO= E(o)
If the target is not too thick, as in most surface phenomena, and if the cross section does not vary too rapidly, this expression is readily evaluated. In practice, there are two ways of evaluating the correction factor. (a) Determining the experimental yield for two different target thicknesses and extrapolating the plot of the yield per unit depth ( N I X ) to,zero thickness. It can be assumed that the relationship between depth and NIX is linear for such targets. In fact, E(x) = Eo - 6E where 6E = KX
K being the average energy loss per unit depth. Range-energy and specific energy loss relations can be found for various substances in (24). Hence writing
I
0
%Its
Figure 8. Example of correction factor for thickness of Ta206 target using 01s(p,a)N15reaction
and integrating
a
I
I50
100
50
= u(E0)
1 dlog u 1 - - __ 2 dE
[
hence
Bombarding energy near 500 KeV. Counting rate per unit thickness -Le., total counts, divided by voltage required to produce the labeled oxide film anodically-plotted against thickness in volts (related to thickness in A by function 16 A/Vj
f = l - - 1- d l o g u (Eo)KX 2 dE
(b) The excitation curve-i.e., the yield N as a function of energy-is measured with a thin standard target in the vicinity of the bombarding energy. The thickness is then determined in terms of the energy lost by the beam-usually expressed in keV units. This can be done by standard techniques-e.g., optical, gravimetric methods, etc.-followed by calculating the equivalent energy loss from the known data on stopping power for the target material at the energy used (24). Alternatively, by depositing an unlabelled target of identical thickness on part of the thin standard target, two values of the yield for standard with and without deposit are obtained, from which the deposited thickness can be evaluated directly (in keV) from the excitation curve. Finally the average value of u is determined by taking the average of the excitation curve over the thickness determined above, For fairly thin targets the energy-depth relation is linear. Hence the average with respect to depth is identical to the average with respect to energy. This greatly simplifies the correction which can readily be made routine. If the energy of the beam is carefully monitored, a single excitation curve can be used for numerous analyses provided the geometry of the experiment is not altered. It should be noted that when the 018/016 ratio is measured directly, the correction factor may be unnecessary. In fact, if the cross sections for 0l8and 0 ’ 6 reactions remain proportional in the interval AE below EO,the same relative contribution exists from both nuclei at any depth. The ratio of the sum of the two peaks is then a good measure of the 0 1 8 / 0 1 B ratio. In other cases the variations of the ratio
are to be considered for the determination of the correction factor. The relevant calculation can be readily deduced from the above considerations. (24) W. Wahling, “Handbuch der Physik.,” Vol. 33, SpringerVerlag, Berlin, 1959, p. 202.
APPENDIX I1
The correction factor for finite target thickness in the special case of an excitation curve of exponential form is evaluated in the following way : If Eo is the bombarding energy then
where uois the cross section at the surface. In this equation E(x) = Eo
- KX
where K is the average energy loss per unit depth. Thus u = goe-aKz
goe-z/l
1
-, which is analogous to Ka a mean free path (I is slowly energy-dependent through K), hence where the length I is defined as I =
Provided Xis small enough with respect to I, this expression can be simplified to
a
= u.[1-
7 1 I] x
This relation, in fact, shows that in this case the correction factor is a linear function of thickness, as was discussed above and as is shown experimentally in Figure 8. Typical values of I near a proton bombarding energy of about 600 keV are Alz03 = 0.65 micron, Si02 = 1 micron, and T a 2 0 j = 0.6 micron for the 018(p,ol)N*5reaction. A thin target, in this case, is therefore defined as one of thickness
