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Very heavy ion track etching in olivine
0191-278X/86 $3.00 + .00 Pergamon Press Ltd.
Nucl. Tracks Radiat. Meas., Vol. 11, Nos 1/2, pp. 73-80, 1986 Int. J. Radiat. Appl. lnstrum., Part D Printed in Great Britain
VERY HEAVY ION TRACK ETCHING IN OLIVINE C. PERRONand M. MAURY Laboratoire de Mineralogie des Roches Profondes et des Meteorites, CNRS and Museum d'Histoire Naturelle, 75005 Paris, France
(Received 25 February 1985; received for publication 7 October 1985) Abstract--Track etch rate has been measured as a function of residual range R for 12-16 MeV/amu Kr, Pb and U ions in olivine, and has been found not to vary with R for Pb and U ions. For this reason, 16 MeV/amu U ions have been used to specificallystudy the influence of the etching process on track etch rate in this mineral. The track etch rate is found to decrease with time, i.e. with etched track length, even for constant radiation damage. Small changes in composition of the WN etchant can considerably modify the olivine bulk etch rate and, to a much lesser extent, the tract etch rate. Evidence is presented for the existence of an etch induction time, with characteristics strikingly similar to those known for plastic track detectors, and of etch blocking features, some of which have been identified with tubular inclusions. An attempt is made to explain and link together these experimental results. The track etch rate is expressed in terms of a competition between two components, one depending on radiation damage, the other on etching chemistry. Implications for the interpretation of track etch rate measurements and for proper revelation of tracks, in particular very long tracks, are pointed out.
1. INTRODUCTION DECIPHERING the cosmic ray track record of extraterrestrial materials has been a challenge, ever since the discovery of tracks of cosmic ray Fe nuclei (Maurette et al., 1964) and, later, transiron nuclei (Fleischer et al., 1967) in meteorites. It requires a good knowledge of all processes involved in track registration and revelation. The best way of improving our knowledge of these processes is to use heavy ions of known energy and atomic number Z, provided by man-made accelerators. Accelerators have long offered only ions of relatively low Z and/or low energy, but a few machines are now capable of accelerating essentially all possible ions, at energies higher than 10 MeV/amu, reaching even 1 GeV/amu in some cases. With the aim of studying the chemical composition of the heaviest cosmic ray nuclei, a calibration--in a broad sense---of olivine as a track detector, is currently carried out in this laboratory, by means of high energy heavy ion irradiations (Perron and Pellas, 1983). Olivine ([Mg, Fe]2 SiO4) has been chosen for the following reasons: ultraheavy cosmic rays are extremely rare, and their tracks are long. Their study will thus require meteorites with long exposure ages, and large (several mm in size) track detectors. These two characteristics are only found in a (rare) class of meteorites called pallasites, which are made up of large (up to .-- 1 cm across) olivine crystals, embedded in an Fe-Ni matrix, and have exposure ages of several times 107yr up to ~ 2 x 108yr. In the course of this calibration work, we realized that the parameters we were measuring--track lengths and track etch rates--seemed to be often more influenced by the chemical etching process itself
than by the radiation damage intensity. We therefore made a series of experiments intended to specifically study the process of track etching in olivine, by measuring the variation of the track etch rate and of the crystal bulk etch rate as a function of time and etchant composition. We found U ions particularly well suited for such a study, for a reason which will be explained below (Section 3.1). The results are presented here. Some of them at least, are probably characteristic not only of olivine but of other mineral detectors as well, and might prove useful, particularly in works on very long tracks produced by very heavy ions.
2. EXPERIMENTAL PROCEDURES Arbitrarily oriented olivine crystals, up to ,-, 5 mm in size, of both terrestrial and meteoritic origin were mounted in epoxy resin, and polished. In meteoritic crystals, fossil cosmic ray tracks were annealed by heating, before mounting, a few crystals of each origin were analyzed by means of an electron microprobe. Their fayalite content was in the range 7-11 mole% and 18.3 + 0.3 mole% for terrestrial olivines from diamond pipes, and the Eagle Station pallasite, respectively. No variation of concentration has been found to clearly lie outside the experimental uncertainties for major (Mg, Fe, Si) or minor (Mn) elements, within a single crystal. No variation has been detected either between crystals, in the case of the meteorite. The crystals were irradiated by heavy ions at the UNILAC accelerator of the G.S.I., Darmstadt. Irradiations have been performed with 12.5 MeV/amu Kr, 14MeV/amu Pb and 15.6MeV/amu U ions (in 73
74
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F]o. 1. Track etch rate as a function of the residual range in olivine, measured with the repolished crystal technique. For Kr ions (a), two different crystal fragments have been used, etched respectively for 15 min (dots) and 120 min (crosses); etching lasted 30 min for Pb ions (b), and 17 rain (c) and 20 min (d) for U ions. See text about the difference between (c) and (d). All crystals are from the Eagle Station pallasite. this last case, the energy of the particles was in fact 16.TMeV/amu, but was degraded by a 1 0 # m thick AI foil, for the need of another experiment). The ion beam made an angle of 30 ° with the target surface, and the fluence was in the range 2-5 x 105 cm -2. Most of the crystals used in this study were then etched, but some of them were first repolished, the new polished surface making a small angle (a few degrees) with the previous one. In this way, the new surface intersects the ion trails at all values of the 1 WN 30
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range from zero to the range of the incoming ions, and one can thus measure the variation of the track etch rate as a function of the ion residual range. This method has been first described by Storzer et al. (1973) and Price et al. (1973). For a clear illustration of how it works, see Figs 1 and 2 of Price et al. (1973). Etching took place in etchants of slightly varying composition and for varying times. The etchants were prepared following the recipes of Krishnaswami et al. (1971) (WN solution) and Otgonsuren et al. (1976). These consist in an aqueous solution of oxalic acid, orthophosphoric acid, sodic ethylenediaminetetracetic acid (EDTA) and sodium hydroxyde. The pH is adjusted by varying the proportion of NaOH. Values between 7.3 and 8.2 were tried. Although etching takes place in the boiling solution, the pH was adjusted at room temperature, to avoid uncontrolled evaporation during this operation. Measurements, performed with a digital pH-meter, have an estimated precision of _ 0.05 pH unit. The etchant used by the Dubna group (Otgonsuren et al., 1976) which we shall call W N D solution (D, for Dubna) differs from that of Krishnaswami et al. (1971) by a four times larger concentration of oxalic acid. This results in an important insoluble residue, and the solution is filtered before pH adjustment (V.P. Perelygin, private communication)• The etching times t I were varied from 10 min to 2 h, which is, in all cases, shorter than the time needed to reveal the tracks to the end of the ion range ( > 4 h), in order to determine track etch rates. After such short etchings, tracks are not in general visible under the optical microscope, or, if they are, they are so thin that it is very difficult to precisely localize their end. Therefore, we applied a method already used by Green et al. (1978), which consists in heating the
VERY HEAVY ION T R A C K E T C H I N G IN OLIVINE crystal after the short etch at a temperature sufficiently high (typically >500°C overnight) to completely anneal the part of the latent tracks that has not yet been etched. A second etching of several hours (t~) then enlarges the tracks to make them clearly visible with a microscope, without increasing their length any further. Of course, in calculating track etch rate v,, only the duration (t~) of the first etching is taken into account, but the total etching time (t~ + t2) is used to derive bulk etch rate vg. To determine the effect of varying the etchant composition or the etching time on the track etch rate, it is necessary not to change other factors which may also have an effect, such as the chemical composition of the crystals, or the orientation of the tracks with respect to the crystal lattice. For this purpose, after irradiation, large crystals were sawn into several pieces with a diamond saw, and etching was performed on each fragment separately. Track lengths and widths were measured at magnification 1600× with an optical microscope whose stage is equipped with 3 orthogonal displacement sensors. The output signals Of the sensors are digitized and fed into a microcomputer, allowing semi-automatic measurements of track lengths and orientations. The resolution of the system is 0.1/am on each of the three coordinates. The uncertainty on track lengths is mainly due to estimating the position of the track ends, and particularly the deeper one. It depends on track width and shape, and thus varies from crystal to crystal. It is in most cases less than + l/am. 3. RESULTS 3.1. Track etch rate vt vs residual range R
These results are presented here because they appear as both a motivation and a justification for the work described in the next sections. More extended work on track etch rate measurements as a function of range will make the subject of a forthcoming paper. The measurements were performed on crystals repolished as described in Section 2. For each track, the etch r a t e r t was obtained by dividing the measured etched track length by the duration t~ of the first, skort etching, and was assigned to a point situated midway down the etched track. The residual range R corresponding to this point was deduced from the measurement of the distance of the track to the intersection of the two polished surfaces. Results obtained with 12.5 MeV/amu Kr, 14 MeV/amu Pb and 15.6 MeV/amu U are shown on Fig. 1, each dot corresponding to an individual track length measurement. For the sake of clarity, points corresponding to fully e~ched tracks (at short residual ranges), from which no etch rate can be deduced, have not been plotted. The first thing to be noted, is the marked difference between the curve for Kr (Fig. 1a), on one hand, and those for Pb (Fig. Ib) and U (Fig. lc), on the other hand. F o r Kr, vt increases rapidly with decreasing R,
75
while for Pb and U, no systematic variation of v, with R can be noted. Such a behavior was predicted by the track model of Dartyge et al. (1981). In this model, the preferential etching of tracks is due to extended defects induced by the incoming ions along their paths. The etch rate is very much larger in what the authors of the model call the core zones, around these defects, than in the gaps between them, which etch only slightly faster than the bulk crystal. The track etch rate should thus follow the linear density of extended defects along the ion path (as in the case of Kr ions), until this density is so high that all core zones overlap. In this case, the track etch rate vt should reach a saturation value, equal to the etch rate of a core zone. This is possibly what we observe for Pb and U ions. Another striking feature is the variable, and sometimes very large, scatter of points that we observed in these experiments. An example of such a large scatter is given in Fig. l(d) for U ions. Broad distributions of etched track lengths are predicted by the model of Dartyge et aL (1981)----and have indeed been observed--as a result of the discontinuous nature of the radiation damage, and the statistical repartition of the defects. However, when all extended defects overlap, the latent track can be considered as a long continuous single defect, so that the etched track length distribution should be much narrowed when v, has reached its plateau value. Therefore, the scatter of the experimental points that we observe, probably does not reflect any characteristic of the radiation damage, but rather, we interpret it as due to inhomogeneities in the chemical etching process itself. It thus appears that this process can have strong effects on track relevation. To correctly interpret the results of track experiments, these effects need to be better known. They can be best studied with very heavy ions, like U and Pb, because of the above demonstrated range-independence of v,, and thus most probably of the radiation damage (at least between ~ 10 # m and ~ 100 # m for U, and ~ 10 # m and ~ 8 0 # m for Pb). For this reason U ion tracks have been used throughout the work described in the next sections. 3.2. Effect o f p H and composition o f etchant on track and bulk etch rates Figure 2 shows U ion track length distributions measured on six fragments of the same terrestrial crystal (diamond pipes), irradiated with 15.6 MeV/ amu U ions, etched for t~ = 1 h, in W N solutions with pH varying from 7.3 to 8.2 (it should be well understood that these are uncompletely etched tracks, whose length reflects the track etch rate v, averaged over time ti, and not the ion range, which is ~ 100 #m). The circles schematically represent the increase of the track cross-section (which actually is not at all a circle), in 1 h. The diameter of each circle is obtained by dividing the track width at the crystal surface, averaged over many tracks, by the total
76
C. P E R R O N and M. M A U R Y
etching time (i.e. 1 h, plus t2, which varies from 2 h for p H = 7 . 3 , to 10h for p H = 8 . 2 ) . It is equal to twice the bulk etch rate Vg, in that particular direction. The effect of varying the etchant pH is clear: Vg strongly increases when pH is decreased (a factor ~ 9, from pH 8.2 to 7.3) while v, increases much more moderately ( ~ 4 0 % on the same pH range). This is in qualitative agreement with the findings of Davie and Durrani (1978), who also reported an increase of vg when the pH of the W N solution was decreased. In addition, the relatively narrow length distribution becomes much broader at pH = 8.2, with a long tail on the short lengths side. Note, however, that there are a few short tracks at nearly all pH values. Very similar results have been obtained with meteoritic crystals, vt and v~ values, and the pH value at which track length distributions broaden, vary somewhat from crystal to crystal, but the picture remains essentially the same. Figure 3 shows the track length distribution obtained from a fragment of the same crystal as above, after 1 h etching in W N D solution at pH = 7.8. The bulk- and track etch rates are similar to those obtained with W N solution at pH = 7.9 and 7.3, respectively. The track cone angle is thus much smaller with W N D than with W N at the same pH, in agreement with the findings of Otgonsuren et aL (1976). 3.3. Variation o f track etch rate as a function o f time In Fig. 4, we show track length distributions obtained for crystal fragments etched in W N with pH = 8.0 (left-hand side) and pH = 7.8 (right-hand side), and etching time tl increasing from 10 rain (top) to 2 h (bottom). All these fragments came from the same meteoritic crystal (from the Eagle Station pallasite). For a given fl, the distributions are broader for the higher pH, as expected from the preceding section. F o r pH = 7.8, these data show a continuous decrease of the time-averaged track etch rate v, from ~ l l 0 # m h -I for f l = 1 0 m i n , to ~ 8 6 # m h -1 for t t = 2 0 m i n , ~ 5 2 # m h -1 for t ~ = l h and ~ 3 6 # m h -~ for q = 2 h . Etch rate measurements on 155 MeV/amu U ion tracks (Perron, 1984) show that this
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FIG. 4. Etched track length histograms obtained for 7 fragments of an olivine crystal from the Eagle Station pallasite, irradiated by 15.6 MeV/amu U ions, after etching in WN solutions at pH = 8.0 (left hand side) and 7.8 (right hand side), and times varying from 10 min (top) to 120 min (bottom). Circles have same meaning as in Fig. 2. trend continues for longer etching times: v, ~ 25/tm h -~ for tl = 3 h and ~ 1 0 # m h -1 for q = 16h (on a different crystal, of course). Note however, that for pH = 8.0, v, apparently increases between tl = 10 min ( ~ 4 2 / ~ m h-1) and tl = 20 min ( ~ 75/~m h-l). Two other observations should be mentioned which show that there are other more discontinuous variations of t,, with time. One is illustrated in Fig. 5, which presents two photomicrographs of U ion tracks in olivine crystals etched for tl = 1 h. In Fig. 5(a), two tracks, shorter than the average, show a sudden change in diameter at about the same depth. A third one stops, also at this same depth. Obviously, although the crystal is optically homogeneous, etching has nearly stopped for these 3 tracks, while the etched channels were still enlarging. Etching finally resumed for 2 of them with probably the normal value of v,. This sort of etch-blocking phenomenon is not uncommon, and has already been observed by other workers (Price et al., 1973). Here, because it happens to 3 neighbouring tracks, and at the same depth, it is clear, apart from the arguments given in Section 3.1, that it is not due to the non-uniform distribution of radiation damage, but to some local inhomogeneity in the crystal. In some cases, the inhomogeneity is indeed visible, as illustrated in Fig. 5b. Here, two tracks stopped at the point where the ion paths crossed a tubular inclusion. There are plenty of such inclusions in this crystal, and the same phenomenon is observed, each time a track encoun-
VERY HEAVY ION T R A C K E T C H I N G IN OLIVINE
77
FIG. 5. Photomicrographs of 15.6 MeV/amu U ion tracks in 2 olivine crystals from diamond pipes (a) and Eagle Station (b). In (a), etching stopped for 3 neighbouring tracks, although the crystal is optically clear and homogeneous. Etching resumed after some time for two of these tracks, leaving a visible sudden change in track width. In (b), etching similarly stopped for two neighbouring tracks, when these encountered a visible tubular inclusion. Scale is the same for both photos. ters one of them. We tried to perform an electron microprobe analysis of one of these inclusions, which goes up to the crystal surface. It appears that it is made up of a mineral (or maybe more than one) with a composition different from olivine, rich in chromium and calcium. Work will be continued on this problem, but it is already quite understandable t h a t tracks are not revealed in this mineral by an etchant specially designed for olivine. The other observation is the following: in several cases, crystals etched according to our usual procedure showed no revealed tracks at all. This means that there is some time-delay before track etching really starts: if it has not yet started at the end of the
first etching step, then no track can be revealed by the longer etching either, since the annealing step completely erases the latent tracks. This occurred for crystals either repolished or not, for tl up to 20 min, and essentially for etchants of high pH ( ~ 8). No such absence of tracks has been observed with WN etchant at pH = 7.6 (i.e. when the bulk etch rate Vg is high), although t~ values as short as 5 min have been used in this case. The length of this time delay (or etch induction time, Ruddy et al., 1977) is highly variable from crystal to crystal (probably due to different orientations of the polished surfaces with respect to the crystallographic planes, leading to different bulk etch rates). There is some evidence that it also
78
C. P E R R O N and M. M A U R Y
effective etching time accordingly. The shorter tl, the larger the relative effect, so that v,, averaged over h, may seem smaller for t~ = 10m in than for 20min. Note that this is not observed at lower pH. The existence of such a time delay, or etch induction time (tin), has already been observed for a variety of plastic track detectors (Monnin and Sanzelle, 1970; Ruddy et al., 1977; Grabez e t a ! . , 1981). An "incubation time" has also been identified for mica by Price and Walker (1962), but it corresponds to a different stage of etching: these authors observed that the damaged trails produced by fission fragments in mica, were etched instantaneously (i,e. in fact in a time less than 4. DISCUSSION I s) over a width of 25-40 A, but then, these channels We have had clear evidence that the rate at which started enlarging further only after some incubation tracks are etched in olivine decreases with time even time. Although the characteristics of t~ that we can when no variation of vt with R is found by the deduce from our experimental results are only of a repolished crystal technique. This is most probably qualitative nature, it is striking that they are the same due to the increasing difficulty of renewing the etch- as those reported for plastics (Ruddy et al., 1977; ant inside the etched channels, as etching proceeds: Grabez et al., 1981): tin increases when either the bulk the longer the etched track, the more difficult it is for etch rate, or the radiation damage intensity decreases. the etch products to diffuse outside and leave room Ruddy et al. (1977) suggested that the existence of tin to fresh etchant, so that v~ decreases for chemistry might be due to the effect of the vacuum-to-solid reasons, although a higher etch rate value could be interface on the stopping power of the ions. Obviallowed by the radiation damage intensity. One can ously, at least in our case, this explanation does not thus schematically view the measured etch rate v, as hold, since we noticed an etch induction time even for resulting from the competition between two com- crystals which had been repolished, i.e. for which the ponents. One, which we shall call the "intrinsic etch etched surface was not the irradiated surface, but rate" v,~, would be the observed etch rate if fresh was rather an internal part of the crystal, at the time etchant had free access to the tip of the tracks (this of the irradiation. Most probably, the explanation is approximately realized when etched tracks are should be sought again in the etching chemistry. still very short). For a given etchant, vt~ essentially This is supported by the observation of Price and depends on the radiation damage intensity and is Walker (1962), since their incubation time applied to therefore range-dependent. Its variation as a function undamaged material. of R is a characteristic of the ion involved. The The above discussion has several important consecond component, which we shall call the "chemical sequences. A measured variation of v, as a function etch rate" vtc, is the maximum etch rate allowed by of R may be strongly biased by chemical effects, and the renewing of the etehant in the etched channels, v,c only slightly reflect the variation of vt~, hence of the is of course range-independent, but depends on the radiation damage intensity. One should therefore be length of the etched track: it is very high, when tracks very careful when using v, measurements to check are still very short, then decreases as the tracks get track formation models. For instance, the comlonger. At any time, the effective etch rate is given by: parison of the variation of v, vs R for high energy U ions in olivine, with that of the primary ionization, vt = M i n ([3ti , l)tC ) • made by Pellas and Perron (1984) now appears not F o r relatively light ions, like Fe ions and fission to be really meaningful, v, measurements are only fragments, for which tracks are short and va has valid for the experimental conditions (revelation techmoderate values, etching is essentially governed by nique used, duration of etching) under which they vti. On the contrary, for the heaviest ions, for which have been obtained. It is clear that the v, variation vt~ has high values over considerable lengths, v,c plays measured for U ions with the repolished crystal the major role, except when etching the "high energy technique (v, plateau, Fig. lc) cannot be applied to end" of the tracks, where v, is low, and also at the the sequential etching of tracks from the irradiated very beginning of etching, when the etched portion of surface (continuous decrease of v,, Fig. 4). Also, the tracks is short, and v,c high. curves obtained with the same repolished crystal The initial increase of vI observed between h = technique, but for different etching times, may 10min a n d h = 20rain for W N at pH = 8.0 (Fig. 4) be quite different, as attested by results on high may seem paradoxal in this context. Recall, however, energy U ion tracks etched for 3 and 16 h (Perron, that we have evidence that etching may be consider- 1984). ably delayed at high pH values. The observation is Because of the continuous decrease of vtc with time, then easily explained if one assumes a time delay of very long tracks cannot be completely revealed with several minutes before start of etching, reducing the classical techniques, which is of great importance for depends on the radiation damage: when measuring etch rate versus residual range, for Kr ions, no tracks were revealed for R > 40/zm, with t~ = 15 rain. However, with t~ = 2 h, tracks were visible for R up to ~ 70 # m, and with lengths such that they should also have been seen on about this whole range for t~ about 8 times shorter, if there was no time delay (see Fig. la). Similar results have been obtained with 155 MeV/amu U ions (Perron, 1984), but in this case, for R > 1200/xm, tracks only appeared after more than 3 h of etching.
VERY HEAVY ION T R A C K E T C H I N G IN OLIVINE studies of Ultraheavy Cosmic Ray tracks in meteorites. Indeed, Perron (1984) has shown that U ion tracks are revealable over a length of about 3 mm, but was unable 'to reveal them on more than about 1.3 ram, when the etchant could reach the tracks at only one point. Moreover, since vt, should' not depend on the nature of the ion, the revealed portion of the tracks would most probably have been of about the same length, had the incoming particles been say Pb or Pt ions. Conversely, a measurement of the etched track lengths would not permit to identify the ions which produced them. We plan to experimentally check this point. If it is true, the only way out is to use techniques which allow the etchant to reach every track at many points along the ion path, so that the whole etchable track length can be revealed. Such techniques are presently under study in this laboratory. Although all the experiments we have reported have been performed on olivine crystals, it is likely that the previous discussion also applies to other mineral detectors. On the contrary, the results concerning the etchant composition are certainly more specific of olivine. It is remarkable that small changes in the etchant composition may have important effects on track revelation. In particular, the fact that vs varies much more with pH than v,, implies that a small variation of pH may significantly change the etchable track length, and the critical angle arcsin (Vg/V,) belo~v which tracks are not revealed, and thus the etching efficiency. The large scatter of experimental points that we often observed in track etch rate measurements is apparently due to our using etchants of high pH (around 8). Although we know how to avoid this now (use lower pH), the reason why it is so is not entirely clear. The scatter may be due to the etch induction time, as it probably varies somewhat from track to track. Since t~, is shorter for etchants with low pH, the effect is expected to be less important with these etchants. It may also be the result of the same sort of phenomenon as that illustrated in Fig. 5, the latter being just an extreme example: in optically homogeneous crystals, there may exist heterogeneities in structure and/or chemical composition, which can cause large variations of track etch-rate. In this hypothesis, WN solution would be more efficient at low pH, to overcome these obstacles. In any case, the fact that track etching can be blocked at some places for relatively long times, is another reason for revealing long tracks from many points, evenly distributed along the ion trail. We now systematically use WN solution with pH = 7.6 when we make track etch rate measurements, i.e. etchings of short duration, for which we want narrow length distributions and short etch induction time. The fact that the bulk etch rate is rather high in this case, is in general not disturbing, and it makes tracks become visible faster. On the other hand when performing long etchings, to reveal
79
long tracks, we use W N D solution with pH = 7.8, which gives a much smaller cone angle. 5. CONCLUSION The results of the experiments that we have described have confirmed that track etching in minerals---and particularly in olivine---is a complex process, which we certainly do not fully understand yet. They nevertheless provide some hints which may be useful in finding ways to avoid undesired phenomena, and correctly reveal tracks. This is specially true in the case of the very long tracks produced by very heavy ions, for which the decrease of the track etch rate with time, and etch blocking features might lead to totally confusing results. Acknowledgements--We are grateful to R. Spohr and the G.S.I. staff for their willingness to perform the heavy ion irradiations and their efficiencyin conducting them, to M. Bourot-Denise for the electron microprobe analyses, and to P. Pellas for constant support and invaluable discussions. One of us (C.P.) is indebted to V. P. Perelygin for many discussionsduring a stay in Dubna in 1979, which were the origin of the work presented here.
REFERENCES Dartyge E., Duraud J. P., Langevin Y. and Maurette M. (1981) A model of nuclear particle tracks in dielectric minerals. Phys. Rev. B23, 5213--5229. Davie I. W. and Durrani S. A. (1978) Anisotropic track etching in olivine crystals using WN solution. Nucl. Track Detection 2, 199-205. Fleischer R. L., Price P. B., Walker R. M., Maurette M. and Morgan G. (1967) Tracks of heavy primary cosmic rays in meteorites. J. Geophys. Res. 72, 355-366. Grabez G., Vater P. and Brandt R. (1981) The etchinduction time (T~) and other registration properties in CR-39 detectors for well-definedions. Nucl. Tracks 5, 291-297. Green P. F., Bull R. K. and Durrani S. A. (1978) Particle identification from track etch rates in minerals. Nucl. Instrum. Meth. 157, 185-193. Krishnaswami S., Lal D., Prabhu N. and Tamhane A. S. (1971) Olivines: revelation of tracks of charged particles. Science 174, 287-291. Monnin M. and Sanzelle S. (1970) Le chlorure de polyvinyle:un d~tecteur de traces int6ressant. Radiat. Effect 5, 125-128. Maurette M., Pellas P. and Walker R. M. (1964) Cosmic ray induced particle tracks in a meteorite. Nature, Lond. 204, 821-823. Otgonsuren O., Perelygin V. P., Stetsenko S. G., Gavrilova N. N., Fieni C. and Pellas P. (1976) Abundances of Z > 52 nuclei in galactic cosmic rays: long-term averages based on studies of pallasites. Astrophys. J. 210, 258-266. Pellas P. and Perron C. (1984) Track formation models: a short review. Nucl. Instrum. Meth. Phys. Res. B1, 387-393. Perron C. (1984) Relativistic 23sUion tracks in olivine and cosmic ray track studies. Nature, Lond. 310, 397-399. Perron C. and Pellas P. (1983) Can we get the cosmic ray actinide abundance from the study of tracks in meteorites? Proc. 18th Int. Cosmic Ray Conf., Bangalore, Vol. 9, pp. 127-130. Tata Institute of Fundamental Research, Bombay.
80
C. P E R R O N
a n d M. M A U R Y
Price P. B. and Walker R. M. (1962) Chemical etching of charged particle tracks. J. Appl. Phys. 33, 3407-3412. Price P. B., Lal D., Tamhane A. S. and Perelygin V. P. (1973) Characteristics of tracks of ions of 14 < Z < 36 in common rock silicates. Earth Planet, Sci. Lett. 19, 377-395. Ruddy F. H., Knowles H. B., Luckstead S. C. and Tripard
G. E. (1977) Etch induction time in cellulose nitrate: a new particle identification parameter. Nucl. Instrum. Meth. 147, 25-30. Storzer D., Poupeau G. and Kr~tschmer W. (1973) Trackexposure and formation ages of some lunar samples. Proc. 4th Lunar Sci. Conf. Geochim. Cosmochim. Aeta 3, Suppl. 4, 2363-2377.
Nucl. Tracks Radiat. Meas., Vol. 11, Nos 1/2, pp. 73-80, 1986 Int. J. Radiat. Appl. lnstrum., Part D Printed in Great Britain
VERY HEAVY ION TRACK ETCHING IN OLIVINE C. PERRONand M. MAURY Laboratoire de Mineralogie des Roches Profondes et des Meteorites, CNRS and Museum d'Histoire Naturelle, 75005 Paris, France
(Received 25 February 1985; received for publication 7 October 1985) Abstract--Track etch rate has been measured as a function of residual range R for 12-16 MeV/amu Kr, Pb and U ions in olivine, and has been found not to vary with R for Pb and U ions. For this reason, 16 MeV/amu U ions have been used to specificallystudy the influence of the etching process on track etch rate in this mineral. The track etch rate is found to decrease with time, i.e. with etched track length, even for constant radiation damage. Small changes in composition of the WN etchant can considerably modify the olivine bulk etch rate and, to a much lesser extent, the tract etch rate. Evidence is presented for the existence of an etch induction time, with characteristics strikingly similar to those known for plastic track detectors, and of etch blocking features, some of which have been identified with tubular inclusions. An attempt is made to explain and link together these experimental results. The track etch rate is expressed in terms of a competition between two components, one depending on radiation damage, the other on etching chemistry. Implications for the interpretation of track etch rate measurements and for proper revelation of tracks, in particular very long tracks, are pointed out.
1. INTRODUCTION DECIPHERING the cosmic ray track record of extraterrestrial materials has been a challenge, ever since the discovery of tracks of cosmic ray Fe nuclei (Maurette et al., 1964) and, later, transiron nuclei (Fleischer et al., 1967) in meteorites. It requires a good knowledge of all processes involved in track registration and revelation. The best way of improving our knowledge of these processes is to use heavy ions of known energy and atomic number Z, provided by man-made accelerators. Accelerators have long offered only ions of relatively low Z and/or low energy, but a few machines are now capable of accelerating essentially all possible ions, at energies higher than 10 MeV/amu, reaching even 1 GeV/amu in some cases. With the aim of studying the chemical composition of the heaviest cosmic ray nuclei, a calibration--in a broad sense---of olivine as a track detector, is currently carried out in this laboratory, by means of high energy heavy ion irradiations (Perron and Pellas, 1983). Olivine ([Mg, Fe]2 SiO4) has been chosen for the following reasons: ultraheavy cosmic rays are extremely rare, and their tracks are long. Their study will thus require meteorites with long exposure ages, and large (several mm in size) track detectors. These two characteristics are only found in a (rare) class of meteorites called pallasites, which are made up of large (up to .-- 1 cm across) olivine crystals, embedded in an Fe-Ni matrix, and have exposure ages of several times 107yr up to ~ 2 x 108yr. In the course of this calibration work, we realized that the parameters we were measuring--track lengths and track etch rates--seemed to be often more influenced by the chemical etching process itself
than by the radiation damage intensity. We therefore made a series of experiments intended to specifically study the process of track etching in olivine, by measuring the variation of the track etch rate and of the crystal bulk etch rate as a function of time and etchant composition. We found U ions particularly well suited for such a study, for a reason which will be explained below (Section 3.1). The results are presented here. Some of them at least, are probably characteristic not only of olivine but of other mineral detectors as well, and might prove useful, particularly in works on very long tracks produced by very heavy ions.
2. EXPERIMENTAL PROCEDURES Arbitrarily oriented olivine crystals, up to ,-, 5 mm in size, of both terrestrial and meteoritic origin were mounted in epoxy resin, and polished. In meteoritic crystals, fossil cosmic ray tracks were annealed by heating, before mounting, a few crystals of each origin were analyzed by means of an electron microprobe. Their fayalite content was in the range 7-11 mole% and 18.3 + 0.3 mole% for terrestrial olivines from diamond pipes, and the Eagle Station pallasite, respectively. No variation of concentration has been found to clearly lie outside the experimental uncertainties for major (Mg, Fe, Si) or minor (Mn) elements, within a single crystal. No variation has been detected either between crystals, in the case of the meteorite. The crystals were irradiated by heavy ions at the UNILAC accelerator of the G.S.I., Darmstadt. Irradiations have been performed with 12.5 MeV/amu Kr, 14MeV/amu Pb and 15.6MeV/amu U ions (in 73
74
C. P E R R O N and M. M A U R Y i
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F]o. 1. Track etch rate as a function of the residual range in olivine, measured with the repolished crystal technique. For Kr ions (a), two different crystal fragments have been used, etched respectively for 15 min (dots) and 120 min (crosses); etching lasted 30 min for Pb ions (b), and 17 rain (c) and 20 min (d) for U ions. See text about the difference between (c) and (d). All crystals are from the Eagle Station pallasite. this last case, the energy of the particles was in fact 16.TMeV/amu, but was degraded by a 1 0 # m thick AI foil, for the need of another experiment). The ion beam made an angle of 30 ° with the target surface, and the fluence was in the range 2-5 x 105 cm -2. Most of the crystals used in this study were then etched, but some of them were first repolished, the new polished surface making a small angle (a few degrees) with the previous one. In this way, the new surface intersects the ion trails at all values of the 1 WN 30
p H = 7.3
i ETCHANT t 1 = 60rnn
~2o
=L@ I
~
i
pH=7.6
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FIG. 2. Etched track length histograms obtained for 6 fragments of a terrestrial olivine crystal, irradiated by 15.6 MeV/amu U ions, after 1 h (t0 etching in boiling WN solutions of different pH. The crystal has been sawn after irradiation, pH values refer to solution at room temperature. The circles schematically represent the increase of track width in I h.
range from zero to the range of the incoming ions, and one can thus measure the variation of the track etch rate as a function of the ion residual range. This method has been first described by Storzer et al. (1973) and Price et al. (1973). For a clear illustration of how it works, see Figs 1 and 2 of Price et al. (1973). Etching took place in etchants of slightly varying composition and for varying times. The etchants were prepared following the recipes of Krishnaswami et al. (1971) (WN solution) and Otgonsuren et al. (1976). These consist in an aqueous solution of oxalic acid, orthophosphoric acid, sodic ethylenediaminetetracetic acid (EDTA) and sodium hydroxyde. The pH is adjusted by varying the proportion of NaOH. Values between 7.3 and 8.2 were tried. Although etching takes place in the boiling solution, the pH was adjusted at room temperature, to avoid uncontrolled evaporation during this operation. Measurements, performed with a digital pH-meter, have an estimated precision of _ 0.05 pH unit. The etchant used by the Dubna group (Otgonsuren et al., 1976) which we shall call W N D solution (D, for Dubna) differs from that of Krishnaswami et al. (1971) by a four times larger concentration of oxalic acid. This results in an important insoluble residue, and the solution is filtered before pH adjustment (V.P. Perelygin, private communication)• The etching times t I were varied from 10 min to 2 h, which is, in all cases, shorter than the time needed to reveal the tracks to the end of the ion range ( > 4 h), in order to determine track etch rates. After such short etchings, tracks are not in general visible under the optical microscope, or, if they are, they are so thin that it is very difficult to precisely localize their end. Therefore, we applied a method already used by Green et al. (1978), which consists in heating the
VERY HEAVY ION T R A C K E T C H I N G IN OLIVINE crystal after the short etch at a temperature sufficiently high (typically >500°C overnight) to completely anneal the part of the latent tracks that has not yet been etched. A second etching of several hours (t~) then enlarges the tracks to make them clearly visible with a microscope, without increasing their length any further. Of course, in calculating track etch rate v,, only the duration (t~) of the first etching is taken into account, but the total etching time (t~ + t2) is used to derive bulk etch rate vg. To determine the effect of varying the etchant composition or the etching time on the track etch rate, it is necessary not to change other factors which may also have an effect, such as the chemical composition of the crystals, or the orientation of the tracks with respect to the crystal lattice. For this purpose, after irradiation, large crystals were sawn into several pieces with a diamond saw, and etching was performed on each fragment separately. Track lengths and widths were measured at magnification 1600× with an optical microscope whose stage is equipped with 3 orthogonal displacement sensors. The output signals Of the sensors are digitized and fed into a microcomputer, allowing semi-automatic measurements of track lengths and orientations. The resolution of the system is 0.1/am on each of the three coordinates. The uncertainty on track lengths is mainly due to estimating the position of the track ends, and particularly the deeper one. It depends on track width and shape, and thus varies from crystal to crystal. It is in most cases less than + l/am. 3. RESULTS 3.1. Track etch rate vt vs residual range R
These results are presented here because they appear as both a motivation and a justification for the work described in the next sections. More extended work on track etch rate measurements as a function of range will make the subject of a forthcoming paper. The measurements were performed on crystals repolished as described in Section 2. For each track, the etch r a t e r t was obtained by dividing the measured etched track length by the duration t~ of the first, skort etching, and was assigned to a point situated midway down the etched track. The residual range R corresponding to this point was deduced from the measurement of the distance of the track to the intersection of the two polished surfaces. Results obtained with 12.5 MeV/amu Kr, 14 MeV/amu Pb and 15.6 MeV/amu U are shown on Fig. 1, each dot corresponding to an individual track length measurement. For the sake of clarity, points corresponding to fully e~ched tracks (at short residual ranges), from which no etch rate can be deduced, have not been plotted. The first thing to be noted, is the marked difference between the curve for Kr (Fig. 1a), on one hand, and those for Pb (Fig. Ib) and U (Fig. lc), on the other hand. F o r Kr, vt increases rapidly with decreasing R,
75
while for Pb and U, no systematic variation of v, with R can be noted. Such a behavior was predicted by the track model of Dartyge et al. (1981). In this model, the preferential etching of tracks is due to extended defects induced by the incoming ions along their paths. The etch rate is very much larger in what the authors of the model call the core zones, around these defects, than in the gaps between them, which etch only slightly faster than the bulk crystal. The track etch rate should thus follow the linear density of extended defects along the ion path (as in the case of Kr ions), until this density is so high that all core zones overlap. In this case, the track etch rate vt should reach a saturation value, equal to the etch rate of a core zone. This is possibly what we observe for Pb and U ions. Another striking feature is the variable, and sometimes very large, scatter of points that we observed in these experiments. An example of such a large scatter is given in Fig. l(d) for U ions. Broad distributions of etched track lengths are predicted by the model of Dartyge et aL (1981)----and have indeed been observed--as a result of the discontinuous nature of the radiation damage, and the statistical repartition of the defects. However, when all extended defects overlap, the latent track can be considered as a long continuous single defect, so that the etched track length distribution should be much narrowed when v, has reached its plateau value. Therefore, the scatter of the experimental points that we observe, probably does not reflect any characteristic of the radiation damage, but rather, we interpret it as due to inhomogeneities in the chemical etching process itself. It thus appears that this process can have strong effects on track relevation. To correctly interpret the results of track experiments, these effects need to be better known. They can be best studied with very heavy ions, like U and Pb, because of the above demonstrated range-independence of v,, and thus most probably of the radiation damage (at least between ~ 10 # m and ~ 100 # m for U, and ~ 10 # m and ~ 8 0 # m for Pb). For this reason U ion tracks have been used throughout the work described in the next sections. 3.2. Effect o f p H and composition o f etchant on track and bulk etch rates Figure 2 shows U ion track length distributions measured on six fragments of the same terrestrial crystal (diamond pipes), irradiated with 15.6 MeV/ amu U ions, etched for t~ = 1 h, in W N solutions with pH varying from 7.3 to 8.2 (it should be well understood that these are uncompletely etched tracks, whose length reflects the track etch rate v, averaged over time ti, and not the ion range, which is ~ 100 #m). The circles schematically represent the increase of the track cross-section (which actually is not at all a circle), in 1 h. The diameter of each circle is obtained by dividing the track width at the crystal surface, averaged over many tracks, by the total
76
C. P E R R O N and M. M A U R Y
etching time (i.e. 1 h, plus t2, which varies from 2 h for p H = 7 . 3 , to 10h for p H = 8 . 2 ) . It is equal to twice the bulk etch rate Vg, in that particular direction. The effect of varying the etchant pH is clear: Vg strongly increases when pH is decreased (a factor ~ 9, from pH 8.2 to 7.3) while v, increases much more moderately ( ~ 4 0 % on the same pH range). This is in qualitative agreement with the findings of Davie and Durrani (1978), who also reported an increase of vg when the pH of the W N solution was decreased. In addition, the relatively narrow length distribution becomes much broader at pH = 8.2, with a long tail on the short lengths side. Note, however, that there are a few short tracks at nearly all pH values. Very similar results have been obtained with meteoritic crystals, vt and v~ values, and the pH value at which track length distributions broaden, vary somewhat from crystal to crystal, but the picture remains essentially the same. Figure 3 shows the track length distribution obtained from a fragment of the same crystal as above, after 1 h etching in W N D solution at pH = 7.8. The bulk- and track etch rates are similar to those obtained with W N solution at pH = 7.9 and 7.3, respectively. The track cone angle is thus much smaller with W N D than with W N at the same pH, in agreement with the findings of Otgonsuren et aL (1976). 3.3. Variation o f track etch rate as a function o f time In Fig. 4, we show track length distributions obtained for crystal fragments etched in W N with pH = 8.0 (left-hand side) and pH = 7.8 (right-hand side), and etching time tl increasing from 10 rain (top) to 2 h (bottom). All these fragments came from the same meteoritic crystal (from the Eagle Station pallasite). For a given fl, the distributions are broader for the higher pH, as expected from the preceding section. F o r pH = 7.8, these data show a continuous decrease of the time-averaged track etch rate v, from ~ l l 0 # m h -I for f l = 1 0 m i n , to ~ 8 6 # m h -1 for t t = 2 0 m i n , ~ 5 2 # m h -1 for t ~ = l h and ~ 3 6 # m h -~ for q = 2 h . Etch rate measurements on 155 MeV/amu U ion tracks (Perron, 1984) show that this
u3
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LENGTH
50
60
(,IJm)
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FIG. 4. Etched track length histograms obtained for 7 fragments of an olivine crystal from the Eagle Station pallasite, irradiated by 15.6 MeV/amu U ions, after etching in WN solutions at pH = 8.0 (left hand side) and 7.8 (right hand side), and times varying from 10 min (top) to 120 min (bottom). Circles have same meaning as in Fig. 2. trend continues for longer etching times: v, ~ 25/tm h -~ for tl = 3 h and ~ 1 0 # m h -1 for q = 16h (on a different crystal, of course). Note however, that for pH = 8.0, v, apparently increases between tl = 10 min ( ~ 4 2 / ~ m h-1) and tl = 20 min ( ~ 75/~m h-l). Two other observations should be mentioned which show that there are other more discontinuous variations of t,, with time. One is illustrated in Fig. 5, which presents two photomicrographs of U ion tracks in olivine crystals etched for tl = 1 h. In Fig. 5(a), two tracks, shorter than the average, show a sudden change in diameter at about the same depth. A third one stops, also at this same depth. Obviously, although the crystal is optically homogeneous, etching has nearly stopped for these 3 tracks, while the etched channels were still enlarging. Etching finally resumed for 2 of them with probably the normal value of v,. This sort of etch-blocking phenomenon is not uncommon, and has already been observed by other workers (Price et al., 1973). Here, because it happens to 3 neighbouring tracks, and at the same depth, it is clear, apart from the arguments given in Section 3.1, that it is not due to the non-uniform distribution of radiation damage, but to some local inhomogeneity in the crystal. In some cases, the inhomogeneity is indeed visible, as illustrated in Fig. 5b. Here, two tracks stopped at the point where the ion paths crossed a tubular inclusion. There are plenty of such inclusions in this crystal, and the same phenomenon is observed, each time a track encoun-
VERY HEAVY ION T R A C K E T C H I N G IN OLIVINE
77
FIG. 5. Photomicrographs of 15.6 MeV/amu U ion tracks in 2 olivine crystals from diamond pipes (a) and Eagle Station (b). In (a), etching stopped for 3 neighbouring tracks, although the crystal is optically clear and homogeneous. Etching resumed after some time for two of these tracks, leaving a visible sudden change in track width. In (b), etching similarly stopped for two neighbouring tracks, when these encountered a visible tubular inclusion. Scale is the same for both photos. ters one of them. We tried to perform an electron microprobe analysis of one of these inclusions, which goes up to the crystal surface. It appears that it is made up of a mineral (or maybe more than one) with a composition different from olivine, rich in chromium and calcium. Work will be continued on this problem, but it is already quite understandable t h a t tracks are not revealed in this mineral by an etchant specially designed for olivine. The other observation is the following: in several cases, crystals etched according to our usual procedure showed no revealed tracks at all. This means that there is some time-delay before track etching really starts: if it has not yet started at the end of the
first etching step, then no track can be revealed by the longer etching either, since the annealing step completely erases the latent tracks. This occurred for crystals either repolished or not, for tl up to 20 min, and essentially for etchants of high pH ( ~ 8). No such absence of tracks has been observed with WN etchant at pH = 7.6 (i.e. when the bulk etch rate Vg is high), although t~ values as short as 5 min have been used in this case. The length of this time delay (or etch induction time, Ruddy et al., 1977) is highly variable from crystal to crystal (probably due to different orientations of the polished surfaces with respect to the crystallographic planes, leading to different bulk etch rates). There is some evidence that it also
78
C. P E R R O N and M. M A U R Y
effective etching time accordingly. The shorter tl, the larger the relative effect, so that v,, averaged over h, may seem smaller for t~ = 10m in than for 20min. Note that this is not observed at lower pH. The existence of such a time delay, or etch induction time (tin), has already been observed for a variety of plastic track detectors (Monnin and Sanzelle, 1970; Ruddy et al., 1977; Grabez e t a ! . , 1981). An "incubation time" has also been identified for mica by Price and Walker (1962), but it corresponds to a different stage of etching: these authors observed that the damaged trails produced by fission fragments in mica, were etched instantaneously (i,e. in fact in a time less than 4. DISCUSSION I s) over a width of 25-40 A, but then, these channels We have had clear evidence that the rate at which started enlarging further only after some incubation tracks are etched in olivine decreases with time even time. Although the characteristics of t~ that we can when no variation of vt with R is found by the deduce from our experimental results are only of a repolished crystal technique. This is most probably qualitative nature, it is striking that they are the same due to the increasing difficulty of renewing the etch- as those reported for plastics (Ruddy et al., 1977; ant inside the etched channels, as etching proceeds: Grabez et al., 1981): tin increases when either the bulk the longer the etched track, the more difficult it is for etch rate, or the radiation damage intensity decreases. the etch products to diffuse outside and leave room Ruddy et al. (1977) suggested that the existence of tin to fresh etchant, so that v~ decreases for chemistry might be due to the effect of the vacuum-to-solid reasons, although a higher etch rate value could be interface on the stopping power of the ions. Obviallowed by the radiation damage intensity. One can ously, at least in our case, this explanation does not thus schematically view the measured etch rate v, as hold, since we noticed an etch induction time even for resulting from the competition between two com- crystals which had been repolished, i.e. for which the ponents. One, which we shall call the "intrinsic etch etched surface was not the irradiated surface, but rate" v,~, would be the observed etch rate if fresh was rather an internal part of the crystal, at the time etchant had free access to the tip of the tracks (this of the irradiation. Most probably, the explanation is approximately realized when etched tracks are should be sought again in the etching chemistry. still very short). For a given etchant, vt~ essentially This is supported by the observation of Price and depends on the radiation damage intensity and is Walker (1962), since their incubation time applied to therefore range-dependent. Its variation as a function undamaged material. of R is a characteristic of the ion involved. The The above discussion has several important consecond component, which we shall call the "chemical sequences. A measured variation of v, as a function etch rate" vtc, is the maximum etch rate allowed by of R may be strongly biased by chemical effects, and the renewing of the etehant in the etched channels, v,c only slightly reflect the variation of vt~, hence of the is of course range-independent, but depends on the radiation damage intensity. One should therefore be length of the etched track: it is very high, when tracks very careful when using v, measurements to check are still very short, then decreases as the tracks get track formation models. For instance, the comlonger. At any time, the effective etch rate is given by: parison of the variation of v, vs R for high energy U ions in olivine, with that of the primary ionization, vt = M i n ([3ti , l)tC ) • made by Pellas and Perron (1984) now appears not F o r relatively light ions, like Fe ions and fission to be really meaningful, v, measurements are only fragments, for which tracks are short and va has valid for the experimental conditions (revelation techmoderate values, etching is essentially governed by nique used, duration of etching) under which they vti. On the contrary, for the heaviest ions, for which have been obtained. It is clear that the v, variation vt~ has high values over considerable lengths, v,c plays measured for U ions with the repolished crystal the major role, except when etching the "high energy technique (v, plateau, Fig. lc) cannot be applied to end" of the tracks, where v, is low, and also at the the sequential etching of tracks from the irradiated very beginning of etching, when the etched portion of surface (continuous decrease of v,, Fig. 4). Also, the tracks is short, and v,c high. curves obtained with the same repolished crystal The initial increase of vI observed between h = technique, but for different etching times, may 10min a n d h = 20rain for W N at pH = 8.0 (Fig. 4) be quite different, as attested by results on high may seem paradoxal in this context. Recall, however, energy U ion tracks etched for 3 and 16 h (Perron, that we have evidence that etching may be consider- 1984). ably delayed at high pH values. The observation is Because of the continuous decrease of vtc with time, then easily explained if one assumes a time delay of very long tracks cannot be completely revealed with several minutes before start of etching, reducing the classical techniques, which is of great importance for depends on the radiation damage: when measuring etch rate versus residual range, for Kr ions, no tracks were revealed for R > 40/zm, with t~ = 15 rain. However, with t~ = 2 h, tracks were visible for R up to ~ 70 # m, and with lengths such that they should also have been seen on about this whole range for t~ about 8 times shorter, if there was no time delay (see Fig. la). Similar results have been obtained with 155 MeV/amu U ions (Perron, 1984), but in this case, for R > 1200/xm, tracks only appeared after more than 3 h of etching.
VERY HEAVY ION T R A C K E T C H I N G IN OLIVINE studies of Ultraheavy Cosmic Ray tracks in meteorites. Indeed, Perron (1984) has shown that U ion tracks are revealable over a length of about 3 mm, but was unable 'to reveal them on more than about 1.3 ram, when the etchant could reach the tracks at only one point. Moreover, since vt, should' not depend on the nature of the ion, the revealed portion of the tracks would most probably have been of about the same length, had the incoming particles been say Pb or Pt ions. Conversely, a measurement of the etched track lengths would not permit to identify the ions which produced them. We plan to experimentally check this point. If it is true, the only way out is to use techniques which allow the etchant to reach every track at many points along the ion path, so that the whole etchable track length can be revealed. Such techniques are presently under study in this laboratory. Although all the experiments we have reported have been performed on olivine crystals, it is likely that the previous discussion also applies to other mineral detectors. On the contrary, the results concerning the etchant composition are certainly more specific of olivine. It is remarkable that small changes in the etchant composition may have important effects on track revelation. In particular, the fact that vs varies much more with pH than v,, implies that a small variation of pH may significantly change the etchable track length, and the critical angle arcsin (Vg/V,) belo~v which tracks are not revealed, and thus the etching efficiency. The large scatter of experimental points that we often observed in track etch rate measurements is apparently due to our using etchants of high pH (around 8). Although we know how to avoid this now (use lower pH), the reason why it is so is not entirely clear. The scatter may be due to the etch induction time, as it probably varies somewhat from track to track. Since t~, is shorter for etchants with low pH, the effect is expected to be less important with these etchants. It may also be the result of the same sort of phenomenon as that illustrated in Fig. 5, the latter being just an extreme example: in optically homogeneous crystals, there may exist heterogeneities in structure and/or chemical composition, which can cause large variations of track etch-rate. In this hypothesis, WN solution would be more efficient at low pH, to overcome these obstacles. In any case, the fact that track etching can be blocked at some places for relatively long times, is another reason for revealing long tracks from many points, evenly distributed along the ion trail. We now systematically use WN solution with pH = 7.6 when we make track etch rate measurements, i.e. etchings of short duration, for which we want narrow length distributions and short etch induction time. The fact that the bulk etch rate is rather high in this case, is in general not disturbing, and it makes tracks become visible faster. On the other hand when performing long etchings, to reveal
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long tracks, we use W N D solution with pH = 7.8, which gives a much smaller cone angle. 5. CONCLUSION The results of the experiments that we have described have confirmed that track etching in minerals---and particularly in olivine---is a complex process, which we certainly do not fully understand yet. They nevertheless provide some hints which may be useful in finding ways to avoid undesired phenomena, and correctly reveal tracks. This is specially true in the case of the very long tracks produced by very heavy ions, for which the decrease of the track etch rate with time, and etch blocking features might lead to totally confusing results. Acknowledgements--We are grateful to R. Spohr and the G.S.I. staff for their willingness to perform the heavy ion irradiations and their efficiencyin conducting them, to M. Bourot-Denise for the electron microprobe analyses, and to P. Pellas for constant support and invaluable discussions. One of us (C.P.) is indebted to V. P. Perelygin for many discussionsduring a stay in Dubna in 1979, which were the origin of the work presented here.
REFERENCES Dartyge E., Duraud J. P., Langevin Y. and Maurette M. (1981) A model of nuclear particle tracks in dielectric minerals. Phys. Rev. B23, 5213--5229. Davie I. W. and Durrani S. A. (1978) Anisotropic track etching in olivine crystals using WN solution. Nucl. Track Detection 2, 199-205. Fleischer R. L., Price P. B., Walker R. M., Maurette M. and Morgan G. (1967) Tracks of heavy primary cosmic rays in meteorites. J. Geophys. Res. 72, 355-366. Grabez G., Vater P. and Brandt R. (1981) The etchinduction time (T~) and other registration properties in CR-39 detectors for well-definedions. Nucl. Tracks 5, 291-297. Green P. F., Bull R. K. and Durrani S. A. (1978) Particle identification from track etch rates in minerals. Nucl. Instrum. Meth. 157, 185-193. Krishnaswami S., Lal D., Prabhu N. and Tamhane A. S. (1971) Olivines: revelation of tracks of charged particles. Science 174, 287-291. Monnin M. and Sanzelle S. (1970) Le chlorure de polyvinyle:un d~tecteur de traces int6ressant. Radiat. Effect 5, 125-128. Maurette M., Pellas P. and Walker R. M. (1964) Cosmic ray induced particle tracks in a meteorite. Nature, Lond. 204, 821-823. Otgonsuren O., Perelygin V. P., Stetsenko S. G., Gavrilova N. N., Fieni C. and Pellas P. (1976) Abundances of Z > 52 nuclei in galactic cosmic rays: long-term averages based on studies of pallasites. Astrophys. J. 210, 258-266. Pellas P. and Perron C. (1984) Track formation models: a short review. Nucl. Instrum. Meth. Phys. Res. B1, 387-393. Perron C. (1984) Relativistic 23sUion tracks in olivine and cosmic ray track studies. Nature, Lond. 310, 397-399. Perron C. and Pellas P. (1983) Can we get the cosmic ray actinide abundance from the study of tracks in meteorites? Proc. 18th Int. Cosmic Ray Conf., Bangalore, Vol. 9, pp. 127-130. Tata Institute of Fundamental Research, Bombay.
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Price P. B. and Walker R. M. (1962) Chemical etching of charged particle tracks. J. Appl. Phys. 33, 3407-3412. Price P. B., Lal D., Tamhane A. S. and Perelygin V. P. (1973) Characteristics of tracks of ions of 14 < Z < 36 in common rock silicates. Earth Planet, Sci. Lett. 19, 377-395. Ruddy F. H., Knowles H. B., Luckstead S. C. and Tripard
G. E. (1977) Etch induction time in cellulose nitrate: a new particle identification parameter. Nucl. Instrum. Meth. 147, 25-30. Storzer D., Poupeau G. and Kr~tschmer W. (1973) Trackexposure and formation ages of some lunar samples. Proc. 4th Lunar Sci. Conf. Geochim. Cosmochim. Aeta 3, Suppl. 4, 2363-2377.