Influence of temperature on track registration in olivine

0191-278X/89 $3.00 + .00 Pergamon Press plc

Nucl. Tracks Radiat. Meas., Vol. 15, Nos. 1--4, pp. 45--46, 1988 Int. J. Radiat. Appl. lnstrum., Part D Printed in Great Britain

INFLUENCE OF T E M P E R A T U R E ON TRACK REGISTRATION IN OLIVINE C. PERRON a n d M. BOUROT-DENISE

CNRS and Museum d'Histoire Nsturelle 75005 Paris, France

Abstract - Track etch rate and total etchable track length have been compared for Xe ion tracks registered in ollvlne crystals, at two different temperatures (160K and 293K). A small registration temperature effect has been found. It appears to vary with the crystallographic orientation of the tracks, beeing nearly inexlstent in some cases. Because of its small amplitude, this effect should not pose any serious problem for the study of ultraheavy cosmic ray tracks In meteorites.

I. INTRODUCTION The influence of the temperature of a track detector at the moment tracks are recorded, on the response of this detector - known as the registration temperature effect (RTE) - has been recognized for several years now,~2for plastic detectors. It had dramatic consequences for cosmic ray experiments. ~ Whether such an effect also existed in mineral track detectors was not known, but has been implicitly ignored by workers in this field. A large RTE in minerals, however, could not a priori be excluded. Apart from the information it would provide on the basic processes which lead to track formatlon~ it would have important implications for studies on U fission tracks in terrestrial rocks~ and Pu fission tracks and cosmic ray tracks in meteorites and lunar samples. In particular, it would preclude the study of ultraheavy cosmic rays through the measurement of tracks in meteoritic ollvine, in which several groupsp including ourselves~ are engaged, since the temperature of meteorites must have considerably varied during their exposure to cosmic rays, on their way from the asteroid belt to the earth orbit. We have therefore carried out experiments to try to settle this point. Our results are brlefly presented here. A more detailed article will be soon submitted for publication. It must be pointed out t h a ~ very recently, James and Durrani ~5 reported on experiments demonstrating the existence of a RTE in several minerals. We shall see that our results agree, at least qualitatively~ with theirs. 2. EXPERIMENTAL Terrestrial and meteoritic olivine crystals were irradiated at 2 different temperatures (160K, which approximately corresponds to the temperature in the asteroid belt~ and 293K) by 25 MeV/u Xe ions at GANIL (Caen, France). Some of the latent Xe ion tracks were then partially annealed, under various conditions. The comparison of the detector response was made on pairs of fragments of the same crystals, for which, except the irradiation temperature, all experimental parameters were identical (track direction with respect to the crystallographic planes, anneallng temperature and tlme, etching, etc...). Two sorts of measurements were performed: i) track etch rate as a function of residual range, by the repollshed crystal technique. %7 This gives the most complete information on the revealable defects induced in the crystals by the passage of the ions. li) total etchable track length, by the TINT technlque~ This was done in a way much slmilar to that which would be followed in a study of cosmic ray tracks in meteorites. In this case, crystals were irradiated a second time~ at UNILAC~ by 16 MeV/u U ions, at right angle to the direction of the Xe ions. The tracks of these U ions are etchable from the crystal surface, and allow the etchant to reach and reveal the latent Xe ion tracks. Details on the experimental techniques can be found elsewhere.S, I°

3. RESULTS AND DISCUSSION Typical results are presented in flg.l and 2. Fig.l shows the variation of the measured track etch rate as a function of residual range R, for an unannealed crystal. Each point corresponds to the measurement of an individual track length. In order for the etch rate "curves" not to be too much distorted by effects due to chemical etching (etch induction time effects and diffusion limitation of etch rate), the data have been obtained for different etching times, as has been proposed earlier?: points below R~100~m correspond to 1 hour etching time, points above to 5 hours. In spite of these precautions, the etch rate is NT 15~I/4-E

45

46

C. PERRON and M. BOUROT-DENISE

likely to be still somewhat underestimated at low ranges (limitation by diffusion) and high ranges (long etch induction time). This, however, should not prevent the comparison of the two curves. The curve corresponding to an irradiation at low temperature clearly lles below that corresponding to an irradiation at room temperature. Similar measurements have been made on 10 different crystals (i.e. presumably 10 different directions of the tracks with respect to crystallographic planes), and 4 different annealing conditions for each crystal (includlng no annealing). The difference between the 2 curves is variable. In some cases, they are undlstlngulshable~ but in no case is the low T r curve above the high T r curve. 4O

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Fig. 2 - Total etchable track length distribution in an olivine crystal, for 2 registration temperatures. Tracks annealed as indicated.

F i g . 2 shows t h e t o t a l e t c h a b l e t r a c k l e n g t h d i s t r i b u t i o n s f o r a c r y s t a l a n n e a l e d a t 450"C during 5 hours. The distribution for low T r is slightly shifted towards low track lengths with respect to the high T r distribution. This type of measurement has also been performed on I0 crystals. The amplitude of the shift is variable, and close to zero for several crystals, but there is no example of a larger mean track length for the lower ~ . It thus appears that there does exist a small RTE in ollvlne: the etch rate is lower, and the track length shorter for the lower temperature irradiation, whether the tracks have been annealed or not. The amplitude of the effect seems to vary somewhat, probably with the orientation of the tracks with respect to the crystallographic axes. The difference in mean track length remains less than 10Z for the 133K temperature difference of our experiment. These results are not yet really understood, but may be an important clue to understanding track formation mechanisms in minerals. For studies of cosmic ray tracks in meteorltes~ the RTE found is too small to pose a real problem, particularly if compared with the variation of the mean track length from crystal to crystal, for a given ion. ~ The evidences presented by James and Durranl ~,s suggest the existence of an RTE in several other minerals, in the same direction as the one we found for olivine, although with an apparently larger amplitude. This may thus be a general characteristic of tracks in minerals.

4. ACKNOWLEDGEMENTS We are much indebted to E. Balanzat and J.M. RaILillon (CIR~L/GAN[L) Vetter for invaluable help with the heavy ion irradiations.

and R. Spohr and J .

REFERENCES D. O'Sullivan and A. Thompson. Nucl. Tracks 4, 271 (1980). A. Thompson and D. O'Sulllvan. Nucl. Tracks Pad. Meas. 8~ 567 (1984). D. O'Sulllvan, A. Thompson and P.H. Fowler. Nucl. Tracks Rad. Meas. 11, 95 (1986). K. James and S.A. Durranl. Nucl. Tracks Pad. Meas. 13, 143 (1987). K. James And S.A Durranl. Earth Planet. Scl. Left. 87~ 229 (1988). P.B. Price, D. Lal, A.S. Tamhane~ and V.P. Perelygln. Earth Planet. Scl. Lett. 19, 377 (1973). 7. D. Storzer, G. Poupeau, and N. Kr~tschmer. Geochlm. Cosmochlm. Acta 3, Suppl. 4, 2363 (1973). 8. D. Lal, R.S. PaJan, and A.S. Tamhane. Nature 221, 33 (1969). 9. C. Perron and M. Bourot-Denlse. Nucl. Tracks Rad. Meas. 12, 29 (1986). I0. C. Perron. Nucl. Tracks Rad. Meas. 12, 367 (1986). 11. C. Perron, M. Bourot-Denise, V.P. Perelygln, W. Birkholz, S.G. Stetsenko~ R. Dersch~ T.C. Zhu~ P. Vater, and R. Brandt. These proceedings. 1. 2. 3. 4. 5. 6.