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Heavy-Ion Track Spectra in Lunar and Analogous Materials S, R

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Heavy-Ion Track Spectra in Lunar and Analogous Materials S, R HEAVY-ION TRACK SPECTRA IN LUNAR AND ANALOGOUS MATERIALS S, R. Malik, S. A. Durrani, A. Aframian and J. HeFremlin, Department of Physics, University of Birmingham, Birmingham B15 ZTT, England. In an effort to understand the heavily-charged external particle phenom- ena in lunar materials, heavy-ion bombardments were carried out on a number of solid- state track detectors (SSTD) (soda-lime glass, muscovite mica, Makrofol plastic and oli 'ne crystals, in ddition to luntt; pb~sses).The beams used were those of 56FeYf+ (total energy- 40 MeV) and 0 (total E-35 MeV), acceleratef6in 9+12 MeV Van de Graaff ('Tandem') Generator at AERE, Harwell, and Fe (- 9.6 MeV per nucleon: total EN 537 MeV) from the University of Manchester Linear Accelerator ('Linac'). In the Tandem generator, th ion beam, scattered by a thin Au foil (~100pg/cm ) inside an evacuated chamber, was made to fall perpendicularly at a series of SST detectors arranged in parallel strips. The Linac beam, agprogriatl- ly defocussed, fell directly on the SSTD. Track densities of -10 - 10 cm- were obtained with both machines. To investigate the effects of lunar erosion and churning, as well as those of intervening layers of lunar material on the etchable damage of incident particles, the detector strips were covered with a step-wise arrangemert of different thicknesses of A1 foil to degrade the ion energies by differing, know amounts. The mean etch-pit diameters in the SSNTD materials, corresp- onding to the residual ion energies, were measured by etching each strip (minus the A1 foils) under standard conditions. Theee conditions were as follows: Glass, 48.0 vo1 %HF at 21.0 + 0.05~~(controlled to within + 0. 1°c), for 5.5 sec; mica, as for glass, but B?ched for 25 min; MFrofol (type E, 1 -400 pm thick), 6.25 N NaOH at 60°c for 60 min; olivine, the etchant tWN' (EDTA, NaOH, oxalic and orthophosphoric acids), pH6, at 105'~for 5.2- hr. In Fig.l,set (b) shows the variation in the mean e&h-pit diameter, d, as a function of the residual energy E of the 40 MeV Fe ions, in all the SSTD materials; 3 gradually falls as the ionic charge gradually diminishes. The damage remains etchable to Fe energies as low as -2 MeV in all the SSTD except olivine. (The curves for glass and mica also show two points for recoiling Au ions; they fall at positions expected from the energetics.) Set (a) for glass shows how t hable damage, in fact, increases at first (the IBragg effect') as the Feaft ions slow down from very high energies (E-53'7 MeV), followed, after a broad maximum, by a decrease as their charge is progressively neutralized (as in (b)). An extra point on curve (a) is obtained by grinding off -93 pm from the top surface of a bare glass detector before etching. The inset (d) depicts the response of Makrofol plastic to high ener- gy Fe ions; here a stack of 8 sheets of Makrofol (type KG), each 22pm thick, was used, the etch-pitodiameters being measured on successive layers (but etching was done at 22 C for 3 hr). The results are not dissimilar in char- 0 Lunar and Planetary Institute m Provided by the NASA Astrophysics Data System HEAVY-ION TRACK SPECTRA S. R. Malik acter to those in glass. Fig. 2 summarizes some of the results obtained with a light-brown lunar spherule (-170 pm in diameter) from sample 15271, 90. Surfaces a, b and c, at depths-10, 21 and 51 pm below an initial external surface, were highly polished and etched under standard conditions (for 5.5 sec). Inset (i) shows the variation of total track density (observed with an optical microsc..; ope at 450 x) with depth; a distinct gradient is evident, suggesting that heav- ily- charged external particles (assuming U- distribution, and hence fossil fission, to be constant over the spherule) are responsible for a substantial fraction of the observed tracks (see also ref. 2). In inset (ii), the etch-pit diameter distribution at various depths is shown. At level c (-51 pm), the proportion of high-diameter etch pits is distinctly greater than at levels a and b; this is compatible with Fig.l(a) which shows an increase in the etch- able damage as a high-energy (9.6 MeV/nucleon) Fe ion slows do-:m. The histograms in set (iii) show the results of annealing the spherule at 500~~ for 10 min, followed by etching at level d(~63pm); for comparison the hist- ogram of unannealed tracks at c is reproduced. Histogram marked d pre- sumably represents the residue of the high-diameter parts of c. The prop- ortion of surviving tracks (73070) is surprisingly high, when one compares - the results with those shown in Fig 3 for 40 MeV Fe ions in soda-lime glass. (The fraction surviving in a tektite3 after such a degree of annealing isrl5%). One has to conclude that either the etchable damage after a further 10 pm of traversal beyond surface c is quite substantial, or, more probably, that some other property (e. g. high silica content) makes this spherule highly resistent to track annealing. As discuseed in a companion paper (ref. 2), the density of heavily-charged particle tracks in a lunar spherule can be used to estimate the cosmic-ray dose rate at the mean depth of exposure if proper allowance is made for the effects of annealing due to ambient lunar temperature. References 1. Krishnaswami S. , La1 D. ,Prabhu N. , and Tamhane A. S. (1971) Olivine : revelation of tracks of charged particles. Science, 74, 287- 291. 2. Khan H. A. ,Durrani S. A. , and Fremlin=9%) Radiation history of some Apollo 16 >mar glasses by track annealing. This volume. 3. Durrani S.A. and Khan H.A. (1970) Annealing of fission tracks in tektites. Earth Planet. Sci. Lett. , -9, 431-445. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System HEAVY-ION TRACK SPECTRA S. R. Mallik 111.2 ~a~rrld herled Track Spctwn md ProUle In *pollo I5 GLI.. (Sample 15271.90) 1100 - (ii) Light brown ephcrula 1501 b (Etched In 88.0% HF at 1200 '------ 2l.o~0.5 C) Ma~niHcaUon:450 x N- 1000 . Track density pmUlt - a aoo . -:: -d 5 600 -.-! - - C d t roo . o 10 LO 30 40 50 M c Depth below initial murfacc (Irm) (<I (iii) -. - , -. - .' LOO . (4 ! 4 LOO L - O 7 0 .5 1:s 2:s 3.5 1.5 Etch-Pit Diameter, dkm) .5 1.5 2. 3.5 1.5 Etch-Plf Diameter. d1un.i 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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