p. /8 SECONDARY ELECTRON EMISSION CHARACTERISTICS OF LUNAR SURFACE FINES. M. Anderegg, B. Feuerbacher, B. Fitton, L. Laude, R.F. Willis, Surface Physics Division, European Space Research Organisation, Noordwijk, Holland. Calculations to date1 s2 394 have shown that the steady-state electro- static potential and charge distribution of the sunlit lunar surface is deter- mined primarily by the photoemissive properties of the lunar surface material and the impacting solar wind flux. Only a small fraction of the emitted photo- electrons can escape, which implies that the sunlit lunar surface attains a small positive electrostatic potential, the magnitude of which is determined by the energy distribution of the photoemitted electrons and the solar wind particle velocity distribution and density. Secondary electron emission due to the absorption of solar wind particles has been assumed to be negligible in view of the low energies of solar wind electrons (charged particle lunar environment experiment (CPLEE)~have shown that, in addition to a stable low- energy photoelectron flux, observed for energies ranging from 200 eV down to 35 eV, rapidly varying fluxes of low energy electrons of magnetospheric origin occur, with intensities greater than that of the photoelectron background. When the lunar surface is illuminated, the electron spectrum between 40 and 100 eV is dominated by the photoelectron continuum, but in the higher energy ranges, prominent peaks in the electron flux density were observed in the ranges 300 to 500 eV and at several keV respectively. These higher energy electrons will give rise to secondary electron emission at the lunar surface, which in turn, will affect the lunar surface charge and potential distribution depending on the secondary emission characteristics of the lunar material. We have therefore measured the secondary electron yield and energy dis- tribution of lunar dust sample 14259.116 over a primary electron energy range of 50 to 2000 eV in order to obtain a more complete description of the low energy electron layer and the electrostatic potential of the sunlit lunar surface. Detailed Auger electron analysis7 has, in addition, provided infor- mation concerning the elemental composition of the sample. The samples were handled in dry nitrogen atmosphere and the secondary electron yield and energy distribution measurements were made using an hemispherical collector system as described previously8. Bakeout of the ultra-high vacuum system and sample was carried out at 1500 C for one week in order to simulate lunar thermal condi- tions and to remove adsorbed nitrogen. Preliminary results for the total secondary electron yield and energy distribution curves for primary incident electron energies in the above range are shown in Fig. 1 and Fig. 2 respec- tively. The secondary electron yield curve, figure 1, shows a maximum of 1.4 at approximately 30b-400 eV, which is somewhat lower than that observed9 for insulating materials such as Si02, A1203, CaO, FeO and MgO, the major consti- tuents of the sample. This is expected in view of the particulate nature of the sample from which the secondaries cannot escape due to scattering within microscopic cavities. Also no elastically reflected electrons were detected,

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System SECONDARY ELECTRON EMISSION CHARACTERISTICS OF LUNAR SURFACE FINES. M. Anderegg

Fig. 1 Secondary electron yield from lunar sample 14259.1 16 for incident primary energies of 50 to 2000 eV.

PRIMARY BEAM ENERGY ( eV )

even at primary beam energies below 50 eV which suggests that the primary electrons are either completely absorbed or, what is more likely, are only reflected back along the direction of the incident beam. This behaviour is very similar to that observed for the lunar albedo variation with phase angle being maximum at zero phase angle1'.

Fig. 2 Energy distribution curves of secondary electrons emitted from lunar sample 14259.116 for various primary energies. Note the reduced size of the low energy secondary peak and the distorted spectra compared with the curve at 1750 eV primary energy due to charging of the sample.

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System SECONDARY ELECTRON EMISSION CHARACTERISTICS OF LUNY SURFACE FINES, M. Anderegg

P&o Values of the yield greater than 1 cause the dust to charge positively i.e., for incident electron flux energies in the range 100-200 eV to 1500-2000 eV. The effect of this charging can be seen in the reduced size of the low energy secondary peak and the distortion of the energy distribution spectra for primary energies in this range, fig. 2. The 'true' secondary electron energy distribution is that shown for a primary energy of 1750 eV, for which the yield is unity. The curve is typical of that observed for insulating materials, i.e., the greater number of secondaries occur with energies less than about 4 or 5 eV. Preliminary measurements indicate the maximum positive charge on the specimen to be less than 20 eV. It is significant that the in- creased electron flux density observed by the CPLEE at energies of 300 to 500 eV when the moon passes through the magnetospheric tail, is also the energy at which the lunar dust sample possesses maximum secondary yield, which implies that such events will cause the lunar surface to charge even more positive and so modify both the photoelectron and secondary electron energy distributions. Incident electron energies below 100 eV, for which the secondary yield will be negligible, and above 2,000 eV, at which increased electron flux density has also been observed6, will cause the surface to charge negatively. These preliminary results therefore, indicate that the lunar surface charge and potential will depend not only on the solar wind flux and the photoemissive properties of the lunar material, but also on the energy distri- bution of the magnetospheric electrons and secondary electron emission of the lunar surface. Future calculations will be based on such considerations and the data presented here.

References : 1. E.J. Opik and S.F. Singer, J. Geophys. Res. 65, 3065 (1960). 2. E.J. opik, Planetary Space Sci. 9, 221 (1962c 3. H. Heffner, Rept. TE-7 of the '~ycho'Study Group, University of Minnesota, Minneapolis (1965). 4. W.D. Grobman and J.L. Blank, J. Geophys. Res. 74, 3943 (1969). 5. J.C. Brandt in "Introduction to the Solar wind", W.H. Freeman and Co. , San Francisco (1970). 6, B.J. O'Brien and D.L. Reasoner, P. 193, Apollo 14 Preliminary Science Report, NASA SP-272 (1971). 7. For a review of Auger electron spectroscopy see: C.C. Chang, Surface Sci. 25, 53 (1971). 8. M. hdZegg, B. Feuerbacher, B. Fitton, L. Laude, R.F. Willis, to be presented at the Third Lunar Science Conference, Houston ( 1972). 9. D. J. Gibbons in "Handbook of Vacuum Physics", ed. A.H. Beck, Vol. 2, Part 3, Pergamon Press (1966). 10. G. Rougier, Astronomie -48, 224 (1934).

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System