|Budget Amount *help
¥2,200,000 (Direct Cost : ¥2,200,000)
Fiscal Year 1995 : ¥800,000 (Direct Cost : ¥800,000)
Fiscal Year 1994 : ¥1,400,000 (Direct Cost : ¥1,400,000)
We have been measuring positron lifetime spectra using an energy valuable pulsed positron beam. It enabled us to measure a positron lifetime reflecting a surface electron density under the condition the positron stops in the neighborhood of the surface. We had found out a specific long lifetime on the HOPG (highly oriented pyrolytic graphite) surface compared with that in the bulk. When the sample was heated, it had become shorter and a positronium emitted from the surface into the vacuum was observed. Thereby, we had once considered that a positron occupies the surface trapped state, and it can be exited by heating and finally emitted into the vacuum as positronium capturing an electron.
However, from the present study introducing a positron Auger spectroscopy under the UHV condition, we have newly found out that the above hypothesis resulted from adsorbed impurities on the surface. Even for the sample which we regarded as clean from an electron Auger spectroscopy, We could observe a c
lear signal from oxygen atoms as well as carbon atoms which is constituent of HOPG by positron Auger spectroscopy. It utilizes a vacany produced by a pair annihilation of a positron with a core electron of the constiuent atoms of the surface. It is regarded that a positron itself tends to approach an oxygen atom of H_2O or hydrocarbons on the surface. When the sample was heated, these oxygen signals reduced, the positron lifetime became shorter, and an emitted positronium increased. From these apparent correlations, we concluded that the reason why the positron's behavior on the surface was not so well understood is the unexpected adsorbants whose quantities are less than the detection limit of the ordinary technique.
Only a positron Auger spectroscopy with the pulsed beam enables us to detect atoms with smaller atomic numbers which are important for practical applications, because of the beam intensity, and such was done for the first time in the world. Moreover, we can detect with very high sensitivity the oxygen of a very small amount which is undetectable by the ordinary technique. This suggests that it is a powerful tool for the surface diagnosis of impurities of a very small amount.