Low-temperature measurements of the fraction of re-emitted positronium from a Cu(111)+S surface

Positron reflection from the surface potential

“…Both the present data and annihilation line-shape measurements 18 show that the surface trapping rate is v^« 10 5 ms~l with practically no temperature dependence. Our findings are direct observations of quantum-mechanical reflection, where we have utilized the fact that positrons have thermalized, in all three dimensions, down to at least 24 K. 26 Inappropriate separation of the thermal and epithermal contributions from the measured data is one of the reasons explaining the failure of the early experiments 7,8 to observe e + reflection. 18 increase in the e yield with 18 As more accurate trans- Reflection will also affect other experimental surface studies: E.g., the value of the sticking coefficient for lowenergy helium scattering at low temperatures is a longstanding problem.…”

“…6 Thus the surface should become fully opaque at 0 K for any abruptly changing surface potential, and e + and Ps yields should vanish at low temperatures. However, experiment did not support these predictions, showing only weakly temperature-dependent e + and Ps yields down to 30 K. 7,8 To account for this discrepancy Wilson 9 proposed that all surface processes are dominated by inelastic processes, while Neilson, Nieminen, and Szymanski 10 explained the high yields by allowing the reflected e + many encounters with the surface.…”

Positron Quantum Reflection in Thin Metal Films and Efficient Generation of High Brightness Low Energy Positron Beams at 4.2 K

“…Furthermore, the energy spread, De, of the emitted e 1 's have been measured as a function of temperature [6,7], showing a significant reduction of De as the temperature is lowered. The temperature dependence of f 1 has also attracted much theoretical attention [8,9] as has e 1 thermalization in cold solids [10].In Ref.[5] it was demonstrated that the reemitted e 1 yield, f 1 , and the Ps yield, f ps , follow the same temperature dependence, and it was shown that both quantities for Cu(111) and Al(110) reduces by a factor of 2.5-3 when the temperature is lowered from 300 to 20 K. Extrapolation of these results show that f 1 and f ps are extremely small at 4.2 K.Other experimental data on f ps [3,4] have shown only a weak temperature dependence at temperatures below room temperature. The latter results were obtained by implanting rather low energy e 1 's into the samples under investigation and may therefore contain effects due to epithermal e 1 's.…”

“…The understanding of QR is also important for the production of high brightness low energy e 1 beams. It is clear, at least in principle, that the energy spread of the elastically emitted e 1 's can be narrowed considerably by operating the b 1 moderator at 4.2 K as opposed to room temperature.There exist several measurements of the yield of low energy e 1 , f 1 , from various metals versus temperature [3][4][5]. Furthermore, the energy spread, De, of the emitted e 1 's have been measured as a function of temperature [6,7], showing a significant reduction of De as the temperature is lowered.…”

“…There exist several measurements of the yield of low energy e 1 , f 1 , from various metals versus temperature [3][4][5]. Furthermore, the energy spread, De, of the emitted e 1 's have been measured as a function of temperature [6,7], showing a significant reduction of De as the temperature is lowered.…”