Detection System for KeV Proton Beam Scattered from an ESD-surface with Monolayer Pits of Alkali Halide Crystal

Abstract

To observe specular oscillations in ion-surface scattering originating from layer-by-layer desorption of an electron stimulated desorbed (ESD) surface, a 15 keV proton beam was impinged on the surface with an angle of incidence less than 1°. We prepared an experimental system to examine the scattering yields of re‰ecting protons by conveniently employing a ‰uorescent screen to view scattering yields (scattering patterns) and a commercial digital camera for yield measurements. Because the present energy of the proton beam was too low to illuminate the screen, a micro-channel plate was inserted in front of the screen. Measurements with electron irradiation show obvious damping specular oscillations; the widths of angular distributions of scattered protons simultaneously oscillated with increasing electron ‰uence. From the measured periods of the oscillations, the ESD rates of thermal desorption from KBr(001) at 1.5 keV electrons irradiation were obtained for sample temperatures.

Desorption Yields and Irradiation-Condition Dependence of Layer-by-Layer Removal for KBr(001) Surface Stimulated by Electrons

“…Szymonski et al observed damping oscillations for the yields of desorbed atoms, both alkali and halogen (a thermal component) ones, which provided evidence of layer-by-layer desorption at the ESD surface 2,3) . These measurements reveal the following scenario, introduced briefly in our previous work 4) :…”

“…The main part of the experimental setup has been described in our previous work ; hence, only a brief outline is mentioned here 4) .…”

“…The sample surface was irradiated by 1.5-, 2-, and 3-keV electrons from a gun connected to a port of the UHV chamber with an incidence angle of 60°m easured from the (001) surface. At the entrance of the chamber port, the electron beam was collimated by a 1.5×10 mm 2 slit and scanned in two dimensions using a modulated line scan method 4) . The electron beam in front of the sample was spread over an area of 30×30 mm 2 , and the current density was adjusted in the range of 20-70 nA cm -2 , which was monitored using a Faraday cup (FC) with a fluorescent screen that also operated as an electron suppressor.…”

“…The intensity distributions of the scattered patterns were obtained from the B values of the RGB color values of the images measured with a bias voltage of 850 V for the MCP. In total, 13 min were required to obtain each distribution set for the irradiation fluence 4) . As the current density of the incident beam was too large to be measured by the MCP, the bias voltage of the MCP was suppressed to 600 V 4) .…”

“…Recently, an experiment was performed to observe the specular oscillation caused by layer-by-layer desorption using a 15-keV proton beam, and some results concerning the ESD rates for KBr(001) were introduced 4) . Despite a long history of research into this phenomenon, there are few reports of measurement of the absolute yields of emitted atoms per incident electron.…”