Thickness dependence of positron induced secondary electron emission in forward geometry from thin carbon foils

Abstract

a b s t r a c tSecondary electron (SE) emission from thin carbon foils induced by 1-20 keV positrons has been investigated over a range of nominal foil thicknesses from 1:0 to 5:0 lg=cm 2 . The measurement of SEs was carried out in forward geometry using a microchannel plate as a detector. The SE yield c has been measured as a function of beam energy and compared with our Monte Carlo simulation results. We also present in this paper the material parameter K ¼ c=ðdE=dxÞ and the emitted SE energy spectra. For incident positron energy of 5 keV or higher, the distribution is found to be characterized by the Sickafus form, AE Àm and m is close to 1. For low energy incident positrons, however, another form, BexpðÀE=tÞ, is proposed for describing the SE distribution.

Characterization of a transmission positron/positronium converter for antihydrogen production

“…The reflection of electrons with a bias of À200 V is in agreement with the expected energy of secondary electrons forward-emitted from thin (5-23 nm) carbon films after 1-20 keV positron implantation, which is less than 100 eV as reported in Ref. [28]. Electrons emitted from the carbon side produce a spot well distinguishable from the one due to e þ , even if no attractive potential is present on the MCP surface.…”

“…8). Given the distance of 3 cm between the target and the MCP, the corresponding kinetic energy for the electrons ranges from a fraction of eV up to few eV (100 ns TOF corresponds to a kinetic energy of $0.3 eV, while 50 ns TOF corresponds to a kinetic energy of $1 eV), in good agreement with previous measurements performed by implanting e þ with an energy ranging from 1-20 keV in thin carbon foils [28].…”

“…Electrons released by positron impact can be emitted into vacuum if they reach the surface before their energy becomes lower than the electron work function of the material [27,28]. In SiO 2 , the electron work function is quite high, 10-12 eV [16].…”

“…Therefore, only hot electrons produced in silica can overcome the surface barrier and leave the target. The emission energy of almost all these electrons is expected to range from a fraction of eV to some tens of eV; emission of secondary electrons with an energy higher than a few hundreds of eV is negligible [27,28]. As the columnar silica structure is highly irregular, the electrons have no preferred emission direction.…”

“…Due to its limited thickness, a fraction of positrons are expected to cross the target after partial thermalization. Moreover, secondary electrons are produced by e þ interaction with the material and possibly emitted by the target [27,28]. The presence of charged particles emitted in the direction of the antiprotons could pose a problem for the stability of the plasma; it could heat up by interaction with positrons and electrons [29].…”