Workshop on Polarized Electron Sources and Polarimeters

of surface cleaning for NEA photoemitters. Good results were achieved if a thermal hydrogen
cracker was used, which may be due to the absence of energetic ions and neutrals
in the atomic beam of this source.
HYDROGEN CLEANING APPARATUS
We have set up a dedicated apparatus, called HABS - Hydrogen Atomic Beam Source -
which allows for the following operations
• Hydrogen cleaning by a beam of thermally cracked hydrogen atoms.
• Activation to NEA state and q.e. measurement in order to test cleaning effect.
• Transfer of samples under UHV to other apparatuses for further investigations.
The apparatus consists of a bakeable all metal UHV system (fig. 1). The hydrogen source
is a commercial one, similar to the design in [4]. It consists of a heated tungsten capillary
which reaches a temperature of about 2150 C at a heating power of 300 watts. H2 flowing
through the hot capillary is dissociated. A beam of atomic hydrogen then hits the sample
surface which is located at a distance of about 10 cm from the end of the capillary. The
dissociator is enclosed in a water cooled copper shielding which reduces the radiative
heating of the inner surfaces and of the sample itself. Clean hydrogen is provided by
using pure H2 gas and further purification is achieved by letting it stream through
a getter. H2 is then stored in an all metal enclosure and introduced by a leak valve.
Hydrogen pressure during treatment was 2 · 10 −6 mbar stabilized by a turbo pump (fig.
1). Typical exposure times were about 5 minutes during which the samples are heated
to 550 ◦ C. Then, the activation procedure is started, consisting of a 30 minute long heat
treatment at 550 ◦ C and co-deposition of cesium and oxygen after cooling down the
sample. The q.e. achieved may be compared with the one from untreated samples (fig. 3).
Once a promising result is achieved, the samples are cleaned again without subsequent
activation. Afterwards they are transported to one of the 100 keV sources in our labs
for further investigations which are not possible within HABS (e.g. polarimetry). For
this purpose the samples are transferred into the transport vessel by a manipulator. The
vessel is equipped with an ion pump. An intermediate loading chamber is pumped by
the turbo already mentioned above. After shutting off the transfer vessel by a valve the
loading chamber is vented and the vessel may be removed. Meanwhile, another (closed)
valve allows to keep the vacuum in the HABS chamber. The vessel is then transported
to one of the electron sources where similar loading chambers exist. All stages of the
transport are performed under UHV.
HABS RESULTS
Photocathodes: The structure of the superlattice (SL-)cathodes labelled 7-395 and 7-
396 is presented in fig. 2. In both cases the working layer consists of 12 periods of double
layers with a composition In.2Al.19Ga.61As/Al.4Ga.6As, yielding a total thickness of
91 nm for the working layer of the cathode. Due to the fact that all layers contain Al it is
necessary to inhibit interaction with oxygen by placing a very thin layer of GaAs on top