Workshop on Polarized Electron Sources and Polarimeters

FIGURE 3. Sequence of activations in the HABS apparatus
maximum q.e. seems to remain constant for a few activations, even without further
treatment, before a reduction is observed, see activations 12-16. The behavior of the
SL’s concerning hydrogen treatment is similar to bulk GaAs. The DBR cathode 7-396
has achieved the highest q.e., yielding more than 10% at 650 nm. 7-395/7-396 cathodes
were transferred to our test source, a 7-395 went to the source at the MAMI accelerator.
Results in Production source: These tests were performed with a high intensity laser
system operated at wavelengths of 785 and 802 nm, close to the maximum polarization
response of the cathode. We observed a ten times increased q.e. at low intensities for
the HABS treated sample if compared with an untreated one. Moreover, the saturation
effects at high intensities had disappeared resulting in a factor 50 gain of maximum
current. Tests were performed at a q.e. of 0.6% and 200 µA of average current (density
of ≈ 200mA/cm 2 ) without showing nonlinear effects. Beam polarization was measured
with a Mott polarimeter that offers a reproducibility of about 1%, whereas the absolute
accuracy is ∆P/P = ±4%. After one week of operation at the accelerator the q.e. had
dropped by a relative 15% which resulted in a relative increase of polarization by 6%,
yielding an absolute value of P = 85.5 ± 3.2%. This value is within the highest ever
observed at our production source.
Results in test source: Comparisons of maximum polarization for treated/untreated
samples were done at the test source. We found no difference (at the 3% (relative) level)
between the different samples once the initial q.e. of the hydrogen cleaned one was
slightly reduced (see above). This provides evidence that hydrogen treatment has no
detrimental effect on the bulk properties of the superlattice structure. A tunable laser
was used for spectrally resolved measurements. The lower curve in figure 4 shows the
results for an untreated sample of 7-395, the upper one those from a fresh one measured
at St. Petersburg. The results for a hydrogen treated sample are close to the St. Petersburg
measurements. Again, saturation effects were present for the non treated sample which
would make the discrepancy in figure 4 even larger at high laser intensities. The DBR