As a first test of the performance of the apparatus in the low resolution
(non-decel) mode
we measured the absolute double differential cross section
(DDCS) for the production of Binary
Encounter electrons (BEe) in collisions of 20 MeV F 
H 
.
The experimental procedure followed has been discussed in detail elsewhere
[15, 14]. It has been used to perform
an accurate in-situ calibration of
the overall electron detection efficiency for
spectrometers [15, 14] and spectrographs [16].
In order to cover the energy range of the Binary Encounter peak in
20 MeV fluorine collisions with H , two overlaping spectra had to
be accumulated as shown in Fig. 3 using two different
tuning energies 
. The two spectra were
then energy and efficiency calibrated and joined
together at the overlapping energy regions.
To perform the absolute overall efficiency calibration
we first use bare 20 MeV F 
ions in collisions with
H . In the case of BEe production by bare ions,
measured DDCS at zero-degrees to the beam direction
are known to be well described by the Rutherford scattering of quasi-free
electrons of H within the elastic scattering model (also known as the
Impulse Approximation) [15, 14].
Thus, the measured bare ion
BEe DDCS were normalized to theory from which the overall
electron detection efficiency as a function of position
along the PSD was obtained. Then using this efficiency we correct the measured F 
BEe double-differential yields and convert them to DDCS.
In Fig. 3 theory and measurement for 20 MeV F +H
BEe production are compared. The theory is the result of a quantum-mechanical
calculation for quasi-free electron scattering on F ions within
the elastic scattering model [17]. It is seen that
theory is off by a factor of 1.12 from
our in-situ calibrated F DDCS. This is within the
20% estimated absolute uncertainty of our data.
Figure: Low resolution Binary Encounter electron spectrum, recorded in two
steps, for 20 MeV F H . Theory based on the elastic scattering
model was multiplied by 1.12 to best fit the data. Absolute error about
20%.
The Auger peak seen on the high energy shoulder of the BEe peak
is primarily due to the formation of the F 
state by
Resonance Transfer Excitation or RTE [18]. From the FWHM of
this peak an energy resolution of 
was obtained
showing that even in the non-decel mode fairly good resolution is obtained
by using the lens to define a much smaller virtual entrance slit.
After subtracting the BEe ``background 
and transforming the LAB spectrum to the CM frame we obtained
a value of 13.6( 
1.9) 
for the zero-degree
SDCS at a mean CM energy of 596.7( 0.9) eV.
The reported theoretical value is 11.5 [18].
In conclusion we have reported on the performance of a new non-typical
hemispherical spectrograph used at zero-degrees to the beam direction
in low resolution mode to measure
Binary Encounter electrons. These were then used to calibrate the
overall absolute electron detection efficiency.
Using this efficiency we determined the absolute DDCS for BEe production
for F ions and found it to be in good agreement with theory.
The double focusing properties of the
hemispherical analyser together with the substantial savings in collection time
compared to the voltage-scanned mode used with the more common
parallel-plate tandem electron spectrometers,
greatly improve the overall efficiency
of electron detection making it ideal for low intensity high resolution DDCS
measurements.
In the future we plan to use our apparatus in deceleration mode
to record much higher resolution spectra.