With certain modifications to the anode readout structure, it is possible
to measure the position of a localized charge cluster in a micro-detector with
pads that are wider than have been used so far. A thin high surface resistivity
film is glued to a separate readout pad plane and is used for the anode (fig.
1). The resistive anode form a 2-dimensional RC network with respect to the
readout pad plane. Any localized charge arriving at the anode structure will
be dispersed. With the initial charge dispersed and covering a larger area
with time, wider pads can be used for the signal pickup and position determination.
Below is a summary of the studies.
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The signal on a readout strip can be computed by integrating the charge density function over the strip width. Furthermore, the finite extent of the initial charge cluster, the intrinsic detector rise time as well as the rise and fall time characteristics of the front-end electtronics determine the shape of the measured pulse. All these parameters need to be included in the model to compare to experiment.
Model calculations were done for a GEM detector with a resistive anode readout
with 1mm wide strips. The anode resistivity and anode-readout gap in the simulation
were chosen to limit the charge dispersion over pads to about 700um, comparable
to transverse diffusion in a high magnetic field TPC. Simulated signals for
the readout strip directly below the initial charge cluster and for the 4 adjacent
strips are shown in fig. 3 as well as the simulated pad response function.
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Fig. 4 shows the experimental setup for the space point resolution studies. The initial ionization is provdied by x-ray photon conversion in the gas. A ~40um pinhole in a thin brass sheet was used to produce a miniaturized focal spot image in the GEM drift gap. The size of the xray spot at the detector is estimated to be ~70 um. After avalanche multiplication and diffusion, the RMS size of the electron cloud reaching the resistiv anode was ~400um. The gas gain was about 3000.
Signals
were read from 7cmx1.5mm strips. The frontend electronics consised of
Aleph TPC wire charge preamplifiers followed by receiver
amplifiers. Signals from 8 adjacent strips were digitized and recorded using
two 4-channel Tektronix oscilloscopes. The x-ray tube is mounted on a
motorized
stage that was used to move the x-ray in small steps over the width of a single
pad. At each x-ray position 1000 events are recorded. The observed shape
of a
pulse depends on the strip location with repect to the primary charge cluster
on the resistive anode. Fig 5 shows an event where x-ray spot is located
directly
above the centre of a readout strip. Pulses closer to the cluster have a faster
risetime and higher maximum amplitude than those that are further away.
An early
induced pulse is observed for all pulses. The pulses are produced by the motion
of the electrons in the GEM induction gap. Although they have demonstrated
position sensitivity, they require the se of high-speed pulse shape sampling
and
are specific to GEM detectors with large induction gaps. The induced pulse has
not been used in these studies.
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The centres of gravity for the events in the calibration data were
computed from the measured pulse heights. The correction function for
the bias in the centre of gravity method (fig. 7) was determined by
plotting the mean calculated result against the known x-ray position.
Having found the polynomial coefficients and bias correction function.
The individual events are used for resolution studies. As indicated
earlier, the pulse amplitudes are determined by fitting a fixed polynomial
shape to the data points. Having found the pulse amplitude for the
three channels of interest, the centroid calculation is made and is
converted to its true position using the bias correction function.
Fig 8 shows the results for the bias corrected centres of gravity calculations
at two x-ray positions. Fig 9 shows the spatial resolutions and position
residuals.