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7.5. The Mesh Analysis
The performance of pnCCDs as X-ray detectors is characterized by electronic noise, energy
resolution and a usable energy range with the lowest and highest detectable X-ray energy in
particular. Usually these performance parameters determine the suitability of a specific CCD
for a given application. However, these parameters do not define in which way the pnCCD pixel
array reacts to incident X-ray photons when hitting the pixel structure at their boundaries.
To view an individual description of the X-ray response, the signal amplitude of neighbouring
pixels surrounding the pixel with the highest electron density can be parameterized with
the so called mesh analyis: It shows how the signal charge generated by an X-ray photon is
distributed over the pixel array of a CCD. By using the mesh analysis the position resolution of
X-ray pnCCDs is improved to the order of one micrometers. Furthermore it is also employed
to correct, verify and optimize device simulations and the realization of pnCCD designs.
The Japanese group around H. Tsunemi employed
mesh analysis to enhance the position resolution
of X-rays in MOS-CCDs. The method was later
adapted and extended for the development of
pnCCDs by MPI HLL. Scanning the CCD surface
with a narrow X-ray beam (of about three to five
μm) in an energy range of 0.2 to 10 keV with
several 100,000 individual X-rays per pixel is very
time consuming and inefficient.
These difficulties can be overcome by placing
an opaque metal foil – called mesh – with a regular hole grid in front of the pnCCD (Figure 2).
A slight rotation of the mesh with respect to
the pixel structure ensures that every hole has
a different position relative to the pixel below
(Figure 1).
The geometrical mesh parameters are tailored
to the pixel sizes of pnCCDs of 150 μm, 75 μm
as well as 51 μm. As the hole distance is one,
two or three times the pixel size, data analysis
shows a Moiré pattern (Figure 1), resulting from
the fact that hole positions near the middle of a
pixel repeat in a regular pattern caused by the
small rotation angle of the mesh.
Our advanced mesh data analysis reconstructs all
occurring positions of mesh holes on the pnCCD.
The mesh position and data measurements are
then translated into a virtual scanning process of
one reconstructed pixel. This is facilitated by the
uniformity of the pixel array of a pnCCD, i.e. all
pixels react in the same way to a given position
of incidence of an X-ray photon.
The foil deployed by MPI HLL is of gold,
10 μm thick, with a hole diameter of 5 μm,
every hole being displaced by 150 μm from
the adjacent hole. It remains opaque for X-rays
with energy of up to 5.4 keV, being the energy
of the Cr-K emission line. The size of the mesh is
11 mm by 16 mm.
One result of the mesh analysis is a charge
collection map. It shows the reconstructed
charge collection function (CCF) of a pnCCD
for a given X-ray energy. The CCF is the relative
amount of signal charge being collected in a
pixel depending on the incidence position of
an X-ray photon (Figures 5 to 7).
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pnCCDs
Figure 1.
Schematic drawing of the mesh on
top of a pixel array. Bright shades
indicate pixels with large signals.
Figure 2.
Schematic drawing of X-ray tube
setup with CCD and mesh.
Figure 3 and 4.
Comparison of the measured and
simulated CCF in the charge transfer
direction. Left plot the results for a
pixel size of 75 μm are shown, the
right plot features results for a pixel
size of 51 μm. Both examples are for
a photon energy of 4.5 keV/ Ti-K .
Figure 1.
Figure 2.
A charge collection function (CCF) is displayed in
a three by three pixel map as a pixel also shows
a signal if an X-ray photon has an incidence
position lying below a certain distance outside
of the pixel border (Figures 5 to 7).
(Figures 3 and 4). This way, the accuracy of the
simulations was improved by the application of
the correct model for the drift and diffusion of
signal electron clouds.
A detected X-ray photon can cause a signal
pattern of up to four neighbouring pixels. In
combination with the charge collection function,
the detected signal pulse heights in the pattern
are used to reconstruct the incidence position
of the photon with a precision of down to one
micron.
The CCF of a pnCCD can be simulated with
the device simulator program TeSCA. The
simulations of the pnCCDs used for mesh
measurements have been compared to the
reconstructed charge collection functions
Figure 3.
MPI HLL
Recently the mesh analysis has been applied
successfully to the DEPMOSFET-pixel arrays
(see Chapter 8. and 8.1.) produced in the MPI
HLL. They show very clearly the regions within
a pixel where charge spreading is enhanced.
By comparing the measured data with the
simulations we are able to reconstruct details of
the potential distribution within a pixel.
In the future all position resolving X-ray
detectors made by MPI HLL will be analyzed
with this method.
Figure 4.
pnCCDs
Figures 5 to 7.
Charge collection functions (CCF)
for X-ray energy of the Ti-K emission
line with mean photon energy of
4.5 keV. From top down CCF maps
of devices with pixel sizes of 150
microns, 75 microns and 51 microns
are shown.
Figure 5.
Figure 6.
Figure 7.
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