First Performance Results from a Real Size Silicon Pad Sensor Module for a Small
Animal Micro PET
A six layer prototype PCB board has been designed for the readout of a full size silicon
pad sensor with 1040 pads of 1mm x 1mm area each. The external dimensions of the
detector are 2.9 cm x 4.5 cm. The active area of the detector is 2.6 x 4.0 cm2. Outside this
area is a multiple guard ring structure. The detector is fabricated with very high resistivity
silicon substratel of 1 mm thickness from TOPSILL. Of the 1040 pads 1024 can be read
out with 8 VATAGP3 chips. It should be noted that this chip was custom designed for
Compton Camera applications. For PET application a similar front-end chip architecture
can be used, but finally with much faster amplifiers. However a number of interesting
aspects for PET application can be addressed also with this readout chip.
The fully assembled module is shown in Fig. 1.
Fig 1 Fully assembled PET prototype module
Fig 1a
Fig 2: Bond connections between Detector and chip, on detector side
Fig. 3 Routing lines on second metal level from p+ pads to bond pads

Fig 2 show a micrograph of bond connections from detector to chip
Fig. 3 demonstrates the double metal technology with routing of sensor pads to the
periphery of the detector to allow for conventional wire bonding to readout chips.
The pad detector has an average leakage current in one pad of about 400 pA above the
full depletion voltage of 150 V. The capacitive load of one pad is dominated by the
routing lines and is estimated to be around 5 pF. Due to the particular layout of the
routing lines the capacitance of all pads is nearly identical.
Some data were taken with different  and X-ray sources. Fig 4 shows the hit map of the
module illuminated with the 57Co source. The source was positioned in the middle of the
detector with respect to the bond pad rows and the long sides. The hit distribution in Fig 2
shows the regular pattern of 20 pads in each of the 26 pad columns being routed to the
edge.
Fig. 4 Hit map on detector with 57Co source
Fig. 5 shows the total spectrum ( energy in ADC counts) in one channel obtained with
the 57Co source. The data are taken in sparse readout mode. Serial readout mode gives
very similar results
The  lines at 122 keV and 136 keV are at ~300 and ~330 ADC counts. At about 75
ADC counts ( corresponding to ~50 keV) the Compton edge, resulting from the signals of
the Compton recoil electrons in the silicon, rises very steeply and extends to about 25
keV. The cut-off here is due to a relatively high threshold setting for the VATAGP3
chips. The lowest possible threshold with this module is around 14 keV. The lower cutoff near 0 ADC counts is an artifact of a pedestal shift in this readout mode.
Fig 6 is a fit to the 122keV peak giving a sigma of ~2.5 ADC counts equivalent to a
noise of 240 e- ENC.
Fig. 5 The 57Co spectrum
Fig. 6 Gaussian fit to 122 keV line
This test proves that a full size PET pad detector of 1 mm thickness can be operated with
self-triggering front-end chips. A second module will be built to demonstrate in a test setup with two detectors that the spatial resolution in PET mode of 500 micron FWHM in
the image plane can be achieved.
For further Compton PET prototype studies a very fast readout chip based on a similar
architecture as the VATAGP3 chip will be required. Also the technology for very dense
packaging of detectors needs to be developed further.