CALICE MAPS Meeting, RAL, 27/02/09
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Present: Jamie Crooks, Paul Dauncey, Renato Turchetta, Mike Tyndel,
 Nigel Watson

Minutes: Paul


Minutes of previous meeting: The meeting was at Coseners, not RAL.
 Otherwise, no comments or corrections.


Sensor V1.2: The FDR took place immediately before this meeting. The
 sensor will be submitted for fabrication within a week and is expected
 back around the end of Apr.


Sensor tests; Jamie showed some slides on sensor measurements; see usual
 web page. The plots on slide 2 are in terms of the laser coordinates,
 so observing the same shape despite rotating the sensor means the tail
 to the lower right is due to the laser, not some asymmetry in the
 sensor. This also means the asymmetry in the response from the four
 diodes within the sensor seems to be a laser effect, not due to any
 pixel asymmetry.

 Jamie has not yet checked the comparator response on a TPAC1.0 test
 pixel sensor but intends to do so. He is also still intending to look
 into whether the gain can be reduced on TPAC1.0 to allow a direct
 observation of the high energy Fe55 peak in the bulk pixels.

 The alternative is to use a lower energy photon source. Mike described
 a "turret" source which uses alphas to generate specific photon lines
 from foils; sulphar would be suitable for us. However, the rates would
 be likely to be too low for a per-pixel study. Using Diamond would be
 another possibility. This would have a large rate and the beam can be
 selected to be monoenergetic using a diffraction grating. The energy
 can be varied over a wide energy range (up to 20keV). The beam spot
 can be focussed to around 1mu, so would allow a detailed mapping of the
 pixel response. Mike and Renato have a contact, Nicola Tartoni, in the
 Diamond group and they will set up a meeting to discuss feasibility.

 Mike showed some slides on Fe55 results from Marcel; see the usual
 web page. The results seem to confirm the idea that the lower energy
 peak is due to a broad plateau in the centre of the pixel (see below),
 but the relative hit rates for the two peaks will be needed to confirm
 this.


Sensor modelling: Paul showed some results using a simple diffusion
 model for the charge spread. This implies the low energy peak is due to
 a uniform region between the pixels, which has a larger area for
 deposits further into the epitaxial layer. The expected rate is ~30% of
 all hits in a given pixel.

 A rough estimate of the hit rate in the high energy peak was made. The
 diode is an octagon with a diameter of 1.8mu but the depletion region
 extends around 1-2mu further around it and a similar distance into the
 epitaxial layer. Assuming an effective diameter of 5mu and depth of
 1.5mu would give a volume of ~30mu3. Each pixel has four diodes, so
 giving a total volume of ~120mu3. The total pixel epitaxial volume is
 ~50x50x12mu3 ~30000mu3 which means that ~0.4% of photon hits would
 result in a hit in the high energy peak. If ~30% of hits give a hit
 in the 0.3 fraction peak, then the ratio of the number of hits in the
 two peaks should be around 80:1.


Next meeting: This will be at 10am on Tue 31 March, at RAL.