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It is interesting to derive the rate of diagonally attached monopixels
(two monopixels touching by a corner and globally isolated.
See Fig. 1, top centre).
Those are produced only by piled-up monopixels (and Si escape,
which can be recognized because one of the energies is that of Si).
Therefore they can reliably be counted as two monopixels.
The exclusion area corresponding to clean (not piled-up) diagonal events,
derived in the same way as in 2.2, is 14 for monopixels,
18 for bipixels, 21 or 22 for tripixels (depending on their orientation),
and 23 for quadripixels.
The production probability is the square of that of monopixels,
times 4 (there are 4 corners), both for (total)
and for (clean).
Note that the rate of diagonal *events*
(comprising two monopixels) is half that.

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For clean bipixels (Fig. 1, bottom left)
the exclusion areas are [12, 15.5, 19, 20].
15.5 for bipixels is actually 15 for bipixels parallel
to that considered and 16 for perpendicular bipixels.
For (all bipixels)
one must remove the two pixels of the pattern for monopixels,
and one pixel for parallel bipixels, from the exclusion areas.
The probability to produce a bipixel pattern of a given orientation
(vertical, say), is that of having at least one vertical bipixel there (*p _{2}*)
plus in the remaining cases (1-

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For tripixels there is a complication since the easiest thing to compute
is the probability to get a geometrical tripixel, but what
one is really interested in is the probability to get a
tripixel with maximum charge at the corner
(other tripixels cannot be confused with a regular X-ray
and can be rejected straight away).
(and *T _{3}*) is the probability to construct a tripixel from
mono- and bipixels. It is written as the sum of that with no bipixel
(requiring three monopixels),
plus that with one (or more) bipixel of a single orientation
(requiring one additional monopixel),
plus that with bipixels of both orientations.
I obtain an estimate of

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For clean quadripixels (Fig. 1, bottom right)
the exclusion areas are [16, 20, 24, 25].
For (all quadripixels) one must remove the four pixels of the pattern
for monopixels, two pixels for bipixels of any orientation
and one pixel for tri- and quadripixels, from the exclusion areas.
*Q _{3}* below is the probability to get a quadripixel event from mono-, bi-
and tripixels.

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