-- SuDong - 2009-09-02

Pixel Insertable B-layer (IBL) Material Estimate

Beam pipe

The beam pipe inner radius with IBL is deemed sufficiently safe to go down to R=25mm from the present 29mm. The new beam pipe envelop outer radius is set to R=29.7mm with actual beam pipe physical outter surface at R=29mm. The initial IBL beam pipe description in Geometry database as ATLAS-SLHC-00-00-00/BeamPipe-07 is as follows:

Name	Inner Radius (mm)	thickness (microns)	Radiation length (%)
"Getter" liner	24.998	2	0.009
Be beam pipe	25.000	800	0.227
Kapton	25.875	25	0.009
"Inconel" heater *	25.900	10	0.065
Kapton	26.000	120	0.042
Aerogel **	26.120	2710	0.293
Kapton	28.830	120	0.042
Aluminum foil	28.950	50	0.056
Total (Z=0)			0.678

* Note that the Inconel heaters are only present for |Z|>5cm while not present for the central +-5cm.
** Scaling the current beam pipe down to the IBL era envelope, implies a thinner aerogel insulation of 2.7mm (previously 4mm) which still needs verification whether it is possible.

Stave Layout

The current baseline layout is a 14 single-pipe single staves in "inverted turbine" geometry with sensors facing beam pipe for the best impact parameter resolution for being at the lowest possible radius (picture of layout and flex connection scheme). A critical issue is the electrical service topology for connecting the flex to the modules. The miniature Panasonic AXE connector is introduced for the flex flap to connect to the module flap. This allows the modules being tested standalone and avoids sharp bends to the flap.

Stave Flex

One of the design options from the SLAC design study is to use the kapton flex with all copper traces to carry the signal/cmd+clk/HV/DCS connections. The stave flex consists vertically paired traces for signal, cmd+clk lines to best control of the impedance minimize unwanted couplings. On the other side of a thin middle ground plane, the HV and DCS lines are placed side by side. (A picture of the flex cross section to be added).

The LV metal conductor is the major source of material which is inevitable due to the 0.2V max voltage drop allowed on the stave. Aluminum as LV conductor is clearly more preferrable to reduce material radiation length, but the risk of breaking thin Al traces when buried in the flex led to the choice of carrying LV on separate Cu-clad Al wires which can be attached to the outside of the flex. Because the LV lines to the center of the stave is longer than those going to the end section, to equate the voltage drop along the stave, the thinnest allowed wires with different sizes are used for each of the 4 2-module LV sections:

2-module section	Wire length (mm)	Wire diameter (mm)
0	120	0.31
1	200	0.40
2	280	0.48
3	360	0.54

If pure copper wires were used to give the same voltage drop, it would be nearly 3 times more material.

The flex flap connection with Panasonic AXE connectors has the advantage of small size, low profile and firm connection. The actual weight of a 40pin connector heade+plug pair is 34mg. A detailed estimate of the metal (Cu) pin mass based on exact pin cross section and estimated lengths and the mold (LCP - assumed kevlar) gave 24.0mg pin + 10.6mg mold=34.6mg, assuming the mold is solid:void=40%:60%. The connection width is 2.2mm and height is 0.8mm. The connector pair material estimates for tracks perpendicular to connector face:

Pins	length (mm)	local rad len (%)
10	4.5	0.58
26	7.7	0.83
40	10.5	0.92
50	12.5	0.96

where the mold is only contributing 0.11% radiation length while most of the material is from the pins. There are 8 flap connection each half stave, one per 2-FE module. The on stave flap connection is a separate 10pin connector for HV to avoid pin spacing problems, and a 26 pin for everything else (8x2 LV, 2x2 signal, 2x2 cmd+clk, 1x2 DCS).