1、1,BaBar Silicon Tracker Perspective at High Luminosity,G. Calderini,2,Which will be the performance of the BaBar SVT when the lumi increases?,Performance of the present SVT with minor modifications: (Short term perspective: n x 1034),Strategy to cope with future physics programs: (Long term perspect
2、ive: n x 1035 to 1036),Main issues: radiation damage, occupancy,3,Present detector: Short Term extrapolation,Extrapolation for the short term is based on expectations for currents,Step 1,Currents are used to calculate instantaneous dose rates, by using background studies,4,Instantaneous dose,Integra
3、ted dose (radiation damage),Step 2,Occupancy (performance),G.Rizzo, G.C.,B.Petersen,5,Occupancy (performance),Step 3,extrapolated hit efficiency,extrapolated hit resolution,M.Mazur,M.Mazur,6,Extrapolations suggest that a detector like it is now, can work well up to a 2-4 x 1034,Which are the limits
4、imposed to the tracker by more aggressive physics programs?,The 1035 scenario,The 1036 scenario,7,The (1-2) x 1035 and 1036 scenarios present similar concerns:,Machine-related background (continuous injection!),Radiation damage,Rate,Physics backgrounds,But they are completely different worlds,8,In a
5、 1035 world, a BaBar-like tracker with SVT-DCH is somehow possible,More phase-space for solutions!,In a 1036 world, life is different, more effort necessary in the design,DCH,7MRad/y,SVT,100% Occupancy,9,Vertexing and Tracking at high luminosity,The inner tracker,The beam-pipe,The beam-pipe radius i
6、s a big issue, the choice may depend strongly on the machine design,KEK-B plans on 1cm more performing PEP-II plans on 1.5-2 cm safer,One or two layers of pixels very close to beam-pipe mainly required for background suppression, integrated by a few additional layers of silicon strip detectors (vert
7、exing, impact parameter resolution, low-P tracking),The central tracker: two options,a) More silicon layers b) Small cell/fast gas drift chamber, combined with normal drift chamber,10,Some keypoints,Radiation hardness: possible using LHC technology,Material budget: current hybrid pixel layers are th
8、ick; the all-silicon solution can get pretty heavy,Rate capability: effects on silicon segmentation and drift chamber cell size,11,Babar possible approach to tracking,All silicon tracker, with lampshade shaped modules to reduce material Start to explore different options Main issue is material Need
9、R&D on thin DSSD and pixels,Pixel (2 layers),Intermediate DSSD (3 layers),Central Silicon Tracker(4 layers) R(outer) = 60 cm,12,Pixels (I),A) Hybrid pixels,In hybrid pixel systems the readout chip is connected to the sensor through solder or Indium bumps,Separate development of readout electronics a
10、nd sensors,Use best available technology for each component,Complexity and reliability issues in assembly,Material budget is high due to overlap of sensor and readout chip.,13,Example: Pixels at LHC,Radiation hardness and rate capability are high They should be OK for a Super B-factory as well. Mate
11、rial budget is serious: At least 1-2% X0 per layer (current Babar Si is around 0.4% X0),Atlas pixel modules,Overlap of: Sensor Front-end chip Flex hybrid with control chip, caps Mechanical structure and cooling,LHC experiments use hybrid pixels,14,What dominates resolution?,Here material budget is c
12、ritical !,Impact parameter resolution is dominated by resolution on first hit,s(point)2 = s(mult.scatt.)2 + s(detector)2,Typical SVT detector resolution at BaBar is s = 12 - 14 mm at 90,For p=1 GeV/c, for R=3 cm, X(beampipe+1st layer) = 1.4% X0 s(mult. scatt.) = 50 mm at 90,15,Model for resolution,W
13、e can model the SVT performance using a resolution-weighted average of the detector radii.,Data,1 GeV58mm 3 GeV25mm,1 GeV55mm 3 GeV23mm,F.Forti,16,Where can we gain ?,1 GeV12mm 3 GeV7 mm,We could gain a lot by reducing the beam pipe radius and the detector + beam pipe thickness. The point resolution
14、 can be improved,F.Forti,17,Proven by the success of CMOS video cameras, replacing CCDs.,B) Monolithic Active Pixels (MAPS),Pixels (II),Sensor and electronics on the same substrate.,Possible approaches:,Integrate electronics on the high resistivity substrate usually employed for sensors,The fabricat
15、ion process is highly non-standard with large feature size (1-2 mm),Signal is high quality, and large,Active components are not of the best quality,Use the low resistivity substrate of standard CMOS process as sensor,Small signal due to the collection mechanism,Standard sub-micron process with state
16、-of-the-art electronics,18,CMOS MAPS,Use epitaxial layer of CMOS low-resistivity substrate to collect charge (thermal diffusion),Potential for low cost and very small thickness (reduced substrate).,Radiation hard if using sub-micron CMOS process,Until now: miniscule pixel size (a few um) prevents us
17、age in large system,Low power-consumption (circuitry active only during read-out),19,Pixel: ongoing R&D,Conventional hybrid pixels,Reduce thickness,It doesnt seem possible to reduce too much preserving also the mechanical stability,MAPS,Develop large-area detectors (already some results),Development
18、 on-going in several places: LEPSI, LBNL, Japan, Perugia,Project launched by Pisa-Pavia-Bergamo-Trento-Trieste-Modena to the Italian Ministry for Education and Scientific Research,- Main goal is to develop a submicron CMOS MAPS that can be used on large area systems,- Time frame is 2-3 years,20,Stri
19、ps,In the central silicon tracker momentum resolution dominated by material budget,1) Reduce the thickness of the active silicon,Signal reduction (1 MIP 8000 e/100um Si),2) Reduce the amount of inactive material,Bring the signal out of the active tracking volume,Mechanical issues: silicon greatly co
20、ntributes to module stiffness,Done already for the present SVT,21,Reduce thickness of readout electronics,For the chip themselves is mainly a mechanical problem, could be solved.,It is harder to do for the hybrids (capacitors, traces, etc.,),Reduce power dissipation (ie cooling),Very hard if one has
21、 to improve the S/N ratio to be able to readout smaller signals,One more reason to go for sub-micron process,Need some local signal amplification,The same technique cannot be used in the larger volume Central Silicon Tracker,22,All-silicon performance,Momentum resolution at low-p is dominated by mul
22、tiple scattering in silicon material To keep a reasonable performance we need 100um thick silicon, which isnt quite ready yet: R&D on thin silicon modules On this large area it will be impossible to keep all the electronics outside the active volume R&D on thin, low power electronics How much the re
23、quirement on momentum resolution at low momentum can berelaxed ? More physics studies,s(1/pt) (GeV-1),23,Summary and conclusions,Pixel layers,Current LHC pixel would work, but too much material.,Develop monolithic pixels,Large structures, thickness (back-thinning), radiation damage,Central tracker,M
24、ost likely it wont be possible to achieve the same performance of current systems at low momentum,It would require a 100um equivalent thickness for a 9 layers (total) all Si tracker.,Explore the possibility of gaseous detectors, such as a small-cell DCH.,Some R&D has started, but we need to proceed fast if we want to design a realistic system in 2-3 years.,
