Study of FPCCD Vertex Detector 12 Jul. th ACFA WS Y. Sugimoto KEK

FPCCD Vertex Detector Accumulate hit signals for one train and read out between trains (Other options read out 20 times per train) Fine pixel of ~5  m (x20 more pixels than “standard” pixels) to keep low pixel occupancy Fully depleted epitaxial layer to minimize the number of hit pixels due to charge spread by diffusion Two layers in proximity make a doublet (super layer) to minimize the wrong-tracking probability due to multiple scattering Tracking capability with single layer using cluster shape can help background rejection Three doublets (6 CCD layers) make the detector Operation at low temperature to keep dark current negligible (r.o. cycle=200ms)

Tracking efficiency Large number of background hits may cause tracking inefficiency: mis-identification of signal hit with background hit  : Background hit density  0 : Multiple scattering angle For a normal incident track, the probability of mis-identification of hit is given by; Angular and momentum dependence;

Tracking efficiency cos  p mis d=10mm,  =40/mm 2 d=2mm,  =40/mm 2 d=10mm,  =2/mm 2 Mis-identification Probability (p=1 GeV/c,t Si =50  m) Expected hit density 40/mm 2 /train (TESLA) 20/mm 2 /train (Nominal) 15/mm 2 /train (LowQ) at R=20 mm, B=3 T

Cluster shape Effective background rejection will be possible by requiring angles of hit clusters in two layers of a doublet and the angle of the line connecting the two hits to be consistent with the extrapolated track. (tracking capability with single layer!) Standard pixel FPCCD

Baseline design

Advantages of FPCCD Free from beam-induced RF noise  x ~1.4  m even with digital readout Simple structure : advantageous for large size Active circuit on one edge : easy to control temperature Readout speed: 15MHz is enough (128(V)x20000(H)/200ms=12.8MHz) (CPCCD:>50MHz) CCD MAPS/FAPS/ISIS

Impact parameter resolution Full simulation study by Jupiter Parameters: Old standard option: R=24, 36, 48, 60 mm t = 330  m/layer  = 4  m FPCCD option: R=20, 22, 39, 41, 58, 60 mm t = 80  m/layer  = 2  m Be beam pipe (common): R=18 mm t = 500  m

Impact parameter resolution p=1, 3, 5, 10, 20, 50, 100 GeV/c T. Nagamine

Lorentz angle Lorentz angle in depleted-layer tan  =  n B  n : electron mobility Carrier velocity saturates at high E field:  n =0.07 m 2 E=1x10 4 V/cm  n =0.045 m 2 E=2x10 4 V/cm Small angle can be cancelled by tilting the wafer May not be a serious problem Number of hit pixels does not increase so much B=3TB=5T E=1x10 4 V/cm  =12deg  =19deg E=2x10 4 V/cm  =7.7deg  =13deg

Lorentz angle Calculation of E-field in epi-layer Tools FEMLAB (COMSOL in Japan) 3.1 Solve Poisson equation by finite element analysis (FEA) Parameters Material is assumed fully depleted (No free charge) n-layer: N D =1x10 16 /cm 3 =1.6x10 3 C/m 3 Epi-layer: N A =1x10 13 /cm 3 =-1.6 C/m 3 V G =4 V t SiO2 =100 nm t n =1  m t epi =15  m

Lorentz angle Result of E-field calculation

Lorentz angle Result of E-field calculation – Potential

Lorentz angle Result of E-field calculation – Summary Almost constant E-field of ~10 4 V/cm in epi-layer can be achieved E-field in epi-layer depends on gate voltage Higher (positive) gate voltage gives higher E-field Positive gate voltage should be applied during train crossing in order to get saturated carrier velocity and less Lorentz angle (Inverted (MPP) mode can be maintained for ~1ms) The Lorentz angle of 12 degrees is expected at B=3T

Summary and outlook We propose FPCCD option for the ILC Vertex Detector Fully depleted CCD with 5  m-square fine pixel size Accumulate 2820 BX and readout between trains Two layers make a doublet (super layer) to pick up signal hits out of background hits If the thickness of the wafers is less than 80  m, the impact parameter resolution better than  b = 5  10/(p  sin 3/2  )  m can be achieved with R in =20 mm and B=3T Electric field in the epitaxial layer has been calculated. From the results, the Lorentz angle of ~12 degrees at B=3T is expected Tracking efficiency under beam background is the most critical issue for FPCCD. Simulation study is urgent.