S. Taroni

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Performance of irradiated CMS forward pixel detectorSilvia Taronion the behalf of the CMS collaboration07/10/09 11th ICATPP Conference 2Outline● CMS forward pixel detector● Test-beam setup● Performance of irradiated detector● Charge collection● Detection efficiency● Charge sharing characteristic● Charge – track impact point correlation● Cluster size07/10/09 11th ICATPP Conference 3CMS Forward Pixel detector● 4 disks:● Z = ±34.5, 46.5 cm● Inner radius: 6.1 cm● Outer radius: 15.0 cm● Si sensor produced by SINTEF:● Type: n+/n● Pixel cell: 100x150 μm2 ● Thickness: 270  μm ● Open p-stop ring isolation technology● Maximum dose: 7Mrad/year at the inner disk at LHC design luminosity● Dose decreasing ~r -1.807/10/09 11th ICATPP Conference 4Test – beam● A plaquette of 2x4 ROCs was exposed to a peak dose of 45Mrad at the Indiana University Cyclotron Facility using a 200 MeV proton beam.The beam was centered on one edge and it was roughly Gaussian in shape with  σ~2cm● Results are compared with a non – irradiated single chip plaquette● Test with 120 GeV proton beam at Fermilab:● 6 plane pixel telescope – 50x400  μm2 cells – Extrapolated track resolutions on the CMS pixel plane: σx=4.9 μm, σy=6.2 μm● Bias voltages:● 200 V  reference detector (ROC 0)● 500 V irradiated plaquette.– It is below the depletion voltage for the most irradiated region (ROC 8)– 500 V maximum allowed by power supply● Thresholds: 3800e- for reference detector and 3300e- for irradiated plaquette07/10/09 11th ICATPP Conference 5Charge Collection● Collected charge as a function of the track impact point distance from the two adjacent pixel divide.● Fiducial cuts are applied to exclude sharing with other  pixels but those under study● RED points: total charge collected by the two adjacent pixels.● BLACK points: charge collected by the sole pixel pointed by the track. Asymptotic points, far away from the divide, correspond to single hit. ● Left column plots are for adjacent pixels on  the same row, right column for the adjacent pixels on the same column● Reference/non-irradiated detector shows a very good charge collection efficiency even in presence of sharing.referencedetectorCollected charge, two adjacent pixels on a rowCollected charge, two adjacent pixels on a columnHigher irradiati on07/10/09 11th ICATPP Conference 6Charge Collection● First of all● Single hit charge collection efficiency degrades moving toward higher irradiated regions – ~100 % in ROC5, ~95 % in ROC6, ~75 % in ROC8.● A clear drop in collected charge is present whenever charge is shared between the two adjacent  pixels.● The drop is relatively larger for sharing between two pixels along a  row, e.g.  31% vs 28% for ROC6– This is due to the presence of the break on the p-stop ring between pixels on the same column. ● It is interesting to notice that in case of sharing, the percentage of lost charge wrt the single hit value becomes smaller  moving toward the most irradiated region, 40% in ROC5 (row) vs 19% in ROC8● This apparently  anomalous behavior can be explained by the drastic change of the electric field at high radiation doses.● The formation of a new junction on the n+ side, as a result of the type-inversion,  radically changes the field configuration between adjacent pixel-implants.referencedetectorCollected charge, two adjacent pixels on a rowCollected charge, two adjacent pixels on a columnHigher irradiati on07/10/09 11th ICATPP Conference 7Detection efficiency● Inefficiency is confined near the pixel corners: this is due to two main reasons● Signal in each pixel could be small and below the threshold● Signal could be outside the synchronization time window because of the time walk● Anyway, the maximum achievable efficiency is limited to ~99%, because of ~1% systematic inefficiency of the readout systemROC ε %0 98.61±0.155 97.69±0.106 98.78±0.058 97.46±0.0607/10/09 11th ICATPP Conference 8η distribution – I● Charge sharing between two pixels is investigated using the variable:Ql: charge collected by the pixel at the left sideQr: charge collected by the pixel at the right side● Moving toward the most irradiated regions, the continuous effective increase of the threshold  shrinks the central part of the η distribution ● ROC6 (left): slope due to the non perfect orthogonality of the detector to the beam tracks● ROC6 (right): effect of the non orthogonality superimpose to those from the p–stop break which unbalances the charge sharing=QrQrQlHigher irradiati onreferencedetector two adjacent pixels on a row two adjacent pixels on a column07/10/09 11th ICATPP Conference 9η – distribution – II● Correlation between the track impact point and measured  η value is investigated using two methods:● Mean value of signed distance vs η bin (plots on the right)● Indirect method: for a certain value of η, the signed distance x(η) is given by● The two results match each other∫−pxdN xdxdx=∫0 dN d d Higher irradiati onreferencedetector two adjacent pixels on a row two adjacent pixels on a column07/10/09 11th ICATPP Conference 10Cluster size● Lower charge collection efficiency induced by radiation damage corresponds to a higher effective threshold● The number of sharing events in ROC 8 is close to that of  ROC 0 which has a higher threshold (3800 e- wrt 3300 e-)Cluster size ROC0 (%) ROC5 (%) ROC6 (%) ROC8 (%)1 85.42 ± 0.70 81.78 ± 0.43 82.03 ± 0.28 85.57 ± 0.272 12.75 ± 0.66 16.00 ± 0.40 15.17 ± 0.26 12.73 ± 0.263 0.75 ± 0.17 1.15 ± 0.12 1.29 ± 0.08 0.84 ± 0.074 0.83 ± 0.18 0.72 ± 0.09 1.05 ± 0.07 0.55 ± 0.065 0.08 ± 0.06 0.12 ± 0.03 0.21 ± 0.03 0.14 ± 0.0307/10/09 11th ICATPP Conference 11Summary and Conclusions● CMS FPix performance tested up to a maximum dose of 45Mrad● Loss of ~25% of the signal relesed by a MIP● High efficiency: ~99%– Only marginal drop near the pixel corners● η distribution symmetry altered by the p–stop breaks● Despite the observed damages, the detector remains fully operational and suitable to accomplish the CMS physics goal for years inside the LHC07/10/09 11th ICATPP Conference 12Backup slides07/10/09 11th ICATPP Conference 13Fiducial cuts● Fiducial cuts are applied to exclude sharing with other  pixels but those under study Two adjacent pixels on the same column: fiducial cut at 20 microns from the adjacent pixel in the row (X direction)Two adjacent pixels on the same row: fiducial cut at 30 microns from the adjacent pixel in the column (Y direction)30 μm20 μmSingle pixel:  fiducial cuts at 30 microns in Y and 20 microns in X from the pixel boundaries30 μm20 μm07/10/09 11th ICATPP Conference 14Charge collection I● Single hit and double hit spectra are investigated separately● Less irradiated region (ROC5): the sensor can still collected all the charge if it is released in one pixel only● In the most irradiated region (ROC8) the collection  efficiency is limited:  Landau peak ~75% of  the expected value (one pixel)● Intermediate region (ROC6): Landau peak  ~95% of the expected  value (one pixel)ROC MPV (e-) Single hitMPV (e-) same rowMPV (e-) same col0 21956 ± 89 20853 ± 305 23374 ± 2875 22521 ± 71 13490 ± 179 14388 ± 1006 20854 ± 33 14317 ± 118 14945 ± 698 16663 ± 23 13436 ± 95 13898 ± 5207/10/09 11th ICATPP Conference 15Charge Collection IIROCCharge loss (%) Same row clusterCharge loss (%) Same col cluster5 40.10 ± 4.42 36.11 ± 4.516 31.35 ± 3.34 28.34 ± 3.388 19.36 ± 3.38 16.59 ± 3.41● The percentage of lost charge wrt  single hit value decreases moving forward ROC8. ● The charge collection efficiency degrades with a lower rate in the inter–pixel region, indication of drastic change in the expected electric field at high radiation dose07/10/09 11th ICATPP Conference 16Charge sharing● The charge sharing correlation between two adjacent pixels deteriorates moving toward the most irradiated region● Corner regions are excluded in the study● Correlations between two adjacent pixels on the same column are reported as example

Performance of irradiated CMS forward pixel detector

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Performance of irradiated CMS forward pixel detector

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