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Monitoring radiation damage in the LHCb Tracker Turicensis

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Abstract

This paper presents the techniques used to monitor radiation damage in the LHCb Tracker Turicensis from 2011 to 2017. Bulk leakage currents in the silicon sensors are monitored continuously, while the full depletion voltage of the sensors is estimated at regular intervals by performing dedicated scans of the charge collection efficiency as a function of the applied bias voltage. Predictions of the expected leakage currents and full depletion voltages are extracted from the simulated radiation profile, the luminosity delivered by the LHC, and the thermal history of the silicon sensors. Good agreement between measurements and predictions is found.

Figures and captions

Layout of the LHCb detector: vertex locator (VELO), Ring-Image Cherenkov counters (RICH1 and RICH2), tracking stations upstream (TT) and downstream (T1-T3) of the spectrometer dipole magnet, scintillating pad detector used for trigger purposes (SPD), pre-shower detector (PS) and electromagnetic calorimeter (ECAL), hadronic calorimeter (HCAL), and muon stations (M1-M5). LHCb uses a right-handed cartesian coordinate system with its origin at the nominal interaction point, the $z$-axis pointing along the LHC beam axis and the $y$ axis pointing upwards.

LHCb.pdf [1 MiB]
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LHCb.pdf

Layout of the TT, with the LHC beam pipe passing through an opening in the centre of the detection layers. The four detection layers are labelled TTaX, TTaU, TTbV and TTbX. The four different types of readout sectors employed in each of the detection layers are indicated by different shading: readout sectors close to the beam pipe consist of a single silicon sensor, other readout sectors consist of two, three or four silicon sensors that are connected together in series.

TT_layout.pdf [82 KiB]
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TT_layout.pdf

Left: evolution of the integrated luminosity for each year of LHC operation. % Right: expected 1- $\mathrm{ Me V}$ neutron equivalent fluence per proton-proton collision, with the TTaU detection layer overlaid. These results were obtained from a Fluka simulation of the LHCb detector, assuming a proton-proton collision energy of 14 TeV.

lumi_s[..].pdf [21 KiB]
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lumi_squared.pdf
map_tt[..].pdf [467 KiB]
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map_tt_larger_with_grid.pdf

Evolution of the leakage current for the different HV channels connected to one-sensor read-out sectors in TT as a function of (a) the delivered integrated luminosity measured and (b) time. The red curves show channels in the detector layer TTaU, the blue one in TTbV. The predicted evolution is shown in black while the grey band shows its uncertainty, computed from the uncertainty on the model parameters, on the Fluka simulation and on the temperature measurements. The uncertainty does not account for the range of fluence expected across the sectors shown.

TT1_il[..].pdf [547 KiB]
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TT1_ileak_lumi_wide.pdf
TT1_il[..].pdf [899 KiB]
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TT1_ileak_time_wide.pdf

Signal height distribution for hits associated to tracks in the TT for two different bias voltage values. The fitted distribution is described in the text.

TT_sig[..].pdf [36 KiB]
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TT_signal_LV.pdf
TT_sig[..].pdf [37 KiB]
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TT_signal_HV.pdf

Pulse shapes recorded in the TT for two different bias voltage values. The fitted function is shown in Eq. (6).

TT_pul[..].pdf [20 KiB]
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TT_pulse_LV.pdf
TT_pul[..].pdf [20 KiB]
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TT_pulse_HV.pdf

Integrated charge as a function of the applied bias voltage for a central TT read-out sector measured in the CCE scans from (a) April 2012 and (b) September 2017. The decrease of $V_\text{depl}$ is clearly visible.

TT_vol[..].pdf [43 KiB]
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TT_voltage_early.pdf
TT_vol[..].pdf [39 KiB]
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TT_voltage_late.pdf

Distributions for all analysed readout sectors of (a) the ratio $r$ defined in Eq.7; (b) the relative difference between $V_\text{depl}$ and $V_\text{depl}^\text{2011-07-04}$ as discussed in the main text; (c) the differences in $V_\text{depl}$ obtained from a fifth-order spline and from a linear interpolation. The solid black line in (a) highlights the value of the average $\overline{r}$.

TT_rat[..].pdf [13 KiB]
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TT_ratio_hist.pdf
TT_sys[..].pdf [14 KiB]
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TT_systematics_r.pdf
TT_sys[..].pdf [13 KiB]
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TT_systematics_spline.pdf

Measured effective depletion voltage (red points) in the area of two TT sensors: one just above the beam pipe (a) and one further away (b), whose position is indicated in Figure 11(a). The black point and error bars corresponds to the CCE scan used to calibrate the ratio $r$. The statistical contribution to the total uncertainty is indicated by the solid error bars. The predicted evolution of the depletion voltage, based on the initial depletion voltage measured after sensor production, the running conditions and the Hamburg model, is shown as a solid black line. The grey bands show the uncertainty on the predicted evolution of $V_\text{depl}$, while the black dashed lines account for the $\pm 2.5$ V uncertainty on the measurement of the initial depletion voltage $V^0_\text{depl}$ in CV scans.

TT_tim[..].pdf [94 KiB]
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TT_time_evolv_inner.pdf
TT_tim[..].pdf [89 KiB]
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TT_time_evolv_outer.pdf

Measured evolution of $V_\text{depl}$ as a function of the 1-MeV neutron equivalent fluence obtained from the running conditions and Fluka for a TT sector in the area close to the beam pipe (a) and for all of the read-out sectors in the TT (b). The innermost sectors are subdivided in concentrical annular rings with increasing distance from the beam axis. The error bars of the data points display the sum of the statistical and systematic uncertainty. The solid black curve shows the predictions based on the stable damage part of the Hamburg model, the grey shaded region its uncertainty due to the parameter uncertainty of the model. The initial depletion voltage $V^0_{\text{depl}}$ for the Hamburg model prediction in (b) is averaged among all sectors, and the dashed black lines in show the standard deviation of the distribution of the initial $V^0_\text{depl}$ values.

TT_dep[..].pdf [24 KiB]
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TT_deplVphiN_sector.pdf
TT_dep[..].pdf [53 KiB]
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TT_deplVphiN.pdf

Sketch highlighting the TT sectors analysed during the various CCE scans (a). The greyed out sectors were excluded due to poor statistics. The two sectors highlighted with dashes, just above the beam pipe and below the beam pipe on the left hand side, are those which evolution is shown in Figure 9(a) and Figure 9(b), respectively. Absolute change in $V_\text{depl}$ in the innermost region of TT in September 2017 (b).

TT_sectors.pdf [14 KiB]
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TT_sectors.pdf
TT_map[..].pdf [151 KiB]
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TT_map_late.pdf

Animated gif made out of all figures.

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Tables and captions

Parameters for the leakage current predictions.

Table_1.pdf [61 KiB]
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tex code
Table_1.pdf

Summary of CCE scans dates and corresponding integrated delivered luminosity.

Table_2.pdf [34 KiB]
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Table_2.pdf

Parameters for the depletion voltage simulation by the Hamburg model [8].

Table_3.pdf [61 KiB]
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Table_3.pdf

Created on 12 October 2019.