Timing performance of a multi-pad PICOSEC-Micromegas detector prototype

Timing performance of a multi-pad PICOSEC-Micromegas detector prototype - Aune, S. et al - arXiv:2012.00545

(left) Sketch of the PICOSEC-Micromegas working principle. (right) Electrical diagram of the detector power supply and signal read-out. The signal of each anode pad is individually amplified with a CIVIDEC C2-HV broadband amplifier.

(left) Technical drawing of the PCB. (right) Technical sketch of the mechanics that holds the Cherenkov crystal with photocathode in place.

(left) Photograph of the multi-pad chamber during assembly in the clean room. The hexagonal pad structure of the readout is visible in the centre. (right) A schematic diagram of the anode segmentation. Notice that there is a gap between adjacent pad edges, represented by the thick black lines. The pads No. 4, 7, 8 and 11 are fully instrumented and their signals are digitized by the oscilloscope channels as indicated. The red axes, labelled as $X_{L}$ and $Y_{L}$, represent the local coordinate frame, while the blue axes, labelled as $X_{B}$ and $Y_{B}$, represent the global tracking coordinate frame (or else beam-frame). The green axes, labelled as $X_{S}$ and $Y_{S}$, represent the symmetry frame, which is used in the alignment procedure as described in the text.

Layout of the experimental setup (not to scale) during the beam tests. The incoming beam enters from the right side of the figure; events are triggered by either the coincidence of two 5 $\times$ 5 mm$^{2}$ scintillators in anti-coincidence with a ‘‘veto’’ scintillator (small area trigger) or by the coincidence of two 5 $\times$ 5 cm$^{2}$ scintillators (large area trigger). Three GEM detectors provide tracking information of the incoming charged particles, and the MCP-PMTs provide the timing reference for the PICOSEC-Micromegas measurements. Details are given in the text.

The 2D distribution in plot-a represents the beam profile that illuminates the area around the instrumented PICOSEC-Micromegas pads. The distributions below show the mean value of the electron-peak charge versus the $\{ x_{B},y_{B} \}$ coordinates where the MIPs traverse the detector. The distributions in plots b, c, d and e correspond to the response of pad No. 4, pad No. 7, pad No. 8 and pad No. 11, respectively.

Electron-peak charge distributions of the instrumented pads, corresponding to tracks passing within 6 mm from the respective mean charge centroids (see text).

The mean value of the electron-peak charge, when the pad responds to MIPs entering the PICOSEC-Micromegas radiator at points with $x_{S}$ (plots a and c) or $y_{S}$ (plots b and d) coordinate, relative to the symmetry axes (see text). The upper and lower rows correspond to pads No. 7 and 8 respectively.

Electron-peak charge distributions of pads No. 4 (plot a), No. 7 (plot b), No. 8 (plot c) and No. 11 (plot d), when respond to MIPs passing closer than 2 mm from the respective pad centre. The solid lines represent fits with a Gamma Distribution function, with estimated mean values equal to 6.54 ($\pm$ 0.04) pC, 5.29 ($\pm$ 0.03) pC, 6.70 ($\pm$ 0.05) pC and 7.20 ($\pm$ 0.05) pC for pads No. 4, 7, 8 and 11, respectively.

The radial deformation of the multi-pad PICOSEC-Micromegas anode has been measured with a ZEISS O-INSPECT 863 (length measurement error in $\textrm{\selectlanguage{greek}m\selectlanguage{english}m}$: 2.2 + L/150mm) at the CERN Metrology service. The shown measurements have been performed on the PCB installed on the aluminium flange.

SAT distributions of pad No. 7 (plots a and b) and pad No. 11 (plots c and d) signals, corresponding to incoming MIPs passing within 2 mm (plots a and c) or between 2 mm and 4.33 mm (plots b and d) from the respective pad centres. The black points represent raw SAT measurements while the red points show the same measurement distributions after applying the flatness corrections described later in Section \ref{spad}. The solid lines represent fits to the data points with the sum of two Gaussian functions sharing the same mean value.

The time resolution of a single pad versus the respective $Q_{e}$ for MIPs passing within 2 mm (black), between 2 mm and 4.33 mm (red) and between 4.33 mm and 7.5 mm (green) from the pad centre, respectively. The left plot corresponds to the central pad No. 7 and the right plot to the peripheral pad No. 11.

Illustration of the test and reference axes used for investigating the symmetry properties (see text) of the mean SAT and the time resolution of a single pad.

The mean signal arrival time (SAT) induced by MIPs to pad No. 8 versus the coordinates of the respective seeds on the test axis. Each point in the graphs corresponds to tracks passing within 0.5 mm around a seed, as it is described in the text. Every plot is marked by a value of the angle $\Theta$, which is the angle between the respective test-axis and the reference axis for pad No. 8. The solid lines represent empirical parametrizations. (For a better illustration, an offset of 2.019 ns has been subtracted from all the raw SAT measurements used in the plots)

The spread (RMS) of SAT distributions induced by MIPs to pad No. 4 versus the coordinates of the respective seeds on the test axis. Each point in the graphs corresponds to tracks passing within 0.5 mm around a seed, as it is described in the text. Every plot is marked by a value of the angle $\Theta$, which is the angle between the respective test-axis and the reference axis for pad No. 4. The solid lines represent empirical parametrizations.

(left) The time resolution versus $Q_{e}$, after applying the flatness correction to signals of pad No. 11. (right) The mean SAT versus $Q_{e}$, after applying the flatness correction to signals of pad No. 11. For better illustration, an offset of 2 ns has been subtracted from the SAT values. Both plots follow the same colour code as in Fig. \ref{fig:dist_resol} to denote the regions of proximity of the respective MIP impact points to the pad centre. The solid curves represent fits to the data points from all proximity regions.

The time resolution (left) and the mean SAT (right), after applying the flatness correction to the peripheral pad signals, as a function of the scaled electron-peak charge (see text). External time delays have been subtracted from the SAT measurements of each pad. The black points correspond to the central pad while the other, coloured points correspond to the peripheral pads. The solid curves represent fits of the central pad data.

(left) Distribution of fully corrected arrival time measurements of all pad signals induced by MIPs passing within 2 mm from the respective pad centre. The solid line represent a double Gaussian fit to the data points with an RMS of $25.8 \pm 0.6$ ps. (right) Distribution of the fully corrected SAT measurements normalized to their expected error (pull). The solid line represents a Gaussian fit with estimated mean and $\sigma$ values consistent to 0 and 1 respectively.

Distributions of fully corrected arrival time measurements of signals (SAT) induced on the pads No. 4 (plot a), No. 7 (plot b) and No. 8 (plot c) by MIPs passing within 2 mm of their common corner (see text). The solid lines represent fits of the data points with the sum of two Gaussian functions. The mean value of the distributions is consistent with zero, as it is expected after correcting for shifts by subtracting the calibration function $ \tau\left( Q_{e}^{m}\right)$. The corresponding RMS values are 71.3 $\pm$ 2.5, 66.5.0 $\pm$ 2.5 and 68.0 $\pm$ 2.5 ps for pads No. 4, 7 and 8, respectively. The plots d, e and f show the corresponding pull distributions of the fully corrected single-pad SAT measurements normalized to the expected measurement errors. The solid lines represent Gaussian fits, which are consistent with standard normal distribution functions.

(left) Distribution of the arrival time of MIPs, passing within 2 mm of a common pad corner (pads No. 4, 7 and 8), estimated by Eq. (\ref{eq9}) combining the individual single-pad measurements and their expected errors. The solid line represents a fit to the data points by a sum of two Gaussian functions corresponding to an RMS of 32.2 $\pm$ 0.5 ps. (right) Pull distribution of estimated arrival times by Eq.(\ref{eq9}). The solid line represents a Gaussian fit to the data points, consistent with mean and $\sigma$ values equal to 0 and 1 respectively.

The resolution of the multi-pad PICOSEC-Micromegas prototype in timing the arrival of MIPs that impact the detector plane along the line connecting the centres of pad No. 11 and pad No. 4. In the left plot, the red circle, 1 mm in radius, displays the area used to select MIPs associated with the sampling-point shown at its centre. The green circle, 3 mm in radius, illustrates the Cherenkov disc of a track passing through a point on the periphery of the red circle. In the right plot, the dots connected by red line segments represent the resolution in timing the arrival of MIPs passing around a sampling-point at distance L from the centre of pad No. 11. The dots connected with blue line segments denote the best single-pad timing performance among the active anode pads. The points at L = 5.25 mm and L = 10.75 mm display the respective time resolution for tracks passing around the common corners of three pads.

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