Study of Edgeless TimePix Pixel Devices Dylan Syracuse University
2
3 Million-Dollar Question Universe is made of matter Particle decays putatively produce equal amounts of matter and antimatter Where did all of the antimatter go? Where did all of the matter come from?
4 CP Violation CP Symmetry: laws of physics are the same if a particle is interchanged with its antiparticle and spatial coordinates are inverted CP Violation is necessary to explain the matter-antimatter imbalance (Sakharov, 1967) Standard Model of particle physics allows for small amount of CP violation in the CKM matrix New physics phenomena sought in order to solve the puzzle! The Cabibbo-Kobayashi-Maskawa (CKM) matrix. A B 0 s decay.
5 LHCb Detector at the LHC Large Hadron Collider (LHC) smashes protons together at highest energy level in history LHCb is one of four experiments at the LHC which studies the decay products Decays of B-mesons offer an avenue of exploration for CP violation Artist s depiction of the Large Hadron Collider. Reconstructed tracks from LHCb. The beam pipe.
6 LHCb Detector The LHCb detector from the side. 800+ people from 60 institutions in 16 countries collaborate on the LHCb detector SU HEP group largely responsible for Vertex Locator (VELO) component of detector VELO is the first of many detector devices in the LHCb detector One half of the Vertex Locator (VELO) detector. Close proximity to collision point demands high spatial accuracy and radiation hardness
7 LHCb Upgrade LHC will run at higher energies and higher luminosity in the future More events, more complex events Plan to upgrade the LHCb detector to increase granularity in space and time to keep up Several novel technologies have been researched as candidates for VELO replacement Planar arrays of silicon pixel detectors replace microstrip devices
8 Silicon Detectors A charged particle of sufficient energy passing through a bulk of silicon causes ionization Electrons are freed from the silicon atoms, which then experience vacancies called electron holes Silicon detectors operate by converting this ionization activity into an electronic signal Reverse bias is applied causing charge carriers to drift toward collecting electrodes This provides information on where the charged particle crossed the detector Doping the silicon with different atoms can add extra electrons or holes Grossly simplified diagram showing basic mechanism of silicon detectors. Different types of sensors combine doped silicon in various ways to form junctions which modify the carrier mobility, depending on desired behavior
9 TimePix Sensors LHCb collaboration is interested in silicon pixel detectors with an active edge region Current VELO sensors are 5 cm in size with 2,048 channels Deconstruction of a typical p-on-n type silicon pixel detector. TimePix sensors are 1.4 cm in size with 65,536 channels! Minimizes the dead area of a sensor array Needs to be assessed for performance! Visual example of a TimePix sensor.
10 Test Subjects Two test subjects were studied: F08, H08 Experimental prototypes produced by VTT Technical Research Centre of Finland Simulation predicts the spatial resolution for this geometry is optimized at an angle of approx. 15.4 (Turchetta, 1993) F08 H08 Thickness 200 μm 200 μm Pitch 55 μm 55 μm Sensor Type n-on-n n-on-n Pixel-to-Edge Distance 55 μm 100 μm, floating guardring Predicted Optimal Angle 15.4 15.4 Summary of properties of test subjects F08, H08. Active edge region maximizes area of effectiveness Device performance near the edge may suffer from distorted electric field
11 The Testbeam Telescope Testbeam telescope allows the careful analysis of the device under test (DUT) using several other devices in parallel as reference Beam of charged subatomic particles applied through the telescope (in this case, pions) mimicking actual detector physics Tracks fitted through hits on reference planes, then associated with hits on DUT (software) Ratio of associated or found hits to total hits on the DUT gives the efficiency in a region Depiction of the Testbeam telescope. Distance from fitted track intercept to the position of its associated hit gives the track residual
12
13 Charge Calibration Nonlinear relationship between deposited charge and electronics response Calibration which addresses this issue works toward improving device resolution Calibration data taken by collaborators at NIKHEF Calibration of surrogaus parameters over the H08 sensor. Substantial variation across the chips Average calibration across pixels used for the testbeam analysis Surrogate function which describes nonlinear charge weighting, convolved with Gaussian to account for noise. TOT stands for Time over Threshold, or how long the channel response exceeds a minimum value. Above, the relationship between TOT and the actual deposited charge in the sensor is ascertained from calibration data averaged across the H08 sensor.
14 Eta Correction η represents charge sharing when a particle ionizes 2 or more adjacent pixels Non-linear η is found because of nonlinear broadening: pixel size is large compared to diffusion width of drifting electron holes Calculation of hit position suffers Example of the inverse eta fit used in the correction. Empirical correction of η may be applied after charge calibration 5 th -degree polynomial function fitted to inverse η distribution of small independent data sample Inverse η function applied to the complementary data to complete the correction Effect of the empirical eta correction on the eta distribution.
15 Residuals for F08 Residual distributions for F08 before and after corrections. Note the change in shape of the 2- pixel-wide cluster contribution In particular. Residuals for F08 shown, including contributions from hits of different cluster widths Applying charge calibration followed by η correction improves the residual distribution, the standard deviation of which defines the spatial resolution Hit clusters of 2-pixel and 3- pixel width enjoy substantial improvement in resolution Best spatial resolution on the order of 4 µm
16 Residuals for H08 Residual distributions for H08 before and after corrections. Residuals for H08 are consistent with F08 and display similar improvement due to corrections Best spatial resolution on the order of 4 µm
17 Resolution versus angle, F08 Angle scan indicates evaluating the resolution at different angles of beam incidence Gaussian function is fit to each residual distribution giving the spatial resolution as a function of angle Prediction of minimum resolution fulfilled in vicinity of 15 Angle scan done at several operating thresholds Angle scan of F08 at 1000 electron threshold.
18 Resolution versus angle, F08 continued Angle scan of F08 at 750 electron threshold. Angle scan of F08 at 2000 electron threshold.
19 Resolution versus angle, H08 Angle scan of H08 with bias voltage -40V. Angle scan of H08 with bias voltage -60V.
20 Resolution versus angle, H08 continued Angle scan of H08 with bias voltage -80V. Angle scan of H08 with bias voltage -100V.
21 Is the detector efficient all the way up to its geometrical edge?
22 Hit Maps Data taken with beam oriented at edge of DUT reveals substantial distortion effects near the edge 4/29/2014 Both F08 and H08 exhibit an inflamed second-to-last row of pixels at the edge with abnormal number of hits Hit maps of F08 and H08 devices with beam concentrated near the edge. The spectrum denotes number of hits in a given pixel.
23 Edge Efficiency 4/29/2014 The ratio of associated hits to total hits is calculated across the chip and binned as a distribution using the hit position The cross section of this 2D distribution near the doped edge regions can be fitted with a sigmoidal function H(x) Falls below 90% 12.3 µm from the edge Falls below 90% 2.2 µm from the edge Efficiency versus sensor X coordinate for F08, H08 devices. Dashed lines denote pixel boundaries. The last region before efficiency dropoff is the active edge region. The turquoise lines denote the physical extent of the chip. H x = 0.5 erf ±k[x x o ] + 0.5
24 Conclusions Spatial granularity of the devices is adequate for an upgrade to the LHCb vertex detector Active edge regions are useful and efficient Edgeless TimePix sensors satisfy demands for numerous state-of-the-art applications: Particle physics Medical imaging X-ray spectroscopy
25