1. Section Two Requires e-h pair creation data from Section One and electric field model from Maxwell software package (Fig. 6 - left). The induced strip signals are evaluated using the Shockley-Ramo Method. Front-end electronics simulated using pre- amp/shaper setup ( Kyle Snavely & Daniel Mannarino ). Section One Simulates deposition of energy by a particle (e.g. p, e, π) moving through the silicon bulk. At each step, the deposited energy is decomposed into the equivalent number of electron hole charge carrier pairs that are generated at a given location. Introduction In the near future, CEBAF (the Continuous Electron Beam Accelerator Facility) at Jefferson Lab in Newport News, Virginia, will be undergoing an upgrade. This upgrade will increase the energy regime of the electron beam with which experiments are performed from 6 GeV to 12 GeV. Such an increase in the beam energy for experiments calls for a similar increase in detector capabilities to utilize the upgrades of the accelerator. In Hall B, the CLAS (CEBAF Large Acceptance Spectrometer) collaboration is upgrading the entire detector array, including the inner tracking system. One part of this is the Silicon Vertex Tracker (SVT), the detectors responsible for locating and disentangling the interaction points, or vertices, of events. These detectors need to have high spatial precision, so an important effect to understand within the detectors is charge sharing and how it varies with and affects different aspects of operation. CLAS12 Upgrade Benefits Understand more fully the substructure of nucleons (e.g. protons) through various nuclear processes. Technologies developed for such experiments can be applied to other fields like medical imaging technologies with more immediate benefits. Acknowledgements I gratefully thank the following individuals for their guidance and assistance throughout the course of my project. Maurik Holtrop (UNH) Sarah Phillips (UNH) M.N. Mazziotta (Bari, Italy) Characterization of Charge Sharing in Silicon Vertex Tracking Detectors for CLAS12 Samuel Meehan (Advisors: Maurik Holtrop, Sarah Phillips) Department of Physics, University of New Hampshire, Durham, NH 03824 For more information concerning Hall B and the CLAS12 collaboration, refer to: http://www.jlab.org/Hall-B/clas12/ Simulating Charge Sharing To simulate charge sharing, I have modified code provided by M. Mazziotta [1] to perform a full scale Monte Carlo simulation of the detector operation. This simulation considers an incident energetic particle passing through the detector (Fig. 3) and calculates the individual electronic output from each strip. The two main sections of the simulation are described below. Charge Sharing Charge sharing occurs when an incident energetic particle causes several strips to respond to the deposited energy from ionization of the silicon bulk. Understanding and characterizing this effect is of particular importance at this stage of development. Variation of Incident Angle During a typical experimental event where particles pass through the detector, it is likely that the particle will be incident upon the detector at an angle that is not normal to the surface of the detector. I have simulated several such scenarios of non-normal incidence to investigate the response of the detector. Optimization of Strip Separation One of the key design parameters that affects the resolution of the detector is the strip separation, or pitch. I have simulated three detector designs using pitch widths of 37.5 μm (blue), 75 μm (red), and 112.5 μm (green), to determine if the present pitch is an optimal choice for the detector. Future Study Test physical detectors for comparison to simulated results. Model charge distributions from particles of varying type, energy, and incident angle to use for testing readout electronics design. Use novel methods of curve fitting to untangle particle entry location and incident angle from charge distribution and obtain resolution of detector. Modify existing simulation to study energy resolution of detector. Conclusions The shifting and skewing of the charge distribution when varying the incident angle indicates that this is a non-negligible effect and needs to be tested experimentally. The variation of the strip pitch shows expected behavior for the charge sharing but indicates only small variations between the 37.5 μm, 75μm, and 112.5 μm strip pitch configurations and so the present 75 μm strip pitch is working sufficiently well. Fig.1 – (Above) Schematic layout showing entire CLAS12 detector system. Fig. 4 – Cross section of SVT showing the detection of an energetic particle as it ionizes the silicon bulk, creating electron hole (e-h) pairs. The holes migrate towards the strips (red) and cause for a voltage signal on multiple strips. This single particle causing multiple strips to output is the basic principle of interstrip charge sharing. Fig. 3 – (Above) Cross section of silicon vertex tracker showing silicon bulk substrate (1), strips (2), and other important design features. This detector is 300μm thick and the strips are separated by 75 μm in the present design. However, it is important to note that only every other strip is a readout strip and so the functional strip separation is 150 μm. Figs. 5 & 6 – (Above) Particle trajectories of ten incident particles in the detector as simulated in Section One. (Below) Example of electric potential used in Section Two that determines how charge carriers migrate and the voltage outputs on strips. Four different incident particle angles were simulated over a range of input positions across the detector (Fig.7 left). The three input ranges tested were directly on top of the central strip (Fig.8 right), 0.5 pitch units on either side of the central strip (Fig.9 down-left), and 1.5 pitch units on either side of the central strip (Fig.10 down-right). In each, the average strip responses are shown for angles of incidence at 0 o (black), 30 o (red), 45 o (blue), and 60 o (green). Note how as the angle of incidence increases, the average strip response shifts laterally to the left. A comparison of the average strip response of the three detector setups with pions input over range spanning 1.5 pitch units on either side of the central strip. Fig. 12 (below) is a focused range of Fig. 11 (above) showing that as expected, a larger fraction of the charge is carries by strips away from the central strip when the pitch length is smaller. Fig. 13 (above) Shows a comparison of the number of holes detected by each readout strip on the three detectors. This is one of the primary causes of charge sharing and helps explain the variation in strip response, as the larger pitch length configuration shows detected holes to be more centralized near the central strip than for smaller pitch configurations. Fig. 14 – Comparison of the average distance that a hole charge carrier drifts while in the detector. Although varied in shape, the three distributions are roughly equivalent in spread. So, by spacing the strips closer together, more holes will be detected over a broader number of strips. [1] M. Brigida et al., Nucl. Instr. and Meth. A 533 (2004), p. 322. Fig.2 – (Right) Inner tracking system of detector whose primary purpose is to reconstruct the interaction point dynamics of an event. Areas of Focus Presented here, we have studied: Effect of (readout pitch - Fig. 3) on charge sharing in the detector. Use of charge sharing to study effect of particle with non-normal angular incidence. Using similar techniques, we can also study: The effect of different particles incident at varying energies different energies. The use of charge sharing for spatial resolution. 1 222 Readout Pitch Implant Pitch