Slide 1

A full Monte Carlo simulation code for silicon strip detectors M. Brigida, C. Favuzzi, P. Fusco, F. Gargano, N. Giglietto, F. Giordano, F. Loparco, B. Marangelli, M. N. Mazziotta, N. Mirizzi, S. Rain, P. Spinelli Bari University & INFN 9th Topical Seminar on Innovative Particle and Radiation Detectors May 23-26, 2004- Siena, Italy Slide 2 Welcome Luciafrancesca ! Slide 3 The simulation chain Charged particlesPhotons Ionization energy loss Photoelectric absorption Primary e-h pairsSecondary e-h pairs Drift of charge carriers Induced current signals Electronics chain Electronic noise Voltage signals on the readout strips Interaction with silicon Propagation of carriers Electronics simulation Slide 4 Energy loss of charged particles in silicon The energy loss in Si is evaluated from the collision cross section (E) (H. Bichsel, Rev. Mod. Phys. 60, 663) M-shell (~17 eV) L-shells (~150 eV) K-shell (~1850 eV) The number of collisions per unit path length is evaluated as: Slide 5 Generation of e-h pairs in silicon Ionizing particle Si atom Virtual Primary e-h pairs Secondary e-h pairs Phonon scattering Slide 6 Silicon energy levels Energy K-shell E k = -1839 eV L-shells E L2-3 = -99.2 eV E L1 = -148.7 eV Valence band E V = [-12, 0] eV Conduction band Energy gap E g = 1.12 eV @T=300K Slide 7 Generation of e-h pairs Primary carriers: are produced in the primary collisions of the incident particle with the silicon absorber, with the absorption of virtual photons by the medium. Secondary carriers: are produced by the subsequent energy losses of primary (and secondary) carriers. The relative absorption probabilities depend on the photon energy. For energies above the K- shell there is a 92% probability of absorption by the K-shell and an 8% probability of absorption by the L 1 -shell Slide 8 Primary e-h pairs Absorption by an inner shell (x=K, L 1, L 23 ): A hole is left in the shell with energy E h =E x A photoelectron is ejected with energy E pe =E -E x -E gap Absorption by the valence band (M shell): A hole is left with an energy E h random distributed in the range [0,E V ] (E V =12eV) A photoelectron is ejected with energy E pe =E -E h -E gap The relaxation process following photon absorption yields electrons and vacancies in the K, L 1 and L 23 shells. Slide 9 Silicon shells relaxation trees K-shell vacancy and photoelectron: E h =E K E pe =E -E K -E gap Auger emissions (95.6%)K-shell fluorescence (4.4%) Transition Chain Proba bility Vacancies Proba bility Photon energy Vacancy KL 1 L 1 19.2%L1L1L1L1 59.3%1740 eVL3L3 KL 1 L 23 38.9%L 1 L 23 29.6%1740 eVL2L2 KL 23 L 23 23.3%L 23 11.1%1836 eVM KL 1 M7.5%L1ML1M KL 23 M10.4%L 23 M KMM0.8%MM L 1 -shell vacancy and photoelectron: E h =E L1 E pe =E -E L1 -E gap Transition Chain ProbabilityEmissionVacancies L 1 MM2.5%AugerMM L 1 L 23 M97.5% Coster- Kroning L 23 M L 23 -shell vacancy and photoelectron: E h =E L23 E pe =E -E L23 -E gap Transition Chain Probabi lity EmissionVacancies L 23 MM100%AugerMM Electron and hole energies are assigned according to Sholze et al, J. Appl. Phys. 84 (1998), 2926 Slide 10 Production of secondary e-h pairs A primary electron (hole) with E > E thr (E thr =3/2 E gap ) can interact with the Si absorber by ionization or by phonon scattering. The ratio between the ionization rate and the phonon scattering rate is: where A=5.2 eV 3 and E 0 is the phonon energy (E 0 =63 meV @ T=300 K) The generation of secondary pairs is a cascade process, that is simulated with a MC method. At the end of each step, a carrier can emit a phonon or can cause ionization. In this case a new e-h pair is created. Slide 11 Pair creation energy & Fano factor Pairs generated by electrons (holes) Pairs generated by photons The Fano Factor approaches the limit F =0.117 for large primary energies The pair creation energy approaches the value W =3.645 eV for large primary energies Slide 12 Pair distribution along the track =5 electron tracks in 400 m silicon Slide 13 SSD geometry p h w d n bulk p + strips The p strips are grounded, the back is kept at a positive voltage V 0 "Small pitch" geometry: d=325 m, p=25 m w=12 m, h=5 m V 0 =100 V "Large pitch" geometry: d=400 m, p=228 m w=60 m, h=5 m V 0 =100 V Slide 14 The electric field "Large pitch" configuration The electric field has been calculated by solving the Maxwell equation: in an elementary detector cell with the following boundary conditions for the potential: The calculation has been performed using the ANSOFT MAXWELL 2D field calculator. Slide 15 Motion of charge carriers After being produced, electrons and holes will drift under the action of the electric field towards the n back and the p strips, according to the equation: where the mobility is related to the E field by the parameterization: The parameters v m, and E c are different for electron and holes and depend on the temperature. During their drift, carriers are diffused by multiple collisions according to a gaussian law: Slide 16 Induced current signals The current signals induced by the moving carriers on the readout electrodes (p strips) are calculated using the Shockley- Ramo's theorem: The weighting field E k describes the geometrical coupling between the moving carrier and the k-th electrode. It has been evaluated by solving the same Maxwell's equation as for the electric field with =0 and with the boundary conditions: Slide 17 Weighting potential "Large pitch" configuration Readout strip Adjacent strips Back electrode Slide 18 Simulation of the electronics Input current signal i(t) Front-end electronics H(s) Output voltage signal V(t) The output signals are evaluated in the time domain by solving the inverse Laplace transform with the finite difference approximation for the time derivatives The transfer function can be expressed as a ratio of polynomials Noise contributions are added by taking into account the proper noise transfer functions Slide 19 Front-end electronics DetectorPreamplifierShaper Slide 20 Noise simulation The electronic noise is due to the detector and to the electronic front-end. Shot noise due to the leakage current: i 2 nd =2eI L Thermal noise due to the bias resistor: i 2 nb =4KT/R b Thermal noise due to the feedback resistor: i 2 nf =4KT/R f Electronic noise due to the amplifier: i 2 na = 0 v 2 na = 2.7KT/g m Slide 21 Charge sharing analysis (1) To study the charge sharing a sample of MIPs has been simulated, crossing the detector with null zenith angle, in the region between two strips The charge sharing has been studied with the function: Slide 22 Charge sharing analysis (2) Both the distribution are symmetric around the value =0.5 In the large pitch geometry the peaks are located at 0 and 1 weak coupling between adjacent strips In the small pitch geometry the peaks are located at 0.2 and 0.8 strong coupling between adjacent strips Slide 23 Comparison with experimental data A beam test has been carried out exposing a 400 m thick SSD with 228 m strip pitch to a 3 GeV/c beam @ CERN-PS T9 beam facility Experimental data are in good agreement with the MC prediction Slide 24 Conclusions We have developed a new MC full simulation code that includes all the physical processes taking place in a SSD The MC code can be used with different detector geometries and front-end electronics The temperature dependence of the physical processes is taken into account, thus allowing a study of the SSD performance with the temperature (an example will be given in S. Rain's talk) A charge sharing analysis has been performed, showing that the MC predictions are in good agreement with experimental data Our MC code allows to study the efficiency and the space resolution of SSDs (an example will be shown in M. Brigida's talk) For further details: http://www.ba.infn.it/~mazziot/article.pdfhttp://www.ba.infn.it/~mazziot/article.pdf