arXiv subjects

S. Mukhopadhyay

Publications and source records attributed to S. Mukhopadhyay.

At least 91 records · Page 5Linked to original sources

arXiv2007

LAMBDA: Large Area Modular BaF2 Detector Array for the measurement of high energy gamma rays

A large BaF2 detector array along with its dedicated CAMAC electronics and VME based data acquisition system has been designed, constructed and installed successfully at VECC, Kolkata for studying high energy gamma rays (E>8 MeV). The array consists of 162 detector elements. The detectors were fabricated from bare barium fluoride crystals (each measuring 35 cm in length and having cross-sectional area of 3.5 cm X 3.5 cm X 35.0 cm). The basic properties of the detectors (energy resolution, time resolution, efficiency, uniformity, fast to slow ratio etc.) were studied exhaustively. Complete GEANT3 monte carlo simulations were performed to optimize the detector design and also to generate the response function. The detector system has been used successfully to measure high energy photons from 113Sb, formed by bombarding 145 and 160 MeV 20Ne beams on a 93Nb target. The measured experimental spectra are in good agreement with those from a modified version of the statistical model code CASCADE. In this paper, we present the complete description of this detector array along with its in-beam performance.

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arXiv2007

From chiral vibration to static chirality in ^{135}Nd

Electromagnetic transition probabilities have been measured for the intra- and inter-band transitions in the two sequences in the nucleus ^{135}Nd that were previously identified as a composite chiral pair of rotational bands. The measurements are in good agreement with results of a new combination of TAC and RPA calculations. The chiral character of the bands is affirmed and it is shown that their behavior is associated with a transition from a vibrational into a static chiral regime.

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arXiv2007

High-spin structure and Band Termination in $^{103}$Cd

Excited states of the neutron deficient $^{103}$Cd nucleus have been investigated via the $^{72}$Ge($^{35}$Cl, p3n) reaction at beam energy of 135 MeV by use of in-beam spectroscopic methods. Gamma rays depopulating the excited states were detected using the Gammasphere spectrometer with high-fold $γ$-ray coincidences. A quadrupole $γ$-ray coincidence analysis ($γ^{4}$) has been used to extend the known level scheme. The positive parity levels have been established up to $J = 35/2\hbar$ and $E_{x} = 7.071$ MeV. In addition to the observation of highly-fragmented level scheme belonging to the positive-parity sequences at E$_{x}\sim$ 5 MeV, the termination of a negative-parity sequence connected by $E2$ transitions has been established at $J = 47/2 \hbar$ and $E_{x} = 11.877$ MeV. The experimental results corresponding to both the positive- and negative-parity sequences have been theoretically interpreted in the framework of the core particle coupling model. Evidence is presented for a shape change from collective prolate to non-collective oblate above the $J^π = 39/2^{-}$ (8011 keV) level and for a smooth termination of the negative-parity band.

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arXiv2007

Characterization of fragment emission in ^{20}Ne (7 - 10 MeV/nucleon) + ^{12}C reactions

The inclusive energy distributions of the complex fragments (3 $\leq$ Z $\leq$ 7) emitted from the bombardment of ^{12}C by ^{20}Ne beams with incident energies between 145 and 200 MeV have been measured in the angular range 10$^{o} \leq θ_{lab} \leq$ 50^{o}. Damped fragment yields in all the cases have been found to be the characteristic of emission from fully energy equilibrated composites. The binary fragment yields are compared with the standard statistical model predictions. Enhanced yields of entrance channel fragments (5 $\leq$ Z $\leq$ 7) indicate the survival of orbiting-like process in ^{20}Ne + ^{12}C system at these energies.

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arXiv2007

Fast and Precise 3D Computation of Capacitance of Parallel Narrow Beam MEMS Structures

Efficient design and performance of electrically actuated MEMS devices necessitate accurate estimation of electrostatic forces on the MEMS structures. This in turn requires thorough study of the capacitance of the structures and finally the charge density distribution on the various surfaces of a device. In this work, nearly exact BEM solutions have been provided in order to estimate these properties of a parallel narrow beam structure found in MEMS devices. The effect of three-dimensionality, which is an important aspect for these structures, and associated fringe fields have been studied in detail. A reasonably large parameter space has been covered in order to follow the variation of capacitance with various geometric factors. The present results have been compared with those obtained using empirical parametrized expressions keeping in view the requirement of the speed of computation. The limitations of the empirical expressions have been pointed out and possible approaches of their improvement have been discussed.

physics.comp-ph↗

arXiv2007

Computation of Electrostatic and Gravitational Sag in MultiWire Chambers

A numerical method of determining the wire sag in a multiwire proportional chamber used in RICH (ALICE) by solving the second order differential equation which governs the wire stability has been presented. The three point Finite Difference Method (FDM) has generated a tridiagonal matrix equation relating the deflection of wire segments to the force acting on it. The precise estimates of electrostatic force has been obtained from accurate field computation using a nearly exact Boundary Element Method (neBEM) solver.

physics.ins-det↗

arXiv2007

Simulation of Pickup Signal in a Resistive Plate Chamber

The signal induced by an electron on a pickup strip has been calculated in a RPC used in INO calorimeter following Ramo's theorem. An accurate estimation of weighting field has been obtained using a nearly exact Boundary Element Method (neBEM) solver while the electron velocity from the real field values. The calculation has shown a growth and subsequent fall of pickup signal with duration about 90ps.

physics.ins-det↗

arXiv2007

Estimation of Weighting Potential for a TPC

In this work, we have computed the three dimensional weighting potential, field and pad response function (PRF) of typical time projection chambers (TPC) using a recently formulated and developed boundary element method solver, namely, the nearly exact BEM solver. A realistic geometry of the device is found to have significant influence on the estimation of signal generation.

physics.ins-det↗

arXiv2007

3D Simulation of New Generation Gas Detectors

Simulation of new generation gas detectors is rendered complicated due to the non-trivial nature of the electric field and simultaneous presence of several length-scales. Computation of the electrostatic field, however, is extremely important since streamers in gas volume and streamers across the dielectric surfaces are known to cause serious damage to Micro Pattern Gas Detectors (MPGD)and are the main factors in limiting their gain. In this paper, we present the use of a nearly exact Boundary Element Method (neBEM) solver that overcomes most of the limitations of FEM packages and is capable of producing accurate results.

physics.ins-det↗

arXiv2006

Factors Affecting the Precision of Electrostatic Computation of 3D MEMS Structures

Micro-Electro-Mechanical Systems (MEMS) normally have fixed or moving structures (plates or array of thin beams) with cross-sections of the order of microns and lengths of the order of tens or hundreds of microns. Electrostatic forces play a very major role in maneuvering these devices, and hence, a thorough understanding of the electrostatic properties of these structures is of critical importance. Recently, a nearly exact boundary element method (neBEM) solver has been developed and used to solve difficult problems related to electrostatics of various devices. Because of the exact foundation expressions, this solver has been found to be very accurate while solving critical problems which normally necessitate special formulations involving elegant, but difficult mathematics. In this work, we investigate the effects of various possible approximations on the 3D electrostatic solutions obtained for MEMS structures. In particular, we investigate the effects of discretization, omission of surfaces with small amount of charge accumulation on the final results such as the charge distribution on various surfaces or capacitance of a given MEMS structure.

physics.comp-ph↗

arXiv2006

Use of the neBEM solver to Compute the 3D Electrostatic Properties of Comb Drives

Micro-Electro-Mechanical Systems (MEMS) comb drives are used for both as sensors and actuators. As a result, they have been considered to be very important in MEMS technology and has been under intense study for last few years. The actuation and the sensitivity are both dependent in a major way on the electrostatic configuration of the comb structure. As a result, accurate estimation of the electrostatic configuration of comb drives is crucial in both design and interpretation phases. The nearly exact Boundary Element Method (neBEM) solver has been developed recently and used successfully to solve difficult problems related to electrostatics. This solver uses exact analytic expression for computing the influence of singularity distributions instead of adopting the conventional and convenient approximation of nodal concentration of charges. Due to the exact foundation expressions, the solver has been found to be exceptionally accurate in the complete physical domain, including the near field. In this work, we explore the possibility of using the neBEM solver to solve 3D electrostatic problems related to comb drives. In particular, we investigate the relationship between the accuracy achieved and the computational expenses incurred for a realistic comb drive geometry. In the process, we estimate the charge density distribution, potential distribution and the capacitance of the comb structure. The study has led us to the conclusion that the neBEM solver can yield very accurate estimates of all the properties of interest at a reasonable computational expenditure.

physics.comp-ph↗

arXiv2006

Study of Dissipative Collisions of $^{20}$Ne ($\sim$7-11 MeV/nucleon) + $^{27}$Al

The inclusive energy distributions of complex fragments (3 $\leq$Z $\leq$ 9) emitted in the reactions $^{20}$Ne (145, 158, 200, 218 MeV) + $^{27}$Al have been measured in the angular range 10$^{o}$ - 50$^{o}$. The fusion-fission and the deep-inelastic components of the fragment yield have been extracted using multiple Gaussian functions from the experimental fragment energy spectra. The elemental yields of the fusion-fission component have been found to be fairly well exlained in the framework of standard statistical model. It is found that there is strong competition between the fusion-fission and the deep-inelastic processes at these energies. The time scale of the deep-inelastic process was estimated to be typically in the range of $\sim$ 10$^{-21}$ - 10$^{-22}$ sec., and it was found to decrease with increasing fragment mass. The angular momentum dissipations in fully energy damped deep-inelastic process have been estimated from the average energies of the deep-inelastic components of the fragment energy spectra. It has been found that, the estimated angular momentum dissipations, for lighter fragments in particular, are more than those predicted by the empirical sticking limit.

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arXiv2006

Evidence of large nuclear deformation of $^{32}$S$^{*}$ formed in $^{20}$Ne + $^{12}$C reaction

Deformations of hot composite $^{32}$S$^{*}$ formed in the reaction $^{20}$Ne ($\sim$ 7 -- 10 MeV/nucleon) + $^{12}$C have been estimated from the respective inclusive $α$-particle evaporation spectra. The estimated deformations for $^{32}$S$^{*}$ have been found to be much larger than the `normal' deformations of hot, rotating composites at similar excitations. This further confirms the formation of highly deformed long-lived configuration of $^{20}$Ne + $^{12}$C at high excitations ($\sim$ 70 -- 100 MeV) -- which was recently indicated from the analysis of the complex fragment emission data for the same system. Exclusive $α$-particle evaporation spectra from the decay of hot composite $^{32}$S$^{*}$ also show similar behaviour.

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arXiv2006

Simulation of Three Dimensional Electrostatic Field Configuration in Wire Chambers : A Novel Approach

Three dimensional field configuration has been simulated for a simple wire chamber consisting of one anode wire stretched along the axis of a grounded square cathode tube by solving numerically the boundary integral equation of the first kind. A closed form expression of potential due to charge distributed over flat rectangular surface has been invoked in the solver using Green's function formalism leading to a nearly exact computation of electrostatic field. The solver has been employed to study the effect of several geometrical attributes such as the aspect ratio ($λ= \frac{l}{d}$, defined as the ratio of the length $l$ of the tube to its width $d$) and the wire modeling on the field configuration. Detailed calculation has revealed that the field values deviate from the analytic estimates significantly when the $λ$ is reduced to 2 or below. The solver has demonstrated the effect of wire modeling on the accuracy of the estimated near-field values in the amplification region. The thin wire results can be reproduced by the polygon model incorporating a modest number of surfaces ($\geq 32$) in the calculation with an accuracy of more than 99%. The smoothness in the three dimensional field calculation in comparison to fluctuations produced by other methods has been observed.

physics.ins-det↗

arXiv2006

Computation of Nearly Exact 3D Electrostatic Field in Gas Ionization Detectors

The three-dimensional electrostatic field configuration in gas ionization detectors has been simulated using an efficient and precise nearly exact boundary element method (NEBEM) solver set up to solve an integral equation of the first kind. This recently proposed formulation of BEM and the resulting solver use exact analytic integral of Green function to compute the electrostatic potential for charge distribution on flat surfaces satisfying Poisson's equation. As a result, extremely precise results have been obtained despite the use of relatively coarse discretization leading to successful validation against analytical results available for two-dimensional MWPCs. Significant three dimensional effects have been observed in the electrostatic field configuration and also on the force experienced by the anode wires of MWPCs. In order to illustrate the applicability of the NEBEM solver for detector geometries having multiple dielectrics and degenerate surfaces, it has been validated against available FEM and BEM numerical solutions for similar geometries.

physics.ins-det↗

arXiv2006

Development of a BEM Solver using Exact Expressions for Computing the Influence of Singularity Distributions

Closed form expressions for three-dimensional potential and force field due to singularities distributed on a finite flat surface have been used to develop a fast and accurate BEM solver. The exact expressions have been investigated in detail and found to be valid throughout the complete physical domain. Thus, it has been possible to precisely simulate the complicated characteristics of the potential and force field in the near-field domain. The BEM solver has been used to compute the capacitance of a unit square plate and a unit cube. The obtained values have compared well with very accurate values of the capacitances available in the literature.

physics.comp-ph↗

arXiv2006

Computation of Electrostatic Properties of 3D MEMS Structures

Micro-Electro-Mechanical Systems (MEMS) normally have fixed or moving structures with cross-sections of the order of microns ($μm$) and lengths of the order of tens or hundreds of microns. These structures are often plates or array of thin beams which, owing to their smallness, can be moved or deflected easily through the application of low voltages. Since electrostatic forces play a very major role in maneuvering these devices, a thorough understanding of the electrostatic properties of these structures is of critical importance, especially in the design phase of MEMS. In many cases, the electrostatic analysis of MEMS is carried out using boundary element method (BEM), while the structural analysis is carried out using finite element method (FEM). In this paper, we focus on accurate electrostatic analysis of MEMS using BEM. In particular, we consider the problem of computing the charge distribution and capacitance of thin conducting plates relevant to the numerical simulation of MEMS. The reason behind the accuracy of the solver is the fact that it uses closed-form analytic expressions that are valid seamlessly throughout the physical domain for computing the influence coefficients. Thus, it is possible to avoid one of the most serious approximations of the BEM, namely, the assumption that the effect of a charge distributed over a boundary element can be approximated by charge located at the centroid of the element. Comparison with other results available in the literature seems to indicate that the present results are more accurate than the existing ones.

physics.comp-ph↗

arXiv2006

Computation of Electrostatic Field near Three-Dimensional Corners and Edges

Theoretically, the electric field becomes infinite at corners of two and three dimensions and edges of three dimensions. Conventional finite-element and boundary element methods do not yield satisfactory results at close proximity to these singular locations. In this paper, we describe the application of a fast and accurate BEM solver (which usesexact analytic expressions to compute the effect of source distributions on flatsurfaces) to compute the electric field near three-dimensional corners and edges. Results have been obtained for distances as close as 1$μm$ near the corner/edge and good agreement has been observed between the present results and existing analytical solutions.

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