Searcharxiv⌕ Search

arXiv subjects

P. Bhattacharya

Publications and source records attributed to P. Bhattacharya.

14 recordsLinked to original sources
arXiv2020

Electron transparency of a Micromegas mesh

Measurements of the electron transparency of a Micromegas mesh are compared to simulations. The flux conservation argument is shown to lead to inaccurate estimates of the transparency, the importance of accurate geometric modelling of the mesh is discussed and the effect of the dipole moment of the mesh is demonstrated. This study provides a validation of the microscopic simulation methods specifically developed for micropattern devices where the characteristic dimensions are of the same order of magnitude as the electron mean free path in the gas.

physics.ins-det↗
arXiv2019

Novel Resistive-Plate WELL sampling element for (S)DHCAL

Digital and Semi-Digital Hadronic Calorimeters (S)DHCAL were suggested for future Colliders as part of the particle-flow concept. Though studied mostly with RPC-based techniques, investigations have shown that MPGD-based sampling elements could outperform. An attractive, industry-produced, robust, particle-tracking detector for large-area coverage, e.g. in (S)DHCAL, could be the novel single-stage Resistive Plate WELL (RPWELL). It is a single-sided THGEM coupled to the segmented readout electrode through a sheet of large bulk resistivity. We summarize here the preliminary test-beam results obtained with 6.5 mm thick (incl. electronics) {$48 \times 48\,\mathrm{cm^2}$}~RPWELL detectors. Two configurations are considered: a standalone RPWELL detector studied with 150 GeV muons and high-rate pions beams and RPWELL sampling element investigated within a small-(S)DHCAL prototype consisting of 7 resistive MICROMEGAS sampling elements followed by 5 RPWELL ones. The sampling elements were equipped with a Semi-Digital readout electronics based on the MICROROC chip.

physics.ins-det↗
arXiv2018

Ion backflow studies with a triple-GEM stack with increasing hole pitch

Gas Electron Multipliers have undergone a very consistent development since their invention in 1997. Their production procedures have been tuned in such a way that nowadays it is possible to produce foils with areas of the order of the square meter that can operate at a reasonable gain, uniform over large areas and with a good stability in what concerns electrical discharges. For the third run of LHC, they will be included in the CMS and ALICE experiments after significant upgrades of the detectors, confirming that these structures are suitable for very large experiments. In the special case of Time Projection Chambers, the ion backflow and the energy resolution are sensitive issues that must be addressed and the GEM has shown to be able to deal with both of them. In this work, a stack of three GEMs with different pitches has been studied as a possible future approach for ion-backflow suppression to be used in TPCs and other detection concepts. With this approach, an ion backflow of 1 % with an energy resolution of 12 % at 5.9 keV has been achieved with the detector operating in an Ar/CO2 (90/10) mixture at a gain of ~ 2000.

physics.ins-det↗
arXiv2018

Fission fragment mass distribution in $^{210}$Po and $^{213}$At

Background: The influence of shell effect on the dynamics of the fusion fission process and it's evolution with excitation energy in the pre-actinide Hg-Pb region in general is a matter of intense research in recent years. In particular, a strong ambiguity remains for the neutron shell closed $^{210}$Po nucleus regarding the role of shell effect in fission around $\approx$ 30 - 40 MeV of excitation energy. Purpose: We have measured the fission fragment mass distribution of $^{210}$Po populated using fusion of $^{4}$He + $^{206}$Pb at different excitation energies and compare the result with recent theoretical predictions as well as with our previous measurement for the same nucleus populated through a different entrance channel. Mass distribution in the fission of the neighbouring nuclei $^{213}$At is also studied for comparison. Methods: Two large area Multi-wire Proportional Counters (MWPC) were used for complete kinematical measurement of the coincident fission fragments. The time of flight differences of the coincident fission fragments were used to directly extract the fission fragment mass distributions. Results: The measured fragment mass distribution for the reactions $^{4}$He + $^{206}$Pb and $^{4}$He + $^{209}$Bi were symmetric and the width of the mass distributions were found to increase monotonically with excitation energy above 36.7 MeV and 32.9 MeV, respectively, indicating the absence of shell effects at the saddle. However, in the fission of $^{210}$Po, we find minor deviation from symmetric mass distributions at the lowest excitation energy (30.8 MeV). Conclusion: Persistence of shell effect in fission fragment mass distribution of $^{210}$Po was observed at the excitation energy $\approx$ 31 MeV as predicted by the theory; at higher excitation energy, however, the present study reaffirms the absence of any shell correction in the fission of $^{210}$Po.

nucl-ex↗
arXiv2016

Fission fragment mass distributions in reactions populating 200Pb

The fission fragment mass distributions have been measured in the reactions 16O + 184W and 19F+ 181Ta populating the same compound nucleus 200Pb? at similar excitation energies. It is found that the widths of the mass distribution increases monotonically with excitation energy, indicating the absence of quasi-fission for both reactions. This is contrary to two recent claims of the presence of quasi-fission in the above mentioned reactions.

nucl-ex↗
arXiv2016

Long-term study of Mkn 421 with the HAGAR Array of Telescopes

Context:The HAGAR Telescope Array at Hanle, Ladakh has been regularly monitoring the nearby blazar Mkn 421 for the past 7yrs. Aims: Blazars show flux variability in all timescales across the electromagnetic spectrum. While there is abundant literature characterizing the short term flares from different blazars, comparatively little work has been done to study the long term variability. We aim to study the long term temporal and spectral variability in the radiation from Mkn 421 during 2009-2015. Methods: We quantify the variability and lognormality from the radio to the VHE bands, and compute the correlations between the various wavebands using the z-transformed discrete correlation function. We construct the Spectral Energy Distribution (SED) contemporaneous with HAGAR observation seasons and fit it with a one zone synchrotron self Compton model to study the spectral variability. Results: The flux is found to be highly variable across all time scales. The variability is energy dependant, and is maximum in the X-ray and Very High Energy (VHE) bands. A strong correlation is found between the Fermi-LAT (gamma) and radio bands, and between Fermi-LAT and optical, but none between Fermi-LAT and X-ray. Lognormality in the flux distribution is clearly detected. This is the third blazar, following BL~Lac and PKS~2155$+$304 to show this behavior. The SED can be well fit by a one zone SSC model, and variations in the flux states can be attributed mainly due to changes in the particle distribution. A strong correlation is seen between the break energy $γ_b$ of the particle spectrum and the total bolometric luminosity.

astro-ph.HE↗
arXiv2015

No influence of a N=126 Neutron Shell Closure in Fission Fragment Mass Distributions

Mass distributions of the fragments in the fission of $^{206}$Po and the N=126 neutron shell closed nucleus $^{210}$Po have been measured. No significant deviation of mass distributions has been found between $^{206}$Po and $^{210}$Po, indicating the absence of shell correction at the saddle point in both the nuclei, contrary to the reported angular anisotropy and pre-scission neutron multiplicity results. This new result provides benchmark data to test the new fission dynamical models to study the effect of shell correction on the potential energy surface at saddle point.

nucl-ex↗
arXiv2015

Characterizations of GEM detector prototype

At NISER-IoP detector laboratory an initiative is taken to build and test Gas Electron Multiplier (GEM) detectors for ALICE experiment. The optimisation of the gas flow rate and the long-term stability test of the GEM detector are performed. The method and test results are presented.

physics.ins-det↗
arXiv2015

Direct Evidence of Washing out of Nuclear Shell Effects

Constraining excitation energy at which nuclear shell effect washes out has important implications on the production of super heavy elements and many other fields of nuclear physics research. We report the fission fragment mass distribution in alpha induced reaction on an actinide target for wide excitation range in close energy interval and show direct evidence that nuclear shell effect washes out at excitation energy ~40 MeV. Calculation shows that second peak of the fission barrier also vanishes around similar excitation energy.

nucl-ex↗
arXiv2008

Electronic transport driven spin-dynamics

We propose a model to explore the dynamics of spin-systems coupled by exchange interaction to the conduction band electrons of a semiconductor material that forms the channel in a ferromagnet/semiconductor/ferromagnet spin-valve structure. We show that recent observation of the novel transient transport signature in a MnAs/GaAs/MnAs spin-valve structure with paramagnetic Mn impurities [D. Saha et al., Phys. Rev. Lett., 100, 196603 (2008)] can be quantitatively understood in terms of current driven dynamical polarization of Mn spins. Using our model of spin polarized transport through Schottky barriers at the two ferromagnet/semiconductor junctions in a spin-valve structure and a dynamical equation describing the paramagnetic impurities coupled to conduction band electrons we explain the scaling behaviour of observed transient features such as the magnitude and time-scale with temperature.

cond-mat.mes-hall↗
arXiv2008

Sharp change over from compound nuclear fission to shape dependent quasi fission

Fission fragment mass distribution has been measured from the decay of $^{246}$Bk nucleus populating via two entrance channels with slight difference in mass asymmetries but belonging on either side of the Businaro Gallone mass asymmetry parameter. Both the target nuclei were deformed. Near the Coulomb barrier, at similar excitation energies the width of the fission fragment mass distribution was found to be drastically different for the $^{14}$N + $^{232}$Th reaction compared to the $^{11}$B + $^{235}$U reaction. The entrance channel mass asymmetry was found to affect the fusion process sharply.

nucl-ex↗
arXiv2005

Evidence of microscopic effects in fragment mass distribution in heavy ion induced fusion-fission reactions

Our measurements of variances ($σ_{m}^2$) in mass distributions of fission fragments from fusion-fission reactions of light projectiles (C, O and F) on deformed thorium targets exhibit a sharp anomalous increase with energy near the Coulomb barrier, in contrast to the smooth variation of $σ_{m}^2$ for the spherical bismuth target. This departure from expectation based on a statistical description is explained in terms of microscopic effects arising from the orientational dependence in the case of deformed thorium targets.

nucl-ex↗
arXiv2005

Microscopic effects in fragment mass distribution in fusion-fission reactions of light projectiles with heavy targets

Microscopic effects in fragment mass distributions in fusion-fission reactions of light projectiles (C, O and F) on deformed thorium and spherical bismuth targets in near and below Coulomb barrier energies are investigated. Precisely measured mass distribution shows a sudden anomalous increase in variances of mass distributions ($σ_{m}^2$) near Coulomb barrier energies for all three projectiles with deformed thorium target, in contrast to a smooth variation of $σ_{m}^2$ with energy for spherical bismuth target. Macroscopic effects of change in mass flow or prolonged mass equilibration time can not explain the observed variation in $σ_{m}^2$ with energy. Microscopic effects due to change in entrance channel shape compactness for projectiles hitting the polar region of prolate thorium target is postulated to reach a almost symmetric saddle without complete fusion for events of anomalous fragment widths. Quantitative estimates of mass widths mixed for the two processes explains the observed variation of $σ_{m}^2$ with excitation energy.

nucl-ex↗