arXiv subjects

A. Pandey

Publications and source records attributed to A. Pandey.

34 records · Page 2Linked to original sources

arXiv2024

A Novel Diamond-like Carbon based photocathode for PICOSEC Micromegas detectors

The PICOSEC Micromegas (MM) detector is a precise timing gaseous detector based on a MM detector operating in a two-stage amplification mode and a Cherenkov radiator. Prototypes equipped with cesium iodide (CsI) photocathodes have shown promising time resolutions as precise as 24 picoseconds (ps) for Minimum Ionizing Particles. However, due to the high hygroscopicity and susceptibility to ion bombardment of the CsI photocathodes, alternative photocathode materials are needed to improve the robustness of PICOSEC MM. Diamond-like Carbon (DLC) film have been introduced as a novel robust photocathode material, which have shown promising results. A batch of DLC photocathodes with different thicknesses were produced and evaluated using ultraviolet light. The quantum efficiency measurements indicate that the optimized thickness of the DLC photocathode is approximately 3 nm. Furthermore, DLC photocathodes show good resistance to ion bombardment in aging test compared to the CsI photocathode. Finally, a PICOSEC MM prototype equipped with DLC photocathodes was tested in muon beams. A time resolution of around 42 ps with a detection efficiency of 97% for 150 GeV/c muons were obtained. These results indicate the great potential of DLC as a photocathode for the PICOSEC MM detector.

physics.ins-det↗

arXiv2024

Single channel PICOSEC Micromegas detector with improved time resolution

This paper presents design guidelines and experimental verification of a single-channel PICOSEC Micromegas (MM) detector with an improved time resolution. The design encompasses the detector board, vessel, auxiliary mechanical parts, and electrical connectivity for high voltage (HV) and signals, focusing on improving stability, reducing noise, and ensuring signal integrity to optimize timing performance. A notable feature is the simple and fast reassembly procedure, facilitating quick replacement of detector internal components that allows for an efficient measurement strategy involving different detector components. The paper also examines the influence of parasitics on the output signal integrity. To validate the design, a prototype assembly and three interchangeable detector boards with varying readout pad diameters were manufactured. The detectors were initially tested in the laboratory environment. Finally, the timing performance of detectors with different pad sizes was verified using a Minimum Ionizing Particle (MIP) beam test. Notably, a record time resolution for a PICOSEC Micromegas detector technology with a CsI photocathode of 12.5$\pm$0.8 ps was achieved with a 10 mm diameter readout pad size detector.

physics.ins-det↗

arXiv2024

Bipartite Sachdev-Ye Models with Read-Saleur Symmetries

We introduce an SU(M)-symmetric disordered bipartite spin model with unusual characteristics. Although superficially similar to the Sachdev-Ye model, it has several markedly different properties for M>2. In particular, it has a large non-trivial nullspace whose dimension grows exponentially with system size. The states in this nullspace are frustration-free, and are ground states when the interactions are ferromagnetic. The exponential growth of the nullspace leads to Hilbert-space fragmentation and a violation of the eigenstate thermalization hypothesis. We demonstrate that the commutant algebra responsible for this fragmentation is a non-trivial subalgebra of the Read-Saleur commutant algebra of certain nearest-neighbour models such as the spin-1 biquadratic spin chain. We also discuss the low-energy behaviour of correlations for the disordered version of this model in the limit of a large number of spins and large M, using techniques similar to those applied to the SY model. We conclude by generalizing the Shiraishi-Mori embedding formalism to non-local models, and apply it to turn some of our nullspace states into quantum many-body scars.

cond-mat.stat-mech↗

arXiv2024

Science with a small two-band UV-photometry mission III: Active Galactic Nuclei and nuclear transients

In this review (the third in the series focused on a small two-band UV-photometry mission), we assess possibilities for a small UV two-band photometry mission in studying accreting supermassive black holes (SMBHs; mass range $\sim 10^6$-$10^{10}\,M_{\odot}$). We focus on the following observational concepts: (i) dedicated monitoring of selected type-I Active Galactic Nuclei (AGN) in order to measure the time delay between the far-UV, the near-UV, and other wavebands (X-ray and optical), (ii) nuclear transients including (partial) tidal disruption events and repetitive nuclear transients, and (iii) the study of peculiar sources, such as changing-look AGN, hollows and gaps in accretion disks, low-luminosity AGN, and candidates for Intermediate-Mass Black Holes (IMBHs; mass range $\sim 10^2$-$10^5\,M_{\odot}$) in galactic nuclei. For tidal disruption events (TDEs), high-cadence UV monitoring is crucial for distinguishing among different scenarios for the origin of the UV emission. The small two-band UV space telescope will also provide the information about the near- and far-UV continuum variability for rare transients, such as repetitive partial TDEs and jetted TDEs. We also discuss the possibilities to study and analyze sources with non-standard accretion flows, such as AGN with gappy disks, low-luminosity active galactic nuclei with intermittent accretion, and SMBH binaries potentially involving intermediate-mass black holes.

astro-ph.HE↗

arXiv2023

Wavelength-resolved reverberation mapping of intermediate redshift quasars HE 0413-4031 and HE 0435-4312: Dissecting Mg II, optical Fe II, and UV Fe II emission regions

We present the wavelength-resolved reverberation mapping (RM) of combined MgII and UV FeII broad-line emissions for two intermediate redshifts (z$\sim$1), luminous quasars - HE 0413-4031 and HE 0435-4312, monitored by the SALT and 1-m class telescopes between 2012-2022. Through this technique, we aim to disentangle the Mg II and FeII emission regions and to build a radius-luminosity relation for UV FeII emission, which has so far remained unconstrained. Several methodologies have been applied to constrain the time delays for total MgII and FeII emissions. In addition, this technique is performed to quantify the inflow or outflow of broad-line region gas around the supermassive black hole and to disentangle the emission/emitting regions from lines produced in proximity to each other. The mean total FeII time delay is nearly equal to the mean total Mg II time delay for HE 0435-4312 suggesting a co-spatiality of their emissions. However, in HE 0413-4031, the mean FeII time delay is found to be longer than the mean MgII time delay, suggesting that FeII is produced at longer distances from the black hole. The UV Fe II R-L relation is updated with these two quasars and compared with the optical FeII relation, which suggests that the optical FeII region is located further than the UV FeII by a factor of 1.7-1.9 i.e. $R_{\rm FeII-opt}\sim(1.7-1.9)R_{\rm FeII-UV}$. We detected a weak pattern in the time delay vs. wavelength relation, suggesting that the MgII broad-line originates a bit closer to the SMBH than the UV FeII, however, the difference is not very significant. Comparison of MgII, UV, and optical FeII R-L relations suggests that the difference may be larger for lower-luminosity sources, possibly with the MgII emission originating further from the SMBH. In the future, more RM data will be acquired to put better constraints on these trends, in particular the UV FeII R-L relation.

astro-ph.GA↗

arXiv2020

Interaction potentials and ultracold scattering cross sections for the $^7$Li$^+$-$^7$Li ion-atom system

We calculate the isotope independent Li$^+$-Li potential energy curves for the electronic ground and first excited states. Scattering phase shifts and total scattering cross section for the $^7$Li$^+$-$^7$Li collision are calculated with emphasis on the ultra-low energy domain down to the $s$-wave regime. The effect of physically motivated alterations on the calculated potential energy curves is used to determine the bound of accuracy of the low-energy scattering parameters for the ion-atom system. It is found that the scattering length for the A$^2Σ_u^+$ state, $a_u$ = 1325 a$_0$, is positive and has well-constrained bounds. For the X$^2Σ_g^+$ state, the scattering length, $a_g$ = 20465 a$_0$ has a large magnitude as it is sensitive to the restrained change of the potential, due to the presence of a vibrational state in the vicinity of the dissociation limit.

physics.atom-ph↗

arXiv2020

On the singular nature of the elastocapillary ridge

The functionality of soft interfaces is crucial to many applications in biology and surface science. Recent studies have used liquid drops to probe the surface mechanics of elastomeric networks. Experiments suggest an intricate surface elasticity, also known as the Shuttleworth effect, where surface tension is not constant but depends on substrate deformation. However, interpretations have remained controversial due to singular elastic deformations, induced exactly at the point where the droplet pulls the network. Here we reveal the nature of the elastocapillary singularity on a hyperelastic substrate with various constitutive relations for the interfacial energy. First, we finely resolve the vicinity of the singularity using goal-adaptive finite element simulations. This confirms the universal validity, also at large elastic deformations, of the previously disputed Neumann's law for the contact angles. Subsequently, we derive exact solutions of nonlinear elasticity that describe the singularity analytically. These solutions are in perfect agreement with numerics, and show that the stretch at the contact line, as previously measured experimentally, consistently points to a strong Shuttleworth effect. Finally, using Noether's theorem we provide a quantitative link between wetting hysteresis and Eshelby-like forces, and thereby offer a complete framework for soft wetting in the presence of the Shuttleworth effect.

cond-mat.soft↗

arXiv2020

Heat radiation reduction in the cryostat with multilayer insulation technique

Multilayer insulation (MLI) is an important technique for the reduction of radiation heat load in cryostats. The present work is focused on investigation for the selection of suitable reflective layer and spacer material in MLI systems. In our analysis, we have selected perforated double-Aluminized Mylar (DAM) with Dacron, unperforated DAM with Silk-net and perforated DAM with Glass-tissue for their evaluation as the reflective layer as well as spacer materials in MLI technique. Current work would discuss the calculation of the effect of layer density and the number of layers on the heat load. Knowing the key parameters of MLI, we have compared the heat load generation in spherical as well as cylindrical cryostats and the effect of layering near and outer surface on the heat load.

physics.ins-det↗

arXiv2019

The relationship between viscoelasticity and elasticity

We consider models for elastic liquids, such as solutions of flexible polymers. They introduce a relaxation time $λ$ into the system, over which stresses relax. We study the kinematics of the problem, and clarify the relationship between Lagrangian and Eulerian descriptions, thereby showing which polymer models correspond to a nonlinear elastic deformation in the limit $λ\rightarrow\infty$. This allows us to split the change in elastic energy into reversible and dissipative parts, and thus to write an equation for the total energy, the sum of kinetic and elastic energies. As an illustration, we show how the presence or absence of an elastic limit determines the fate of an elastic thread during capillary instability, using novel numerical schemes based on our insights into the flow kinematics.

physics.flu-dyn↗

arXiv2019

Characterizing optical variability of OJ 287 in 2016 - 2017

We report on a recent multi-band optical photometric and polarimetric observational campaign of the blazar OJ 287 which was carried out during September 2016 -- December 2017. We employed nine telescopes in Bulgaria, China, Georgia, Japan, Serbia, Spain and the United States. We collected over 1800 photometric image frames in BVRI bands and over 100 polarimetric measurements over ~175 nights. In 11 nights with many quasi-simultaneous multi-band (V, R, I) observations, we did not detect any genuine intraday variability in flux or color. On longer timescales, multiple flaring events were seen. Large changes in color with respect to time and in a color--magnitude diagram were seen, and while only a weak systematic variability trend was noticed in color with respect to time, the color--magnitude diagram shows a bluer-when-brighter trend. Large changes in the degree of polarization, and substantial swings in the polarization angle were detected. The fractional Stokes parameters of the polarization showed a systematic trend with time in the beginning of these observations, followed by chaotic changes and then an apparently systematic variation at the end. These polarization changes coincide with the detection and duration of the source at very high energies as seen by VERITAS. The spectral index shows a systematic variation with time and V-band magnitude. We briefly discuss possible physical mechanisms that could explain the observed flux, color, polarization, and spectral variability.

astro-ph.HE↗

arXiv2018

Characteristic Study and Development of Surface Resistivity Measuring Device for Resistive Plate Chamber Detector

The India-based Neutrino Observatory (INO) is an approved multi-institutional experiment of India. It will use maximum 30,000 Resistive Plate Chamber (RPC) of size 2x2m as an active detector in which each will be sandwiched between two iron plates. Each RPC detectors will be made up of 3mm thick glass (or Bakelite) plates whose one surface will be coated with graphite paint to work as electrode. To achieve effective and uniform electric field, the coated graphite paint must have uniform distribution over the glass surface. Uniformity can be demonstrated by measuring the surface resistance of the coated graphite paint. However, manual measurement of 60,000 electrodes with the help of multimeter and the jig demands huge amount of person-hour with compromised accuracy, and therefore that requires the need and importance of the automatic scanning system. An advanced automatic scanning system (ASS) has been designed & developed to fulfill the required needs and overcome the shortcomings of the initially developed system. Measurement are sensitive to the contact pressure between the jig and RPC electrode's surface. In this paper, various parameters of the ASS have been precisely fixed. Obtained results are compared with the standard one and found satisfactory within error. It has been observed that for obtaining the correct and safe value of surface resistance, applied pressure value in terms of force should be in the range of 9-11 Newton.

physics.ins-det↗

arXiv2017

Cusp-shaped Elastic Creases and Furrows

The surfaces of growing biological tissues, swelling gels, and compressed rubbers do not remain smooth, but frequently exhibit highly localized inward folds. We reveal the morphology of this surface folding in a novel experimental setup, which permits to deform the surface of a soft gel in a controlled fashion. The interface first forms a sharp furrow, whose tip size decreases rapidly with deformation. Above a critical deformation, the furrow bifurcates to an inward folded crease of vanishing tip size. We show experimentally and numerically that both creases and furrows exhibit a universal cusp-shape, whose width scales like $y^{3/2}$ at a distance $y$ from the tip. We provide a similarity theory that captures the singular profiles before and after the self-folding bifurcation, and derive the length of the fold from large deformation elasticity.

cond-mat.soft↗

arXiv2016

Multiband optical variability of the blazar OJ 287 during its outbursts in 2015 -- 2016

We present recent optical photometric observations of the blazar OJ 287 taken during September 2015 -- May 2016. Our intense observations of the blazar started in November 2015 and continued until May 2016 and included detection of the large optical outburst in December 2016 that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of 9 ground based optical telescopes of which one is in Japan, one is in India, three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the USA. These observations were carried out in 102 nights with a total of ~ 1000 image frames in BVRI bands, though the majority were in the R band. We detected a second comparably strong flare in March 2016. In addition, we investigated multi-band flux variations, colour variations, and spectral changes in the blazar on diverse timescales as they are useful in understanding the emission mechanisms. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux, colour and spectral variability.

astro-ph.HE↗

arXiv2016

Inverted Cheerios effect: Liquid drops attract or repel by elasto-capillarity

Solid particles floating at a liquid interface exhibit a long-ranged attraction mediated by surface tension. In the absence of bulk elasticity, this is the dominant lateral interaction of mechanical origin. Here we show that an analogous long-range interaction occurs between adjacent droplets on solid substrates, which crucially relies on a combination of capillarity and bulk elasticity. We experimentally observe the interaction between droplets on soft gels and provide a theoretical framework that quantitatively predicts the migration velocity of the droplets. Remarkably, we find that while on thick substrates the interaction is purely attractive and leads to drop-drop coalescence, for relatively thin substrates a short-range repulsion occurs which prevents the two drops from coming into direct contact. This versatile, new interaction is the liquid-on-solid analogue of the "Cheerios effect". The effect will strongly influence the condensation and coarsening of drop soft polymer films, and has potential implications for colloidal assembly and in mechanobiology.

physics.flu-dyn↗

arXiv2000

Role of intergranular silver in modulating the aperiodicity in disordered Josephson junction arrays : impact on relaxation of intergranular critical state

Relaxation of the intergranular critical state has been observed at a very low applied magnetic field (10-50 Oe) over a temperature regime of 20-77 K in bulk polycrystalline YBa(2)Cu(3)O(7-x) (YBCO) and Bi(1.75)Pb(0.35)Sr(2)Ca(2)Cu(3)O(10+x) (BPSCCO) samples. In such a disordered Josephson junction network, the relaxation is slower than the intragranular relaxation and, hence, it yields higher flux pinning energy U0 than the corresponding intragranular values. Silver addition seems to have given rise to much uniformity in the grain boundary characteristics which results in sharp drop in the flux pinning energy since it depends on the variation of the junction coupling energy EJ across the network. While U0 ~ 0.5 and 0.55 eV for the parent BPSCCO and YBCO samples, respectively, in silver added (10-15 wt%) samples the corresponding values are ~0.15 and 0.27 eV. The distribution functions m(U) for the flux pinning energy and n(theta) for the grain boundary mis-alignment angle(theta) have been evaluated from the experimentally observed patterns of magnetic relaxation and variation of the grain boundary critical fields with temperature. The distribution functions become narrower in the case of silver added samples reflecing a reduction in the degree of disorder. The variation of the effective vortex mass m* with the variation in the degree of disorder is observed. Considering the width of the superconducting transition delTc as the measure of the degree of disorder (inhomogeneity), it has been shown that the transport critical current density Jc follows a relationship Jc exp(-delTc) while the flux pinning energy U ~ delTc. These relations may help in devising a suitable strategy for achieving the desired effect : high Jc yet slower decay rate, i.e., large U.

cond-mat.supr-con↗

arXiv2000

On the factors affecting the high temperature insulator-metal transition in rare-earth manganites

The measurement of resistivity across a wide temperature range - from 15 to 1473 K - in rare-earth manganite series of compounds reveals a very interesting feature : normally observed insulating pattern beyond Tc (Curie Point) gives way to a reentrant metallic pattern around a characteristic temperature T*. The transport activation barrier Ea collapses to zero around T*. T* is found to be dependent on the carrier concentration or the concentration of the Jahn-Teller-active Mn(3+) ions as well as on the average A-site radius for a fixed carrier concentration. These factors govern the effective lattice distortion and hence lead to the variation in the conduction bandwidth. Our data cover a wide range - from T*>>Tc for smaller bandwidth to T* tending towards Tc for larger bandwidth. These results seem to provide evidence of the onset of lattice distortion at high temperature (around T*) and its variation. Since lattice distortion governs the magnetic, transport and other important behaviors significantly, our data assume importance as they offer a new measure of the effective distortion and its tunability.

cond-mat.str-el↗