Warm Dark Matter Simulation with thermal (left) and non-thermal (right) velocity. Large structures are formed prior to the small structures if we invoke warm dark matter. For cold dark matter, the process is vice-versa. Watch the simulation here.

 
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Study of Si-32 background for CDMS II, SuperCDMS Collaboration
[ Jan. 1, 2018 - July 31, 2019 ]

Supervisor : Prof. Bedangadas Mohanty

National Institute of Science Education and Research (NISER),

Bhubaneswar

CDMS is a direct dark matter search experiment aiming to measure the nuclear recoil energy for a  dark matter-nucleus elastic scattering. This scattering produces charge pairs and phonons (lattice vibration) in the crystal. CDMS measures this charge and phonon signals to get the nuclear recoil energy. The above picture is the detector of SuperCDMS at Soudan, USA. The 'grooved' like lines in the top surface are basically 4 phonon channels as well as charge channels. Our detector measures charge and phonon signals. The next generation of CDMS experiment called SuperCDMS SNOLAB will be focussed for low mass WIMPs ( < 10 GeV ). This needs a careful study of the low energy background. Si -32 will be the dominant background in SuperCDMS SNOLAB. This radioactive isotope is present right from the detector fabrication. It undergoes beta decay to produce P-32 with an end-point energy 227.2 keV. P-32 again emits beta and produce stable S-32. These beta particles can create charge signals as background. Since CDMS II used Si detectors, we are trying to estimate this estimation of Si-32 decay using CDMS II data. Not only this analysis will be relevant for SuperCDMS SNOLAB, but also other Si-based dark matter experiments like DAMIC, SENSEI etc. This long-term project is still ongoing.

Neutron detector for dark matter search experiments
[ Jan. 1, 2018 - July 31, 2019 ]

Supervisor : Prof. Bedangadas Mohanty , Dr. V. Kashyap

National Institute of Science Education and Research (NISER),

Bhubaneswar

Neutron mimics WIMPs signal as they also interact with detector nuclei producing charge and phonon signals. Understanding the neutron spectrum in mK temperature is thus essential. This leads to revamp our neutron veto. In this project, I'm trying to understand how one can separate a neutron and gamma signal as gamma is also a  major background in dark matter search experiment. One can use pulse shape discrimination (PSD) method to discriminate these two pulses. I've used the charge integration method to determine PSD using digitizer in lieu of conventional electronics. Along with this, I've simulated our detector using GEANT4. Deposited energy spectrum for known sources is used to calibrate the detector. After that, using these experimental energy spectra and the simulated energy spectra for those sources, we can determine our detector resolution and the response matrix so on. The motivation of this study is to make a neutron veto for dark matter search experiments.



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Exploration of modified Poisson's equation and it's possible physical application

[ May 12 - June 5, 2017 ]

(Thanks Google for this nice cartoon!)


Supervisor: Prof. Banibrata Mukhopadhyay

Indian Institute of Science (IISc), Bangalore

In this theoretical project, we were trying to see the solutions of Poisson's equation after taking into some modifications. The left side of Poisson's equation has the information of underlying space whereas the right-hand side contains the information of matter. Hence, one can extract the space-time information from Poisson's equation. We modified the space part and obtained the modified Lane-Emden equation for a spherically symmetric star having hydro-dynamically equilibrium by invoking stellar structure equations. Original Lane-Emden equation leads to Chandrasekhar mass limit for white dwarfs in relativistic case. The top plot is nothing but the dimensionless density vs. dimensionless radius of a white dwarf in the Lane-Emden equation. We've got the dimensionless Chandrasekhar radius (see the figure) 6.901 which leads to mass limit 1.436 of the mass of the sun. By modifying the space part (left side) of the Poisson's equation, we were trying to see the modified Chandrasekhar mass limit from the classical regime. The above results aren't included in the report as we're still on the prepping of the solutions. Supplemental part can be found here.

Characterization and testing of Multi Wire Proportional Chamber (MWPC)
[ Dec. 13 - 30, 2016 ]

Supervisor : Dr. Tilak Kumar Ghosh

Variable Energy Cyclotron Centre (VECC) , Kolkata

In my M.Sc. first year, I'd an opportunity to do a short-term winter project on the gas detectors in the CPDA lab, VECC. They had made a Multi-Wire Proportional Chamber (MWPC). Basically, that was a little bit modification of MWPC called Breskin detector, which has two position sensing plates like a grid, unlike normal MWPC which has one position detection plates. The advantage of this type of detector is that one can measure the particle trajectory in more accurate by using the information of particle's positions (x,y coordinates) and time (delay line chip was used for this). In this project, the first time I was introduced various types of detectors and the interaction of charged particles with matters. In the first part of my project, I had learned about the construction and detection principle of MWPC. We'd used Cf-252 as a source to test our MWPC. We got the energy spectrum as well as measured the noise. We optimized the noise upto ~ 50 mV for anode while < 100 mV for X,Y plates. We found that the position sensing plates were working well. Also, we applied symmetric and asymmetric bias voltage across the electrodes and found that asymmetric bias leads to better response under very low pressure (~3.2 torr).

''Masters' Thesis''

Study of Si-32 background for CDMS II and

neutron detector for dark matter search experiments

The nature of dark matter is an open question in the area of astroparticle physics. The Super Cryogenic Dark Matter Search (SuperCDMS) is one of the leading direct low-mass dark matter search experiments in the world. The CDMS experiment looks for elastic scattering of dark matter with the detector material. These detectors measure charge and phonon signals from the interaction of dark matter with the detectors. Due to very low dark matter-nucleon interaction cross-section and the nuclear recoil energy (∼ few keV), identifying dark matter detection becomes very challenging. To detect this rare and weak signal, a rigorous understanding of backgrounds is essential. Si-32 exists in the detectors right from the time of its fabrication. Si-32 is an isotope that shows beta decay, which will be the dominant source of background in the future based experiment that employs silicon detectors. The first part of this thesis describes the analysis procedures to estimate Si-32 background activity.

The second part of this thesis is focussed on the study of the neutron detector as a neutron is another major background in dark matter search experiments. A liquid organic scintillator detector is used for neutron studies at NISER. To characterize the detector, various gamma sources are used along with Am-Be neutron source. To obtain the unfolded neutron spectrum in neutron recoil energy scale, a simulation study is needed due to unavailability of the monoenergetic neutron source. GEANT4 toolkit has been used for this purpose. Gaussian broadening is used to determine the detector resolution, which can be used further to unfold the neutron spectrum in the future.

 
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©2020 by Rik Bhattacharyya, Texas A&M University, USA.