V. Nanal

Highly efficient and electrically robust carbon irradiated semi-insulating GaAs based photoconductive terahertz emitters

We demonstrate here an efficient photoconductive THz source with low electrical power consumption. We have increased the maximum THz radiation power emitted from SI-GaAs based photoconductive emitters (PCEs) by two orders of magnitude. By irradiating the SI-GaAs substrate with Carbon-ions up to 2 μm deep, we have created lot of defects and decreased the lifetime of photo-excited carriers inside the substrate. Depending on the irradiation dose, we find 1 to 2 orders of magnitude decrease in total current flowing in the substrate, resulting in subsequent decrease of heat dissipation in the device. This has resulted in increasing maximum cut-off of the applied voltage across PCE electrodes to operate the device without thermal breakdown from ∼35 V to >150 V for the 25 μm electrode gaps. At optimum operating conditions, carbon irradiated (1014 ions/cm2) PCEs give THz pulses with power about 100 times higher in comparison to the usual PCEs on SI-GaAs and electrical to THz power conversion efficiency has improv...

Characterization and modeling of a low background HPGe detector

Abstract A high efficiency, low background counting setup has been made at TIFR consisting of a special HPGe detector ( ~ 70 % ) surrounded by a low activity copper+lead shield. Detailed measurements are performed with point and extended geometry sources to obtain a complete response of the detector. An effective model of the detector has been made with GEANT4 based Monte Carlo simulations which agrees with experimental data within 5%. This setup will be used for qualification and selection of radio-pure materials to be used in a cryogenic bolometer for the study of Neutrinoless Double Beta Decay in 124Sn as well as for other rare event studies. Using this setup, radio-impurities in the rock sample from India-based Neutrino Observatory (INO) site have been estimated.

Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection

We report here a photoconductive material for THz detection with sub-picosecond carrier lifetime made by C(12) (Carbon) irradiation on commercially available semi-insulating (SI) GaAs. We are able to reduce the carrier lifetime of SI-GaAs down to sub-picosecond by irradiating it with various irradiation dosages of Carbon (C(12)) ions. With an increase of the irradiation dose from ~10(12) /cm(2) to ~10(15) /cm(2) the carrier lifetime of SI-GaAs monotonously decreases to 0.55 picosecond, whereas that of usual non-irradiated SI-GaAs is ~70 picosecond. This decreased carrier lifetime has resulted in a strong improvement in THz pulse detection compared with normal SI-GaAs. Improvement in signal to noise ratio as well as in detection bandwidth is observed. Carbon irradiated SI-GaAs appears to be an economical alternative to low temperature grown GaAs for fabrication of THz devices.

Study of radioactive impurities in neutron transmutation doped Germanium

A program to develop low temperature (mK) sensors with neutron transmutation doped Ge for rare event studies with a cryogenic bolometer has been initiated. For this purpose, semiconductor grade Ge wafers are irradiated with thermal neutron flux from Dhruva reactor at Bhabha Atomic Research Centre (BARC), Mumbai. Spectroscopic studies of irradiated samples have revealed that the environment of the capsule used for irradiating the sample leads to significant levels of 65 Zn, 110m Ag and 182 Ta impurities, which can be reduced by chemical etching of approximately � 50 µm thick surface layer. From measurements of the etched samples in the low background counting setup, activity due to trace impurities of 123 Sb in bulk Ge is estimated to be � 1 Bq/g after irradiation. These estimates indicate that in order to use the NTD Ge sensors for rare event studies, a cooldown period of � 2 years would be necessary to reduce the radioactive background to � 1 mBq/g.

Development of NTD Ge sensors for low temperature thermometry

The development of NTD Ge sensors for use in cryogenic bolometric detector to search for neutrinoless double beta decay (Ovββ) in 124Sn is reported. The samples made from device grade Ge wafers are irradiated with thermal neutrons at Dhruva reactor, Bhabha Atomic Research Centre (BARC), Mumbai. The carrier concentration in irradiated Ge samples is estimated by Hall effect measurement at 77K. The fast neutron induced defects are studied using Positron Annihilation Lifetime Spectroscopy and Channeling. It is found that vacuum annealing of the samples at 600°C for 2 hours is necessary to cure the defects. Sensors are made from annealed NTD samples using Au-Ge Ohmic contact. Preliminary measurements have shown a significantly large dR/dT ~ 2.3 kΩ/mK at 100 mK. Details of these measurements are presented.

Continuous wave terahertz radiation from antennas fabricated on C 12 -irradiated semi-insulating GaAs

We demonstrate continuous wave (CW) terahertz generation from antennas fabricated on C12-irradiated semi-insulating (SI) GaAs substrates. The dark current drawn by the antennas fabricated on irradiated substrates is ∼3 to 4 orders of magnitude lower compared to antennas fabricated on un-irradiated substrates, while the photocurrents decrease by only ∼1.5 orders of magnitude. This can be attributed to the strong reduction of the carrier lifetime that is 2.5 orders of magnitude, with values around τ(rec)=0.2  ps. Reduced thermal heating allows for higher bias voltages to the irradiated antenna devices resulting in higher CW terahertz power, just slightly lower than that of low-temperature grown GaAs (LT GaAs)at similar excitation conditions.

Development of Cryogenic Bolometer for 0νββ in 124Sn

Cryogenic bolometer detectors, with their high resolution spectroscopy capability, are ideal for neutrino mass experiments as well as for search of rare processes like neutrinoless double beta decay (0νββ) and dark matter. A feasibility study for investigation of 0νββ in 124Sn at the upcoming underground facility of India based Neutrino Observatory (INO) has been initiated. This paper describes endeavors towards cryogenic tin bolometer development.

A versatile PC based control system for channeling experiments

Abstract A PC based control system for aligning the low index directions of a single crystal target with that of a charged particle beam from an accelerator has been designed. Two stepper motors are employed to control the tilt (θ) and rotation (φ) of the target mounted on a double axis high precision goniometer. A user friendly software using BASIC language is developed for controlling the motors and acquiring data with two detectors simultaneously. Many additional features are also included in the design of the system.

New limit for the half-life of double beta decay of 94Zr to the first excited state of 94Mo

Abstract.Neutrinoless double beta decay is a phenomenon of fundamental interest in particle physics. The decay rates of double beta decay transitions to the excited states can provide input for Nuclear Transition Matrix Element calculations for the relevant two neutrino double beta decay process. It can be useful as supplementary information for the calculation of Nuclear Transition Matrix Element for the neutrinoless double beta decay process. In the present work, double beta decay of 94Zr to the

Estimation of low energy neutron flux (En ⩽ 15 MeV) in India-based Neutrino Observatory cavern using Monte Carlo techniques

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