S. Mathimalar

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.

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.

Characterization of Neutron Transmutation Doped (NTD) Ge for low temperature sensor development

Abstract Development of NTD Ge sensors has been initiated for low temperature (mK) thermometry in The India-based TIN detector (TIN.TIN). NTD Ge sensors are prepared by thermal neutron irradiation of device grade Ge samples at Dhruva reactor, BARC, Mumbai. Detailed measurements have been carried out in irradiated samples for estimating the carrier concentration and fast neutron induced defects. The Positron Annihilation Lifetime Spectroscopy (PALS) measurements indicated monovacancy type defects for all irradiated samples, while Channeling studies employing RBS with 2xa0MeV alpha particles, revealed no significant defects in the samples exposed to fast neutron fluence of ∼ 4 × 10 16 / cm 2 . Both PALS and Channeling studies have shown that vacuum annealing at 600xa0°C for ∼ 2 xa0h is sufficient to recover the damage in the irradiated samples, thereby making them suitable for the sensor development.

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

The neutron flux at low energy (En ≤ 15 MeV) resulting from the radioactivity of the rock in the underground cavern of the India-based Neutrino Observatory is estimated using Geant4-based Monte Carlo simulations. The neutron production rate due to the spontaneous fission of 235, 238U, 232Th and (α, n) interactions in the rock is determined employing the actual rock composition. It is shown that the total flux is equivalent to a finite size cylindrical rock (D=L=140 cm) element. The energy integrated neutron flux thus obtained at the center of the underground tunnel is 2.76 (0.47) × 10−6 n cm−2 s−1. The estimated neutron flux is of the same order (~10−6 n cm−2 s−1) as measured in other underground laboratories.

Study of neutron-induced background and its effect on the search of 0νββ decay in 124Sn

Neutron-induced background has been studied in various components of the TIN.TIN detector, which is under development for the search of Neutrinoless Double Beta Decay in 124Sn. Fast neutron flux ~106 n cm-2s-1 covering a broad energy range ~0.1 to ( ~18 MeV) was generated using 9Be(p,n)9B reaction. In addition, reactions with quasi-monoenergetic neutrons were also studied using 7Li(p,n)7Be reaction. Among the different cryogenic support structures studied, Teflon is found to be preferable compared to Torlon as there is no high energy gamma background Eγ > 1 MeV) . Contribution of neutron-induced reactions in nat, 124Sn from other Sn isotopes (A = 112–122) in the energy region of interest, namely, around the Qββ of 124Sn (E ~ 2.293 MeV), is also investigated.