  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 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, $^{110}$Ag and
$^{182}$Ta impurities, which can be reduced by chemical etching of
approximately $\sim50 \mu$m 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 $\sim$ 1 Bq/gm after
irradiation. These estimates indicate that in order to use the NTD Ge sensors
for rare event studies, a cool down period of $\sim$ 2 years would be necessary
to reduce the radioactive background to $\le$ 1 mBq/gm.
