Question

12. If the speed of sound in helium at 0°C be 960 m s ,
then what will be the speed of sound in hydrogen at
the same temperature ? The values of y for He and H2
are respectively 1.67 and 1.40 and the ratio of their
molecular masses is 2:1. Ans. 1243 m s.​

Answer 1

Answer:

There is strong evidence from astrophysical and cosmological observations, that

only 5 % of the Universe is made out of ordinary matter and radiation, while

about 27 % is in a form of cold, non-luminous matter, whose intrinsic nature is

still unknown. The other 68 % in form of dark energy, that is responsible for the

accelerated expansion of the Universe.

Since dark matter is predicted to interact only very weakly with ordinary matter,

its presence is inferred by the gravitational effects on large scale objects such as

galaxies, clusters and super-clusters of galaxies, and it played a fundamental role in

the formation of these structures. Theories beyond the Standard Model of particle

physics predict that dark matter is in form of a new, stable or long-lived particle,

produced in the hot Big Bang. Their unknown nature motivates the strong existing

experimental efforts to detect these new kind of particles and eventually to study

their properties.

Several detectors, to directly observe the scattering of such particles with atomic

nuclei, have been built and operated during the last decades, and no conclusive evidence of dark matter detection has been reported. The new generation XENON1T

detector has been installed at the Laboratori Nazionali del Gran Sasso (LNGS) in

Italy at the end of 2015, and is operating since the start of 2016. After an initial

commissioning phase it started the first scientific run in November 2016, and with

∼ 34 days of life-time, it placed the strongest exclusion upper limit on the interaction of weakly massive particles (WIMPs) with ordinary matter. In the next two

years of data acquisition it is expected to improve the sensitivity to WIMP-nucleon

cross section to 1.6 × 10−47 cm2

, about 2 orders of magnitude lower compared to

its predecessor XENON100.

The work that lead to this thesis comprises three main studies: the radioassay

campaign of XENON1T, the study of the cosmogenic production of radionuclides

in natural xenon by atmospheric cosmic rays, and the development of a small

liquid xenon time-projection chamber for study of the liquid xenon response to

neutron-induced nuclear recoils with energies below 10 keV.

The radioactive contamination of the detector construction materials is one of

the most dangerous background sources that might compromise the final detection

sensitivity of the experiment. For this reason, a radioassay campaign for the

selection of detector components was performed with the low-background, highiii

iv

purity germanium (HP-Ge) γ-ray spectrometry technique, which allowed to detect

radioactive contaminations in materials down to 10−17 g/g of 238U and 10−10 g/g

of 232Th. Within the radioassay campaign the new highly radio-pure Hamamatsu

R11410-11 photomultipliers, employed as the photosensors in XENON1T, have

been developed, resulting in the light sensor with highest radio-purity per sensitive

area available to date. The prediction of the background based on the radioassay

measurements are discussed, showing that γ-ray background from the selected

materials is expected to contribute less than 5 % of the total background, and

the background from radiogenic neutrons is estimated to be ∼ 1.2 events/y. It

is concluded that the experiment will be able to probe WIMP interactions with

nucleons down to a cross section of ∼ 1.6 × 10−47 cm2

for a WIMP mass of ∼

50 GeV/c

2 with an exposure of ∼ 2 t y.

The first dedicated experimental study of the cosmic ray induced activation of

a natural xenon sample, together with the activation of an oxygen-free high conductivity copper sample for benchmark control, is presented. The results achieved

for both samples are compared with the predictions performed with the available

codes, Activia and Cosmo, and in the case of copper also with the only existing

experimental study of cosmogenic activation found in the literature. It was found

that while for the copper the calculations predict values in substantial agreement

with the measurement, in the case of xenon the predictions strongly underestimate

the production yields of many observed cosmogenic radionuclei. This is most likely

due to the poor knowledge of the cross sections of the nuclear processes for the