242 Chapter 4. Liquid Filled Detectors
the concept of indirect detection through Cherenkov light. A neutrino can elastically
scatter off an orbital electron in heavy water and set it free. Since the neutrinos
coming from the sun have high enough energy, they provide the electron with so
much energy that their velocity crosses the threshold of equation 4.6.2. As a result
the electron produces Cherenkov light. The cone of light spreads and is ultimately
detected by the photomultiplier tubes (see Fig.4.6.1).
e +
ν
ν
Detection
Transparent
Array Container
Photodetector
Cone
Cherenkov
Medium
Figure 4.6.1: Sketch of the work-
ing principle of a neutrino detector.
The neutrino is shown to scatter off
an electron from the medium. If
the scattered electron moves with
a velocity higher than the veloc-
ity of light in that medium, it pro-
duces Cherenkov light in the form
of a cone. The light photons thus
produced are detected by an ar-
ray of photomultiplier tubes in-
stalled around the spherical con-
tainer of the detection medium.
Such a detector has been built at
the Sudbury Neutrino Observatory
in Canada.4.7 Bubble Chamber
Bubble chamber is one of the earliest and extremely successful imaging detectors.
It was built for tracking particles in high energy particle collisions.
A conventional bubble chamber is made of a sealed container filled with a lique-
fied gas. The chamber is designed such that pressure inside can be quickly changed.
The idea is to momentarily superheat the fluid when the particles are expected to
pass through it. This is accomplished by suddenly lowering the pressure, which de-
creases the boiling point of the liquefied gas, thus converting it into a superheated
liquid. When particles pass through this fluid they produce dense tracks of localized
electron ion pairs. The energy delivered to the liquid during this process produces
tiny bubbles along the particle’s track. The whole chamber is then illuminated and
photographed by a high definition camera. The photograph is then analyzed offline
for particle identification and measurements. Bubble chambers were highly success-
ful in the early days of high energy physics research, where application of external
magnetic field allowed measurements of particle momenta and thus facilitated par-
ticle identification. Fig.4.7.1 shows a typical photograph obtained from a bubble
chamber.
The obvious disadvantage of bubble chambers is that it is extremely difficult
to use it for online analysis and triggering. The bubble chambers have now been
replaced with other types of electronic trackers, most of which are based on sili-