Physics and Engineering of Radiation Detection

4.6. Cherenkov Detectors 241

and the refractive indexnof the medium through the relation

cosθc =

c

vn

=

1

βn

, (4.6.1)

whereβ=v/cis the ratio of the velocity of the particlein the mediumto the

velocity of lightin vacuum.


The threshold velocity of the particle to produce Cherenkov light as obtained

from the above relation is

vth=

c

n

. (4.6.2)

The angle of the cone is proportional to the energy of the particle producing
the radiation.
The last three points above are very important in terms of building a detector
based on Cherenkov emission. The unique conic geometry of the emitted radiation
can be used not only to tag events (that is, differentiate them from the background)
but also to estimate the energy of the particle producing the radiation.
A liquid filled Cherenkov detector uses a liquid to produce Cherenkov light either
directly by the particle to be detected or by another particle interacting with it.
The light thus produced is detected by detectors surrounding the liquid. Though
any type of detector can be used to detect light photons but the general practice is
to use photomultiplier tubes due to their high photon detection efficiency. We will
learn more about these tubes in the chapter on photo detectors.

Example:

Calculate the critical angle of Cherenkov cone produced by the passage of

electrons through light water. Assume the average velocity of the electrons to

be 0. 92 c,wherecis the velocity of light in vacuum.

Solution:

Substitutingβ=0.92 andn=1.33 (for light water) in equation 4.6.1, we get

cosθc =

1

βn

=

1

(0.92)(1.33)

=0. 817

⇒θc =35. 20.

Fig.4.6.1 shows a Cherenkov detector consisting of a spherical ball containing a
liquid and photomultiplier tubes surrounding it. Such a detector has been built for
detecting neutrinos at the Sudbury Neutrino Observatory (SNO) in Canada. The
neutrinos, as we saw in chapter 1, have extremely low interaction cross section and
therefore detecting them is quite a challenging task. The SNO detector uses heavy
water to detect these elusive particles. There are three modes of interaction of
neutrinos that are exploited at this facility. We will look at one of those to explain