Subatomic Physics

4.10. Calorimeters 69
Figure 4.18: Example of the use of ˘ Cerenkov detectors: the SNO detector consited of 1000 Tons of
heavy water and 9456 photomultiplier tubes in a spherical array. Left: artist’s view of the detector,
which was located deep underground. Right: the appearance of a neutrino event is shown. Each
dot indicates a photomultiplier tube that detected ˘ Cerenkov light. [Courtesy J. Wilkerson]
The number of photons emitted per unit path length and energy is (11)
d 2 N
dxdE
�
αz2
= 1 −
�c
c2
v2n2 �
, (4.13)
where ze is the charge of the particle and α is the fine structure constant α =
e 2 /(�c) =1/137.
A recent spectacular application of ˘ Cerenkov detectors has been their use to
detect neutrinos. In a typical situation a neutrino scatters from electrons in water
and the electrons generate ˘ Cerenkov ‘rings’ that are detected with photomultiplier
tubes. Figure 4.18 shows an example of a neutrino event from the SNO detector.
4.10 Calorimeters
Modern high energy accelerators and heavy ion accelerators both produce a multitude
of events per collision. Calorimeters are used to measure the energies of
particles by stopping them and thus having them deposit all of their energy inside
the detector. Thus a large mass detector is required. There are two types of
calorimeters, namely those for electrons and those for hadrons.
As discussed in Section 3.4, high energy electrons slow down primarily by bremsstrahlung,
with the photons then producing electron-positron pairs; these pairs
produce further photons and the process results in a “shower” of e + e − pairs. In
one radiation length, X0, the inital energy E0 leads to two particles of energy E0/2.
After n iterations or in a distance of nX0, there will be 2 n particles with an average
11 See Jackson, Sect. 13.4.