Elementary Particles
387
Emulsion Chamber
Charged particles interact with photographic emulsions in the same way as
photons, and hence the emulsions can be used for recording the tracks of charged
particles. Photographic emulsions such as silver bromide (which has a density
of about 4 g/cc) is an efficient stopper of high-energy ionizing particles. Several
hundreds of layers of these emulsion sheets, each about 1/2 mm thick, may be
exposed to cosmic rays or to high energy particles from accelerators. After the
development of the sheets, the path of the particles can be traced from layer to
layer.
Geiger Counter
This is a very compact and sturdy instrument used for detecting energetic
particles. It consists of a metal tube with a wire along its axis. It is filled with a
suitable gas mixture at low pressure. The tube is insulated from the wire and a
high potential difference is maintained between the tube and the wire. When an
energetic particle enters the tube, it produces some ions in the gas. This causes
a small discharge current to flow between the wire and the tube. Recording of
these signals allows us to count the number of incident particles. Since the ions
and the electrons take a time of about 10
–3
s to recombine, the counter can
record only a few hundred counts per second. Its main advantage is that it is
very inexpensive and easy to operate.
10.7 EXAMPLES
A few examples are discussed in this section.
Example 1
It is interesting to note that several particles in the SU (3) multiplet scheme,
such as the η-meson, were discovered only after the scheme predicted the
existence of these particles. Particularly noteworthy is the Ω
–
which is a member
of the baryon decuplet. Gell-Mann predicted it to have spin of 3/2, strangeness
–3, and a mass of about 1676 MeV. Since there is no baryonic system with
strangeness –3, and which is lighter than 1676 MeV, the Ω
–
is stable against
decay through strong interaction (which conserves strangeness). The Ω
–
can
decay via weak interaction (which does not conserve strangeness) but would
have a lifetime of about 10
–10
s characteristic of weak decays. A particle with
precisely these characteristics, with a mass of 1675 MeV was observed in 1964,
through its weak decay
Ω
–
→ Ξ
–
+ π
0
(10.46)
which does not conserve strangeness.
387
Emulsion Chamber
Charged particles interact with photographic emulsions in the same way as
photons, and hence the emulsions can be used for recording the tracks of charged
particles. Photographic emulsions such as silver bromide (which has a density
of about 4 g/cc) is an efficient stopper of high-energy ionizing particles. Several
hundreds of layers of these emulsion sheets, each about 1/2 mm thick, may be
exposed to cosmic rays or to high energy particles from accelerators. After the
development of the sheets, the path of the particles can be traced from layer to
layer.
Geiger Counter
This is a very compact and sturdy instrument used for detecting energetic
particles. It consists of a metal tube with a wire along its axis. It is filled with a
suitable gas mixture at low pressure. The tube is insulated from the wire and a
high potential difference is maintained between the tube and the wire. When an
energetic particle enters the tube, it produces some ions in the gas. This causes
a small discharge current to flow between the wire and the tube. Recording of
these signals allows us to count the number of incident particles. Since the ions
and the electrons take a time of about 10
–3
s to recombine, the counter can
record only a few hundred counts per second. Its main advantage is that it is
very inexpensive and easy to operate.
10.7 EXAMPLES
A few examples are discussed in this section.
Example 1
It is interesting to note that several particles in the SU (3) multiplet scheme,
such as the η-meson, were discovered only after the scheme predicted the
existence of these particles. Particularly noteworthy is the Ω
–
which is a member
of the baryon decuplet. Gell-Mann predicted it to have spin of 3/2, strangeness
–3, and a mass of about 1676 MeV. Since there is no baryonic system with
strangeness –3, and which is lighter than 1676 MeV, the Ω
–
is stable against
decay through strong interaction (which conserves strangeness). The Ω
–
can
decay via weak interaction (which does not conserve strangeness) but would
have a lifetime of about 10
–10
s characteristic of weak decays. A particle with
precisely these characteristics, with a mass of 1675 MeV was observed in 1964,
through its weak decay
Ω
–
→ Ξ
–
+ π
0
(10.46)
which does not conserve strangeness.