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LIGHT and SPECIAL
RELATIVITY MATTER AND ANTIMATTER |
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SUMMARY Every
type of particle has an associated antiparticle with the same mass but with
opposite electrical charge. For example, the antiparticle of the electron is the positron. Some
particles, such as the photon, are their own antiparticle. Otherwise, for
each pair of antiparticle partners, one is designated as normal matter
(most objects in the universe) and the other as antimatter. Particle–antiparticle
pairs can annihilate
each other, producing photons. Since the charges of the particle and
antiparticle are opposite, total charge is conserved. For example, the
positrons produced in natural radioactive decay quickly annihilate themselves
with electrons, producing pairs of gamma rays, a process exploited in positron emission
tomography. Pair production occurs when an electron - positron pair is created when a gamma
ray interacts with a nucleus when traveling through matter. Pair production
is a direct conversion of electromagnetic energy to matter (energy |
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Particle – Antiparticle Interactions A
particularly interesting feature of the equivalence of mass and energy is the
existence of antimatter. For each charged particle that exists in nature,
there is its corresponding antiparticle which has the same mass but opposite
charge. For example, the antiparticle for an electron
(e-) is a positron (e+). Antimatter
is frequently created in particle accelerators, where particles collide at
speeds approaching the speed of light. Electron –
positron annihilation When
an electron (e-) and a positron (e+) meet, they
annihilate converting their mass into energy in the form of gamma rays (g) e- + e+ ® g
+ g energy
of gamma rays = 2me c2 = 1.02 MeV Momentum,
energy and charge are conserved in the reaction. In matter-antimatter
annihilation, the particles vanish in a burst of radiation.
Fig. 1. The particle / antiparticle annihilate each other generating two high
energy gamma ray photons. Charge, energy and mometum are all conserved in the
annihilation process. The kinetic energy and the rest energy of the electron
and positron are converted into the energy of the zero mass photons. Electron-positron annihilation is the basis
for the diagnostic technique called positron-electron tomography (PET). PET
scanning is used to examine biological processes in the body. In a PET scan
of the brain, a patient is injected with glucose that has been tagged with
radioactive traces. These radioactive traces emit positrons in a nuclear
reaction. These positrons collide with electrons in the brain and undergo
annihilation. The two gamma rays emitted are detected by the instrumentation
surrounding the patient. The recordings are analysed by a computer to produce
coloured images showing the glucose metabolism levels within the brain.
Fig. 2. PET scanning. The powerful diagnostic tools of PET
scanning rely on the annihilation of matter and antimatter in a
person’s brain. Pair Production A
1.02 MeV gamma ray can produce an electron - positron pair in a process known
as pair
production. g ® e- + e+
Fig. 3. Cloud chamber tracks: Pair production.
From the
charged particle tracks, what can you conclude about the magnetic field of
the cloud chamber? For
pair production to occur, the electromagnetic energy, in a discrete quantity
called a photon,
must be at least equivalent to the mass of electrons plus mass of positron.
The mass m of
a single electron is equivalent to 0.51 MeV (E0 = m c2). Hence, to produce electron – positron pair, the photon
energy must be at least 1.02 MeV. Photon energy greater 1.02 MeV is converted
into the kinetic energy of the electron-positron pair. If pair production
occurs in a track detector (cloud chamber figure 3) to which a magnetic field
is properly applied, the electron and the positron curve away from the point
of formation in opposite directions in arcs of equal curvature. In this way
pair production was first detected (1933). The positron that is formed
quickly disappears by reconversion into photons in the process of
annihilation with another electron in matter. |
