15.3 Lasers, Nuclear Reactors, Radiation Detectors, Dosimetry & Elementary Particles

Lasers, Nuclear Reactors, Radiation Detectors, Dosimetry & Elementary Particles

Section 15.2 covered photons, atomic spectra, X-rays, radioactive decay, fission and fusion. This section completes the atomic and nuclear chapters with lasers, the practical side of nuclear physics, and the modern picture of what matter is made of.

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1. Lasers

LASER stands for Light Amplification by Stimulated Emission of Radiation.

Three Ways Atoms Interact with Light

Stimulated emission amplifies light, but normally most atoms are in the lower level, so absorption wins. A laser therefore needs:

1. Population inversion: more atoms in the upper level than in the lower level, produced by pumping energy in (electrical discharge, flash lamp or another laser).
2. A metastable state: an excited level with a long lifetime (around $10^{-3}\text{ s}$, compared with about $10^{-8}\text{ s}$ for ordinary excited states), so atoms accumulate there.
3. An optical resonator: two parallel mirrors, one fully reflecting and one partially transmitting, so photons pass back and forth through the medium many times and the beam builds up.

The Helium–Neon Laser

A glass tube contains a mixture of about 85% helium and 15% neon. An electrical discharge excites helium atoms to a metastable level whose energy almost matches an excited level of neon. Collisions transfer the energy to neon atoms, creating a population inversion in neon, which then lases at 632.8 nm (red light).

Properties and Uses of Laser Light

• Monochromatic (a very narrow band of wavelengths), coherent (constant phase relation), directional (very little spreading) and intense.
• Uses: eye surgery (reattaching retinas), cutting and welding, barcode scanners, optical-fibre communication, measuring distances, holography (three-dimensional images recorded by interference), and destroying kidney stones and tumours.

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2. Interaction of Nuclear Radiation with Matter

Radiation loses energy mainly by ionising the atoms it passes.

Gamma rays are not "stopped" at a fixed range; their intensity falls roughly exponentially with thickness, so shielding is described by the half-value thickness.

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3. Radiation Detectors

A counter always records a small background count from cosmic rays and natural radioactivity, which must be subtracted from measurements.

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4. Nuclear Reactors

Each fission of uranium-235 by a slow neutron releases about 200 MeV and two or three neutrons (Section 15.2). If at least one of these neutrons causes another fission, a chain reaction continues. The minimum mass needed for a self-sustaining chain reaction is the critical mass.

The heat produces steam that drives turbines and generators, as in a conventional power station. In a fast breeder reactor, uranium-238 absorbs neutrons and is converted into fissile plutonium-239, producing more fuel than it consumes.

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5. Radiation Dose and Biological Effects

The weighting factor is about 1 for beta and gamma radiation and about 20 for alpha particles, so 1 mGy of alpha radiation inside the body is roughly as harmful as 20 mGy of gamma radiation.

• Somatic effects damage the exposed person (burns, radiation sickness, cancer, cataracts).
• Genetic effects damage reproductive cells and can be passed to future generations.
• Typical natural background dose is a few millisieverts per year. Protection relies on time (minimise exposure), distance (intensity falls with the square of distance from a point source) and shielding.

Uses of Radioisotopes

• Medical tracers: iodine-131 to study the thyroid; technetium-99m for imaging.
• Radiotherapy: gamma rays from cobalt-60 to destroy cancer cells.
• Carbon dating: carbon-14 (half-life about 5730 years) dates once-living material up to tens of thousands of years old.
• Industry and agriculture: thickness gauging, leak detection in pipes, sterilising medical equipment, and tracing fertiliser uptake by plants.

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6. Basic Forces and Building Blocks of Matter

Particles are grouped as:

• Hadrons, which feel the strong force and are built from quarks: baryons (three quarks, such as protons and neutrons) and mesons (a quark and an antiquark, such as pions).
• Leptons, which do not feel the strong force and are not made of quarks: the electron, muon, tau and their neutrinos.

A proton is $uud$: charge $\frac{2}{3} + \frac{2}{3} - \frac{1}{3} = +1$. A neutron is $udd$: charge $\frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0$. In beta-minus decay a down quark changes into an up quark, turning a neutron into a proton and emitting an electron and an antineutrino.