Ionizing radiation and thermonuclear fusion

Radiation is invisible but leaves traces in matter. Nuclei can split or combine, and a particle–antiparticle pair can turn into new particles. In each case, we will follow where energy goes and what changes.

When radiation gives an atom enough energy, an electron may become free and the atom becomes an ion: this is ionization. In molecules, such interactions can alter bonds; in living tissue they can damage cells. Consequences depend on dose, radiation type and conditions, not merely invisibility or the source's name. Dose describes the exposure of matter; assessing effects also requires knowing how and where energy was deposited.

Imagine unchanged exposure conditions: tissue receives the same dose increment every second. Doubling the time then doubles dose, and halving time halves dose. This holds at constant dose rate, the rate of dose accumulation. Express time t in seconds or another consistent unit. If the source weakens, distance changes or shielding is introduced, t alone no longer suffices for comparison.

For external exposure, reducing time, increasing distance or adding suitable shielding changes the energy received from the source. Effectiveness depends on geometry, radiation type and energy. Contamination is distinct: radioactive material is on or inside an object and continues emitting radiation itself. Exposure and contamination are therefore not synonyms; the same shield cannot solve every situation.

Alpha particles quickly lose energy and penetrate outer layers poorly, but alpha-emitting material inside the body can irradiate nearby living tissue. Suitable materials stop beta radiation; gamma radiation usually needs more substantial shielding and is gradually attenuated. 'Penetrates deeper' does not mean 'more dangerous in every situation': we must know where the source is, how much energy it transfers and the dose received by tissue.

Can combining nuclei release energy rather than splitting them? In fusion, light nuclei form heavier products. Energy is released if their total mass is below that of the initial particles; c² relates the difference to energy. This is the same energy-release criterion as in fission, but composition is rearranged differently. The word 'combining' does not guarantee energy release for every choice of nuclei.

Why do suitable nuclei not combine immediately at ordinary temperature? Their positive charges repel electrically and hinder a close approach. High temperature means greater particle speeds and a higher reaction probability; a precise account needs quantum physics. Substantial thermonuclear fusion also requires sufficient density and confinement time for plasma, matter containing free charges. Brief heating alone does not ensure many reactions.

In stars, fusion of light nuclei can form more tightly bound systems and release energy. An important example in research on Earth is deuterium and tritium, hydrogen isotopes, producing a helium nucleus and a neutron. Charge is conserved: the two initial proton charges go to helium, while the neutron is neutral. This check does not calculate reaction probability, but distinguishes an allowed transformation from a diagram violating charge conservation.

A proton is tiny, but must it be elementary? No: size does not define the term. An elementary particle has no discovered internal structure. The electron is an example; protons and neutrons contain quarks. This conclusion rests on interaction experiments, not on seeing a particle through an ordinary microscope. 'Elementary' marks the limit of discovered structure in the accepted physical description.

The electron's antiparticle is the positron: equal mass, opposite electric charge. Other particle–antiparticle pairs also have equal masses and opposite corresponding charge properties. Yet an antiparticle need not carry electric charge: neutral antiparticles exist. 'Anti means reversing electric charge' is therefore insufficient for all properties; the electron pair is simply the clearest example.

A particle and its antiparticle can interact by annihilation: the initial pair becomes other particles, such as photons. Mass and energy do not simply vanish without trace. The initial system's rest and kinetic energies contribute to product energy, and total momentum is conserved. If the pair as a whole was at rest, product momenta must sum to zero. Photons can carry energy and momentum despite having no electric charge.

Return to half-life and seal a sample in a container. After each half-life, half the current amount of parent isotope remains on average, but decayed nuclei have become other particles and nuclei. Many products may remain inside. A decrease in the parent isotope's mass therefore does not mean an equal decrease in the total contents' mass. Specify what is counted: parent nuclei, products, or the full system including transferred energy.

A closed sample initially contains 80 g of one radioactive isotope with half-life 4 days. How much of the parent isotope is expected after 12 days?

The elapsed 12 days and half-life of 4 days use the same units. The number of half-lives is t/T = 12/4 = 3.

Count only the mass of undecayed parent isotope. Its mean halves each half-life: 80 → 40 → 20 → 10 г.

Check: 80·2⁻³ = 10 г, one eighth of the initial isotope. The other parent nuclei became decay products, which may remain in the closed sample.

An average of 10 g of parent isotope remains. This is not the total mass inside the container: decay products may also be present.

Ionization explains energy transfer to matter, but effects depend on dose, radiation type and exposure conditions.

Fission and fusion rearrange nuclei differently; energy release in either is associated with a lower total product mass.

Annihilation transforms particles while conserving energy and momentum; decay likewise does not mean matter vanishes without trace.

Next year, we will revisit these ideas with more precise models of motion, matter and fields.