Radiation and Matter: Production, Uses and Hazards

Radiation changes matter — and matter produces radiation

Electromagnetic radiation transfers energy. When matter absorbs radiation, its atoms or molecules may gain energy or undergo a change. The reverse can also happen: changes within matter can release energy as electromagnetic radiation.

Different changes produce different regions of the spectrum. Changes involving molecules can emit or absorb infrared, changes involving atomic electrons can produce visible light, ultraviolet or X-rays, and changes within atomic nuclei can emit gamma rays.

Electrons absorbing and releasing energy

An atom has a small, positively charged nucleus surrounded by negatively charged electrons. The electrons occupy energy levels, associated with different distances from the nucleus. Electrons in higher levels have more energy than those in lower levels.

When an atom absorbs a suitable amount of energy from electromagnetic radiation, an electron can move to a higher energy level, generally further from the nucleus. The atom is then in an excited state. The electron has not left the atom: it has changed its energy and arrangement within it.

An electron can later move back to a lower energy level. It loses energy, which is emitted as electromagnetic radiation. Depending on the energy difference between the levels, this radiation can be visible light, ultraviolet or X-rays. A larger energy difference produces higher-frequency radiation.

Absorption and emission are therefore connected processes: absorption can increase an electron’s energy, while emission accompanies a decrease in its energy.

Removing an electron: ionisation

If enough energy is absorbed, an outer electron can leave the atom completely. This is ionisation, rather than just movement to another energy level.

A neutral atom has equal numbers of positive protons and negative electrons. Losing a negative electron leaves it with an overall positive charge, so it becomes a positive ion. For example, a neutral sodium atom can lose its outer electron to become a sodium ion:

$$\mathrm{Na \rightarrow Na^+ + e^-}$$

High-energy ultraviolet, X-rays and gamma rays can carry enough energy to remove electrons when absorbed by some atoms. They are therefore examples of ionising radiation. Whether ionisation occurs depends on whether the absorbed radiation supplies enough energy to remove an electron from that particular atom.

Molecules and nuclei also exchange energy

Molecules can absorb and emit infrared radiation through changes in their motion, including vibration. Absorbing infrared increases their energy and can heat a material; emitting infrared transfers energy away. This explains why infrared is useful for heating.

Gamma rays originate in the nucleus, not in the surrounding electrons. A nucleus with excess energy can lose that energy by emitting gamma radiation. This is a different process from the electron energy changes that can produce visible light, ultraviolet and X-rays.

Together, molecular, atomic and nuclear changes account for the generation and absorption of radiation over a wide range of frequencies.

Oxygen, ozone and protection from ultraviolet

Some ultraviolet radiation from the Sun is absorbed by oxygen molecules high in the atmosphere. The absorbed energy can split an oxygen molecule, $\mathrm{O_2}$, into separate oxygen atoms. An oxygen atom can then combine with another oxygen molecule to form ozone, .

Ozone also absorbs ultraviolet radiation. Oxygen and ozone therefore absorb harmful ultraviolet before it reaches the ground, protecting living organisms from damage. They do not remove all ultraviolet: some still reaches Earth’s surface, so atmospheric protection does not make exposure to sunlight risk-free.

How radiation can damage living tissue

Ultraviolet, X-rays and gamma rays can damage human bodily tissues when absorbed. High-energy radiation can ionise atoms and molecules, disrupting chemical bonds and reactions within cells.

Two important consequences are:

• Cell damage or death, which can damage tissue. Large exposures can kill many cells.
• DNA damage, which can cause mutations. Some mutations can lead to uncontrolled cell division and cancer.

Ultraviolet can damage skin, causing sunburn and increasing the risk of skin cancer. X-rays and gamma rays can penetrate into the body and damage internal tissues. Harm is not inevitable after every exposure: risk depends on the radiation, the dose absorbed and the tissues exposed.

Reducing unnecessary exposure lowers risk. For sunlight, shade, covering skin and suitable sunscreen reduce ultraviolet exposure. For X-rays and gamma rays, exposure time can be limited, distance from a source increased where possible, and suitable shielding used, such as lead or concrete.

Practical uses across the spectrum

Different regions are useful because of how their radiation travels and interacts with matter.

The same interaction can be both beneficial and hazardous. Ultraviolet can kill microorganisms, but it can also damage human cells. Gamma rays can destroy cancer cells, but healthy cells may also be damaged.

Medical decisions therefore balance benefit against risk. An X-ray may provide information needed for treatment, justifying a carefully controlled exposure. In cancer treatment, radiation is directed at the tumour and exposure is planned to limit damage to healthy tissue. A useful assessment explains both the benefit and the hazard, then considers whether the benefit justifies the controlled risk.

Production and absorption

• Absorption: an electron gains energy and can move to a higher level.
• Emission: an electron moves to a lower level and releases energy as radiation.
• Atomic electron energy losses can produce visible light, ultraviolet and X-rays.
• Gamma rays: emitted from atomic nuclei losing energy.
• Infrared: emitted and absorbed by molecules.

Ionisation and hazards

Loss of an outer electron makes an atom a .