High-Energy EM Waves: UV, X-Rays, and Gamma Rays

Ultraviolet (UV) radiation, X-rays and gamma rays are the high-frequency, short-wavelength regions of the electromagnetic spectrum, with photon energies E = hf = 1,240 eV·nm ÷ λ high enough to ionize atoms and damage molecules; they are used in medicine, sterilization and industry, and require careful protection. In brief: UV has wavelengths of about 10 to 400 nm, X-rays about 0.01 to 10 nm, and gamma rays below about 0.01 nm (the boundary between X-rays and gamma rays is defined by origin: X-rays come from electron transitions and deceleration, gamma rays from the nucleus). The ionizing radiation begins at photon energies of about 10 eV. The harm they cause depends on dose, and is reduced by time, distance and shielding.

Sunburn, dental X-rays, cancer therapy, sterilization of medical equipment and the study of black holes all involve high-energy photons. This guide describes the origin, properties, uses and hazards of UV, X-rays and gamma rays, compares them in a table, works through photon-energy calculations and corrects common mistakes.

What is ultraviolet radiation?

UV has wavelengths from about 10 nm to 400 nm, divided into UV-A (315 to 400 nm), UV-B (280 to 315 nm) and UV-C (100 to 280 nm). The Sun is the main natural source; the ozone layer absorbs all UV-C and most UV-B. UV-A causes tanning and skin aging; UV-B is responsible for sunburn, damage to DNA and skin cancer; UV-C is strongly germicidal. Effects and uses: production of vitamin D in the skin; sterilization of water, air and surfaces (UV-C lamps); fluorescence (substances absorb UV and emit visible light: security marks on banknotes, fluorescent lamps and white-clothes brighteners); curing of dental resins and inks; forensic and astronomical observations. Sunscreens, clothes and sunglasses protect against UV, and UV exposure should be limited for the skin and eyes (snow blindness is UV damage to the cornea). UV photons have energies of about 3 to 12 eV, enough to break chemical bonds and, at the short end, to ionize atoms.

What are X-rays?

X-rays (about 0.01 to 10 nm, energies from about 100 eV to 100 keV and above) are produced in an X-ray tube: electrons accelerated through a high voltage (30 to 150 kV for medical use) strike a metal target (tungsten), and are decelerated, producing a continuous spectrum (bremsstrahlung), with a sharp maximum photon energy equal to the electron energy eV, plus characteristic lines from transitions of inner-shell electrons. The shortest wavelength is λ_min = hc ÷ eV. X-rays penetrate soft tissue but are absorbed more by dense materials like bone and metals, producing shadow images on a detector (radiography). Uses: medical imaging, CT scans (computed tomography), dental X-rays, airport security, crystallography (X-ray diffraction by crystals reveals atomic structures, as in the determination of the structure of DNA and proteins), and X-ray astronomy. X-rays are ionizing: doses must be kept as low as reasonably achievable.

What are gamma rays?

Gamma rays have the highest frequency and shortest wavelength (below about 0.01 nm, photon energies above about 100 keV, up to many GeV). They come from nuclear transitions (radioactive decay, such as cobalt-60 and technetium-99m), from nuclear reactions, from the annihilation of matter and antimatter (positron emission tomography), and from astrophysical sources (supernovae, pulsars, black holes and gamma-ray bursts). Gamma rays are highly penetrating and ionizing, needing thick lead or concrete as shielding. Uses: radiotherapy for cancer (cobalt-60 or accelerator sources), sterilization of medical equipment and food irradiation, medical tracers and imaging (gamma camera, PET), industrial radiography and gauges, and astronomy.

Attenuation of a gamma beam by lead with a half-value layer of 1.0 cm, I = I₀(1/2)^x for x cm. Each centimeter halves the intensity, so 4.0 cm leaves 1/16, or 6.25 percent. The half-value layer depends on the photon energy and the material.

High-energy regions compared

High-frequency regions of the electromagnetic spectrum (approximate boundaries)
Region	Wavelength	Photon energy	Main source	Typical use or hazard
Ultraviolet	10 to 400 nm	3 to 124 eV	Sun, UV lamps	Sterilization; sunburn
X-rays	0.01 to 10 nm	0.1 to 100 keV	X-ray tubes	Imaging; ionizing
Gamma rays	below 0.01 nm	above 100 keV	Nuclei, cosmic sources	Radiotherapy; highly penetrating

Worked example: photon energies and attenuation

UV photon. λ = 300 nm: E = hc ÷ λ = 1,240 eV·nm ÷ 300 nm = 4.1 eV = 6.6 × 10⁻¹⁹ J. That is enough to break many chemical bonds (a typical bond has 3 to 5 eV) but not enough to ionize most atoms (hydrogen needs 13.6 eV).
Ionization threshold. A photon energy of about 10 eV corresponds to λ = 1,240 ÷ 10 = 124 nm, in the far UV.
X-ray tube. A tube voltage of 60 kV produces photons up to 60 keV, with the shortest wavelength λ_min = 1,240 eV·nm ÷ 60,000 eV = 0.0207 nm = 20.7 pm.
Gamma ray. A 1.0 MeV gamma photon has λ = 1,240 ÷ 1.0 × 10⁶ = 1.24 × 10⁻³ nm = 1.24 pm. The photon energy is 1.6 × 10⁻¹³ J.
Attenuation. For a gamma beam, each half-value layer (HVL) of shielding halves the intensity. If the HVL of lead for a certain gamma energy is 1.0 cm, a thickness of 4.0 cm reduces the intensity to (1/2)⁴ = 1/16 = 6.25 percent, and 10 cm to about 0.1 percent. (HVL depends on the photon energy and the material.)
Distance. The dose rate from a small source falls with the inverse square of the distance: if it is 4.0 µSv/h at 1.0 m, it is 1.0 µSv/h at 2.0 m and 0.44 µSv/h at 3.0 m.

How were they discovered?

Johann Ritter discovered UV in 1801 by the darkening of silver chloride beyond the violet end of the spectrum. Wilhelm Röntgen discovered X-rays in 1895 and took the X-ray image of his wife’s hand, and received the first Nobel Prize in Physics in 1901. Max von Laue showed in 1912 that X-rays are diffracted by crystals, proving their wave nature. Paul Villard discovered gamma rays in 1900 in radiation from radium, and Ernest Rutherford showed in 1914 that they are electromagnetic radiation similar to X-rays. Gamma-ray bursts were first detected in the 1960s by satellites monitoring nuclear test bans.

Which mistakes are common?

Mistake 1: Thinking that all UV is blocked by the atmosphere

UV-A and some UV-B reach the ground. The ozone layer blocks UV-C and most UV-B.

Mistake 2: Saying that X-rays and gamma rays are different types of waves

They are both electromagnetic waves. The distinction is based on the origin and the energy range.

Mistake 3: Believing that radiation makes objects radioactive

Exposure to X-rays or gamma rays does not make an object radioactive (except in rare nuclear reactions at very high energies).

Mistake 4: Thinking that brighter means more energetic

The photon energy depends on frequency. Intensity depends on the number of photons.