gamma ray

A gamma ray is light — the same stuff as radio waves and sunshine — carrying so much energy per photon that it knocks electrons out of atoms and passes straight through solid walls. NASA puts it simply:

Gamma rays have the smallest wavelengths and the most energy of any wave in the electromagnetic spectrum.

Gamma rays are the third of the three classic radiations. Alpha particles are helium nuclei and stop at a sheet of paper; beta particles are electrons and stop at a few millimetres of aluminium; gamma rays are massless, chargeless, and need centimetres of lead to cut down by half.

Defined by where it comes from, not how short it is

Gamma rays and hard X-rays overlap in energy, and there is no wavelength that separates them. The distinction is the source:

• A gamma ray comes out of a nucleus, or out of particle annihilation or decay.
• An X-ray comes from electrons — knocked out of an inner shell, or slowed down in a target.

A 100 keV photon from a nucleus is a gamma ray. A 100 keV photon from an X-ray tube is an X-ray. Same photon, different paperwork.

Where nuclear gamma rays come from

A nucleus that has just decayed is usually left in an excited state — it has the right particles but too much energy. It sheds that energy as a gamma ray, usually within picoseconds — though some excited states, called nuclear isomers, hang on for hours or years. So gammas rarely travel alone: they come stapled to an alpha decay or a beta decay that happened first.

Because nuclear energy levels are sharp, the emitted photons are sharp too. Every nuclide has its own fingerprint of gamma lines, and reading those lines back is how you identify an unknown radioactive source without touching it.

source	gamma energy	emitted per 100 decays	why it is familiar
¹³¹I	364.489 keV	81.5	thyroid treatment and fallout
¹³⁷Cs	661.657 keV	85.1	reactor waste; the standard calibration source
⁶⁰Co	1 173.228 keV	99.85	sterilisation, cancer therapy, steel radiography
⁶⁰Co	1 332.492 keV	99.98	the second line of the same pair
⁴⁰K	1 460.820 keV	10.66	in every banana, and in you

⁴⁰K deserves the note. It is a naturally occurring Isotope of potassium — 0.0117% of all potassium on Earth — with a half-life of 1.248 billion years. Potassium is essential to life, so every living thing carries ⁴⁰K, and 10.66% of its decays throw out a 1 461 keV gamma. Your own body is a weak gamma source, and always has been.

The three ways a gamma ray dies

Gamma rays do not slow down. Each photon travels until it interacts, and then it is gone or badly degraded in one step. Three processes compete, and which one wins depends on energy and on how heavy the absorber is.

1. Photoelectric absorption — the photon is swallowed whole and ejects a bound electron. Dominant at low energy, and it climbs steeply with atomic number, which is why lead is such good shielding.
2. Compton scattering — the photon bounces off an electron, gives it part of its energy, and carries on in a new direction with a longer wavelength. Dominant in the hundreds-of-keV to few-MeV range — which is exactly where most nuclear gammas live. This is also how most gamma detectors work.
3. Pair production — the photon converts into an electron and a Positron in the electric field of a nucleus. It cannot happen below 1.022 MeV, because that is twice the electron's rest energy of 510.999 keV and the pair has to be paid for. Above a few MeV it takes over.

Shielding, in real numbers

Absorption is exponential in distance, the way decay is exponential in time: I = I₀ e^(−µx). The useful quantity is the half-value layer, the thickness that cuts intensity in half.

For lead, using NIST's mass attenuation coefficients and the density NIST assumed alongside them, 11.35 g/cm³:

photon energy	µ/ρ (cm²/g)	µ (cm⁻¹)	half-value layer
100 keV	5.549	62.98	0.11 mm
500 keV	0.1614	1.832	3.8 mm
1.0 MeV	0.07102	0.806	8.6 mm
1.25 MeV (⁶⁰Co average)	0.05876	0.667	10.4 mm
3.0 MeV	0.04234	0.481	14.4 mm

Two things to read off that table. First, lead stops soft gammas almost trivially and MeV gammas only grudgingly — a thirtyfold rise in photon energy costs more than a hundredfold in lead. Second, exponential attenuation has no end point. There is no thickness that stops gamma rays; there is only a thickness that makes what gets through acceptably rare. To cut ⁶⁰Co radiation by a factor of a thousand takes ten half-value layers — about 10 cm of lead.

One caveat on those figures: they are narrow-beam values. A real shield also has scattered photons working their way through it, so practical designs add a buildup factor and end up thicker than the table says.

The sky is full of them

Gamma rays are also the light of the most violent things in the universe. Per NASA, they are

produced by the hottest and most energetic objects in the universe, such as neutron stars and pulsars, supernova explosions, and regions around black holes

and on Earth by "nuclear explosions, lightning, and the less dramatic activity of radioactive decay."

The extreme case is the gamma-ray burst. NASA describes these as the most energetic and luminous electromagnetic events since the Big Bang, capable of releasing more energy in ten seconds than the Sun will emit in its entire ten-billion-year lifetime.

Gamma-ray astronomy needs different instruments from every other kind. Gamma rays cannot be focused: mirrors do not work, because the wavelengths are short enough to slip between the atoms of any surface you point at them. Detectors instead use dense crystal blocks and watch for the charged particles that Compton scattering leaves behind. That is why gamma telescopes see fuzzy and why sky maps like the one above are built from millions of individually reconstructed photons.

Why they matter to people

• Medicine. Gamma knife radiosurgery focuses many weak ⁶⁰Co beams so they only add up at the tumour. Nuclear imaging runs the other way, injecting a gamma emitter and watching where it goes.
• Sterilisation. ⁶⁰Co gammas kill bacteria inside sealed packaging, which is how single-use medical kit is sterilised after it is wrapped.
• Industry. Shining gammas through a weld and photographing the far side finds cracks in steel too thick for X-rays.
• Planetary science. Cosmic rays striking a planet's surface make its elements emit characteristic gammas. Orbiters read the composition of Mars and Mercury this way, without landing.
• Damage. Gamma rays ionise whatever they pass through, including DNA. Penetration is exactly what makes them dangerous: an alpha emitter outside the body is harmless, a gamma source is not.

See also: half-life, radioactive decay, Nuclear binding energy, Neutron star, photon, electromagnetic spectrum.