So I'm aware of this and this, but the question is Are Hard X-rays and Gamma-rays the same thing? If not, then what would be the key difference between them. Moreover, How much would the properties of each type differ from each other?

I'd appreciate a simple answer without much technicalities.

  • $\begingroup$ I don't understand why the two answers you cite in your question leave any doubt in your mind. You should define what you mean by "the same thing". $\endgroup$
    – Bill N
    Jun 23, 2020 at 12:43
  • $\begingroup$ So I'm actually a rookie in this field. In fact, I'm still in high school. So I think I just had to be sure of it which is why I asked the question. By "same thing" what I really wanted to know whether they were just two terms for the same thing (which isn't the case). $\endgroup$ Jun 23, 2020 at 13:01
  • $\begingroup$ Don't use "same thing" to define "same thing." Do you mean "can a photon emitted by a nucleus be 'called' an x-ray?" Or do you mean "is there a way to determine whether a 100 keV photon is an x-ray (by emission process) or a gamma ray without knowing the source?" $\endgroup$
    – Bill N
    Jun 23, 2020 at 13:36
  • $\begingroup$ By the way, there are also high-energy (511-keV) photons which, technically, are neither x-rays nor gamma rays. They are often, casually, called gamma rays, but originate from the combination of an electron and positron. Annihilation photons $\endgroup$
    – Bill N
    Jun 23, 2020 at 13:43
  • $\begingroup$ Once the light reaches you, X-rays and Gamma rays are the same thing. Same as other light, from infrared over visible up to ultraviolet and so on. Light is light, the same thing. From convention (what we as persons has decided, not inherent in the light as such) we call them different things. One difference is the wave-length. Visible light (as we know it) has a longer wavelenghs than X-rays. Another difference might be the energy in the light, compare a sunny day to a candle. And for X-ray or Gamma we name them differently due to the source of light. $\endgroup$
    – ghellquist
    Jun 23, 2020 at 19:41

3 Answers 3


It can be a little confusing because there are two conventions.

The modern convention is to distinguish x-rays from gamma rays by how they are produced. X-rays are produced by electron energy transitions, typically inner orbital transitions, whereas gamma rays are produced by electromagnetic transitions in the nucleus.

Usually, gamma rays have shorter wavelength (and therefore higher frequency and energy) than x-rays, but not always. Some radioactive processes release gamma rays with frequencies in the ultraviolet portion of the electromagnetic spectrum.

However, there is an older convention which distinguishes them by energy. This convention is still common in astronomy and astrophysics. From Wikipedia:

In astrophysics, gamma rays are conventionally defined as having photon energies above 100 keV and are the subject of gamma ray astronomy, while radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy. This convention stems from the early man-made X-rays, which had energies only up to 100 keV, whereas many gamma rays could go to higher energies.

Here's a diagram from that article.

spectrum diagram

In practice, gamma ray energies overlap with the range of X-rays, especially in the higher-frequency region referred to as "hard" X-rays. This depiction follows the older convention of distinguishing by wavelength.

In both conventions, x-rays are electromagnetic radiation with wavelength shorter than ~10 nm (and hence energy ~125 eV) and EM radiation just below that energy is considered to be ultraviolet light. Here's another relevant passage from that Wikipedia article:

Due to this broad overlap in energy ranges, in physics the two types of electromagnetic radiation are now often defined by their origin: X-rays are emitted by electrons (either in orbitals outside of the nucleus, or while being accelerated to produce bremsstrahlung-type radiation), while gamma rays are emitted by the nucleus or by means of other particle decays or annihilation events.

There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet or lower energy photons produced by these processes would also be defined as "gamma rays".

The only naming-convention that is still universally respected is the rule that electromagnetic radiation that is known to be of atomic nuclear origin is always referred to as "gamma rays", and never as X-rays. However, in physics and astronomy, the converse convention (that all gamma rays are considered to be of nuclear origin) is frequently violated.

Bremsstrahlung is braking radiation. It is any radiation produced due to the acceleration of a charged particle.

A table of gamma emitters from Professor Peter Siegel's page on the California State Polytechnic University site lists the energies of a wide range of radioisotopes. The lowest energy in the table is that of Erbium-169, which decays by beta emission (with a half-life of 9.4 days) but some decays also release a gamma ray. The energy of that gamma photon is a mere 8 keV, well below the astronomical threshold of 100 keV. It has a wavelength of 0.155 nm, so a bit shorter than high energy UV, and at the low end of x-ray energies.

  • 1
    $\begingroup$ by this definition (emitted by electrons), wouldn't many forms of light such as visible, UV, IR, etc. also be considered "X-rays"? $\endgroup$
    – Michael
    Jun 23, 2020 at 22:54
  • 1
    $\begingroup$ @Michael Fair point. This question asks about the difference between x-rays & gamma rays. In the lower energy bands of EM radiation produced by electron transitions, the different bands are divided by wavelength / frequency / energy. Thus EM (not produced in the nucleus) with wavelength smaller than 10 nm (about 125 eV) is called x-ray, just below that energy is UV. But as Wikipedia says, the naming convention is frequently violated. It would be great if we didn't have this mess... $\endgroup$
    – PM 2Ring
    Jun 23, 2020 at 23:31
  • $\begingroup$ Also see the nuclear isomer Thorium-229m, which (in theory) has a decay around 8 eV, but actually observing this photon has been problematic... $\endgroup$
    – PM 2Ring
    Apr 24 at 10:38

As your links say: gamma rays are produced in nuclei whereas X rays are produced by atoms.

Typical gamma rays have more energy than typical X rays. But there is an overlap region. And apart from their origin there is no difference between a (relatively low energy) gamma and a (relatively high energy)X ray. They're both just a quantum of EM radiation.

  • $\begingroup$ In astronomy, much of the radiation in this range comes from processes (synchrotron, inverse Compton, annihilation) that are neither atomic nor nuclear. $\endgroup$
    – John Doty
    Jun 24, 2020 at 21:10
  • $\begingroup$ True. I was thinking inside the lab. It would have been better to say 'by processes involving electrons.' $\endgroup$ Jun 25, 2020 at 7:30
  • $\begingroup$ But consider fermi.gsfc.nasa.gov. Most of the "gamma ray" photons it detects are from processes involving electrons. $\endgroup$
    – John Doty
    Jun 25, 2020 at 16:18

Wikipedia is your friend. The convention is that gamma rays originate from the nucleus and x-rays are generated in other ways. https://en.wikipedia.org/wiki/Gamma_ray#Distinction_from_X-rays


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