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I'm reading a NASA paper about the Great Observatories (for grade schoolers) in which is says that "gamma rays can only be detected when they interact with matter." Surely we can only detect anything when it interacts with the matter of our detecting instrument, but some helpful people at Rutgers explain that with gamma rays, what we actually record is the interaction of these rays with other matter.

What makes gamma rays (as opposed to other kinds of electromagnetic radiation) so special that they can't be detected directly, like visible light? Are they directly undetectable in principle, or is it just that we haven't built any instruments that can deal with that high an energy level, or what?

EDIT TAKING COMMENTS INTO ACCOUNT: The things I've been reading make it sound like visible light just sets off a detector, whereas to detect a gamma ray I'd have to set up two devices--one to interact with the gamma ray and one to detect the visible light (or whatever) from that interaction. The first is what I'm calling "direct detection," even though there's a chain of things happening in my detector. Is this a distinction without a difference? I figured there was something different about gamma rays or they wouldn't have bothered mentioning it.

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    $\begingroup$ It doesn't say "only gamma rays can be detected when they interact with matter". Interacting with matter is the only way EM radiation can be detected, and that includes visible light. So "... they can't be detected directly, like visible light" ... what do you mean by "detected directly"? How do we detect light without having some interaction with matter? $\endgroup$
    – garyp
    Commented Oct 11, 2016 at 18:14
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    $\begingroup$ How do you "directly" detect visible light? Usually I detect it when it hits my retina where it sets off a chemical reaction, an action potential... when I use a photo cell, it interacts with an electron, that... You hinted at this already, but ALL detection involves some interaction with matter. Is there really a question here? It's not clear to me that there is. $\endgroup$
    – Floris
    Commented Oct 11, 2016 at 19:28

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The quote is wrong.

“Any photons can only be detected when they interact with matter”. It is how they are detected that changes with the wavelength of light.

Radio and microwaves are usually detected by measuring the electric/magnetic fields directly. The waves interact with matter on a bulk scale. Single photons are too weak to be detected.

Infared, visible, and ultraviolet rays are usually detected as single photons. The photon excites a single atom or molecule which then transfers charge/changes shape/etc in a detectable way. Light is detected as a particle rather than as wave.

Xrays, and gamma rays are also detected at the single photon level but they have so much energy they can excite many atoms at once. This makes detecting easier.

So what was the quote getting at?

The real difference/challenge with gamma-rays is not detection, but a lack of lenses/mirrors to form a coherent image with. A metal screen is a mirror for radio-waves and microwaves (so is an ordinary mirror). Infrared, visible, and ultraviolet can also use lenses. But for higher energy, the per-photon energy is so large that changing the photon's path is like using styrofoam to deflect bullets: the photon tends to just push right through and eventually get absorbed. Xrays use grazing mirrors to lessen the "impact" or multiple lenses to enhance the very weak refraction. For gamma-rays, however, the energy is so high that grazing optics would require too small an angle to be practical; however it remains an active field of research.

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