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Background for Context:

For the science fiction setting I am working on, I was originally intending for spaceships of a specific faction to have solar panel arrays on their larger ships. Originally, it was more as a stylistic choice to fix up some empty spaces. Some time between designing those ships and the present, I decided I wanted that faction to have exclusive access to efficient antimatter production (where the rest of humanity use fusion reactors and vent off the plasma for thrust and certain weapons.)

I got to thinking on how these solar panel arrays would be useful and decided they would be a backup power source for the electromagnetic confinement systems they store their antimatter in so that the antimattter escape if the main reactor fails.

In my research, I discovered that the energy from matter-antimatter annihilation comes in the form of gamma rays which lead me down a different line of thought. Instead of just using the gamma rays to heat water to run a steam turbine like modern nuclear fission power plants, what if the faction in question had technology that could directly absorb gamma rays and convert that energy into usable electricity? That brought me back to the solar panel arrays, wondering if such tech could be applied to them as well.

Question:

If a passive power collection system that was both functionally and visibly similar to a solar panel could absorb gamma radiation and other short wavelength radiation in space, how much could it potentially collect before hitting any thermodynamic limits?

Is there even enough energy to be found short wavelength radiation in space to justify trying to collect it as a power source? Or would it be better to leave those as dedicated solar panels and leave the short wavelength radiation-electricity conversion tech in the antimatter reactor?

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  • $\begingroup$ Is the question answerable? I mean solar pannels have limits depending on their form of collection. Gamma won't be different. But if you would say both are the same but one collects solar and the other Gamma you have your answer. They just produce energy, so they would have similar limits. Changing the wavelength doesn't change the power inside. Just how it reacts with matter. $\endgroup$
    – Trioxidane
    Jul 22 '20 at 21:08
  • $\begingroup$ I saw it as more of a question of whether or not there was more energy to be gathered in short wavelength radiation as opposed to visible light and whether or not there was enough short wavelength radiation in space to justify setting up such panels over conventional solar cells. $\endgroup$
    – Arvex
    Jul 22 '20 at 21:15
  • $\begingroup$ I think not much if Wikipedia on non atmospheric radiation tells me anything (en.m.wikipedia.org/wiki/Sunlight). Gamma isn't even on the chart. Maybe someone else can say more about it, but it seems you can get very little energy that way. $\endgroup$
    – Trioxidane
    Jul 22 '20 at 21:20
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    $\begingroup$ From Wikipedia: Although the Sun produces gamma rays as a result of the nuclear-fusion process, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun's surface and are emitted out into space. $\endgroup$
    – Trioxidane
    Jul 22 '20 at 21:23
  • $\begingroup$ Are you sure those panels aren't cooling fins? Ships which generate more power must dissipate more heat. The only way for a space ship to get rid of heat is by radiating it away. The amount of heat you're capable of dissipation is proportional to your surface area. The ISS uses cooling radiators. $\endgroup$
    – Luke
    Jul 22 '20 at 21:35
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It's not really a significant energy source. If you measure wattage carried by photons that pass through a particular volume of space, gamma rays are not very significant. Visible and infrared light are the wavelength where you find the most wattage. That ratio is basically the same whether you're close to a sun or not. In deep space, far from any sun, you'll get very little energy from any form of light.

It's different if you're near a neutron star or active black hole which emit more energy in the short wavelengths. Though they still have more energy in x-rays than gamma rays.

But, your gamma ray absorbing panels would make excellent radiation shields.

One of the problems with gamma rays is that they go through everything. Gama rays don't interact with things very often, which means they will likely go through solar panels without being absorbed. If these fictional gamma panels are effective at absorbing gamma rays, then they may be more valuable as shielding than as power collection.

Real world gamma ray shielding involves massive blocks of lead, concrete, water, or whatever. The only way to block gamma rays is to put enough stuff in front of it that it gets absorbed eventually. If you had a lighter, thinner mechanism to block gamma rays that might be a big deal. Especially if your ships frequent high radiation environments or if their enemies utilize gamma-ray lasers.

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  • $\begingroup$ I had intended for the ships to generate their own electromagnetic fields to deflect cosmic radiation and gamma rays similar to the magnetic field Earth has protecting it. In hindsight, that kind of hurt the idea of collecting gamma radiation from space unless I made it so they were positioned around weak points in the magnetic shielding. Easier to just leave them as conventional solar panels. $\endgroup$
    – Arvex
    Jul 23 '20 at 2:55
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    $\begingroup$ @Arvex That's a common misconception. Magnetic fields cannot deflect gama rays. They can't deflect any kind of electromagnetic radiation. The Earth's magnetic field only deflects charged particles. Things like Alpha and Beta radiation. The thing that protects us from cosmic rays is the atmosphere. 100km of air is a pretty good radiation shield. $\endgroup$
    – Luke
    Jul 23 '20 at 7:44
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It depends on the bandgap of your solar panel's semiconductor

Solar panels work by absorbing a photon and using it to "boost" an electron in the silicon lattice. Once boosted, the electron is free to wander around the lattice and do work until it gives up its energy and returns to the lattice (through a process called hole-electron recombination). Thus, an electrical current is created.

Photons are of course quantized, so a single photon can only eject a single electron. Einstein showed this through his explanation of the photoelectric effect. In it he found that the photon must have a minimum energy or no electron would be ejected, in other words, a photon's energy was quantized. This was actually the birth of quantum mechanics.

Getting back to your gamma-ray solar panel, yes, it is feasible to engineer a panel that can absorb gamma rays, but it will not increase the number of electrons available (i.e. the current). In theory, however, it could increase the energy of the electron when it's ejected (i.e. the voltage).

Since power is P = VI, you could, theoretically, increase the power by absorbing gamma rays, but it's not that simple.

Modern photovoltaic cells are only about 20% efficient. Most of the photons falling on them are unable to eject an electron and are thus converted to heat. This isn't necessarily related to the photon's energy, either, it has to do with how the solar panel is engineered. So in the end, the amount of power you can generate with your solar panel is directly related to its efficiency, not the energy of the photons it absorbs.

All of it falls down to what's called the "band-gap energy." This is a measure of how easy it is in your semiconductor to eject an electron, and it has direct implications on the power and voltage that your semiconductor operates at.

There are multiple kinds of band gaps, but the one you care about is the optical bandgap. For pure silicon this is about 1 electron volt. For our purposes, 1eV is equal to 1V, so a single silicon wafer can generate 1 volt of electricity. The amount of electrons is going to be related to the surface area of the cell, and thus so is the current. In order to get higher voltage, you string the panels together in series. To get more current, you would string the panels together in parallel.

For pure silicon, we can calculate the optimal photon wavelength required to eject an electron, and it turns out to be around 1 micrometer, which is not even in the visible spectrum; it's near-infrared!

But one of the useful properties of certain semiconductors is that its bandgap energy can be changed through doping. In silicon, this is very difficult to do, but in other semiconductors such as Gallium Arsenide and Indium Phosphate, it's relatively straightforward.

This band-gap engineering is how engineers control the color of LEDs. Namely, by changing the band-gap energy to be roughly the photon energy (and thus the color of the photon).

For your gamma-ray semiconductor to work, you would need to engineer your solar panels to support the gamma-ray energy photons available. However, this would be in direct conflict with attempting to make your panel absorb other wavelengths, such as visible light.

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  • $\begingroup$ Gamma rays have energies starting in the hundreds of keV. Semiconductor bandgaps max out around 10 eV. Gamma rays have energies similar to the binding energy of the particles in atomic nuclei, electron bandgaps are largely irrelevant to their physical interactions. $\endgroup$ Feb 20 at 17:56
  • $\begingroup$ @ChristopherJamesHuff Not entirely true. The wafer can be engineered to "downshift" the gamma rays through luminescence. This type of engineering is used quite often in PV design to shift higher frequency photons down to usable frequencies. The gamma rays are absorbed at one frequency then released as a photon of a lower frequency. In addition, since the photoelectric effect still applies for gamma rays, it is possible to engineer a gamma photovoltaic. $\endgroup$
    – stix
    Feb 22 at 17:55
  • $\begingroup$ Fluorescence can be used to shift the frequency down by a factor of 2 or so. We're talking about 5+ orders of magnitude with gamma rays. Your scheme fails at "the gamma rays are absorbed", because absorbing any significant fraction of them requires a large thickness of material. Electrons ejected by the photoelectric effect or (more realistically at these energies) Compton scattering will be set loose in the middle of the bulk material, not at its surface. The end result is heat, not an electric current. $\endgroup$ Feb 22 at 18:27
  • $\begingroup$ It's a moot point anyway since as Luke pointed out in his answer, there's no significant power to be generated by converting gamma radiation. If there were, it wouldn't conflict with converting visible light, because that can be done with a thin film that gamma rays will pass through with very little loss. $\endgroup$ Feb 22 at 18:37
  • $\begingroup$ @ChristopherJamesHuff I'm not sure where you're getting your physics or engineering, but downconverting gamma rays to visible wavelengths is something we've been able to do ever since we discovered gamma rays well over a hundred years ago. Zinc sulfide is a simple chemical that luminesces under gamma exposure. As for the moot-ness of my answer, that's irrelevant to your comments about the feasibility of my answer, and the OP's question of whether or not it could be done. $\endgroup$
    – stix
    Feb 22 at 19:28

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