I'm thinking of adding a neutron beam cannon to my hard sci-fi setting (basically works by accelerating a bunch of positively-charged deuterium ions and then shooting them at a beryllium foil target (see here).

However, I don't want it to be completely overpowered, so there needs to be some way to defend against it.

Because this cannon is extremely unwieldy, small ships can easily run away from it, so the only ships that need to protect against it are large capital ships. These can usually afford the extra mass of some shielding, but not enough to protect against a high-intensity neutron beam. Water-based shielding (such as a double hull with water inbetween) is infeasible mass-wise.

Since they can't be deflected magnetically, heavy armored plating is prohibitively massive, and I'm guessing that a plasma shield would be too sparse, I can't really think of a way to defend against this weapon, though...

EDIT: Some more details since the original question was quite open.

This weapon is quite large, unwieldy and energy-intensive, and has a very low thermal efficiency (= produces a lot of thermal waste). It is employed by a technologically superior navy to take out capital ships operated by an enemy that outguns the former army significantly and operates a lot more ships.

All ships in the setting draw their power from a nuclear fusion reactor. Fusion is complicated, so let's not worry about how those reactors work in detail, but they're effectively stellarators. I'm willing to accept that at this point, their working principle is basically just sci-fi magic and not really grounded in actual research. They produce fairly little neutron radiation and as such aren't too heavily shielded to avoid excess mass.

The idea is that a technologically superior but outnumbered navy can use the neutron beam to take out the reactors on slow-moving enemy carriers to render them unable to utilize their engines and come close enough to deploy the smaller corvettes and fighters they carry.

Protecting against this weapon with heavy shielding like water or otherwise is infeasible to due to mass constraints, since carriers are already hard to maneuver as they are, so either some kind of light, neutron-absorbing shielding or an active mode of defense are necessary.

Question: As far as I know, this means that there are no effective ways of defending againts this weapon. Am I wrong?

  • $\begingroup$ @ErikHall Yeah, that's basically the conclusion I had arrived at as well. Thanks for confirming. It is most definitely going to be a heavy mobile weapon, especially since its thermal efficiency is probably way below even 1%. The idea was that it was going to be super-heavy mobile weapons platform that can be used to take out the reactor of an enemy capital ship. Given all that, it's pretty much a guaranteed kill if it manages to strike its target, so I guess the best balancing mechanic is just to make prohibitively difficult to employ in most combat situations $\endgroup$
    – Jake
    Commented Dec 21, 2023 at 21:24
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    $\begingroup$ Without knowing the specifics of the weapon, the context within which it will be used, and what we're trying to defend how do you expect us to come up with a "defense" for it. A bb, a 7.62, and an APFSDS are all metal projectiles, and the defenses against all 3 depend greatly upon the specific context. $\endgroup$
    – sphennings
    Commented Dec 21, 2023 at 21:26
  • $\begingroup$ The answer is in the question. More mass. In particular hydrogen rich material, for maximum transfer of energy in collisions (perfectly elastic). $\endgroup$ Commented Dec 21, 2023 at 21:32
  • $\begingroup$ Re your comment about "take out the reactor of an enemy capital ship" - while you have not defined your power generation technology it would seem that a "reactor" would have seriously massive physical shielding anyway, even if it is mass-prohibitive to armour the entire ship's exterior with that much shielding. So it's up to you whether it's a "guaranteed kill". $\endgroup$ Commented Dec 21, 2023 at 21:32
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    $\begingroup$ May I ask how do you collimate the beam? Collimate means directing or focusing, since neutron do not carry electric charge. I just wanna throw the ball back to you 😁 $\endgroup$
    – user6760
    Commented Dec 22, 2023 at 10:26

3 Answers 3


You are vastly over-estimating your neutron beam weapon effectiveness.

Per the article you referenced

A very thin foil beryllium target was used (0.1 µm) this means it will very rapidly be destroyed by the energy striking the foil to produce the neutron beam. Not nearly long energy to be useful as a weapon. Making the foil thicker will reduced the efficiency and spread out the beam further. There's a reason they chose an ultra-thin film.

Though the article mentions

"The calculated deuteron beam is highly directional along the laser propagation direction"

You should have kept reading

The computed neutron spectra show that a forward directed neutron beam is generated with an opening angle of ∼1 sr

A steradian spread means this is not a focused beam weapon source, especially not at the distances required for use in space battle.

Your 1% efficiency in generation of neutron flux at the source, will result in efficiency millions or billions of times worse in terms of energy delivered to the military target.

The commander of the vastly superior enemy fleet will have no trouble taking out your mildly annoying superweapon. Recommend you consider switching to a mass-driver super-weapon instead. You can throw a mix of BB's and large slugs to complicate defense.


A Double hull of Water is too expensive Mass wise, but Water would otherwise provide a solution?


A Trophy-esque Active Protection System, using high-tech space Water guns

No, I'm actually being quite serious.

Let's talk limitations of the Weapon:

  • Big
  • Unwieldy
  • Therefore, Kinda obvious where it is (think a Battleships main armement, even a 4 YO can reasonably guess which ones are the important ones)
  • Due to lots of excess heat when used, we can presume that it has a limited firing time - lets say 10 seconds for the sake of argument.
  • It works by Damage Over Time - the longer it's on target, the worse it gets.

Okay - since we know where the Weapon is going to be mounted, even if it's in a fixed mount like the A10s Cannon or it's in a Turret - just by observing it, we roughly know where it's pointing.

We can also assume that a Capital Ship will have some form of Armor - not enough to stop this weapon, but certainly enough to withstand it for a small period of time. And if you think this isn't reasonable - current Body armour will stop a Rifle round, it may even stop multiple rifle rounds, but if you don't get out of the line of fire, sooner or later one of them is going to get through.

Due to the heat - as I said a limited firing time, but also there is a significant cool-down/reload time - say 10 seconds of firing for 2 minutes of cooling (Battleships had a reload time of between 30 seconds to a minute, depending on factors)

And finally, because of the mechanism of Damage, interrupting the beam enough until it has to cool down and then either re-positioning or getting the hell out of dodge is going to be our primary tactic.

Now to the 'Armour' - You said water would be great - so lets stick with that. Having a layer of Water all over the ship, is as you say, expensive in terms of Mass - however a smaller tank (and remember, this doesn't need to be potable water, could be waste water - for extra insult points to the enemy) is much more feasible. This would then run to a series of nozzles across the ship, you could design the ship with as many as are needed, I'd recommend a minimum of 6 (Fore, Aft, Port, Starboard, Up, down) - but you could add as many as plausibly needed.

The defensive system would track where the weapon was aimed at all times, then once the firing started, all the Nozzles that can 'see' the beam, would turn, aim and fire a laminar stream of water in a burst, the goal to have 1 or more intersecting streams of water right at a point where the Beam is passing through - thus disrupting the Beam enough that any remaining particles that get through can be handled by the Armour.

The reason why this is a Burst mode and not a continous mode is because you want to make the most efficient use of water, the Armour can handle a bit of a scratching, so you just need to periodically disrupt (so like every 100 milliseconds or so) for the duration of the attack, then once the weapon firing sequence has completed, you can either run away or press the attack.

Add in some AI type stuff to maximize where the bursts are aimed and fired to maximize the efficiency and you have a system that is much lighter in terms of Mass than dumb armour, yet still provides the protection needed.


You're looking at this the wrong way

First of all, unless you're just having fun, one assumes you're trying to tell a story. That means that a great deal of what you're doing (whether you like it or not, and this is true even for hard sci-fi stories) has more to do with the needs of the story than it does the so-called reality of science. When you apply hard science to shows like Intergalactic and The Eclipse and stop believing the marketing hype about their being so scientifically accurate what you'll discover is that while their effort was admirable, no, they weren't. Let's ignore this...

... But what we can't ignore is that you want to be in a hard sci-fi setting. That means you're required to deal with all the weaknesses along with the strengths. The weapon you're postulating is enormously impractical and almost trivially susceptible to drone/fighter/missile attacks (Star Wars wasn't actually wrong about that). The more you do to protect it, the less valuable it is. Worst of all, it uses so much energy that it's a ticking time bomb waiting for the right moment to literally level the playing field. But the actual hard sci-fi truth behind all this is that all military forces succeed by taking advantage of the weaknesses of their opponents. Not, as Hollywood might glamorously portray, by bring some overwhelming counter force to the fight.

OK, sometimes there is an overwhelming counter force. When the U.S. dropped nuclear bombs on Hiroshima and Nagasaki it was a rare moment in history when the whole world was stunned by what had just happened. How could Japan have defended against those bombs? They couldn't. They didn't know they were coming. How could they today? (a) Diplomacy, (b) economic superiority, (c) strategic alliances... and that assumes they're not (d) building their own nukes. Japan has the tech to build them, but a century of treaties and the simple reality that they're a small island in a big world has given them the wisdom to pursue other ways of defending themselves. Post WWII Japan is a great example of how to defend against substantially superior military forces... and in many ways, they're kicking the world's collective butts.

What are some of the weaknesses of all weapons?

  • Weapons are either easy to move and weak or hard to move and strong.

  • As size and complexity increases, so does maintenance and logistics.

  • All weapons depend on at least one limited resource (muscles, electricity, gunpowder...).

  • All weapons require some amount of time to use.

And what are some of the weaknesses of your weapon?

  • Beam intensity is limited by the strength of the power source (can be cut), energy storage capacity (all storage is unstable), and the capacity of the heat sink (easy to shoot away).

  • Beam duration is limited by the same things, just different ratios.

  • Directing and focusing the beam is complex.

  • Focus distance is limited, beam dispersion is not.

  • It takes energy to use the weapon and to move the weapon.

Now let's defend!

All of the mentioned weaknesses can be used to defend, but some are better than others and a comprehensive list is beyond the scope of this site.

  1. As trivial as it may sound, don't get in the way of the weapon. That's a sassy way of saying strategy is usually more valuable than armor. To paraphrase General Patton, excessive armor is a monument to someone's stupidity. Aircraft carriers enjoy supporting ships for many reasons. Your capital ships will, too. Use speed and agility against your ship. They'll attack its logistic chain (hard sci-fi, you don't have infinite energy) and distract it by attacking its flanks.

  2. The best defense is a good offense (yeah, it sounds like #1). Your weapon's single biggest weaknesses are (a) it's inability to point anywhere and everywhere quickly, (b) engines and maneuvering thrusters, and (c) those radiators. You have radiators. The heat must go somewhere. Yes, you could use Seebeck Generators to convert the excess heat to electricity, except you really can't because space isn't actually cold. Yeah, radiators... destroy enough of them and that weapon can't be used without melting the hull.

  3. If your ship has enough energy to focus a neutron beam, the enemy can have enough energy to refocus the beam. The more I think about it, the less impressed I am with neutron beams as weapons. They can be focused using magnetic fields. So defending ships need either a single strong magnetic field to move the neutron beam around the ship or a bunch of smaller magnetic nodes to disperse it. Remember, if your ship can have the power to focus the beam, so can they. The argument against this is that a rail gun uses magnetic fields to propel a shell, and it's a good point, but rail guns can shed the shell around the projectile, removing the magnetically susceptible component. Neutrons can't.

  4. Take advantage of the scientific limitations of the weapon, like its recharge cycle. The weapon can only continuously fire at a low enough energy level to not stress the fusion reactors or overwhelm the radiators. If your weapon is so powerful that this condition is ultimately deadly to all intended opponents, then narrative necessity needs to kick in because that's a "godlike character" problem and the only way to overcome a godlike character is for that character to have a weakness. Therefore, in its powerful condition, your weapon needs recharge time, has limited duration, and the weak points are the fusion reactors (and their support systems, like coolant), energy storage and those radiators.


  1. I'm holding tight to your hard sci-fi goal. In a hard sci-fi universe, capital ships don't hang around at a couple of hundred kph separated by only a couple of dozen kilometers and trivially hit each other with instantaneous beams of energy. What they're really doing is moving whomping fast, which means they can't get very close to one another lest somebody physically connect. You have a fraction of a second to cast a beam of light that will take seconds to minutes to connect and if you miss you'll be hours, days, or a week coming around for another shot. In for a penny....
  • $\begingroup$ Neutrons are not able to be focused by EM fields. $\endgroup$ Commented Dec 22, 2023 at 3:56
  • $\begingroup$ @GaryWalker Focusing of a neutron beam is commonly achieved through reflection on curved or inclined supermirror assemblies, refraction, or magnetic fields. This shouldn't surprise you, it's how supercolliders work. $\endgroup$
    – JBH
    Commented Dec 22, 2023 at 4:21
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    $\begingroup$ You're correct, I misspoke, neutrons are not easily able to be focused by EM fields, in particular how it was described in the answer is not practical (as I read it at least). I am not aware of any supercollider that uses neutrons. Neutron focusing with EM fields is too my knowledge, not very effective since it can only use the spin of the neutron, not the electric charge. Weirdly, this didn't save my @JBH link, $\endgroup$ Commented Dec 22, 2023 at 4:35
  • $\begingroup$ @GaryWalker You're missing the point. In the OP's world of a far-future technology they have the ability to manipulate neutrons. Limiting the OP's world based on the limitations we have today is contrary to the point of this Stack. $\endgroup$
    – JBH
    Commented Dec 22, 2023 at 15:58
  • $\begingroup$ Question mentioned a hard science fiction though it was not an explicit tag. I can think of no theoretical hard science basis improvement that will make EM manipulation of neutrons sufficiently effective, in fact that is explicit in the question. $\endgroup$ Commented Dec 22, 2023 at 21:45

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