Building a better particle weapon.

[Borgholio]

Inspired by the discussion on energy weapons vs mass drivers, I decided to think for a bit on what would make an ideal particle weapon design. Taking a phaser, as an example, I thought about how to supercharge it to do the most damage it possibly could to a target. The phaser is basically a long, flat emitter that spits exotic particles at the target. These particles seem to induce some kind of nuclear chain-reaction in the target to amplify the damage output.

To improve the damage of a phaser, the most obvious routes are to increase the density of the beam (focus the same amount of energy into a smaller point), and to increase the speed of the particles so they inflict greater kinetic damage on the target. The simplest way to do this is with a particle accelerator.



A particle accelerator...well...accelerates particles. It can move them close to the speed of light before emitting them in a beam towards the target. In the case of CERN, it's used to smash atoms. In combat, it could be used to smash into an enemy target.

So to put 2 an 2 together, instead of just emitting a phaser beam from a strip, why not emit it into a particle accelerator coil shaped something like this:



Ideally, the accelerator will pack the particles into a tight, focused beam that leaves the barrel of the weapon at nearly the speed of light.


Now, two questions.

1. If you take a phaser beam, focus it, and accelerate the particles to near lightspeed...would that actually increase the damage output? Or would the additional kinetic impact of the particles be negligible?

2. Modern particle accelerators are HUGE, because it takes us a fair distance to accelerate a particle that fast. Can it be reasonably assumed that a futuristic civilization could do the same thing using an accelerator that can fit in a reasonably-sized gun turret?

[Eternal_Freedom]

Since the phaser effect is explicitly some kind of chain-reaction then no, I don't think speeding them up would do more. Hand phasers can vaporise people with no recoil and no momentum imparted on the target, so they clearly don't rely on kinetic force.

EDIT: As to your second point, I suppose it's possible that it could be miniaturised but you would pay some huge penalties in terms of life expectancy. Consider how much power you would have to channel through such small components to achieve any measureable gain. Either you would need to swap out the magnets to keep them melting on a regular basis (not good in a combat situation to have to replace key components of your weapon system) or create an insanely capable cooling system (which requires space, coolant, more power etc).

Frankly, if you had magnetic accelerator technology that good, you'd probably be better off just building coilguns as a weapon rather than slapping it into some particle beam.

[Sea Skimmer]

Not all modern particle accelerators are that huge. The ring ones are huge because they aim for the highest possible energy levels of specific particles, which is unlikely to be the path to the optimal weapon energy output. Free electron lasers expect to use ~16m long linear particle acclerators and such devices already exist, just not with enough power to be megawatt class weapons yet.

Also its possible for particles to keep going around and around rings, so you don't need a spiral. Electromagentic fields can be used to steer stuff in and out of the rings. CERN already does this. Below is a map of the actual paths stuff take; also a reason why it was so bloody expensive.



Since we know so little about how phasers work it is impossible to know if any of this is applicable or not. They might be generating the beams out of functionally solid state devices for all we bloody know. Higher velocities could well make the beams incapable of inflicting the atomic bond disruption effect's they've been seen to have.

Also since the federation already made pulsed phasers with fixed mounts, and other powers like the Klingons long had fixed barrel equipped cannons that's most likely the best path to evolving Trek beam weapon firepower.

[Simon_Jester]

Also, a phaser is not a typical 'particle beam' weapon.

When we talk about particle beams in SF we usually mean charged particles- electrons, protons, or ions, as in real research accelerators. This allows us to use intense EM fields to manipulate the beam, both to steer it and to accelerate it... and it also makes it fairly practical to simply use a continuous beam, something that is often ignored.

A long linear barrel is almost certainly better than a circular barrel. Circular accelerators work the way they do because of the desire to keep a beam stored- the technical term for them is "storage rings." If you have enough power to keep the accelerator spitting out fresh beam continuously, there is very little advantage to a huge storage ring. And a compact storage ring has its own problems, not least something called 'synchrotron radiation,' which guarantees that dangerous EM radiation (X-rays and gamma) is getting sprayed out tangentially wherever you curve the beam path. Synchrotron radiation is especially nasty for light charged particles like electrons.

Incidentally, there is one reason to optimize for very high per-particle energy in a particle beam, and that is range. The internal repulsive forces acting on a beam as each proton/whatever repels all the like-charged proton/whatevers around it can be pretty intense and tend to disperse the beam. Containment magnets help as long as you're in the accelerator pipe but do nothing once the beam leaves the muzzle.

The faster the beam travels, the farther it gets before it disperses. And for ultrarelativistic speeds which are quite attainable, in effect time dilation is reducing dispersion, because in the beam's frame of reference it may only have, say, one millisecond to fall apart in a time where the rest of the universe sees it taking ten milliseconds to hit the target.

Personally I tend to picture really nasty particle cannon as being spinal mount weapons, with the ship being optimized to engage targets in that direction (i.e. most of the armor protection is a big slab up front and so on). Shorter barreled turreted weapons are certainly possible, but are unlikely to be nearly as dangerous as a spinal beam mounted on the same hull.

[Borgholio]

And a compact storage ring has its own problems, not least something called 'synchrotron radiation,' which guarantees that dangerous EM radiation (X-rays and gamma) is getting sprayed out tangentially wherever you curve the beam path. Synchrotron radiation is especially nasty for light charged particles like electrons.

How does that work? Is it the interaction of the particle beam with the magnetic field that generates the radiation?

Personally I tend to picture really nasty particle cannon as being spinal mount weapons, with the ship being optimized to engage targets in that direction (i.e. most of the armor protection is a big slab up front and so on). Shorter barreled turreted weapons are certainly possible, but are unlikely to be nearly as dangerous as a spinal beam mounted on the same hull.

They actually did something like that on an alternate E-D, where it was slung underneath the saucer section. It punched right through a Klingon warship with it's first shots.

[Simon_Jester]

And a compact storage ring has its own problems, not least something called 'synchrotron radiation,' which guarantees that dangerous EM radiation (X-rays and gamma) is getting sprayed out tangentially wherever you curve the beam path. Synchrotron radiation is especially nasty for light charged particles like electrons.

How does that work? Is it the interaction of the particle beam with the magnetic field that generates the radiation?

More or less; particles emit synchrotron radiation whenever someone accelerates them in any direction. You get more such radiation from bending the beam than from simply boosting it in a straight line though.

They actually did something like that on an alternate E-D, where it was slung underneath the saucer section. It punched right through a Klingon warship with it's first shots.

Yes, although it is unwise to assume that the relevant weapon was in fact a charged particle beam weapon. Basically, there are a lot of different ways to build a "giant-ass cannon," if I might speak informally. It just so happens that "giant-ass cannon" is arguably the best way to build a charged particle beam weapon... that doesn't mean all such cannons are charged particle beam weapons.

[Lord of the Abyss]

Incidentally, there is one reason to optimize for very high per-particle energy in a particle beam, and that is range. The internal repulsive forces acting on a beam as each proton/whatever repels all the like-charged proton/whatevers around it can be pretty intense and tend to disperse the beam. Containment magnets help as long as you're in the accelerator pipe but do nothing once the beam leaves the muzzle.

Which is why if you have the technology what you want to use are neutral particle beams, since neutral particles won't repel each other. We can't do that, at least not very well; but a setting like Star Trek with gravity control and so forth probably finds it quite easy.

[Borgholio]

I wonder if even in ST or SW, they can control gravity precisely enough to replace a magnetic field in a miniaturized particle accelerator...

[Batman]

Given that in Wars they use repulsorlift technology to have smoothly moving doors (ref 'Truce at Bakura') on occasion I'd say they, at least, have a good chance of having that level of control. Trek is a bit more of a maybe, but even they have gravity control pretty well in hand (what's the one onboard system that never ever fails even if you're one minute away from suffocation and/or freezing to death, both of which you're going to do in the dark because life support cranked out thanks to somebody shooting up the hangar bay? Yup-artificial gravity )
I seem to recall individual AG cargo-handling rigs from The Motionless Picture (or at least the novelization) and the TOS episode with the ludicrously powerful manportable AM charge too.

[Eleventh Century Remnant]

Particles respond so much more strongly to everything else- gravity is such a hopelessly weak force- that it's hard to see the logic, or efficiency, in a gravitic particle accelerator, unless you're acelerating something that you can't make go with electromagnetism.

There are a few of those, way out in the particle zoo; hm. What happens when you hit something with a beam of Higgs bosons?

One idea I like is the wake field accelerator; https://portal.slac.stanford.edu/sites/ ... /rpwa.aspx compact, extremely powerful, and leaves a nice dramatic 'muzzle blast' of plasma.

The obvious zapgun of megadoom would be- and in some original SF that I still have hopes for and won't show on the net, is frequently used as a spinal mount- a http://en.wikipedia.org/wiki/Strangelet- strangelet cannon.

In practise, charged particle beam weapons are going to be monstrously ugly and legally dubious things to use- they effectively kill people and electronics with hard radiation.

Neutral particle beams work, as far as I know, by accelerating a charged beam and then neutralising it at the muzzle; what you'd probably do is have a NPB weapon, fire it as such in space, and for planetary bombardment simply not neutralise it again. Size and weight- they might be a shade more efficient than lasers, throwing off less waste heat, which might matter if we're being hard- SF about this; but they would not be small and light.

[Eternal_Freedom]

The trouble I see is that, yes, while it's probably possible to build an effective particle beam weapon, it's easier to build other weapons: coilguns for instance, or missiles, or powerful lasers.

Hell if you want a decent plasma cannon you can always use the old casaba howitzer idea. Nuclear powered plasma cannon is highly effective.

[Simon_Jester]

Which is why if you have the technology what you want to use are neutral particle beams, since neutral particles won't repel each other. We can't do that, at least not very well; but a setting like Star Trek with gravity control and so forth probably finds it quite easy.

Gravity is intrinsically a fairly weak force and likely to be harder to manipulate than EM fields that apply comparable acceleration. You do need to transfer a large amount of kinetic energy into the beam to make it effective, regardless of whether you use RF exciter cavities, gravitic pressors, or the outrage of honest politicians to make the things go.

Also, the only neutral particle beam I can think of which is practical would be neutrons, which present their own problems.

Given that in Wars they use repulsorlift technology to have smoothly moving doors (ref 'Truce at Bakura') on occasion I'd say they, at least, have a good chance of having that level of control.

Batman, making a particle beam work effectively is a whole different order of problem. A door is doing well if it's precise to the millimeter; with particle beams the tolerances are measured in micrometers.

There are a few of those, way out in the particle zoo; hm. What happens when you hit something with a beam of Higgs bosons?

...I truly wish I knew, now that you mention it.

In practise, charged particle beam weapons are going to be monstrously ugly and legally dubious things to use- they effectively kill people and electronics with hard radiation.

For militaries this is sort of a moot point- especially since among the obvious alternatives are nuclear-tipped missiles and X-ray lasers, neither of which is much more appealing if you're radiophobic. And when you start talking about highly energetic beams and soft-SF sized ships, it becomes very much conceivable that a particle beam could do very serious damage to a ship while sheltered compartments in the interior do *not* receive severe doses of radiation.

Neutral particle beams work, as far as I know, by accelerating a charged beam and then neutralising it at the muzzle;

The problem with that is that you don't get a collimated beam, or at least not one that holds together over long range. Imagine a weapon that relied on, say, taking the stream of bullets from a machine gun and having it ricochet off a plate. While good engineers could probably at least arrange for all the ricochets to go in the same general direction, there's no way you'd get anything like the kind of tight grouping possible with the gun itself.

The trouble I see is that, yes, while it's probably possible to build an effective particle beam weapon, it's easier to build other weapons: coilguns for instance, or missiles, or powerful lasers.

This may or may not prove true. As noted, missiles have the disadvantage of being relatively fragile and visible- targets in their own right, and effectively impossible to armor against even low-energy weapons designed to blot them out. Coilguns are surely effective but the limited accurate range issue becomes a problem if ship accelerations and/or combat ranges get high enough. Lasers are the most credible competitor because they have most of the advantages of a particle beam, while avoiding some of the disadvantages.

The most conspicuous disadvantage I can think of to lasers in near-visible wavelengths is that it's very practical to build materials that reflect or otherwise resist the effect of a laser beam operating on a known wavelength. Lasers are also more vulnerable to various types of diffusion and deflection.

There's really not much you can do to "fine-tune" your armor against a steel bar coming in at 100 km/s, and X-rays or high-relativistic protons aren't going to go away just because you painted your ship pink or whatever. Lasers are more temperamental in terms of their effectiveness against a target designed to resist them.

Hell if you want a decent plasma cannon you can always use the old casaba howitzer idea. Nuclear powered plasma cannon is highly effective.

Trouble is, Casaba Howitzer has to be moved into effective range- the range limits on shaped nuclear charges are highly classified as is the entire theory behind the weapon, but as far as I can tell it amounts to a way to ensure that the entire nuclear blast passes through a circle a couple of kilometers across, but a few hundred kilometers away. It's a flamethrower, not an antitank gun, and hardened targets have a good chance of resisting except at (for space combat) extremely short ranges.

At least, as long as you stay within the realm of what Casaba Howitzer was actually supposed to do and invoke no extra technology. Last time I had occasion to write space battles, I invoked such technology shamelessly, so whatever. Hey, it worked for Weber.

[Batman]

Given that in Wars they use repulsorlift technology to have smoothly moving doors (ref 'Truce at Bakura') on occasion I'd say they, at least, have a good chance of having that level of control.

Batman, making a particle beam work effectively is a whole different order of problem. A door is doing well if it's precise to the millimeter; with particle beams the tolerances are measured in micrometers.

Hence the 'good chance' part of the comment?

[StarSword]

There are a few of those, way out in the particle zoo; hm. What happens when you hit something with a beam of Higgs bosons?

...I truly wish I knew, now that you mention it.

Somebody later in the Star Carrier books uses meson beams, the particle that carries the nuclear strong force. You get hit by that thing and your entire atomic structure gets compacted.

You can do some really weird shit once you start moving away from basic baryons.

[biostem]

To be honest, I would love to see a Sci-Fi movie/show that moved away from the continuous beam energy weapons, and instead had extremely powerful pulsed weapons. If you can deliver the same amount of energy in a very short period of times, the per-shot yield goes up dramatically. I'd also like to see things like lasers depicted as truly lightspeed.

As for the mechanism behind the weapon itself - wasn't there a concept for a 747-based weapon that actually used chemicals to somehow produce the light or energy for the laser? I wonder if you could do one better, and simply convert the matter in a cartridge of some sort into energy, to power or provide the actual energy bolt itself. Perhaps instead of a complicated accelerator of sorts, the matter to energy conversion process provides all the acceleration you need...

[Starglider]

Somebody later in the Star Carrier books uses meson beams, the particle that carries the nuclear strong force.

Strictly the exchange particle for the strong nuclear force is the gluon, although pions exchange does bind nucleons into atomic nuclei. The damage mechanism for mesons is primarily the hard gamma from the quark/antiquark anhiliation, although the high-energy electrons won't help either. ' Atomic structure gets compacted' no.

[Starglider]

As for the mechanism behind the weapon itself - wasn't there a concept for a 747-based weapon that actually used chemicals to somehow produce the light or energy for the laser?

ABL and later YAL-1 yes.

I wonder if you could do one better, and simply convert the matter in a cartridge of some sort into energy

Sounds like the 'pure energy' fallacy, energy is not a 'thing', anhiliating matter generally produces ultra high energy photons (as in a nuclear weapon).

to power or provide the actual energy bolt itself.

There is no such thing as an 'energy bolt'. There are lasers and there are particle beams. You could use fusing plasma or bomb-pumped crystals as the lasing medium in which case you have a 'matter to energy conversion powered laser', but the photon energy is still coming from electron excitation. Using handwavium you might be able to magnetically redirect and collimate the fusion reaction products themselves into a beam, but that is a highly focused plasma rocket aka particle beam, not an 'energy bolt'.

[Eleventh Century Remnant]

Not convinced, Simon- look at it this way on the NPB, you're firing maybe ionised hydrogen, possibly 2+ state helium nuclei. Or alpha particles, if you prefer. The electrostatic grid that donates electrons to neutralise the beam perturbs the beam by what, two electron masses, maybe 1/3680~ of the total?

Yes, there is a jiggling that costs fine focus, but worse than the beam blowing itself apart? Back when I used to do this sort of thing for fun, I got a two hundred thousand kilometre range out of an NPB that only delivered about eight hundred in charged mode-
granted it would have been about three hundred thousand in atmosphere, but if you can arrange a column of sea level density air a light second long between your gun and the target on a moment's notice, you may be a bit beyond playing with second generation SDI hardware.

Relatively speaking, you're far better off with NPB in space. The other problem being that by definition you are firing radiation- you'd practically have to run the thing in reverse to get the beam down to below 200 ev. (GURPS Transhuman Space had particle beam spinal mounts, this was in around 2100 in a nano-soaked, singularity age setting, and it rated their output in rads, not in physical damage- an obsolete unit of measurement, but you were looking at reaching ld50 in about 0.4 of a second from a continuous beam weapon. For early particle beam weapons, radiation is the primary kill mechanism, not a side effect. Later on, you get more power out of the beam by firing, ah, let me see, even more radiation.)

Speaking of light seconds I seem to remember Rick Robinson rambling on about gamma ray free electron lasers, and that it actually would take a fairly respectable- kilometre diameter- cyclotron to get the electrons up to having enough energy for a gamma beam; in the plausible midfuture, of course. Your lethal range from the resulting beam would be systemic, though- at least in the light minutes, single digit AU.

And yes, meson cannon featured very heavily in old Traveller, although it was the energetic decay that was the important part- only interacting on a direct hit on the nuclei, so they pass through electron clouds, so what meson cannon did was acelerate a beam of them to a precise relativistic speed, so that they would reach the end of their lifespans and decay explosively inside the target. While this was great fun, the actual lifespan of pi- mesons is so short that I doubt they'd last long enough in the barrel to be accelerated- feasible in principle but not in practise.

[Starglider]

I would note that progress does seem to be continuing towards a literal 'nuclear laser' i.e. stimulating photon emission from a state change in the nucleus rather than the electron cloud. Surprisingly this isn't necessarily gamma radiation although that is the usual depicition in military SF. This is technically matter-to-energy conversion in the same sense that fusion is, in that the net mass of the atom decreases (slightly), but you still have the same number of bayrons that you started with.

[Simon_Jester]

To be honest, I would love to see a Sci-Fi movie/show that moved away from the continuous beam energy weapons, and instead had extremely powerful pulsed weapons. If you can deliver the same amount of energy in a very short period of times, the per-shot yield goes up dramatically. I'd also like to see things like lasers depicted as truly lightspeed.

The energy dumped into the target doesn't change, though: you can't put ten megajoules of energy into the target without a system supplying comparable energy out the muzzle of your weapon. Besides, very few SF settings in modern times focus on continuous-beam weapons, certainly not in visual media where the preference is for stuff that looks like big glowy bullets.

As for the mechanism behind the weapon itself - wasn't there a concept for a 747-based weapon that actually used chemicals to somehow produce the light or energy for the laser? I wonder if you could do one better, and simply convert the matter in a cartridge of some sort into energy, to power or provide the actual energy bolt itself. Perhaps instead of a complicated accelerator of sorts, the matter to energy conversion process provides all the acceleration you need...

See powerguns from David Drake's Hammers' Slammers setting.

Not convinced, Simon- look at it this way on the NPB, you're firing maybe ionised hydrogen, possibly 2+ state helium nuclei. Or alpha particles, if you prefer. The electrostatic grid that donates electrons to neutralise the beam perturbs the beam by what, two electron masses, maybe 1/3680~ of the total?

Yes, there is a jiggling that costs fine focus, but worse than the beam blowing itself apart? Back when I used to do this sort of thing for fun, I got a two hundred thousand kilometre range out of an NPB that only delivered about eight hundred in charged mode...[/quote]Hm. You may have me out-familiaritied. I was originally thinking of spallation neutron sources, actually, which are about as precisely directional as the word "spallation" might lead you to think given its root of "spalling." You're thinking of an entirely different mechanism that I'm unfamiliar with.

And yes, meson cannon featured very heavily in old Traveller, although it was the energetic decay that was the important part- only interacting on a direct hit on the nuclei, so they pass through electron clouds, so what meson cannon did was acelerate a beam of them to a precise relativistic speed, so that they would reach the end of their lifespans and decay explosively inside the target. While this was great fun, the actual lifespan of pi- mesons is so short that I doubt they'd last long enough in the barrel to be accelerated- feasible in principle but not in practise.

The other problem is that they won't all decay at once, but rather all along the beam track...

[Starglider]

The other problem is that they won't all decay at once, but rather all along the beam track...

Yeah that 'precise relativistic speed' thing never made any sense to me either, (very) high particle velocity will just extend the half life. If you're using charged mesons no problem, on reaching the target they will deflect and bleed energy via bremsstrahlung (both producing secondary heating in the target), bringing the decay time back down to 'almost instantaneous'. That said mesons are so hard to make and accelerate that it seems more plausible to just shoot antihydrogen plasma.

[Simon_Jester]

Hm.

Also thinking about the only setting I ever even tried to design-

Really, if you have a spaceship that expects to take fire from particle beam weapons, weapons energetic enough to actually blow holes in the hull by direct energy transfer... the radiation issue is very, very vicious. Because even tiny percentages of the energy it takes to 'vaporize spherical mass of iron' will irradiate that iron until it's something no human can live anywhere near, let alone inside.

Think about the Chernobyl wreckage; only a relatively small amount of matter melted, only about... I don't know, the closest figure I can find is 40 GJ of energy were released. And yet an insane amount of highly radioactive waste resulted, and the actual ground zero of the event is so viciously uninhabitable that you literally can't go there and survive, even for a short time, even with protective gear.

To prevent this from becoming a problem you need, essentially, magic. A means of reliably blocking radiation flux, far beyond any technology now known to man, so that you can have vast numbers of rems on one side of a barrier and zero on the other... but the barrier must be of reasonable thickness to fit in the ship.

Force fields, in other words. And you need these force fields not only on the outside of your hull, but scattered throughout your ship, dividing it into radiation-tight compartments, so that particle radiation which blows a hole in the hull and wrecks the inside of Beam Turret A won't turn the whole ship into something as useless and deadly as the inside of the Fukushima No. 1 reactor.

Come to think of it, Star Trek ships are actually fairly well adapted for handling this because they have internal forcefields, very explicitly, and are prepared to raise them in a hurry. The main site may slam them for relying so much on powered containment systems, but in this context what else could do the trick?

No radiation shielding made out of atoms could take "hull-vaporizing particle flux on one side of this wall" and turn it into "nonlethal radiation dosage on other side of this wall" unless "this wall" is something insane like... I don't know, hundreds of meters thick. A force field conceivably could.

Personally if I'm going to write soft-SF with 'vaporizing' particle weapons, I'll damn well include internal force fields as part of the ship's armor and protective scheme. But that's me.

[Starglider]

Really, if you have a spaceship that expects to take fire from particle beam weapons, weapons energetic enough to actually blow holes in the hull by direct energy transfer... the radiation issue is very, very vicious. Because even tiny percentages of the energy it takes to 'vaporize spherical mass of iron' will irradiate that iron until it's something no human can live anywhere near, let alone inside.

Not necessarily. The main culprit for turning stable materials radioactive is neutron flux; free fast neutrons and alpha particles (containing two neutrons each). High-energy gamma and beta do cause photodisintigation (and at cosmic energy levels photofission) but the amount of radioactive isotopes created per unit energy dumped into the material is vastly lower than shooting neutrons at it.

And yet an insane amount of highly radioactive waste resulted

The really high level stuff was direct fission products, the intermediate and low level stuff was from neutron activation. There won't be any fission products in particle beam targets and there won't be significant neutron activiation if you're shooting hydrogen plasma (secondary only), although there definitely will be some if you're using heavier ions for the beam. For light ions the primary radiation threat is definitely the transient hard gamma when the beam is actually impacting; for plausible linear accelerators it makes more sense to use these because you get a higher beam velocity (hence higher hit probability). If your accelerator is powerful enough to reach high relativistic velocity regardless of particle mass, or you're fighting at close range, then maybe you would use heavy ions specifically to increase neutron hazard.

[Simon_Jester]

You don't consider neutron spallation to be a serious threat, then?

Also, while 40 GJ of energy transfer from a particle beam will surely not be anywhere near as 'dirty' as 40 GJ of energy released during the Chernobyl meltdown, it's still enough to create major radiation hazards, especially in a spacecraft which you can't really evacuate because there's nowhere to run to.

Do you think it feasible to build sufficient thicknesses of material radiation shielding into a ship, that you could have particle beams burning holes in the surface without killing people in the core hull? Because that was the point I was really trying to get at- that I can't really imagine that unless you use layers of radiation shielding so heavy that, slabbed on the outside of the hull as armor, they'd basically have prevented the attack from ever mattering in the first place.

[Starglider]

You don't consider neutron spallation to be a serious threat, then?

If you mean direct spallation of neutrons by proton impact, compared to neutron capture that (a) requires much higher particle energy (to overcome the nucleus charge barrier) and (b) has a much lower reaction cross section (due to deflection and the high probability of the protons getting thermalised by electron interactions before they can impact a nucleus). Consider that the particle energy required for direct spallation is similar to the energy required to fuse the two nuclei, and fusing anything but the lightest ions is really hard with current technology (not that fusing even deuterium is easy). This isn't CERN where you can spend as long as you like slowly accelerating small bunches of particles, in a weaponised application power efficiency and beam luminosity are critical. Using heavier beam ions (alpha particles up) means they have a higher inertia-to-charge to overcome the charge barrier, but at sub-cosmic energies they're still going to have a vastly smaller cross-section than a pure neutron beam (that we see in fission reactor meltdowns).

TLDR beam energies sufficient to generate significant neutron spall mean you have disintigrated the target into fusing plasma and it is probably about to become expanding vapor anyway, especially if you are using hydrogen plasma to get the max velocity / chance of hitting the target.

it's still enough to create major radiation hazards, especially in a spacecraft which you can't really evacuate because there's nowhere to run to. Do you think it feasible to build sufficient thicknesses of material radiation shielding into a ship, that you could have particle beams burning holes in the surface without killing people in the core hull?

I'm not disputing that the radiation hazard is very serious and hence force fields are nice if you can get them. What I'm saying is that the main hazard is the transient radiation spike not the lingering emission from nuclear-activated armour. Particularly because both mechanical and biological systems can tolerate a higher net dose delivered over time than they can instantaneously. Even for nucleon spallation (spalled protons are nearly as bad at these energies as they will tear apart ionic structure), the initial surge is the worst because it occurs in a high-energy jet that will punch deep into the mass of the ship. Later radioactive decay of the armor will produce relatively low-energy omnidirectional radiation - and unlike earth-bound fallout we don't even care about the alpha & beta, because it has no chance of penetrating to anything important.