Comment by jessriedel | Hacker News Reader

Comment by jessriedel | Hacker News Reader

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> plus a huge risk of collateral damage to any life, human or otherwise, that happens to get in the path of the beam.

Why? This doesn't actually deposit that much energy directly in any given volume. The beam is about a meter wide and the power is about a kilowatt (1000 joules per second). The authors say

> This is equivalent to about 1 Sv/sec. We note that this value of the radiation dose is very large, compared with the U.S. Federal off-site limit of 1 mSv/year.

but that seems wrong. Up to a dimensionless factor Q that adjusts for biological details, a sievert (Sv) is 1 joule of radiation energy deposited per kilogram. But that neutrino beam has a mean-free-path of the diameter of the Earth! That means it is roughly distributing the kilowatt of energy over 1 m^2 * 13,000 km ~= 1e7 m^3. At the density of water, that's 1e10 kg, so the deposited power is about a 1e-7 watts per kilogram, or roughly 100 nano sieverts per second. You'd have to point the beam at someone for 3 hours straight to exceed the (conservative) federal limit.

Furthermore, assuming it becomes feasible to develop multiple neutrino beam machines, you could use a technique similar to "Gamma Knife" to reduce the radiation exposure along the beam paths (and to make it harder to detect/track) [0].

[0] https://en.wikipedia.org/wiki/Radiosurgery#Gamma_Knife

My reading of the paper shows they already factored that into the equation. So at the target, in the 1m² cross-section, 1,000 W of energy is being delivered. It takes about 50,000,000,000 W at the beam generator to achieve that.

If it's just 50 gigawatts it is potentially viable, considering the stakes of nuclear warfare. The precise locations of most stationary nuclear weapons systems are already known, though this would run into some trouble finding nukes on submarines unless they were detected by submerged drones or subullites. I'd look into it.

> most

Sounds really perilous. If even one is missed isn’t its risk of usage now astronomically higher?

Well I don't think anyone in charge is really happy about how nuclear weapons are handled these days, but generally speaking I'd say that most people don't want to use them, but they're forced into over-worrying about attack vectors because that's the nature of MAD. Even so, there are so many ways of doing so much damage to an adversary right now I doubt if a major nuclear power would actually deploy[0] offensive neutrino weapons outside of an actual war.

[0] There is a difference between use and deploy in military terminology.

Which part of the paper? I don't know why they would quote power per unit area for a beam that is clearly depositing it proportional to volume. What is the assumed depth?

The assumed depth is the diameter of Earth. That's where the 10^-7 figure comes in. Look at eq. (1). It's dividing the initial energy (1,000 TeV, or 10^15) by the diameter of Earth (10^-7) The beam energy of 1000TeV is chosen to "have approximately single interaction before the neutrino beam hits the bomb". The mechanism is for a neutrino to interact with the Earth a few meters before the bomb, creating a hadron shower that does the real work.

If you look at the equations, they're sub-scripting "E" with "dep", meaning deposited. The figure is arrived at after accounting for the 1/10^7 probability that a given neutrino in the beam actually interacts in the right zone of soil.

> The range of the neutrino is 10^7 meters and the effective neutrino interaction is restricted within a few meters away from the bomb because of the interaction range of the hadrons.

Which I'm reading as: "we're going to lose the vast majority of this energy in the Earth itself, so we need to choose an initial power level so that, by the time the beam is interacting with the dirt just under the bomb, it's delivering ≈ 1kW of power."

Thanks much

I don't know that much about the physics of neutrino radiation, or the fine details of bomb design. I do have a hard time believing that such a beam can dump enough neutron radiation into a subcritical bomb core to trigger a meltdown or fizzle detonation without also giving a dangerous exposure to any human who is in the way of that beam.

I'm also suspicious of the difficult of aiming a beam that's a meter wide at the target distance to hit a nuclear warhead core reliably halfway across the world. If you miss, how do you tell what direction you missed in and correct?

I could imagine a wider “targeting” beam, fired so that a satellite overhead of the target site could catch neutrino scatter from the warhead. I don’t know what ridiculous engineering would be required to get sufficient neutrino collection for that to work, on a satellite, but if the budget covered a 50GW neutrino beam it might stretch to that, too.

>I do have a hard time believing that such a beam can dump enough neutron radiation into a subcritical bomb core to trigger a meltdown or fizzle detonation without also giving a dangerous exposure to any human who is in the way of that beam.

Particle beams act in surprising ways. For example, proton beams are used to treat tumors because they end up dumping most of their energy in a small volume when fired into matter. This allows you to do things like delivering very high doses of radiation into a brain tumor while the tissue along the path of the beam receives a very low dose.

This occurs because at high enough energies, the protons are essentially traveling fast enough that they don't have time to really interact with anything in their way. But as they lose energy they begin to interact more, which causes them to lose more energy, making them interact even more, etc.

The difference is that protons are charged particles while neutrinos are not, but I don't find it surprising that a neutrino beam would act in a similar fashion.

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