# Does cosmic radiation have a predominant 'direction of origin'?

Considering some speculative designs, and am going through radiation-shielding options. It seems that, other than during solar flare activity, the main thing to protect against on a day-to-day basis is cosmic radiation.

Now I know about water / hydrogen rich plastics being the ideal shielding material, but it's the location that I'm working on. And I wondered if there was any need/benefit to focusing the protection layers in one place - eg. if the cosmic radiation predominantly comes from the galactic core, then you could get away with having most of your shielding on the 'side' that faces that direction.

Was just a thought. I've seen conflicting data on this, none of which seemed all that reliable, so thought I'd ask as I'm sure someone knows of an actual paper that would help :)

• This is highly relevant to Space Exploration SE but it's possible that there will be some helpful answers in Astronomy SE as well. It might be a good idea to search there for anything helpful.
– uhoh
May 24, 2021 at 1:17
• Very true, but unfortunately I don't think I can cross-post and I had used this SE before for other questions. I'll go take a look over there though. May 24, 2021 at 1:52
• Ya I would say the question definitely belongs here due to it's relevance to crewed spaceflight and spacecraft and mission design. One possible strategy would be to as a purely astronomy question there: "Angular distribution of cosmic rays between 1 and 2 AU above 50 MeV?" and ask instead hear "Have plans for shielding astronauts on deep space missions ever considered or addressed possible directionality of cosmic rays?". These could be asked in parallel in the two sites and can/should even link to each other. It's what I would do but I'm a particularly active question-poster :-)
– uhoh
May 24, 2021 at 1:57
• Where is this craft intended to operate? LEO, near Earth but outside the Van Allen belts, inner solar system, outer solar system? In most of the solar system, the main source of particles is the Sun. There's some relevant info at physics.stackexchange.com/q/556945/123208 May 24, 2021 at 10:10
• Is that accurate? CMEs and such cause the short-term but high-intensity radiation spikes, but the cosmic rays from outside the solar system are the constant threat (especially as their interactions with the hull of a ship can cause sudden bursts of high-energy radiation). May 24, 2021 at 17:19

Cosmic ray directions are somewhat anisotropic, but I think not enough that you'd be able to exploit this for shielding.

Here are the slides of a presentation which quotes an observed anisotropy of about $$10^{-3}$$. Here is a paper with some plots using data from IceCube and IceTop, which again shows anisotropies $$\sim 10^{-3}$$. Searching for 'cosmic ray anisotropy' will find a lot of results: I've just picked a couple here.

If you're thinking about shielding, then you also need to consider whether the object you are shielding ever might rotate with respect to an inertial frame, and deal with that it if will. Chances are if it's a spacecraft or attached to planet it will (but if it's attached to a planet you don't need to worry about the stuff coming up from underneath you so much).

Although these results apply to high-energy cosmic rays, the isotropy of lower energy cosmic rays tends to be higher. That's because their gyroradius goes down as their energy goes down, so wherever they started from their direction gets essentially randomised. From these lecture notes, a proton with energy $$\approx 1\,\mathrm{TeV}$$, and given a magnetic field of about $$10^{-4}\,\mathrm{G}$$ in the local interplanetary medium, the gyroradius is about $$20\,\mathrm{au}$$ (the radius of the orbit of Uranus is $$\approx 20\,\mathrm{au}$$). So only protons with energies significantly greater than that can maintain their direction once they enter the Solar system. From these lecture notes:

Observations of cosmic rays show that the arrival directions are relatively isotropic, and in fact the lower the energy (down to $$10^{12}\,\mathrm{eV}$$) the more isotropic the distribution of cosmic ray directions.

• These are for extremely high energy cosmic rays, 100's of TeV, which makes them less relevant because they are so relativistic that they are 'minimum ionizing particles though they do cause spallation. I'm not sure these isotropic super high energy particles are the ones most likely to cause health effects but I'm not sure. 1 GeV down to 10's of MeV will make it through a cabin wall and ionize like crazy at lower energy. Is it possible cite work that single out energies most likely to cause trouble?
– uhoh
May 24, 2021 at 9:35
• @uhoh: I've added a note: low-energy particles have relatively small gyroradii so where they appear from is mostly unrelated to where they came from.
– user21103
May 24, 2021 at 11:40
• Yes, but that's kinds my point; magnetic fields do have a well-defined direction, so the directionality of the protons that matter might deviate a lot more than 1E-03 from random. I don't think that that means there's going to be a magic place to put shielding though; your conclusions are certainly fine.
– uhoh
May 24, 2021 at 12:38
• @uhoh: they don't travel along the magnetic field lines: they spiral in some complex way. Quoting from the lecture notes: 'Observations of cosmic rays show that the arrival directions are relatively isotropic, and in fact the lower the energy (down to $10^{12}\,\mathrm{eV}$) the more isotropic the distribution of cosmic ray directions'.
– user21103
May 24, 2021 at 13:08
• An oriented field will not scramble their directions, it will not make them random or isotropic. Instead you may see quite a substantial difference in the flux per unit solid angle perpendicular to the field lines vs parallel to it. It could be a substantial anisotropy, and possibly the perpendicular flux per unit solid angle will be larger perpendicular to it which means you may have two minima along the direction of the field lines. It's a complicated problem because the field in the solar system is also affected by solar wind.
– uhoh
May 24, 2021 at 18:47