Why aren't all Cosmic Ray energies 'effective'?

[ChrisVer]

Pouring a drop of some liquid into the ocean, you don't expect the interactions of other particles with that ocean to change... Just because you get a tail that spreads out doesn't change the composition of the atmospheric gases much... Again I find it a weird topic to discuss, since we are not that much interested into the target (what the cosmic rays strike), since most of those highly energetic strikes will lead to a hadronization (pions, Kaons etc) and so an electromagnetic shower (muons, electrons, gamma rays).
What we care about is identifying those showers and reconstructing the initial UHECR particle's characteristics. For example in the Cherenkov radiation method, they say that the Cherenkov radiation yield signal depends on the initial particle's atomic number squared $Z^2$.

 

[mfb]

You do have to keep in mind that meteors are not just small static dots in the atmosphere for in-flying High-Energy Cosmic rays to hit with a very very unlikely chance; the fact is that they do cover a lot of ground and evaporate / spreading out and leaving a trail which increases the chance of interaction significantly.

It does not matter. Even if with perfectly uniform distribution, and even if all their components stay in the atmosphere for one day for whatever reason, the mass of the atmosphere is 50000000000000 times larger. The chance to hit one atom from a meteorite instead of one out of 50000000000000 in the atmosphere is about 1/50000000000000.

 

[Michel_vdg]

It does not matter. Even if with perfectly uniform distribution, and even if all their components stay in the atmosphere for one day for whatever reason, the mass of the atmosphere is 50000000000000 times larger. The chance to hit one atom from a meteorite instead of one out of 50000000000000 in the atmosphere is about 1/50000000000000.

That's a comparison that doesn't make a lot of sense. Every cosmic ray has a chance of 1/1 to collide with a particle that makes up the atmosphere. If you now have 1 or 1 billion of cosmic rays, the chance stays the same 1/1. Now let's say you have 1 million of iron nuclei vs. 1, than the chance already becomes 1/50.000.000 that one Iron nucleus hits an other iron nucleus coming from a meteoric dust particle.

Next you not only need to look at mass but more at surface as an atmosphere is layer upon layer of particles, and Cosmic Rays shoot right through top to bottom (for those that are being observed), so you should mainly look at the upper layer surface where the particles are the lightest (low mass) and the meteors are the most spread out.

 

[Michel_vdg]

What we care about is identifying those showers and reconstructing the initial UHECR particle's characteristics. For example in the Cherenkov radiation method, they say that the Cherenkov radiation yield signal depends on the initial particle's atomic number squared $Z^2$.

True. What surprises me a little is that as a 'showroom model' they use the spike of a 100 TeV cosmic ray iron nucleus, while one might think that much faster cosmic rays would have been obsereved, because with it's ~4 Tev / proton it isn't something very exceptional.

 

[ChrisVer]

Again I am not sure, but I think that the iron nuclei don't appear at very large energies...
https://alteaspace.wordpress.com/2011/11/27/galactic-cosmic-rays-gcr/
I am talking about a figure as the second above. I am not sure if it's exceptional, but its flux is extremely small. Again I don't remember pretty well, but iron nuclei in cosmic rays, originate/are accelerated by supernovae, and so they have a threshold of energies they can reach. For the even higher energies we are not sure about the source mechanisms and there is a large literature dealing with different candidates.

Also I think you have a misconception when it comes to particle interactions.
The # of interaction events depends on the cross section (probability of a given interaction) times a factor that we call integrated luminosity. The integrated luminosity in this case depends on the incoming flux (which is fixed for every such kind of interaction=flux of incoming cosmic rays) and the density of the target...
The # of events for hitting an iron will get suppressed by the factor mfb wrote (1/500more zeroes) because the particular density of the iron in the whole atmosphere is very low. Nobody says that this can't happen, but it's very unlikely to happen by the factor mfb wrote down = you get very few events... if for example you have 1billion events in total in the atmosphere, you will have less than 1 of them coming from hitting an iron nucleus.

 

[mfb]

@Michel_vdg: Every cosmic ray hits one nucleus of the atmosphere at random (the collision products then hit other nuclei and so on, but the initial collision is a single nucleus). The chance to hit a nucleus of a specific type is (approximately) proportional to the amount of nuclei in the atmosphere. Out of 50000000000000 atoms in the atmosphere, about 39000000000000 are nitrogen, 10000000000000 are oxygen, 500000000000 are Argon, ..., and 1 atom is from a meteorite. The chance that a specific high-energetic particle from space hits an atom from a meteorite instead of one of the 50000000000000 atoms from the remaining atmosphere is tiny.
Sure, given the large number of cosmic rays, it happens sometimes, but the rate is completely negligible.

 

[Michel_vdg]

Again I am not sure, but I think that the iron nuclei don't appear at very large energies...
https://alteaspace.wordpress.com/2011/11/27/galactic-cosmic-rays-gcr/

According to the graph on the linked-to page it seems to be the same; protons, alpha particles (helium nuclei), electrons, Carbon nuclei and Iron nuclei (Fe) on the inside, they are only at each stage exponentionally less dense:

... if for example you have 1billion events in total in the atmosphere, you will have less than 1 of them coming from hitting an iron nucleus.

Yes that's very rare but so are UHECR's in general:

"These particles are extremely rare; between 2004 and 2007, the initial runs of the Pierre Auger Observatory detected 27 events with estimated arrival energies above 5.7×10¹⁹ eV, i.e., about one such event every four weeks in the 3000 km² area surveyed by the observatory." - Wiki

The question of course is how much collisions does the observatory process, billions ... I guess they only look for those above a certain threshold generating enough Cherenkov radiation ...

Anyway I think they would specifically say if it there are iron on iron collisions, and like 'mfb' already pointed out it's not really mentioned. I guess it is a difficult field of research with a limited amount of precision, measuring collisions up to 50 km in the sky that are initially happening at the nano meter scale.

 

[ChrisVer]

Yes that's very rare but so are UHECR's in general:

Then even worse, since the flux is itself low... but still when you are looking for UHECR your flux is fixed (either they hit the A nucleus in the atmosphere or the iron, the flux is still the same). What that means is that the rate of events is low by itself, and asking for a particular struke target which is very "rare", will make the events you want to look into even less...

My "billion" number was just an example.

There is no need to mention the iron target [I don't think they have to mention the target at all]... it's totally negligible and it's not a difficult field of research... I'd call it a waste of money research (even if you had the extreme sci-fi technology to distinguish the event)...because there's nothing to study out of it that can help you understand the nature, composition and different characteristics of the UHECR.

 

[Michel_vdg]

There is no need to mention the iron target [I don't think they have to mention the target at all]... it's totally negligible and it's not a difficult field of research... I'd call it a waste of money research (even if you had the extreme sci-fi technology to distinguish the event)...because there's nothing to study out of it that can help you understand the nature, composition and different characteristics of the UHECR.

That's like saying that the LHC is useless because we don't learn anything about the characteristics of protons. Of course there's little use for that, the importance here is the interaction between the different sorts of Cosmic rays and the different targets. As an example Parity Violation was discovered by shooting Beta rays at a cobalt target; or even more basic the nucleus was discovered by shooting Alpha rays at a metal target. Ray and target go hand in hand.

 

[ChrisVer]

There is some difference between in-laboratory experiments and cosmic rays ones...Otherwise we wouldn't need accelerators since we have the cosmic ray interactions.
One big difference is that in the lab we have a pretty good picture of what is happening...

 

[mfb]

There is absolutely no way to find and identify an iron/iron collisions in 10^13 iron/nitrogen collisions. Also, you would have to cover a significant fraction of the surface of Earth with detectors to get 10^13 recorded events in the first place.
Separating the small rate of iron/nitrogen collisions from proton/nitrogen collisions is hard enough and those are way easier to distinguish.

 

[Michel_vdg]

There is absolutely no way to find and identify an iron/iron collisions in 10^13 iron/nitrogen collisions. Also, you would have to cover a significant fraction of the surface of Earth with detectors to get 10^13 recorded events in the first place.

The problem is indeed collecting enough data, but one has to be careful with saying 'absolutely no way', we can now discover exoplanets which was also unthinkable years ago.

Separating the small rate of iron/nitrogen collisions from proton/nitrogen collisions is hard enough and those are way easier to distinguish.

Mh, according to this article in Nature it shouldn't be too hard:

Cosmic-ray theory unravels

"... they are seeing small air showers that are indicative of iron nuclei, rather than the larger showers that point to protons."

and

"The group revealed new data that weaken the link between the high-energy particles and the AGN (active galactic nuclei) ... the team has found evidence that these highest-energy cosmic rays might be iron nuclei, rather than the protons that make up most cosmic rays."

 

[mfb]

The problem is indeed collecting enough data, but one has to be careful with saying 'absolutely no way', we can now discover exoplanets which was also unthinkable years ago.

It was not unthinkable years ago. The idea is as old as the insight that our sun is a star like others, and the two most successful methods of today were proposed decades ago.
This is orders of magnitude easier than finding one event in 10¹³. We are doing this for rare Higgs decays at the LHC, for example, I know how hard it is. And we have the detector all around the interaction point, and a Higgs decay looks completely different from most of the other collisions.

Mh, according to this article in Nature it shouldn't be too hard:

Cosmic-ray theory unravels

If by "not too hard", you mean 10 or more work years: yes sure, it is not too hard.
And the result is still "just" a statistical evidence. Given all events, they can tell that some come from heavy nuclei, but they cannot say "this event was an iron nucleus for sure".

 

[Michel_vdg]

That's all true.

An interesting article I found was this one:

How Astrophysicists Are Turning The Entire Moon Into A Cosmic Ray Detector
The $1.5 billion plan breaks ground in 2018 and should be complete by 2025
https://medium.com/the-physics-arxi...-moon-into-a-cosmic-ray-detector-6dd20a6acc62

"These cascades also generate another signal. The rapid acceleration and deceleration of charged particles produces radio waves. So another signature of the impact of an ultra-high energy cosmic ray is a brief burst of radio waves, known as the Askaryan effect after the Soviet-American physicist who proposed it in the early 1960s.

It is this signal that astronomers hope to pick up from the Moon. The idea is that ultrahigh energy cosmic rays should smash into the lunar surface generating a cascade of other particles and a short burst of radio waves less than a nanosecond long."

--

After that they can fly to the moon and dig up the collision sites.