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

[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.