Cosmogenic Nuclides and Dating

Posted January 29th, 2009 by admin

Last year I had really interesting lecture on this topic – this post is an altered version of what appears over on Snell-Pym– at first I thought I was going mad as it seemed to me that I had remembered fission track stuff incorrectly from my undergraduate but no it is a similar related technic and when i asked about it Peter Vermeesh said said I was trying to jump straight to the more advance applications of the subject. I have to confess I was eeek maths, difficult panic and when he stopped for the break and asked if anyone had any questions we all said no and then me and two others dashed out to get a cup of ‘tea’ which actually involved bitting knuckles and looking at each other in dispair and asking if the other two had understood – we all shook our heads and started stressing. But…. the lecture had followed us out and had a)over heard us and b) had realised he’d lost the class – he’d said it happens to him as well – infact it had happened to him at the Tuesday Seminar the night before!

We said we thought we got the concept but it was the equations that were scaring us – then he said there was only one little bit of maths – we looked at him increduosly. But I then had a cool disgussion with him about fission tracks. I can not find wikipedia link on either of these two concepts/techniques as the pages do not exist yet – ok gosmogenic nuclids are like highly charged particals from like cosmic rays – hmmm…. ok I’ll just delve into the notes I made and hope that they make scense! There are two types of cosmic rays – GCR (Galatic Cosmic Rays) which are highly energetic and come from super novas, the others are solar rays. The GCR are 38% something I’ve written as +P which will either be protons or positrons (but I normally write these as +e) or a cation of P(but I feel this option unlickely if anyone knows let me know!), they are also 3% electrons.

Now apparently the amount of energy in these +P is alot – they travel at near the speed of light and form black wholes in our upper atmosphere all the time – I think I want to see more evidence for that particular claim personally! These cosmic rays are deflected by the earths magnetic field and due to how the feild forms there is more protection/sheilding at the equators that there is at the poles – at the poles the low energy solar cosmic rays can sneak in giving us the aurora.

Even with the high energy particles there is a defernce in how many of the rays/particles can crash into the ground giving us somehting to count – this gives us geomagnetic latitudes with the effects. Just help make things more complicated there are secondary particles produced when a +P enters our atmosphere ie it hits/collids with oxygen, nitrogen molecules and the atom they hit sort of explodes into a host of secondary particles – this is called spillation (sigh there is no wikipedia article on this either 🙁 ) So from these collisions in the atmosphere we now have nuetrons,protons, alpha particles and many more – these are secondary cosmic rays and you get a comic ray cascade – sort of the priamid selling of the sub-atomic particle world.

There is a big increase in the number of nuetrons as they are the most abundent secondary particle. Of cource the secopndary particles hit other molecules and atoms in the atmosphere and so the cascade propogates through the atmosphere but the energy of the origonal cosmic ray is being spread out and there is a corrasponding – and basically exponential decrease in the energy.

This means that not many of the cosmogenic nuclids actually hit the ground – those that hit the ground are called in-situ and until recently (the 1980’s) these could not be measured! Anyway new elements are made in this way – now 99% of those will be useless for dating rocks as there will be so much of that element around on the earth you can not detect the new from old! This means we need the rarer elements and then there is all complexiaty with the expected amounts of that element in a sample and the actual amount then there is weather they are radio active etc… Then there is the fact that the cosmic rays only penitrate the very surface rocks – this is actually cool as it gives you burial and erosion rates. We ploted a graph which looks at the rates of burial and exposure – when done on a log scale it is known as the Banana Plot! (no wikipedia again :'( ) this is becuase it looks like a banana!

Anyway I got very excited about the applications of this and asked about the nuclear disastor/test factor and think I might have agreed to look for a cesium spike in one of my milk teeth – though I did get a bit too excited and started on about brizil nuts affecting the results – not sure what I was going on about there! Still I also asked about extraterrestrial material and therefore made the class late leaving yet again! But this was sooooo interesting – becuase in space there is no or little shielding the rate of rays hitting the surface is higher and you can therefore get a ‘burial’ age for say a meteorite which has landed on the earth due to the atmophere stopping most of the cosmic rays! – now this is info I can use!

Oh and I got told off for trying to solve an equation – I didn’t initially know what I was doing with the ploting of the graph – as always I thought he was asking for somehting much harder than what he actually wanted – I am awear that this is a really really bad explanation of this subject but I’m still sorting it out in my own head :/

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