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[Music]

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as has been mentioned I'm going to tell

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you a little bit more about radiation

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and I talk which is subtitled cosmic

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rays bite the dust so hopefully you'll

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see why in a bit all right I can advance

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this slide all right so in order to talk

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about radiation chemistry and radiation

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chemical processes we have to talk about

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stopping theory so what do we mean by

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stopping theory well I'll get around to

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it well kind of do its OOP circuitous

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route first we'll start with a

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historical overview in 1896 all Rebecca

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rel discovered radioactivity except a

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they didn't know that it was

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radioactivity and it wasn't called

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radioactivity what beck rail found was

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that uranium salts were messing up his

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photographic plates well he was doing

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experiments and he thought they were

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making x-rays turns out they're making

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something which were called immediately

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afterwards Becquerel rays and later in

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1998 Marie Curie and Pierre Curie

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started working on this they called it

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radioactivity and Marie Curie made the

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very astute observation that the Alpha

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rays that she was seeing were losing

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velocity as they traveled through matter

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one of the remarkable things about this

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observation is that she made it before

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we had the modern model of atomic

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structure so they had no idea what alpha

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rays were or how they could be losing

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matter they just knew that from

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experiments that's what they were doing

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all right moving a little bit forward in

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time to 1912

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JJ Thompson discovers the electron or he

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discovered the electron previously and

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then in that year he wrote the first

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theoretical paper on the how charged

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particles lose energy now the field has

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evolved a bit but we still use a lot of

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and in 1913

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niels bohr wrote a paper with the very

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short title on the decrease of velocity

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of moving electrified particles on

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passing through matter and he swiftly

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followed it up with a paper with the

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radically different title on the

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decrease of velocity of swiftly moving

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electrified particles in passing through

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matter now these were landmark papers

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they sort of helped make his career and

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we still use a lot of the theory that he

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developed in talking about how charged

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particles lose energy here's one of the

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things which we still use a lot and is

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to divide the types of collisions which

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a charged particle and

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counters on moving through matter into

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two main categories one of them which

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one is the laser

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there's laser conveniently colored and

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we call nuclear or elastic collisions

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and this is when the charged particle is

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losing energy to the nuclei in what we

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call the target the other is called

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electronic or inelastic collisions and

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this is when the charged particle is

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losing energy to the electrons so this

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comes from Bohr and we still use it so

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finally in our historical overview in

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1932 Hans bethe applied the very latest

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in quantum mechanics and relativity to

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stopping theory and the bethe

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formula for stopping power is still

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widely used oh okay now that works right

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so now that I've told you about how the

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theory of stopping power has developed

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I'm going to tell you what it looks like

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when you put all the physical processes

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together and so you can divide the

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timeline of what happens when a particle

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is moving through some matter into three

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main stages so in this first stage the

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physical stage molecules are ionized and

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excited

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this is very fast and so what happens is

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you produce a lot of charged species and

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species in electronically excited States

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which go on in the physicochemical stage

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to recombine electronically so charges

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are neutralized and this results in

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dissociations and then some of the

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excited species can also dissociate when

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you're talking about radiation and

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solids species on or near the surface

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can also get kicked off

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process called sputtering which is very

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complicated so in the physicochemical

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stage what you're left with are a bunch

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of really reactive fragments and

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radicals and they can do chemistry in

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the chemical stage so you'll notice this

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is the longest it's also been the

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hardest to simulate but in this case

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what happens is at least in solids the

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species that you've made you can sort of

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move around as you saw in Brandon's talk

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and if they encounter one another and

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can react they can do so and form

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strange new molecules alright so here's

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what it looks like all together now

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for reasons which will become hopefully

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obvious later on imagine that this blue

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block is an o2 ice and you've got a

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proton coming in and what happens well

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as I mentioned during the physical stage

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it can ionize species or excite them and

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these electrons produced in the

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ionizations are referred to as secondary

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electrons confusingly here electron a is

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called a first generation secondary

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electron and if it has enough energy it

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can go on and further ini species

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creating also confusingly what's known

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as a second generation secondary

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electron and these can excite species as

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well now when they lose enough energy

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through collisions these secondary

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electrons enter what's called the sub

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excitation regime and can do strange and

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bizarre things like where you can have a

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for evey electron breaking a 5e v bond

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thanks to the magic of potential

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surfaces all right so that's track

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structure now I'm going to tell you a

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little bit about a new model I wrote

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which can do the physical the

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physicochemical and the chemical stages

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of irradiation is called serous serous

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it's not Chris that would be a little

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bit too vain it stands for the chemistry

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of ionizing radiation and solids as you

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can see I could have called it Chris but

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that would have been a little bit too

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too rich so it's the microscopic Monte

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Carlo model which as far as I know is

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the first one which is able to combine

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the atomic physics calculations again

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and the chemistry they're able to do

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that long

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cool phase as well where you have things

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moving around and so again we use the

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kinetic Monte Carlo method for that

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here's the paper recently came out where

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I described it and just as a quick side

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note the model was written in Fortran is

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a matter of local pride that I have to

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mention that in 1956 the Fortran project

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at IBM was headed by this fellow John

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Backus who was a UVA chemistry undergrad

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just a little bit of Fortran trivia

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alright back to the science so in trying

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to simulate tracks my initial attempts

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were not very impressive so what do we

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have here well imagine you have an o2

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ice and you have a proton entering right

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here it's moving through and randomly

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since this is a Monte Carlo model it's

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having collisions and so these points

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represent either elastic collisions or

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inelastic collisions or which can be

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ionization of excitations so that's nice

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and I was certainly very happy when my

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model started to do something I wanted

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it to do but it's not very impressive so

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you may be slightly more impressed with

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what we can do currently if I can get

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uh-huh okay there you go all right so so

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here's what we have now circa 2017 what

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we have is similar to the first plot I

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showed you I'll replay that you have a

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particle coming in here a proton moving

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through your o2 ice except now you can

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see all of the secondary electron tracks

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so these meander around randomly and do

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those excitation and ionization 's and

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in those funny things they do and they

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lose enough energy so we can do a little

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bit better today all right back to

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theorists are the worst at using

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technology sometimes alright so yes so

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there it is so now I'll finally tell you

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why I've been talking about Oh to ice

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and that's because chemically for an

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irradiation for an irradiated system

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it's about as simple as you can get so

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we found this experiment it we wanted to

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simulate a well constrained experiment

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rather than the fairly well

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unconstrained interstellar medium and so

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we found this one which was done here at

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UVA

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that wasn't a requirement but it was

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certainly a perk where OSA our oxygen

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ice was irradiated with 100 kV protons

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and ozone was synthesized that the ice

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was kept at 5 Kelvin under ultra-high

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vacuum and was about 10 microns thick

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keep this number in the back of your

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mind all right so here's our $64,000

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question once we have the tracks could

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we combine it with the chemical model

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and actually do chemistry this was the

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part which had never been done before so

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let's talk about our chemical model a

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Monte Carlo model is a little bit like

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waiting in line to board a plane it's

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like the snow rate and a game of

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three-dimensional chess all right so how

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is it like a game of three-dimensional

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chess well this is just a more detailed

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version of what you saw earlier in

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Brandon's talk you can have imagine this

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is looking down from an aliens eye view

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on the surface of an ice you can have

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things coming in from the gas phase onto

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the grain dis orbing or hopping around

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and again if they encounter a co

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reactant they can react and then imagine

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this being sort of a dull house top to

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bottom slice and the yellow the blue

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squares represent lattice sites we would

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call them normal sites and the yellow

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squares are interstitial sites and these

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enable diffusion through out the solid

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so as you see here things can move

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around and react when they encounter a

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co reactant that's just a simple picture

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of how it's like a three-dimensional

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chess game so putting it all together

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here's the basic algorithm of the cirrus

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model so when you start your program the

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first thing which happens is it gets

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smacked with the pro

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on and so you calculate the track then

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you evaluate your ending condition and

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if the model isn't over yet you go back

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and figure out what the next thing is

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since it's a Monte Carlo model

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everything has to wait for everything

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else only one thing can happen at a time

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and you just keep going through this

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loop until you reach your exit condition

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but now when a neutral species hops this

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is where the chemistry happens because

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as I mentioned if it encounters

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something it can react with you form a

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new product so we have the chemistry

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over here we have the track calculations

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over here it combines all together into

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a model which can do some cool stuff all

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right

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so chemistry as I mentioned the o2

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system you can think of it as having a

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fairly simple chemistry you have your o2

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getting broken apart by X here meaning

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radiation and two two oxygen atoms those

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oxygen atoms can combine with molecular

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oxygen to form ozone stable stabilized

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by a third body the ozone can get broken

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apart and can also react with oxygen so

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it looks like a fairly simple system but

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now this is from the point of view of a

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typical rate equations type chemical

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model what does it look like from the

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point of view of a detailed Monte Carlo

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based model where you're dealing with

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excited species and ionized species so

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here is the reactions table from our

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paper we have more than an order of

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magnitude more reactions than that

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simple system that I showed you before

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we have as I mentioned we have ions we

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have excited species and these become

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very useful in actually doing the

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chemistry because you have to make a lot

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of approximations in this sort of regime

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so I beating around the bush but here

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are our final results here is ozone

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concentration here's fluids fluence just

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means how many particles you smacked

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into your solid and you can see in our

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blue are the experimental data in yellow

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we have our standard model results and

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you can see we get the steady state

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chemists Bundys of ozone early well so

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we were happy about this but we noticed

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we were at higher fluencies than the

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experiment so we thought about it a

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little bit

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and realized oh it's probably because

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what we're doing this is a thinner ice

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than the experiment actually used so

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you're depositing less energy per

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particle into it and making less ozone

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per particle so this was a point one

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micron ice we simulated the effects of

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having a 1 and 10 micron ice and the 10

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micron ice reproduced the experimental

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data very nicely and as you remember

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from the earlier slide the experimental

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ice was about 10 microns so not only do

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we predict the steady-state abundance

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but we also are able to roughly predict

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the thickness of the ice so we are very

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happy about that all right so with that

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I'd like to thank my advisor former

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fellow group members collaborators and

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I'll leave you with this last slide from

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this may be a very nice question but so

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in your we had your proton that hits

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your eyes we have all these tracks they

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seem to have the same width I was

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wondering if you have more energy when

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you come in you have more secondary

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electrons and we will be larger at the

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beginning and yell again yeah that's a

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good question

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so the will go back so there's a funny

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relationship where the higher the energy

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of the primary particle coming in the

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smaller the cross-section so the fewer

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secondary electrons they're going to

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make now the sort of width of this is

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actually determined by the average

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secondary electron energy now you can

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get very energetic electrons coming off

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this as well those are called delta rays

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and so since this is random you see some

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of them don't go very far some of them

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have more energy so since this is a

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stochastic model you get the luck of the

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draw but this is as it is in nature as

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well it's based on the average electron

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energy which is sort of a skewed

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Gaussian type distribution all right

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Chris I got a question for you cool so

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can you do this with a water ice yes

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there's a so we just that's one of the

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next things that we're looking to do is

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water ice because clearly water ice is

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the most astrophysically relevant ice

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and so the reason why we didn't want to

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do this water ice first is because

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hydrogen's make everything complicated

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and Oh two is relatively simple as you

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saw at least in the rate equations type

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approach so that's the next step

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my follow-up is what happens when you

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add desk to when you add us to so you

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mean a physical surface underneath the

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ice with more stopping power with more

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stopping power so what happens is there

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are some cases where because the dust

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grains are just on average point 1

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microns in diameter you're not going to

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lose a lot of energy if you have say 100

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kV proton you can try

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through the dusk rain come out the other

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side and sort of them bar the other side

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of the ice so there will be cases where

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the cosmic ray will bite the dust and in

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those cases you can actually completely

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obliterate your grain and just blast

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everything off into the gas phase which

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is an interesting thing which I am going

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to be looking at in the future at some

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point but yeah generally speaking the

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cosmic rays will just zip right through

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the dust grain they also have questions

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all right that case let's thank Chris