Shooting an electron beam through air

today on applied science i want to show you one of the coolest things i've seen in a long time with this apparatus it can accelerate electrons and shoot them out through a special aperture into air and we can see the beam there and then also we can put things in front of it like a phosphor screen this one came out of an oscilloscope and actually see the effect working just like a cathode ray tube but it's in air so in today's video i'm going to show you what this special aperture is that allows this to happen how i built the thing and industrial uses for electron beams in air you might be surprised but in your house there's probably multiple items that were manufactured by being irradiated with electrons let's talk about how this thing is built we're going to create the electrons in the conventional way in a vacuum area so we need a pretty good vacuum setup so i have a turbo molecular pump here this cross fitting a vacuum gauge up here some electrical pass-through back here and then here's the business end of this thing in this glass tube i've got this convenient swift seal here so you can kind of unclamp it and then just pull the glass tube out and there's an o-ring that seals inside there and then as you can see my filament that's actually boiling off the electrons is just a tiny little light bulb that i've crushed the glass envelope and i soldered its base to a piece of brass and so that when the brass is inserted into the tube like this it fits pretty tightly and centers the filament in the tube so we'll just hang him here and then the glass tube i cut on the lathe with a diamond wheel just so i'd get a very smooth consistent edge and the other end is made from a piece of aluminum which i also cut off on the lathe and this piece has to be very flat to interface with this special aperture i used hysol 1c which is a special kind of epoxy that's made for high vacuum applications it's actually really high quality epoxy that you can use for all kinds of things and it's i also bought it on amazon it's about 15 i think so let's talk about this special aperture what is it about this thing that allows electrons to come through let me show you and here it is believe it or not it's an off-the-shelf part that i bought from ted pella and they are specifically made to allow x-rays and electrons through it's a window and it's a super super thin window it's almost the thinnest window that you can get and the reason i say that is because these are only 100 nanometers thick which is like what 500 atoms thick so i've seen these windows available in thinner versions i think you can get them down to maybe 30 nanometers but eventually you just kind of run out of atoms and you can't make it any thinner so this is really getting down to that level and as you can imagine it's very fragile you can't touch them or anything but they are strong enough to hold vacuum so 100 nanometers thick with a one by one millimeter aperture which is what these are are strong enough to hold vacuum forces in other words we can have vacuum on one side and one atmosphere of pressure on the other and they don't break but they're getting close to it you probably couldn't have two or three atmospheres of pressure you'd be surprised i mean silicon nitride is like a glassy or a crystal material and it's weird to think how much these things bow under under the pressures of atmospheric you know air pushing against it but they do hold together and it does work although you can see i only have eight left so i actually did destroy one of these unfortunately not on camera but it wasn't that spectacular of a failure anyway i i started the voltage before i was fully pumped down and there was a plasma formed in the tube and for some reason the plasma caused the window to pop and you know obviously a lost vacuum so i did learn that exposing the turbo molecular pump to air at least through a one by one millimeter aperture does not kill it so at least that's a existence proof that doing that does not mean instant death for your turbo although it's probably not good for it in the long term one other trick if you decide to do this the the apertures have a flat side and a etched side and i've heard that the etched side could go towards vacuum and the flat side should go towards your atmospheric pressure this was about 140 dollars for the 10 apertures so 14 bucks a piece is actually quite reasonable for something like this here's a schematic of how this thing is built we've got our glass tube here our aluminum plate here and the filament from the crushed light bulb here and in today's setup i'm going to put the plate at plus 25 kilovolts and the filament at zero and there's about a two volt 2 volts across the filament to heat it up and then we've got our aperture here so the question is how do we know 100 nanometers is thin enough or thick enough for this aperture well it all comes down to how fast the electrons are going how dense the material is in the aperture and how thick it is so for an arbitrary thickness we could just speed the electrons up and force them through if you have time on the large hadron collider a lot of things look transparent at those velocities because the particles are just so powerful they just force their way through anything so the thing that limits us in the home shop is our acceleration voltage without a rf powered linear accelerator or something like that we're going to be limited to you know the tens of kilovolts range and in that range the materials have to be well under a few microns for this to work here's a couple rules of thumb just to get a sort of an idea of what materials will do to an electron beam so if we had a 1 mega electron volt beam which is relatively high power the distance that electrons will travel in air is about 3.5 meters and in water about 4.2 millimeters and so on as you can see for some other materials here but today we're working at 25 kilo electron volts and so in air the distance is about eight millimeters which yeah that actually agrees almost perfectly with uh experimental results that i got and you can see some other results for different materials so silicon nitride is maybe a little bit more dense than aluminum so it should go maybe 30 microns in silicon nitride so in theory this window could be tens of microns or 10 microns and still work but there's another consideration if the window is catching let's say half the beam energy that's coming out here it might absorb so much energy that it destroys itself so even though our you know system requirements might be okay like if we want a beam that's two watts and we have an emitter that's doing four watts that would meet our challenges from an engineering point of view but if the window is catching two watts and it's only a micron thick or whatever it might destroy itself just due to so much energy being absorbed so the window actually has to be super super thin just so it allows all those electrons through and doesn't end up catching so much energy so you might also be wondering you know with all these electrons flying around in the high vacuum is this thing producing x-rays yeah you betcha i mean it's built exactly like an x-ray tube the thing is today we're going to be using a supply that's maxes out at 300 microamps so it's significantly less powerful than a medical imaging tube and also we don't have any hard metal targets out here so this is aluminum if it were tungsten or something we'd be producing much harder x-rays here but yeah keep in mind it produces a lot of x-rays now having said that the intensity of the electron beam coming out here might blow your mind compared to other beta sources so here's the little beta source that i have this is strontium 90. and it says on here it's hard to see it's actually 0.1 micro curies of activity and by definition one curie is 3.7 times 10 becquerels which is decays per second and i think for a lot of materials this is essentially how many particles are coming out or the maximum number that could possibly be coming out for this analysis if it's close it's not going to matter so we also know that we know how many electrons are in a coulomb and we know that an amp is one coulomb per second so let's just say we were only running our tube here at 100 microamps that ends up being 6 times 10 to the 14 electrons per second and so it's four orders of magnitude more than one curie of beta decays or beta particles coming out of a radioactive material but this check source is 0.1 micro curies so another seven orders of magnitude so putting it all together our beta source here is 11 orders of magnitude more plentiful in electrons than what's coming out of this check source here and even compared to those little tritium vials that you get those fluorescent tritium vials even those typically have one curie or less of tritium gas in there and again we're four orders of magnitude already at 100 microamps it's pretty crazy um one other consideration is that the speed of the electrons coming out of this strontium 90 source or in the mega electron volt range while we're in the 25 kilo electron volt range so not exactly comparable but still it's kind of cool to be able to build something that's actually significantly more powerful than a radioactive source and it's also at the same time safer in a way because you can just turn the power off and throw all this in the trash and that's totally safe you don't have to create or transport or dispose of any special radioactive materials in here and this is actually why industrially they use electron beams to irradiate things again because it's safe as soon as you turn it off it doesn't activate things it doesn't cause things to become radioactive it just produces this very controllable source of radiation that has really good known penetration depths so as we've seen here even 100 microns of water is enough to completely stop our 25 kilo electron volt electrons and sure enough even a post-it note a thin piece of paper is more than enough to stop the beam in air so that makes sense it's good that everything's lining up here i decided to try a few other things too that might be interesting i put a magnet behind the screen thinking that that would deflect the spot a little bit and i think it's just not enough distance basically to see it i think there was a teeny effect but it was really hard to see so that one wasn't too interesting and then i started looking around the shop for other objects that would be interesting you know that would glow in an electron beam so i tried three different glow powders that i attached to a piece of packing tape and this worked out really well they were even phosphorescent which is pretty cool and then the next thing i found that's cool is this manganese doped piece of calcite so this is a naturally occurring material and it has this nice orange phosphorescence that's triggered both by ultraviolet light and by electrons and so you should definitely check out this video i'm going to link in the description to see what happens to a piece of calcite when you shoot it with a mega electron volt beam at high currents the electrons actually stay in the material and cause it to keep fluorescing for a long time it's really quite something and then i thought i had a really good one worked out i've got some homemade aerogel and i thought putting that in the beam would be absolutely spectacular because there would be a deep penetration and there might be like you know lightning discharges kind of throughout the material unfortunately none of this happened and all we can see is just the beam of incandescent light coming through the window and shining through there so that one didn't turn out so good i also tried some other materials like ruby and silicon carbide and did not get any sort of a electroluminescent effect out of these and so finally what i wanted to do is make these lichtenberg figures and so you've probably seen these cool paperweights where it looks like there's lightning trapped in plastic and the way they make these is to send a chunk of plain old acrylic through a particle accelerator through an electron beam and the electrons get trapped deep in the deep in the plastic and then when they discharge it on purpose with like a sharp metal grounded rod all of this charge you know exits the material and you end up with this destructive kind of lightning within the material and i knew that i was kind of on the edge here as we can see in plastic you know we're only getting maybe 90 microns of penetration but i still thought it would work unfortunately it didn't so i left the acrylic in front of the beam for a little while and then came over when onto a grounded plate with a grounded screwdriver and tapped it with a hammer and i couldn't see anything at all so unfortunately that one didn't work out so i mentioned that cancer treatments are actually something that makes use of this limited penetration depth if there's a tumor just below the skin it's actually better to use electron radiation beta rays as opposed to gamma because you don't want the energy to go any further it's actually nice to have this controllable depth and similarly they use electron beams to sterilize medical instruments like face masks for example and it's great because you can adjust the power of the beam and like i say as soon as you turn it off everything is safe so it's better than handling and storing radioactive material but there's a couple other applications that i think you might find surprising a very large amount of pvc wire insulation is cross-linked with beta radiation and it makes it more fireproof apparently and similarly heat shrink tubing is made with beta radiation i think they stretch the plastic and then irradiate it and that causes it to go back to its original shape after you've warmed it up that whole process i think is enabled by beta radiation and similarly a lot of things are cross-linked so you can buy paints that are cured by beta radiation again that's great because you don't have to have a volatile organic compound in the paint you can spray it without any worry of hurting the atmosphere you just shine your beta rays on there and the beta rays penetrate through the paints pigments which ultraviolet light cannot and so then you can cure opaque paints with this method pretty cool i'll link to a paper that details 10 or 20 other industrial processes you might be surprised where this thing crops up so anyway give me suggestions for what you want me to put in the beam other than my finger of course and we'll put that together and it'll come up in a future video okay hope you found it interesting and i will see you next time bye you

Channel: Applied Science

Views: 313,952

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Id: m3oonk1wnHY

Length: 14min 5sec (845 seconds)

Published: Sun Sep 20 2020