Negative Temperature

Sorry I haven't posted much this week. I've been really busy in and outside of the lab. Negative temperature is something I encountered in my Statistical Mechanics course, but I didn't really understand it until I stumbled upon the wikipedia article on it.

First off, we're dealing with the Kelvin scale. There's nothing special about negative Farenheit or Celsius. Celsius is based off the freezing point of water and Farenheit is based off of... something? The important part is that the Kelvin temperature scale has it's zero as absolute zero. Absolute zero is a theoretical "cold limit" in that no physical system can actually reach this limit. Scientists have gotten to very low temperatures (.004 K is fairly common in condensed matter physics) but none have achieved absolute zero. So, with this in mind, when I say "negative kelvin temperature" you should be thinking something like "Wha...?"

Another aside: Negative temperature is actually a consequence of the definition of temperature and can only occur under a set of certain circumstances. No macroscopic system would ever be able to obtain such a state.

First let's look at the definition of temperature. Temperature is defined as the relationship between the change in energy of a system and the corresponding change in entropy. In calculus speak, T = dq/dS.

The system that we're going to consider is a nuclear spin system, that is a system of nucleons where we are only considering the energy associated with their spins. One caveat of negative temperature is that there must be a finite number of states in the system. For a spin system, that limit is two. A nucleon can either be spin up, or spin down. Nothing else. Also, we are assuming that the spin system is isolated from other sources of energy contribution (i.e., other degrees of freedom like vibrations, rotations, etc). We can make this assumption because the time scale at which this spin system receives energy from these other degrees of freedom is very large compared to the time scale in which we are considering this system. It's like assuming a radioactive element with a half life of 4 billion years won't decay during your experiment that will take 4 hours. Technically, it could decay since the probability for it to decay at each second is equal... but the probability is so low that you can safely ignore it. Now, moving forward...

Now let's say we apply a magnetic field to the system. What this does is break the degeneracy of the system. Before the magnetic field is applied both states of spin-up and spin-down have the same energy. With the magnetic field, those spins that align parallel to the field have a higher energy than the ones that align anti-parallel. Now, the second law of thermodynamics says that a system will evolve over time such that entropy is maximized (order to disorder). How this manifests itself in this system is an even distribution of spin-ups and spin-downs... a 50/50 split.

This is where the magic happens. There are certain techniques that allow you to flip the spin of the nucleons (say, from down to up) by using radio waves. By using these radio waves to move away from the 50/50 split, we are DECREASING the entropy of the system (since the maximum point of entropy is at 50/50) by ADDING energy, thereby creating negative temperature!

An interesting consequence of this is that negative temperature is hotter than positive! Heat will flow from a source of negative temperature to a source of positive temperature. So, the temperature scale goes (from cold to hot) like: 0 to +infinity to -infinity.

Work at the lab is going good. I've probably said this a billion times, but I think we're finally ready to wrap up the gas delivery system. I'll be glad to move on to something else :)

The week is almost over already?!

Time flies when you're doing really cool shit all the time. I'm almost at the end of my third week working at ORNL! It sure doesn't feel that way, though...

My adviser today told me that he's trying to get a collaboration group together that would solely fund research based on the advancement of nuclear energy. Since that's the exact field I'm going into, that was excellent news to hear!

There's just not too much to say today, really. I'm still working on the gas delivery system and we've got three possible solutions. They would all work fine so the deciding factor now will be cost. Some of this equipment is stupid expensive... $1300 for a valve! Granted, it's a SUPER fancy valve that has no detectable leaks down to 10^(-11) torr*liters/second and it was meant to operate in ultra high vacuum... The other mass flow controller we are looking at is $1800! Science is EXPENSIVE!

I'm heading back to Atlanta tomorrow for the weekend, so I'm super excited about that. There's a psytrance show at CoLab on Saturday night, so I'll get to see all my hippy friends and I'll get to spin fire! I'm stoked!

Setback

So, I kind of screwed up. When going over the final design for my gas delivery system to JJ today, he inquired about the accuracy of one of the flowmeters we were planning on buying. The accuracy is rated at +- 5% at the maximum flow rate, which you can pay extra to reduce to +-1%. That accuracy, however, only applies to the maximum flow rate. The further you get from the maximum rate, the worse your accuracy gets. When we calculated the uncertainty at the flow rate we'd be using we got 360% for the +-5% and 76% for the +-1%... both totally and completely unacceptable. Another reason this comes out so poorly is that the resolution of these flowmeters is given in minutes and we need good resolution down to seconds. At least we caught this error before we paid $400 something for the flowmeter. That's one more lesson learned in the world of experimental physics. It's not a huge setback, but it is going to take a few days to find another flowmeter. We're currently looking at electronically controlled ones since I can't find a manual one with the resolution and flow rate that we need.

I started the application process for graduate school on Monday. I sent in the main application (name, birthday, address, etc) and applied to be a Ph.D. student in Nuclear Engineering. I've got my transcripts being mailed to them and I've gotten all the people I need to write my recommendation letters. All I have to do now is wait... Aghhh!

Something interesting about nuclear structure that I read the other day. I'm sure most of you are familiar with the 4 forces that govern what happens in our world: gravity, electromagnetic, nuclear strong, and the nuclear weak forces. The nuclear strong and weak only manifest themselves inside the nucleus, but they are many orders of magnitude more powerful than gravity or the electromagnetic forces. Here's an interesting comparison between the gravitational and electromagnetic force. If the strength of gravity could be represented by the length of my forearm, the strength of the electromagnetic force would be represented as the length of the known universe. That' BIG. Back to the nuclear forces, though...

Here's something crazy I read the other day about nuclear structure. Protons and neutrons are both fermions, which means that in a given energy level no two particles can exist in the same quantum state. Also, within the nucleus, protons and neutrons are much closer together than electrons are which means that collisions between nucleons (protons and neutrons) is a fairly likely event. If a collision occurs, energy is transferred from one particle to the next. This energy transferal would be manifest in one of the nucleons moving up to an excited state (nuclear structure theory has a shell-like model, just like atomic theory and electrons). Now consider some nucleons in the ground state and further consider that there are many filled states above ground. Since all of the states above the ground are filled, if a nucleon were to transition to a higher state it would have to be given enough energy to transition to the very last energy level (like jumping from the bottom step in a staircase to the very top in one jump). What I find wild is that if a collision between two particles would not yield enough energy for such a jump, the particles are not allowed to collide! How #$%@ing wild is that?!

Something like this also happens with superfluids. I can't remember the specifics without looking at my book (which is in Georgia >_> ), but it involves particles being unable to move to excited states because collisions would not provide enough energy (below a critical velocity, in this case). I love physics.

Flat Tire amber ale is delicious.

The Helium-Jet Ion Source pt. 2

So now that we've got a beam of neutral atoms, we need to ionize them. That's where Argon plasma comes into play. Recall that a plasma is a partially ionized gas that has it's electrons on the outside with the ions and neutrals in the center. As this beam of neutral atoms stream in, the electrons strike the neutral atoms causing an electron from the neutral atom to be stripped off, which is called electron impact ionization. The chamber that houses this plasma is also negatively charged. Since the ions we care about are positively charged, they get sucked out of the chamber creating an ion beam! Now, I've skipped a lot of the details, but that's basically how this thing works.

Switching gears. This entire week has been devoted to designing the argon and helium gas delivery systems. Since very little argon is actually used, which means the tank is small and light, we plan on mounting that on a sheet of metal or wood (haven't made our decision yet) along with all the transport lines and a flow regulator. We also want the flow regulator for the helium as well. Since we can't be in the room where all of this is located during the experiment (since the room is being flooded with radiation) we want all of this in a nice, compact design such that we can put a video camera and monitor things from a control room. I've already figured out what components we're going to use, but now I've got to put it all together. I had heard of Google Sketchup, but had never used it before. It's a free 3D modeling program that's good for when you need a simple visualization. I read that it has all the doohickeys and doodads that other 3D CAD software has, but I also hear that it's atrocious for real engineering drawings. Thankfully, I just wanted a rough "sketchup" of the design.

I also got to meet with the rest of my group yesterday. I really only work with JJ and a lot of the other people in the group are in buildings so I'd never met many of these people before. I got to see a seminar on nuclear astrophysics from a potential new hire as well, which was fascinating. I was surprised that I was able to follow as much of the presentation as I did. I guess I did learn something during my undergraduate after all! I also got to network with some people that I could do research with as a grad student later on.

Speaking of grad school... I'm still trying to figure that one out. I'm certain that I want a Ph. D., but I'm trying to decide between nuclear physics and nuclear engineering. I like the idea of helping build things that will pave the way to the future, but I also like the idea of doing the science necessary to build those things. I'm going to be looking into possible programs with the Department of Defense and Department of Homeland Security as well. That's where all the cool classified stuff goes down. Tennessee's state legislature is also voting on a bill right now that will, if passed, provide funding in the hopes of getting more Ph. D. seeking students working at ORNL, specifically those interested in nuclear physics/engineering. That's good for me :D

I hear this pizza place around here called "Big Ed's" is awesome. Pizza being the best thing invented since best things were invented, Jeff, my roommate, and I are going to go check it out. Mmmm.... pizza....

The Helium-Jet Ion Source

I've talked a little about what I've learned and about a few tasks I've finished/I'm currently working on, but I haven't said much about the project I'm working on. So I think I'll do that now!

JJ and I are trying to develop a helium-jet ion source so that we can examine ions of refractory metals. That's a lot of jargon, eh? First off, what is an ion source? Well, it's a source of ions! Ions are atoms that have a charge due to gaining or losing electrons. How do we get these ions, though? There are lots of ways, but our method involves using a Tandem accelerator to send protons to collide with a uranium target that causes proton induced fission. Protons collide with uranium nuclei and break it into two separate, usually radioactive, nuclei.

When something is radioactive, that means that the nucleus is unstable and it must emit something until the nucleus becomes stable. Depending on what is emitted determines what kind of radiation it is: alpha, beta-plus, beta-minus, gamma, etc. Alpha radiation is when a nucleus spontaneously emits 2 protons and 2 neutrons bound together (which actually is the nucleus of a Helium atom) and the original nucleus loses those protons and neutrons. Since an element is defined by how many protons it has, the original nucleus is now a different element! The other types of decay involve protons converting to neutrons, or vice-versa, photons being emitted, and a plethora of other things. The point I'm trying to make is that radiation involves things flying off of radioactive elements.

Back to my project. So these nuclei that are shot off from the uranium target are themselves decaying into other nuclei, which then decay further, etc. etc. What we now have is a ton of different elements flying around. We use an isotope separator to select the specific element we want and then those atoms are shot towards the ion source, which is what I'm working on.

We now have a beam of electrically neutral atoms that we want to turn into ions by stripping them of one or more electrons. Normally, the helium jet isn't needed but it's currently the only way we think we can ionize certain elements called refractory metals. Normally, the beam is directed straight to the ion source, but with refractory metals they end up sticking to the walls of the transport tubes (due to some chemistry that none of us understand).

To fix this problem, we employ the use of a sodium chloride (NaCl) aerosol. Any pressurized spray can (like spray paint) consists of an aerosol inside and this NaCl aerosol relies on the same concept. We then direct the beam through a chamber of the NaCl aerosol, moving the atoms using a stream of helium, and the atoms stick to this aerosol. Since the aerosol + atom clusters are so much heavier than the helium, they all flow in the center of the tube (called laminar flow) and don't bounce around, which means they don't stick to the walls. I'll save what happens next for tomorrow!

I also figured out today our setup for the Argon gas delivery system! We ended up going with the Matheson setup. I'm particularly excited about this part of the project because it's mainly been designed by me. I'd report to JJ every so often and he'd poke and prod me in the right direction, but most of the research has been done by myself. I've learned a lot about how one should go about designing experimental setups and it's really awesome to know that this system will be one of many parts that make this project work. Woo!

Numbers make me cry

My job this week has been working on a delivery system for the Ar-36 gas that will be "plasmatized" (as of now, that is official jargon). I've got to come up with a way to get the gas from a tank into the ion source. It's not as simple as it sounds, unfortunately, and has been quite taxing on my sanity. The difficulty is being able to have this gas come into the ion source at a specific rate of flow, 0.02 std-cc/s specifically.

My first thought was to go to this company called Vacuum Technology Inc. (VTI), who operates in Oak Ridge, and have them design a specific reservoir and capillary leak (that's technical jargon for "tank that spews out gas") that fits our needs. My first calculation turned out to be super wrong when I estimated that given their maximum capacity tank we would only be able to run an experiment for around 14 hours before needing to refill. Since our experiments could run for weeks at a time, this isn't feasible.

I immediately threw that idea out and started looking into their open style capillary leaks, which differ in that you hook them up to your own source. The problem here started with the specs I have to provide to them: pressure coming into the device (known), the rate of flow (known), and the pressure coming out of the device (kind of known). Now, eventually the capillary goes into the ion source which is at vacuum. However, since we are transporting this gas a relatively long distance, there will be a difference in pressure from the exit of the leak to the entrance of the ion source. This pressure drop will affect the flow rate, which means if we want to have a final specific flow rate I need to figure out exactly how much pressure to have them calibrate this device to output.

It's easy, right! Just apply Poiseuille's equation, which relates pressure difference to rate of flow! Well, as I've come to find out, there are two different measures of flow rate: volumetric flow rate (measured in volume/second) and some other flow rate (measured in pressure*volume/second, which actually comes out to Joules/second). Poiseuille's equation uses volumetric flow rate and for most of the day I had been using the other kind. I also ran into numerous problems with determining the exact viscosity of Argon at the conditions we'd have it at as well as a cluster$%@# of unit conversions. Aghhh!

After my problems with the open capillary leak, my boss told me how to actually calculate how long my original plan would work for and it came out to 70 days or so. That's assuming a 1800 psi tank, which is approximately 122 atm, based on some specs I got from VTI. Thinking about it later, 122 atm is retardedly high for pressure so I'm going to have to call back and get those numbers again. Even if we can only squeeze 20 days out of a tank, that would work. Getting the capillary with reservoir calibrated is much easier than the open one so I'm hoping this works out.

I also examined getting one of the huge Argon gas tanks, getting a pressure regulator, and then attaching a flowmeter and adjusting the flow that way. But that wouldn't work because the flow rates possible with their tanks are way higher than what we need! Later I came to find out that I read their rates in std-cc/MINUTE and I was comparing that to std-cc/SECOND. So now that might actually be a viable option! Arghhhh!

The day wasn't wasted... I've learned a bit about how to be more careful with calculations and what not. I'm also that JJ (my boss) didn't get frustrated with me and wa shappy to help me out. I'm going to crunch the numbers again and give him a better report in the morning. Wish me luck? Please?

Plasmas!

These next few posts will be fairly technical in nature, though I'll do my best to not seep them with jargon. However there will be several posts (I've already got them in my head!) that will be chock full of jargon and these will be mainly for my physics friends back home.

I'm going to talk a bit about plasmas, the 4th state of matter. The ions that we're studying are created in plasmas (Ar-36 plasmas, to be exact) and these plasmas are controlled via magnetic fields. Obviously, it seemed to behoove me to understand them at least on a conceptual level. We're all familiar with the solid, liquid, and gas phases of matter. The plasma phase of matter is most closely related to the gas phase in that a plasma is a partially ionized gas. If you reach back to high school chemistry (a disgusting time, I know) you'll recall that an ion is an atom that has an electric charge either by stripping it of one or more electrons (giving it a positive charge) or by adding one or more electrons (giving it a positive charge). Depending on the field of science/engineering that you're working in, a "plasma" could need to be fully ionized (all the atoms are ionized) simply fractionally ionized. I'm not sure on what the True Physics Definition (TM) is, but that's really not important.

How are plasmas created? I'm not sure in general, but I know how we create our Ar-36 plasma: by applying a potential difference between the two surfaces between which the gas inhibits and causing an electric discharge from one surface to the next. In essence, we shock the shit out of it. By giving the gas atoms enough energy, electrons are stripped off the atoms. Where these electrons go and what happens to the ions is what makes plasmas so special. We now have this plum model picture of 3 things floating about: Neutral atoms, positively charge ions, and electrons (the ones stripped from the neutrals). One thing that's awesome about electrically charged things is that we can control them with a magnetic field. By applying a magnetic field with the right geometry and magnitude we can cause the plasma to flow around it's container in a nice, orderly fashion.

This flow has distinct, separate layers. Since the electrons have much lower mass than the ions/neutrals, they flow on the outer most layer (called the plasma boundary) while the ions and neutrals flow within this boundary. Now, due to conservation of charge, the plasma as a whole is electrically neutral, but that doesn't hold true for its constituent parts. With the electrons on the outside and the positive ions on the inside, a potential difference is created. This potential difference is what keeps the inner section inside the barrier. Imagine marbles rolling around in a bowl. To escape the bowl, you'd have to flick them, roll them, or somehow provide them with some external energy to escape the bowl; this is essentially what is happening.

I will go into why plasmas are important to my project in future posts.

First Week

I've finished my first week as an intern at Oak Ridge National Laboratory (ORNL) in Oak Ridge, TN. I'm working with Dr. J. J. Das and Dr. Kennon Carter in the Physics division at the Hollifield Radioactive Ion Beam Facility (HRIBF) on developing a Helium-Jet Ion source. I'll go into the science behind it a bit later. First, a bit of background information.

I graduated from Georgia Tech in December of 2009 with my bachelor's in Physics. I applied to the Science Undergraduate Laboratory Internships (SULI) program, an internship program funded by the Department of Energy (DOE), and was selected for a 4 month internship. There were other people selected as well, though we're all in separate divisions. I ended up rooming with one of them in a nice apartment about 15 minutes from ORNL, Jeff G., and that's been working out wonderfully. All in all, I'm very happy up here. Going to work is a pleasure and something I look forward to and I've learned so much about nuclear physics and ion sources this week.

This first week has mainly been devoted to reading papers on ion sources and basic nuclear physics. Being able to read professionally written papers from Real Scientists (TM) and learn from them is a huge ego boos; it makes me feel as if I actually learned something while I was an undergraduate. Not only am I learning a lot, but I'm enjoying it immensely! I have never found a specific field in physics that I was this excited about. Nuclear physics is @#$%ing awesome! I spent two semesters in Quantum I and Quantum II learning about atomic physics, but I never studied anything about the nucleus of the atom. Learning about how the nucleus works is only one part of our field. All the different types of radiation, how they work, and what data collected about these different types implies is also packaged into this. In this first week, because of this new found fascination, I've been reconsidering what to do with my future.

I'm now considering going back to my original plans of getting a Ph. D. in physics. My biggest reason for changing those plans was that I was both turned off by my undergraduate research experience as well as being unable to find a field of physics that I was excited about. Graduate school for physics is all about specialization and I wasn't able to find something I enjoyed enough to specialize in. Unfortunately, it's too late for me to get into graduate school for physics in 2010. I still haven't taken the Physics GRE as well as getting a bunch of other documents in check.

It's not all bad news, though! Dr. Das has stated several times that he would like to keep me on this project after my internship. If we can figure out a way to make that happen I'm going to stay up here for a while and continue the project. Getting the ion source working is only the first step... after that we could be exploring nuclei that no other human being has ever observed or studied before. How can I say no to that? I can't! If I can stay on here for at least a year I'll have more time to learn as well as getting very good references for my grad school application. I'm looking at the University of Tennessee at Knoxville (UTK) since they have such close ties to ORNL. All of that is a long way off, but I'm trying to get everything in line as soon as I can.

My next few posts will go more into the science of what exactly I'm doing up here. I'll try to make it accessible as possible, but my physics friends would never forgive me if I didn't wank just a little bit using as much jargon as possible, lol.