Detectors. What?

Okay, so I've talked briefly about what I'm working on (LUX dark matter direct detection experiment, if you weren't listening), and I've talked at relative length about dark matter. However, I haven't really described the actual detection process. This is, as you can imagine, rather important. Detectors don't just tell you 'yes, I found what you're looking for!', or 'no, nothing, check back later.' Instead, detectors are extremely complicated devices with extremely complicated signals to interpret. Later I'll talk about the latter (signals), here I'll discuss the former (detector itself).

The art of detection (okay, science), is really a complex collection of different steps. Let's take an example that you're probably familiar with (unless you're reading this in braille): vision. You're eyes are your light detectors. They convert photons of light that enter the eye into electrical signals that your brain (DAQ) interprets. Now, with dark matter, you can't just look at it. Mostly because dark matter is dark and so is invisible to light. But it does interact with the nuclear force (bumps into an atom's nucleus). So when you want to detect something, you have to find a way to interact with it. For LUX, we use a big bucket of liquid xenon, which is a massive nucleus that provides a large target for the dark matter to interact with. When dark matter bumps into a xenon atom it creates a flash of light (the light escapes as the interaction energy). Now, light is something that we can detect. However, the flash of light is so small that we can't see it with our eyes. So now what? We need a quantitative way to detect small amounts of light. With experiments our goal is to turn some interaction evidence (in this case, the light) into an electrical signal that we can record with the DAQ computer. This can then be analyzed. But first, to the problem at hand: turning the light, into an electrical signal. There are several ways that we do this in the physics world. The predominant method is to use something called a photomultiplier tube (PMT). This device has a special piece of metal that, when hit with a photon of light, ejects an electron (this is like billiards, the photon has a lot of energy in it, and when it bumps into the electron, it knocks it right out of the metal, known as a photocathode). This electron is accelerated with a strong electric field into another piece of metal (called a dynode), knocking out more electrons, and so on with several stages of dynodes, each ejecting more electrons. By the time this is all over, one photon that converts into a single electron on one end, becomes over a million electrons on the other end! This signal is easily recorded by a computer. In LUX, we have 120 of these PMTs (60 on the top of the bucket, and 60 on the bottom) so that we catch all of the light that comes out when a dark matter particle (WIMP) interacts with a xenon atom. So in the next issue I'll talk about how we interpret and analyze these 120 signals. Eventually I'll also discuss an alternative to PMTs that I have been working on, known as an Avalanche Photodiode (APD). So now at least, you know a bit about what 'detectors' are and how they work (sort of).

the physics of graduate school

Still in graduate school, I haven't failed out yet! In fact, a few weeks ago I managed to pass my finals and in so doing, finish the six core courses. Now all I need is to pass the Qualifying exam at the end of August and I will officially be a candidate for the Ph.D. No small matter, I also have a lot of actual physics to do this summer. I will be spending a total of three weeks working on the LUX 0.1 detector at Case Western Reserve in Cleveland, OH. I'll also be working on the DAQ for the full LUX detector, as well as experiments at Brown with Avalanche Photodiodes. So, it's a busy summer, and as always, it doesn't look as thought it will be letting up anytime soon. Wish me luck, and forgive me for updating infrequently.

Physics? Really?

So I just finished a terrible midterm on Electricity & Magnetism taught by a nice guy but... bad teacher. Awful teacher. I'm so sick of being taught physics by theorists. I get the feeling that if I wanted to be an experimental physicist I should have just gone into engineering.

Along those lines, looking on courses to take for next semester, the options are bleak in physics. Solid State, and Advanced Quantum Mechanics. Solid State would be nice because it can help me get a job, but that's about it. Engineering courses on the other hand, are awesome. Vehicle Design, Photonics, Feedback Controllers, etc. What the hell? Rather than learning really cool things that are real, I'll be learning about Group Theory and the Theory of Representations, which is an entire course dedicated to what happens when you, get this, turn something slightly. Ack.

more of the same. waaaay too much of the same.

Hi, I'm back, it's been awhile, I know. I've been putting in a lot of loooong hours at work, and just got back from a week at Case Western Reserve University, where the first stage of LUX is being built (dubbed LUX 0.1). We got a lot of work done, learned a lot, but glad to be home now. Missed the misses quite a lot, so I'm staying put for awhile. I'm technically on vacation now for spring break, but instead of being lazy, I have to make up work for my classes and study a lot so that I don't fail. Failing would be bad. However, the semester is flying by in a good way, pretty soon it'll be summer and I'll be able to spend all my time on research. Also, working at the camp as 'Mr. Physics' in which I will run 1 hour physics experiments/demos for kids in Michael's camp. I'm nervous because I'm notoriously bad with kids, but the money is very attractive, especially since I get paid so little for summer work.

Anyway, before then we have some vacations to look forward to. One will be a trip to Arizona to visit my Dad, and another will be a trip to Seattle to visit Erin's Dad. We're also hoping to be able to afford a trip in June to my cousin's wedding in Long Beach, but we may not come up with the money for it. Oh well, such is life when first starting out.

Speaking of which, Erin is taking more classes, this time for photography! Also, Erin's sister Meghan got into Harvard for mind, brain and education, and her mother is going to finish her dissertation (finally) on music stuff (Franz Liszt). So everybody is back to school!

Now if you were reading this blog for the science, then here, some pictures of my trip to Case.

Sorry this is sideways. This is the top two flanges for the detector can. The bottom flange has a smaller can that bolts to it, and that's where the liquid xenon, pmts, etc. go. The top flang has a larger can that bolts onto it over the smaller one and that's the vacuum and cryostat for cooling. The cans are shown...


... in this picture


This is Dave and I working in the clean room... We broke something, now we're trying to fix it...


Here is more of the detector constructed. The aluminum blocks are to take up space so that we don't use a lot of liquid xenon. The idea is that this is a smaller version of the larger, full LUX, so we use the same detector system, just use a tinier amount of PMTs and xenon. The big hole in the back is where the instrumented xenon will be, with the four PMTs observing it. Ask questions if you don't know what I'm talking about.


This should give you a better idea of what everything should look like, only this will have an even larger can over it, and then the whole contraption will go in a large bucket of water, because water is good at shielding against neutrons that can create noise in the detector.


That's it for now! Bye!

still married, still in graduate school.

Hi there, it's been awhile. So far, nothing really new. Still married, Erin is still in school for graphic and web design, and I'm still in school for physics. This last one was (and still is sometimes) on the edge. I am finding things about graduate school that may not be what I want. That is, what I want may not be worth the work to get there, and I can get some job doing what I want (engineering and physics R&D) with only a master's degree, and not the whole Ph.D. So, the plan is to go ahead and get my master's in physics (which should be complete by January of next year) and then take another look and see if I can stay on for another 4 years, or if I will put it off and get a job. Of course if that happens I can always go back and finish a Ph.D, or get a master's in engineering (two master's equals a Ph.D, everyone knows that). In any event, I enjoy the research, the classes kind of suck, but I have to do them. Maybe by the time classes are over I'll be content to help discover dark matter. Anyway, that being said, back to work. Weekends are for homework.

By the way, still working on the data acquisition system for the LUX dark matter experiment, and now also working on research on avalanche photodiodes as an application for dark matter detection. Fun stuff.

Stay savvy.

Married.

So there, I did it. We did it. Erin and I got married. That's right, I'm now Mr. Jeremy Chapman!

Of course, most of you who read this were probably there, and those who weren't... either it's your fault, or .... haha. Sorry.

Anyway, here's a sample picture from the wedding...


Compliments of Grazier Photography. I think it exemplifies what our marriage will be like: classy, artsy, fun, and really, really goofy.

Fantastic 5th

I'm watching the latest Fantastic 4 movie with Gwerin, and the defense department asked Reid Richards to do a science thingy for national security. Instead, he refers the good General to Dr. So and So, head of the physics department at Brown. That's right, Brown!! Now I know I made the right decision to come here! I mean, basically, the Fantastic 4 refer to, in some way or another, ME. I'm practically the Fantastic 5th! Now what powers do/could I have...

In the interest of blog participation, please, tell me what power you think I should have, and/or what power you would have, had Reid Richards referred the welfare of the world to you...