Experimental techniques.

I studied in physics in York. The three guys in charge of the demonstration experiments were Matt, Pete and Ian. They were all quite accustomed to students and professors who couldn't get things to work, but were always keen to remind you how stupid you were when you appeared with a newly broken piece of apparatus. In their office they had a framed cartoon which summed up their suffering. I made a quick reproduction of this cartoon, which referred to the famous tale of Galileo and his cannon balls.



The cause of my new-found solidarity with lab technicians was an encore of the Stern-Gerlach experiment which was held today. Prof. Dr. X decided to include the experiment in his lecture, and was determined to run the experiment himself, despite the fact that there were four perfectly qualified people standing around ready to press the necessary buttons. The experiment worked, but not as well as it could have. An hour of preparation was almost screwed up because he lacked the coordination to press a key and flick a switch together. Of course he then had several suggestions for how the experiment could be improved. It's not that he's wrong: of course you could improve the experiment. It's just the pomposity of assuming that he's the first person to think of these things.

Success!

Today I helped run the Stern-Gerlach experiment, and it worked even better than I hoped. Physicists make a distinction between things they are looking for (signal) and things they don't want to see (background). What the layman would call a needle in a haystack, a physicist calls a signal/background ratio of 0.00001.

If you look at this plot then the maximum value (0.21) is nearly four times larger than the minimum value (0.06). This means that the signal is almost three times as large as the background: so this is more of a lamppost in a flower arrangement than a needle in a haystack (if you are a layman) or a signal/background ratio of 3:1 (if you are a physicist).

This background of 0.06 nA doesn't come from the beam and is always present, so in principle you could just subtract this value from all the measurements to see how many atoms come from the beam (this is then your signal). The trouble is that the 0.06 is sometimes 0.058 and sometimes 0.062, and these fluctuations cause the measurement to jump up and down from point to point. The only way to get a really nice measurement is to keep this background very low. This is where the liquid nitrogen cold-trap comes in handy: the more stuff which freezes on to this trap, the less background is seen by the detector, and the smoother your curves are.

For one night only...

Tomorrow is the big day for the Stern-Gerlach experiment.

Roughing pumps: check
Pressure gauge: check
Turbo pump: check
Oven heating: check

The detector is on standby, the liquid nitrogen is still liquid...

As you can see I am a bit excited by the whole thing. To tell you the truth I never really got over the loss of my nice big Stern-Gerlach experiment: If you look at the photo you can see part of the "Atomic beam source" which is a souped up version of the good old Stern-Gerlach device. I'm the guy on the left caressing a cryo pump. It was an emotional affair between me and the Atomic Beam Source, and I got in way too deep. It's not healthy for one man to spend so much time and energy looking after one gizmo.



But tomorrow, for one day, I can be an expert again, and not the poor fucker with broken nerves and broken dreams who does the baker's run. I can be important, needed, coolly guiding the detector wire with an expert hand into the beam of polarized atoms, tuning the oven for optimal intensity...

Please only leave a comment if you have a degree in psychology, or at least saw a talk show with the theme "Men who fall hopelessly in love with plasma-driven turbo-pumped atomic mother-fucking beam sources".

Liquid Nitrogen

Today I tested the cold-trap of the Stern-Gerlach experiment. This is just a steel container which holds liquid nitrogen, and is represented as the blue thing in the above diagram. Well it's not quite just a steel container, it's actually a steel container with a pipe welded through it. Only a small proportion of atoms which leave the oven will fly in the wanted direction (straight forwards and through the pipe). Most of the others will hit the cold-trap, and stick to the cold walls. This is good, because they can't cause problems by then flying somewhere they shouldn't.

As part of the intensive research that goes into every post I write here (or actually every post I write soberly here) I typed "liquid nitrogen" into google. What do you think is the top hit? The ubiquitous Wikipedia, maybe, or a company which delivers liquid nitrogen, or a site explaining chemistry and phase transitions of nitrogen? Wrong, wrong, wrong, the top hit is of course "1001 things to do with liquid nitrogen". (Hello Mr. googlebot, by the way).

I'm not really sure if the liquid nitrogen cold-trap is really needed, as the turbo pump is pretty nifty alone. However, someone already explained to me that it is never a bad thing to have liquid nitrogen in your experiment. Even if the experiment doesn't work, you can freeze things, or blow them up, or pretend you're in a disco. And when you've finished you can clean the floor with it. If I ever build a house it will be designed to be cleaned by liquid nitrogen. I think I will need a rather understanding wife.

Stern-Gerlach woohoo!

That is more like it. The guys at the mechanical workshop here rebuilt the motor system so that the detector moves automatically across the beam. Together with Fabian I put everything together and it works. woohoo!

If you've not been following this, check out these previous posts!

Introduction to the Stern-Gerlach experiment.

First measurements, with an explanation of what you can see in the plot.

Stern-Gerlach results

The first results for the Stern-Gerlach experiment are in. Our data production has been rushed through, and the results have been analysed. The data transmission from the experimental area to the main site (my office) was found to be rather slow. The exact rate was 0.144 MB per minute, but could be doubled if I didn't stop for a coffee on the way over with my floppy disk.

Anyway, here are the results: The black points show the profile of an atomic beam as the detector is slowly moved across the beam. I pushed the detector by hand, so you can't really compare the positions of the maxima for the four measurements. The width of the beam is a couple of millimetres. When the inhomogeneous magnetic field is turned on by passing a current through the electromagnet the beam is split into two. You would expect that the two beams have the same intensity, but one is always larger than the other: maybe something is cutting the beam off for positions in the left of the plot. The next step will be to reattach a motor which moves the detector automatically, and to check the alignment of all the components.

An honest day's work

For the first time in god knows how long I actually worked properly today. I took the Stern-Gerlach experiment apart and stuck it back together again with a new vacuum chamber, which has been built by the workshop. Vacuum systems are a bit like a child's building set. Each component has a kind of connection at the end, which (hopefully) fits the connections of other components. This is commonly called a "flange". This means that once you get enough pieces you can build and rebuild your system in any way you want.

With a bit of luck it will work soon. The down-side of this is that I sacrificed the afternoon coffee and cake break.

Stern-Gerlach sort of lives

I have been quite busy trying to get the Stern-Gerlach experiment to work. After fixing the leaky flange I tried to run the experiment. Yesterday I saw the first signs of a beam of potassium atoms. It wasn't really a beam, more of a diffuse flow from the oven to the detector, but I guess that once you measure some potassium atoms it is a matter of time until you get the thing working.

I think that the reason I didn't see a clear beam was because I didn't put the detector together properly. The detector consists of a hot tungsten wire and a collector plate. The plate has a hole in it to let the beam of potassium atoms through. They should then hit the hot wire where they lose their outer electron. The remaining potassium ions are then attracted to the plate, which is at a potential of -9V compared to the wire.



The problem is that when I took another look at the detector the wire wasn't directly beneath the hole in the plate, but had slipped to the side. This means that atoms from the beam could not be detected, only those which had somehow reflected off a wall somewhere.

Leak Chasing

Yesterday I found that the Stern-Gerlach apparatus had a vacuum leak. Molecules in the atmosphere travel only about a tenth of a micrometer before they hit another molecule and go off in a different direction. If you want to get a molecule to fly in a straight line for a whole metre then you have to get rid of most of the other molecules (removing roughly 99.9999999% of them should do the trick).

To do this you need a container and a sucking device (the pump). The pump has the job of removing the molecules, and the container has the job of not letting new ones get in. If everything goes well you will have a vacuum container with practically nothing in it.

The trouble with vacuum containers is that they tend to leak. And they follow Murphy's laws of vacuum leaks.

1/ A vacuum leak will never be quite large enough that you can hear the air whistling through it, but will be large enough to screw up your measurement.
2/ A vacuum leak will always occur at the point in a vacuum system furthest away from a pressure gauge.
3/ A vacuum leak will always occur at point closest to the most sensitive device in your system.

Laws 2 and 3 combine to form the vacuum-system exclusion principle: It is impossible to mount a pressure gauge next to your most sensitive device, usually because you have the wrong connector.

Fortunately laws 2 and 3 can also be used to divine the exact position of your leak. If you want to be really sure you can buy yourself a nice Helium leak chasing device

In the bowels of the institute

By far the most interesting part of the physics institute is the basement. This is where people used to work. One of the characteristics of physicists is that they don't tidy up after themselves. They go home and leave stuff lying around. And one fine day they go home and never come back, leaving their things for future generations of physicists or archaeologists to find. Today I found a very nice connection piece for a vacuum system, which made me very happy. I will need this piece for the Stern-Gerlach experiment.

Today summed up my present role in physics. Within earshot of the "kick-off" meeting of the next big thing (a neutrino telescope beneath the sea) I was fiddling around with an imitation of an 84 year old experiment which comfortably sits on a desktop. I am rather happy with this state of affairs. Although I wish the km3netters the best of luck, it is not for me. It may be stimulating to build one sensor, but after you build, test and install several hundred you probably get a bit fed up with them. Once you get into the thousands I can't bear to think.

If I may quote Arthur Roberts (you really want to check out this link)

“Oh, dammit – engineering isn’t physics – isn’t that plain?
Take, oh take your billion dollars. Let’s be physicists again.”

Stern-Gerlach Experiment

Today I started playing with a Stern-Gerlach demonstration experiment, which looks very much like the one above (thanks to Wikipedia). The one here runs with Potassium, though, and has an ionizing wire as a detector instead of photographic film.

The experiment is notable in at least two ways not usually mentioned. Firstly it was something of an accident that it worked at all. Trying to find the quantised orbital angular momentum predicted by the Bohr model, Otto Stern and Walter Gerlach chose an atom (Silver) which has zero orbital angular momentum, which was not predicted and could not be accounted for by Bohr's model. Fortunately enough they chanced upon the property of electron spin, although it took three more years for anyone to postulate spin, and another two for anyone to realise that Stern and Gerlach had measured it.

The other interesting point is that Otto Stern and Walther Gerlach, in a real-life homage to their famous experiment, went their seperate ways. Stern was part jewish and resigned from Hamburg university in 1933 to emigrate to the USA. Gerlach stayed in Germany and after working on Torpedos was put (officially) in charge of the nuclear weapons program. Fortunately for Gerlach and the rest of us, he seems to have been rather overwhelmed by the job. I quote from "the Virus House" by the controversial historian David Irving

Admirable though [Gerlach's] personal qualities were, he did not abound in energy and drive. Those who knew him then, and those who visit him now, retain an impression of an outstanding scientist imprisoned behind a desk heaped with reports, papers and documents through which he laboriously works, while never quite getting to the bottom. The calendars on his wall are over eight years old, the surface of his desk is seldom seen. Very soon the [Reichsforschungsrat] was sending [Gerlach] exasperated letters asking for the long-overdue bimonthly reports on nuclear physics to submit to [Hermann] Goering. They were met with a wall of silence from Gerlach's flower and plant bedecked Munich institute

In 1943 Stern received the Nobel prize in physics for "for his contribution to the development of the molecular ray method and his discovery of the magnetic moment of the proton". Gerlach has the distinction of being the eighth most nominated physicist to never receive a Nobel prize. Presumably Hitler's 1936 ban on Germans receiving Nobel prizes didn't exactly help his cause.

Stern and Gerlach both survived the war, continued their work as physics professors and met just the once afterwards.

Links:
Description of the experiment:

A complete history of the experiment:

A free download of "The Virus House" by David Irving, who may be a holocaust denier, but is one of the few historians who can speak german, understand physics and write entertaining prose.
Click Here

A very entertaining analysis of Nobel recipients, which reveals who the unluckiest physicist of all time is:
Click Here