For technical reasons, it turns out that alkali metal atoms are particularly good candidates for laser cooling. Rubidium is probably the most favorable of all of them– some atomic physicists jokingly refer to it as “God’s atom”– but all of the alkalis, even Francium, have been cooled and trapped.<\/p>\n
Of course, alkali metal elements are also the ones that explode when they come in contact with water. They’re insanely reactive, so you have to be very careful handling them. As a result, they’re usually shipped either in vacuum-sealed ampoules, or as chunks of metal packed in some oily liquid, to keep them from reacting with the water vapor in the air.<\/p>\n
This combination means that pretty much anyone who has ever done a laser cooling experiment has some True Lab Stories about alkali metal mishaps. My personal story isn’t all that impressive (as an undergrad cleaning out a vacuum system that had been used with Rb, I was advised to use ethanol rather than water on the first pass, because it reacts more slowly. Of course, it’s also flammable, so when we filled the arm of the chamber that had held the 5-g slug of rubidium metal with ethanol, we got a cheery little blaze…), but there was an impressive incident in the lab when I was in grad school, that I’ll relate after the cut…<\/p>\n
<\/p>\n
The lab I worked in had several different experiments going. I was working with noble gases (specifically xenon), which are famously inert, but we had a cesium experiment, plus two sodium experiments.<\/p>\n
One of the sodium experiments had some sort of design flaw that caused it to run through a hellacious amount of sodium in a short period of time, which meant the source needed to be re-filled on a regular basis. This was kind of a headache, since it involved opening the vacuum system up, removing the oven (sodium needs to be heated to a few hundred degrees to get a reasonable amount of vapor), adding more sodium, closing everything back up, and pumping the system back down. The pump-down takes a few hours, so they tended to do the re-filling at the end of the day (so they could turn the pumps on, go home, and come back in the morning to find everything ready to go).<\/p>\n
On one occasion, the re-fill was done rather late at night, by a post-doc working alone. He got everything closed back up, and started the pumps, and then set about cleaning up. He put the leftover sodium back in the jar (this particular batch was bought as chunks of metal packed in xylene, an oily liquid sort of like kerosene, only less flammable), put the jar and the various other tools in a metal tray, and headed back for the hood room where we kept the alkali metals.<\/p>\n
To get from the lab out into the hallway, you had to pass through the computer room\/ break room, and open two doors, one from the lab into the break room, and one into the hall. He made it into the break room with no trouble, but when he went to open the door into the hall, he had to balance the tray on the edge of a sink to free up a hand. And you can see where this is going…<\/p>\n
In accordance with Murphy’s Law, the tray tipped over into the sink, which was one of those institutional granite sinks that are more or less impervious to anything you might manage to dump into them, but always have about a quarter-inch of standing water somewhere in the bottom. And, of course, the jar fell out of the tray, and shattered, dumping the sodium directly into the water.<\/p>\n
Describing it the next day, he said that he saw the metal skid into the water, and small flames start up, and thought “This is bad…” Then, the xylene went up, in a fireball that reached the ceiling.<\/p>\n
Amazingly, he kept a cool head, and went to get the Met-L-X. You can’t put out alkali fires with regular fire extinguishers, so you need to sort of smother them in anhydrous soda ash<\/a>, which we had on hand. He got the stuff, and got the fire put out without any significant damage.<\/p>\n At this point, he was hugely relieved to not hear loud fire alarms going off, but figured that just to be safe, he should call the Fire Safety office, and tell them what happened. He dialed the emergency number, and the person who picked up the phone said “We were wondering whether you were going to call. The fire crew should be there, well, now.” And when he looked out in the hall, there were fire fighters sprinting down the hall toward him…<\/p>\n It seems they have a silent alarm on the fire sensors in the labs, so they don’t wind up panicking the entire building every time some clumsy chemist makes a little smoke. They need to get alarms in multiple labs (or a phone call) before the big sirens start up, but they will send a crew for a single alarm, especially after hours.<\/p>\n The post-doc in question got in a whole bunch of trouble for that, and we had to agree that nobody would ever re-load the alkali metal sources after hours, but other than that, everything was ok. That wasn’t the worst interaction the group ever had with Fire Safety, though, but that’s a story for another time…<\/p>\n","protected":false},"excerpt":{"rendered":" For technical reasons, it turns out that alkali metal atoms are particularly good candidates for laser cooling. Rubidium is probably the most favorable of all of them– some atomic physicists jokingly refer to it as “God’s atom”– but all of the alkalis, even Francium, have been cooled and trapped. Of course, alkali metal elements are… Continue reading True Lab Stories: The Sodium Incident<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[25],"tags":[],"class_list":["post-147","post","type-post","status-publish","format-standard","hentry","category-lab_stories","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/147","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=147"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/147\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=147"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=147"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=147"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}