Astronomy Picture of the Day for Mar 28 is a time-lapse video entitled The Aurora, taken in Norway. The not-yet-dark sequence at 1:30, with the aurora lighting up the clouds, is way cool.
Bonus timelapse — Stunning winter sky timelapse video: Sub Zero
Both have an effect of the camera slowly translating and their linked pages mention some sort of special dolly used for the effects.
Understating the risks is just as irresponsible as overstating it.
The Mind of Dr. Pion: Don’t believe what the press is telling you!
There is no explicit by-line on this article, but the video contains an interview with BBC reporter Chris Hogg in Tokyo that repeats that a half life of 8 days means “that after 8 days the risk will have dissipated”.
The reporter is WRONG. Twice, because that is also not what the officials said. His ignorance of basic physics, in this case a topic I always teach in a college general education class, led him to misinterpret what was actually said by a government spokesman and hence mislead the public.
Stock tip: invest in adult diaper companies, what with the soiling of undergarments going on about radiation levels in the US.
I’ve run across a number of stories about the worries and the run on iodine tablets, and then saw the California radiation monitoring map which led me to the EPA site. They give radiation levels for select cities, but don’t tell you what the expected background levels are, so all you have is the assurance that the detected levels are small. However, the EPA has set up a section dedicated to the Japan accident, which includes a map with the most recent data for all of their monitoring sites. I eventually found how to get historical data — you click on “Query View” over in the left column — and looked to see what I could find.
I chose Eureka, CA because it’s on the West Coast and I was excited to have found the database, and the beta count rate because that would be indicative of having fallout reach the US; many fission products are beta-emitters. (The gamma data is divided into energy bins, and would have taken longer to analyze.)
Here’s what the radiation levels look like, starting with March 10, up through a half-day’s worth of data on the 25th.
The earthquake happened in Japan on the 11th at 0542 UTC. You might think the first spike, on the 11th, might be caused by the quake/tsunami, but the cooling problems didn’t happen until about 8 hours had elapsed and it would take several days for any fallout to reach California. If you really think that either peak is significant, all you have to do is go into the database and look at a larger data set.
This graph goes back to early February. The two peaks shown on the first graph are near points 750 and 1000. We can see that the radiation levels are showing no unusual behavior.
Because the EPA has labeled levels coming from specific isotopes I have to assume that’s by looking at the spectrum, and they give numbers that are much less than a picoCurie per cubic meter. One Curie is 3.7 x 10^10 decays per second (based on the activity of a gram of Radium-226), which means that a picoCurie is about 2 decays per minute. The EPA isn’t clear that the numbers it gives for gross beta counts are for a cubic meter or a larger volume, but I think it has to be, because 0.0017 pCi (the Anaheim Cs-137 activity) is only about a quarter of a decay per hour, so I imagine they sample a much larger volume.
Vocabulary lesson: many MSM stories are confusing radiation and fallout/contamination. radiation (in this context) is the energetic particle emitted when something decays, e.g. a gamma or a beta. Fallout or contamination refers to the radioactive particles, such as particulate matter that was expelled from the reactor and contains radioactive particles. We aren’t worried about radiation reaching us from Japan, because that is diminished by distance. It would be like complaining that the lights of Tokyo are too bright and though I’m sure Sarah Palin can see them from her home, it’s simply not an issue for us. What matters is the amount of radioactive particles that might reach us, and decay when they are here. But we can’t see any effect on the radiation levels, because any increase is small compared to the background and fluctuations in the background.
To quote Hedley LaMarr, “Gentlemen, Please, Rest Your Sphincters!”
Physicists observe antihelium-4 nucleus, the heaviest antinucleus yet
[S]cientists with the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory have observed another state of antimatter for the first time: the antimatter helium-4 nucleus, which is the heaviest antinucleus observed so far.
Which reminds me that my blogroll disappeared at some point in my layout reshuffling, and I should do something about that someday…
In all the discussion of nuclear power, there’s one bit that hasn’t been discussed in any of the summaries, which may be a good thing because I’m not confident that the general science media would get it right: How can you control the fission reaction in a reactor?
First, a bit of terminology. A critical reactor is one in which each fission result in one neutron, on average, inducing another fission. Which makes the fission rate from this chain reaction constant. So all those times you’ve seen on a TV show or in a movie, where a character shrieks, “OH my GOD! The reactor has gone CRITICAL!” it’s really no big deal. The population of neutrons from fission is constant over some period of time. If you are supercritical this population is increasing, and subcritical means it is decreasing. And the rate of fissioning is proportional to the neutron population.
The neutrons that come from fission (called “prompt neutrons,” for reasons which should become clear shortly) have a lot of energy — an MeV or so, typically, so they are called “fast,” — and they bounce around, scattering off nuclei and losing energy in those collisions. Most of the collisions are with the material that’s there for the express purpose of slowing the neutrons down — the moderator — and if the neutron gets down to thermal energies (“slow” or “thermal” neutrons) before it leaks out of the reactor or gets captured by some nucleus, it can be absorbed by the uranium and induce another fission. And this happens really quickly. A matter of a few tens of microseconds. It’s convenient to look at this in discrete steps, even though the reactions are continuous: each set of fissions releases neutrons which slow down or are lost, and these then induce more fissions. That constitutes a generation with some characteristic lifetime. If the lifetime were that of prompt neutrons alone there’s no way you could ever control a nuclear reactor. Because when a reactor is supercritical, the effect compounds: more than one neutron survives to cause a fission, which means more fissions, which gives you even more neutrons, and so on, ad infinitum. And if your time constant for doing this compounding is 25 microseconds, that’s of order 40,00 generations in just one second. Even if you had just a 0.01% increase in population per generation, you’d increase the population (and power) by a factor of 50 in just one second. There are no systems that could react fast enough to stay within reasonable limits.
But that’s not the whole picture. Not all neutrons come directly from the fission process. Some neutrons appear as the result of beta decays. Beta minus decay occurs in nuclei that have an overabundance of neutrons, and as you look at nuclei that are far from stability (that is, it takes several decays before you would end up with a stable nucleus) there is a tendency for the decays to release more energy and happen quicker.
In beta minus decay, a neutron changes into a proton and a positron and neutrino are emitted from the nucleus. This “reshuffles” the nucleus and the energy states it has; in a lot of cases, the daughter nucleus is not in the ground state, so a gamma is also emitted as it de-excites. For some nuclei far from stability, there is so much energy left over that the nucleus can emit a neutron instead of a gamma. But before this neutron can appear, we have to wait for the beta decay to happen, and that delay is on a time scale best measured in seconds. These are called “delayed neutrons,” and they comprise a little less than 1% of the neutrons from fissions in a critical reactor. But because they take so relatively long to appear, the effective generation time is much greater. Which means only a handful of neutron generations will occur in the time it would take for the system to respond — the reactor is actually subcritical from just the prompt neutrons, and any neutron population increase is relying on decays that won’t occur for around a second.
This works as long as the reactor is subcritical on prompt neutrons. If the reactor went “prompt critical” there would be a spike in power before the system could respond; this is what happened at Chernobyl, because safeguards were intentionally disabled and procedures violated in order to run a test. In a bomb the design is to be highly prompt supercritical using fast neutrons; you aren’t thermalizing neutrons or relying on delayed neutrons for anything. One of many ways a bomb is different from a reactor.
I was dumping some packing peanuts into a trash can and they stopped pouring in — enough charge had built up that the additional peanuts were repelled by the ones in the can. Here’s a couple of attempts to throw peanuts in.
You might be tempted to think you could confine a peanut (or any charged particle) this way — all of the charges repel, so you form a potential well which traps the extra charge. Unfortunately it doesn’t work out. The electric field you get will counteract gravity, but there’s no field directed radially inward, at all points, and no way to get one. Electric fields only converge on charges. The best you could get would be a field that was “leaky” — inward at one point, but outward somewhere else. All of this is shown mathematically in Earnshaw’s theorem. It works for magnetic fields, too, with the loophole that it doesn’t apply to diamagnets. (but Earnshaw didn’t know about those)