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MatC
295940.  Fri Mar 14, 2008 8:05 am Reply with quote

It’s sometimes said that the time taken for uranium to reach critical mass and initiate spontaneous fission* is measured in “shakes of a lamb’s tail.”

If true, did we already know this? If untrue (I can’t find a really reliable source for it, though many semi-reliable ones), is it Gen enough for an Ig?

*(I’ve no idea what this sentence means. Apologies for any howlers contained therein.)

 
Flash
295954.  Fri Mar 14, 2008 8:15 am Reply with quote

I've never heard of that. Bob?

 
dr.bob
296000.  Fri Mar 14, 2008 8:57 am Reply with quote

I've never heard of the term before, but a bit of googling turned up a couple of references. Here's the skinny for those that are interested.

A Uranium atom can be induced to undergo nuclear fission by absorbing a passing neutron. This fission releases a tiny amount of energy, causes the nucleus of the atom to split in two, and also releases a neutron or two.

If a neutron released from one atom undergoing fission then induces another atom to fiss, and this atom releases a neutron which causes a third atom to fiss, and so on, this is a "chain reaction." Normally chain reactions are not possible since the neutron will simply fly off at speed before it hits another atom (hence the use of moderators like carbon rods to slow down neutrons in nuclear reactors). However, if you have a sufficiently large mass of uranium with a sufficiently low surface area (to prevent loss of neutrons), a chain reaction will be self-supporting. This mass is known as the "critical mass."

The average length of time between the neutron being released from one nucleus and being absorbed into another is dependent on the speed of the neutron and the average distance traveled before it is absorbed. In Uranium, the "mean free path" (as it's called) is about 13cm for a 1MeV neutron. This particle will be traveling at 1.4x10^9cm/s so the average time between fission generations is 10^-8 s (10 nanoseconds). This is an important number to know about if you're planning an A-bomb.

During work on the Los Alamos project, this length of time was nicknamed "a shake" after the phrase "a shake of a lamb's tail."

s:
http://tinyurl.com/2ddgoj
http://www.atomicarchive.com/Fission/Fission8.shtml

 
MatC
296006.  Fri Mar 14, 2008 9:09 am Reply with quote

Ah - "nicknamed". Does that mean it's not an official unit?

 
dr.bob
296010.  Fri Mar 14, 2008 9:10 am Reply with quote

It's certainly not one I've ever come across in my Physics career. Given that I'm not usually concerned with fission reaction times, I tend to measure periods of time in "seconds" :)

 
eggshaped
296017.  Fri Mar 14, 2008 9:26 am Reply with quote

I think that if that's the unit they used then it's fair game for us, nickname or no.

How to explain it in a sentence though.

"The average life of a Uranium atom during a nuclear fission chain reaction = 1 shake of a lamb's tail"

Does that work, Bob?

If so, it's going into SQUIRE in two shakes.

Actually, I think that round here we say "two shakes of a gnat's tail" rather than "lamb's tail"; or perhaps I've been mishearing that for years.

 
dr.bob
297130.  Mon Mar 17, 2008 4:50 am Reply with quote

eggshaped wrote:
How to explain it in a sentence though.

"The average life of a Uranium atom during a nuclear fission chain reaction = 1 shake of a lamb's tail"

Does that work, Bob?


Not really. It's the average length of time between a neutron leaving one atom undergoing fission and hitting another to induce fission. Given the number of atoms present in a critical mass, the average life of an atom would be much higher.

Not sure you could even say "the average time between atoms decaying" since there'll be lots of neutrons spreading out and causing many concurrent reactions.

It's really just "when a Uranium atom decays and releases a neutron, the length of time for that neutron to hit another Uranium atom and propagate a chain reaction = 1 shake of a lamb's tail"

Not very snappy, though.

eggshaped wrote:
Actually, I think that round here we say "two shakes of a gnat's tail" rather than "lamb's tail"; or perhaps I've been mishearing that for years.


Do gnats have tails? If so, do they often shake them?

 
MatC
297415.  Mon Mar 17, 2008 9:24 am Reply with quote

MatC wrote:
The modern use of artificial fertilisers in farming began in the mid 19th century with the discovery (or invention?) of superphosphates. The first superphosphate was made by digging up fossilised dinosaur dung and mixing it with sulphuric acid.
S: Organic Gardening magazine, Winter 08


“Phosphates came into use and since these came from pulverised bones there was a gruesome rush to find them: tons of bones were imported, many supposedly from foreign battlefields.”

S: “A little history of British gardening” by Jenny Uglow (Chatto, 2004).

 
MatC
297948.  Tue Mar 18, 2008 8:13 am Reply with quote

Fundamental Questions:

A book review in Fortean Times, apropos of something or other, says this:

“We now agree with Aristotle that there is no such thing as nothing.”

Is that right, then? I rather fancy the question “Alan: what is nothing?”

 
eggshaped
297959.  Tue Mar 18, 2008 8:23 am Reply with quote

I think the point is probably that it is impossible to crate a true vacuum. It will always contain zero point energy.

I have lots of stuff on "nothing" from the radio show, should it be needed.

 
Flash
298055.  Tue Mar 18, 2008 11:02 am Reply with quote

Stephen: Alan, what are you doing?

Alan: Nothing.

KLAXON

 
MatC
298058.  Tue Mar 18, 2008 11:06 am Reply with quote

Oh, yes.

 
MatC
299809.  Fri Mar 21, 2008 5:51 am Reply with quote

Fighter planes? Fashion?

You’ll have heard that the MCC is trialling pink cricket balls (in place of white ones) this year. But some experts have warned that they are barking up the wrong stump; although pink balls might last longer, they might be harder to see.

During WW2, photoreconnaissance Spitfires were painted pink to camouflage them against low cloud which - especially in the evening - often has a pink tinge. So, a pink cricket ball in the air, against a background of low cloud, might be tricky to pick out.

(The ideal colour for a cricket ball, says a retired lecturer in clinical biochemistry, is yellow - since the 10% of boys and men who are colour blind can’t see red balls against the green grass, or pink ones against the blue sky).

But never mind all that - I just rather like the idea of pink Spitfires.

S: The Wisden Cricketer magazine, January 08

 
Flash
299819.  Fri Mar 21, 2008 6:17 am Reply with quote

Yes, that's great, and it sits squarely in the middle of a "Fight or Flight" theme.

 
Flash
299826.  Fri Mar 21, 2008 6:30 am Reply with quote



There's film of a pink Spitfire in a museum at http://www.youtube.com/watch?v=gIvUN539a7A but it seems from the attached comments that this one has now been painted blue (because they used pink for low-level and blue for high-level recce).

And elsewhere:
Quote:
there is no doubt that pilots felt rather vulnerable over Occupied Europe in an unarmed pink aircraft

http://www.spitfiresociety.demon.co.uk/whatmark.htm

Picture researchers, maybe you could get onto this? If there's one in a museum somewhere I should think a field trip is called for.

 

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