[FirstClassSkeptic]

Dr. Louis Slotin
What is tickling the Dragon's tail?


Question:

By the way, what would be the exact quantity of enriched Uranium 239 needed to produce a continuous chain reaction with minimal initiation energy?

Answer:

(from Science Education Partnerships)

My guess is that about 8KG of Uranium would be about enough. (Note from Larry Lavitt: DO NOT) Try this experiment: Get a number of blocks of Uranium, in different sizes. Start with about 1KG pieces, and up to about 5KG in 1KG increments. Don't put them all in the same box.

Take two of the smaller pieces, and record their masses. Then carefully push the two blocks together. If nothing interesting happens, it isn't big enough. Record that fact. When something interesting does happen, you will (briefly) know the mass needed for a chain reaction.

Note: In the old days, they did this in deserted canyons and called it "tickling the dragon's tail". There aren't many deserted canyons nearby, but most anyplace in California will do.

Actually, this is exactly how atomic physics operated during the cowboy stage of World War II in the Manhattan Project to build the Atom Bomb.

They didn't have the powerful electronic calculators which came after the war, so they used women, who were called ... calculators.

And they determined the critical mass of the bombs by exactly the method you describe. It was called "tickling the tail of the dragon", and it was done with a couple of screwdrivers held in the bare hands, working on top of a bare metal lab table in a shed down in a canyon off the mesa at Los Alamos.

They pushed the hemispheres together till they were close enough to produce the beginning of a chain reaction, which they detected by the blue glow set up in the air around the hemispheres by Cerenkov radiation (ionisation of the air itself). Then they pushed the hemispheres apart with the screwdrivers.

While having the hemispheres meet would not have produced a true rapid chain reaction and nuclear explosion, it would have produced a meltdown and a chemical explosion from heat and spread huge amounts of radioactive debris.

After the war, as experimentation continued, it got to be quite a daring sport. At least two workers made mistakes, lost control of the screwdrivers, and died.

One, Louis Slotin, saved a roomful of colleagues from lethal radiation exposure when he slipped and the hemispheres met, by pusing them apart with his bare hands. He died -- nastily -- in about five days. All the others lived.

From: http://www.dandugan.com/waldorf/WCA%20A ... A9711.html (No longer available.)

Created: June 7, 1999

http://web.ncf.ca/lavitt/louisslotin/tickle.html

The test assembly for "tickling the dragon's tail", an experimental determination of the critical mass of the uranium bomb. A small uranium hydride slug was dropped through this almost critical assembly of slugs. For a fraction of a second, the assembly "went critical".

http://www.mphpa.org/classic/LA/Photo-P ... AP-217.htm

Excuse me, but isn't a 'fraction of a second' all it takes?

[rerevisionist]

A friend of mine who worked in nuclear power all his life (and a considerable sceptic about physics - he has some very good ideas) told me he had, himself, seen a reactor go critical, just once. I'm trying to get him to answer some of the questions this site poses, including of course this one, but I don't think he will. SIgh.

[rerevisionist]

In 1999, I had an email exchange with R Kiehn, including material on tickling the dragon's tail. (The subject was whether e=mc^2 had any influence on nuclear weapons which, at the time, I assumed to exist). Kiehn claimed - see below, red - he made critical assemblages, and has some patents, but he seemed mostly interested in computer programming in the surrealist style judging by the website link he provided ---

R KIEHN TO R WEST 6 Sep 1999 Subject: Slotin
FYI (from one who has constructed numerous critical assemblies) Slotin used a screw driver to prop open two hemispheres of fissionable material. He would reduce the gap to cause the neutron counting rate to increase as the assembly approached criticality. He would increase the gap to lower the neutron counting rate. He was showing off to another colleague when the screw driver slipped. The assembly went critical and self disassembled due to thermal expansion. Slotin received a lethal dose >> 500 R His colleague survived, and died much later of natural causes.

***Although you say that E=mc^2 did not make a difference to the development of the nuclear weapons, IMO you are wrong. Indeed a number of experimental results ultimately led to the result, but if it took 1,000,000 tons of uranium to make a 20 kt device it would not have been built. - RMK

** This doesn't show that 'E=mc^2' had any effect. The whole thing was empirical. If no energy had been got from the atomic pile, presumably experiments would have stopped, or something else would have been tried.
To put it another way, if the equation had never been invented, nevertheless there was widespread awareness (cf slow cooling of the earth, energy from radium etc) that, in principle, a lot of energy was there in atoms; there are plenty of pre-Einstein statements to that effect. I see no evidence that the formula made any difference to the actual work on nuclear weapons. Still less, so to speak, did it help with the work. - Regards Rae

R KIEHN TO R WEST 6 Sep 1999 Subject: Re: Slotin
BUT I WAS THERE

You don't make it clear whether you mean with Slotin or just generally involved. As regards Slotin, and no doubt others, the industrial injury side of things isn't very relevant to the main thesis.

The idea that e=mc^2 had a contributing part in atomic bombs seems a myth. Except perhaps in the sense of suggesting a lot of energy 'locked' in matter.

Regards Rae West

R KIEHN TO R WEST 10 Sep 1999 Subject: Re: Slotin

I was not with Slotin during his blue glow accident. I have tickled the dragon's tail. I think you are wrong about the effect of e=mc^2 on the thinking of those who designed nuclear devices. It did influence my thinking.

Have you had experience designing nuclear devices?

R WEST TO R KIEHN Sat, 11 Sep 1999 Subject: Re Slotin
[previous email snipped]
Have you any experience of theoretical physics?

You don't say what you mean by 'influence your thinking'. Have you tried to measure whether there's loss in mass with 'nuclear devices'?

Regards Rae West

R KIEHN TO R WEST Sat, 11 Sep 1999 Subject: Credentials
<< Have you any experience of theoretical physics?>> [snip]

Yes. A bit:
See http://www22.pair.com/csdc/car/carhomep.htm

PhD Physics MIT.

15 years with the AEC, 35 Years plus as a professor of physics. Many publications, a few patents (including Plutonium Fast Breeder Reactors and dual polarized ring lasers).

I have stacked Pu and U235 into critical assemblies of various types, and have conducted experiments that involved nuclear explosions.

Also I have been a programmer for more than 45 years, having written multi-group neutron transport programs at LASL for the design of nuclear devices in the early days of computers.

I made some of the first measurements of neutron inelastic scattering cross sections, and other neutron cross sections, and must say that the Q values associated with neutron reactions were correlated with mc^2 concepts.

Measurements of recoils of charged particles in magnetic spectrometers also justifies the mc^2 idea. The Compton experiment also gives a level of credence to the idea. I have been involved in such experiments.

NOW What are your credentials?
Do you know anything about Theoretical Physics? Do you know anything about experimental Physics?
Have you ever brought an assembly of fissionable material to criticality? Have you ever conducted neutron nuclear reaction experiments.

**Although I have never placed Einstein on a pedestal, I think you are wrong about the influence of E=mc^2 on the development of nuclear devices. The development of Oak Ridge and the Hanford Reactors were things that were started well before the Trinity event. Without the Einstein letter to FDR, it is doubtful that anything would have been started at such magnitudes of investment. In this manner, the popularity of the E=mc^2 concept to the nontechnical mind had an enormous impact.

***You can voice your (journalistic) opinion, but I would suggest you talk more to people who were involved, rather than pontificating your point of view.
Regards RMK

R WEST TO R KIEHN Sat, 11 Sep 1999 Re: credentials
[previous email snipped]
Yes. I co-wrote this piece with Phil Holland, who spent his working life involved in various ways with physics, including many years at Sellafield, having got as I understand it a double first in physics and chemistry from Cambridge.

The question at issue is whether, as is widely believed, e=mc^2 played a crucial part in developing atomic weapons. Not whether Einstein's letter had influence. [Rest of this email snipped, as irrelevant to the criticality issue - rerevisionist]

According to Phil Holland, not one person at Sellafield took the e=mc^2 idea seriously; they all regarded the energy as produced by rearrangement of the nucleus, and nothing to do with destruction of matter.

Regards Rae West

[FirstClassSkeptic]

I made some of the first measurements of neutron inelastic scattering cross sections, and other neutron cross sections, and must say that the Q values associated with neutron reactions were correlated with mc^2 concepts.

Measurements of recoils of charged particles in magnetic spectrometers also justifies the mc^2 idea. The Compton experiment also gives a level of credence to the idea. I have been involved in such experiments.

That doesn't sound like any experiments wherein matter was destroyed, much less the mass destroyed being measured, and the energy released being measured.

http://hyperphysics.phy-astr.gsu.edu/hb ... ptint.html

Doesn't sound like matter is destroyed, or mass measured, in the Compton experiment.

Perhaps no one has ever done such an experiment.