The Life History Of One Uranium Nuclei

One final nugget from E = mc²

Uranium²³⁵ is a nuclei which can undergo spontaneous fission. Radioactivity is a stochastic process and it is not possible to predict exactly when any one particular nuclei will decay. However "mean lifetime" (T) or "decay constant" (reciprocal of mean lifetime) is a measure of the longevity of a nuclei and is proportional to the half life of the isotope; t1/2 = Tln2. U²³⁵ has a half life (t1/2) of about 700 million years. That means that any one U²³⁵ nuclei may remain stable without decaying for hundreds of millions to few billion years....unless it is made to decay by inducing fission...

...which is what happened to a few U²³⁵ nuclei when the bomb hatches of the Enola Gay opened and Little Boy began hurtling towards Hiroshima:

The uranium atoms mined on Earth were each over 4.5 billion years old. Only a very powerful force , before the Earth was formed had been able to squeeze their electrically crackling protons together. Once that uranium had been formed, the strong nuclear force had acted, gluelike, to hold these protons in place over all that long span; while the Earth cooled, and continents formed; as America separated from Europe, and the North Atlantic Ocean slowly filled; as volcanic bursts widened on the other side of the globe, forming what would become Japan. A single extra neutron unbalanced that stability now...

... the density of uranium was enough that a chain reaction started, and soon there weren't just two speeding fragments of uranium nuclei, there were four, then eight, then sixteen, and so on. Mass was "disappearing" within the atoms, and coming out as the energy of speeding nuclei fragments. E = mc² was now under way.

1900 feet over Hiroshima, a few billion years of stable existence for those U²³⁵ nuclei ended. Nature hangs by a precarious balance...

Preparing For The Next Big Oil Spill

David Brooks writes a perceptive column in NY Times in which he argues that preparation for the next big disaster has to include not just better technical and mechanical processes but improvements in understanding human psychology which is not very good at complicated risk assessment:

So it seems important, in the months ahead, to not only focus on mechanical ways to make drilling safer, but also more broadly on helping people deal with potentially catastrophic complexity. There must be ways to improve the choice architecture — to help people guard against risk creep, false security, groupthink, the good-news bias and all the rest. 

This isn’t just about oil. It’s a challenge for people living in an imponderably complex technical society.

He quotes Malcolm Gladwell a lot.... synthesizer of stories on how seemingly disparate small events can link up and cascade into a really big event.

Computers And Sexism In 1920's Harvard

When astronomy student Cecilia Payne arrived at Harvard in 1923:

...She also got a glimpse of what was going on in the back rooms of the Observatory. In 1923, the word computer did not mean an electrical machine. It meant people whose sole job was to compute. At Harvard, it was applied to ranks of slump-shouldered spinsters in those back rooms. A few of them had once had first-rate scientific talent ("I've always wanted to learn the calculus," one said, "but [the director] did not wish it"), yet that was usually long since crushed out of them, as they were kept busy measuring star locations, or cataloging volumes of previous results. If they got married they could get fired; if they complained of their low salaries, they would get fired as well.

...A few of the Harvard "computers" in several decades of bent-back work, succeeded in measuring over 100,000 spectral lines. But what it meant, or how it fitted in with the latest developments in physics, was almost always not for them to understand.

Cecilia Payne though went on to discover that the Sun was largely composed of hydrogen. At that time the understanding was that the Sun is composed mostly of iron and Payne's discovery was dismissed.  The eminent astronomers of that time discredited her and forced her to admit in her thesis:

"The enormous abundance [of hydrogen]...is almost certainly not real".

All this after Cecilia Payne had run away from Britain to escape the stifling discrimination against women pursuing science careers there.

...again from E=mc².

A Quiet Beginning To A Giant Explosion

I am reading E = mc² A Biography of the World's Most Famous Equation by David Bodanis.

Here is a passage about the pivotal moment in 1938 in Sweden when Lise Meitner and her nephew Robert Frisch worked out how much energy is released by the fission of one uranium nucleus:

...Fortunately [ my aunt] remembered how to compute the masses of nuclei.... and in that way she worked out that the two nuclei formed by the division of a uranium nucleus would be lighter than the original uranium nucleus, by about one-fifth the mass of a proton. Now whenever mass disappears, energy is created, according to Einstein's formula E = mc²  ....

..They had crossed a river on their walk out from Kungalv and it was frozen. The village was too far away to hear any market noises. Meitner did the calculation. Frisch remembers later: " One-fifth of a proton mass was just equivalent  to 200 MeV. So here was the source of that energy; it all fitted!"

The atom was open. Everyone has been wrong before. The way in wasn't by blasting harder and harder fragments at it. One women and her nephew, quiet in the midday snow, had now seen that.

The equation is always linked to Einstein and his work... but this book explores the role of physicists and chemists going back to Lavoisier, Faraday and Maxwell among others whose work on mass and energy built the foundation Einstein stood on and a later cast of characters who worked out the broader implications of the equation.

The added attraction is a suspenseful commando raid that destroyed Germany's heavy water supply essential to its growing nuclear program.

..Terrific stuff..

Accretionary Wedge: Geo-Images ..Calcite Cements

Entry for the Accretionary Wedge carnival hosted by Chris and Anne at Highly Allochthonous.

When I viewed these crystals under a microscope for the first time... I felt more relief than elation. I finally had a story to tell my PhD committee!!

These are photomicrographs of pendant calcite crystals which precipitated within meteoric aquifers that developed during late Ordovician sea-level falls....location... Appalachians...northern Georgia.

On left is a view of the crystal stained with potassium ferricyanide. Non ferroan early cements are not stained. Burial Fe rich calcite stains blue. On right is a view of the same crystal in cathodoluminescence (crystals are bombarded with cathode rays in a vacuum chamber).

Cathodoluminescence helps understand pore fluid fluctuations between oxidizing and reducing conditions. In oxidizing pore-fluids, neither Mn+4 or Fe+3 is incorporated into growing calcite crystals, and thus cements are black (non-luminescent). In pore fluids with progressively lower Eh , reduction of Mn first and then Fe leads to their incorporation into the growing cements, giving the crystals a bright to dull luminescence. The image on left shows the changing geochemistry of pore fluids from oxidizing to mildly reducing to reducing as the sequence got buried.
 
All this geochemistry makes more sense when placed in a stratigraphic context. The image below shows a cyclic late Ordovician sequence and the position of calcite cements within it. Cathodoluminescent signatures help constraint the lower limits of the fresh water aquifer that developed during successive sea-level drops.

During the first sea-level drop both vadose and phreatic meteoric conditions are recognized and groundwater fluctuated between oxic to mildly reducing. During the next sea-level drop (Ordovician-Silurian unconformity) the meteoric aquifer was reducing and only bright luminescent phreatic cements precipitated. The vadose zone is not preserved in the younger sequence.

This kind of cement stratigraphy helps understand the geochemistry and the lateral and vertical extent of groundwater systems and fluid flow patterns that develop as sea-level drops and basins are exposed to fresh water infiltration. Fluid - sediment interaction create and destroy porosity and permeability,  properties which in turn influence the hydrocarbon and mineral potential of the sedimentary sequence.