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Old 2020-08-10, 03:02   #221
Dr Sardonicus
 
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Quote:
Originally Posted by ewmayer View Post
I hate such later hagiography where the role of the actual other 'lesser' scientists who made the discoveries get written out in favor of the Big Names. In this case, Joliot-Curie, Enrico Fermi and Leo Szilard, who approached the by-then-elder-statesman Einstein because they figured someone of his stature would be more likely to catch the president's ear.
Yes, this is the story I learned. WRT discovering the potential of uranium, the name Lise Meitner also comes to mind.

You might be amused at a misdiagnosis by Dr. Einstein in this regard, mentioned, e.g. here:
Quote:
In 1932, the great physicist claimed that "there is not the slightest indication that nuclear energy will ever be obtainable. That would mean that the atom would have to be shattered at will."
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Old 2020-08-13, 23:35   #222
ewmayer
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Re. the Shinkolobwe mine, my memory got mixed up re. the natural fission reactor - it's actually at Oklo, Gabon, a couple W African statelets NW of the Shinkolobwe mine:
Quote:
The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials no longer could sustain a chain reaction.
...
A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope U235 made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 96.9% was non-fissile U238.) Because U235 has a shorter half-life than U238, and thus decays more rapidly, the current abundance of U235 in natural uranium is about 0.70–0.72%. A natural nuclear reactor is therefore no longer possible on Earth without heavy water or graphite.

The Oklo uranium ore deposits are the only known sites in which natural nuclear reactors existed. Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water and physical conditions needed to support the chain reaction was unique, as far as is currently known, to the Oklo ore bodies.
Note that the U235/U238 isotopic ratio at the time of Earth's formation was a global constant, due to whatever primordial supernova(e) which created the heavy elements incorporated into the Earth having mixed said elements more or less uniformly throughout the presolar nebula.

Getting back to the question of the % of U and Pu atoms which fissioned in the Hiroshima and Nagasaki A-bombs - this would fit better in the Science subforum, but by way of continuity:

Hiroshima: Wikipedia says: "[Little Boy] contained 64 kg (141 lb) of enriched uranium, although less than a kilogram underwent nuclear fission."

For the Nagasaki A-bomb, Wikipedia says "6.2 kg of Plutonium-239, [of which]about 1 kg fissioned", thus more than 10x more efficient than Little Boy.

Modern fission weapons (most often used as the initial small 'trigger' stage of a fusion bomb) achieve significantly higher efficiencies even than Little Boy via use of fusion boosting, so we could conceivably be talking of on the order of half the atoms of fission core undergoing fission.

Last fiddled with by ewmayer on 2020-08-13 at 23:36
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Old 2020-11-23, 08:47   #223
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Today (23 November 2020), the Dutch city of Haarlem is 775 years old!
(The district of Harlem in New York City was named for our Haarlem, but is much younger.)
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Old 2020-11-23, 22:06   #224
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Quote:
Originally Posted by Nick View Post
Today (23 November 2020), the Dutch city of Haarlem is 775 years old!
(The district of Harlem in New York City was named for our Haarlem, but is much younger.)
Happy Birthday, Haarlem!
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Old 2020-11-26, 12:54   #225
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Today (26.11.2020)
The most interesting what happened this day:

1949 India becomes a sovereign Democratic republic.
1922 Lord Carnarvon and Howard Carter, archeologists, open King Tut's tomb, undisturbed for 3,000 years.
1789 George Washington proclaims this a National Thanksgiving Day in honor of the new Constitution. This date was later used to set the date for Thanksgiving.
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Old 2020-11-27, 01:57   #226
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Quote:
Originally Posted by AJensen89 View Post
Today (26.11.2020)
The most interesting what happened this day:
<snip>
1789 George Washington proclaims this a National Thanksgiving Day in honor of the new Constitution. This date was later used to set the date for Thanksgiving.
Currently, Thanksgiving Day in the USA is the fourth Thursday in November.

Thus, it can occur as early as November 22 (as it did in 2018), and as late as November 28 (as it did in 2019).

Not a religious holiday, but even so, a "moveable feast," in a manner of speaking...
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Old 2020-12-30, 21:24   #227
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When I was a kid, I heard that a fire, the Iroquois Theater Fire, was the reason that exits from public buildings open outward. It turns out it also led to a related invention - "panic bars." They are described in a hardware company piece, How the Deadliest Fire in U.S. History Saved Countless Lives.

I just ran across a mention of the fire in an "On this day" listing. It happened on December 30, 1903.
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Old 2021-01-12, 14:42   #228
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Default January 12

January 12, 1907 - Sergei Korolev, designer of Soviet rocket and space systems, academician, founder of practical cosmonautics, was born.
I think this event is quite important.
January 12, 1955 - the beginning of construction of the Baikonur cosmodrome. This is the first and also the world's largest cosmodrome.
January 12, 2005 - Deep Impact spacecraft launch.
Spacecraft from NASA, designed to study comet Tempel 1. For the first time in history, the spacecraft dropped a probe on the comet, which rammed its surface, having previously photographed it at close range.
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Old 2021-01-19, 05:26   #229
LaurV
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Martin Luther King Jr. day, anybody there?(we were just talking about racism and liberties in another thread...)
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Old 2021-01-19, 12:47   #230
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Quote:
Originally Posted by LaurV View Post
Martin Luther King Jr. day, anybody there?(we were just talking about racism and liberties in another thread...)
His birth date was [Google Google] January 15, 1929. I remember, when I was young, seeing the "Childrens' Crusade" on the news - kids having their clothes shredded, and being pinballed down the streets by the water from fire-pressure fire hoses, while adults were attacked by police dogs and billy clubs. Those images shocked the world.

I remember hearing on the news about three civil rights workers who had gone missing, and whose bodies were later found. I remember hearing about a church being bombed, and four little girls being killed. I remember hearing about "Bloody Sunday" on the Edmund Pettis Bridge. Those images shocked the world.

I remember when he was assassinated.

I remember racist US Senator Jesse Helms opposing the establishment of a Federal holiday honoring Martin Luther King.

Last fiddled with by Dr Sardonicus on 2021-01-19 at 12:49
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