Monday, May 31, 2010

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...


  1. Hi Suvrat,
    I am a totally ignorent person. I have lot of curiosity about this. Can you elaborate little more on 'A single extra neutron unbalanced that stability now...'?
    I always wonder where does devastating energy comes from when a nuclear device is exploded? Why uranium only? And what is a chain reaction?

  2. 'A single extra neutron unbalanced that stability now. - in the uranium nucleus the strong nuclear force holds the nucleus together while the electrical repulsion between the postively charged protons tries to break it apart. That is in a very fine balance and it can be upset by bombarding the nucleus with neutrons. that extra neutrons makes the nucleus stretch and eventually split.

    Why uranium only? doesn't have to be but uranium is a radioactive element that occurs in sufficient quantities to be used for energy production. other examples are plutonium fission and hydrogen fusion

    what is a chain reaction? see this