Atomic Bomb In the history of the world, there is no invention that is more feared and loathed, nor

5.3.4 Atomic Bomb In the history of the world, there is no invention that is more feared and loathed, nor

one that has created more remorse among its inventors. After 2 million years of inventions and human progress, there is now a genuine possibility of being “bombed all the way back to the Stone Age” and an even more apocalyptic vision of wiping out humans on earth. But some believe that the doomsday vision of the end of earth is so frightening that it might have contributed to the current stalemate and absence of major wars since the end of the World War II in 1945, a peace of about 70 years. This uneasy peace should be compared with the only 20 years of peace from the end of the World War I to the start of the World War II.

A great number of advances in physics took place at the end of 1890 when only 92 elements were known, from the lightest hydrogen to the heaviest uranium. For thousands of years, alchemists have sought ways to change one element into another, such as lead into gold, which is called transmutation. By 1890, the atom was thought to be impossible to split, to have no internal structure, and was consid- ered a permanent building block of the universe. The ideas of transmutation and alchemy were also discarded.

Wilhelm R€ ontgen discovered X-ray in 1895 by bombardment of metal targets with cathode ray, which created the mysterious radiation that could penetrate into the flesh but was stopped by the bone. Uranium was found in pitchblende from a mine in Joachimsthal in Czechoslovakia. Antoine Becquerel discovered in 1896 that the uranium salts also emitted a mysterious ray that caused photographic plates in a drawer to fog and demonstrated great penetrating power comparable to X-ray. Then came the discovery of the electron by J.J. Thompson in 1897. Pierre and Marie Curie made further studies with uranium, and in 1898 found another element in pitchblende that is even more radioactive than uranium, and called it radium.

There is a tremendous amount of energy in the nucleus of some atoms, wait- ing to fly apart. In 1906, Ernest Rutherford bombarded thin gold foil with positively charged alpha particles and found that most particles went through but a few were bounced straight back to the source. This suggested that the gold atom has a very small nucleus that contains most of the mass and is positively charged, surrounded by mostly void space occupied by the negatively charged electrons. The structure of an atom was proposed by Rutherford in 1911 to have a small and heavy nucleus with positive charges, surrounded by a large cloud of negatively charged electrons.

It used to be thought that matter and energy are different concepts that are eternal and unchangeable. In 1905, Albert Einstein proposed that matter can

be changed into energy, and described the conversion by the famous equation

E 2 ¼ MC . In this equation C is the speed of light, which is rated at 186,000 miles/s or 300 million m/s so that a tiny amount of mass is equivalent to a great deal of

energy. In fact, if 1 kg (2.2 pounds) of matter were converted entirely into energy, it would produce 25 billion kWh of energy that is equivalent to 2.7 million tons of oil. However, at that time, there were no practical methods to change mass into energy.

179 Nevertheless, in 1914, H.G. Wells wrote a science fiction story called, “The World

5.3 HUMAN VIOLENCE: WAR

Set Free,” which discussed an air-dropped atomic bomb! In 1932, James Chadwick bombarded beryllium with alpha particles and dis- covered the neutron, which has no electrical charge and can be used to penetrate atomic nuclei as it is electrically neutral and would not be repelled by the positive atomic nucleus. In 1933, Leo Szilard in London conceived the idea that if you irradiate a nucleus with a neutron and cause it to create more than one neutron, this could set off a “chain reaction” with rapid increases in neutrons, which is similar to the chemical reactions in flames. Szilard filed for a patent on the concept in the following year, but in subsequent experiments with beryllium, he failed to create more neutrons. In 1934, Enrico Fermi bombarded uranium with neutrons and appeared to have created new elements that were heavier than uranium. For this discovery, Fermi was rewarded with the Nobel Prize in Physics in 1938. He was a physicist, and without the help of a chemist to analyze the properties of the reaction product, he did not realize that he had actually split the atom! The German chemist, Otto Hahn, and his assistant, Lisa Meitner, had worked together for 30 years; but Meitner was Jewish, which was becoming dangerous in 1938 Nazi Germany, so Hahn helped her to leave for Sweden. Hahn and his student, Fritz Strassmann, continued the project to bombard uranium with neutrons. Using chemical methods for analysis, they found barium among the products. Hahn passed the news to Lisa Meitner, who discussed the results with her nephew, Otto Frisch. Since barium has a lower atomic weight of 141 than uranium at 235, this had to mean that the uranium atom had been split in two, which was interpreted as nuclear fission, that is, the nucleus of uranium had split into two smaller atoms, such as barium and krypton.

U 235 92 þn 1 0 ! Ba 141 56 þ Kr 92 36 þ 3n 1 0 þ 180MeV Two numbers are attached to the symbol U that stands for uranium: the upper num-

ber 235 is the atomic mass, which is the sum of the number of neutrons and protons in the nucleus, and the bottom number 92 is the atomic number, which is the num-

ber of just protons. Thus, this atom has 235 92 ¼ 143 neutrons in the nucleus, as well as 92 electrons whirling around the nucleus. Neutron has the symbol n and MeV is the million electron volts, which is a measure of the energy release and an enormous value compared to burning a fuel such as carbon, which makes only about 10 eV. Uranium has two principal forms: U-235 can undergo fission but has a natural abundance of only 0.7%, and U-238 that does not undergo fission and has a much greater natural abundance of 99.3%.

This news about the splitting of the atom was brought by Niels Bohr from Denmark, who went to Princeton University in 1939 to give a seminar and to dis- cuss this development with Albert Einstein. There was great fear that Germany would build an atomic bomb soon, led by Werner Heisenberg, a famous founder of quantum mechanics with a Nobel Prize in Physics. In July 1939, Leo Szilard and Eugene Wigner conferred with Albert Einstein, followed by discussions with the financier, Alexander Sachs. In the fall, Sachs handed a letter from this group to President Roosevelt, urging the United States to work on the atomic bomb.