Nuclear

Part 4 - Alpha Radiation

In 1814, Joseph von Fraunhofer invented the first spectroscope, using a prism, diffraction slit and telescope, to analyze light from the sun, and noticed that the spectrum of frequencies produce distinctive lines. Some frequencies were brighter than others and some frequencies were missing (dark).

An analysis by Gustav Robert Kirchhoff and Robert Bunsen also provided a chemical explanation of stellar spectra by indicating the elements present in the outer layers of the sun and other stars.

The spectroscope was subsequently used to identify elements during chemical research. It also permitted metallurgists to quickly determine the precise amount of impurities and alloying elements in steel, aluminum and other materials.

In 1899, the New Zealand physicist, Ernest Rutherford working at McGill University in Montreal, Canada, discovered that uranium emitted alpha particles spontaneously. He, and Paul Ulrich Villard, a French chemist and physicist working in Paris, identified three types of radiation, later known as alpha, beta, and gamma, based on the distance each penetrated solid material and how they were deflected by a magnetic field.

In 1900, Paul Villard, placed radium salts in a shielded container and examined the radiation emanating from a narrow aperture, using photographic plates to record the intensity. He covered the aperture with a thin piece of lead that would stop alpha rays. He then showed that some of the remaining radiation could be deflected by a magnetic field so it was the beta radiation (electrons) identified by Ernest Rutherford. However, he then found a previously unidentified third type of radiation that could penetrate a substantial amount of lead shielding and was not deflected by a magnetic field. 

 Rutherford confirmed the existence of these new rays and, in 1903, named them gamma rays (very high frequency electro-magnetic radiation). In 1908, Villard pioneered the use of an ionization chamber (known as a dosimeter) to measure ionizing radiation. He defined a unit of dose that was later named the roentgen.

Rutherford also measured the ratio of an alpha particle's mass to its charge, and hypothesized that alpha particles were doubly charged helium ions (helium nuclei missing both electrons).In 1900, he found thorium was emitting a radioactive gas and persuaded Frederick Soddy, a young chemist, to analyze it. It was the inert, non radio-active gas, argon. Thorium was slowly and spontaneously converting itself into argon! 

Rutherford and Soddy systematically analyzed a number of radio-active products and found that each had a characteristic half-life (The time when half of the atoms transmuted themselves into different atom, what Soddy later termed, isotopes). 

The half life of 238-uranium (the more common isotope) is 4.5 billion years while radium has a half-life of 1620 years. Of several decay products of thorium, one has a half-life of 27 days, another only 22 minutes. Some have half-lives of milliseconds. 210-Polonium, with a half-life of 138 days, emits highly radioactive alpha particles (helium nuclei with two protons and two neutrons). 210-Po decays directly to the stable isotope 206-Po and is found naturally in only tiny traces. There are 33 isotopes produced by the decay of 238-U, from 188-Po to 220-Po, all of which are radioactive. 209-Po has the longest half life of about 125 years.

Rutherford demonstrated that beta radiation was identical to high energy electrons (cathode rays). He also realized that these transmutations explained why helium was trapped inside the crystals of uranium. It was a decay product (an alpha particle).

In 1903 Soddy and Rutherford published the first calculations of the energy released. Radio-active transmutations provided between 20,000 and a million times more energy than any chemical reaction.

In 1913, H.G.Wells published The World Set Free. It imagined a world where 'atomic disintegration' destroyed 200 cities with 'atomic bombs.'

In 1905, Otto Hahn was working with Rutherford in Montreal, when Rutherford proved that alpha particles were helium nuclei. In a typically elegant experiment, Rutherford had his glassblower make a tube with an extremely thin wall. He filled this with radon, a gas that emitted alpha radiation, before sealing the tube and placing it inside a larger tube that was sealed after the air had been pumped out. Several days later the outer tube showed a clear spectrum of helium. Alpha particles from the radon decay had easily passed through the thin glass tube and accumulated in the outer tube.