Simple Nature - Light and Matter

It was already known that although alpha particles would bestopped completely by a sheet of paper, they could pass through asufficiently thin metal foil. Marsden was to work with a gold foilonly 1000 atoms thick. (The foil was probably made by evaporatinga little gold in a vacuum chamber so that a thin layer would bedeposited on a glass microscope slide. The foil would then be liftedoff the slide by submerging the slide in water.)e / Ernest Rutherford (1871-1937).f / Marsden and Rutherford’sapparatus.Rutherford had already determined in his previous experimentsthe speed of the alpha particles emitted by radium, a fantastic 1.5×10 7 m/s. The experimenters in Rutherford’s group visualized themas very small, very fast cannonballs penetrating the “cookie dough”part of the big gold atoms. A piece of paper has a thickness of ahundred thousand atoms or so, which would be sufficient to stopthem completely, but crashing through a thousand would only slowthem a little and turn them slightly off of their original paths.Marsden’s supposedly ho-hum assignment was to use the apparatusshown in figure f to measure how often alpha particles weredeflected at various angles. A tiny lump of radium in a box emittedalpha particles, and a thin beam was created by blocking allthe alphas except those that happened to pass out through a tube.Typically deflected in the gold by only a small amount, they wouldreach a screen very much like the screen of a TV’s picture tube,which would make a flash of light when it was hit. Here is the firstexample we have encountered of an experiment in which a beam ofparticles is detected one at a time. This was possible because eachalpha particle carried so much kinetic energy; they were moving atabout the same speed as the electrons in the Thomson experiment,but had ten thousand times more mass.Marsden sat in a dark room, watching the apparatus hour afterhour and recording the number of flashes with the screen moved tovarious angles. The rate of the flashes was highest when he set thescreen at an angle close to the line of the alphas’ original path, but ifhe watched an area farther off to the side, he would also occasionallysee an alpha that had been deflected through a larger angle. Afterseeing a few of these, he got the crazy idea of moving the screen tosee if even larger angles ever occurred, perhaps even angles largerthan 90 degrees.The crazy idea worked: a few alpha particles were deflectedthrough angles of up to 180 degrees, and the routine experimenthad become an epoch-making one. Rutherford said, “We have beenable to get some of the alpha particles coming backwards. It wasalmost as incredible as if you fired a 15-inch shell at a piece of tissuepaper and it came back and hit you.” Explanations were hard tocome by in the raisin cookie model. What intense electrical forces478 Chapter 8 Atoms and Electromagnetism