Nuclear

Part 5 - Proton

1505 Rutherford Gold foil 210619 (1450) Wikipedia Zapscience

Between 1908 and 1913, at the University of Manchester in Britain, Hans Geiger and Ernest Marsden, directed by the New Zealand physicist, Ernest Rutherford, conducted a series of experiments to determine the structure of the atom.

The "plum pudding model", devised by Lord Kelvin and developed by J. J. Thomson postulated that the negatively charged electrons were embedded in a positively charged mass making the atom neutral. But several scientists disagreed with this idea. Japanese scientist Hantaro Nagaoka pointed out that opposing charges could not penetrate each other and suggested that electrons orbited the positive charge like the rings around Saturn.

The existence of protons and neutrons was unknown at this time but scientists knew that atoms were extremely small. (Rutherford thought their diameter might be less than a ten millionth of a millimetre).

Rutherford had already discovered the existence of alpha rays (helium nuclei), beta rays (electrons) and gamma rays (high energy electro-magnetic radiation), and had proved that these came from the spontaneous disintegration of the atoms of some substances such as uranium and radium. 

Rutherford had discovered alpha particles in 1899 and, in 1908, he was trying to precisely measure their charge-to-mass ratio. (Alpha particles are tiny, positively charged particles far too small to be seen with a microscope). However, Rutherford knew that they knocked electrons from the molecules of air (ionizing them) so he and Geiger designed a counting device using two electrodes in a glass tube. 

Every alpha particle that passed through the tube would ionize air molecules and these would flow to the cathode creating a pulse of electricity that could be counted. It was a prototype of the Geiger counter.  Unfortunately, the alpha particles were strongly deflected by their collisions with the molecules of air in the detection chamber and did not all generate the same number of ions, producing erratic readings.

Rutherford was puzzled, because he had assumed that alpha particles (containing 2 protons and 2 neutrons) were just too heavy to be deflected so much, and asked Hans Geiger to find out how much matter would scatter alpha rays.

In 1908 Geiger, made a glass tube nearly two metres long with radium at one end, spontaneously emitting alpha particles. The alpha particles passed through a 0.9 mm-wide slit in a plug blocking the middle of the tube and created a glowing patch of light on a phosphorescent zinc sulphide screen at the far end.

Each impact of an alpha particle on the screen produced a tiny flash of light that lasted long enough for Geiger, working in a darkened laboratory for hours on end, painstakingly counting the scintillations and measured their spread with a microscope.

Geiger pumped all the air out of the tube, so that the alpha particles would not be deflected by air molecules, and this left a sharp image of the slit on the screen. Geiger then placed gold foil over the slit and this made the image fuzzy as did air molecules. This proved that any substance would deflect alpha particles but Rutherford wanted to know if the alpha particles were being scattered by even larger angles . . . and the tube was too narrow.

In 1909 Geiger and Marsden bounced alpha particles off a a metal reflector onto a fluorescent screen on the other side of a lead plate proving that alpha particles could be scattered by more than 90°. At first they used a source containing radon, radium and 214-bismuth all of which emitted alpha particles. Counting the scintillations on the screen they found that metals with higher atomic mass, such as gold, reflected more alpha particles than lighter ones such as aluminum.

Geiger and Marsden then wanted to know the total number of alpha particles that were being reflected but found the different isotopes in the alpha particle source made it difficult to determine the rate of emission. So, they made a new source using only 214-bismuth and bounced the particles off a platinum reflector. They found that only a tiny fraction of the alpha particles bounced off the reflector and onto the screen (1 in 8,000) suggesting that most of the alpha particles were being absorbed or simply passing through the densely packed atoms of the heavy metal reflector!

Rutherford preferred 222-radon as a source of alpha particles, as it was naturally several million times more radioactive than uranium. He also chose gold as a target material as it was heavier than most other materials and could be made very thin (about 400 nanometres).In 1910, Geiger constructed an airtight glass tube with a 222-radon source at one end. Two ports along the tube permitted Geiger to insert plugs with collimating holes and metal targets of various materials. At the far end of the tube was a fluorescent zinc sulfide screen and a microscope fitted with a vernier millimetre scale to count the scintillations on the screen. 

 This allowed Geiger to precisely measure where the flashes of light appeared on the screen so he could calculate the particles' angles of deflection. Geiger concluded that the deflection angle increased with the thickness of the target material and was also proportional to its atomic mass. The angle of deflection decreased with the velocity of the alpha particles and the probability that a particle would be deflected by more than 90° was extremely small.

This unexpected result suggested that most of the atom's volume was empty space!After studying the results, Rutherford developed a mathematical equation that modelled how the foil should scatter the alpha particles if all the positive charge and most of the atomic mass was concentrated in a single point at the centre of an atom. In a 1911 paper Rutherford proposed that the atom contained a very small and very intense electric charge in a tiny nucleus at its centre (in his calculations, Rutherford assumed it was a point charge). In 1913 Geiger and Marsden tried to prove Rutherford's equation. They built an apparatus to accurately measure the scattering pattern of the alpha particles produced by the metal target foil. The microscope and screen were fixed to a rotating cylinder and could be moved around the foil up to 150° so that they could count scintillations from every angle.

They used this to measure how the alpha particle scattering pattern varied in relation to the thickness of the foil, the atomic weight of the material, and the velocity of the alpha particles and found that these agreed with Rutherford's mathematical model. However, they also found that most alpha particles flew straight through the foil with negligible deflection. A few bounced off the metal foil in all directions but only a small fraction of the alpha particles were deflected by more than 90° while very few bounced straight back at the source. Obviously, those particles had directly encountered a very strong electrostatic force.

When Geiger reported to Rutherford that he had seen alpha particles being strongly deflected, Rutherford was astonished. In a lecture he said, 'It was quite the most incredible event . . . It was as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you . . . I realized that this scattering backward must be the result of a single collision . . . it was impossible . . . unless . . . the greater part of the mass of the atom was concentrated in a minuscule 'nucleus.'

In a 1913 paper, based on the result of experiments exploring the scattering of alpha particles in various gases, Rutherford stated that the nucleus was indeed positively charged. However, Rutherford's model did not explain why atoms are stable. According to his model, the electrons would spiral down ultimately shortening the distance between the electron and the nucleus and the atom would collapse.

In 1917, Rutherford and William Kay shot alpha particles into air (which is mainly nitrogen), and noticed that, the scintillation detectors showed the signatures of hydrogen nuclei. The effect was larger when Rutherford used pure nitrogen gas so, in 1919, he assumed that the alpha particle (being four times heavier) merely knocked a proton out of nitrogen, turning it into carbon.

It was not until he saw Patrick Maynard Stuart Blackett's cloud chamber images, in 1925, that Rutherford realized that the alpha particle (4 nucleons) was absorbed by the nitrogen nucleus which promptly ejected a proton (hydrogen nucleus) thereby transmuting 14-nitrogen into the heavy isotope 17-oxygen (14 plus 4 nucleons, minus 1 proton = 17). (The proton was a free hydrogen nuclei, (a proton). This was the first artificially induced transmutation ever recorded. Rutherford announced the discovery of the proton when he reported all his experiments in 1919 and 1925.

SEE   https://zapscience.com/rutherford-gold-foil-experiment/