Monday, April 29, 2013

History of Science -- Part Eighteen: Protons

Ernest Rutherford
Around 460 B.C., a Greek philosopher, Democritus, developed the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.

For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter. Not until the early 1800's did people begin again to question the structure of matter.

Then an English chemist, John Dalton performed experiments with various chemicals that showed that matter seemed to consist of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that the evidence pointed to something fundamental.

The German physicist Johann Wilhelm Hittorf studied electrical conductivity in rarefied gases. In 1869, he discovered a glow emitted from the cathode, the part connected to the negative terminal of a battery. In 1876, the German physicist Eugen Goldstein showed that the rays from this glow cast a shadow, and he dubbed the rays cathode rays.

During the 1870s, the English chemist and physicist Sir William Crookes developed the first cathode ray tube to have a high vacuum inside thereafter called the “Crookes’ tube.” He then showed that the luminescence rays appearing within the tube carried energy and moved from the cathode to the anode, the part connected to the positive terminal of the battery. Furthermore, by applying a magnetic field, he was able to deflect the rays, thereby demonstrating that the beam behaved as though it were negatively charged.

In 1896, the British physicist J. J. Thomson, with his colleagues, performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms, or molecules as was believed earlier. Thomson made good estimates of both the charge e and the mass m, finding that cathode ray particles, which he called "corpuscles," had perhaps one thousandth of the mass of the least massive ion known: hydrogen. He showed that their charge to mass ratio, e/m, was independent of cathode material. He further showed that the negatively charged particles produced by radioactive materials, by heated materials, and by illuminated materials were universal. The name electron was proposed for these particles by the Irish physicist George F. Fitzgerald.

These cathode rays and tubes with cathodes and anodes enclosed in a high vacuum became the “high tech” of the first half of the twentieth century. The so called “vacuum tubes” had a cathode “emitter” that was heated by a light bulb-like filament to “boil off electrons.” For example, the 6L6 vacuum tube has a six-volt filament and the 12AT7 has a 12-volt filament. Between the cathode and the anode are one or more screens called “grids.” A triode has a single grid in addition to the cathode and anode. The “A” battery in old radios would provide the filament power and the “B” battery would provide a positive voltage to the anode or “plate” called B+. Vacuum tubes required several hundred volts B+ to operate.

There are vacuum tube “diodes” with only a cathode and an anode. They can’t be used to amplify a signal, but they can be used like one way valves ("valve" is also the British term for a vacuum tube) to change AC current into DC current.

Another form of cathode ray tube or CRT applied a very large negative voltage to the cathode, over a thousand volts in some cases, and then surrounded the sides of a funnel shaped tube with a grounded anode. The electron beam would fly down the middle of the tube and hit a fluorescent screen at the end. The beam would be bent by magnetic fields in a TV and the CRT is called a “picture tube.”

Comparing mass and charge to the simplest atom of all, the hydrogen atom (with its one electron and one proton) soon convinced scientists that this cathode ray consisted of a component of an atom, and that these electrons could be detached from the atom by heat (and, as additional studies showed, by large electric fields and also the photoelectric effect explained by Einstein.)

Since atoms are electrically neutral, it was obvious that an atom must have some positive charged material to offset the negative charge of the electron.

I’ve described before the various theories of a “rice pudding” like atom and that Ernest Rutherford arrived at the correct structure of the electrons spinning around the nucleus. So what is the nucleus made of?

Was the nucleus one solid substance? Further, since we know electrons are very light compared to even the hydrogen nucleus, what forms the nucleus of larger atoms such as carbon, oxygen, iron, or gold.

One theory was that the more massive (and “larger”) atoms were made up of a combination of hydrogen atoms. This concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout proposed that all atoms are composed of hydrogen atoms (which he called "protyles"), based on a simplistic interpretation of early values of atomic weights called “Prout's hypothesis,” which was disproved when more accurate values were measured. (Prout had failed to account for neutrons, which we'll hear about in another chapter.)

Ernest Rutherford was the son of James Rutherford, a farmer, and his wife Martha Thompson, originally from England. James had emigrated to New Zealand from Perth, Scotland, "to raise a little flax and a lot of children." Ernest was born at Spring Grove (now Brightwater), near Nelson, New Zealand.

He studied at Havelock School and then Nelson College and won a scholarship to study at Canterbury College, University of New Zealand where he was president of the debating society, among other things. After gaining his BA, MA and BSc, and doing two years of research during which he invented a new form of radio receiver, in 1895 Rutherford was awarded an "1851 Exhibition Scholarship" to travel to England for postgraduate study at the Cavendish Laboratory, University of Cambridge.

Under the inspiring leadership of J. J. Thomson he managed to detect radio waves at half a mile and briefly held the world record for the distance over which electromagnetic waves could be detected, though when he presented his results at a meeting in 1896, he discovered he had been outdone by another lecturer by the name of Marconi.

In 1898 Thomson offered Rutherford the chance of a post at McGill University in Montreal, Canada. Rutherford accepted, which meant that in 1900 he could marry Mary Georgina Newton to whom he had become engaged before leaving New Zealand. In 1907 Rutherford returned to Britain to take the chair of physics at the University of Manchester.

Along with Hans Geiger and Ernest Marsden in 1909, he carried out the Geiger–Marsden experiment, which demonstrated the nuclear nature of atoms. It was Rutherford's interpretation of this data that led him to formulate his model of the atom in 1911 — that a very small charged nucleus, containing much of the atom's mass, was orbited by low-mass electrons.

In 1917, Rutherford proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of the proton. Rutherford had earlier learned to produce hydrogen nuclei as a type of radiation produced as a result of the impact of Alpha particles on hydrogen gas, and recognized them by their unique penetration signature in air and their appearance in scintillation detectors.

(Alpha particles are two protons and two neutrons bound together. It is the nucleus of the helium atom stripped of its two electrons. Alpha particles, were known before anyone understood the structure of the atom and the nucleus. So called Beta particles are actually high speed electrons (or their antimatter twin, positrons). These “rays” were first discovered in the study of radioactivity and X-rays, which was also part of the discovery of the atom and its components.)

Rutherford knew hydrogen to be the simplest and lightest element and was influenced by Prout's hypothesis that hydrogen was the building block of all elements. Discovery that the hydrogen nucleus is present in all other nuclei as an elementary particle, led Rutherford to give the hydrogen nucleus a special name as a particle, since he suspected that hydrogen, the lightest element, contained only one of these particles.

He named this new fundamental building block of the nucleus the proton, after the Greek word for "first", πρῶτον. Plus, Rutherford also had in mind the word protyle as used by Prout.

The single proton in the nucleus of hydrogen was easy to understand. But what about helium and heavier atoms such as carbon or oxygen. You see, the problem is, what would hold the nucleus together. Recall that we know that like charges repel. So what holds the two protons in helium together in the nucleus, not to mention the six protons in carbon or the eight in oxygen, or the 82 in lead.

To overcome the repulsion of the electric force would require a force even stronger than the electric force. And if this force was so strong, why hadn't it been detected before? Was it a force that was only present in the nucleus of an atom?

Some of the answers hinted at another particle in the nucleus as well as a new force. Up until this point, only gravity and electromagnetic force were known. The study of the nucleus led to a discovery of a third force called the “strong nuclear force.” However, that has to wait for the 1930s and the discovery of the neutron to really be understood. And that leads to the discovery of a fourth fundamental force, also present in the nucleus.

So many questions. So many chapters. Another coming your way.

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