Ernest Rutherford: Father of Nuclear Physics
Growing Up in New Zealand
The rugged South Island of New Zealand, known for its mountains, glaciers, and lakes, was truly frontier country during the mid-1800’s. Bold settlers from Europe were attempting to tame the land and survive half a world away from their homelands. Ernest Rutherford, who would go on to be the favorite son of this island nation, was born to James and Martha Rutherford on August 30, 1871, in a settlement thirteen miles from the nearest small town of Nelson. James did many things to make ends meet, including: farming, making wagon wheels, running a flax mill, and making rope. Martha tended to her large family of twelve children and was a school teacher. As a young boy Ernest worked on the family farm and showed great promise at the local school. With the help of a scholarship he was able to attend Canterbury College in Christchurch, one of the four campuses of the New Zealand University. At the small college he became interested in physics and developed a magnetic detector for radio waves. He completed his Bachelor of Arts degree in 1892 and continued the following year to complete a masters with first-class honors in physical science and mathematics. During his college years he fell in love with Mary Newton, the daughter of the women he boarded with.
Rutherford was an ambitious young man engrossed in everything science and found few opportunities in a land so far from the intellectual centers of Europe. He wanted to continue his education and participated in a scholarship competition to attend Cambridge University in England. He finished second in the competition but got lucky because the first place winner decided to stay in New Zealand and get married. The news of the scholarship reached Rutherford while he was digging potatoes on the family farm, and as the story goes, he threw down the spade and said “That’s the last potato I’ll dig.” He set sail for England leaving his family and a fiancé behind.
Upon arriving at Cambridge, he enrolled in a plan of study that after two years of study and an acceptable research project he would graduate. Working under Europe’s leading expert on electromagnetic radiation, J.J. Thomas, Rutherford observed that a magnetized needle lost some of its magnetization when placed in a magnetic field produced by an alternating current. This made the needle a form of detector of the newly discovered electromagnetic waves. The electromagnetic waves had been theorized by physicist James Clerk Maxwell in 1864 but had only been detected in the last ten years by the German physicist Heinrich Hertz. Rutherford’s apparatus was more sensitive at detecting radio waves than hertz’s instrument. With further work on the detector, Rutherford was able to detect radio waves up to a half a mile away. He lacked the entrepreneurial skills to make the receiver commercially viable - this would be accomplished by the Italian inventor Guglielmo Marconi, who invented an early version of the modern radio.
The world of physics had many new discoveries at the end of the nineteenth century. In France, Henri Becquerel discovered a strange new property of matter were energy was continually being emitted from uranium salts. Pierre and Marie Curie continued with Becquerel’s work and discover the radioactive elements: thorium, polonium, and radium. At about the same time, Wilhelm Röntgen discovered X-rays which was a form of high energy radiation that was capable of penetrating solid materials. Rutherford learned of these new discoveries and began his own research into the radioactive nature of some elements. From these discoveries, Rutherford would spend the rest of his days unraveling the mysteries of the atom.
McGill University in Canada
Rutherford’s strong research skills won him a professorship at McGill University in Montreal, Canada. In fall of 1898 Rutherford started his position as professor of physics at McGill. During the summer of 1900 after two years of concentrated work on the radioactive nature of thorium, he traveled back to New Zealand to marry his impatient bride. The newlyweds returned to Montreal that fall and began their life together.
Rutherford worked closely with his able assistant Frederick Soddy starting in 1902 and the pair followed up on a discovery by William Crookes who had found that uranium formed a different substance as is gave off radiation. Through careful laboratory research, Rutherford and Soddy demonstrated that uranium and thorium broke down in the course of radioactivity into a series of intermediate elements. Rutherford observed that during each stage of the transmutation process different intermediate elements broke down at a particular rate so that half of any quantity was gone in a fixed amount of time, which Rutherford called the “half-life” – at term still in use today.
Rutherford observed that the radiation emitted by radioactive elements came in two forms, he named them alpha and beta. Alpha particles are negatively charged and wouldn’t penetrate a piece of paper. Beta particles are negatively charged and would pass through several pieces of paper. In 1900 it was found that some of the radiations were not affected by a magnetic field. Rutherford demonstrated the newly discovered radiation to a form of electromagnetic waves, like light, and named them gamma rays.
University of Manchester
Rutherford’s work was beginning to be taken seriously by the scientific community and he was offed a chair of physics at the University of Manchester in England, which boasted a research laboratory second only to the Cavendish Laboratory at Cambridge University. The Rutherfords, accompanied by their young daughter Eileen, arrived in Manchester in the spring of 1907. The atmosphere was a change for Rutherford at Manchester, as he wrote to a colleague: “I find the students here regard a full professor as little short of Lord God Almighty. It is quite refreshing after the critical attitude of the Canadian students.” Rutherford and his young German assistant, Hans Geiger, studied the alpha particles and proved that they were simply a helium atom with its electrons removed.
Rutherford continued his study of how alpha particles are scattered by thin metal sheets that he had begun at McGill University. Now he would make a key discovery on the nature of the atom. In his experimentation, he fired alpha particles at a sheet of gold foil only one fifty-thousandth of an inch thick, thus the gold was only a few thousands of atoms thick. The results of the experiment showed that most of the alpha particles passed through without being affected by the gold. However, on the photographic plate that recorded the path of the alpha particles through the gold film, some were scattered through large angles indicating they had collided with a gold atom and the path of travel was deflected – much like a collision of billiard balls. The discovery lead Rutherford to exclaim, “it was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
From the results of the scattering experiment, Rutherford began to piece together a picture of the atom. He concluded that since the gold foil was two thousand atoms thick, and the majority of the alpha particles passed through deflected, it would seem that the atoms were mostly empty space. The alpha particles that were undeflected through large angles, sometimes greater than ninety degrees, seemed to indicate that within the gold atom there were very massive positively charged regions capable of turning back the alpha particles – much like a tennis ball bouncing off a wall. Rutherford announced in 1911 his model of that atom. In his mind the atom contains a very tiny nucleus at its center, which is positively charged and contains the protons and virtually all the mass of the atom since the proton is much more massive than the electron. Surrounding the nucleus are the much lighter electrons that have an equal number of negative charges. This model of the atom was much closer to the modern view of the atom and replaced the concept of the featureless, indivisible spheres of proposed by the ancient Greek philosopher Democritus, which had held sway for over two millennia.
Rutherford continued to work on radioactive material and devised a method to quantify the amount of radioactivity a material possessed. Rutherford and Geiger used a scintillation counter to measure the amount of radioactivity produced. By counting the number of flashes on a zinc sulfide screen where on flash indicated a colliding subatomic particle, he and Geiger could tell that a gram of radium ejects 37 billion alpha particles per second. Thus, a unit of radioactivity was born, named after Pierre and Marie Curie, a “curie” which represents 37 billion alpha particles per second. Rutherford would have his own unit of radioactivity named after him, the “Rutherford”, which represents one million breakdowns per second.
Like a drill Sargent inspecting his troops, Rutherford made regular rounds to each of the laboratories to check on the progress of his students. The students knew he was approaching as he often sang his off-key rendition of “Onward Christian Soldiers” in a thunderous voice. He would probe the students with questions such as “Why don’t you get a move on?” or “When are you going to get some results?” delivered in a voice that rattled the student and the equipment. One of his students later commented “At no time did we feel that Rutherford had a contempt for our work, although he might be amused. We might feel that he had watched this sort of thing before and this was the stage we had to go through, but we always had the feeling that he did care, that we were trying the best we could, and he was not going to stop us.”
In 1908, Rutherford was awarded the Nobel Prize in Chemistry “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances” – the nuclear decay work that he had done back at McGill. As was the custom, Rutherford gave a speech at the Nobel award ceremony in Stockholm, Sweden. The audience was filled with past award winners and dignitaries. At thirty-seven, Rutherford was a youngster, at least in this crowd. His large thin frame with a head full of bushy blond hair stood out. After the formal ceremony there were banquets and celebrations, starting in Stockholm, then Germany, and finally the Netherlands. Rutherford recalled of that exciting period “Lady Rutherford and I had the time of our lives.”
World War I
The outbreak of World War I in Europe in 1914 drew the young men into the war and virtually emptied his laboratory of students and assistants. Rutherford worked as a civilian for the British military on the development of sonar and antisubmarine research. Toward the end of World War I in 1917, Rutherford began to make quantitative measurements of radioactivity. He experimented with alpha particles from a radioactive source to shot through a cylinder into which he could introduce various gases. The introduction of oxygen into the chamber caused the number of scintillations on the zinc sulfide screen to drop off, indicating the oxygen absorbed some of the alpha particles. When hydrogen was introduced into the chamber, noticeable brighter scintillations were produced. This effect was explained because the nucleus of the hydrogen atom consisted of single protons and these were knocked forward by the alpha particles. The protons from the hydrogen gas that were launched forward produced a bright scintillation on the screen. When nitrogen was introduced into the cylinder, the alpha particle scintillations were reduced in number, and occasional scintillations of the hydrogen type appeared. Rutherford concluded that alpha particles were knocking protons out of the nuclei of the nitrogen atoms, making the nuclei that was left that of an oxygen atoms.
Rutherford had accomplished what alchemists had been trying to accomplish for centuries, that was, convert one element to another or transmutation. Alchemists, of which Sir Isaac Newton was one, sought among other things to convert base metals into gold. He had demonstrated the first “nuclear reaction” although it was a very inefficient process with only one in 300,000 nitrogen atoms being converted to oxygen. He continued his work on transmutation and by 1924 he had managed to knock proton out of the nuclei of most of the lighter elements.
The Cavendish Laboratory
With the retirement of J.J. Thomson in 1919 from the Cavendish Laboratory Rutherford was offered the job as head of the laboratory and took the position. The Cavendish Laboratory which was part of Cambridge University and was Great Britain’s premier physical sciences laboratory. The lab had been funded the wealthy Cavendish family and was set up by its first director by the famous Scottish physicist James Clerk Maxwell.
As his fame spread Rutherford had many occasions to give public lectures; one such occasion was the 1920 Bakerian lecture at the Royal Society. In the lecture he spoke of the artificial transmutations he had recently induced with assistance of alpha particles. He also gave a prediction regarding the existence of a yet undiscovered particle that resides in the atom: “Under some conditions it may be possible for an electron to combine [with a proton] much more closely [than in the case of a hydrogen atom], forming a kind of neutral doublet. Such an atom would have very novel properties. Its external field would be practically zero, except very close to the nucleus, and in consequence it should be able to move freely through matter…The existence of such atoms seems almost necessary to explain the building of the heavy elements.”
It would be a dozen years before Rutherford’s “neutral doublet” or neutron as it would be called would be discovered. Rutherford’s second in charge at the Cavendish, James Chadwick, who followed him from Manchester, would take up the search for the elusive new particle. Chadwick’s road to discovery of the neutron was long and troublesome. The electrically neutral particle did not leave observable tails of ions as they passed through matter, essentially, they were invisible to the experimenter. Chadwick would take many wrong turns and go down many blind alleys on his quest for the neutron, telling one interviewer “I did a lot of experiments about which I never said anything…Some of them were quite stupid. I suppose I got that habit or impulse or whatever you’d like to call it from Rutherford.” Finally, all the pieces of the nuclear puzzle fell into place and in February of 1932, Chadwick published a paper titled “The Possible Existence of a Neutron.”
Rutherford’s model of the atoms was now in focus. At its core, that atom had positively charged protons, along with neutrons, and surrounding the core or nucleus, were electrons, equal in number to the protons, that completed the outer shell of the atom.
At this point, Rutherford had become one of the most eminent scientists in Europe and was elected as president of the Royal Society from 1925 to 1930. He was knighted in 1914 and was created Baron Rutherford of Nelson in 1931. He had become a victim of his own success – little time for science, more time spent in the tedium of administration and on occasion, uttering the prognostications only a sage could deliver.
Ernest Rutherford died on October 19, 1937 from complications from a strangulated hernia and was buried in Westminster Abby near Sir Isaac Newton and Lord Kelvin. Shortly after his death, Rutherford’s old friend James Chadwick wrote “He had the most astonishing insight into physical processes, and in a few remarks he would illuminate a whole subject…To work with him was a continual joy and wonder. He seemed to know the answer before the experiment was made, and was ready to push with irresistible urge to the next.”
Asimov, Isaac. Asimov’s Biographical Encyclopedia of Science and Technology. 2nd Revised Edition. Doubleday & Company, Inc. 1982.
Cropper, William H. Great Physicists: The Life and Times of Leading Physicists From Galileo to Hawking. Oxford University Press. 2001.
Reeves, Richard. A Force of Nature: The Frontier Genius of Ernest Rutherford. W.W. Norton & Company. 2008.
West, Doug. Ernest Rutherford: A Short Biography: Father of Nuclear Physics. C&D Publications. 2018.