Just as the light microscope enabled biologists to investigate the structure of living cells, so now the development of accelerators allowed physicsts the opportunity to study the structure of the atomic nucleus. Accelerators allowed physicists to explore properties of nuclei which were not stable.

In 1932 John Cockcroft and Ernest Walton produced the first man-made nuclear transmutations using protons accelerated by an accelerator they had built.
They studied, among other reactions, the following:

The repulsive barrier height for this reaction is about 1.35MeV (million electron volts). They were able to detect the production of helium at a bombarding energy as low as 100 KeV ( 100,000 electron volts). They also were able to verify the energy/mass relationship of Einstein's theory of special relativity, E = mc². More kinetic energy came out of the above reaction than went into it. Due to the mass difference between the initial reactants and the final products there is 17.347 MeV of energy available for kinetic energy of the helium nuclei.

The presentation speech below is from the following site:
http://www.nobel.se/physics/laureates/1951/press.html
highlights in red ( K. Aniol)

Nobel Prize in Physics 1951 Presentation Speech by Professor I. Waller, member of the Nobel Committee for Physics Your Majesties, Royal Highnesses, Ladies and Gentlemen.  By giving the Nobel Prize in Physics of this year to Sir John Cockcroft, Director of the Atomic Energy Research Establishment at Harwell, and Professor Ernest Walton of Dublin University, the Swedish Academy of Sciences has rewarded a discovery which stands out as a milestone in nuclear research.  At the beginning of this century, the study of the naturally radioactive substances had shown that their property of emitting radiation is connected with spontaneous transmutations of their atoms. It appeared, however, to be beyond human power to influence the course of these processes.  The radiation emitted by a radium source contains swiftly moving and positively charged helium atoms. By investigating the way in which these particles are deflected by other atoms, the great nuclear scientist Rutherford found in 1911 that an atom has a positive nucleus which is very small compared to the whole atom but contains most of its mass. Besides the nucleus, the atom contains negative electrons, moving around the nucleus.  Continuing these investigations, Rutherford was able in 1919 to produce transmutations of atomic nuclei by bombarding nitrogen with helium nuclei from a radium source. Some of the helium nuclei had enough energy to overcome the repelling electric field and to penetrate into the nitrogen nucleus, in those rare cases when they struck such a nucleus. The nitrogen nucleus thereupon turned into an oxygen nucleus, while a hydrogen nucleus was emitted.  Thus it became possible by external means to transform nitrogen into oxygen, i.e., to transmute one element into another.  However, only a very few nuclear transmutations could be produced by these natural projectiles, the helium nuclei from radioactive substances. In order to produce nuclear transmutations on a larger scale, and thus obtain further insight into the structure of atomic nuclei, a more powerful stream of projectiles was needed.  Accordingly, the end of the 1920's saw investigations of the possibility of accelerating charged particles to high energies, with the ultimate aim of using these particles to produce nuclear transmutations. This year's Nobel Laureates in Physics were the first to succeed in this task, by their joint work at the Cavendish Laboratory in Cambridge, of which Rutherford was at that time the director. In planning this work, they realized the importance of certain contemporary theoretical studies by Gurney and Condon, and by Gamow. This work had shown that, because of the wave properties of matter, there is a certain probability for a positively charged particle to penetrate into a nucleus even if, according to ordinary mechanical concepts, the velocity of the particle does not suffice to overcome the electric repulsion from the nucleus. Cockcroft had emphasized that the conditions are particularly favourable if hydrogen nuclei are used as projectiles, and that an accelerating voltage of only a few hundred thousand volts should suffice to give observable transmutations of light elements.  The work of Cockcroft and Walton was a bold thrust forward into a new domain of research. Great difficulties had to be overcome before they were able to achieve their first successful experiments at the beginning of 1932. By then, they had constructed an apparatus which, by multiplication and rectification of the voltage from a transformer, could produce a nearly constant voltage of about six hundred thousand volts. They had also constructed a discharge tube in which hydrogen nuclei were accelerated. Causing these particles to strike a lithium layer, Cockcroft and Walton observed that helium nuclei were emitted from the lithium. Their interpretation of this phenomenon was that a lithium nucleus into which a hydrogen nucleus has penetrated breaks up into two helium nuclei, which are emitted with high energy, in nearly opposite directions. This interpretation was later fully confirmed.  Thus, for the first time, a nuclear transmutation was produced by means entirely under human control.  In order to get a detectable transmutation of lithium, a voltage of little more than one hundred thousand volts was required. The number of transmutations rose quickly as the voltage was increased. The corroboration obtained in this way for the theory which Gamow and others had propounded, and which was referred to above, was of great importance.  The analysis made by Cockcroft and Walton of the energy relations in a transmutation is of particular interest, because a verification was provided by this analysis for Einstein's law concerning the equivalence of mass and energy. Energy is liberated in the transmutation of lithium, because the total kinetic energy of the helium nuclei produced is greater than that of the original nuclei. According to Einstein's law, this gain in energy must be paid for by a corresponding loss in the mass of the atomic nuclei. This assertion was satisfactorily confirmed by Cockcroft and Walton, experimental errors being taken into consideration. Somewhat later, more exact investigations based on the same principles gave a complete verification of Einstein's law. Thus a powerful method was obtained for comparing masses of atomic nuclei.  In subsequent work, Cockcroft and Walton investigated the transmutations of many other atomic nuclei. Their techniques and results remain a model for nuclear research. As projectiles, they also used the nuclei of heavy hydrogen, which had then just been discovered. As end products, several atomic nuclei were obtained which had not been known previously. Following the discovery of artificially radioactive elements, by Frédéric and Irène Joliot-Curie, they found that such elements can also be produced by irradiation with hydrogen nuclei.  The investigations of Cockcroft and Walton disclosed a new and fertile domain of research, consisting of the study of nuclear transmutations of various types.  Their discoveries initiated a period of rapid development in nuclear physics. Besides the apparatus of Cockcroft and Walton, the cyclotron constructed by Lawrence, and various other particle accelerators played important roles. By its stimulation of new theoretical and experimental advances, the work of Cockcroft and Walton displayed its fundamental importance. Indeed, this work may be said to have introduced a totally new epoch in nuclear research.  Sir John Cockcroft, Professor Ernest Walton. The great nuclear scientist Rutherford, with whose work your discovery is closely connected, sometimes used to say: "it is the first step that counts". This saying may be applied in the truest sense to your discovery of the transmutations of atomic nuclei by artificially accelerated particles. Indeed, this work of yours opened up a new and fruitful field of research which was eagerly seized upon by scientific workers the world over. It has profoundly influenced the whole subsequent course of nuclear physics. It has been of decisive importance for the achievement of new insight into the properties of atomic nuclei, which could not even have been dreamt of before. Your work thus stands out as a landmark in the history of science.  On behalf of the Royal Swedish Academy of Sciences may I extend to you our warmest congratulations. I now ask you to receive your Nobel Prize from the hands of His Majesty the King.  From Nobel Lectures, Physics 1942-1962.  Last modified June 16, 2000  © Copyright The Nobel Foundation

E. O. Lawrence - Inventor of the Cyclotron

Nobel presentation speech - 1939
source:http://www.nobel.se/physics/laureates/1939/press.html

Nobel Prize in Physics 1939 The following account of Lawrence's work is by Professor K. M. G. Siegbahn, member of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences In 1919 Lord Rutherford discovered that nitrogen can be brought to emit protons by bombardment with alpha particles, according to the nuclear reaction equation:    This discovery meant the initiation of a new era in natural sciences. However, as long as one was limited to the use of alpha radiation of naturally radioactive substances for carrying out nuclear reactions, very strict limits were set to further development both with regard to the substances which could produce these reactions, as well as to the quantitative yield of the reactions.  How then would it be possible, by some method other than the use of radioactive substances, to make available projectiles with sufficient energies to bring about nuclear reactions in an artificial way? Fortunately, the quantum-mechanical treatment of this problem, developed in the meantime, implied that the energy of the particles need not be as high as might be expected from classical theories. Among all the proposals and experiments carried out in different quarters to produce sufficiently fast particles for nuclear experiments, those carried out at the Cavendish Laboratory on Rutherford's initiative were the first to yield a positive result (1932). In this case use was made of a high electrical voltage, up to about 600 kV, to accelerate protons which, upon bombarding lithium, caused a nuclear reaction:    Two years earlier (September, 1930), however, Lawrence had indicated an entirely new method to obtain fast particles, i.e. the so-called magnetic resonance acceleration. This method is based on a brilliant combination of a constant homogeneous magnetic field and an oscillating electrical field with constant frequency, whereby the ions move about in circular orbits with ever-increasing radii, through stepwise acceleration. The communication on the first simple experimental model of the "cyclotron" was published in the same year as the aforementioned experiment with artificially produced nuclear reactions at the Cavendish Laboratory. Under Lawrence's guidance and with the assistance of a large number of skilled collaborators the cyclotron method soon proved suitable for rapid development towards an exceptionally effective tool for research in this field. The energies of the particles, successively obtained by the further development of the cyclotron method, surpassed significantly that which had been obtained by other means. The maximum energy of the particles accelerated in the cyclotron even considerably exceeded the energy values present in alpha rays of naturally radioactive substances. While the latter energy is of the order of magnitude of 7 to 8 MeV, the energy of alpha particles supplied by the cyclotron is, according to latest reports (November, 1939), up to 38 MeV.  Experiments with heavy hydrogen nuclei as projectiles, with which Lawrence and his collaborators could produce nuclear reactions with practically all elements, proved to be particularly successful.  With regard to the intensities of the radiation produced in the cyclotron, it can be mentioned that a current of over 150 microamperes has been attained, corresponding to the alpha radiation of 30 kg radium. As a comparison it may be mentioned that the entire world stock of purified radium can be estimated at 1 kg.  With the powerful means given to nuclear research by the cyclotron, an explosive development took place in this field. Nowadays, cyclotron installations are built or planned in a large number of laboratories throughout the world. The number of publications on the results obtained with the use of cyclotrons has grown with the speed of an avalanche.  The greatest significance the cyclotron has had is in the production of artificially radioactive substances. True, the discovery of active isotopes was made by the Curie-Joliots in 1933 with the use of alpha particles from naturally radioactive substances, but only with the cyclotron was it possible to produce active isotopes in large quantities. This was, among other things, an essential condition for the use of active elements for biological and medical purposes. On this terrain, where such splendid achievements had already been made, a new field for research and practical applications has been opened, thanks to the cyclotron. To appreciate the strength of the radioactive sources produced for the last-mentioned purposes, the following data may be given. Using deuterium in his cyclotron Lawrence was able, already in 1936, to produce daily quantities of active sodium, which, with regard to gamma radiation, were equivalent to 200 mg radium. The later cyclotrons of larger dimensions (1939) have a production capacity of about 10 times this value.  Finally, it may be mentioned that the cyclotron offers possibilities of producing neutron radiation of great intensity, as a result of which quantitative research on the physical and biological effects of this radiation has been carried out. With regard to therapeutic applications, these preliminary investigations are rather encouraging.  Within the history of the development of experimental physics, the cyclotron takes an exceptional position. It is, without comparison, the most extensive and complicated apparatus construction carried out so far. As to the scientific results achieved, we can scarcely find anything similar among the other experimental tools in physics. It is also evident that the operation and testing of an apparatus of this type, with such a multitude of details, cannot be the merit of one man alone. As promotor and leader of this almost gigantic work, Lawrence has shown such merits in the field of physics that the Royal Swedish Academy of Sciences has considered him as having fulfilled to the highest degree the requirements implied in the award of the Nobel Prize*.      *Owing to the war conditions, the Prize was handed over to Professor Lawrence at a ceremony in Berkeley on February 29, 1940. Among the speeches delivered was a thorough account of Professor Lawrence's work by the physicist R. T. Birge. A report of the ceremonies in Berkeley has been published in "Les Prix Nobel en 1939", Stockholm, 1942.      From Nobel Lectures, Physics 1922-1941.