Chapter 1 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT - An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

Radioactivity was discovered by the French scientist Henri Becquerel in 1896 while he was investigating the radiation emitted from the uranium salts, which he noticed in the 1880s while preparing potassium uranyl sulfate. He began to investigate this phenomenon again upon the 1895 publication by K.W. Röntgen who reported about a new type of penetrable radiation (X-rays). Becquerel then experimented with possible formation of fluorescence and X-rays by UV-radiation of sunlight in uranium salts. He placed uranium salt in a package and exposed it to sunlight while it was on top of a photographic plate. The photographic plate was exposed, which he at first interpreted as fluorescence until the realization that the uranium salts exposed the plate without exposure to sunlight. Becquerel also noted that the radiation emitted by the uranium salts discharge electroscope charges and made the air conductive.

Not too long after this Marie Curie, Becquerel’s student, showed that not only potassium uranyl sulfate emits radiation but also other uranium salts and their solutions. In addition, the amount of the radiation in the samples was seen to be proportional to the amount of uranium. Based on these observations it was clear that the emission of radiation was a property of uranium regardless of what compound it was present as. Marie Curie, along with her husband Pierre Curie, coined the term “radioactivity” to refer to the radiation emitted by uranium.

Henri Becquerel (December 15, 1852 – August 25, 1908) (Public Domain, https://commons.wikimedia.org/w/index.php?curid=294735)

Marie Curie mapped out the ability of the elements, known then, to radiate and concluded that only thorium, in addition to uranium, was radioactive. She, however, noticed that the pitchblende mineral, from which uranium can be separated, had an even higher radioactivity level than pure uranium and concluded that it must also contain an even more active element than uranium. She dissolved pitchblende into acid and precipitated it into different fractions of which the radioactivity was measured. In order to measure the activity levels the Curies used the electroscope, which was designed by the University of Cambridge professor J.J. Thompson as a device for measuring the X-ray radiation. During her investigations, Marie Curie noticed that a new highly radioactive element coprecipitated along with bismuth, which she decided to name polonium in honor of her home country Poland. The amount of separated substance was, however, insufficient to determine the chemical properties and atomic weight. The same experiment also revealed that another element coprecipitated with barium and it was determined to be 900 times more radioactive than uranium. Due to this high level of radiation, Curie named the element radium.

Since pitchblende only contains a very small concentration of radium, the Curies decided to separate it in macro quantities from two tons of pitchblende. In 1902, after major efforts they extracted 0.1 g of radium chloride, allowing the determination of atomic weight. They determined the value to be 225, which we now know to be 226.0254. Later, in 1910, Marie Curie electrolytically separated radium as a pure metal as well. Together with her husband Pierre Curie and Henri Becquerel, she received the Nobel Prize for physics in 1903 and then in 1911 she was the sole recipient of the Nobel Prize for chemistry.

Maria Skłodowska-Curie (Marie Curie) (November 7, 1867 – July 4, 1934) (Public domain, https://commons.wikimedia.org/wiki/File:Mariecurie.jpg)

Soon after the first observations of radiations were made, the nature of radiation was examined. In 1898, Ernest Rutherford showed the presence of two types of radiation, alpha and beta radiation. A third type, gamma radiation, was detected in 1903 by Paul Villard. Both alpha and beta radiation proved to bend in a magnetic field, though in opposite directions, and were therefore determined to be charged particles, with alpha radiation being positively and beta radiation negatively charged. Gamma radiation did not bend in a magnetic field and Rutherford demonstrated it to be an electromagnetic form of radiation with even shorter wavelengths than X-ray radiation. Research on the mass and charge of alpha and beta radiation showed alpha radiation to be helium ions and beta radiation to be electrons.

Ernest Rutherford (August 30, 1871 – October 19, 1937) (Public domain, https://commons.wikimedia.org/wiki/File:Ernest_Rutherford_(Nobel).jpg)

The radiochemical separation of radium and polonium by Curie sparked a continuous finding of new radioactive elements in the 1910s. Precipitations were made of uranium and thorium salt solutions with various reagents and the activity levels of the resulting precipitates were measured. For example, Rutherford and his colleague precipitated a substance from a thorium solution, which they named thorium-X (later identified to be ²²⁴Ra, t_1/2 3.6 d). It lost its radioactivity in a month, while after the precipitation the activity of the remaining thorium solution increased from the lowered activity level back to the original level. At first, only small quantities of new radioactive substances were collected so neither their atomic weight nor optical spectrum could be measured. They were identifiable only by the type of radiation they emitted, alpha and/or beta radiation, and the rate at which their radiation levels diminished. For these reasons the new radiating substances could not be named and were referred to be based on their parent nuclide, for example U-X, Ac-D, or Ra-F. These new substances were shown to belong to three decay series, which began with uranium and thorium. Around 1910 a total of 40 new radiating substances were known, all of which ended up as the stable nuclides Ra-G, Th-D, and Ac-D, later identified to different isotopes of lead. The formation of alpha particle emitting radioactive gases, called emanation and later identified as radon gas, occurred in all of the decay series.

Rutherford and Soddy had already hypothesized in 1902 that observed phenomena were explainable by the spontaneous decay of radioactive elements into other elements, and that the rate of decay was exponential. The concept of the atom as the smallest indivisible particle began to break down. In 1913, both Kasimir Fajans and Frederick Soddy independently concluded that radioactive decay series starting with uranium and thorium always resulted in elements with an atomic number two units lower in alpha decay and one unit higher in beta decay. Many radioactive substances found in radioactive decay series were, however, found to be chemically identical to each other in spite of their atomic weights. Fajans and Soddy determined that elements could occur as different forms with differing masses, which Soddy named isotopes. Thus, for example, it was possible to identify the uranium series member formerly known as ionium to thorium and thus an isotope to previously known element thorium. The 40 members of the natural radioactive decay series could now be categorized using the isotope concept into eleven elements and their different isotopes. The full understanding of the isotope concept, however, still required the discovery of neutrons.

In 1911, when examining the passage of alpha radiation through a thin metallic foil, Rutherford found that most of the alpha particles passed through the foil without changing direction. Some of the particles, however, changed direction, a few up to 180 degrees. From this, Rutherford concluded that atoms are mostly sparse, particle-permeable space and they have a small positively charged nucleus from which the alpha particles scatter. From the angle of the scattering alpha particles, he calculated that the diameter of the nucleus is approximately one-hundred-thousandth of the diameter of the whole atom.

Soon after this, Niels Bohr presented his theory of atomic structure. According to him, atoms have a small, positively charged nucleus around which the negatively charged electrons orbit. Electrons, however, do not orbit the nucleus randomly, but in predefined shells with a specified energy: the closer to the nucleus the higher the energy and lower in the outermost shells. The number of electrons in the atom is equal to the charge of the nucleus, and is the same as the atomic number of the element in the periodic table. The nuclear charge was, however, not yet able to be directly measured. The values were then determined by Henry Mosely and his group bombarding elements with electrons, which led to removal of the shell electrons from their orbits and formation of X-ray radiation as the electron holes were filled with electrons from upper shells. Characteristic X-ray spectra were obtained for each element. From these, it was found that the frequencies (f) of the emitted X-rays correlated with the systematics f = constant × (Z-1)² in relation to the atomic number (Z) of the elements, from which all elements’ atomic numbers could be calculated. For example, the atomic number of uranium was able to be determined as 92.

In 1920, the first artificial nuclear reaction was triggered by Rutherford. He targeted nitrogen with alpha particles and determined that hydrogen nuclei were emitted from the nitrogen atoms, which he called protons. Until 1932, it was expected that atoms consisted of positively charged protons in the nucleus and orbiting electrons. James Chadwick then identified previously discovered penetrating radiation to neutrons, particles with the same mass as protons but no charge. With this knowledge, Bohr’s atomic model could be completed: the nucleus contains the atomic number of positively charged protons as well as a variable number of neutrons. Elements with nuclei containing a different amount of neutrons, and therefore having a different atomic weight, are isotopes of these elements.

The first particle accelerators were developed during the 1930s and in 1932 the first accelerated particle(proton)-induced nuclear reaction, ¹H + ⁷Li → 2 ⁴He, was accomplished. Also in 1932, the husband and wife team, Frederic and Irene Joliot-Curie, accomplished creating the first artificial radioactive nucleus by bombarding boron, aluminum, and magnesium with alpha particles. Bombarding aluminum produced ³⁰P, which decayed by positron emission with a 10 minute half-life. Positrons, particles with the same mass as an electron but an opposite charge, were identified two years earlier.

Already in the first half of the 1930s, following the discovery of accelerators attempts were made to make heavier elements by bombarding uranium with neutrons. Otto Hahn, Lise Meitner, and Fritz Strassman also attempted this, but in 1938, they found that uranium nuclei split into lighter elements when bombarded with thermal neutrons. This process was proven to release an extremely large amount of energy. At the beginning of the Second World War, when it was demonstrated that the harnessing of this energy could be used by the armed forces, the US government began developing a nuclear weapon. The venture was called the Manhattan Project and was headed by Robert Oppenheimer. In the first stage, Enrico Fermi and his team started the first nuclear reactor in Chicago in 1942, which was used as the basis for the building of reactors in the subsequent years for plutonium weapon production. Within the nuclear reactor, a controlled chain reaction of uranium fission by neutrons is generated. At the same time, it was proven that the thermal neutrons do not arise from the fission of the prominent uranium isotope ²³⁸U, but from the fission ²³⁵U. The latter isotope only accounts for 0.7% of naturally occurring uranium. In order to provide enough ²³⁵U for nuclear weapons an isotope enriching plant was built in Oak Ridge, Tennessee, in which the enrichment process was based on the different diffusion rate of the hexafluoride molecules of the two uranium isotopes.

In addition to uranium, the Manhattan Project also used a new, heavier element that undergoes fission more sensitively than ²³⁵U, plutonium, for the development of nuclear weapons. In the mid-1930s, Meitner, Hahn, and Strassman had already suspected that there were elements heavier than uranium when they verified that ²³⁹U, which they got by bombarding ²³⁸U with neutrons, decayed by beta emission. They knew that the decay produced element 93, but were unable to prove it. At Berkeley University, Edwin McMillian and his team confirmed its existence and named the element neptunium. In 1941, they were also able to confirm the existence of element 94, ultimately called plutonium, which then began to be produced in Hanford, Washington as a material for weapons. Plutonium was separated from uranium, irradiated in a nuclear reactor, by the PUREX method, which is still used at the nuclear fuel reprocessing plants. In this method, the spent fuel is dissolved in nitric acid and extracted by tributyl phosphate allowing uranium and plutonium to transfer into the organic phase while other substances remain in the acid. When plutonium is reduced to trivalent state, it can be extracted from uranium back to the aqueous phase and further reduced to metal. The Manhattan Project also developed a large number of other radiochemical separation methods for the separation of radionuclides, some of which are still in use.

The sad conclusion of the Manhattan Project occurred in the summer of 1945, first with the test explosion in New Mexico and then with the annihilation of Hiroshima and Nagasaki in August.

Synthesis of new, heavier than uranium elements did not end with the development of plutonium. At first, they were produced in Berkeley in Lawrence Livermore Laboratory under the direction of Glenn T. Seaborg. In the 1940s and early-1950s they found americium (element 95), curium (96), berkelium (97), californium (98), einsteinium (99), fermium (100), and mendelevium (101). Later other nuclear centers, particularly Dubna in Russia and Darmstadt in Germany, joined the race. So far, the heaviest element that has been identified is number 116. All of these super heavy elements are very short-lived.

Until the development of nuclear weapons, radioactive research was at the level of basic research. As early as 1912, however, de Hevesy and Paneth used ²¹⁰Pb (RaD) to determine the solubility of lead chromate. De Hevesy was also responsible for the first biological radionuclide experiment: in 1923, using the ²¹²Pb tracer he studied the uptake and distribution of lead in bean plants. The invention of accelerators, and its use in the production of artificial radionuclides, brought researchers new opportunities to use radionuclides in investigations. Until the introduction of nuclear reactors, however, large-scale production of radionuclides was not possible. Already in 1946, the Oak Ridge nuclear center sold radionuclides, which began to be more widely used in research towards the end of the 1940s.

After the war, the nuclear reactors that were originally built for production of nuclear weapons were also used for production of electricity. The first nuclear reactors producing electricity were introduced in the Soviet Union in 1954 and in England in 1956. The output of the aforementioned reactors were 5 MV and 45 MV, while modern reactors run at a capacity of 500-1500 MV.

Presently (2014), there are a total of 437 electricity producing nuclear power plants in 31 countries. In a few countries, France, Belgium, Hungary and Slovakia, over half of the electricity is nuclear energy.

As previously stated, Bequerel, like many other early investigators, used photographic plates for the detection of radiation. The second radiation detection device of that time was the electroscope, which had a metal rod with two metal plates hanging from it inside a glass ball. An electrical charge was applied to the plates, pushing them apart. When the radiation ionized the air within the glass ball and made it conductive, the charge between the plates was discharged and the shortening of the distance between the two plates was proportional to the amount of radiation hitting the ball. In 1903, William Crookes took a device called a spinthariscope into use, which for the first time utilized the scintillation effect. In this device, the alpha radiation hit the screen of the zinc sulfide layer, which caused excitation and the formation of light upon relaxation. The light had to be detected visually. In the late 1940s, a photomultiplier tube was invented, allowing the light to be transformed into an electrical pulse able to be counted electronically. Scintillation crystals (e.g. NaI(Tl)) and liquid scintillation counting are now common radiation measurement methods.

Before the Second World War the most prominent radiation measuring devices were the Geiger-Müller counter and the Wilson cloud chamber. The Geiger-Müller counter, which prototype H. Geiger and W. Müller developed in 1908, is based on the ability of radiation to penetrate a very thin window (previously enamel, presently beryllium) to a gas filled metal cylinder, which is connected to the voltage between the anode wire in the center of the cylinder and the metallic cylinder wall as a cathode. Radiation particles or rays ionize gas within the tube and cause an ion/electron cascade that can be detected and counted as electrical pulses in an external electric circuit. The Geiger-Müller counter is still used as a dosimeter in radiation protection and also in beta counting.

In the Wilson cloud chamber, radiation is directed through a window into an enclosed space filled with water vapor. When the volume of the chamber is suddenly extended with a piston, the steam cools and the chamber becomes supersaturated. The radiation ionizes the gas and the generated ions act as the water vapor condensation centers. The phenomenon lasts for a couple of seconds and can be detected by the track of the water droplets along the glass wall, which can be photographed.

Geiger counters were the most common tool for radiation measurements still in the 1950s. Later, they were replaced in most cases by liquid scintillation counting in beta detection, as well as the scintillation and semiconductor detectors for the counting and spectrometry of alpha particles and gamma radiation. Scintillation crystals and the photomultiplier tubes used for their pulse amplification were developed in the late 1940s; the semiconductor detector was only developed in the early 1960s. Development of multichannel analyzers has also been important step in the radiation measurements.