Category Science

Although the idea for the machine had already been suggested by other scientists, the Geiger counter was perfected by a German physicist called Hans Geiger (1882-1945).

The German physicist Hans Wilhelm Geiger is best known as the inventor of the Geiger counter to measure radiation. In 1908, Geiger introduced the first successful detector of individual alpha particles. Later versions of this counter were able to count beta particles and other ionizing radiation. The introduction in July 1928 of the Geiger-Muller counter marked the introduction of modern electrical devices into radiation research.

Geiger, the eldest of 5 children of a professor of philology, was born on September 30, 1882, in Neustadt an der Hardt, Rhineland-Palatinate state in western Germany (about 20 miles southwest of Mannheim). He studied physics at the universities of Munich and Erlangen in Bavaria, Germany, and received the PhD degree from the latter university in 1906. At the University of Erlangen, he worked with Eilhard Wiedemann (1852-1928) and wrote a thesis on electrical discharges through gases.

Geiger, the eldest of 5 children of a professor of philology, was born on September 30, 1882, in Neustadt an der Hardt, Rhineland-Palatinate state in western Germany (about 20 miles southwest of Mannheim). He studied physics at the universities of Munich and Erlangen in Bavaria, Germany, and received the PhD degree from the latter university in 1906. At the University of Erlangen, he worked with Eilhard Wiedemann (1852-1928) and wrote a thesis on electrical discharges through gases.

In 1913, Geiger was joined by two physicists, Walther Bothe (1891-1957), later the 1954 Nobel Prize winner in physics, and James Chadwick (1891-1974), later Sir James Chadwick and winner of the 1935 Nobel Prize in physics. Bothe investigated alpha scattering, and Chadwick counted beta particles. The work was interrupted in 1914, with the beginning of World War I (1914-1918). Geiger served in the German army in the field artillery.

After the war, Geiger returned to his work, and in 1924, he used his device to confirm the Compton Effect, namely, the increase in wavelength of electromagnetic radiation, especially of an X-ray or gamma-ray photon, scattered by an electron. The Compton Effect was discovered by the American physicist Arthur Holly Compton (1892-1962), for which he was awarded the 1927 Nobel Prize in physics.

In 1925, Geiger accepted his first teaching position, which was at the University of Kiel, Germany. Here, he and Walther Müller improved the sensitivity, performance, and durability of the counter, and it became known as the “Geiger-Müller counter.” It could detect not only alpha particles but also beta particles (electrons) and ionizing photons. The counter was essentially in the same form as the modern counter.

In 1929, Geiger moved to the University of Tübingen (Germany), where he was named professor of physics and director of research at the Institute of Physics. In 1929, while at the Institute, Geiger made his first observations of a cosmic-ray shower. Geiger continued to investigate cosmic rays, artificial radioactivity, and nuclear fission after accepting a position in 1936 at the Technische Hochschule in Berlin, a position he held until his death. In 1937, with Otto Zeiller, Geiger used the counter to measure a cosmic-ray shower.

During World War II (1939-1945), Geiger participated briefly in Germany’s abortive attempt to develop an atomic bomb. In June 1945, Geiger fled the Russian occupation of Berlin and went to nearby Potsdam, where he died on September 24, 1945, at the age of 62 years, less than 2 months after the American atomic bomb was dropped on Hiroshima, Japan.

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A Geiger counter measures the radiation being given off by a substance. It has both a dial, giving a reading, and a loudspeaker that transmits a regular clicking sound if radiation is detected. The faster the clicking, the more radiation there is.

A Geiger counter is a metal cylinder filled with low-pressure gas sealed in by a plastic or ceramic window at one end. Running down the center of the tube there’s a thin metal wire made of tungsten. The wire is connected to a high, positive voltage so there’s a strong electric field between it and the outside tube.

When radiation enters the tube, it causes ionization, splitting gas molecules into ions and electrons. The electrons, being negatively charged, are instantly attracted by the high-voltage positive wire and as they zoom through the tube collide with more gas molecules and produce further ionization. The result is that lots of electrons suddenly arrive at the wire, producing a pulse of electricity that can be measured on a meter and (if the counter is connected to an amplifier and loudspeaker) heard as a “click.” The ions and electrons are quickly absorbed among the billions of gas molecules in the tube so the counter effectively resets itself in a fraction of a second, ready to detect more radiation. Geiger counters can detect alpha, beta, and gamma radiation.

The sound of a Geiger counter is often associated with nuclear weapons and fallout. While it is useful in these situations, it is also used every day for the detection and control of nuclear waste, by-products and exposure in nuclear power plants, hospitals and even mines.

These ingenious devices allow anyone to detect potentially harmful radiation around them, using the power of electrons and the degradation of unstable radioactive atoms.

The detector is the main part of the Geiger counter. It is responsible for capturing, detecting and then signalling that a radioactive particle, known as a radioactive isotope, has passed through the detector.

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Nuclear fusion releases so much energy that it is hard to control. At the moment, only nuclear fission is used to give nuclear power. In power stations with pressurized water reactors, a radioactive substance, such as uranium, is bombarded with neutrons so that its atoms split and release energy. This energy heats water. The resulting steam turns a turbine to create electricity. Nuclear power has also been used to power submarines. One problem with nuclear power is that the waste material left behind is still radioactive and must be disposed of safely.

Nuclear energy is the energy in the nucleus, or core, of an atom. Atoms are tiny units that make up all matter in the universe, and energy is what holds the nucleus together. There is a huge amount of energy in an atom’s dense nucleus. In fact, the power that holds the nucleus together is officially called the “strong force.”

Nuclear energy can be used to create electricity, but it must first be released from the atom. In the process of nuclear fission, atoms are split to release that energy.

A nuclear reactor, or power plant, is a series of machines that can control nuclear fission to produce electricity. The fuel that nuclear reactors use to produce nuclear fission is pellets of the element uranium. In a nuclear reactor, atoms of uranium are forced to break apart. As they split, the atoms release tiny particles called fission products. Fission products cause other uranium atoms to split, starting a chain reaction. The energy released from this chain reaction creates heat.

The heat created by nuclear fission warms the reactor’s cooling agent. A cooling agent is usually water, but some nuclear reactors use liquid metal or molten salt. The cooling agent, heated by nuclear fission, produces steam. The steam turns turbines, or wheels turned by a flowing current. The turbines drive generators, or engines that create electricity.

Rods of material called nuclear poison can adjust how much electricity is produced. Nuclear poisons are materials, such as a type of the element xenon, that absorb some of the fission products created by nuclear fission. The more rods of nuclear poison that are present during the chain reaction, the slower and more controlled the reaction will be. Removing the rods will allow a stronger chain reaction and create more electricity.

As of 2011, about 15 percent of the world’s electricity is generated by nuclear power plants. The United States has more than 100 reactors, although it creates most of its electricity from fossil fuels and hydroelectric energy. Nations such as Lithuania, France, and Slovakia create almost all of their electricity from nuclear power plants.

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Nuclear weapons were first used in the Second World War. Two bombs were dropped on the Japanese cities of Nagasaki and Hiroshima, killing hundreds of thousands of people.

On August 6, 1945, the United States dropped an atomic bomb on the Japanese city of Hiroshima. It killed or wounded nearly 130,000 people. Three days later, the United States bombed Nagasaki. Of the 286,00 people living there at the time of the blast, 74,000 were killed and another 75,000 sustained severe injuries. Japan agreed to an unconditional surrender on August 14, 1945; it also resulted in the end of World War II. Scientists at Los Alamos had developed two distinct types of atomic bombs by 1945—a uranium-based design called “the Little Boy” and a plutonium-based weapon called “the Fat Man.”

While the war in Europe had ended in April, fighting in the Pacific continued between Japanese forces and U.S. troops. In late July, President Harry Truman called for Japan’s surrender with the Potsdam Declaration. The declaration promised “prompt and utter destruction” if Japan did not surrender.

On August 6, 1945, the United States dropped its first atomic bomb from a B-29 bomber plane called the Enola Gay over the city of Hiroshima, Japan. The “Little Boy” exploded with about 13 kilotons of force, leveling five square miles of the city and killing 80,000 people instantly. Tens of thousands more would later die from radiation exposure.

When the Japanese did not immediately surrender, the United States dropped a second atomic bomb three days later on the city of Nagasaki. The “Fat Man” killed an estimated 40,000 people on impact.

Nagasaki had not been the primary target for the second bomb. American bombers initially had targeted the city of Kokura, where Japan had one of its largest munitions plants, but smoke from firebombing raids obscured the sky over Kokura. American planes then turned toward their secondary target, Nagasaki.

Citing the devastating power of “a new and most cruel bomb,” Japanese Emperor Hirohito announced his country’s surrender on August 15—a day that became known as ‘V-J Day’—ending World War II.

In subsequent years, the United States, the Soviet Union and Great Britain conducted several nuclear weapons tests. In 1954, President Jawaharlal Nehru of India called for a ban on nuclear testing. It was the first large-scale initiative to ban using nuclear technology for mass destruction.

In 1958, nearly 10,000 scientists presented to United Nations Secretary-General Dag Hammarskjold a petition that begged, “We deem it imperative that immediate action be taken to effect an international agreement to stop testing of all nuclear weapons.”

France exploded its first nuclear device in 1960 and China entered the “nuclear arms club” in October 1964 when it conducted its first test. The United States, Soviet Union and some sixty other countries signed a treaty to seek the ends of the nuclear arms race and promote disarmament on July 1, 1968. The treaty bars nuclear weapons states from propagating weapons to other states and prohibits states without nuclear weapons to develop or acquire nuclear arsenal. It permits the use of nuclear energy for peaceful purposes. It entered into force in 1970 and was extended indefinitely and unconditionally on May 11, 1995.

In 1974, India conducted its first nuclear test: a subterranean explosion of a nuclear device (not weapon). India declared it to be a “peaceful” test, but it announced to the world that India had the scientific know-how to build a bomb.

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The half-life of a radioactive substance is a measure of the rate at which the nuclei of its atoms are breaking up or decaying. It is the time it takes for half the atoms in a sample to decay. Thorium, for example, has a half-life of 24 days, while radium-221 has a half-life of only 30 seconds. Uranium has a half-life of 4.5 thousand million years. Of course, as each isotope of an element has a certain number of protons and neutrons in its nucleus, it changes as it decays, forming other elements. For example, plutonium-242 decays to become uranium-238, which in turn breaks down to become thorium-234.

The half-life of a radioactive substance is a characteristic constant. It measures the time it takes for a given amount of the substance to become reduced by half as a consequence of decay, and therefore, the emission of radiation.

Archeologists and geologists use half-life to date the age of organic objects in a process known as carbon dating. During beta decay, carbon 14 becomes nitrogen 14. At the time of death organisms stop producing carbon 14. Since half-life is a constant, the ratio of carbon 14 to nitrogen 14 provides a measurement of the age of a sample.

In the medical field, the radioactive isotope Cobalt 60 has been used for radiotherapy to shrink tumors that will later be surgically removed, or to destroy cancer cells in inoperable tumors. When it decays to stable nickel, it emits two relatively high-energy gamma rays. Today it is being replaced by electron beam radiation therapy systems.
The half-life of isotopes from some sample elements:

Oxygen 16 – infinite
uranium 238 – 4,460,000,000 years
uranium 235 – 713,000,000 years
carbon 14 – 5,730 years
cobalt 60 – 5.27 years
silver 94 – .42 seconds

In the illustration above, 50% of the original mother substance decays into a new daughter substance. After two half-lives, the mother substance will decay another 50%, leaving 25% mother and 75% daughter. A third half-life will leave 12.5% of the mother and 87.5% daughter. In reality, daughter substances can also decay, so the proportions of substance involved will vary.

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The isotope called carbon-14 has a half-life of 5730 years. All living things on our planet contain this form of carbon, but they stop taking it in when they die. Scientists can examine ancient substances to see how much the carbon in it has decayed. They can then give a fairly accurate date for when the substance was alive. This is particularly useful for archaeologists and historians, who can date objects they find, helping to build up a picture of the past.

Radiocarbon dating is a method of what is known as “Absolute Dating”. Despite the name, it does not give an absolute date of organic material – but an approximate age, usually within a range of a few years either way. The other method is “Relative Dating” which gives an order of events without giving an exact age: typically artifact typology or the study of the sequence of the evolution of fossils.

There are three carbon isotopes that occur as part of the Earth’s natural processes; these are carbon-12, carbon-13 and carbon-14. The unstable nature of carbon 14 (with a precise half-life that makes it easy to measure) means it is ideal as an absolute dating method. The other two isotopes in comparison are more common than carbon-14 in the atmosphere but increase with the burning of fossil fuels making them less reliable for study; carbon-14 also increases, but its relative rarity means its increase is negligible. The half-life of the 14C isotope is 5,730 years, adjusted from 5,568 years originally calculated in the 1940s; the upper limit of dating is in the region of 55-60,000 years, after which the amount of 14C is negligible. After this point, other Absolute Dating methods may be used.

Today, the amount of carbon dioxide humans are pumping into Earth’s atmosphere is threatening to skew the accuracy of this technique for future archaeologists looking at our own time. That’s because fossil fuels can shift the radiocarbon age of new organic materials today, making them hard to distinguish from ancient ones. Thankfully, research published yesterday in the journal Environmental Research Letters offers a way to save Libby’s work and revitalize this crucial dating technique: simply look at another isotope of carbon.

Carbon-12 is a stable isotope, meaning its amount in any material remains the same year-after-year, century-after-century. Libby’s groundbreaking radiocarbon dating technique instead looked at a much rarer isotope of carbon: Carbon-14. Unlike Carbon-12, this isotope of carbon is unstable, and its atoms decay into an isotope of nitrogen over a period of thousands of years. New Carbon-14 is produced at a steady rate in Earth’s upper atmosphere, however, as the Sun’s rays strike nitrogen atoms.

Radiocarbon dating exploits this contrast between a stable and unstable carbon isotope. During its lifetime, a plant is constantly taking in carbon from the atmosphere through photosynthesis. Animals, in turn, consume this carbon when they eat plants, and the carbon spreads through the food cycle. This carbon comprises a steady ratio of Carbon-12 and Carbon-14.

When these plants and animals die, they cease taking in carbon. From that point forward, the amount of Carbon-14 in materials left over from the plant or animal will decrease over time, while the amount of Carbon-12 will remain unchanged. To radiocarbon date an organic material, a scientist can measure the ratio of remaining Carbon-14 to the unchanged Carbon-12 to see how long it has been since the material’s source died. Advancing technology has allowed radiocarbon dating to become accurate to within just a few decades in many cases.

Carbon dating is a brilliant way for archaeologists to take advantage of the natural ways that atoms decay. Unfortunately, humans are on the verge of messing things up. The slow, steady process of Carbon-14 creation in the upper atmosphere has been dwarfed in the past centuries by humans spewing carbon from fossil fuels into the air. Since fossil fuels are millions of years old, they no longer contain any measurable amount of Carbon-14. Thus, as millions of tons of Carbon-12 are pushed into the atmosphere, the steady ratio of these two isotopes is being disrupted. In a study published last year, Imperial College London physicist Heather Graven pointed out how these extra carbon emissions will skew radiocarbon dating.

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