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i09

Radioactivity

Radioactive decay

Radioactivity is the phenomenon that unstable nuclei of atoms (radionuclides) spontaneously decays into another kind of atom, usually coupled with the emission of nui09 Radioactivity Radioactive decay Radioactivity is the phenomenon that unstable nuclei of atoms (radionuclides) spontaneously decays into another kind of atom, usually coupled with the emission of nuclear radiation: alpha, beta and/or gamma radiation. The decay product, also called the decay daughter, can be radioactive in itself or can be stable. In nature on earth a few radioactive kinds of atoms occur in low concentrations. Important are the elements uranium and thorium, which are formed billons of years ago in supernova explosions. These radionuclides decay via a series of other radionuclides into stable lead or bismuth atoms. During fission of uranium large amounts of radioactive isotopes of nearly all known elements are formed.

Ionizing radiation

Nuclear radiation, also called ionizing radiation, strongly interacts with matter and is harmful to living organisms, for it destroys biomolecules. Alpha and beta radiation can be blocked by thick paper respectively aluminum foil, so these rays may seem not very harmful to man. However radionuclides radiating alpha or beta rays inside the human body are extremely dangerous, because the living cells are not protected by the skin or clothes. A dose of only a few nanograms of the alpha-emitter polonium-210 in the human body is lethal. A complicating factor is that alpha and beta radiation are not detectable by hand-held counters, which can only detect gamma rays. Radionuclides that emit weak or no gamma rays are invisible to these detectors. A number of biologically very active radionuclides fall within this category, such as tritium (radioactive hydrogen) and carbon-14 (radioactive carbon).

Half-life

The rate of radioactive decay is characteristic to each kind of radionuclide and cannot be decelerated or accelerated by any means. Radioactivity cannot be destroyed nor made harmless to man and other living organisms. Radionuclides occurring in nature, such as uranium anf thorium, have very long half-lifes measured in billions of years. These nuclides have been formed in stellar explosions long before the Earth came into being. Man-made radionuclides have much shorter half-lifes, ranging from seconds to millions of years. The specific radioactivity of a radionuclide with a short half-life is higher as the half-life is shorter.

Nuclear bomb equivalents

Nuclear power generates immense amounts of radioactivity, irrevocably and irreversibly. During fission of uranium atoms many dozens of different kinds of radioactive atoms are coming into being, called the fission products. In addition non-radioactive construction materials become radioactive by neutron radiation. The amount of man-made radioactivity is a billion times the radioactivity of the fresh uranium entering the reactor. One nuclear reactor generates each year an amount of radioactivity equivalent to roughly 1000 nuclear bombs of the yield of the Hiroshima bomb. All radioactive wastes ever generated during the nuclear era are still stored in temporary storage facilities. These facilities are leaking all kinds of radioactivity into the environment at an increasing rate, due to the unavoidable deterioration of the materials and structures of the containing facilities, and are vulnerable to natural disasters, accidents and terroristic attacks.

Once generated, radioactivity cannot be influenced by any means. The radioactivity resulting from nuclear power decreases by natural decay only. For some components of the man-made radioactivity the decay rate can be measured in seconds to hours, for other components time scales of years to hundreds of thousands of years are involved.

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Tdecay

Figure 09-1. Radioactive decay of tritium.

Tritium, symbols T or H-3, is a heavy isotope of hydrogen, with one proton and two neutrons in the nucleus. When a tritium atom decays, it emits a beta particle (an electron) at high speed. After decay the nucleus contains two protons and one neutron, the nucleus of a helium-3 atom, which captures a second electron and becomes a neutral helium-3 atom. The sum of electric charges remains constant, a minute fraction of the mass is converted into energy.

decay

Figure 09-2. Decay of a radionuclide.

One half-life period after creating of a given amount of a certain radionuclide at time t = 0, half of the radionuclides has decayed into another kind of, non-radioactive, nuclides. A second half-life period later half of the remaining radionuclides has decayed into stable nuclides. And so on. The total mass of matter remains almost constant during the decay process.

symbolradioact

Figure 09-3. Symbol of nuclear radiation.

This pictogram symbolizes three kinds of lethal nuclear radiation: alpha, beta and gamma radiation.

halflifeT

Figure 09-4. Half-life of the radioactive decay of tritium.

Mass of a given amount of tritium as function of the time. After one half-life half of the initial number of tritium atoms has decayed into helium-3 atoms. During the second half-life period half of the remaining tritium atoms decay, not the other half of the initial amount of tritium atoms. Radioactive decay is a stochastic process.

cumulative

Figure 09-4. World radioactivity inventory.

All man-made radioactivity still exists in a mobile form in the human environment. The world inventory of man-made radioactivity passed the 10 million nuclear bomb equivalent mark in 2010 and is rising nearly linearly with 370 000 nuclear bomb equivalents a year.