Download
1 / 35
350 likes | 362 Vues
Radioactivity. This dish measured 20 millirad /hour when the counter was placed directly on it. The mantle measured 9 mr/hr. The No Salt has a lower level of radioactivity from the potassium chloride it contains, measuring 0.2 mr/hr. Nuclei decay because they are unstable.
E N D
This dish measured 20 millirad /hour when the counter was placed directly on it. The mantle measured 9 mr/hr. The No Salt has a lower level of radioactivity from the potassium chloride it contains, measuring 0.2 mr/hr.
Nuclei decay because they are unstable • Proton to neutron ratio is close to 1 in stable nuclei. • The further from 1, the more unstable the nucleus is.
First order kinetics • All nuclear processes follow first order kinetics • No way to change half-life • U238 Th234 + • C14 N14 +
Alpha Decay • Measurement of distance traveled by alpha particles is used to identify isotopes.
Alpha Radioactivity Composed of 2 protons and 2 neutrons, the alpha particle is a nucleus of the element helium. Because of its very large mass (more than 7000 times the mass of the beta particle) and its charge, it has a very short range. Not suitable for radiation therapy - its range is less than a tenth of a millimeter inside the body. Mainradiation hazard comes when it isingested;it has great destructivepower in its short range.
Beta particles are ejected at much greater speeds than alpha particles, and thus will penetrate considerably more matter, although the mechanism by means of which they are stopped is essentially similar. • Unlike alpha particles, however, beta particles are emitted at many different speeds, and beta emitters must be distinguished from one another through the existence of the characteristic maximum and average speeds of their beta particles.
The distribution in the beta-particle energies (speeds) necessitates the hypothesis of the existence of an uncharged, massless particle called the neutrino, and neutrino emission is now thought to accompany all beta decays. • n p+ + β- + ν
Penetration of Matter • Though the most massive and most energetic of radioactive emissions, the alpha particle is the shortest in range because of its strong interaction with matter. • The electromagnetic gamma ray is extremely penetrating, even penetrating considerable thicknesses of concrete. • The electron of beta radioactivity strongly interacts with matter and has a short range.
Positron emission • Identical with electrons but opposite in charge. • The positron-emission process is sometimes classified as a beta decay and termed beta-plus emission to distinguish it from the more common negative-electron emission. • Positron emission is accomplished through the conversion, in the nucleus, of a proton into a neutron, resulting in a decrease of the atomic number by one unit. • p+ n +β+ • Annihilates resulting in 2 gamma rays of 0.51 MeV
K-electron capture • The capture of an electron by the nucleus, followed by the transformation of a proton to a neutron. • The net result is thus a decrease of the atomic number by one unit. • The process is observable only because the removal of the electron from its orbit results in the emission of an X ray.
Cu-64 12.7 h EC, β- 0.578 Β+ 0.653 1.346 E- .578 E+ 1.657
Penetration of Matter • Though the most massive and most energetic of radioactive emissions, the alpha particle is the shortest in range because of its strong interaction with matter. • The electromagnetic gamma ray is extremely penetrating, even penetrating considerable thicknesses of concrete. • The electron of beta radioactivity strongly interacts with matter and has a short range.
Spontaneous-fission • The nucleus is split into two fragments. • Example: isotopes of radium of masses 222, 223, and 224 emit carbon-14 nuclei rather than decaying in the usual way by emitting alpha radiation.
Units • The activity of a source is measured in Becquerels (Bq) One Becquerel is one decay per second. The amount of radiation that your cells absorb is measured in grays (Gy),One gray is one Joule of energy absorbed by 1kg of your body. This is the dose you receive.
Dose • To measure the harm done to you, we need to remember that particles ionise very strongly, and cause 20 times more cell damage than the same dose of β particles, rays or X-rays. • We measure "dose equivalent" in sieverts (Sv). A dose of 1 gray of β particles, rays or X-rays will give you a dose equivalent of 1 sievert. • A dose of 1 gray of particles will give you a dose equivalent of 20 sieverts Usually work in millisieverts (mSv) or microsieverts (mSv).
Naturally occurring radioactive species Radon gas is produced from naturally occurring Uranium-238 in the soil. Radon is a problem in some areas today because homes are much more air-tight than they used to be. The radon gas enters the house through the basement. Thorium-232 also exists in the soil. Uranium and Thorium decay into numerous other radioactive isotopes before finally decaying into a stable element such as lead. And all this occurs naturally. In fact, the decay of uranium and thorium is the principle source of energy that heats the center of the Earth.
Decay series • Radon (Rn222) does an alpha decay into Polonium (Po218) with a half life of 3.824 days. • Polonium (Po218) does an alpha decay into Lead (Pb214) with a half life of 3.05 minutes. • Lead (Pb214) does a beta decay into Bismuth (Bi214) with a half life of 26.8 minutes. • Bismuth (Bi214) does a beta decay into Polonium (Po214) with a half life of 19.8 minutes
Polonium (Po214) does an alpha decay into Lead (Pb210) with a half life of 164 microseconds. • Lead (Pb210) does a beta decay into Bismuth (Bi210) with a half life of 22.3 years. • Bismuth (Bi210) does a beta decay into Polonium (Po210) with a half life of 5.01 days. • Polonium (Po210) does an alpha decay into Lead (Pb206) with a half life of 138.38 days. • Lead (Pb206) is stable.
Determining geologic time • Based on the fact that in many uranium and thorium ores, all of which have been decaying since their formation, the alpha particles have been trapped (as helium atoms) in the interior of the rock. • By accurately determining the relative amounts of helium, uranium, and thorium in the rock, the length of time during which the decay processes have been going on (the age of the rock) can be calculated. • Another method is based on the determination of the ratio of uranium-238 to lead-206 or of thorium-232 to lead-208 in the rocks (that is, the ratios of concentration of the initial and final members of the decay series). These and other methods give values for the age of the earth of between 3 billion and 5 billion years.
Applications • Calcium - 47: • studying the cell function and bone formation of mammals. • research to ensure that potential new drugs are metabolized without forming harmful by-products. • Cesium - 137: • Used to treat cancers.. • and to measure and control the liquid flow in oil pipelines • Chromium - 51: • Used in research in red blood cell survival studies. • Cobalt - 60 : • Used to sterilize surgical instruments...to improve the safety and reliability of industrial fuel oil burners...and to preserve poultry fruits and spices.
Ionization Detectors(Smoke detectors) • Use an ionization chamber and a source of ionizing radiation to detect smoke. • Inexpensive and good at detecting the smaller amounts of smoke produced by flaming fires. • Inside an ionization detector is a small amount (0.2 mg, 0.9 microcurie) of americium-241. • Americium has a half-life of 432 years, and is a good source of alpha particles.
The ionization chamber is two plates with a V between them. • The alpha particles ionize the O2 and N2 in the chamber. • A current is thus generated and detected. • Smoke particles attach to the ions and neutralize them, reducing the current.
Californium-252 • An isotope with a half-life of about 2.6 years, is a very strong neutron source. One microgram of californium-252 produces 170,000,000 neutrons per minute. • It is being used as a neutron source to identify gold and silver ores through a technique known as neutron activation. • It is also being used in devices known as neutron moisture gauges that are used to find water and oil bearing layers in oil wells.
Neutron moisture gauge • Neutrons from a neutron source such as Americium 241/Beryllium (Am-Be), are moderated (slowed down/thermalized) by water molecules and reflected into the detector volume. • The detector in the gauge is only capable of detecting moderated neutrons. • Hydrogen is a very effective neutron moderator and the number of moderated neutrons is proportional to the presence of hydrogen.
Cobalt - 60 : • Used to sterilize surgical instruments and to preserve poultry fruits and spices. • Iodine - 131: • Used to diagnose and treat thyroid disorders • Polonium - 210: • Reduces the static charge in production of photographic film and when weighing small amounts
Technetium - 99m • Most widely used radioactive isotope for diagnostics in nuclear medicine. Different chemical forms are used for brain, bone, liver,spleen and kidney imaging and for blood flow studies. • Technetium was the first artificially produced element. • Technetium is produced by bombarding molybdenum-98 with neutrons. Molybdenum-98 becomes molybdenum-99 when it captures a neutron. Molybdenum-99, with a half-life of 65.94 hours, decays into technetium-99 through beta decay. While technetium has never been found to occur naturally on earth, its spectral lines have been observed in S-, M- and N-type stars. • Tc-99m has a 6 h half life, Tc-99 has a half life of 2 x 105 y • Technetium's most stable isotope, technetium-98, has a half-life of about 4,200,000 years. It decays into ruthenium-98 through beta decay.
Neutron activation analysis • Multielement analytical method for the accurate and precise determination of elemental concentrations. • Based on the detection and measurement of characteristic gamma rays emitted from radioactive isotopes produced in the sample upon irradiation with neutrons.
Unknown samples together with standard materials of known elemental concentrations are irradiated with thermal neutrons in a nuclear reactor. • After some appropriate decay period, high resolution gamma ray spectroscopy is performed to measure the intensity and energies of the gamma lines emitted.
