c2-p1-28.pdf

Production of radionuclides  There are more than 2700 known radionuclides.  Radionuclides used in medical imaging are produced artificially, in the following ways: 2 Production of radionuclides A) If an additional neutron is forced into a stable nucleus  a neutron excess  This process occurs in...

Production of radionuclides  There are more than 2700 known radionuclides.  Radionuclides used in medical imaging are produced artificially, in the following ways: 2 Production of radionuclides A) If an additional neutron is forced into a stable nucleus  a neutron excess  This process occurs in a nuclear reactor. For example, with Molybdenum: 98Mo +n 99Mo  The atomic number of the nucleus remains unchanged, but its mass has been increased by one. 3 Production of radionuclides B) If an additional proton is forced into a stable nucleus, knocking out a neutron  a neutron deficit  This process occurs in a cyclotron, which accelerates positively charged ions (protons, deuterons or alpha particles) on the target material. For example, with oxygen: 18O +p 18F +n  The mass number has not been changed, but its atomic number has increased by one to become fluorine. 4 Production of radionuclides  Radionuclides produced in a cyclotron are short-lived (i.e. with half-lives ranging from less than a minute to a couple of hours).  So, it is only possible to use them reasonably close to the cyclotron.  Medical minicyclotrons have been designed specially for the production of short-lived radionuclides such as fluorine-18 (18F) at or near the hospital site. 5 Production of radionuclides Cyclotron 6 Production of radionuclides C) Radioactive fission products may be extracted from the spent fuel rods of nuclear reactors 238U  99Mo + other fission by-products  As the molybdenum is different chemically from the other products, it can be separated and prepared in a very pure form. 7 Production of radionuclides D) Some radionuclides are daughter products obtained from generator that contain a longer-lived radioactive parent.  Useful daughter products: Molybdenum-99 (99Mo) generator  Technetium-99m (99mTc) Germanium-68 (68Ge) generator  Gallium-68 (68Ga) 8 Radioactive transformation (Decay) 9 Radionuclides with a neutron excess (β- decay)  Radionuclides with a neutron excess may lose energy and become stable by a neutron changing into a proton plus an electron.  The electron is ejected from the nucleus with high energy and is referred to a negative beta particle n  p + β- 10 Radionuclides with a neutron excess (β- decay) For example: Iodine-131 (131I) with atomic number 53, decays in this way to xenon-131 (131Xe), with atomic number 54.  There has been no change of mass number, but the atomic number has increased by one. 11 Radionuclides with a neutron excess (β- decay)  Isomeric transition  in the case of some radionuclides, the gamma ray is not emitted until an appreciable time after the emission of the beta particle. For example: 99Mo decays by the emission of negative beta particle, the daughter nucleus technetium remains in the excited state for a variable length of time, which averages a matter of hours. - It is said to be metastable and is written as 99mTc 12 Radionuclides with a neutron excess (β- decay) Parent nuclide  emission (half-life) daughter nuclide  99mTc decay to the ground state, technetium-99 (99Tc), and the decay happens with the emission of a gamma ray of energy 140 keV.  Both 99mTc and 99Tc are said to be isomer, nuclei having different energy states and halflives, but otherwise indistinguishable as regards mass number, atomic number and other properties. half-life = 6 hours  decay to 99Tc  unstable 99Tc half-life = 211000 years  decay to ruthenium-99 (99Ru)  stable 99mTc 13 Radionuclides with a neutron excess (β- decay) 14 Radionuclides with a neutron deficit: β+ decay or K-electron capture β+ decay  Radionuclides with a neutron deficit may lose energy and become stable by a proton within the nucleus changing into a neutron and a positive electron.  The positive electron is ejected from the nucleus with high energy and is referred to a positive beta particle (i.e. positron) p  n + β+ 15 Radionuclides with a neutron deficit: β+ decay or K-electron capture  For example: Fluorine-18 (18F) with atomic number 9, transforms into oxygen-18 (18O), with atomic number 8.  There has been no change of mass number, but the atomic number has decreased by one. 16 Radionuclides with a neutron deficit: β+ decay or K-electron capture K-electron capture  the nucleus may increase its number of neutrons relative to the number of protons by capturing an extranuclear electron from the nearest K-shell. p + e-  n  The daughter nuclide will emit K-characteristic X-rays when an electron from an outer shell fills the created in the K-shell. 17 Radionuclides with a neutron deficit: β+ decay or K-electron capture  For example: Iodine-123 (123I) decays wholly by electron capture and emits 160 keV gamma and 28 keV X-rays but no positive beta particles. 18 Gamma rays  The gamma rays emitted during radioactive decay of a given radionuclide have at most a few specific energies (forming a line spectrum) that are characteristic of the nuclide that emits them.  Gamma rays have identical properties to X-ray. 19 Gamma rays For example: Iodine-131 (131I) emits mostly 364 keV gamma rays. 20 Positron emitters  Positive electrons (i.e. positrons) have a very brief existence.  When a positive beta particle comes to the end of its range, it combines with a nearly negative electron.  The opposite charges neutralise each other.  The combined masses of the two electrons are wholly converted into energy. 21 Positron emitters  According to Einstein's formula (E = m c2 ) for the equivalence between energy E and mass m, the mass of each electron is equivalent to 511 keV.  When the positive and negative electrons annihilate each other, the energy is emitted as two photons of annihilation radiation (each of 511 keV) travelling in opposite directions.  Positron emitters are used in positron emission tomography (PET) imaging. 22 Positron emitters 23 Radioactive decay  Radioactive disintegration is a stochastic (random) process.  It is impossible to predict which of the unstable nuclei in a sample will disintegrate in the next second.  BUT it is possible to be quite precise about the fraction that will disintegrate on account of the large numbers of nuclei the sample contains. 24 Activity  A radioactive nucleus does not make its presence known until it decays and emits a beta or gamma rays, or both.  The quantity of radioactivity is measured not by the ‘population’, the mass or number of radioactive atoms, BUT by the transformation rate, i.e. the number that represents disintegrate per second (also known as the decay rate). 25 Activity  The activity of radioactive sample is the rate of disintegration, the amount of disintegrating per second.  The SI unit is the Becquerel (Bq) = 1 disintegration per second.  In medical gamma imaging, most radionuclide administrations are measured in megabecquerels (1 MBq = 106 Bq).  The activity of radionuclide generators in gigabecquerels (1 GBq = 109 Bq).  A unit found in old textbooks is the curie (Ci)  1 mCi = 37MBq 26 Activity  When the gamma rays enter a detector, they may be registered individually as counts.  The count rate [number of counts per second (cps)] measured by a given instrument is less than the activity, because the greater proportion of the rays usually miss the detector and some pass through it undetected. However, there is a proportionality: Count rate α activity α number of radioactive atoms in the sample 27 Activity  The fundamental law of radioactive decay states that the activity of a radioactive sample decreases by equal fractions (percentage) in equal interval time. This is referred to as the exponential law.  However long the time, the activity of a radioactive sample never falls to zero. 28

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