[ Pobierz całość w formacie PDF ]
.5).While two gamma energies are usually associated with the decay of io-dine-131, the spectrum is quite complex.A 0.364 MeV photon is emittedin 82 percent of the beta disintegrations and a 0.637 MeV photon in 6.5percent, but small percentages of photons with higher and lower energies131are also emitted.The I photons have a much lower penetration in lead60than the photons from Co, since a half-value layer is only 0.3 cm.Thethickness of water required is 5.8 cm, or 19 times as great.Thus the lead ismuch more effective than water in attenuating the incident radiation atthese energies.125The gamma photons emitted in I decay are of low energy.They donot accompany beta decay, but are emitted in a decay process known aselectron capture.In this transformation (Fig.2.8), an electron in the inner-most orbit of the atom is captured by the nucleus, and the energy madeavailable by this reaction is equal to 0.035 MeV.In 6.7 percent of the dis-integrations, this energy is emitted as a 0.035 MeV gamma photon.Therest of the time, it causes the release of electrons, known as internal conver-sion electrons, from the shells surrounding the nucleus.Electron capture and internal conversion processes are always accompa-nied by the emission of x rays9 from the inner shells of the atom.About 1.4x rays with energies between 0.027 and 0.032 MeV are emitted per disin-125tegration of the I nucleus.Note the most unusual correspondence be-tween the energies of the gamma rays (emitted from energy transitions in8.Note the analogy between half-value layer and half-life (introduced in section 6.1.1).See sections 8.4 and 21.4 for methods of using these concepts in attenuation and decay cal-culations.9.Photons originating in the inner orbits of the atom are called x rays, and photons orig-inating in the nucleus are called gamma rays, although they are identical if of the sameenergy. |38 TWO Principles of Protection against Ionizing Particles2.8 Mechanisms of energy release during125electron capture, as illustrated with I.(a)Nucleus captures electron, usually from inner-³(0.035 MeV)e-53p+ 52p+most shell, producing 0.035 MeV excess en-72n 73nergy in resulting tellurium-125 nucleus.(b) In7%6.7 percent of the disintegrations, a 0.035 MeV125I 125Tegamma photon leaves the atom.(c) In 93 per-cent of the disintegrations, an electron is(a) (b)ejected from one of the inner shells in a pro-cess known as internal conversion.The energye-e-of the internal-conversion electron is equal to(IC)x-raythe difference between 0.035 MeV and the en-93%ergy required to remove it from the atom.The52p+ 52p+(0.027 MeV)73n 73nenergies required for removal from the two in-nermost shells are 0.032 MeV and 0.005 MeV.(d) The vacancies left by electron capture and 125Te 125Teinternal conversion are filled by the transfer of(c) (d)electrons from the outer shells.The energymade available by the transfer is emitted as x-125ray photons characteristic of Te or is usedto eject additional orbital electrons, known asAuger electrons.the nucleus) and the x rays (originating from electron transitions in theshells outside the nucleus).Lead is extremely effective in attenuating the low-energy photons emit-125ted by I.It takes only 0.0037 cm of lead to reduce them by a factor of 2.Because of their low energies, other materials of higher atomic number,such as steel or brass, are also very effective, and preferable to lead becauseof its toxicity.Water is much less effective because of its low atomic num-ber and density, requiring 2.3 cm, or 620 times the thickness of lead, forthe same degree of attenuation of the incident photons.8.2 Positron-Emitting Radionuclides and Annihilation RadiationThe electron with a single negative charge has a counterpart in anotherparticle, with the same rest mass but with a single positive charge.Thispositive electron is called a positron.When apart from the electron, thepositron is stable, but a free positron quickly comes in contact with anelectron with disastrous consequences for both.They both disappear,their mass being completely transformed into energy as two 0.51 MeVphotons, which fly apart in exactly opposite directions.The electron and positron are antiparticles.Although the positron doesnot normally exist on earth, it is created in some forms of radioactive de-cay, as well as in reactions involving high-energy radiations.For example, |8 Gamma Rays 39the nucleus of fluorine-18, atomic number 9, contains 9 protons and 9neutrons.Nuclei with odd numbers of both neutrons and protons are usu-ally unstable, and fluorine-18 is radioactive with a half-life of 110 minutes.Ninety-seven percent of the decays are by positron emission a proton inthe nucleus is transformed into a neutron with the emission of a positivebeta particle (positron), maximum energy 0.634 MeV and fluorine-18 istransformed into oxygen-18.The other 3 percent of the time, decay is intooxygen-18 by electron capture, in which an inner electron from the atomcombines with a proton in the nucleus to reduce the atomic number byone.This decay process has already been discussed for iodine-125.Positrons have the same ranges in matter as electrons of the same ener-gies because they undergo similar collision processes.Since the positronmust ultimately vanish through annihilation with an electron, positronemitters can be traced not only through the positrons but also through thetwo 0.51 MeV gamma photons resulting from the annihilation.Because positron decay in matter is accompanied by the simultaneousemission of two photons moving in opposite directions, the origin of thephotons can be determined by appropriate measurements.This has led tothe development of a very useful medical imaging technique using posi-tron-emitting radiopharmaceuticals called positron emission tomography,or PET.PET has many applications in the diagnosis of medical conditions,including the visualization and measurement of blood flow.An example ofhow it is used in both medical treatment and research is the study of bloodflow in the brain, as affected by diseases such as epilepsy or by the taking ofdrugs.PET scans are also used to study blood flow in the heart.The positron-emitting radionuclides commonly used in medicine andtheir half lives are fluorine-18 (110 min), carbon-11 (20 min), nitrogen-13(10 min), and oxygen-15 (2 min)
[ Pobierz całość w formacie PDF ]