Radiation Physics and Dosimetry MPHY360 PDF

Radiation Physics and Dosimetry MPHY360 3. Radioactivity Prof. Azza Helal Structure of the atom Revise structure of the atom (Introduction)  Matter is composed of atoms. Atom has positive charge nucleus (Protons and neutrons (nucleons)...

Radiation Physics and Dosimetry MPHY360 3. Radioactivity Prof. Azza Helal Structure of the atom Revise structure of the atom (Introduction)  Matter is composed of atoms. Atom has positive charge nucleus (Protons and neutrons (nucleons) & negative charge electrons revolve in orbital shells. Electrons are negatively charged (e- ), Protons are positively charged (p+ ) Neutrons have no charge (n±) The mass of an electron is 1/2000 the mass of a hydrogen atom. The mass of a proton is equal to the mass of a hydrogen atom and is taken as 1 unit. The mass of a neutron is equal to the mass of a hydrogen atom and is and is taken as 1 unit. (n±)  A nucleus is designated as ZXA. The atomic number (Z): is the number of protons present in the nucleus of the atom. Number of electrons equal to the number of protons. The mass number (A): is the sum of the number of protons and neutrons (nucleons) present in the nucleus of the atom. Number of neutrons equal to the difference between the mass number (A) and the atomic number (Z). 12C: 6 electrons, 6 protons, and 6 neutrons 13C: 6 electrons, 6 protons, and 7 neutrons 14C: 6 electrons, 6 protons, and 8 neutrons  Isotopes:- atoms of the same element having the same atomic numbers but different mass numbers. Isobars: atoms of different elements having different atomic numbers but same mass numbers Isotones: atoms of different elements having different atomic numbers and mass numbers but same number of neutrons. Metastable state: atoms of the same element having same atomic & mass numbers but different energy state. Dr Azza Helal  Stable & unstable nuclei 1500 nuclides exist Stable nuclei: 300 They have equal no of protons (P) & neutrons (N) P/N ratio (1-1). Unstable nuclei (radioactive) : 1200 65 natural occurring and the rest are human-made. They have some unbalance in the number of protons & neutrons (neutrons excess or deficit)  A radioactive / unstable nucleus has excess energy. To achieve a state of low energy, one of particle may gain enough energy to escape from nucleus. (α, B & γ ) Ejection of a particle changes the nucleus from one form to another. The emission of particle may still leave the nucleus in an excited state, so emits more particles or γ rays to reach low energy state and so to stable or ground state.  XA Y A + W+Q Z Z X = parent, Y = daughter & W = radiation particle, Q is provided by the difference in mass between the parent nucleus and sum of mass of daughter nucleus and particle emitted.  Radioactivity * * Modes of Radioactive Decay  Alpha decay ( α decay ) For high Z nuclei (heavy atoms) the coulomb force of repulsion between protons becomes large enough to overcome nuclear force that binds nucleons together.  Alpha decay ( α decay )  Alpha particle (α) is positive charge deflected by electromagnetic field and stopped by few sheets of paper or few cm in air.  Beta decay (B ± rays) A) Nuclides with neutron excess (-1B0 decay): The nucleus is unstable as it has high neutrons number. Nucleus may lose energy and becomes stable by neutron changing into proton and electron. Electron (negative B particle) is ejected from the nucleus (not from the atomic orbital shell).  Negative B particle (e) has same mass and charge of electron, deflected by EM field & stopped by few sheets of AL V neutrino has no charge, practically no mass so probability of its interaction is very small B) Nuclides with neutron deficit (+1B0 decay): The nucleus is unstable as it has low number of neutrons. Nucleus lose energy and becomes stable by proton changing into neutron & positive electron (positron). Positive B particle is ejected from the nucleus with high energy Positive electron combines with a nearby negative electron. They neutralize each other and the combined masses of two electrons are converted into energy (2 photons of annihilation radiation) Annihilation process C) K Electron Capture (EC) neutrons deficit Nucleus may increase its number of neutrons relative to protons by capturing an electron from k shell. Daughter nuclide emits characteristic X rays when hole created in k shell is filled by electron from outer shell. It is an alternative mechanism to β+ decay. - neutrons deficit  Gamma decay (γ)  So gamma rays are emitted either instantaneously with the emission of beta or alpha particle, or γ rays emission may take an appreciable time after emission of B & α particle. Isomeric transition A) Isomeric Transition (IT) pure gamma decay It occurs if γ rays are not emitted immediately but it takes an appreciable time after emission of B particle. ZX + γ * XA (M) A Z B- IT Mo 99 Tc 99m Tc 99 + γ …………….stable nuclide 42 43 43 99m & 43Tc99 are isomers of same atom. 43Tc Isomers are nuclei which have different energy states but otherwise have same A , Z, No of protons and neutrons and other properties. B) Internal Conversion (IC) The energy liberated from isomeric transition can be delivered to an electron ejected from the atom. Then electrons rearrange to fill the vacancy left by the I.C. electron resulting in characteristic x-rays and/or Auger electrons. Spontaneous fission Summary of decay processes So XA YA + W+Q Z Z A A-4 + α +Q (α decay) ZX Z-2Y XA YA + B±+V+ Q (B± decay) Z Z±1 XA YA + B±+ γ (γ decay) Z Z±1 XA (M) XA+ γ (IT) Z Z XA + e- YA +V+ Q (EC) Z Z-1 Modes of decay α β γ β- decay Excess of neutrons Neutron transformed into a proton and an electron Electron (beta particle) ejected from the nucleus β+ decay Excess of protons Proton transforms into a neutron and a positron Positron ejected from the nucleus α decay Excess of nucleons (heavy nuclei) Nucleus ejects alpha particle (helium n= 2 protons + 2 neutrons Decay series: Emission of more particles or γ rays from the nucleus to reach low energy or ground state as emission of one particle may still leave the nucleus in an excited state. B- B- B- 54Xe → 55Cs 139 139→ Ba139 → La139 56 57 0.5min 2min 86min stable Exponential law of decay  Disintegration is a (stochastic) random process, no way to decide when any particle will disintegrate.  The amount of sample that remains after certain time can be calculated by  Physical T1/2  Exponential law of decay Exponential law of decay N=N0e-λt N0/2= N0e-λt1/2 ½= e-λt1/2 Ln1/2= -λt1/2 -0.693= -λt1/2 T1/2= 0.693/λ N is the number of radioactive atoms at time t, N0 is the number of radioactive atoms at time t0 (initial quantity) λ is decay, disintegration, transformation constant. The value of the transformation constant is inversely related to half life time. Physical half life time (T1/2 ) is the time interval required for a given nuclei to decay to half of its original value. If the radionuclide is stored in bottle, it decays with its physical t1/2 The value of the transformation constant is inversely related to half life time. Isotope T1/2 Decay Isotope T1/2 Decay Kr81m 13sec IT Tc99m 6hrs IT Th201 73hr EC Xe 133 5days γ&B I123 13hr EC I131 8days B- G67 78hr EC F18 110 min B+ T1/2 of different isotopes. It is necessary to know the T1/2 of different atoms to know For how long we store the radionuclide before it becomes sufficiently harmless. For how long the patient is radioactive after isotopic scan ( gamma imaging). Biological half life time: it is the time required for the body to eliminate half of the dose of any substance by regular processes of elimination (by usual metabolic turn over and excretion). Effective half life time: it is the time required for the radioactivity from given amount of a radioactive element deposited in the tissues or organs to diminish to 50% as a result of combined action of radioactive decay and loss of the material by biologic elimination.  Effective t1/2 is shorter than physical or biological t1/2 due to the simultaneous effect of radioactive decay and metabolic turn over and excretion 1/ effective t1/2=1/ biological t1/2 + 1/physical t1/2  Units of radioactivity Curie is the radioactivity of a sample which decays at a rate of 3.7X1010 dis/sec. It is the activity of one gram of radium 226. It is the older unit of radioactivity. Becquerel is the radioactivity of a sample which decays at a rate of one disintegration /sec. It is SI unit of radioactivity Ci=37GBq Radiation Physics and Dosimetry MPHY360 4. Decay Equilibrium Prof. Azza Helal Decay Equilibrium: A condition established in a parent/daughter mixture when both parent and daughter are radioactive and when the daughter’ s half life is shorter than that of the parent.  Secular equilibrium is a situation in which equilibrium is reached by a parent-daughter radioactive isotope pair where half-life of the daughter radionuclide is much shorter than half-life of the parent. (parent half-life >> daughter half-life). It occurs after 6 half-lives of daughter. The decay rate of parent and hence the production rate of daughter is approximately equal, because the half-life of parent is very long compared to that of daughter. So that the daughter appear to decay with half life of the parent.  Transient equilibrium is a situation in which equilibrium is reached by a parent- daughter radioactive isotope pair where the half-life of the daughter is shorter than the half-life of the parent. Transient equilibrium occurs after four half-lives of daughter. (parent half-life > daughter half-life) Example is 99Mo decay (67h) to 99mTc decay (6h)