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DEPT. OF NUCLEAR MEDICINE SVIMS-University, TIRUPATI. Presented by Dr.K.R.Venkata Rajesh Reddy

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Radioactive Decay Radioactivity

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Alpha decay was discovered by Marie and Pierre Curie in 1898 in their efforts to isolate radium. it was first described by Ernest Rutherford in 1899. Ernest Rutherford first characterized beta decay in 1899 The difference in the energy released during decay, and that possessed by the negatron, threatened the concept of energy conservation for several years. In 1933 Wolfgang Pauli suggested that a second particle was emitted during each decay that accounted for the energy not carried out by the negatron. This particle was named the neutrino (Italian for “little neutral particle”) by Enrico Fermi . Ernico fermi Wolfgang Pauli Marie and Pierre Curie

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Gamma rays were discovered by the French physicist Paul Villard in 1900. Rutherford and Andrade confirmed in 1912 that γ rays and x rays are similar types of radiation. The emission of positrons from radioactive nuclei was discovered in 1934 by Irene Curie (daughter of Marie Curie) and her husband Frederic Joliet.

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Radioactive Decay: 1.Alpha Decay 2.Beta Decay 3.Gamma Decay or Isomeric Transition

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Alpha Decay: The reason alpha decay occurs is because the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a Helium nucleus is emitted. Helium Nucleus Two Protons and two Neutrons Decreases atomic number by 2 Decreases mass number by 4 Highly ionizing Stopped by skin or clothing

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Beta decay: three common forms of beta decay: a)Electron Emission b)Positron Emission c)Electron Capture

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Unstable nuclei which have too many neutrons (i.e., neutron-rich nuclei) tend to decay by the emission of negative beta particles. Beta minus Decay: a)ELECTRON EMISSION: The negative beta particle, also known as a negatron, is an electron formed in an unstable nucleus through the following nuclear conversion:

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An example of this type of decay occurs in the iodine-131 nucleus which decays into xenon-131 with the emission of an electron

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This nuclear transformation results in the loss of a neutron and the addition of a proton to the nucleus, thereby altering the neutron to proton ratio. The nuclear electron (-) and the antineutrino (  ) are ejected from the unstable nucleus. What Is Negative Beta Particle? A beta particle (negatron) is an electron ejected from a radioactive nucleus which is neutron-rich.

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Commonly Used - Emitters in Medical Research and Therapy

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Positron Emission: Positive beta particles (+), known as positrons, may be emitted from radioactive nuclei which have too many protons, i.e., those which are proton-rich. BETA PLUS DECAY: What Is a Positron? A positron is a positive electron emitted from proton-rich radioactive nuclei.

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Positrons are formed by the following nuclear transformation: A nuclear proton is converted into a neutron (altering the N to P ratio) followed by the emission of the + and the .

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Positron Decay Equation:

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Biologically Useful Positron Emitters:

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What happens to those unstable nuclei that are proton-rich but cannot meet the energy requirement for + emission? c)Electron Capture: (EC) inverse beta decay When an unstable nucleus decays by EC, an inner shell electron is captured by the unstable nucleus. Its neutron to proton ratio is adjusted through the following transformation. Important Note: Proton rich nuclei can decay either by electron capture only or positron capture only or by both depending on specific radionuclide

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Radionuclides That Decay by Positron Emission or EC

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c)Gamma Decay: Isomeric Transition Gamma decay involves the emission of energy from an unstable nucleus in the form of electromagnetic radiation. Isomeric transition: isomer – (excited state) Decay of excited state to a lower energy state or ground state known as isomeric transition and proceeds through either of two processes 1)High energy photon emission 2)Internal conversion Nuclei which are in an excited energy state may be designated by an asterisk (*) or by the letter “m” in the A - number. The “m” designates a metastable (i.e, almost stable) state

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1)High Energy Photon Emission: Excess energy of isomer is released in the form of gamma ray. (energy emitted from nucleus is gamma ray , energy emitted by transition of electrons is X rays). 2)Internal Conversion: Nuclide in excited state transfers its excess energy directly to orbital electron (in the K,L,M,and N shells) In internal conversion an electron is emitted from the inner shell of an atom ( K, L, or M) a vacancy is created in that shell. this vacancy is subsequently filled by electrons from higher shells leading to emission of an x-ray or auger electron.

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Radionuclides That Emit Gamma Radiation

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Henry bequerel Marie curie

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RADIOACTIVITY: Number of disintegrations per unit of time (decay rate) is called radioactivity. Radioactive Decay Law: decay will depend on overall number of nuclei, N, and also on the length of the brief period of time. In other words the more nuclei there are the more will decay and the longer the time period the more nuclei will decay. Let us denote the number which will have decayed as dN and the small time interval as dt. So we have reasoned that the number of radioactive nuclei which will decay during the time interval from t to t+dt must be proportional to N and to dt. In symbols therefore:

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Turning the proportionality in this equation into an equality we can write: where the constant of proportionality, λ, is called the Decay Constant Dividing across by N we can rewrite this equation as: So this equation describes the situation for any brief time interval, dt. This final expression is known as the Radioactive Decay Law. It tells us that the number of radioactive nuclei will decrease in an exponential fashion with time with the rate of decrease being controlled by the Decay Constant.

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Radioactive Decay Law: The Law tells us that the number of radioactive nuclei will decrease with time in an exponential fashion with the rate of decrease being controlled by the Decay Constant. The Law is shown in graphical form in the figure below: The graph plots the number of radioactive nuclei at any time, Nt, against time, t. We can see that the number of radioactive nuclei decreases from N0 that is the number at t = 0 in a rapid fashion initially and then more slowly in the classic exponential manner.

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The influence of the Decay Constant can be seen in the following figure: All three curves here are exponential in nature, only the Decay Constant is different. Notice that when the Decay Constant has a low value the curve decreases relatively slowly and when the Decay Constant is large the curve decreases very quickly.

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Decay Constant is characteristic of individual radionuclides. Some like uranium-238 have a small value and the material therefore decays quite slowly over a long period of time. Other nuclei such as technetium-99m have a relatively large Decay Constant and they decay far more quickly. Half-Life: This indicator is called the Half Life and it expresses the length of time it takes for the radioactivity of a radioisotope to decrease by a factor of two.

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Note that the half-life does not express how long a material will remain radioactive but simply the length of time for its radioactivity to halve.

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Relationship between the Decay Constant and the HalfLife when the Decay Constant is small the Half Life should be long and correspondingly when the Decay Constant is large the Half Life should be short. But what exactly is the nature of this relationship? We can easily answer this question by using the definition of Half Life and applying it to the Radioactive Decay Law.Once again the law tells us that at any time, t: definition of Half Life tells us that:

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We can therefore re-write the Radioactive Decay Law by substituting for Nt and t as follows: These two equations express the relationship between the Decay Constant and the Half Life.

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