Course : Inorganic Pharmacy II Course code: PHR 107 Course Teacher : Zara Sheikh Radioactivity and Radiopharmaceuticals

ISOTOPES : Isotopes are atoms of the same element with the same number of protons but different number of neutrons. The isotopes of a particular element have the same chemical and physical properties. Examples of isotopes are : C, C 6 6

TYPES OF ISOTOPES : Two major types of isotopes are found in nature : stable isotopes and unstable (radioactive) isotopes. Stable isotopes maintain their elemental integrity and do not decompose to other isotopic or elemental forms. E.g. 12 C Unstable or radioactive isotopes decompose by emission of nuclear particles (alpha particles, beta particles, gamma rays, x-rays) into other isotopes of the same or different elements. E.g. 14C

Stable and Unstable Nuclei The nuclei of elements of low atomic number are most stable if the number of neutrons is equal to or slightly greater (1 more) than the number of protons. If the neutron number is less or significantly greater (2 or more) than the number of protons, the nucleus is unstable e.g for carbon Protons Neutrons Stability 6 4 unstable 6 5 unstable 6 6 stable 6 7 stable 6 8 unstable For elements of higher atomic number the most stable neutron-proton ratio approximates 15:1 Unstable nuclei have excess energy and by discharging this in the form of particles or radiation they achieve a stable structure.

Alpha particles, α α particles which constitute alpha radiation consists of 2 protons and 2 neutrons. Characteristics : a) They are equivalent to the nuclei of helium atoms 4 He 2 b) They are heavy and positively charged (+2). c) The particles move at a relatively slow speed, averaging 0.1 the speed of light (3x10 10 ) cm/s. d) Their penetrating power is very low and can be stopped by a sheet of paper or a very thin sheet of aluminium foil. e) These particles will travel only 3 to 8 cm in air. f) Energy value, typically 4 Mev.

Alpha particles, α g) α radiation is usually emitted only from elements having atomic numbers greater than 82. h) The emission of alpha radiation is illustrated below with radium -226 (the radium isotope having a mass number of 226) : Ra ———> Rn + α (or He) The low penetrating power of alpha particles makes isotopes emitting this type of radiation not useful for biological applications because these particles cannot penetrate tissue.

Beta particles, β β radiation is of two types because there are two types of electrons : negative electron (negatron) and positive electron (positron). The positron is identical with the negatron in all respects except for its charge of +1 instead of -1. It is also known as the antiparticle of the electron. When these electrons are emitted from radioactive nuclei, they are called β -particles. Negatrons (β – ). are emitted by unstable nuclei having neutrons in excess of protons. So, a transformation of a neutron to a proton occurs with the emission of beta radiation (β – ).

Beta particles, β Elements undergoing this type of transformation will decay to the element having the next highest atomic number. 1 1 n ———> p + β – 0 1 An example of β decay is shown below : C ———> N + β – 6 7

Beta particles, β Positrons ( β + ) are emitted from nuclei having a proton/ neutron ratio above stable limits. So a proton can be transformed into a neutron, with the emission of beta – radiation ( β + ) 1 1 p ———> n + β An example of β decay is shown below : Zn ———> Cu + c Positrons are not important in biological applications because they are short-lived and undergoes reactions with electrons to produce gamma radiation. β + + e – ———> 2 γ

Beta particles, β Characteristics: They have the mass of an electron (approximately 9.1 x g). They move at a faster velocity. Their emissions from elements do not alter the atomic mass but changes the atomic number. They have more penetrating power than alpha particles and can travel from 10 to 15 mm in water or penetrate almost 1-inch thicknesses of aluminium. Maximum energy is 1.5 Mev and mean energy is 0.6 Mev. Many isotopes emitting β – have useful biological applications as the radiation will penetrate tissues.

Gamma Radiation, γ Gamma radiation is electromagnetic but α and β radiations are particulate. It means γ radiation demonstrates both wave and particle properties. Gamma rays are radiated as discrete packets of energy (quanta). These are also known as photons. Characteristics: The rays are of a short wavelength and travel at the speed of light. Since it is of electromagnetic radiation, it has no mass or charge. They have very high energy (2 Mev). They have excellent penetrating power and very thick lead or concrete shielding is required to protect against this radiation.