Introduction to Radiochemistry NUSC 341-3

Forces in Matter and the Subatomic Particles Chapter 1

What is Nuclear Science? Nuclear science: study of structure, properties, and interactions of atomic nuclei at fundamental level. nucleus – contains almost all mass of ordinary matter in a tiny volume understanding behavior of nuclear matter under normal conditions and conditions far from normal a major challenge extreme conditions existed in the early universe, exist now in the core of stars, and can be created in the laboratory during collisions between nuclei (TRIUMF) Nuclear scientists investigate by measuring the properties, shapes, and decays of nuclei at rest and in collisions. This course covers low energy, or low temperature, nuclear science => properties of the nucleus

Why should we bother?

Interactions Electromagnetic e - (lepton) bound in the atoms by the electromagnetic force Weak interaction Neutrino observed in beta decay. Strong interaction Quarks are bound in together by the strong force in nucleons. Nuclear forces bind nucleons into nuclei. Gravitation Gravitational interaction between the elementary particles is in practice very small compared to the other three.

Interactions forcesstrengthrange (fm)exchange particlemass (eV)chargespin gravitational6x infinitegraviton?002 weak1x x10 -3 W±, Z91x10 9 ±1,01 electromagnetic7x10 -3 infinitephoton001 strong11.5pion35x fm = m The forces of elementary particle physics are associated with the exchange of particles. An interaction between particles is characterized by both its strength and its range. Force between two objects can be described as exchange of a particle – particle transfers momentum and energy between the two objects, and is said to mediate the interaction graviton – not yet observed pions or pi mesons – between nucleons

Standard Model Attempts to explain all phenomena of particle physics in terms of properties and interactions of a small number of three distinct types. Leptons: spin-1/2 Quarks: spin-1/2 Bosons: spin-1; force carriers These are assumed to be elementary.

Standard Model

Hadrons Hadrons: any strongly interacting subatomic particle; composed of quarks. There are 2 categories: Baryons: fermions, make of 3 quarks Mesons: bosons, made of quark, antiquark

Antiparticles Electron (e-) – Positron (e+) Particles and antiparticles (such as the pair highlighted in pink) are created in pairs from the energy released by the collision of fast-moving particles with atoms in a bubble chamber. Since particles and antiparticles have opposite electrical charges, they curl in opposite directions in the magnetic field applied to the chamber.

Antiparticles

Building Blocks Molecules consists of atoms. An atom consists of a nucleus, which carries almost all the mass of the atom and a positive charge Ze, surrounded by a cloud of Z electrons. Nuclei consist of two types of fermions: protons and neutrons, called also nucleons. Nucleons consists of three quarks. e = x C

1 fm = m 1 Å = m

m p = x kg = MeV = u m n = x kg = MeV = u Charge: e Charge: 0 3 quarks baryons

The Nucleus The atomic nucleus consists of protons and neutrons Protons and neutrons are generally called nucleons A nucleus is characterized by: A: Mass Number = number of nucleons Z: Charge Number = number of protons N: Neutron Number Of course A=Z+N Determines the Element Determines the Isotope Usual notation: 12 C Element symbol – defined by charge number C is Carbon and Z = 6 Mass number A So this nucleus is made of 6 protons and 6 neutrons

Mass Nuclear and atomic masses often given in u: atomic mass unit u = 12 daltons mass of a neutral 12 C atom 1 u = x kg Mass and energy are interchangeable – E = mc 2 where energy usually expressed in MeV 1 MeV = x J 1 u = MeV/c 2

Isotopes: same Z 40 Ca, 42 Ca, 44 Ca often, ‘isotope’ used instead of ‘nuclide’ isotopes have same Z, so same number of electrons => same chemistry use radioactive isotope in place of stable one – can monitor decay for tracer studies Isotones: same N 40 Ca, 42 Ti, 44 Cr Isobars: same A 42 Ca, 42 Ti, 42 Cr Isodiaphors: same neutron excess 42 Ca, 46 Ti, 50 Cr isodiaphors isotopes isobars isotones Z

Classification of Nuclides Stable nuclei: 264; 16 O Primary natural radionuclides: 26; very long half-lives; 238 U with t 1/2 = 4.47 x 10 9 y Secondary natural radionuclides: 38; 226 Ra t 1/2 = 1600 y decay of 238 U Induced natural radionuclides: 10; cosmic rays; 3 H t 1/2 = 12.3 y; 14 N(n,t) 12 C Artificial radionuclides: , 60 Co, 137 Cs…

Periodic Table

Chart of Nuclei plot of nuclei as a function of Z and N “Not just one box per element”

Chart of Nuclides

…or Segre Chart plot allows various nuclear properties to be understood at a glance, similar to how chemical properties are understood from the periodic chart ~ 2500 different nuclei known 270 stable/non-radioactive theorists guess at least 4000 more to be discovered at higher neutron numbers, higher mass limits – proton-rich side (left of stable): proton dripline cannot add another proton, it “drips” off dripline is known/accessible to experiments neutron-rich side (right of stable): neutron dripline cannot add another neutron, it “drips” off dripline is unknown – neutron-rich nuclei difficult to produce/study mass (above stable) – cannot add another proton or neutron limit unknown – again, difficult to produce/study “island of stability” predicted near Z = 114; not yet observed

Natural Decay Chains

Thorium Decay Chain (4n + 0) 1.4 x y

(4n + 2) 4.5 x 10 9 y

The Actinium Decay Series (4n +3) 235 U  …  207 Pb (7 alphas and 4 betas) 7.04 x 10 8 y

An Extinct Natural Decay Chain Neptunium decay series (4n + 1) 237 Np (t 1/2 = 2.14 x 10 9 y )  …  209 Bi

End of Chapter 1 Any questions?