Introduction to Introduction to RadiochemistryRadiochemistry

NUSC 341-3NUSC 341-3

Forces in Matter Forces in Matter and and

the Subatomic Particlesthe Subatomic Particles

Chapter 1Chapter 1

What is Nuclear Science?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 volumeunderstanding 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 collisionsbetween 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?Why should we bother?

InteractionsInteractions• Electromagnetice- (lepton) bound in the atoms by the electromagnetic force• Weak interactionNeutrino observed in beta decay.• Strong interactionQuarks are bound in together by the strong forcein nucleons. Nuclear forces bind nucleons intonuclei.• GravitationGravitational interaction between the elementary particlesis in practice very small compared to the other three.

InteractionsInteractions

forces strength range (fm) exchange particle mass (eV) charge spin

gravitational 6x10-39 infinite graviton? 0 0 2

weak 1x10-6 2x10-3 W±, Z 91x109 ±1,0 1

electromagnetic 7x10-3 infinite photon 0 0 1

strong 1 1.5 pion 35x106 0 1

1 fm = 101 fm = 10-15-15 m m

The forces of elementary particle physics are associated with the exchange of particles. 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.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 transfersmomentum and energy between the two objects, and is said to mediate the interaction

graviton – not yet observedpions or pi mesons – between nucleons

Standard ModelStandard 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 ModelStandard Model

HadronsHadrons

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

AntiparticlesAntiparticles• 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.

AntiparticlesAntiparticles

Building BlocksBuilding 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 = 1.6022 x 10-19 C

1 fm = 10-15 m

1 Å = 10-10 m

mp = 1.6726 x 10-27 kg = 938.26 MeV = 1.007276 u

mn = 1.6749 x 10-27 kg = 939.55 MeV = 1.008665 u

Charge: e Charge: 0

3 quarksbaryons

The NucleusThe 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 ElementDetermines the Isotope

Usual notation:

12CElement symbol – defined by charge numberC is Carbon and Z = 6

Mass number A

So this nucleus is made of 6 protons and 6 neutrons

MassMass

• Nuclear and atomic masses often given in u: atomic mass unit• 12.000 u = 12 daltons mass of a neutral 12C atom• 1 u = 1.6605 x 10-27 kg• Mass and energy are interchangeable –

E = mc2

where energy usually expressed in MeV• 1 MeV = 1.602 x 10-13 J• 1 u = 931.5 MeV/c2

Isotopes: same Z 40Ca, 42Ca, 44Caoften, ‘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 monitordecay for tracer studies

Isotones: same N 40Ca, 42Ti, 44Cr

Isobars: same A 42Ca, 42Ti, 42Cr

Isodiaphors: same neutron excess 42Ca, 46Ti, 50Cr

11 NA

Z E NAZ E11

1

21

NAZ E

11

NAZYN

AZY1

1

N

AZY

121

NAZ X N

AZ X11

11 N

AZ X

isodiaphors

isotopes

isobars

isotones

Z

Classification of Nuclides Classification of Nuclides

• Stable nuclei: 264; 16O

• Primary natural radionuclides: 26; very long half-lives; 238U with t1/2 = 4.47 x 109 y

• Secondary natural radionuclides: 38; 226Ra t1/2 = 1600 y decay of 238U

• Induced natural radionuclides: 10; cosmic rays; 3H t1/2 = 12.3 y; 14N(n,t)12C

• Artificial radionuclides: 2-4000, 60Co, 137Cs…

Periodic TablePeriodic Table

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

Chart of Nuclides Chart of Nuclides

http://www.nndc.bnl.gov/chart/

……or Segre Chartor Segre Chart• plot allows various nuclear properties to be understood at a glance, similarto 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

Nat

ura

l Dec

ay C

hai

ns

Nat

ura

l Dec

ay C

hai

ns

Thorium Decay Chain (4n + 0)Thorium Decay Chain (4n + 0)

1.4 x 1010 y

(4n + 2)

4.5 x 109 y

The Actinium Decay Series The Actinium Decay Series (4n +3)(4n +3)

• 235U … 207Pb (7 alphas and 4 betas)

7.04 x 108 y

An Extinct Natural Decay ChainAn Extinct Natural Decay Chain

• Neptunium decay series (4n + 1)

• 237Np (t1/2 = 2.14 x 109 y ) …209Bi

End of Chapter 1

Any questions?