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Introduction to Radiochemistry

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Introduction to Radiochemistry

NUSC 341-3

Forces in Matter and the Subatomic Particles

Chapter 1

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

- 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.

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.

1 fm = 10-15 m

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

- 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.

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

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.

- 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 Å = 10-10 m

1 fm = 10-15 m

3 quarks

baryons

mn = 1.6749 x 10-27 kg

= 939.55 MeV

= 1.008665 u

mp = 1.6726 x 10-27 kg

= 938.26 MeV

= 1.007276 u

Charge: 0

Charge: e

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

Determines the Element

Determines the Isotope

Of course A=Z+N

Usual notation:

Mass number A

12C

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

So this nucleus is made of 6 protons and 6 neutrons

- 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

isodiaphors

Z

isotopes

isobars

isotones

Isotopes: same Z40Ca, 42Ca, 44Ca

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 N40Ca, 42Ti, 44Cr

Isobars: same A42Ca, 42Ti, 42Cr

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

- 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…

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

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

1.4 x 1010 y

(4n + 2)

4.5 x 109 y

- 235U … 207Pb (7 alphas and 4 betas)
7.04 x 108 y

- Neptunium decay series (4n + 1)
- 237Np (t1/2 = 2.14 x 109 y ) …209Bi

End of Chapter 1

Any questions?