Nuclear Physics

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Rutherford's alpha scattering experiment

Most of the fast, highly charged alpha particles went whizzing straight through undeflected. This was the expected result for all of the particles if the plum pudding model was correct.

Some of the alpha particles were deflected back through large angles. This was not expected.

A very small number of alpha particles were deflected backwards! This was definitely not as expected.

The strong nuclear force holds protons together in the nucleus.

Properties of SF

Forces decrease according to an inverse square law for distance.

Acts between nucleons

Nucleons very close together it is a force of repulsion.

It acts between quarks.

Nucleon number is number of nucleons in a nucleus.

Proton number is number of protons in nucleus.

Isotopes are two nuclides with same number of protons but different numbers of neutrons.

Nuclear reactions things are conserved;

mass/energy

momoentum

charge

Fission is when a slow neutron collides with a uranium nucleus, resulting in high speed neutrons being emitted.

Heavy particles; protons and neutrons are baryons

Light particles such as electrons and neutrinos are leptons

Mesons have intermediate mass

Antiparticles

Any known particle has an opposite particle

They are made of antimatter

If a particle and antiparticle meet it causes annihilation releasing a large amount of energy

Hadrons can be broken down into quarks and influenced by the SF

Baryons madeof 3 quarks

Mesons made of a quark and a antiquark

properties of quarks are up down and strange.

Leptons experience no SF.

proton; uud charge +e, b1, s0

neutron; udd charge e0, b1 s0

antiproton; uUd charge -e, b-1, s0

Most alpha particles pass straight through gold foil - The atoms of gold foil are mostly empty space.

Few are deflected back towards observer - positive charge is concentrated in small - nucleus

Diameter of nucleus 10-15m

Diameter of atom 10-10m

e=1.6x10-19C

Beta decay - in B- Decay a neutron emits an electron and becomes a proton

n => p + e- +-v +y

in B+ decay up quark becomes a down quark.

p => n + e+ + v +y

Alpha decay - nucleus emits an alpha particle

Beta minus decay - nucleus emits a beta particle; electron, antineutrino also released

Beta plus decay - nucleus emits a beta particle; positron, and a neutrino

Gamma decay - a gamma photon may be released during alpha or beta decay

E= m x c2

c = speed of light; 3 x 10^8m/s

mass of particles before decay> mass of particles after decay.

The binding energy of a nucleus is the energy needed to separate a nucleus into individual nucleons.

Nuclear fusion - small nuclei bind together to form a bigger nucleus; energy is released and the nucleons are more tightly bound.

High temperatures and pressures are required.

Nuclear fission; large nucleus splits to form two or more smaller nuclei. Energy is released and again the nucleons are more tightly bound.

In the reactor core

Fuel rods are surrounded by a moderator which slows the neutrons, so that they interact more strongly with the nuclei. Control rods absorb nucleons strongly, so they can stop the reaction. The coolant carries the heat away to the boiler.

Alpha - helium nucleus, absorbed by thin paper, most ionising

Beta is an electron, absorbed by few mm of aluminium, moderately ionising

Gamma is EM radiation, absorbed by few cm of lead, least ionising.

The activity of a sample is the average number of decays per second.

1 Becquerel = 1 decay per second.

Decay constant for a particular isotope is the probability that an individual nucleus will decay in unit time.

A =hN

Activity = decay constant x no. of decayed nuclei

Radioactive decay is exponential decay.

The decay equation;

x = x0e-ht

A = A0e-ht

The half life, t 1/2, is the average time for the no. of nuclei to decay to half its original value.

ht1/2:- 0.693

Radiocarbon dating

Living material contains tiny fraction of carbon -14.

Death of organism, fraction of C-14 gradually decays.

Measuring the fraction remaining the time since death can be found.

Types of decay

Energy conservation

Radioactive decay

Quarks and the Strong force

Nuclear Reactions

NUCLEAR PHYSICS

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