Particle Physics Flashcards
Overview of the Alpha Scattering Experiment
A narrow beam of alpha particles, of the same kinetic energy, was directed at a thin sheet of metal foil
Alpha particles scattered by the foil were detected by a detector which could be moved at a constant distance from the point of impact
Details of the Alpha Scattering Experiment
Alpha particles must have the same speed - slower alpha particles would be deflected more than faster ones
Chamber must be evacuated else the alpha particles would be stopped by air molecules
The foil must be very thin, else alpha particles will be scattered more than once
Half-life of source of alpha particles must be long - otherwise later readings would be lower than earlier readings due to radioactive decay of source
Results from Alpha Scattering Experiment
Most alpha particles passed straight through the foil with little or no deflection
1 in 2000 alpha particles were deflected through small angles
A small percentage of alpha particles were deflected through angles greater than 90 degrees
Alpha Scattering Experiment Conclusions
Most of an atom’s mass is concentrated in a small nucleus in the centre
The nucleus is positively charged
What can be used to determine the size of a nucleus
Distance of closest approach
What happens to the distance of approach the more energetic the alpha particles
The distance of approach gets smaller
What happens as the distance of approach gets smaller
The electrostatic force of repulsion is greater and the deflection is greater
What happens to Kinetic Energy as an alpha particle approaches a nucleus
Kinetic Energy is transferred to electrostatic PE
How to calculate the distance of the smallest approach
Energy conversion
When alpha particle stops before rebounding - Ke = EPE
Gives the upper limit for the radius of a nucleus
What did Rutherford conclude
Nucleus had a radius of around 10^-14 m
What is the Atomic Mass Unit
1 Atomic Mass Unit is 1/12 the mass of a neutral Carbon-12 atom
What does Nuclear Radius depend on
Nuclear Mass Number
What is the Strong Nucleus Force
Counters the Electrostatic Force of Repulsion between protons in a nucleus - ensuring the nucleus remains stable
Acts between all nucleons
Strongest of the 4 fundamental forces and has the shortest range - approximately 3 femtometres
When is the Strong Nucleus Force attractive
Up to 3fm
When is the Strong Nuclear Force repulsive
At separations less than 0.5fm to prevent protons and neutrons being pushed into each other
What are antiparticles
Particles with the same mass as their corresponding particle - but are opposite in every other quantum number
When a particle interacts with its corresponding antiparticle - they annihilate - releasing energy
What are the 4 Fundamental Forces
Gravity
EM Force
Strong Nuclear Force
Weak Nuclear Force
What are Hadrons
Particles that can interact through the strong force
