8p1 Nuclear and Particle Physics Flashcards
What is a nucleon number
The nucleon number is how many protons and neutrons are in the nucleus (protons and neutrons)
What is a proton number
Number of protons in the nucleus
Describe the alpha scattering experiment setup
Rutherford set up a stream of alpha particles to be fired at a gold foil, when the particles striked a fluorescent screen on the other side of the foil, a visible flash was given off indicating how much the particles were deflected at and allowing them to measure angles
Explain the results of this experiment and what it showed
Most alpha particles went straight through showing the atom is mainly empty space, some were deflected through large angles showing the centre of the atom has a large positive charge to repel the electrons. Very few angles are deflected more than 90 degrees so the diameter of the nucleus is much smaller compared to the diameter of the atom. The deflections only occur with a collision of a higher mass so most of the atom’s mass is in the nucleus
Describe how our understanding of the atom has changed over time
Democritus originally came up with the idea of the atom in the 5th century BC. JJ Thompson then discovered electrons can be removed from the atom so disproving that atoms are the smallest matter. JJ Thompson suggested that the spheres were a positive charge with small negative electrons stuck on it. After the alpha scattering experiment the model changed to the nuclear model
Describe the nuclear model
Inside the atom there is a positive nucleus containing neutrons and protons, electrons are negative particles that orbit this core in shells. Nucleus is 10000th size of the atom and electrons orbit it at vast distances. Electrons have very little mass so most mass is centred in the nucleus.
How are electrons released in thermionic emission
When a metal is heated, the free electrons gain thermal energy, with enough energy they can break out of the metal surface which is called thermionic emission
How are electrons accelerated by magnetic and electric fields?
Since electrons are negative they can be accelerated by using a positive charge to attract electrons which can be done using electric or magnetic fields
If the first tube in a linac is positive, what’s the charge of the second and third tube
Second is negative and third is positive - always alternating
Why are the electrodes connected to an alternating pd supply in a linac
So the charge of each electrode is continuously changing between positive and negative - therefore the electric field between the electrodes are switching direction each time
Why is the ac pd timed in a linac
So that the particles are always attracted to the next electrode tube and repelled from the previous one
How do the length of tubes change as you go along the linac and how does this affect the time electrons spend in the tube
The tube lengths increase, since the electrons are gaining speed they spend equal time in each tube despite the increase in length
What are the applications of an electron gun?
To pass the electron beam through an applied magnetic field to direct the electrons - e.g in an electron microscope the electrons are focussed onto the sample
What are cyclotrons made up of?
Two semicircular electrodes with a uniform magnetic field acting perpendicular to the plane of electrodes and an alternating pd applied between the electrodes
How do cyclotrons work?
Charged particles are fired into one of the electrodes from the centre - the magnetic field makes them follows circular path - applied potential difference accelerates particles across the gap so they enter the other electrode - since the electron has greater speed therefore greater momentum, the radius of the circular path will be larger - the potential difference is reversed so the particle accelerates before entering the other electrode again - this continues so the particle spirals outwards
When do particles leave tracks
When a charged particle passes through a substance, it causes ionisation as electrons are knocked out of atoms which leaves a trail of ions
How do cloud chambers work to detect charged particles
Cloud chambers use supercooled vapour (still gas below condensation temperature), the ions left by particle cause the vapour to condense to get vapour trails
How to identify alpha vs beta particle tracks in a cloud chamber
Heavy short tracks mean lots of ionisation which is from alpha particles - whereas fainter longer tracks are from beta particles that travel further but are less ionising
How do bubble chambers work to detect charged particles
Bubble chambers store hydrogen (kept as a liquid above its boiling point caused by putting it under pressure), if the pressure reduces due to the ion trails then the bubbles of gas start to form in the those places
Disadvantage of both these chambers?
They only show up charged particle
When does a charged particle follow a curved track?
When it is moving in a magnetic field so it experiences a force that causes it to curve
How are V shaped tracks formed?
It forms because a neutral particle decays into two oppositely charged particles
What forms a little spiral coming from a straight track and what do they show?
It shows a knock of electron that’s been kicked out of a hydrogen atom, they show which way the particles are going and the direction that the negative particles curve
Which detectors do physicists use now?
Detectors that give out electrical signals sent straight to a computer such as drift chambers
