Nuclear Physics Flashcards
Which experiment disproved the plum pudding model?
The Rutherford Scattering experiment
What was Rutherford’s scattering experiment?
A stream of alpha particles are fired from a radioactive source at very thin gold foil. When the alphas strike a fluorescent screen, there is tiny visible flash of light on screen/ The scientists recorded the numbers of flashes on screen and their angles.
What did the scientists first expect when doing Rutherford’s scattering experiment, and what were their results?
If the Plum pudding model was right, all flashes should have been seen within a small angle of the beam. They saw most particles went straight through the foil, a few scattered at angles greater than 90, deflected back the way they came.
What were the results of the Rutherford scattering experiment?
Most alpha particles went straight through the foil. Some were deflected at large angles. Very few were deflected at angle greater than 90, coming back the way they came.
Conclusion learnt from most of the alpha particles going through the gold foil in Rutherford Scattering experiment?
Atom is mostly empty space
Conclusion learnt from some alphas being deflected at large angles in Rutherford experiment?
Center must have large, positive charge, the nucleus, to repel the particles
Conclusion learnt from the very few alphas being deflected so much (greater than 90) that they came back the way they came
Nucleus is more massive than alpha particles, as the alphas could only be scattered by something more massive than themselves.
Conclusion learnt from very few alphas deflected at angle greater than 90, coming back from where they came, while most went through
Diameter of nucleus must be tiny when compared to the diameter of the atom
What was the conclusion about the structure of the atom after Rutherford’s experiment
Most mass and positive charge in atom must be contained in tiny, central nucleus.
What is the structure of an atom
There is a positive nucleus containing nucleons of protons, which are positive charge, and neutrons, neutral charge. Electrons orbit the nucleus
What is the charge of an electron
-1.6 x 10^-19 C. Equal and opposite to the charge of a proton, +1.6 x 10^-19 C.
What is the position of protons, neutrons and electrons in atom
Electrons orbit nucleus at VAST distances, so atoms are mostly empty space. Proton and Neutrons are much more massive than electrons, so nucleus makes up almost all of the mass of the atom
What is proton number and its symbol
Number of protons in an atom. Symbol: Z.
What is the relationship between protons and electrons in a neutrally charged element
Number of protons = number of electrons
What is a proton number used to identify
Defines element. No 2 elements have the same proton number.
What is nucleon number or atomic mass, it’s symbol and qualities
Number of nucleons in atom (protons and neutrons). Symbol: A
How is nucleon number the same as atomic mass
Protons and neutrons have an atomic mass of 1, and electron mass is negligible as it is very small, so nucleon number is same as atomic mass
What is nucleon number used for
To know number of neutrons and protons in atom, as well as the mass of the atom
What is electric current caused by
The flow of negatively charged electrons
What is the relationship between electric current and magnetic fields
The charged particles in a current are affected by magnetic field, so a current carrying wire can experience a force in a magnetic field.
What do you use F=BQv to find
The force acting on a single charged particle moving through a magnetic field
How do charge particles in a magnetic field move in particle accelerators
They are deflected in a circular path using the principle that magnetic field is always perpendicular to direction of motion.
How does a particle follow a circular path in a cyclotron
Charged particles are fired into one of the electrodes. The magnetic field makes them follow a semicircular path and then leave the electrode.
How does the particle jump between electrodes in a cyclotron
Potential difference is applied between electrodes, which accelerates the particles across the gap until they enter the next electrode.
Why does the particle have a larger turning radius each time it circles in the cyclotron
Since the particle was accelerated within the gap, it now has a higher speed, which means momentum is higher. p=mv and v is higher. So the particle follows a circular path with a larger radius than last time before leaving the electrode.
How is the charged particle attracted to each electrode despite each D having opposite charges
The p.d is on the electrodes is reversed, so as, i.e, the negative particle has just left the positive charged D and is in the gap, the negative D now alternates in current, becoming positively charged, while the first D becomes negative. So particle is pulled in by the positive D and repelled by the negative D, accelerating it.
How does the particle exit the cyclotron
The positive, negative alternation of electrodes repeats as particle spirals outwards due to increasing momentum and so increasing radius, before eventually exiting the cyclotron
In a cyclotron, what happens once the particle reaches relativistic speeds
At that point, the turning radius has reached its maximum and cannot increase more, as speed has reached it’s maximum. But mass is now increasing, so in order to keep the same turning radius, magnetic field strength must keep increasing to maintain the radius, otherwise, momentum decreases since p=mv……idk ask chris tomorrow
What is a cyclotron
A type of particle accelerator, made of two D shaped electrodes (insert image)
What is an electron volt
The kinetic energy carried by an electron after it has been accelerated through a p.d of 1 volt.
What is the equation for electron volt
eV = 1/2mv^2
What is the energy in eV of an electron accelerated by a p.d
energy gained by electron (eV) = accelerating p.d. (V)
How much joules is 1eV
1.60 x 10^-19 J
What is a linear accelerator
A linac is a long, straight tube containing a series of tube shaped electrodes. The charge on each electrode alternates along the tube, so positive, negative, positive… (INSERT IMAGE)
In a linac, how does the charged particle accelerate between electrodes if they are oppositely charged
The electrodes are connected to an alternating p.d supply so the charge of each continuously changes between + and -. This means electric field between each pair of electrodes is continuously changing direction. So the particle is always going to be attracted to the next electrode, and repelled by the previous.
How do the electrodes switch charge at the exact time the particle exits the previous electrode in a linac
The alternation is timed so particles are always attracted to the next electrode, and repelled from the previous one.
Linac, If the particle is accelerating each time it is in a gap between electrodes, so speed is increasing, how does the charge switch exactly when it needs to, if it is in a timed switch sequence?
To compensate for increasing speed, the length of the electrodes increases as the particle travels down the accelerator, so that the particle spends the same amount of time in each electrode. So the charge switch will happen every 5 seconds for example, but the tubes get longer so the particle always spends 5 seconds within each tube despite having higher velocity each time.
In a linac, do the tubes continue increasing in length once the particle reaches relativistic speeds, close to the speed of light
At that point, the tubes stay at the same size, because the speed increase is so very tiny, it is negligible. The tubes keep increasing in length until we hit relativistic speeds, at which point the tubes are the same size. There is a maximum length.
What do particles do once they leave the linac?
High energy particles leaving a linac collide with a fixed target at the end of the tube.
What is thermionic emission
When you heat a metal, its free electrons gain a lot of thermal energy. If you give them enough energy, they break free from the surface of the metal. This is thermionic emission.
How does an electron gun work
A heating coil heats the metal cathode. Electrons are accelerated towards the cylindrical anode by the electric field set up by high p.d. Some electrons pass through a little hole in the anode, making a narrow electron beam. Electrons in the beam move at a constant velocity, because there’s no field beyond the anode -i.e no force.
INSERT ELECTRON GUN IMAGE HERE
What is an electron gun used for
Accelerating electrons with an electric field. Often, the beam of electrons produced by the gun are directed with an applied magnetic field towards something. An example is an electron microscope, where electrons can be focused onto a sample using a magnetic field.
What is a proton number used to identify
Defines element. No 2 elements have the same proton number.
What is the relationship between protons and electrons in a neutrally charged element
Number of protons = number of electrons
What is a hadron
Any particle which is made up of quarks
What are examples of a hadron
Protons, Neutrons, Mesons
What are quarks
Fundamental particles. There is nothing a quark is made of. A quark is the most fundamental building block
What are the types of hadrons
Baryons and Mesons
What is a baryon
Protons and Neutrons are both baryons. Think of them like the same particle but with different charge. Sigmas are also baryons.
What is baryon decay
All baryons except protons decay to a proton. All of them are unstable, which means they decay to become other particles. The end result of a decaying particle depends on what it started as, but it always includes a proton. Protons are the only stable baryon, they don’t decay
What are antibaryons
Antiparticles of protons and neutrons, antiprotons and antineutrons.
What do anti-baryons do
Antiparticles are annihilated when they meet the corresponding particle. This means you don’t find anti-baryons in ordinary matter, as they have all been annihilated.
What is a baryon number
The number of baryons. It is similar to how protons have a +1 charge and electrons have a -1 charge. This value can only be a whole number. Protons, neutrons and sigmas have a baryon number of 1. Antiprotons, antineutrons have a baryon number of -1. Non-baryon particles like electrons have a baryon number of 0.
How can you use baryon number to check if a reaction is feasible
Baryon number must always be conserved in any interaction. So baryon number before an interaction must be the same as after an interaction. You can use this to predict whether an interaction will happen. If the numbers don’t match, it can’t happen. Total baryon number in any particle interaction never changes. (Before and after must be equal)
What are mesons
They are hadrons which are unstable, and have baryon number=0, so they are not baryons
What are pions
Pions (pi-mesons) are a type of meson. They are the lightest mesons.
What are the type of pions you get
You get π+, π0, π-, these have different electric charges.
Where do pions occur
You get lots of pions in high-energy particle collisions like those studied in CERN. Both pions and kaons were discovered in cosmic rays, cosmic ray showers are a source of both particles.
How long is a meson lifespan
Short, they are unstable so they only exist briefly before decaying
