Particles and Radiation: Matter and Radiation Flashcards
What is a nucleon?
A proton or a neutron in the nucleus.
What is the diameter of a nucleus and an atom?
Nucleus diameter (10^-15) is x10^5 smaller than atom diameter (10^-10)
What is a nuclide?
Each type of nucleus is called a nuclide.
Eg nuclide of O-16 is different to nuclide of C-14 is different to nuclide of C-12.
What is specific charge of charged particles?
charge (x 1.6 x 10^-19) / mass (x atomic mass unit)
A stable isotope has nuclei that don’t disintegrate - there must be a force holding them together. What is this force?
This force is strong nuclear force bc it overcomes electrostatic force of repulsion between protons in nucleus and keeps protons and neutrons together.
What is the range of strong force (compared to electrostatic force of repulsion)?
Range is no more than 3-4fm (≈ diameter of small nucleus) vs electrostatic which ahs infinite range (but decreases with increasing range).
How does strong force affect p and n differently?
Has same effect between a p and an n, a p and a p, a n and a n.
When is strong force repulsive, and why?
At 0.5fm and less, strong force is repulsive to prevent n and p being pushed into each other.
What is gamma radiation and when is it emitted?
Em radiation emitted by nucleus with too much energy following an α or β emission.
What is β radiation?
When does β- decay occur?
β radiation consists of fast-moving e-s.
A β- particle is emitted instantly when a n–>p. This change happens to nuclei that have too many neutrons.
X —> Y + β- + ν ¯
Energy spectrum of β particles showed β particles were released with Ek’s up to a max that depended on the isotope. What dies this tell us?
We know that each unstable nucleus loses a certain amount of E therefore some of the Ek must be carried away by other particles - neutrinos and antineutrinos.
When are em waves emitted?
Emitted by a charged particle when it loses energy:
-when a fast-moving e- is stopped (eg in X-ray tube) or slows down or changes direction
- when an e- in a shell moves to a diff shell of lower energy
What’s a photon?
A packet (short burst) of em waves. Photons leave source in different directions.
What’s the eq for the power of a laser beam?
P = nhf , where n is the number of photons in the beam passing a fixed point each second
How do PET (positron-emitting tomography) scans work?
Positron emitting isotopes administered to patient, reaches brain via blood, annihilation w e-s, releasing 2 γ that are sensed by detectors. Builds image of where positron-emitting nuclei are in the brain.
When does positron emission take place?
Occurs when p—>n in an unstable nucleus with too many p.
X —> Y + β+ + ν
How does positron emission take place?
Positron emitting isotopes don’t occur naturally. Manufactured by placing stable isotope (l) or (s)in path pf p beam. Some nuclei absorb extra p’s and become unstable positron emitters.
What is rest mass? Relate to energy.
The mass of a particle when it’s stationary (rest mass) corresponds to rest energy (mc^2) locked up as mass.
What is annihilation?
Annihilation is when a particle and corresponding antiparticle meet and their mass is converted into radiation energy (photons).
What happens when annihilation occurs?
A particle and corresponding antiparticle (of the same rest mass and exact opposite charge) meet, producing two photons of same rest energy and opposite direction (bc conservation of momentum).
What is the minimum energy of the two photons in relation to rest mass?
2hf = 2E0
(where E0 is the rest mass of the particle/antiparticle)
∴ min E of each γ = hf(min) = E0
What is pair production?
If a γ of sufficient energy passes near a nucleus or an e-, it can suddenly change into a particle-antiparticle pair, which would separate from each other.
What’s the minimum energy of γ needed?
E(min) = 2E0
In a cloud chamber, ionising particles leave a visible trail. When a magnetic field is applied to chamber:
+ve particle is deflected by magnetic field in opposite direction to -ve p[article travelling in same direction;
the slower is went the more it bent.
When two objects interact, they exert = - forces on each other. What is transferred between the objects by these forces?
Momentum is transferred.
Why is there an em force between two charged objects?
Due to the exchange of virtual particles. They’re called virtual particles because they can’t be detected directly.
Weak nuclear force is weaker than strong nuclear force. Why?
Because otherwise would affect stable nuclei.
In both β- and β+ decay, a new particle and a new antiparticle is created (but not corresponding particles). What are they?
An e- or e+ and a ν or ν ¯.
Neutrinos and antineutrinos rarely interact with other particles, but sometimes they do eg…
ν with n —-> p, β- created and emitted;
ν ¯ with p —> n, β+ created and emitted.
These interaction are due to…
exchange particles called W bosons.
Unlike γ, W bosons (3):
- have non-zero rest mass
- are positive or negative
- have very short range (0.001fm)
Draw out a neutron-neutrino interaction.
n to p, ν to β- , exchange of W- from n.
Draw out a proton-antineutrino interaction.
p to n, ν ¯ to β+ , exchange of W+ from p.
In each case, what did the W boson do?
W boson met ν or ν ¯, changing them into β- or β+.
What if no ν or ν ¯ was present?
W- boson decays into β- and ν ¯
W+ boson decays into β+ and ν
Draw β- decay when no ν present?
n to p, release W-, decays into β- and ν ¯
Draw β+ decay when no ν ¯ present?
p to n, release W+, decays into β+ and ν
What is electron capture?
Sometimes a proton in a proton- rich nucleus turns into a neutron as a result of interacting through the weak interaction with an inner shell e- from outside the nucleus.
Draw electron capture.
p to n, e- to v, exchange of W+ from p.
Same change can happen when p and e- collide at vey high speed.
Other instance this can happen?
Overall change can also occur when W- is exchanged from e- to p, provided e- has sufficient energy.
What are force carriers?
γ and W bosons are force carriers bc they’re exchanged when em force and weak force act respectively.
Pions are exchange particles for strong nuclear force.
Gravitons are carriers of force of gravity.
