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.
