Particles and Radiation: Matter and Radiation

Question 1

What is a nucleon?

Answer 1

A proton or a neutron in the nucleus.

Question 2

What is the diameter of a nucleus and an atom?

Answer 2

Nucleus diameter (10^-15) is x10^5 smaller than atom diameter (10^-10)

Question 3

What is a nuclide?

Answer 3

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.

Question 4

What is specific charge of charged particles?

Answer 4

charge (x 1.6 x 10^-19) / mass (x atomic mass unit)

Question 5

A stable isotope has nuclei that don’t disintegrate - there must be a force holding them together. What is this force?

Answer 5

This force is strong nuclear force bc it overcomes electrostatic force of repulsion between protons in nucleus and keeps protons and neutrons together.

Question 6

What is the range of strong force (compared to electrostatic force of repulsion)?

Answer 6

Range is no more than 3-4fm (≈ diameter of small nucleus) vs electrostatic which ahs infinite range (but decreases with increasing range).

Question 7

How does strong force affect p and n differently?

Answer 7

Has same effect between a p and an n, a p and a p, a n and a n.

Question 8

When is strong force repulsive, and why?

Answer 8

At 0.5fm and less, strong force is repulsive to prevent n and p being pushed into each other.

Question 9

What is gamma radiation and when is it emitted?

Answer 9

Em radiation emitted by nucleus with too much energy following an α or β emission.

Question 10

What is β radiation?
When does β- decay occur?

Answer 10

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 ¯

Question 11

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?

Answer 11

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.

Question 12

When are em waves emitted?

Answer 12

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

Question 13

What’s a photon?

Answer 13

A packet (short burst) of em waves. Photons leave source in different directions.

Question 14

What’s the eq for the power of a laser beam?

Answer 14

P = nhf , where n is the number of photons in the beam passing a fixed point each second

Question 15

How do PET (positron-emitting tomography) scans work?

Answer 15

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.

Question 16

When does positron emission take place?

Answer 16

Occurs when p—>n in an unstable nucleus with too many p.
X —> Y + β+ + ν

Question 17

How does positron emission take place?

Answer 17

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.

Question 18

What is rest mass? Relate to energy.

Answer 18

The mass of a particle when it’s stationary (rest mass) corresponds to rest energy (mc^2) locked up as mass.

Question 19

What is annihilation?

Answer 19

Annihilation is when a particle and corresponding antiparticle meet and their mass is converted into radiation energy (photons).

Question 20

What happens when annihilation occurs?

Answer 20

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).

Question 21

What is the minimum energy of the two photons in relation to rest mass?

Answer 21

2hf = 2E0
(where E0 is the rest mass of the particle/antiparticle)
∴ min E of each γ = hf(min) = E0

Question 22

What is pair production?

Answer 22

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.

Question 23

What’s the minimum energy of γ needed?

Answer 23

E(min) = 2E0

Question 24

In a cloud chamber, ionising particles leave a visible trail. When a magnetic field is applied to chamber:

Answer 24

+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.

Question 25

When two objects interact, they exert = - forces on each other. What is transferred between the objects by these forces?

Answer 25

Momentum is transferred.

Question 26

Why is there an em force between two charged objects?

Answer 26

Due to the exchange of virtual particles. They’re called virtual particles because they can’t be detected directly.

Question 27

Weak nuclear force is weaker than strong nuclear force. Why?

Answer 27

Because otherwise would affect stable nuclei.

Question 28

In both β- and β+ decay, a new particle and a new antiparticle is created (but not corresponding particles). What are they?

Answer 28

An e- or e+ and a ν or ν ¯.

Question 29

Neutrinos and antineutrinos rarely interact with other particles, but sometimes they do eg…

Answer 29

ν with n —-> p, β- created and emitted;
ν ¯ with p —> n, β+ created and emitted.

Question 30

These interaction are due to…

Answer 30

exchange particles called W bosons.

Question 31

Unlike γ, W bosons (3):

Answer 31

• have non-zero rest mass
• are positive or negative
• have very short range (0.001fm)

Question 32

Draw out a neutron-neutrino interaction.

Answer 32

n to p, ν to β- , exchange of W- from n.

Question 33

Draw out a proton-antineutrino interaction.

Answer 33

p to n, ν ¯ to β+ , exchange of W+ from p.

Question 34

In each case, what did the W boson do?

Answer 34

W boson met ν or ν ¯, changing them into β- or β+.

Question 35

What if no ν or ν ¯ was present?

Answer 35

W- boson decays into β- and ν ¯
W+ boson decays into β+ and ν

Question 36

Draw β- decay when no ν present?

Answer 36

n to p, release W-, decays into β- and ν ¯

Question 37

Draw β+ decay when no ν ¯ present?

Answer 37

p to n, release W+, decays into β+ and ν

Question 38

What is electron capture?

Answer 38

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.

Question 39

Draw electron capture.

Answer 39

p to n, e- to v, exchange of W+ from p.

Question 40

Same change can happen when p and e- collide at vey high speed.
Other instance this can happen?

Answer 40

Overall change can also occur when W- is exchanged from e- to p, provided e- has sufficient energy.

Question 41

What are force carriers?

Answer 41

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.