Particle Physics Flashcards
(41 cards)
1
Q
protons
A
positively charged particles found in the nucleus. not a fundamental particle. composed of 3 quarks. charge: +1, weight: 1
2
Q
neutrons
A
uncharged particles found in the nucleus. not a fundamental particle. composed of three quarks. charge: 0, mass: 1
3
Q
electrons
A
negatively charged particles orbiting nucleus. fundamental particle. charge: -1, mass: 0
4
Q
specific charge
A
specific charge = charge (c) / mass (kg)
5
Q
representing atoms
A
6
Q
isotopes
A
atoms of the same element that contain different no. of neutrons. physical properties differ slightly.
7
Q
forces acting on nucleons
A
At very short distances of less than 0.5fm, the strong force causes nucleons to repel one another strongly. there is also strong electrostatic repulsion.
at about 1.5fm (typical nucleus radius) the strong force becomes strongly attractive and holds the nucleons together, balancing the repulsive electrostatic force.
further from the centre, beyond 3fm, the strong force drops off rapidly. alotugh electrostatic repulsion has also decreased, the strong force cannot hold the nucleon in the nucleus.
8
Q
alpha decay
A
the equivalent of a He nucleus being given off. an unstable nucleus, x, emits an alpha particle (2p + 2n).
easily stopped, sheet of paper. dangerous is swallowed.
9
Q
beta - decay
A
gives off electron and antineutrino.
10
Q
the strong force
A
- 1 of 4 fundamental forces of nature, the others being gravitational, electromagnetic, weak nuclear force
- provides attractive force between nucleons with a range of ~3fm
- overcomes the repulsive electrostatic force exerted by positively charged protons on each other
- at distances <0.5fm the strong force is repulsive and prevents the nucleus imploding/collapsing
11
Q
gamma decay
A
electromagnetic radiation emitted from an unstable nucleus.
- gamma rad often occurs straight after α / β decay. the child nuclide formed often has excess energy whis is released by gamma emission.
- the transition isn’t perfect between stages in α decay. in β there’s only 1 jump
- no change occurs to nucleon numbers as a result
- gamma is pure energy
12
Q
neutrinos
A
always emitted with β decay.
- β- decay results in antineutrino
- β+ decay results in a neutrino
- neutrinos are very difficult to detect as they have nerly 0 mass and no charge. they barely interact with matter. billions of these particles that have been emitted from the sun, sweep through our bodies every second.
13
Q
what stops the radiation types
A
α stopped by gold leaf.
β stopped by ~8mm Al
γ stopped by several cm of Pb
- why Pb? well. it’s the heaviest non-radioactive element. because of its large mass it hinders the radiation. smth even heavier would be better though it’s radioactive itself.
14
Q
electricmagnetic radiation
A
radiation emitted by charged particles losing energy.
- e- decreasing in energy inside an atom (light)
- the biggest source of radiation available to us is the SUN
- e- losing kinetic energy when stopped by a solid material (X-rays)
the radiation consists of two linked electric and magnetic field waves which are
- at right angles to each other
- are in phase (peak together)
15
Q
why are γ rays dangerous
A
γ rays are dangerous because their wavelength is about as big as a nucleus. they just pass right through, penetrating just about everything
16
Q
the wave equation
A
wave speed = frequency * wavelength
17
Q
photon
A
- electromagnetic radiation is emittd as short bursts
- each packet of waves is called a photon
- each photon contains a set amount of energy and is proportional to the frequency of the ER
photon energy EQ:
E = planck constant * freq.
18
Q
antimatter
A
all particles of normal matter (p,n,e-) have a corresponding particle that:
- as the same mass as the normal particle
- has the opposite charge as the normal particle
- will undergo annihilation with the normal particle if they meet
19
Q
antiparticles
A
- antiproton: neg. charged p
- positron: pos. charged e-
- the antineutrino produced in β- decay
- other particle properties are also reversed; uncharged antiparticles such as the antineutron
- two particles w/ same mass and opposite charges, not necessarily particle-antiparticle pair
- antimatter symbol: bar above letter
- certain man-made isotopes are made in order to provide a source of antimatter. eg. positrions are needed for PET scans
20
Q
annihilation
A
- when a particle and its corresponding antiparticle meet together annihilation occurs
- all of their mass and kinetic energy is converted into two photons of equal freq. that move off in opp. directions
21
Q
pair production
A
- opposite of annihilation
- the energy of one photon can be used to create a particle and its corresponding antiparticle
- the photon ceases to exist afterwards
22
Q
electron volt
A
- the electron volt (eV) is a very small unit
- eV = 1.6 * 10^-19 J
- eV = kE gained by an e- when it is accelerated by a potential difference of 1V
23
Q
particle rest energy
A
energy equivalent of mass -> E = mc^2
masses of subatomic particles quoted in energy terms in MeV.
24
Q
early atom models
A
-
1800’s : Dalton’s model.
- each element its own specific atom w/ specific weight and indivisible.
- 1897: discovery of subatomic particles- e-
- the plum pudding model. positive body w/ negative dots in it
-
rutherford’s model
- mostly empty atom w/ a core: nucleus
- proved wrong because the attraction between - and + would be too strong and collapse on itself
- bohr’s model 1973
- neg. charged e- in circular orbits. planetary model. as e- goes up/down valence shells, quantum leap.
25
what is radioactive decay
the random disintegration of an unstable nucleus by the emission of particles or electromagnetic radiation. after radioactive decay has taken place, the resulting nucleus is mores stable.
the activity A is the no. of decaying nuclei/second. unit is becquerel (Bq). 1Bq = 1 decay/s.
26
decay equation
N0 is the no. of radioactive nuclei at t=0.
N = the no. of radioactive particles present at time t
27
half life
the time taken for the number of radioactive particles present, N, or activity A, to halve from an original value.
* the A of a radioactive sample decreases over time
* the half life of a radioactive sample is the average time taken for half of the original mass of the sample to decay
28
common half lives
uranium 238: 4500 mill yrs
plutonium 239: 24100 yrs
carbon 14: 5600 yrs
hydrogen 3: 12 yrs
radon 224: 60 seconds
29
background radiation
always present in the environment. it comes from various natural and artificial sources.
most comes from the air (e.g radon gas). then medical (x-ray machines), then the ground and buildings, then food and drink, then cosmic rays. the least is nuclear power plants, provided everything goes well
30
nuclear instability
as protons increase, neutrons increase too. heavier isotopses (N\>20) stable nuclei have more neutrons than protons. this is to push the + protons a little further apart.
**α**
* some nuclei are unstable b/c **too large**. the strong force cannot match electrostatic force. these nuclei decay by α emission to become more stable.
**β-**
* some nuclei are unstable b/c **too many neutrons**. these nuclei decay by β- emission, converting a neutron into a proton.
**β+**
* some nuclei are unstable b/c **too many protons compared to neutrons**. these become stable by converting a proton into a neutron.
31
Excited states and gamma emission
Nuclei can still be unstable if they have too much energy after α or β decay. The nucleus is said to be in an **excited state**.
in this case it emits a gamma ray photon to move to the lower energy **ground state**. this does not affect the nucleus.
32
Atomic mass unit
1/12 of mass of C12 atom
1u = 1.66 \* 10^-27kg
33
Mass defect
from the definition of an atomic mass units, 1 C atom has a mass of 12u. the mass of a nucleus is always **less** than the totoal mass of the nucleons from which it's made. the difference between the mass of a nucleus and the total mass of its nucleons is called the mass defect.
mass defect = (mass of nucleons) - (mass of nucleus)
34
binding energy
energy required to separate all of the nucleons in a nucleus. energy equivalent of mass defect.
**binding energy (J) = mass defect (kg) \* c^2**
35
fission & fusion
**fission**
* **splitting** of large _unstable_ nuclei to form smaller ones. this can happen spontaneously, or it can be encouraged to happen, as in a nuclear reactor -\> _induced fission_.
**fusion**
* **combining** of two smaller nuclei to create a large one. this can only happen if the nuclei have enough energy to _overcome_ the _electrostatic repulsion_ between their protons so that they can get close enough to be _attracted_ by the _strong nuclear force_.
**!! fusion only goes as far as to Fe !! iron is the most stable atom, heavier than that doesn't fuse.**
36
nuclear fission & fusion
**nuclear fission**
* Uranium235 + neutron -\> fission fragments + neutrons + energy. in power plants, energy is used to heat water -\> steam -\> turn turbines
**nuclear fusion**
* energy emitted by a star comes from fusion
* the core temp- has to be incredibly high: 10 million K at least
* combination of lighter isoptopes of hydrogen to form He
* once these reactions have begun, the energy released maintains the temp. in the star, and fusion continues until all the reactants have been used
37
binding energy and nuclear fusion
energy is released in a nuclear reaction when the binding energy of the products is greater than the bidning energy of the reactants.
* very _small_ nuclei release energy by increasing in size
* very _big_ nuclei release energy by decreasing in size
the stability of a nucleus is based not on the binding energy but on the _binding energy per nucleon_. this is equal to the binding energy divided by the no. of nucleons. it indicates how tightly bound each nucleon is on average.
**fe = inert material**
38
nuclear chain reactions
when U-235 undergoes induced fission after collision with a neutron, it breaks uo into 2 smaller nuclei and two neutrons. these are called **_fission neutrons*.*_**
these fission neutrons can go on to cause further fission events which will produce further neutrons etc. causing a _chain reaction_. there's no stopping it until supply runs dry.
39
critical mass
the _smallest amount of mass needed to sustain a chain reaction_ is called **critical mass**. but at the same time there mustn't be too much either.
the critical mass _depends on_:
* mass of fissile material
* density
* shape
* temperature
* the concentration of isotopes that have not yet decayed
40
energy from induced fission
a nuclear reactor uses a controlled chain reaction to produce heat to produce steam for a generator.
1. reactor produces heat
2. hot coolant from core heats water to produce steam
3. steam powers turbine generators
4. steam cooled and condensed by cooling tower
41
nuclear reactor core parts
1. fuel rods. contains enriched uranium.
2. control rods (cadmium or boron). can be lifted up/down. control how many are in. control temp and constant fission rate.
3. moderator (graphite). slows down fission neutrons.
4. coolant. water flows around the core transferring heat away from the fuel to be used to produce steam to drive the turbine. nasty water.
