Nuclear And Particle Physics Flashcards
What is radioactive decay?
Radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting particles and/or energy and so potentially becomes more stable. It is a random process meaning that it is impossible to predict which of a number of identical nuclei will decay next.
State the three types of radiation emitted by radioactive materials, in order from most to least ionising
- alpha particles
- beta particles
- gamma rays
State the characteristics of an alpha particle
An α-particle is a helium nucleus consisting of 2p + 2n. It has a mass of 4u, a charge of +2e and can penetrate 2cm of air, skin and paper
State the characteristics of a beta particle
A β-particle is a high speed (high energy) electron. It has a mass of 1/2000u (negligible), a charge of -e, and can penetrate a few metres of air, aluminium and perspex
State the characteristics of gamma rays
A γ-ray is a high energy photon of electromagnetic radiation. It has no mass, no charge, and can penetrate thick lead and concrete
What does a Geiger-Muller tube do, and how does it work?
A Geiger-Muller tube is a device which is used in conjunction with an amplifier and counter (called a scalar) to detect alpha, beta and gamma radiation. It works by detecting ionisation caused by these radiations as they pass through the air
Describe Rutherford’s alpha-particle scattering experiment and the results he obtained
Rutherford carried out an experiment where a narrow beam of alpha-particles were fired at thin gold foil (approximately 400 atoms thick) under a vacuum. The deflected alpha-particles were detected on all sides by a ring of scintillators. He observed that:
1. Most of the particles go straight through undeflected
2. A few are deflected through small angles (about 1 in 2000)
3. A very small number are deflected through large angles (about 1 in 10,000)
State what was concluded about the atom from Rutherford’s experiment
All the positive charge and most of the mass of the atom are concentrated in the nucleus, diameter 10^-14m rather than an atom’s 10^-10m diameter. In other words, he showed that most of the mass in an atom was in a diameter only a ten thousandth that of a typical atom.
Describe the simple nuclear model of the atom
The atom consists of a small, dense, positively charged nucleus surrounded by a cloud of electrons. The nucleus consists of protons and neutrons collectively called nucleons.
What are isotopes, and how does their stability vary?
Isotopes are atoms of the same element with the same number of protons but differing numbers of neutrons, which causes the stability of their nuclei to differ greatly
What does a nucleon refer to?
A subatomic particle that resides in the nucleus of the atom, either a proton or a neutron.
What do atomic (proton) number and mass number represent?
Mass number = number of nucleons in a nucleus (neutrons + protons)
Atomic number = number of protons in a nucleus
State the four fundamental forces in order from weakest to strongest
- gravitational force
- weak nuclear force
- electromagnetic force
- strong nuclear force
State the properties of the gravitational force
The gravitational force acts on all particles with mass. It is always attractive, has an infinite range and is very weak (relative strength of 10^-40)
State the properties of the electromagnetic force
The electromagnetic force acts on static and moving charged particles. It can be both attractive and repulsive, it has an infinite range and has a relative strength of 10^-3.
State the properties of the weak nuclear force
The weak nuclear force is the force responsible for beta decay. It acts to change quark types over very small distances (range of 10^-18m) and has a relative strength of 10^-6.
State the properties of the strong nuclear force
The strong nuclear force acts between all nucleons and all quarks, and counteracts the repulsive electrostatic forces between protons in the nucleus. It is attractive at small distances up to about 3fm and repulsive at incredibly small distances below about 0.5fm, and has a limited range (about 10^-15m). It has a relative strength of 1.
Describe the nature of the strong nuclear force and how it varies with nucleon separation
Within a nucleus, for a typical nucleon separation of 1.3fm, the strong nuclear force is very attractive. Beyond 1.3fm separation, the strong nuclear force quickly reduces to zero. Therefore the strong nuclear force is a very short-range force. When the separation exceeds about 2.5fm the electrostatic force between protons dominates, and they will be propelled apart.
What is 1fm in m?
1fm = 10^-15m
What is the volume of a nucleus equal to?
V = 4/3πR³, where R is the nuclear radius
Since R = R₀A^1/3
We can also write this as V = 4/3π(R₀A^1/3)³
Or V = 4/3πR₀³A, where A is the mass number
State the equation for nuclear radius (R)
R = R₀A^1/3, where R₀ is a constant of proportionality
State the equation for mass of a nucleus
m = Au, where A is the mass number and u is the atomic mass unit
Derive the equation for nuclear density
ρ = m/V
Since m = Au and V = 4/3πR₀³A
ρ = Au/(4/3πR₀³A)
ρ = 3u/4πR₀³
Give four particles (matter) and their corresponding antiparticle (antimatter), and state the symbol for each
Electron (e-) - positron (e+)
Proton (p) - antiproton (pbar)
Neutron (n) - antineutron (nbar)
Neutrino (𝜈) - antineutrino (𝜈bar)
State the similarities and differences between matter and antimatter
Antiparticles have the same rest mass as the corresponding particles, but if they are charged, they have equal and opposite charge to the corresponding particle. All other quantum numbers are also opposite.
What happens when a particle and its corresponding antiparticle meet at a point in space
They annihilate to produce energy in the form of photons. The rest energy and kinetic energy of the particle and antiparticle is transferred into the energy of the photons
Describe the classification of particles
Particles which experience both the strong and weak nuclear force are called HADRONS.
Particles that experience the weak nuclear force but not the strong nuclear force are called LEPTONS.
HADRONS can be split into two groups, BARYONS and MESONS. BARYONS contain the proton in their decay products whereas MESONS do not.
Describe the fundamental particle make up of baryons, anti-baryons, mesons and leptons
Baryons are made up of three quarks
Anti-baryons are made up of three antiquarks
Mesons are made up of a quark-antiquark pair
Leptons are themselves fundamental particles, they do not consist of any smaller particles
State 6 types of quark/anti-quark and their corresponding charge, baryon number and strangeness
Up quark (u) +2/3e, +1/3, 0
Down quark (d) -1/3e, +1/3, 0
Strange quark (s) -1/3e, +1/3, -1
Anti-up quark (ubar) -2/3e, -1/3, 0
Anti-down quark (dbar) +1/3e, -1/3, 0
Anti-strange quark (sbar) +1/3e, -1/3, 1
Give three examples of leptons
Electrons, positrons and neutrinos
State the β- decay equation for a neutron in terms of particles, and in quark notation
neutron -> proton + electron + anti-neutrino
or n -> p + e- + 𝜈bar
udd -> uud + e- + 𝜈bar
or d -> u + e- + 𝜈bar
State the β+ decay equation for a proton in terms of particles, and in quark notation
proton -> neutron + positron + neutrino
or p -> n + e+ + 𝜈
uud -> udd + e+ + 𝜈
or u -> d + e+ + 𝜈
State and define the two types of meson
Pion - meson not consisting of either a strange or anti-strange quark
Kaon - meson with one strange or one anti-strange quark
State the quark model of the proton and neutron
Proton - uud
Neutron - udd
Define half-life
The half-life, t1/2 of a radioactive isotope is the average time it takes for half of the number of active nuclei in the sample to decay.
Define activity (A) of a radioactive isotope, and state its unit
Activity of a radioactive isotope is the number of nuclei of the isotope that disintegrate per second
Unit - becquerel (Bq)
Define decay constant, and state its unit
The decay constant, λ, is the probability of an individual nucleus decaying per second
Unit - s^-1
State the equation for activity
A = λN
Where λ is the decay constant and N is the number of undecayed nuclei
State the exponential decay equations for number of undecayed nuclei (N), mass of a radioactive isotope (m), and activity of a radioactive isotope (A)
N = N₀e^-λt
m = m₀e^-λt
A = A₀e^-λt
Derive an equation that relates decay constant (λ) and half-life (t1/2)
We know that N = N₀e^-λt
When t = t1/2, N = N₀/2
So N₀/2 = N₀e^-λt1/2
1/2 = e^-λt1/2
ln(1/2) = -λt1/2
ln2 = λt1/2
t1/2 = ln2/λ
What is rest energy?
Rest energy is the energy due to rest mass m₀ and is given by Einstein’s mass-energy equation
E = m₀c²
Describe and explain pair production
A very high energy gamma photon can change into a particle-antiparticle pair. Pair production is only possible when the photon has energy equal to or greater than the rest energy of the particle/antiparticle.
In other words, a single gamma photon with energy in excess of 2m₀c² can produce a particle-antiparticle pair, each of mass m
Define binding energy
Binding energy of a nucleus is the work which must be done to separate the nucleus into its constituent neutrons and protons.
Define mass defect
Mass defect (∆m) of a nucleus is the difference between the mass of the separated nucleons and the mass of the nucleus
Explain why separate nucleons have a greater mass than the original nucleus
As work is done in separating the nucleus into its constituent parts, more energy is “stored” as potential energy in the separate parts than the original nucleus.
Using E = m₀c², if the separate nucleons have more energy, they must also have more mass
Define binding energy per nucleon
The binding energy per nucleon of a nucleus is the average work done per nucleon to remove all the nucleons from a nucleus. This is a measure of the stability of the nucleus.
