IB Physics SL Topic 7 (copy)

Atomic, Nuclear, and Particle Physics

Absorption spectrum

• missing discrete frequencies / wavelengths;

• in otherwise continuous spectrum;

Activity

the number of __radioactive disintegrations__ per unit time.

Alpha-particle

helium nucleus

Antiparticle

a particle with the __same rest mass__ but opposite quantum numbers/charge/state;

Artificial (induced) transmutation

a particle is fired at a nucleus;

a different nucleus/nuclide/element forms;

Binding energy of a nucleus

Alt 1: (minimum) energy required to «completely» separate the __nucleus__ into its constituent/component __nucleons__

Alt 2: energy released when a __nucleus__ is formed from its constituent __nucleons__

{Allow neutrons AND protons for nucleons. Don’t allow constituent parts. Do not allow reference to “atom”. Award \[0\] for “energy to assemble nucleus”. Do not allow “particles”, “constituents” or “components” for “nucleons”. 2013 allowed the equivalence to mass defect with the equation, but other years have not}

Elementary particle

a particle with no internal structure / cannot be broken down further / a particle that cannot be made from any smaller constituents/particles

Exchange particle

• a short-lived/virtual particle/(gauge) boson; {This point is not mentioned in all previous exams.}

**Alt 1:**

• that transfers energy/momentum/(fundamental) force between interacting particles;

**Alt 2:**

• that mediates/carries/transmits one of the fundamental forces / is exchanged between two particles when undergoing one of the fundamental interactions

Half-life

**Alt 1:**

• the time required for the activity to reduce/drop to half;

**Alt 2:**

• the time taken for the number of unstable nuclei/activity to halve;

{Accept atom/isotope. Do not accept mass/molecule/amount/substance.}

ionization

removal (addition) of electron from atom/molecule;

Ionization radiation

• the radiation “knocks off” electrons from neutral atoms;

• thus creating an ion pair-free electron and positive ion;

Isotope

nuclide with same number of protons and different numbers of neutrons/nucleons;

{2014 doesn’t include nucleon as an option}

Mass defect

difference between mass of a nucleus and the sum of mass of nucleons / constituents / particles;

Nucleon

a proton or neutron;

Nuclide

• nucleus (or atom) characterized by specified by the constituents of its nucleus;

• Particularly, the number of protons and neutrons;

Nuclear fusion

• combination of two light __nuclei__; (do not allow “particles” or “atoms”)

• to form a new nuclide with greater mass/larger nucleus/greater number of nucleons; {mass is most often used}

• with the release of energy or with a greater total binding energy

Nuclear fission

• splitting of a heavy nucleus;

• to form two new lighter nuclei of similar mass;

• with the release of energy or with a greater total binding energy

Quantized energy

• **Alt 1:** energy that takes certain values and not others;

• **Alt 2:** energy values that are not continuous;

• **Alt 3:** energies that give rise to a discrete line spectrum;

Quantization of energy in atoms

• appropriate reference to the energy of electrons within the atom (e.g. the electrons of an atom have energy);

• not all energy values are possible (within an atom) / energy can only take discrete values;

Law of Radioactive Decay

the rate of decay is proportional to the amount of (radioactive) material remaining;

Radioactive decay \[2\]

**Alt 1:**

• decay of unstable __nuclei__ in a __spontaneous/random__ process;

• which emits radiation (from the nucleus) and __forms a different nucleus__;

**Alt 2:**

• unstable nuclei/nuclides change spontaneously/randomly and emit energy;

• by the __emission of alpha particles and/or electrons and/or gamma rays__;

{accept α, β and γ particles/radiation}

{To award max, reference must be made to nuclei and to spontaneous/random}

Rest mass

**Alt 1:** mass of an object measured in the object’s rest frame/when not in motion;

**Alt 2:** invariant mass;

{Alt 1 is mostly used}

Random decay \[2\]

• cannot predict __which nucleus__ will decay next;

• cannot predict at __what time__ a nucleus will decay;

Quark Confinement \[2\]

• quarks cannot be directly observed as free particles/must remain bound to other quarks/quarks cannot be isolated;

• energy given to nucleon creates other particles rather than freeing quarks;

Standard model

the (presently accepted) theory that describes the electromagnetic and weak interactions of quarks and leptons / the electromagnetic, weak interactions of quarks and electrons and the strong interaction of baryons;

(current atomic model including electromagnetic and weak interactions between quarks and leptons)

Pair production

process by which an electron and positron are produced

Spontaneous decay

the decay cannot be influenced/modified in any way

Unified atomic mass unit

one twelfth of the mass of a carbon-12 __atom__

{do not allow nucleus}

Virtual particle

**Alt 1:** a particle that mediates one of fundamental forces;

**Alt 2:** a particle that appears as an intermediate particle in a Feynman diagram;

**Alt 3:** a particle that is not observed and may violate energy and momentum conservation at a vertex;

Radioactive decay is said to be “random” and “spontaneous”. Outline what is meant by each of these terms

• random: it cannot be predicted which nucleus will decay OR it cannot be predicted when a nucleus will decay

• spontaneous: the decay cannot be influenced/modified in any way

Rutherford constructed a model of the atom based on the alpha particle scattering experiment results. Describe this model. \[3\]

• «most of» the mass of the atom is confined within a very small volume/nucleus;

• «all» the positive charge is confined within a very small volume/nucleus;

• electrons orbit the nucleus «in circular orbits»

Rutherford and Royds identified the helium gas in a cylinder by observing its emission spectrum. With reference to atomic energy levels, outline how an emission spectrum is formed \[3\]

• electron drops from high energy state/level to low state;

• energy levels are discrete;

• wavelength/frequency of a photon is related to the difference in energy and is also discrete

{allow quotes to E=hf or E=hc/λ}

State the quark structures of a meson and a baryon

• Meson: quark-antiquark pair

• Baryon: 3 quarks or antiquarks

Distinguish between hadrons and leptons. \[2\]

• hadrons experience strong force OR leptons do not experience the strong force

{accept weak force; accept “interaction” for “force”};

• hadrons are made of quarks/not fundamental OR leptons are not made of quarks/are fundamental;

• hadrons decay «eventually» into protons OR leptons do not decay into protons

Particles can be used in scattering experiments to estimate nuclear sizes.

(i) Outline how these experiments are carried out.

(ii) Outline why the particles must be accelerated to high energies in scattering experiments.

(i) • «high energy particles incident on» thin sample

• detect the angle/position of deflected particles

• reference to interference / diffraction / minimum / maximum / numbers of particles

{allow “foil” instead of thin}

\
(ii) **Alternative 1:**

• E = hc/λ so λ ∝ 1/E;

• high energy gives small λ;

• to match the small nuclear size

**Alternative 2:**

• higher energy means a closer approach to the nucleus;

• to overcome the repulsive force from the nucleus;

• so greater precision in the measurement of the size of the nucleus;

State the type of emission, one in each case, that:

(i) is not affected by electric and magnetic fields

(ii) produces the greatest density of ionisation in a medium

(iii) does not directly result in a change in the proton number of the nucleus

(iv) has a range of energies, rather than discrete values

(i) gamma / γ

(ii) alpha / α

(iii) gamma / γ

(iv) beta / β

Outline how the evidence supplied by the Geiger–Marsden experiment supports the nuclear model of the atom. \[2\]

• most undeflected/pass straight through hence mostly empty space;

• few deflected hence small positive/positively charged dense nucleus;

Outline why classical physics does not permit a model of an electron orbiting the nucleus. \[3\]

• electron accelerated / mention of centripetal force;

• should radiate EM waves/energy;

• and spiral into the nucleus;

Energy is supplied to a meson in order to separate the quark from the antiquark. With reference to quark confinement, predict the likely outcome of this experiment. \[2\]

• quarks are confined / a single quark cannot be observed/exist outside a nucleon as the interaction strength increases with separation;

• energy supplied will create a hadron/meson rather than a free quark;

Outline how atomic absorption spectra provide evidence for the quantization of energy states in atoms. \[2\]

• an atom will only absorb a photon if the photon energy corresponds to an __energy difference between two energy levels__;

• the absorption of energy takes place in discrete (quantum) quantities;

State the role of the Higgs boson in the standard model.

• responsible for giving masses (to quarks, leptons and the exchange particles of the weak interaction);

Other than conservation of mass-energy and conservation of momentum, state the names of three other conservation laws \[3\]

• (electric) charge;

• strangeness;

• lepton number;

• parity;

• baryon number;

• angular momentum;

• isotopic spin;

Distinguish between fission and radioactive decay.

**fission:**

• nucleus splits;

• into two parts of similar mass;

**radioactive decay**

• nucleus emits;

• a particle of small mass and/or a photon;

Explain why the temperature and pressure of the gases in the Sun’s core must both be very high for it to produce its radiant energy.

• high temperature means high kinetic energy for nuclei;

• so can overcome (electrostatic) repulsion (between nuclei);

• to come close together / collide;

• high pressure so that there are many nuclei (per unit volume);

• so that chance of two nuclei coming close together is greater

(two of the above choices)

State two properties of alpha-particles \[2\]

• range is a few cm in air or sheet of thin paper

• speed up to 0.1 c

• causes dense ionization in air

• positively charged or deflected in magnetic or electric fields

(any two, 1 each to max 2)

State the properties of gamma-particles

• high penetration ability / absorbed by thick, dense materials

• speed up to 1c

• very low ionization ability

• no deflection in magnetic or electric fields

State the properties of beta-particles

• range is a 30 cm in air/sheet of aluminum

• speed up to 0.9 c

• light ionization ability

• charged particle leading to deflection in magnetic or electric fields