Atomic, Nuclear and Particle Physics Flashcards
Rutherford’s (Geiger - Marsden) Experiment - aim & method
aim: to investigate the distribution of charge within the atom
Method: fired alpha particles (helium nuclei) at a thin gold foil. a fluorescent screen detected where they went.
Why gold? (GM experiment)
- can be hammered into very thin sheets (approx 1000 atoms thick)
- large atom
Why alpha particles? (GM experiment)
- large enough mass not to be deflected by electrons
- positive charge to investigate charge distribution
Expected Results vs actual (GM Experiment)
expected: if Thompson’s model was correct then the alpha particles would pass straight through undeflected (as charge evenly distributed)
actual: most alpha particles passed straight through as most of the atom is empty space
a significant number pf alpha particles had their path deflected as they passed through the foil as the positive charge of an atom is concentrated in its centre
some particles bounced straight back from gold foil (hit nucleus) therefore most of the mass of an atom is in the nucleus
What are atomic energy levels?
electrons can only exist within a series of discrete energy levels around a nucleus
electrons can move between energy levels by either absorbing or emitting photons of light (packets of energy)
Continuous Spectrum
from a light source producing photons of all wavelengths (and frequencies)
shine a light through slit to produce beam then through a prism
Emission Line Spectrum
only photons of specific wavelengths are emitted (black with coloured bands)
pass light through a hot gas then through a slit
then shine through a prism
Absorption Line Spectrum
only photons of specific wavelengths are absorbed (continuous with black bands)
shine a light through a cold gas and then through a slit
then shine through a prism
How are protons and neutrons held together?
a strong nuclear force which:
- acts at a short range
- is balanced by repulsion between protons that acts at a long range (Coulomb interaction)
if too many protons repulsion > strong nuclear force
if too many neutrons strong nuclear force is not strong enough to hold whole nucleus together
Nucleon
is any particle in the nucleus (e.g. proton or neutron)
Isotopes
are different nuclei of an element that have different number of neutrons
Alpha Radiation
helium nucleus He4/2
range of about 5cm
penetration is stopped by paper
very high ionising ability (high Ek and +ve charge)
Beta Radiation
Beta -ve = an electron emitted from the nucleus (neutron -> proton) 0/-1 accompanied by an antineutrino
Beta +ve = a positive electron or positron (proton -> neutron) 0/1 accompanied by neutrinos
range of 30 cm
stopped by aluminium foil
low ionising ability
Gamma Radiation
EM radiation (photon)
very large range
stopped by 10cm of lead
very low ionising ability
always accompanies alpha and beta radiation
Radioactive decay is:
- spontaneous (cannot be modified in any way)
- random (don’t know when a particular nucleus will decay or which particular nucleus will decay)
Half Life
the time taken for half of the nuclei in a sample to decay (or for the activity to fall to 50% of the initial value)
unit is the becquerel (Bq)
after n half lives, 1/2n of the original substance remains
Background Radiation
constantly exposed to radiation - must be subtracted when taking measurements
natural:
- cosmic, terrestrial and internal (food and air)
or artificial:
- medical, industrial / occupational, nuclear fall out
Rules of Nuclear Equations
- conservation of mass: the total mass number of each side of the equation must be the same
- conservation of charge: the total atomic number on each side of the equation must be the same
Alpha particle
helium nucleus emitted during radioactive decay
Antineutrino
particle emitted with a beta negative particle
Atomic number (Z)
number of protons in a nucleus
baryon
formed from three quarks (or antiquarks)
beta negative particle
an electron emitted from the nucleus
beta positive particle
a positive electron (positron) emitted from the nucleus
