Nuclear physics Flashcards
Nucleon
particle in the nucleus
Atomic number (Z)
number of protons
N
number of neutrons
Mass number
protons and neutrons
Nuclide
a particle with a particular mass number
Isotopes
nuclides of the same element, same number of protons but different neutrons
evidence for isotopes
bainbridge mass spectrometer;charged atoms are fired at a specific velocity passed by an opposite plate and landing at different places reveals having different isotopes
Interactions in a nucleus
Coulomb interactions and nuclear interactions
Coulomb interactions
A charged particle will never enter the nucleus because as the distance to it reached 0, then the repulsion would be infinitely large.
Nuclear interactions
Where the coulomb repulsion is overcome, as on sun, but much larger gravities.
Stability of nuclei
if strong force and coulomb repulsion are balanced; if not, unstable
for it to be stable neutrons must be ≥ number of protons (because coulomb force goes further than strong force)
unified atomic mass unit
1/12 of the mass of a carbon-12 atom
Energy-mass equivalence
mass and energy are equivalent in that a change in mass can also be regarded in the same change in energy
E=mc^2
MeVc^-2
equivalent to 1 atomic mass unit
mass defect
difference in mass between mass of nucleus and the mass of separate nucleons
nuclear binding energy
energy required to separate the nucleus into different components (MeV)
E=mc^2 (binding energy= mass defect* c^2)
radioactive decay
when an unstable nucleus emits an alpa or beta particle or gamma ray photon resulting in a daughter nucleus which is more stable
characteristics of radioactive decay
random and spontaneous (with less nuclei, probability does not change and the number of decays reduce)
half-life
time taken for the initial activity of the radioactive sample to halve
radioactive decay law
the rate of decay is proportional to the number of undecayed nuclei (dN/dt=-λ(decay constant)*N(number of undecayed nuclei))
N=N0e^(-λt)
Activity (rate of decay same)
A=-dN/dt=λN=λN0e^(-λt)
Measuring the half-life of an isotope (long)
number of decays by time
mass of the sample and calculating the number of atoms N
using A=λN to calculate λ constant
using T1/2= ln2/λ
carbon-14 dating
once an organism has died, the amount of carbon-14 will fall with decay, which can be used to tell how old it is
why is C-14 dating possible
c14 is unstable, but c12(also present) is stable (proportion constant in living)
neither can be replenished after death
c14 decays
limitations of Carbon-14 dating
after 5000 years, the activity is too small to be measured
Measuring geological time
age of rocks can be determined by the decay of Uranium-238 to lead-206
(the rock must contain uranium as an impurity in its structure, but reject lead so all that is left is results of the decay)
the half life is approximately the age of the earth
alpha decay
what, why, how
an alpha particle (2 protons+2 neutrons) (also described as the nucleus of a helium-4)
causes transmutation
tends to be emitted by nuclides with too many neutrons for stability
transmutation
alpha decay causing a change in element
properties of alpha particles
travel at about 10^7ms^-1
deflected by electric and magnetic fields (double positive charges)
low penetration
causes intense ionization (change of charge from e- changes)
beta decay for a proton in a nucleus
beta + particles (positrons, or antimatter of an electron (same mass, opposite charge)) and neutrino (v)(neutral particle with little or no mass) are emitted
beta decay for a neutron
beta particle (electron) and antineutrino
beta decay
both types are transmutations
beta+ because of too few electrons
beta because of too many electrons
properties of beta particles
typically at 10^8ms^-1
deflected by electric and magnetic fields as expected for a single negative
causes moderate ionization
medium penetration
gamma decay
gamma ray (part of electromagnetic spectrum) does not cause transmutation after alpha or beta decay, the exited state may gamma decay to reduce energy
properties of gamma rays
travel at 3*10^8
not deflected by electric and magnetic fields
cause little ionization
high penetration
nuclear energy levels (evidence)
alpha particles always emitted at same speed from a nuclide. gamma rays always emitted with same energy.
biological effects of ionizing radiation
may damage dna (can be repaired, but if not could cause cancerous cells)
short term: could cause burns in high dosage; could kill organs
long term: risks of cancer, but no link between dosage and type of cancer
induced nuclear reactions
the reactions can be forced by by striking a stable nucleus with another nucleus, particle or gamma ray photon
nuclear fission
when a stable nucleus absorbs a slow moving neutron making it unstable and causing a split into two large fragments and some neutrons
the importance of neutron emission is possible chain reactions
estimation of the energy produced by fission/fusion
=the increase in binding energy (binding energy of mother nucleus is seen as negative)
Nuclear fusion
combing of two nuclei, which results in high levels of energy emission
evaluation of fusion
possible because the binding energies of two low mass nuclei can fuse to create a larger nuclei of higher binding energy.
coulomb repulsion prevents it working, and requires the collision to happen at high speeds causing the energy released to mainly be kinetic energy
attractive because of large quantities of deuterium and tritium (H-2+H-3) and much less radioactive waste, but extremely hard to make it happen and remain constant
thermonuclear reactions
reactions that require immense heat to keep the reaction occurring, happens in a star.
problems involving mass defect and binding energy
E=mc^2
Mass defect= total mass of separated nucleons- mass of nucleus
Sometimes given in atomic masses, so incorporate electrons
Atomic mass= nuclear mass+(Zmass of electron); therefore Mass defect= (Zmass of proton)+(Nmass of a neutron)- (Atomic mass-(Zmass of electron)
Measuring a short half life
lnA by time
beta energy spectra
continuous with a range of speeds (thus energy levels) up to a maximum. the anitneutrino takes remaing energy away
