Nuclear physics Flashcards
how was the alpha scattering experiment setup?
- alpha particles used as projectiles to study the structure of gold atoms
- a very thin gold foil was used as the target for the alpha particles
-a fluorescent screen was used as the detector, when alpha particles struck the screen a flash of light was produced allowing the team to see the trajectories
what was the alpha scattering experiment measuring?
- the number of alpha particles deflected by an angle theta
why did the apparatus have to be evacuated?
- to stop the alpha particles being stopped by air
what was the first Rutherford conclusion?
- atom has a very small positively charged nucleus
- responsible for the repulsion force on the positively charged alpha particle, causing it to change direction
what was the second Rutherford conclusion?
- nucleus contains nearly all the mass of the atom
- conservation of momentum explains this
MaVa + Mn x 0 = MaVa
nucleus barely moves after repulsion of a particles so Mn>Ma
what was the closest distance of approach?
- minimum distance an alpha particle can get to the nucleus before being repelled due to Coulombs Force
what was the equation for closest distance of approach?
kinetic energy at start = potential energy at repulsion
what was electron scattering
- experiment used to study the structure of matter
- high energy electron beams fired at a target
- deflection was analysed giving conclusions
what are the three equations for electron scattering?
lambda = h / p
p = E / c
sin theta = 1.22 lambda / d
what is meant by an empirical equation?
- an equation based entirely on experimental results
what was the empirical equation for radius of nucleus?
R = Ro x A^1/3
derive how i can plot this graph
R = Ro x A^n
ln R = lnRo + lnA^n
ln R = n lnA + lnRo
whats the equation for nuclear density
p = 3/(4piR^3) x m
what is radiation?
- when an unstable nuclei emits energy
give the nature of each three types of radiation
alpha particle - helium nucleus
beta particle - fast electron
gamma ray - electromagnetic photon
give the charge of each of the three types of radiation
alpha particle is +2e
beta particle is -e
gamma ray is 0
give the masses of each of the three types of radiation
alpha particle - 6.6x10^-27kg
beta particle - 9.1x10^-31kg
gamma ray - 0
give the speed of each of the three types of radiation
alpha particle - 5% of c
beta particle - 98-99% of c
gamma ray - c
give the ions per mm of air for a particle of 3MeV
alpha particle - 10000
beta particle - 100
gamma ray - 1
give the detection for each of the three types of radiation
alpha particle - slight deflection in electric and magnetic fields
beta particle - significant deflection in electric and magnetic fields
gamma ray - no deflection in electric and magnetic fields
all affects photographic film
give the ionisation of each of the three types of radiation
alpha particle - strongly ionising
beta particle - much less ionising than a particles
gamma ray - very weakly ionising
how does the charged gold leaf electroscope illustrate the strong ionising power of alpha radiation?
- alpha particles create a large number of positive and negative ions in the air
- the charge of the electroscope becomes neutralised depending on the inital charge
( free electrons attracted if positively charged )
( positive ions attracted if negatively charged ) - as the charge on the electroscope is reduced, the gold leaf gradually collapses
what is background radiation?
there is always radiation present in the environment, such as rocks that contain radioactive uranium
the sun also emits protons which can create ions in the atmosphere
how does a smoke alarm work?
- alpha particles ionise the air between plates so a current flows
- smoke particles absorb the alpha particles so the air is no longer ionised and current decreases
- this sets off the alarm
describe the emission of radiation from a nucleus
- the emission is both random
and spontaneous
what is the decay constant?
- probability of a nucleus decaying per unit time
what is the significance of the minus sign in the equation
- the number of radioactive nuclei in a sample of material decreases with time
what is the activity of a radioactive source
- the number of disintegrations per second
give the equation to predict the number of nuclei at any time
N = No x e^-kt
where k = decay constant
what is half life
- one half life is the time taken for half of a sample of radioactive nuclei to decay
how is half life connected to the decay constant?
T1/2 = ln2 / decay constant
what can the half life of a radioactive nucleus be used for?
- dating of objects
- medical diagnosis
how does dating of objects work?
- nuclei with long half lifes such as carbon 14 can be used to date organic objects
- done by measuring the current amount of carbon-14 and comparing it to the initial amount
how does medical diagnosis work
- nuclei with relatively short half-lives are used as radioactive tracers in medical diagnosis
- eg pure gamma emitters, half life of 6 hours, short enough to limit exposure but long enough for tests to be carried out
why do radioactive nuclei have to be stored in a certain way
- to prevent nuclei from damaging the environment and the people that may be living around them hundreds of years into the future
what is meant by electron capture?
when an unstable nucleus absorbs one of its own orbiting electrons, to stabilise itself
why can electromagnetic radiation be given off sometimes when decaying
the daughter nucleus may be excited
what are the two ways in which i can take safety precautions when dealing with radioactive sources?
- minimise time of exposure
- keep as big a distance between you and the source
what is nuclear stability
- nucleus held together by strong force
- protons experience a force of repulsion
- if forces are out if balance, nuclei will become unstable
what decay occurs if a nuclei has too many neutrons?
- beta minus
what decay occurs if a nucleus has too many protons
beta plus
electron capture
what decay occurs if a nucleus has too many nucleons?
- alpha decay
what happens if a nucleus has too much energy
it will decay and emit a gamma ray
what is the equation linking count and area
( area / surface area ) x count rate
why us technetium used in medicine
- metastable isotope, so it emits gamma rays to become more stable
- physical half life is short
what are the dangers of nuclear radiation , alpha radiation
- outside body does little harm
- inside body very dangeous
- never used for medicine as danger of inhaling / digesting
what are the dangers of beta radiation
- more penetrating than alpha radiation
- able to penetrate inside and outside the body
what are the dangers of gamma rays?
- in high doses, dangerous in and outside the body
how does a Geiger tube work
- radiation interacts with gas knocking off electrons
- electrons are accelerated towards the anode via e field
- as the electron accelerates it collides into more gas molecules, causing more ionisation
- avalanche creates a sudden surge of current, detected as a pulse of electricity
- pulse corresspondes to a single ionising event
- external circuit count pucks up number of times
