Nuclear physics - Radioactivity Flashcards
(39 cards)
Describe setup of Rutherford’s α scattering experiment, inc explanation for each point (esp explaining details abt α particle). 5
Narrow beam of α particles (bc +ve), all of the same Ek (bc otherwise slow α particles would be deflected more than faster α particles on same initial path), in a vacuum (bc otherwise α stopped by air particles), fired at a thin gold foil (bc otherwise α scattered more than once). Detectors can move around and detect scattered α particles. α source must have long half life otherwise readings would be lower than earlier readings due to radioactive decay.
Results of Rutherford’s experiment? 2
-most α passed straights through foil with little/no deflection (about 1/2000 deflected).
-small percentage of α (1/10,000) were deflected through angles > 90°.
Interpretation of results of Rutherford’s experiment? 2
-most of atom’s MASS is concentrated in a small region, the nucleus, at the centre of the atom
-nucleus is +vely charged to repel α particles (+ve) that approach too closely.
Paths of some α particles which pass near a fixed nucleus.
Draw:
A
B
C
D
E
A, B, D deflected at diff angles. the closer the α particle, the greater its deflection bc electrostatic force of repulsion increases with decreasing distance between them. (A barely deflected, B deflected backwards, D deflected forwards).
C collides head-on w the nucleus and rebounds (deflected at 180°).
E doesn’t approach nucleus closely enough to be deflected.
Estimating size of nucleus:
1: 1/10000 α particles deflected by more than 90°.
2: Thin foil so each α scattered once.
3: A typical value for number of layers of atoms is 10^4
The probability of an α particle being deflected by a given atom is 1/10000n, where n is the number of layers of atoms. This probability depends on the effective cross-sectional area of the nucleus to that of the atom.
For a nucleus of diameter d in an atom of diameter D, the area ratio si equal to 1/4πd^2 / 1/4πD^2 = d^2/D^2
∴ d^2 = D^2/10000n.
A typical value for n = 10^4 gives d = D/10000
Rutherford found that radiation… (3 properties - found via experiment).
-ionises air, making it conduct electricity .
-was of two types: α more easily absorbed, β more penetrating (γ discovered a year later).
-magnetic field deflects α and β in opposite directions, and has no effect on γ - α is +ve and β is -ve (γ later shown to consist of high energy photons).
Radioactivity experiments: 4
Ionisation
Cloud chamber observations
Absorption tests
Range in air
Ionisation - explain (clue pA)
Ions created are attracted to oppo charge electrode where they’re discharged. E-s pass through pA as a result of ionisation. I ∝ number of ions created per second ∴ can see:
-α most ionising and if move source a few cms away the current ceases,
-β weaker ionisation and range varies up to a metre or more,
-γ least ionising bc photons carry no charge
Cloud chamber observations
(α vs β tracks and explain)
Contains air saturated w a vapour at a very low temp. Due to ionisation of air α/β passing through leave visible track.
α: straight track, easily visible, same length track if same isotope (α have same range). α particles from given isotope always emitted w same Ek bc α particles and nucleus move apart w equal and opposite momentum.
β: wispy tracks, easily deflected when collide w air molecules, less ionising ∴ harder to see tracks. in β decay electron antineutrino emitted as well ∴ the nucleus, β and neutrino share the energy released in variable proportions.
Absorption tests
Using Geiger tube and counter to find corrected count rate of source.
Keeping distance constant, can measure absorbance by using no absorber then absorber of diff thickness etc
Range in air - how and explain observations
Geiger tube and counter- vary distance.
-α few cm then sharp decrease in count rate bc all same Ek
-β up to a metre, gradual decrease bc range of Ek up to a max
-γ unlimited range in air, gradual decrease in count rate bc radiation spreads out in all directions. Proportion of γ photons from source entering tube decreases according to inverse square law (all same E).
β radiation consists of fast moving e-s. How was this proven?
By working out specific charge of β using electric and magnetic fields - same as e- specific charge.
γ radiation consists of photons with wavelength of… ?
How was this discovered?
10^-11 or less.
Discovery made by using a crystal to diffract a beam of γ radiation.
Inverse square law for γ radiation:
Intensity of radiation =
-Intensity, I, of radiation = radiation energy per second passing normally through unit area.
-For a point source emitting nγ photons per sec, radiation energy per sec = nhf
-At a distance r from source, all photons emitted pass through total area 4πr^2
∴ I=nhf/4πr^2 =k/r^2 ∴ I ∝1/r^2
why might results of experiment not follow expected inverse-square law
-random nature of radiation count
-dead-time in G-M detector
-d is not the real distance between the source and detector
-assume no absorption between source and detector
Hazards of ionising radiation (explain fully)
how can we monitor the radiation dose (+define)?
Destroys cell membranes –> cell dies (at high doses) or damages DNA –> cell divides + grows uncontrollably, causing tumour which may be cancerous. Damaged DNA in sex cells –> mutation which is passed on.
Monitor ionising radiation (eg x-ray, α etc) by wearing a FILM BADGE.
radiation dose = energy absorbed per unit mass of matter from the radiation.
Define activity, A, of a radioactive isotope?
Units?
Activity, A is the number of nuclei of the isotope that disintegrate per second.
Units becquerel (Bq)
Graph of count rate - time shows how A decreases with time because..
since activity is proportional to corrected count rate.
Energy transfer per second from source = AE = power, where E is energy of each particle/photon.
Mass of radioactive isotope ∝
Mass of radioactive isotope ∝ number of nuclei of isotope ∝ activity.
N = N.e^-λt (cam be A or cr too)
Decay constant, λ, is?
Units?
λ = the probability of an individual nucleus decaying per second.
Units s^-1
Half life, T1/2 = In2/λ
Uses of radioactive isotopes:
1 - a,b
2
3 - a,b,c
Radioactive dating
-carbon dating
-argon dating
Radioactive tracers
Industrial use of radiation
-engine wear
-thickness monitoring
-power sources for remote devices
Radioactive dating - carbon dating:
How 14C formed?
Half-life significance?
How to calculate age?
Cosmic rays knocking out p from nitrogen nuclei:
n + 14N —> 14C + p
CO2 from atmosphere taken up by living plants ∴ small percentage is 14C.
14C has huge half-life ∴ negligible decay during lifetime of plant. Measuring activity of dead sample enables age to be calculated if know activity of same mass of living wood.
Radioactive dating - argon dating:
K decays into Ar or Ca
-Ancient rocks contain trapped argon gas as a result of decay of radioactive isotope of K.
-40K + e- –> 40Ar + ν
-But also 40K –> β + 40Ca + ν ¯ which is 8x more likely decay.
-for every N 40K atoms now present, if there’s 1 40Ar present, there must’ve been N+9 40K originally.
Radioactive tracers
use?
properties of radiation used?
A radioactive tracer is used to follow the path of a substance through a system.
Should have half life which is stable enough fir measurements to be mad and short enough to decay quickly after use.
Should emit β or γ so can be detected outside flow path.
