chapter 25 - radioactivity Flashcards
ionising radiation
can ionise atoms by removing some of their electrons, leaving positive ions eg alpha, beta, gamma
cloud chamber
- used to detect the presence of ionising radiation
- contains air saturated with vapour at a low temp
- when air molecules are ionised liquid condenses onto the ions to leave tracks of droplets marking the path of radiation
alpha radiation
consists of 2 protons and 2 neutrons - helium nucleus
2+ charge
strong ionising power
absorbed by few cm of air + paper
constant energy
beta radiation
fast moving electron or positron
beta minus has -1 charge beta plus has +1
less ionising
absorbed by thin aluminium
energy varies for each source
gamma radiation
consist of high energy photons
travel at speed of light
carry no charge
least ionising
absorbed by thick lead
constant energy
electric and magnetic fields on radiation
alpha - deflected towards neg plate
beta - deflected towards pos plate - less due to greater mass
gamma - not deflected due to no charge
absorption experiments
use a GM tube and counter
- keep at fixed distance
- count background radiation
- then but an absorber between and count
- corrected count rate = count - background rate
dangers of radioactivity
all radiation causes ionisation
- can damage living cells
must
- store radiation in lead
- use tongs to transfer with long handles
- never handle with bare hands
transmutation
when the nuclei of one element emit alpha they change (transmutate) into to nuclei of another element
nucleus before is parent and after is daughter
when does nuclear decay stop
when stable nuclei are formed
alpha decay
the mass no. loses 4
the atomic no. loses 2
beta minus decay
caused by weak nuclear force
one neutron decays into a proton and an electron is released with an electron anitneutrino
mass no. same
atomic no. +1
beta plus decay
cause by weak nuclear force
one proton decays unto a neutron - a positron is released with an electron neutrino
mass no. same
atomic no. -1
gamma decay
gamma photons are emitted if a nucleus has too much energy after alpha or beta emission
doesnt change mass or atomic no
random
- cant predict when a nucleus will decay or which will decay next
- each nucleus has the same change of decay /time
radioactive decay is described as
random and spontaneous
spontaneous
decay is not affected by
- presence of other nuclei
- external factors eg pressure
- cant be sped up or slowed down
half life
average time it take for half the number of active nuclei in a substance to decay
activity
the rate at which nuclei decay
= number of nuclei decaying /time
1 decay / sec = 1Bq
decay constant
λ
the probability of the decay of an individual nucleus per unit time
A = λN
dN/dt = -λN
exponential decay
ΔN/Δt = -λN
N = N0 e^-λt
m = m0 e^-λt
A = A0 e^-λt
decay constant and half life
when t = t1/2, N = N0/2
so using the decay formula you can solve for half life
t1/2 = ln2/λ
carbon dating
- all living things contain carbon
- once they die it stops taking in carbon - carbon decays and isnt replenished
- so if you know the half life of carbon12 and the ratio of carbon14:carbon12 of the dead thing you can find its age
carbon-14
- high speed protons in cosmic rays collide with atoms in the upper atmosphere to make neutrons
- neutrons collide with nitrogen14 to make carbon 14
- C14 eventually emits beta- and becomes N14 again
neutron + N13-> C14 + proton
limitation of carbon dating
- activity is very small and close to background
- assume ratio of C14:C12 is constant
can be changed by - increased emission of CO2 due to fossil fuels
- volcanic eruption
- solar flares
- testing nuclear bombs
dating rocks
- cant use C14 bc half life not long enough
- use rubidium 87
- emit beta minus and transform into stable strontium 87
count rate
number of decaying nuclei detected
