Cjapter 25 Radioactivity Flashcards
What are three types of radiation and info about them
Alpha radiation = is a helium nucleus, two protons and two. Dutton’s , hence 4/2, and has a charge of +2
Beta radiation = consists of either fast moving electrons B- or positrons which are B+
Gamma radiation is just EM radiating
What are all these three type of raids it on referred to as ionising?
Energy enough to cause an electron to be kicked of atoms, making them POSITIVE IONS
Beta radiation, what is positrons?
This is thr ANTIMATTER PART OF AN ELECTRON
How will all 4 types of radiosonde (b- and b+) interact in the presence of a UNIFROM ELECTRIC FIELD
Supplied by two parallel charged plates
1) gamma = will just go through, because uncharged
2) b+ will deflect towards the negative , and B- towards positive plate, both in exact mirror opposite trajectories, as everything else is same (mass etc)
3) the alpha will also deflect towards negative, but because more MASS, will deflect less (even tho charge is twice, mass is 4, so )
Remember when using a GM tube ti measure radiation, WHAT MUST ALWAYS DO TI GET PROPER RESDINGS
Measure the BACKGROUND RADIATION count rate first, and subtract from measured count rate in edperiemnt
IONISING POWER vs RANGE IN AIR for each 3
Alpha has highest mass charge so HIGHEST IONISING POWER
- but the lowest range in air thus (as ionises quick)
BETA has less mass so Less ionising power
- thus better range in air than alpha
Gamma has no charge so even LESS IONISING than beta
- range is longggg
What stops each radiation in air (materials’
Alpha stopped COMPLETELY by a thin sheet of PAPER, or few cm of air
- BETA stopped by a few mm of ALUMINIUM
- Gamma must need LEAD, few cm
How to be safe with radioactive sources, what are they usually stored in
Why they dangerous?
Ionising can cause damage ti cells, so always stored in LEAD,
# should wear tongs and gloves keep far away as possible
Hat is charge of position.
+e
What happens in an ALPHA emission (change in mass charge what emitted)
Alpha nucleus emitted that’s it
Mass number decrease by 4, and proton by 2
What happens in a BETA - EMISSION
HERE too many NEUTRONs so unstable
= a NEUTRON converts into a proton, and releases an electron and an ELECTRON ANTI NEUTRINO
Thus mass decreases but increases so nothing happens to mass
Charge on proptin increases so +1
New element formed
What happens in BETA + EMISSION
here too many protons so unstable
= a proton changed into a neutron and releases a position, and electron anti neutrino
Thus mass stays the same again
Proton number DECREASES , so new element
And positron and anti neutrino rrlease
What happens with GAMMA emission
No charge or mass , as it’s just em wave
Energy is given off as gamma, and new nucleus stabke
Remember what happens as soon as the Proton number changes
Here the ELEMENT WILL CHANGE
Will an unstable nucleus keep decaying forever? What normally happens
It will decay a few times, and of different decays, until it’s finally stable
How ti describe the decay of a radioactive source as spontaneous and random
Decay is random bevause
- can’t predict which nucleus will decay
- each nucleus has the SAME chance of decaying p
It’s spontaneous bevause
- not affected by presence of other nuclei
- or external factors like pressure
What is half life
What kinda decay does this show
The average time taken for half of the active nuclei remaining to decay
Exponential decay
What is ACTIVITY A
Units
What does it depend on
This is the rate at which nuclei are decayed per second
This is bq, 10 bq means 10 per second
Depends on number left snd half life, csn model using exp decay
Explain A = decay constant x N formula
Basicslly the rate of decay of nuclei is proprtinsl to the number there is left
So chnage propritnsk to - N, because the N is decreasing
If we put a constsnt in to make it an equation, we know chnage is just the activity
A = kN
Where K is the decay constsnt
What definition can we give to the decay constsnt
Units?
This is DIFFERENT FOR each isotope, so unique
Thus it’s the PROBABILITY of decay of any nucleus per unit time , units 1/s
As N decreases exponentially, and as A proprtinsl to N , what two equations can we write to model their decrease
N = N0 e ^-Kt
A= A0 e^-kt
What is equatuin fir half life (how to derrive)
Ln2 / k = half life
Derrive by trying to find the time taken for half N using an equatuin
How to do questions to calculate change in activities
Use info you have to find the decay constant
Now use activity equation
What 2 things yiu need to measure half life then
And how
Just the number of. H lei usign mass spec
And activity which can be measured using detectors
How does CARBIN dating work
Here the ratio of CARBIN 14 which is unstable to Carbon 12 in thr atmosphere and LIVING ORGANISMs is said ti be constsnt
Thus if you measure some dead organic matter, and compare the nuclei of carbon 14 or the ratios of activities (because activity is oroeotinsl to N), you can work out the time taken presuming you know the half life of carbon 14 , then you can work out time
How is CARBON 14 which is used for dating formed in the earths atmospher
1) high speed protons from cosmic rays collide with atoms to produce neutrons
2) these collide with nitrogen 14 nuclei to form carbon 14 nuclei
How does carbon 14 decay
It decays by beta - and become nitrogen again
(Yiu saying it will decay until you find material again, less nuclei means less activity, compare activity or nuckeinetc
What are TWO LIMITSTIONS TO CARBON DATING
1) make the assumption the ratio of cabrinn14 to cabrinn12 in living matter has REMAINED CONSTANT since you were dating it (it might have changed now)
2) the activists of CARBIN 14 in organic matter is similar to the background rate so hard to tell
Why can’t we use CARBIN 14 to date ROCKS
What material is needed instead
rocks are much older than life and so CARBIN 14s half life is TOO SMALL to have been in the rocks
Need a material whose half life is big enough to have been able to decayed and still be in the source
