Chapter 25 Radioactivity Flashcards
Why could cabron dating not work? (2) main reasons
And what about if the sample is too old / too young?
1) background activity is comparable to observed rate of living wood so hard to tell!
2) sample mass is still small perhaps
3) cannot guarantee the activity of the object was the same when made… could’ve have been cut and then sometime LATER MADE
1) if too old then too little activity as half life would have decayed it so much, so now probably comparable with background and not accurate
2) if too young not decayed enough and it’s difficult to measure this accurately when small time could mean none decayed at all , so hard to tell
How to calculate half life from graph?
It’s not going to be constant remember, but should be roughly the same
Best way is to find MULTIPLE half life’s, and average them out!
Remember the A = KN formula can only be used for what activity?
The activity at T = 0!!!
so if they give you the N at t = 0, then look for the activity at t=0 too to find decay constant rather than ln2 / t half
Remmeber 2 reason why gamma used instead of alpha?
Gamma used because can penetrate and is very low ionising power so won’t risk damage to patients
If alpha is in body, it’s over
Generic answer to why the half life is approptie to be used in medical imaging?(2)
Lomg enough to make measurements
Short enough so it will not stay in the patients body in large quantities for long periods of time due to decaying.
What damage does ionisations fo
Causes damage to cell and dna etc
Can cause mutations
Remember N can’t be grams so what to do to convert
Divide by molar mass x avagadro
The archaeologist obtained an estimate of the volume knowing that similar empty
flasks have an average mass of 1.5 kg and that mass of the flask and liquid was 5.2
kg. Compare the estimate that the archaeologist could obtain from these masses
with the volume calculated in part 4.3 and account for any difference.
For this q you got 3.5 litres, and they got 3,7kg, what’s the reason for differenc.e
Assuming liquid is water that means it is 3,5kg
Thus you can say the liquid is not water and the actual liquid is one if higher density than water!
Can you interchange mass for atoms of same isotope why
YES, because they are proprtinsl obviously
Why is TOTAL ACTIVITY after a sample has decayed actually bigger than caculated!
Think about the daughters
The daughters are gonna be RADIOACTIVE TOO!
So they have activity and it adds up
If they ask for charger and mass for anti neutrino in equation?
0,0
List 3 reasons why alpha more ionising than beta?
WHY?
1) more mass
2) twice charge
3) moves slower than beta at same ke
Why?
1) more mass = means more volume as same density . As a result more LIKLEY TO COLLIDE WITH ATOMS and thus remove electrons
2) MORE CHARGE = MORE CHANCE OF ATTRACTING AND RIPPING ELECTRONS OFF
3) slower means more ionisations happen in same distance!
What reasons could it be to say measuring the activity usign GM tube of like a rock not effective
- It’s half life so big count rate will be very small, and thus COMPARABLE to background rate
- daughter nuckei could be radioactive, don’t know for sure what’s coming out
HOW WOULD COUNT RATE AGAINST TIME FOR GAMMA LOOK LIKE
What about beta / alpha, and why!
VERY IMPORTANT
Count rate vs gamma is INVERSE SQAURE LAW due to intensity with EM radiation
I = p/ area, where area is 4pir2 m thus intensity is proprtinsl 1/r2. So INVERSE SQUARE LAW
2) however, alpha and beta PENETRATION POWER is too weak, as a result their activity at certain distances will be LOWER than what the inverse LAW suggests!!!
- so they DONT follow inverse sqaure !
