P4 Half-life (page 198) Flashcards
How quickly unstable nuclei decay is measured by?
using activity and half-life. Two very important terms
Radioactiv substances give out what?
radiation from the nuclei of their atoms (no matter what).
What is radiation from the nuclei measured with
a Geiger-Muller tube and counter.
What do the Geiger-Muller tube and counter record?
the count-rate - the number of radiation counts reaching it per second.
Radioactive decay is entirely random. What do this mean?
you can’t predict exactly which nucleus in a sample will decay next, or when any one of them will decay.
Can you find out the time it takes for the amount of radiation emitted by a source to halve? Explain why?
Yes. This is known as the half=life. It can be used to make predictions about radioactive sources, even though their decays are random.
What can Half-life be used to fine?
the rate at which a source decays - its ACTIVITY.
Activity measured in what unit?
it is measured in Becquerels, Bq (where 1 Bq is 1 decay per second).
Do the radioactivity of a source Increase or decrease over time?
Decrease.
Each time a radioactive nucleus decays to become a stable nucleus, what happens to the activity?
as a whole it will decrease. (older sources emit less radiation).
How long do Isotopes take before all the ustable nuclei have decayed?
For soe isotopes it takes just a few hours before nearly all the unstable nuclei have decayed, whist others last for millions of years.
What is the problem with trying to measure isotopes decaying?
the problem is that the activity never reaches zero, which is why we have to use the idea of half-life to measure how quickly the activity drops off.
What is meant by half-life?
it is the time taken for the number of radioactive nuclei in an isotope to halve.
Half-life can also be described as what?
as the time taken for the activitty (and so count-rate) to fall to half of its initial value.
The initial activity of a sample is 640 Bq. Calculate the final activity as a percentage of the initial activity after two half-lives?
1) Find the activity after each half-life
half-life: 640 ÷ 2 = 320.
2) Now divide the final activity by the initial activity, then multiply by 100 to make it a percentage.
half-lives: 320 ÷ 2 - 160
(160 ÷ 640) x 100 = 0.25 x 100 = 25%
(always double check what the question is asking for - it may want a fraction, ratio or a percentage).
Can you measure Half-life using a Graph?
Yes
If you plot a graph of activity against time, what will happen?
it will always be shaped like the one on page 198.
How is the half-life found from the graph? (see graph on page 196)
by finding, the time interval on the bottom axis coresponding to the halving of the activity on the verticle axis.
Half-life is…..
the time for the number of radioactive nuclei, the activity or the count-rate to halve.
The initial count-rate of a sample is 40 cps. Show that the ratio of its final count rate to its initial count rate is 1:8 after three half-lives (3 marks).
After one half-life the count-rate will be
40 ÷ 2 = 20 cps (1 mark).
After a second: 20 ÷ 2 = 10 cps
After a third@ 10 ÷ 2 = 5 cps (1 mark)
so the ratio is 5:40 = 1.8 (1 mark).
