· 7.13 describe the uses of radioactivity in medical and non-medical tracers, in radiotherapy and in the radioactive dating of archaeological specimens and rocks
<<Uses of radioactivity.pptm>>
Medical Tracers
Radioactive Dating
<<DJFPh106carb2.swf>>
<<DJFPh108dating5.swf>>
1. What happens to the amount of ‘mother’ nuclei as time passes?
Decrease
2. What sort of radioisotope will decay the fastest - one with a long half life or one with a short half life?
One with a short half life
3. Does half life tell us exactly when a particular nucleus in a radioisotope will decay?
No. It is a random process - we can not know exactly when a particular nucleus will decay
4. What are the two definitions of half life?
a. The time taken for the activity of a sample to halve
b. The time taken for the number of radioactive atoms in a sample to halve
5. What does the activity of a source mean?
The number of decays per second
6. What is the unit of activity?
Becquerel (Bq)
7. What will happen to the number of ‘mother’ nuclei after two half lives?
1 è ½ è ¼
1xhalf life 2xhalf life
The number of mother nuclei will be ¼ of the original number
8. What will happen to the activity of a source after two half lives?
1 è ½ è ¼
1xhalf life 2xhalf life
The activity will be ¼ of the original activity
7.10 to 7.12 calculation answers
12 January 2012
10:24
1. A radioisotope has a half life of 12 years. What fraction of the radioisotope will be left after 60 years?
Fraction remaining:
1 è ½ è ¼ è 1/8 è 1/16 è 1/32
1xhalf life 2xhalf life 3xhalf life 4xhalf life 5xhalf life
12 years 24 years 36 years 48 years 60 years
2. If the activity of a sample falls to 1/64th of its original level after 2 hours, what is the half life of the sample?
1 è ½ è ¼ è 1/8 è 1/16 è 1/32 è 1/64
1xhalf life 2xhalf life 3xhalf life 4xhalf life 5xhalf life 6xhalf life
Decay takes 2 hours (= 120 minutes)
This is 6 half lives
So 1 half life = 120 minutes / 6 half lives = 20 minutes
3. The background radiation in a laboratory is 7 Bq. The count rate from a radioisotope is measured and it has a reading of 119 Bq. If the half life of the radioisotope is 10 minutes, what will be the reading 20 minutes later?
Initial count rate at detector = 119 Bq
But Background rate = 7 Bq
So activity of radioisotope = 119 – 7 = 112 Bq
1 è ½ è ¼
1xhalf life 2xhalf life
10mins + 10mins = 20mins
112Bq è 56Bq è 28Bq = activity of radioisotope after 20 mins
But this doesn’t include the background rate!
Detector reading = 28Bq + 7Bq = 35Bq
4. Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock?
Proportion of K: 1 è ½ è ¼
Proportion of Ar: 0 è ½ è 3/4
1xhalf life 2xhalf life
So after 2 half lives there will be 3 times as many Ar atoms as K atoms
Age of rocks = 2 x half life = 2 x 1.3x109 = 2.6 billion years
starter
01 February 2012
13:08
Tell the person next to you…
· What is the most dangerous type of radiation outside the body? Why?
· What is the most dangerous type of radiation inside the body? Why?
· Gamma radiation is the most dangerous type of radiation outside the body because it is extremely penetrating. (Alpha radiation is not dangerous because it is stopped by skin)
· Alpha radiation is the most dangerous type of radiation inside the body because it is highly ionising due to its high charge (+2) and high mass (RAM=4). (Gamma radiation is only very weakly ionising)
7.14
12 January 2012
10:24
· 7.14 describe the dangers of ionising radiations, including:
· radiation can cause mutations in living organisms
· radiation can damage cells and tissue
· the problems arising in the disposal of radioactive waste
<<Dangers of radioactivity.pptm>>
Chernobyl Disaster
Hiroshima and Nagasaki - the first nuclear bombs
After the Hiroshima bomb
· Smoke detectors use 241Am to emit alpha particles which pass through a small air gap before being detected. If smoke particles are present they interrupt the beam of alpha particles and this triggers the alarm to go off
· Tomorrow, will the 241Am still be as radioactive?
· Next year, will the 241Am still be as radioactive?
· In a thousand years, will the 241Am still be as radioactive?
Answers
· To answer the questions, we need to know the half life of Americium-241 which is 432 years
· Tomorrow and even next year its activity will hardly have changed at all (sensible for a smoke detector - you don't want it to suddenly stop working!)
· In a thousand years its activity will have dropped to about a quarter
7.10 to 7.12
12 January 2012
10:24
· 7.10 understand that the activity of a radioactive source decreases over a period of time and is measured in becquerels
· 7.11 recall the term ‘half-life’ and understand that it is different for different radioactive isotopes
· 7.12 use the concept of half-life to carry out simple calculations on activity
<<Half life.pptx>>
<<interactive simple half life calculations.swf>>
<<Decay of Balonium - exponential graph.swf>>
7.10 to 7.12 questions
01 February 2012
09:46
1. What happens to the amount of ‘mother’ nuclei as time passes?
2. What sort of radioisotope will decay the fastest - one with a long half life or one with a short half life?
3. Does half life tell us exactly when a particular nucleus in a radioisotope will decay?
4. What are the two definitions of half life?
5. What does the activity of a source mean?
6. What is the unit of activity?
7. What will happen to the number of ‘mother’ nuclei after two half lives?
8. What will happen to the activity of a source after two half lives?
7.10 to 7.12 calculation questions
12 January 2012
10:24
1. A radioisotope has a half life of 12 years. What fraction of the radioisotope will be left after 60 years?
2. If the activity of a sample falls to 1/64th of its original level after 2 hours, what is the half life of the sample?
3. The background radiation in a laboratory is 7 Bq. The count rate from a radioisotope is measured and it has a reading of 119 Bq. If the half life of the radioisotope is 10 minutes, what will be the reading 20 minutes later?
4. Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock?
· 7.6 describe the effects on the atomic and mass numbers of a nucleus of the emission of each of the three main types of radiation
· 7.7 understand how to complete balanced nuclear equations
<<Balanced nuclear equations.pptx>>
| |||
| <<alpha decay of Am241 animation.swf>> | ||
| Did you spot the deliberate mistake on this animation? | ||
|
<<beta decay of C14 animation.swf>>
Answer
The symbol for Neptunium is Np not NP!
7.6 and 7.7 Plenary
12 January 2012
10:24
<<interactive alpha and beta decay eqns.swf>>
<<Balanced nuclear equations plenary mulichoice question.pptx>>
