Chapter 9: Atomic structure
The chapter problem: the hospital scanning unit
A hospital uses radioactive isotopes to look inside the body and to treat some illnesses. One isotope is a tracer. It is injected into a patient, and a detector outside the body follows where it goes.
These are the facts to use all the way through the chapter.
- Each nucleus of the tracer has 43 protons and 56 neutrons.
- The tracer emits gamma radiation.
- The half-life of the tracer is 6.0 hours.
- A freshly prepared sample has a count rate of 800 counts per second.
- The background count rate in the scanning room is 20 counts per second.
- The hospital also uses a source that destroys unwanted tissue.
- The hospital's electricity is generated by a nuclear fission reactor.
By the end of the chapter you will work out how many protons, neutrons and electrons the tracer has. You will find how its count rate falls during a day. You will explain why the tracer emits gamma radiation, why its half-life is chosen carefully, and how the hospital's reactor makes electricity. You will also decide whether a tracer scan is worth its small risk.
First contact: no working yet. Predict. A sample of the tracer is prepared at 08:00 and used at 20:00. By then, how much of the original count rate is left: none, one half, one quarter, or one sixteenth? Write your prediction down, with a reason.
If you are not sure where to start
The half-life is 6.0 hours. Count how many 6.0-hour intervals fit between 08:00 and 20:00. In each interval, the count rate halves.
By the end of this chapter
You will be able to:
- describe the structure of an atom, and name the particles inside it;
- state the atomic number, the mass number and the number of electrons of an atom or an ion;
- explain what isotopes are, and describe how the model of the atom changed;
- describe alpha, beta and gamma radiation, and write balanced decay equations;
- explain what half-life means, and use it to find a count rate or an activity;
- compare contamination with irradiation, and describe the hazard of each;
- describe background radiation, and explain why the dose a person receives varies;
- describe and evaluate the medical uses of nuclear radiation;
- describe nuclear fission and nuclear fusion, and explain where each happens.
Where it is used
Radioactive isotopes are used in medicine to image organs and to treat some cancers. They are used in industry to measure the thickness of paper and metal. A smoke alarm uses a tiny alpha source. Nuclear reactors make electricity without releasing carbon dioxide while they run. The same ideas also explain the radiation that reaches us from rocks, from space and from the air.
Your turn to connect it
Where have you met radioactivity, or a use of it, outside this book? It might be in a hospital, in a news story, or in a smoke alarm at home. Write one example. Say whether the radiation is being used to help people, or controlled as a hazard.