Ionizing Radiation From The Nucleus
Resumen
Put a radiation counter on a bench in an empty room, switch it on and walk away. It clicks. Not steadily, and not because anything in the room is broken: a click now, two more a second later, then a pause, then another. Those clicks have been arriving since long before anyone built a counter to hear them, and they are coming from the bricks in the wall, from the food in your bag, from the air you are breathing and from the depths of space. Somewhere inside each of those sources, a nucleus that had held together for a million years chose this particular second to fall apart, and nothing that has ever been done to it changed that choice by a fraction of a second.
This lesson is about that behaviour and about how physicists learned to predict it without ever being able to predict any single event. You will find out what an unstable nucleus throws out and why, how to balance a decay equation using nothing harder than subtraction, how a sheet of paper stops one kind of radiation while several centimetres of lead only weaken another, and how a quantity called half-life turns a completely random process into something a hospital can schedule around. By the end you will be able to look at a table of isotopes and say which one belongs in a smoke alarm, which one belongs in a patient and which one nobody should let near either.
Objetivos
- Some atomic nuclei are unstable. The nucleus emits particles or radiation and the nucleus changes to that of a different element and becomes more stable. This is a random process called radioactive decay.
- Energy is emitted by changes in the nucleus.
- Unstable nuclei emit alpha particles, beta particles, or neutrons, and electromagnetic radiation as gamma waves. Neither chemical nor physical processes affect this behaviour. These substances are said to be radioactive and although the general process follows a pattern this radioactive decay is a random process. It is impossible to predict when a particular atom might decay.
- Background radiation is around us all of the time. It comes from a range of sources, such as radioactive substances in the environment, from space or from devices such as X-ray machines in hospitals.
- An alpha particle consists of two neutrons and two protons (ie a Helium nucleus). A beta particle is a high speed electron ejected from the nucleus as a neutron turns into a proton. Gamma radiation is electromagnetic radiation from the nucleus.
- Nuclear equations are used to represent radioactive decay. Students will be required to balance equations for single alpha and beta decay, limited to the completion of atomic number and mass number. The identification of daughter elements from such decays is not required.
- Properties of the alpha, beta and gamma radiations are limited to their relative ionising power, their penetration through materials and their range in air.
- Radioactive decay is random, but with a large enough number of nuclei it is possible to predict how many will decay in a certain amount of time. The half-life of a radioactive isotope is: the average time it takes for the number of nuclei of the isotope in a sample to halve; the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.
- Radioactive contamination is the unwanted presence of radioactive atoms on other materials. The hazard from contamination is due to the decay of the contaminating atoms. The type of radiation emitted affects the level of hazard. Irradiation is the process of exposing an object to ionizing radiation. The irradiated object does not become radioactive. Suitable precautions must be taken to protect against the hazards of the radioactive source used in irradiation. Students should be able to compare the hazards associated with contamination and irradiation.
- Radioactive isotopes have a very wide range of half-life values. The most unstable nuclei have the shortest half-lives; decay is rapid with a lot of radiation emitted in a short time. The least unstable nuclei have the longest half-lives; hey emit little radiation each second but emit radiation for a long time. There are uses and dangers associated with each type of nuclear radiation. Students should be able to evaluate the possible hazards associated with the use of different types of ionizing radiation and the effect of half-life.
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Nota de la lección
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- What is an alpha particle? A. A high speed electron from the nucleus B. Two protons and two neutrons C. Electromagnetic radiation from the nucleus D. A neutron ejected from the nucleus Answer: B
- A nucleus has mass number 210 and atomic number 84. It emits an alpha particle. What are the mass number and atomic number of the nucleus produced? A. Mass number 206, atomic number 82 B. Mass number 210, atomic number 85 C. Mass number 206, atomic number 86 D. Mass number 209, atomic number 83 Answer: A
- Which radiation has the greatest ionising power and the shortest range in air? A. Alpha B. Beta C. Gamma D. All three are the same Answer: A
- Which statement about irradiation is correct? A. The irradiated object becomes radioactive B. The hazard continues after the source is removed C. The object is exposed to radiation but does not become radioactive D. Radioactive atoms are transferred onto the object Answer: C
- The corrected count rate from a sample falls from 800 counts per minute to 100 counts per minute in 9 hours. What is the half-life of the isotope? A. 1.5 hours B. 3 hours C. 4.5 hours D. 9 hours Answer: B
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