Lesson Notes By Weeks and Term v4 - SHS 3
NUCLEAR PHYSICS
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NUCLEAR PHYSICS
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Subject: Physics
Class: SHS 3
Term: 1st Term
Week: 1
Grade code: 2.4.2.LI.2
Strand code: 4
Sub-strand code: 2
Content standard code: 2.4.2.CS.1
Indicator code: 2.4.2.LI.2
Theme: ATOMIC AND NUCLEAR PHYSICS
Subtheme: NUCLEAR PHYSICS
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Welcome, future scientists and engineers! Today, we are delving into a fascinating part of nuclear physics: radioactive decay and half-life. You may have heard about radioactivity in the news, perhaps in relation to medical treatments at Korle Bu Teaching Hospital, or discussions about nuclear energy for Ghana. The concept of half-life is the key to understanding how we can use these powerful atomic processes safely and effectively. It helps us predict how long a radioactive material will remain dangerous and allows us to use it for dating ancient artifacts, treating diseases, and much more. This lesson will equip you with the fundamental knowledge to calculate this crucial quantity.
This topic revolves around three core ideas: Activity, Decay Constant, and Half-life. Let's break them down one by one. A. What is Radioactive Decay?
Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation (like alpha particles, beta particles, or gamma rays). This process is: Spontaneous: It happens on its own, without any external influence like temperature or pressure. Random: It is impossible to predict which specific nucleus will decay next. We can only talk about the probability of decay for a large number of nuclei.
Think of it like popping popcorn. You know that after a few minutes, many kernels will have popped, but you can't point to a specific kernel and say, "You're next!" B. Key Terms and Definitions Activity (A) Definition: The Activity of a radioactive sample is the rate at which its nuclei decay. It tells us how "radioactive" the sample is at a particular moment. Formula: `A = - dN/dt`, where `dN` is the number of nuclei that decay in a time `dt`. The negative sign indicates that the number of original nuclei (N) is decreasing. Another Formula: Activity is also directly proportional to the number of undecayed nuclei (N) present: `A = λN`, where λ is the decay constant. Unit: The SI unit of activity is the Becquerel (Bq). 1 Bq = 1 decay per second. An older unit, the Curie (Ci), is still sometimes used. `1 Ci = 3.7 x 10¹⁰ Bq`. Decay Constant (λ) Definition: The decay constant is the probability that a single nucleus will decay per unit of time. It is a constant value for a specific radioactive isotope. A large decay constant means the substance decays quickly, while a small decay constant means it decays slowly. Unit: The unit of the decay constant is `per second (s⁻¹)`, `per minute (min⁻¹)`, or `per year (yr⁻¹)`. The unit depends on the time unit being used. Half-Life (T½) Definition: The half-life of a radioactive isotope is the time it takes for half of the initial number of undecayed nuclei in a sample to decay. After one half-life, the number of radioactive nuclei is reduced by half. After two half-lives, it is reduced to a quarter (1/2 x 1/2). After three half-lives, it is reduced to an eighth (1/2 x 1/2 x 1/2), and so on.
Visualizing Half-Life: