Online reading assignment: radioactive decay rates

Physics 205B, spring semester 2014
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on radioactive decay rates.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"I do vaguely remember the radioactive decay equation from pre-calculus. I get that the more atoms there are the older the object being tested."

"Radioactive atoms decay into daughter atoms. In one half-life, half of the radioactive atoms in a sample decay into their daughter atoms."

"You can reset the solidification age of a radioactive material if you melt it. This means it will have no daughter atoms when it resolidifies."

I liked the M&M's™ example. It really showed me how much probability plays into decay."

"I understand the average lifetime before decay and the half life and how they relate to one another."

"Didn't get much done over break."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"The toy model of a radioactive substance was a little confusing. Also I couldn't really wrap my mind around a time constant and how it could be equal to approximately the average lifetime of a single M&M™ before it decays."

"The equations will confuse me as soon as I try to use them."

"Why are the equations so different than from an exponential decay problem from calculus?"

"I don't know why the M&M's™ don't have a 0.50 half-life."

"It seems confusing to find the radioactive age of a substance. I understand the explanation but I would benefit from practice."

State the SI units for activity (radioactive decays per time).
"Probability/ second."


"SI unit for activity is the becquerel. 1 becquerel (abbreviated 'Bq') is 1 disintegration per second."

Explain what the Greek letter τ ("tau") refers to when used in radioactive decay calculations.
"Tau is the time constant, which is 1/λ. It is the average time that a nucleus survives before decaying."

What is the mathematical relationship between τ and half-life for radioactive decays?
"They are inverses of each other."

"T1/2 = τ·ln(2)."

"I'm not sure. I'm confused on this."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I'm radioactive, radioactive." (Is this what you kids listen to these days? Or did you mean this video?)

"Yes...M&M's™. Nom nom nom."

"What did you do for spring break?" (I managed to both impress Mrs. P-dog and avoid killing myself...USING PHYSICS.)

"I think after I see some sample problems I will be okay with this section."

"What is decay referring to? When a nucleus decomposes and splits?" (That sounds like fission. A decay is just a conversion of a nucleus to a slightly more stable form, usually by converting or ejecting protons and/or neutrons, as we'll see in the next class.)

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