I disagree with my biology teacher

[Shellman035]

What is the meaning of "radioactive decay"?

What is the proper use of the meaning of "radioactive decay"? My teacher states it is the amount of decay that has occurred and I think it is a measurement of the "rate" of decay while it is being measured. I have re-phrased this question from an earlier post which gives the specifics of the disagreement. I know the definition of the term but what is its usage,,,, Who's right?


To whom it may concern:
I am having a debate with my biology teacher. I was asked the following multiple choice question:

Question: Radioactive isotopes are used in dating materials from the distant past. Which of the following statements is accurate about radioactive dating techniques?

Answer:
1. It gives relative ages of rock strata.

2. It gives exact ages of rock strata.

3. It uses a technique in which the degree of radioactive decay is measured, the younger the rock the more radioactive decay.

4. It uses a technique in which the degree of radioactive decay is measured, the older the rock the more radioactive decay.

5. It does not work well with fossil remains that have not absorbed radiation because scientists cannot measure the radiation with any degree of certainty.


This is the question exactly as it was posed on the test. I picked answer 3. and got it wrong. The teacher states the corect answer is 4. I think he is wrong. The accepted use of the term "radioactive decay" is not how much decay HAS occurred but rather how much decay is going on when it is being measured. There is no such device as a "Decay-o-meter" and his use of the term "radioactive decay" is in error. He has improperly phrased the answer and no reference to "past-tense" is infered Who's right in this matter? Please help.
Thanks
Todd

 

[arunma]

Well, as things get older, the number of radioisotopes decrease. And fewer radioisotopes means fewer radioactive decays. Assuming that I'm understanding this definition of radioactive decay correctly, it looks to me like the answer is #4.

Then again, #2 would also be a good choice, assuming that the phrase "exact ages" allows for the fact that all scientific data have an associated error.

 

[Shellman035]

I am hung up with the first part of the sentence which states it is being measured. Therefore, wouldn't the measured rate of decay be less the older the rock gets?

 

[Shellman035]

What is the proper use and meaning of "Radioactive decay"? How much decay HAS occured? or how much decay IS occurring?

 

[Crosson]

The teacher is right, and in the context of the problem #4 is the best answer. I would call your concept "the rate of radioactive decay".

 

[cesiumfrog]

Y'know, you could have just got your English teacher to explain this one.

 

[Gokul43201]

The teacher is WRONG. And that's only because the wording of choices 3 and 4 is ambiguous, and from a physics point of view, gibberish.

The teacher is applying the meaning that (for #4) the greater the age the more the object has decayed. You have interpreted it to say (in the case of choice #3) that the smaller the age, the greater is the decay rate. Of these two interpretations, I'd pick yours any day over the teacher's (which would be the better interpretation if this were a layman conversation, not a science question). Think about what the teacher's interpretation means and you'll see that were the question intended that way, choice #4 would be trivially true, and the question would not, in any way, test your knowledge of science. On the other hand, your interpretation requires the student to understand that the rate obeys first order kinetics, or that it is porportional to the amount of undecayed stuff left. That's a question that tests understanding of a scientific concept.

Besides, any measurement of radioactivity only measures the decay rate, from which amounts that have decayed are calculated. This alone is sufficient to show (reading carefully, the first part of the choices 3,4) that the OP is correct and the teacher wrong.

 

[Andrew Mason]

The teacher is WRONG. And that's only because the wording of choices 3 and 4 is ambiguous, and from a physics point of view, gibberish.

The teacher is applying the meaning that (for #4) the greater the age the more the object has decayed. You have interpreted it to say (in the case of choice #3) that the smaller the age, the greater is the decay rate. Of these two interpretations, I'd pick yours any day over the teacher's (which would be the better interpretation if this were a layman conversation, not a science question). Think about what the teacher's interpretation means and you'll see that were the question intended that way, choice #4 would be trivially true, and the question would not, in any way, test your knowledge of science. On the other hand, your interpretation requires the student to understand that the rate obeys first order kinetics, or that it is porportional to the amount of undecayed stuff left. That's a question that tests understanding of a scientific concept.

Besides, any measurement of radioactivity only measures the decay rate, from which amounts that have decayed are calculated. This alone is sufficient to show (reading carefully, the first part of the choices 3,4) that the OP is correct and the teacher wrong.

I would have to disagree.

As I understand radiometric dating, the measurement of age of a rock is not determined by its present radioactivity but by the relative concentrations of different nuclei. A rock with a high concentration of U will be more radioactive than a rock with a low concentration of U but the first may be much older than the latter. This relative proportion of nuclei is a measure of how much decay has occurred since the rock crystals were formed.

In Uranium-Lead dating, for example, Zircon crystals incorporate U when formed but not lead. So the Pb present in the Zircon is from the decay of U to Pb. By measuring that proportion of U to Pb (not the absolute amount of U or Pb or its present radioactivity) we can determine how long it has been since the Zircon crystals were formed.

AM

 

[Panda]

I agree with Gokul - The 1st reading of the question I picked out 4, then reread 3 to see why it might be right.

Of course I score very low in aural attention
e.g. "Your driving a bus at an average of 30mph, at the 1st stop 3 people get on 2 people get off, at the 2nd stop 4 people get on 1 person gets off, What is the name of the driver?"
Always gets me...

I would say that whilst 3 maybe correct the wording of the question allows for ambiguity so unless it is testing your skills at interpretation and grammer it is a poor question.

Award yourself an honoury A+ and develop a deep rooted social problem based around this injustice. I did, never did me any harm.

 

[Sojourner01]

The question is ambiguous. All those answers can be correct based on perfectly reasonable and correct interpretations of the term 'radioactive decay'. The term has been improperly used in this question - radioactive decay is a process, you don't have an 'amount' of it, that doesn't make any sense.

 

[vanesch]

I would have to disagree.

As I understand radiometric dating, the measurement of age of a rock is not determined by its present radioactivity but by the relative concentrations of different nuclei. A rock with a high concentration of U will be more radioactive than a rock with a low concentration of U but the first may be much older than the latter. This relative proportion of nuclei is a measure of how much decay has occurred since the rock crystals were formed.

In Uranium-Lead dating, for example, Zircon crystals incorporate U when formed but not lead. So the Pb present in the Zircon is from the decay of U to Pb. By measuring that proportion of U to Pb (not the absolute amount of U or Pb or its present radioactivity) we can determine how long it has been since the Zircon crystals were formed.

AM

I tend to agree with Andrew on this one, although I also agree with Gokul that the least one can say, is that this question is badly and ambiguously phrased.

Indeed, what matters is the amount of decay that occurred from an initial amount of "unstable" material. So the necessary quantity to be measured is "what fraction has decayed" independently of how much there is overall (and hence what is the actual decay rate, which is only dependent on the total amount of parent material present, and is not a function of how old the stuff is for equal amounts of final parent product). Usually, the "current decay rate" is almost unmeasurable for very long-lived isotopes, and it is a chemical analysis which indicates how much decay product has formed. So the more decay product has formed, AS COMPARED TO AMOUNT OF PARENT NUCLEUS that is somehow estimated (that's the difficult part!), the older the rock.

 

[russ_watters]

I'm going to say that I don't think that answers 3 and 4 are poorly phrased at all. Gokul, you threw the word "rate" in there, but that word isn't there in either usage and the context doesn't imply it should be (more on that below). The words "degree of radioactive decay" is another way of saying "the more the object has decayed". Key word: degree. A degree is not a rate, it is a discrete point on a scale. http://dictionary.reference.com/browse/degree

1. any of a series of steps or stages, as in a process or course of action; a point in any scale.
2. a stage or point in or as if in progression or retrogression: We followed the degrees of her recovery with joy.
3. a stage in a scale of intensity or amount: a high degree of mastery.
4. extent, measure, scope, or the like: To what degree will he cooperate?
5. a stage in a scale of rank or station; relative standing in society, business, etc.: His uncouth behavior showed him to be a man of low degree.
6. Education. an academic title conferred by universities and colleges as an indication of the completion of a course of study, or as an honorary recognition of achievement.
7. a unit of measure, as of temperature or pressure, marked off on the scale of a measuring instrument: This thermometer shows a scale of degrees between only 20° and 40° C.
[there are other irrelevant defintions]

Now s/he left the word "degree" off the second time s/he used it, but since it is still in the same sentence, it is reasonable to expect people to consider the usage to be the same.

In fact, since an understanding of how radioactive dating works precludes the use of a rate in either case, the word "degree" isn't even required the first time. The words "radioactive decay" can mean either a rate or a total, depending on the context - and in this context, it is always a total.

The only quibble I have with the question is that I don't see what is wrong with answers 1&2 - both look correct to me. But if I can only pick one, it is clearly answer #4 based on how specific it is.

 

[Gokul43201]

Okay, admittedly, I'm completely wrong on what is the best way to interpret "degree of decay". But, in that case, the question is silly and translates to: 'Given that something is decaying with time, does it take more time to have decayed more?'

Russ, I threw the word "rate" into the interpretation to help the question qualify as science.

And then there's this:

1. http://www.google.com/search?hl=en&q=phys+rev+decay+radioactive&btnG=Search
Nearly 400,000 hits on Google for search terms: phys+rev+decay+radioactive

2. http://www.google.com/search?hl=en&q=phys+rev+"degree+of+decay"+radioactive&btnG=Search
5 hits on Google for search terms: phys+rev+"degree of decay"+radioactive

Dictionary definitions are virtually useless in real science. All that matters is how the scientific community defines a term (not how Merriam-Webster does).

 

[russ_watters]

But, in that case, the question is silly and translates to: 'Given that something is decaying with time, does it take more time to have decayed more?'

Well, it is high school... :tongue:

Russ, I threw the word "rate" into the interpretation to help the question qualify as science.

Yeah, I know - absent other contextual clues, there really are two different ways to interpret the words "radioactive decay".

And then there's this:

Dictionary definitions are virtually useless in real science. All that matters is how the scientific community defines a term (not how Merriam-Webster does).

Yes, a geologist likely would have worded the question differently.

 

[billiards]

I see the ambiguity, although I think it has been established that #4 is the correct answer. I found this link in the Earth sciences forum, it basically explains the methods of radiometric dating to a layman.

http://www.asa3.org/aSA/resources/Wiens.html

 

[arunma]

I am hung up with the first part of the sentence which states it is being measured. Therefore, wouldn't the measured rate of decay be less the older the rock gets?

I see your point, and I confess that the question was poorly worded. As others have suggested, this seems to be a problem for English majors and grammar nazis rather than physicists.

Let me just say this: it is true that older objects will tend to be less radioactive. But ultimately, the age is determined by the ratio of C-14 to other isotopes of carbon. I haven't taken English since high school (yes, I even got out of Freshman comp in college), so I won't say anything beyond that for fear of confusing you.

 

[Andrew Mason]

I see your point, and I confess that the question was poorly worded. As others have suggested, this seems to be a problem for English majors and grammar nazis rather than physicists.

Let me just say this: it is true that older objects will tend to be less radioactive. But ultimately, the age is determined by the ratio of C-14 to other isotopes of carbon. I haven't taken English since high school (yes, I even got out of Freshman comp in college), so I won't say anything beyond that for fear of confusing you.

Carbon 14 dating is only used to date living tissue and it does not work for geological time. It works because we know what the relative proportion of Carbon 14 to carbon 12 is when the organism dies (thereafter no new C14 is taken up) and we know what the C14 half life is so we can determine how long it has been since death occurred.

For rocks, it is not so easy. And it is not necessarily true, but it is generally true, that radioactivity decreases with age. An exception would be if there is natural fission - which has occurred at least once in Earth's history.

AM

 

[cincirob]

I disagree with your teacher also.

It's pretty clear that you understand the idea of radioisotope dating and that is what should count. Unfortunately your teacher chooses to use multiple choice questions rather than essay. This is a poor choice. The ability to explain concepts should be the goal of a science course and that is what you should be asked to do. Writing a number of sound-alike answers to try to trick you is a poor educational device.

 

[vanesch]

Unfortunately your teacher chooses to use multiple choice questions rather than essay. This is a poor choice.

I agree with you in principle, but from the teacher's PoV, multiple choice questions have two big advantages:
1) it grades quickly and easily (right/wrong)
2) there shouldn't be any discussion about poorly worded half maybe correct essays.

Now, there's nothing worse than a confusingly worded multiple choice question, as in this case.

From the student PoV, it is of course a bad and horrible technique, but it is not the student who decides

 

[Chaos' lil bro Order]

I think answer #4 is most correct

#4. 'It uses a technique in which the degree of radioactive decay is measured, the older the rock the more radioactive decay.'

To me when he says the 'degree of radioactive decay', he means the total aggregate decay that has happened in the rock. Older rocks on average must have had more decays than newer rocks, relatively of course. As someone pointed out earlier, its still a very real possibility that the newer rock has undergone more decays than the older rock, since the newer rock could have started out with higher concentrations of decaying nuclei than the concentration of decaying nuclei that the older rock did. But, to me the question sort of implies an 'on average' or 'relative to' hidden clause in it. So this is why I think #4 is the best answer to a poorly worded question.

The real question is why this teacher phrases questions like he's writing an english exam; they are so open to interpretation and filled with airy fluff that they are sure to confuse his students. It seems inappropriate to have a pretty rigid scientific discipline such as chemistry, treated with a not so cut-n-dry question.

my two cents

 

[Shellman035]

Thank you everyone for all the feedback, I still would like to know in the context of the question,, What is the accepted meaning and usage of the term "radioactive decay" Does it most often refer to present tense or past tense?
Also, I believe one reply stated that both could be measured, I would like to know these procedures. I don't know of any device that can measure the amount of decay that has occured. Only that a "current" measurement can be take of the present "rate" of decay and the amount of decay that has taken place is a math formula based on this present measurement. Further if it has already decayed (past-tense) how can it be measured. there is nothing left to measure.
I understand this is an argument left to one's own interpretation of the meaning of "radioactive decay" and that there are two ways of defining it, but I am used to the term referring to a "present rate of decay" and interpreted the question as such. this is why I'm trying to see what the "common usage" is.

By the way this isn't high school, it's college biology 182, and I'm trying to convince my bio teacher to rephrase a question that obviously has more than one interpretation. This is an on-line course and this question will surface semester after semester for all future students who take this class unless I can point out the error of phrasing a question such as this.
It sucks to get an answer wrong when you have a good conception and knowledge of how it works.

Thanks again everyone, as a new user to forums I'm still learning the rules but enjoy it immensly
Todd

 

[disregardthat]

First I would have picked number 4.
But after intense reading I think i'd go with three. Since it can be interpreted as the amount of radioactive decay measured.
I hate tricky questions! :yuck:

 

[Andrew Mason]

Thank you everyone for all the feedback, I still would like to know in the context of the question,, What is the accepted meaning and usage of the term "radioactive decay" Does it most often refer to present tense or past tense?
Also, I believe one reply stated that both could be measured, I would like to know these procedures. I don't know of any device that can measure the amount of decay that has occured. Only that a "current" measurement can be take of the present "rate" of decay and the amount of decay that has taken place is a math formula based on this present measurement. Further if it has already decayed (past-tense) how can it be measured. there is nothing left to measure.
I understand this is an argument left to one's own interpretation of the meaning of "radioactive decay" and that there are two ways of defining it, but I am used to the term referring to a "present rate of decay" and interpreted the question as such. this is why I'm trying to see what the "common usage" is.

By the way this isn't high school, it's college biology 182, and I'm trying to convince my bio teacher to rephrase a question that obviously has more than one interpretation. This is an on-line course and this question will surface semester after semester for all future students who take this class unless I can point out the error of phrasing a question such as this.
It sucks to get an answer wrong when you have a good conception and knowledge of how it works.

Thanks again everyone, as a new user to forums I'm still learning the rules but enjoy it immensly
Todd

"Devices" don't measure the amount of decay that has occurred directly. Devices measure relative populations of different nuclei (isotopes and elements). Working out the age is then done on the basis of a scientific theory.

For example, if a type of rock crystal contained Uranium but no lead when it was formed and if the crystal contains U and Pb now, since U decays ultimately to Pb, we know that the Pb is entirely from the decay of U nuclei. We can determine from the relative proportion of Pb to U how much of the original U has decayed since the rock was formed. Since we know the rate of decay, we can determine the time that it took for that decay to occur. And that gives us the age. It is a bit complicated because there were several isotopes of U to begin with*, each of which have different half-lives, but it can be calculated pretty accurately.

Measuring the present radioactivity or "rate of radioactive decay" of the U in the crystal does not tell us anything. The concentration of U could have been very high or very low to begin with. What we need to know is how much of what was originally present has decayed.

So, in the context of determining the age of rocks, "degree of radioactive decay" has only one interpretation that makes sense. Although "degree of radioactive decay" in another context could refer to the amount of radioactivity present, it does not fit here. This is why #4 is the correct answer and not #3.

AM

* The Earth's U was formed about 5 billion years ago (likely from a super nova somewhere in our galaxy), with a much higher proportion of U235, U234 than presently exists. Over time, the proportion of U235 and U234 has declined because they have shorter half-lives than U238. This has to be factored into the age calculation.

 

[pivoxa15]

What is the proper use of the meaning of "radioactive decay"? My teacher states it is the amount of decay that has occurred and I think it is a measurement of the "rate" of decay while it is being measured. I have re-phrased this question from an earlier post which gives the specifics of the disagreement. I know the definition of the term but what is its usage,,,, Who's right?To whom it may concern:
I am having a debate with my biology teacher. I was asked the following multiple choice question:

Question: Radioactive isotopes are used in dating materials from the distant past. Which of the following statements is accurate about radioactive dating techniques?

Answer:
1. It gives relative ages of rock strata.

2. It gives exact ages of rock strata.

3. It uses a technique in which the degree of radioactive decay is measured, the younger the rock the more radioactive decay.

4. It uses a technique in which the degree of radioactive decay is measured, the older the rock the more radioactive decay.

5. It does not work well with fossil remains that have not absorbed radiation because scientists cannot measure the radiation with any degree of certainty.


This is the question exactly as it was posed on the test. I picked answer 3. and got it wrong. The teacher states the corect answer is 4. I think he is wrong. The accepted use of the term "radioactive decay" is not how much decay HAS occurred but rather how much decay is going on when it is being measured. There is no such device as a "Decay-o-meter" and his use of the term "radioactive decay" is in error. He has improperly phrased the answer and no reference to "past-tense" is infered Who's right in this matter? Please help.
Thanks
Todd

Just from looking at the question and the option. I think 1 and 4 could be right although 4 looks the best as often in multiple choice quesitons more than more answer could be correct and the student is told to choose the best one. A physicist may think about the decay rate of products when thinking about radioactivity but from the question alone, only the word radioactive decay is used. 'degree' is used to describe how much of radioactive decay has occured. You could think of degree as how 'significant' is the decay (decay=loss of protons) which give the impression of how much (in absolute amount) the number of proton turning to neutrons and beta particles that has occurred (not the rate at which it is happening). Clearly the older the rock, the more decay has happened hence less radioactive substance is available. So 4.

 

[billiards]

As a geophysicist I can tell you both (broadly speaking: as in measuring the rate & the record of decay) methods involved with radioactive decay.

The one that your question concerns is called radiometric dating, this is where you measure the ratio of the parent to the daughter. First understand that a rock is made from minerals, and minerals are made from elements. If the mineral is composed of elements that decay then its composition will change with time, the parent decays to the daughter. Basically the more daughter there is relative to the parent then the older the sample. If you know the half-life and the daughter/parent ratio you can work out the age of the sample.

The other method will measure the rate of decay, for example gamma ray spectroscopy will measure the number of gamma rays detected per unit time. This technique is often used in borehole geophysics, because the gamma ray count tells you about the concentration of radioactive elements in the rock below, you can use the gamma ray count to log the bore hole and hence tell you something about the stratigraphic sequence. It works because different layers of rock have different chemistry - this method tells you about the geology, but it does not tell you anything about the age.

 

[pivoxa15]

Here is an extract from Halliday, Resnik, Walker

The radionuclide carbon-14 is produced at a constant rate in the upper atmosphere. Through biological activity such as photosynthesis and breathing, the atoms of atmospheric carbon trade places with the carbon atoms in every living thing. Eventually an exchange of equilibrium is reached at which the carbon atoms of every living thing contain a fixed small fraction of the C-14. This equilibrium persists as long as the organism is alive. When the organism dies, the exchange with the atmosphere stops and the amount of radiocarbon trapped in the organism dwindles (radiates via beta decay) away with a half-life of 5730y. By measuring the amount of radiocarbon per gram of organic matter, it is possible to measure the time that has elapsed since the organism died.

The older the fossil, the less amount of radiocarbon per gram. Or the higher the degree of radioactive decay = the higher the loss of radionuclides.

 

[vanesch]

Here is an extract from Halliday, Resnik, Walker

The radionuclide carbon-14 is produced at a constant rate in the upper atmosphere. Through biological activity such as photosynthesis and breathing, the atoms of atmospheric carbon trade places with the carbon atoms in every living thing. Eventually an exchange of equilibrium is reached at which the carbon atoms of every living thing contain a fixed small fraction of the C-14. This equilibrium persists as long as the organism is alive. When the organism dies, the exchange with the atmosphere stops and the amount of radiocarbon trapped in the organism dwindles (radiates via beta decay) away with a half-life of 5730y. By measuring the amount of radiocarbon per gram of organic matter, it is possible to measure the time that has elapsed since the organism died.

The older the fossil, the less amount of radiocarbon per gram. Or the higher the degree of radioactive decay = the higher the loss of radionuclides.

Yes, however, C-14 dating is special, because you can consider fossil organic material as "essentially" carbon, so you KNOW the initial amount of carbon, and by hypothesis of constant fraction of C-14 in a living thing, the initial amount of mother nucleus. Moreover, as the half life of C-14 is relatively short, and hence its activity relatively high, one can measure directly the rate of radioactive decay (and hence the absolute amount of mother material present).

In many other radioactive dating systems, one doesn't really know a priori the INITIAL amount of mother nucleae: this is one of the quantities to be estimated by one or other method. So ONLY knowing the total amount of mother material (which is what could be measured by measuring the RATE of decay) doesn't tell us anything about the age of the sample, by itself. One needs to know both the initial amount of mother material, AND the final amount of, or, daughter material or mother material. With very low activities, I don't think it is practical to try to measure the amount of mother material in the end by measuring the rate of radioactive decay (unless there's really quite a lot of it). It is much more practical to measure the final amounts of daughter material AND of mother material, because that can be done by chemical analysis. Assuming that the initial quantity was pure mother material (or came in a known initial proportion of daughter and mother material), one can then estimate the age of the sample.

 

[loom91]

Radioactive decay is a process, not a quantity. But I think the teacher should be given the benefit of doubt because the option first uses the term degree of radioactive decay which is unambiguous.

In any case, high school questions (at least where I live) are usually far more ambiguously worded than that and to score marks one must not write a correct answer, but the particular wrong answer favoured by the examiner (of course, different teachers prefer different wrong answers). By wrong, I mean answers that have been known to be absurdly wrong for at least a decade (that can go up to half-a-century). Arguments with them will result in nothing better than getting you in the teacher's bad book.

I'm not exaggerating, this is the state of high-school (and college) education here. In all but a handful of institutions in the whole country that's how things work. Teachers teach (when they bother to do that), don't learn.

Molu

 

[Chaos' lil bro Order]

I think pivoxa's C-14 decay and Andrew Mason's Uranium decay example are excellent summaries of two common decays.

As a side note, does anyone know how the supposed meteorite that crashed into the Gulf of Mexico and killed the dinosaurs was dated? I vaguely recall from a year one University astronomy class that the explosion sent a layer of fine dust up into the atmosphere that settled all around the Earth due to wind currents. Its a cm layer of dust found in the strata of the Earth's near surface called 'Gabio clay'. I am very sketchy on the details as you can see, but I think the theory was that the impact was so hot that it melted sand (?) and turned it into glass, which can be seen in the gabio clay layer.

That said, were there radioactive nuclei in the clay that allowed them to date it, or is it a depth of its location in the crust's strata that determines the dating estimate?

 

[Gokul43201]

Thank you everyone for all the feedback, I still would like to know in the context of the question,, What is the accepted meaning and usage of the term "radioactive decay" Does it most often refer to present tense or past tense?

Don't worry too much about the question itself - it was badly written, and any teacher that disagrees is being dishonest or ignorant. The term "radioactive decay" refers to a phenomenon or process, and NOT to a quantity. Quantities are things that can be quantified and represented mathematically. Also, the term "degree of decay" (which appears in the question) as I mentioned before, is gibberish - while it may appear to have an obvious meaning (due to colloquializations), it certainly is not a well-defined scientific term in the context of radioactivity. It has no place in a science class.