The h Bar - 2014-10-30

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6:44 AM

26

Q: DO NOT FEED THE BEARS

A question was just asked on ELL, and it needs a lot of work. Here's a screen shot. There are two problems with this question: first, the way the initial question was asked; second, the way a follow-on question was asked in a comment. Error Identification I'll start by addressing the ques...

7 hours later…

2:04 PM

@KyleKanos: Concerning e.g. retagging here. The equation-of-state tag seems to have too many meanings in different areas of physics to be really useful as a tag, e.g. one meaning in the area of statistical physics and one meaning in cosmology, and so forth. Thoughts?

The equation of state is simply a relationship between state variables

While the equations used are different (e.g., $w=p/\rho$, $pV=nRT$, etc), the meaning is really the same

This was mentioned a few weeks back: chat.stackexchange.com/transcript/71?m=17064334#17064334

But that seems almost like tagging with e.g. differential-equations. That could apply to a sizable part of all physics questions.

I'm trying to keep the retagging to questions that are only about the equation of state

you're also wiping the active page

I am

Sorry

Perhaps I should (re)edit the EOS excerpt to state that should explain it's only to be used for questions about the EOS and not simply involving it

2:15 PM

@KyleKanos : I just added a line concerning this to the tag wiki of EOS.

Nice

Thanks

2 hours later…

4:39 PM

Guys we are having a conversation in a room where we require a physics person who understands time and space well... can someone from here join us please?

@AwalGarg What's it about? Special relativity?

@ACuriousMind ahem... actually, I have to explain a person that time and space don't require any god.

@AwalGarg What? You can't prove or disprove the notion so why bother?

@KyleKanos I have to show him that we can't disprove it.

@KyleKanos do you know much about radiation detection? :3

I have some (probably silly) questions about my nuclear homework

4:49 PM

Not really, sorry

@AwalGarg I hate debates where I'd have to explain the scientific method all over again (and probably bicker about whether or not "God" is a scientific hypothesis or not). I'll pass.

@ACuriousMind me too :( That's why I wanted someone from here to do that :P

@AwalGarg: If the person needs convincing of that, then they are not worth talking to; alienate yourself from them :)

@JamalS I thought it would be good if I can educate someone...

being asked about measuring multiple half lives at once from a decay chain (one $\varepsilon$ and $\beta^{+}$, one $\varepsilon$), with the question phrased to emphasize their differing decay modes

which was throwing me off

4:53 PM

@JamalS As a devoutly-believing Catholic, I find it hard to believe that you'd simply reject a conversation with me simply because I'm a believer.

@KyleKanos: Of course, I was joking, Kyle :) That'd be extreme.

electron capture and beta+ can both be measured with scintillating detectors, right? :3

@GBeau Anything that gives off radiation of some kind can be detected with scintillators, I think.

@ACuriousMind: Indeed, any ionizing radiation.

@ACuriousMind Astronomically speaking, I think scintillators are only on $\gamma$-ray telescopes.

But that may be due to efficiencies, I suppose

5:00 PM

@KyleKanos I once did a lab course that was aimed at detecting muons with scintillators. The result was horrible, but it is in principle possible

(The supervisor insisted on us fitting a cosine-squared to three (in numbers: 3) data points.)

I'm guessing there is trivially a photopeak unique to the beta+ emission

@ACuriousMind Pfft, you can do that with 2 (zwei) points!

Yeah, but what on earth will it tell you?

Duh: your emission follows a cosine-squared law.

That I can get two points to lie on any function I choose is kinda the point here - but the supervisor insisted that we "check that the data follow the expected distribution".

5:04 PM

user image

(this is my plot)

starting with a sample of the 56Ni solely, which only decays by electron capture

and then the Co is decaying by both electron capture and beta+ emission

@GBeau And your question is?

and given the sample of Ni and that information about their decay modes, how might one measure both half lives

@GBeau Well, the nickel one is straight forward, isn't it?

@ACuriousMind It would get conflated with data from its daughter, no?

@GBeau Unless I'm missing something, Co-56 does not decay into Ni-56

5:09 PM

@ACuriousMind the other way around

Right

the Co-56 only exists because of the Nickel decay

That makes it a bit tricky to get the half-life of the daughter, since it is continually produced anew

since we're only starting with a Nickel sample

is the parent not conflated as well?

But the half-life of nickel is just the slope of the log-plot (divided by ln(2) or something)

5:10 PM

how are you suggesting the half life be measured?

@GBeau Why would it, there's no new Ni-56 being produced, it just decays

scintillating detectors and counting photopeaks is bound to end up detecting things from both the Ni and the Co?

Wait...the plot is not your actual data, is it?

no, it's theoretical data

Your actual data are just counts in a scintillator

5:11 PM

well, it's just a plot from the half lives

Ah, now I understand what you're trying to do

the plot isn't necessary for the question, I just included it to illustrate the situation since I'd already made it

it just assumes solely a sample of 56Ni at 10^5 Bq to start with

and then plots the activity over time

it decays by electron capture to 56Co, so that activity is included, but the daughter of 56Co is 56Fe, which is of course stable

Well, I'd try to derive a formula for the expected activity at any time, and just fit that to the scintillator clicks

@ACuriousMind that formula is exactly what is plotted

(for total)

Yes

So what's wrong with fitting the "total activity" formula to your data?

5:15 PM

well, the reason I ask is the question was phrased as if the fact that the decay modes differed was supposed to be used

"given that ___, "

Hm. Has the plot been done with by including both decay modes?

yes, multimodal decay adds nicely

it wasn't even included separately in Brookhaven's data

Then I don't see what else you'd need to do, but I'm a terrible experimentalist, so don't trust me :P

:P

well, you typically measure the counts at a photopeak after calibrating

for example, to measure 7-Be half life in grass, you would plot the detection counts at a ~478 keV photopeak nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=7BE

So you've got energy information about the decays? If you know the expected spectra, you could fit the individual decay modes seperately

But, well, beta-spectra are broad, so there won't be any nice peaks for them, I think

And if you do not have "impurities" from other radiation sources, I don't see why you couldn't just fit the theoretical total activity to the total counts

I'd probably just do the fit, compare with literature results and if I got nonsense, then I'd think again about this :D

1 hour later…

6:26 PM

@KyleKanos Do you feel that doing physics strengthens your faith, or reinforces it? I could imagine it working both ways...

6:37 PM

@Danu What's the difference between strengthening and reinforcing?

LOL

No idea what happened there - brainfart

Second try: @KyleKanos Do you feel that (your knowledge of) physics strengthens your faith, or makes you call certain aspects into question? I could imagine it working both ways...

Sometimes I do question some aspects, but I usually resolve it (often times much later!)

So I suppose I would say it strengthens it

user54412

7:00 PM

@GBeau I have to run, but do you not have full spectra to look at (at different times)? If so, the gammas associated with the new nuclei going to ground state should be separate peaks. And the Co-56 -> Fe-56 positron should make a nice 511 keV peak. Combined with branching ratios I think this sort of information can be used to quickly get at half lives.

11:20 PM

@ChrisWhite No, the question is hypothetical

(so I have no actual data)

I had actually noted the 511 keV peak in my answer thus far~

(from looking at the Brookhaven data)

user54412

11:52 PM

@GBeau I see.

user54412

Actually what are the branching ratios? Wiki claims they are both pure positron emitters, which definitely doesn't sound right

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