Half-Life and Radioactivity (Need an explanation) :)

[Nadbuddy]

Hi forums! Just wanted to start out with a greeting as i'm new to this forum. I'm 14 and i'm in the 9th grade (Swedish school) and we're starting to learn about half-life and radioactivity. I'd really want to know more about it and if anyone knows a way to briefly explain how these two are related to each other and how they work. We haven't gone into it that much and this is why i'm here. Physics and math are my absolute favorite subjects and i want to explore whatever i find within these two subjects deeper and more complicated. This might be much about me but this is to give you an idea of what kind of person i am and how you would be able to explain it to me.

 

All the knowledge we've received are that half-time is the time it takes for an atom to split in half which relates to radioactivity because radioactive decay is when an unstable atom loses energy by discharging ionizing radiation. I interpret it as both are the same thing, i'm not sure if that's correct or not. I am here looking for help.

 

If there are anything you could advice me when it comes to going deeper in physics (from the state where i am now) feel free to tell me, i don't care if its in school or not. Whatever knowledge there are about physics, i'll learn them all.

 

I hope this is not too basic for a forum like this, i really don't know where else to go since talking to the teachers isn't really a good place to look for answers that has nothing to do with school material. (They just don't have the time i guess).

[Sensei]

Atoms are not "splitting in half".

 

Half-life is time needed to decay half of amount of atoms.

 

f.e. you have 100 particles of unstable isotope at time t0

then 50 particles at time t1

then 25 particles at time t2

 

where (t1-t0) = (t2-t1) = half-life time

 

Obviously while unstable isotope amount is decreasing with time, final product isotope is increasing at the same time.

 

You should start from reading about beta decay

http://en.wikipedia.org/wiki/Beta_decay

 

Alpha decay

http://en.wikipedia.org/wiki/Alpha_decay

 

Proton emission

http://en.wikipedia.org/wiki/Proton_emission

 

Neutron emission

http://en.wikipedia.org/wiki/Neutron_emission

[swansont]

Half-life is time needed to decay half of amount of atoms.

 

f.e. you have 100 particles of unstable isotope at time t0

then 50 particles at time t1

then 25 particles at time t2

 

where (t1-t0) = (t2-t1) = half-life time

 

One thing to keep in mind, though, is that owing to the statistical/probabilistic nature of decay, the equations will only work well when you have a fairly large number of them. Realistically, Sensei's example should have some additional zeroes on the end of them (e.g. 1,000,00 atoms decaying to 500,000 and then 250,000). If you were actually doing an experiment you'd see significant fluctuations away from the prediction with small numbers. Conceptually though, it's correct.

[Janus]

 

Atoms are generally unstable because of either an imbalance of neutrons and protons in the nucleus or just too many of both for the nucleus to hold on to .

The atom can try to remedy this in a couple of ways, One is to eject 2 neutrons and 2 protons as a high speed helium nucleus known as an alpha particle as a way to reduce the number of nucleons.

Sometimes the nucleus will emit an electron (known as a beta particle) and in the process a neutron becomes a proton.

 

More rarely, a nucleus will emit a positron (known as a beta+ particle) and a proton becomes a neutron.

 

In any case, the nucleus changes and we end up with a new element. Many times, the new nucleus itself is not stable and you can get a series of nuclei, each decaying into the other until a stable nucleus is reached.

 

There is no way to predict when a particular nucleus will decay, however statistically, if we have a large number of nuclei, we can work out how long it would take for half of them to decay. This is what a Half-life is, it is the time it would take for half of a sample of nuclei to undergo decay to the next nuclei.

 

So, if for example, you had a kilogram of a radioactive isotope with a half-life of 1 day, after 1 day 1/2 kilogram of it would have decayed and you would be left with 1/2 kilogram of the original isotope. After another day 1/2 of that would have decayed, leaving you with 1/4 kg. each subsequent day leaves you with half of what you had the previous day.

 

I hope this helps.

 

Don't worry about asking "basic" questions. Everyone here has been at your level of understanding at one time, and there are a lot who are more then willing to help someone who is eager and willing to learn.

[Sensei]

More exotic decay modes are double beta decay minus, and double beta decay plus

http://en.wikipedia.org/wiki/Double_beta_decay

 

Electron capture

http://en.wikipedia.org/wiki/Electron_capture

 

Double electron capture

http://en.wikipedia.org/wiki/Double_electron_capture

 

Isomeric transition

http://en.wikipedia.org/wiki/Nuclear_isomer

 

[Nadbuddy]

Thanks for the info, i'll be reading all this and i'll come back and see if i've understood it correctly.

[between3and26characterslon]

Hi forums! Just wanted to start out with a greeting as i'm new to this forum. I'm 14 and i'm in the 9th grade (Swedish school) and we're starting to learn about half-life and radioactivity.'t know where else to go since talking to the teachers isn't really a good place to look for answers that has nothing to do with school material. (They just don't have the time i guess).

 

 

 

I've put in bold to remind people that you are "14" and "just starting" to learn about these things

 

Very simply if you have a counter that counts the number of radioactive decays per second and it reads say 100 decays per second and you carry on measuring until it reads 50 decays per second and it takes 2 weeks for this to happen you now know the half life is 2 weeks.

 

If you measure for another 2 weeks your counter will read 25 decays per second and 2 weeks after that 12.5 decays per second. Half life is the ammount of time it takes something to become half as radioactive.

 

Be careful not to be confused by what Sensei said

 

Atoms are not "splitting in half".

 

Half-life is time needed to decay half of amount of atoms.

 

f.e. you have 100 particles of unstable isotope at time t0

then 50 particles at time t1

then 25 particles at time t2

 

That would be ferociously radioactive for anything with a half life of less than several billion years if that were the measure i.e 238 grammes of U238 = 6x10²³ atoms, divide by a billion years = 6x10¹⁴ decays per year, divide by number of seconds per year = 19,025,875 decays per second

 

EDIT: Also with this definition the rate of decay would be constant 100 clicks at t1, 100 clicks at t2, 100 clicks at t3, until all gone is not half life

Edited November 11, 2014 by between3and26characterslon

[Sensei]

That would be ferociously radioactive for anything with a half life of less than several billion years if that were the measure i.e 238 grammes of U238 = 6x10²³ atoms, divide by a billion years = 6x10¹⁴ decays per year, divide by number of seconds per year = 19,025,875 decays per second

Are familiar with function allowing calculate decays per second at all?

For U-238 (238 g sample) decays per second will be average 2.136 mln/second.

 

Uranium-238 -> Thorium-234 + alpha + 4.26992 MeV

 

4.26992 MeV is 6.84*10^-13 J energy.

2.136*10^6 * 6.84*10^-13 J = 1.4608*10^-6 J/s

 

100,000 times less than LED diode with U=5 V,I=25 mA is consuming energy per second..

Edited November 11, 2014 by Sensei

[between3and26characterslon]

Are familiar with function allowing calculate decays per second at all?

For U-238 (238 g sample) decays per second will be average 2.136 mln/second.

 

Merely pointing out that the definition you gave of time needed to decay half of amount of atoms would not only be extremely radioactive but the rate of decay would not decrease over time.

 

I thought it was an important distinction that it is not the number of particles you start with but the number of decays you detect

[Enthalpy]

Radioactive half-lives span from very long (²³⁸U) to extremely short, without a clear lower limit. No worry with Sensei's example of a half-life equal to 1, even less so because he didn't tell the unit.

 

Here's an example of a chart of the nuclides, where the colours indicate the half-life between 10⁺¹⁵s and 10^-15s:

http://www.nndc.bnl.gov/chart/

[swansont]

 

Merely pointing out that the definition you gave of time needed to decay half of amount of atoms would not only be extremely radioactive but the rate of decay would not decrease over time.

 

I thought it was an important distinction that it is not the number of particles you start with but the number of decays you detect

 

No unit was given, and of course the decay rate would decrease over time. The decay rate is proportional to the sample size. Further, "highly radioactive" is somewhat subjective, but in that example, even if all 100 atoms decayed in a millisecond, it would still only be 100 decays.

 

A standard unit for activity is the curie (Ci), which is 3.7 x 10¹⁰ decays per second. 1 Ci gamma sources give about 1 rem/hour dose at 1 meter, so that's a lot. 1 microcurie, OTOH, would give 9 millirem/year at 1 meter, which is not a large dose. The human body contains radioactive C-14 and K-40 which undergo several thousand decays per second, and you are not a meter away. People are not generally considered highly radioactive.

[between3and26characterslon]

Are familiar with function allowing calculate decays per second at all?

For U-238 (238 g sample) decays per second will be average 2.136 mln/second.

 

Uranium-238 -> Thorium-234 + alpha + 4.26992 MeV

 

4.26992 MeV is 6.84*10^-13 J energy.

2.136*10^6 * 6.84*10^-13 J = 1.4608*10^-6 J/s

 

100,000 times less than LED diode with U=5 V,I=25 mA is consuming energy per second..

 

Having given it some more thought what you said in you first post makes sense, I don't know, maybe it had been a long day but when I first read your post it looked wrong.

[Sensei]

 

Having given it some more thought what you said in you first post makes sense, I don't know, maybe it had been a long day but when I first read your post it looked wrong.

 

See this old thread

http://www.scienceforums.net/topic/83245-a-question-on-radioactive-decay/#entry806204

I am showing how to set up OpenOffice SpreadSheet for decay rate equation.

[Nadbuddy]

Thank you for all the answers and sorry for the late reply. I've read through most of what you people have linked and said on the thread and i cannot be more thankful.

Thank you once again for explaining.