What is Half life: Definition and 125 Discussions

Half-life (symbol t1⁄2) is the time required for a quantity to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable atoms survive. The term is also used more generally to characterize any type of exponential or non-exponential decay. For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the human body. The converse of half-life is doubling time.
The original term, half-life period, dating to Ernest Rutherford's discovery of the principle in 1907, was shortened to half-life in the early 1950s. Rutherford applied the principle of a radioactive element's half-life to studies of age determination of rocks by measuring the decay period of radium to lead-206.
Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a characteristic unit for the exponential decay equation. The accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed.

View More On Wikipedia.org
Recent contents Top users
  1. G

    How do I determine the half life of an alpha particle?

    I am fairly new to nuclear chemistry and I was just curious as how to find the half life of a helium nucleus. Would it be the same as finding the half life of 1 neutron x2 and 1 proton x2? Or is this the completely wrong way to do this. Also would the half life be the same as a helium atom or...
  2. C

    Help with Half Life: Confused on A=DecayConstant*N and ln(2)

    Hi, I'm new to the forum and came here because I could use some help. At the moment I am really confused because I know that A = DecayConstant * N, where A is activity and N is the number of atoms before decay. Isn't the Activity is how many atoms decay after 1 second with a specific value of...
  3. H

    Half life and activation energy

    Homework Statement I have two questions that I just don't even know where to start The activation energy for the reaction CH3CO CH3 + CO is 71 kJ/mol. How many times greater is the rate constant for this reaction at 170°C than at 150°C? A) 0.40 B) 1.1 C) 2.5 D) 4.0 E) 5.0 and A...
  4. P

    How Is Error on Half-Life Calculated Using Poisson Distribution?

    How may the error on the half life (using Poisson) be calculated? Barlow states that the error on the log would be 1/√N. Why is that?
  5. C

    How Does Time Affect the Radioactivity of Substances with Different Half-Lives?

    Question: A sample of radioactive substance K and a sample of another radioactive substances L have the same initial activity. the half life of K is TK and the half life of L is TL. After a time T, the activity of K was half the activity of L. Expression in term of T, TK, and TL.
  6. C

    MHB How Long for 61Co to Reduce to 80% of Its Initial Value?

    Jimson asks over on Yahoo Answers The key idea is that after 100 minutes 1/2 remains after 200 minutes 1/4 remains so the fraction remaining after \(t\) minutes is: \[Q=2^{-t/100}\] So if \(80\%\) remains we need to solve: \[0.8=2^{-t/100}\] which we do by taking logs (the base is...
  7. M

    Understanding Half-Life Accuracy in Radiometric Dating

    I was debating a friend and we started running numbers for the statistical accuracy of half-lives. I am a numbers guy(Accountant/Statistical Analysis), not a chemist. He is a scientist and his wife is a chemical engineer. Here is my question: If we have been studying half lives of...
  8. F

    Calculating half life (Decay Constant)

    Homework Statement The activity of a radioactive nuclide drops 78.5% of it's initial value in 2000 years. Homework Equations I know I need to use the decay constant to work out my answer so I have re-arranged the equation as illustrated in my picture, the only problem is I cannot...
  9. J

    Half-Life Problem: Find Nuclei & Mass After 3 Hours

    Homework Statement Initially, a particular sample has a total mass of 200 grams and contains 256 x 1010 radioactive nuclei. These radioactive nuclei have a half life of 1 hour. (a) After 3 hours, how many of these radioactive nuclei remain in the sample (that is, how many have not yet...
  10. D

    Nuclear u-235 and u-238 isotope half life question

    Homework Statement The present day value of the uranium isotopic ratio U-235/U-238 is 0.00723. The half life of U-238 is 4.51 x 10^9 and the half life of U-235 is 7.13 x 10^8 years. What was the U-235/U-238 isotopic ratio 2 billion years ago when the Oklo natural reactor was active...
  11. B

    Exploring the Concept of Half-Life in Radioactive Decay

    Hello I was wondering if someone could explain why a half life exist? What attribute of radioactive matter requires that on average half of the particle decay within this so called half life period. Why couldn't it vary? thanks
  12. C

    Does Rotting Food Have a Direct Impact on Its Half Life?

    Is the process of rotting food or any organic material directly related to the half life of its components?
  13. J

    Half Life - Calculate Fraction Remaining

    Homework Statement Carbon 14 has a half-life of 5730 years, what fraction will have decayed in 2300 years? Homework Equations Nf/No =e^-kt The Attempt at a Solution Nf/No =e^-1.21 x 10^-4 x 2300 =0.068585374 However if I use Nf =No(1/2)^2300/5730 I get 0.757125224 I...
  14. E

    Radioactivity, disintegrations and Half Life

    Hi, My Physics textbook talks about a nucleus disintegrating. What does this mean? I understand that the nucleus will emit either an alpha, beta or gamma ray but surely the nucleus still remains after this? Does the term disintegrate just mean than the nucleus has got smaller and become a...
  15. S

    Half-Life of He-6 Nucleus: 40 GeV

    Homework Statement The question asks: "What is the half-life of a He-6 nucleus with total(kinetic+rest) energy E=40 GeV?" How can we approach this problem? Homework Equations E=mc2 ; The Attempt at a Solution I thought of getting the mass of the He-6 using M=E/c2...
  16. P

    Understanding Decay and Half-Life of Uranium

    Homework Statement I'm not sure if whether mass changes when atoms decay. Does emitting alpha/beta/gamma cause the atoms to lose mass? I don't know :confused:Homework Equations Uranium-235 has a half life of 7.35x10^5 years and uranium 238 has a half life of 4.5x10^6 years. Compare the two...
  17. T

    Calculating Half-Life Using Calculus: Deriving λ from y = y0e-kt

    Homework Statement This is the equation y = y0e-kt Homework Equations b) Show using your expression for λ that if at time t1 the amount is y1, then at time t1 + λ it will be y1/2, no matter what t1 is. The Attempt at a Solution y1=y0ekt1 that part I got. Then the answer key...
  18. K

    Half life of a nuclear decay via simulation

    I don't know how to solve the questions that my lecturer gave me. I not understand about probability of half life. Can anyone explain to me and help me solve the questions as well? My lecturer ask us to prove the probability as shown in the picture.
  19. A

    If Uranium's half life is 4.5 billion years, why does it become waste

    Okay, If Uranium's half life is 4.5 billion years, why does it become waste at a reactor before it decays 4.5 billion years later... Forgive me if my question is badly worded or really stupid; I am a high-school student who has just finished the topic of "Nuclear Energy" in my physics class...
  20. K

    Radioactive decay and half life?

    Homework Statement I have some data for "Activity(Bq * 10^4)" and "time". I have already found the half life and everything but I can't seem to see any sources of error in the data Homework Equations I need sources of error. The Attempt at a Solution I said, the half life matches the...
  21. Y

    How Long Ago Did the Plant Die Using Carbon-14 Dating?

    Homework Statement half life of carbon 14 is 5730 years. living matter produces 15.3 disintegrations per min per gram of carbon it contains. A 1g sample of a plant from an excavation shows 7 disintegrations per min from carbon-14. how long did the plant die? Homework Equations k = 1/ t...
  22. S

    Converting Half Life to Becquerels

    Can you convert Half Life into Becquerels? How?
  23. B

    Why Half life radiation is constant

    what makes half life(T) of radiation independent of the quantity of radioactive substance? why is it constant whatever the amount of the radioactive substance is?
  24. S

    Rate Law & Half Life: Solve NOBr Reaction

    Homework Statement Consider the following reaction NOBr (g) → NO (g) + ½ Br2 (g) The table below gives the time-dependent concentration of NOBr. Using this data, determine, if the reaction is first- or second-order, with respect to NOBr. Give the rate equation and determine the rate...
  25. J

    Opposite of Half Life: Exponential Growth

    Opposite of half life? If a system is experiencing exponential decay (nuclear decay, capacitors, whatever else), it has a constant half life, and decreases by the same proportion in the same time. I was wondering what term people use to describe systems experiencing exponential growth? Do...
  26. C

    Astronomy, age of a moon rock, half life

    Moon rocks containing uranium 238, decays to lead with a half life of 4.5 billion yrs. The lunar rock sample you're invistigating contains half of the original uranium 238. How old is the rock? How would I go about solving this? What formula should I use?
  27. D

    Half-Life Calculation: How Do Physicists Measure Isotopes?

    Hi there, Someone asked me this earlier when we were talking about radioisotope dating. How do physicists determine the half lives of isotopes when said half life stretches into the millions of years? Random question I know :P -Dan
  28. G

    Discharging capacitor half life

    What is meant by discharging capacitor half life (the description). I seem to be getting different description, I would just like for someone to confirm it here for me please.
  29. M

    Half Life Problem: Solving for 5 Years Starting with 100 Grams

    The problem: A radioactive material has a half-life of 50ln2 years. If you add 5 grams per year to the material, how much material will you have after 5 years if you start with 100 grams? What I've done so far: t = ln2/k = 50ln2 k = 1/50 k = .02 A = A0e^-kt N = 100e^(-.02)(5) = 90 grams...
  30. L

    Half life of a radio active element

    Homework Statement The half life period of N-13 is 10.1 minute. Its life time is ------- The answer has been given as infinity. Could someone help the formula to arrive at this answer 2. The time taken by the radio active element to reduce 1/e times is ----- The answer has been given as...
  31. P

    Finding the Half Life From Activity

    I am meant to deduce the half life from this table (as attached.) I was under the impression that the half life would occur when the initial activity was halved but my answer of 2.34 minutes (from halving the initial activity of 36506) is wrong. Instead, the answer the textbook got was 1 minute...
  32. M

    Age of Seashell: Calculating 14-C Half Life

    Homework Statement seashell HAVE 90% of 14-C as a livining shell of the same size half life is 5570 years, how old is it in years how many yrs did it take for 14-C to diminish from its initial value to 99% of that? Homework Equations The Attempt at a Solution for the first part...
  33. K

    Calculating activity using half life

    The question is that the human body contains 0.2% potassium by weight and the natural abundance of 40K is 0.0118%. The half life of 40K is 1.28x10^9 years and I have to calculate the activity in an adult weighing 75 kg. What I tried was to find the decay constant by 0.693/1.12x10^13 hours =...
  34. A

    Half Life of radioactive element

    It takes an infinite time for complete decay of a radioactive element. but on the same time we calculate the half life of radioactive material i-e T-1/2 = ln 2 / lamda. is that means that if we double the half life time we could reach the total life time of radioactive decay so on contrary we...
  35. D

    Half Life Questions: Solving Physics 30 Issues

    i am taking upgrading, and one of the main reasons i failed physics 30 to start with was due to half life. the book can't explain it good enough to me and either could my teacher. so i have a few questions that i would appreciate a starting point for. How much time elapses before 90% of the...
  36. U

    How to find the mass of a compound given the half life and activity?

    Homework Statement What mass of Na2 SO4 has an activity of 10.0 mCi? Sulfur-38 has an atomic mass of 38.0 and a half life of 2.87h Homework Equations Half life= 0.693/ k Rate= kN The Attempt at a Solution I'm not sure where to begin the sulfur-38 part is throwing me off All I...
  37. C

    How Long Ago Did the Supernova Occur That Created Earth's Heavy Elements?

    All the very heavy atoms found in the Earth were created long ago by nuclear fusion reactions in a supernova, an exploding star. The debris spewed out by the supernova later coalesced to form the sun and the planets of our solar system. Nuclear physics suggests that the uranium isotopes 235U...
  38. I

    Need help understanding Half life, decay constant & activity equations

    Hi guys i'm having much difficulties in understanding the relations between half life, decay constant and activity. I'll explain what i known here. T^{\frac{1}{2}} is the half life which is the time to reduce the nuclides by half. \lambda is the decay constant which can be solved...
  39. 8

    Half-Life of ^{208}_{82}Pb (alpha decay)

    Homework Statement Calculate the half life for alpha decay of ^{208}_{82}Pb (in years) Homework Equations gamow factor transmission probability coloumb potential The Attempt at a Solution...
  40. L

    Half-Life Problem: Solving A0e13K to Get K

    1. Half life of a radioactive substance is 13 days. How long for 400 grams to decay to 300 grams? Solve algebraically and show all work. Give both exact answers and the answer rounded to 4 decimal places. So I manipulated (1/2)A0=A0e13K to obtain K = ln(1/2)/13 Where do I go from here?
  41. C

    Half-Life of Strontium-90 - Solve Here!

    Homework Statement The half life of strontium-90 is 28.0 years. Deduce that the decay constant of strontium-90 is 7.85 * 10^(-10) s^1. Homework Equations k = ln 2 over half-life in seconds The Attempt at a Solution dunno...please help me solve this!
  42. F

    Studying half life and radioactive deca

    Hi guys I am currently doing some homework for physics and I am having a little bit of difficulty understanding the topic. We are currently studying half life and radioactive decay. We completed a practical involving the rolling of X number of dice and removed all dice that rolled the number 2...
  43. F

    How Do You Calculate Half-Life from an Exponential Decay Equation?

    Homework Statement Given Y=Ae-t/(lambda) find the half life in terms of A and lambdaHomework Equations Y=Ae-t/(lambda) The Attempt at a Solution every time t is a multiple of lambda the relationship then becomes (lambda(n)=A/e^n), i can't figure out how that will relate, i also tried...
  44. PrincePhoenix

    Understanding the Randomness of Radioactive Decay

    1-Half life is the time it takes for half of the nuclei in a sample of radioactive material to decay(Am I right?). Why does the first nucleas that decays,decay first and the one that decays in the end, decay in the end? What's the difference between the two nuclei or what causes this the nuclei...
  45. A

    Discovering the Half Life of a Radioactive Substance

    Homework Statement Find the half life (in hours) of a radioactive substance that is reduced by 5% in 65 hours. Homework Equations The Attempt at a Solution So looking around I find that A=A0e^-kt but I am not really sure how to solve this without having an initial ammount.
  46. M

    Calculating the Half Life of Awareness

    This is a strange one, I know. I have background info below. Here is the question: How would one begin to calculate the half life of awareness (the social and political)? What would the equation to calculate that look like? I am a grad student in art and am making work about social...
  47. W

    How Long Until a Radioactive Daughter Isotope Reaches 97% of Its Equilibrium?

    Homework Statement A parent isotope has \tau_\frac{1}{2}=\delta. Its decays through a series of daughters to a final stable isotope. One of the daughter particles has the greatest half life of \tau_\frac{1}{2}=\alpha-- the others are less then a year. At t=0 the parent nuclei has N_0 nuclei...
  48. K

    Half life for beta-gamma decay.

    Homework Statement A sample of radioactive nuclei of a certain element can decay only by gamma -emission and beta -emission. If the half-life for gamma -emission is 24 minutes and that for beta -emission is 36 minutes, what's the half-life for the sample? 2. The attempt at a solution...
  49. S

    How to Calculate Initial Radioactive Activity for Patient Treatment?

    Homework Statement Ok so I'm doing this practise problem but I have no clue what to do. "A radioactive sample intended for irradiation of a hospital patient is prepared at a nearby lab. The sample has a half-life of 83.61 h. What should it's initial activity be if it's activity is to be...
  50. W

    Half Life / Alpha Decay Question - No IDea

    Half Life / Alpha Decay Question -- No IDea Homework Statement A space rock contains 3.20 g of 147 62 Sm and 0.110 g of 143 60 Nd. 147 62 Sm alpha decays to 143 60 Nd with a half-life of 1.06 1011 yr. If the rock originally contained no , how old is it? Homework Equations ...
Back
Top