Half life Definition and 12 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.

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  1. freelooooo

    How does Cmax decrease over time in human body? Use half life?

    I'm interested in chemistry but it's all new for me and I'm just starting out. I tried to do some calculations but got stuck... I was wondering, for example a supplement has the following values: Molar mass: 306.247 g mol-1 Tmax 2-4 hours Cmax 363.3 ng / ml Half life: 11.21 hours How do...
  2. J

    Order of Reaction from Halflife times

    Homework Statement A -> Products The two half-lives are 34 minutes and 68 minutes respectively. Find the order of the reaction. So I and my friends are having disagreements over whether this is first order or second order or not. Homework Equations For first order half-life, t = 0.693/k For...
  3. Mackenzie Cobb

    Chernobyl Cesium 137 Half-Life vs. Chernobyl Contamination

    Let me start out by saying that I have no idea what I'm talking about. I graduated from Indiana University with a Bachelor's in Spanish, and I work as a Loan Review Specialist at a bank, which has NOTHING to do with my degree, and still yet nothing to do with this topic. But lately, I've become...
  4. A

    A Beta Decay Rates Changing by Ionization?

    I recently read about a beta decay isotope (Rhenium-187),whose half life was changed from 42 X 109 years to 33 years, just by stripping the nucleus of all it's electrons. Why does this allow for a faster decay, and does this apply to all beta decay nuclei, or just Rhenium 187?
  5. P

    Protein melt temp/half life relation?

    Is anyone familiar with a way to determine the half-life of a protein from its melt temperature (Tm)? It seems like there ought to be some sort of correlation, be it theoretical or empirical.
  6. F

    Half life question

    Homework Statement A radioactive source emits alpha particles at a constant rate 3.5x10^6 s^-1. The particles are collected for a period of 40 days. BY reference to the half life of the source, suggest why it may be assumed that rate of emission of alpha particles remain constant? The Attempt...
  7. L

    How do I use this equation? Estimating half-life of a decay

    How should I go about using equation 8.18? Link can be found below. In the book, an example is used where Th-220 --> C-12 + Po-208 with a Q value of 32.1 MeV is used, and it is said to yield t1/2 = 2.3x106 but for the life of me I cannot reproduce this result. This is what I did: Going from...
  8. WhiteWolf98

    Activity of 1 g of carbon if 1 in 10^12 atoms are carbon-14

    Homework Statement Carbon-14 decays by β emission and has a half-life of 5570 years. What is the decay constant of carbon-14? What is the activity of 1 g of carbon if 1 in 1012 atoms are carbon-14? After what time will the activity per gram have fallen to 3 Bq? Homework Equations λt½ = ln(2)...
  9. G

    A Nuclear stability equations?

    Are there equations that detail the stability of nuclei against beta decay? On a related point, I'm familiar with the chart that shows all the isotopes and their half-lives (with a good chunk undergoing beta decay), but I was wondering if that can be derived from first principles, just using the...
  10. kiwaho

    A Equivalence equation between cross section & half life?

    Is there equivalence equation between cross section & half life? For beta decay, we usually use half life to describe how fast or slow the decay undergo. For nuclear reaction, we use cross section to describe the possibility of reaction. In a sense, they reflect the same root physics spirit. Is...
  11. T

    B Nuclear Decay with time (half-life,initial activity/ nuclei)

    Hi! I just keep having a hard time looking for the formula stating the relationship between half life, the initial number of unstable nuclei and the initial activity, can someone help me on that ? To be more specific, here is a problem which can be solved using that formula: "A substance has a...
  12. VjStylz

    Half life Problem

    Alright so i have exams in a matter of days and stumbled upon this multiple choice question 7. A radioactive source has 1.6×10^20 atoms of a radioactive isotope, with a half-life of 3 days. How many atoms will decay in 12 days? A. 1.0×10^19 B. 1.2×10^20 C. 4.0×10^19 D. 1.5×10^20 So my first...