Difference between revisions of "Half-life" - New World Encyclopedia

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:''This article describes a scientific and mathematical term. For other meanings see [[half-life (disambiguation)]].''
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[[Image:UraniumUSGOV.jpg|thumb|right|250px|Uranium ore. The most abundant uranium [[isotope]] is <sup>238</sup>U, with a half-life of 4.5 &times; 10<sup>9</sup> years.]]
The '''half-life''' of a quantity subject to [[exponential decay]] is the time required for the quantity to decay to half of its initial value. The concept originated in the study of [[radioactive decay]], but applies to many other fields as well, including phenomena which are described by non-exponential decays.
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If a sample of material decays at a certain rate over time, its '''half-life''' is defined as the time it takes for the sample to decay to half its initial amount. This concept originated when studying the [[exponent]]ial decay of [[radioactive]] [[isotope]]s, but it is applied to other phenomena as well, including those described by non-exponential decay. An exponential decay process, as exemplified by the decay of radioactive isotopes, for example, is simply one in which the number of atoms disintegrating per unit time is proportional to the total number of radioactive atoms present.
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{{toc}}
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In the case of radioactive decay, each different radioactive isotope has a particular half-life that is unaffected by changes in the physical or chemical conditions of the surroundings. This property is the basis for radiometric dating of [[rock]]s and [[fossil]]s. In pharmacology, the half-life of a drug (in a biological system) is the time it takes for the drug to lose half its pharmacologic activity.
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== Example of radioactive decay ==
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Carbon-14 (<sup>14</sup>C) is a radioactive isotope that decays to produce the isotope nitrogen-14 (<sup>14</sup>N). The half-life of <sup>14</sup>C is about 5,730 years. This means that if one starts with 10 grams of <sup>14</sup>C, then 5 grams of the isotope will remain after 5,730 years, 2.5 grams will remain after another 5,730 years, and so forth.
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== Calculation of half-life ==
  
 
{| class="wikitable" align=right
 
{| class="wikitable" align=right
! Number of<br>half-lives<br>elapsed !! Fraction<br>remaining !! As<br> power<br> of 2
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! Number of<br/>half-lives<br/>elapsed !! Fraction<br/>remaining !! As<br/> power<br/> of 2
 
|-
 
|-
 
| 0 || 1/1 || <math>1/2^0</math>
 
| 0 || 1/1 || <math>1/2^0</math>
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The table at right shows the reduction of the quantity in terms of the number of half-lives elapsed.  
 
The table at right shows the reduction of the quantity in terms of the number of half-lives elapsed.  
  
It can be shown that, for exponential decay, the half-life <math>t_{1/2}</math> obeys this relation:
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It can be shown that, for exponential decay, the half-life <math>t_{1/2}</math> obeys the following relation:
 
:<math> t_{1/2} = \frac{\ln (2)}{\lambda} </math>
 
:<math> t_{1/2} = \frac{\ln (2)}{\lambda} </math>
 
where
 
where
:* <math>\ln (2)</math> is the [[natural logarithm]] of 2, and
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:* <math>\ln (2)</math> is the natural [[logarithm]] of 2, and
:* <math>\lambda</math> is the '''decay constant''', a [[negative and non-negative numbers|positive]] constant used to describe the rate of exponential decay.
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:* <math>\lambda</math>, called the ''decay constant'', is a [[negative and non-negative numbers|positive]] constant used to describe the rate of exponential decay.
  
The half-life is related to the [[mean lifetime]] τ by the following relation:
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In addition, the half-life is related to the [[mean lifetime]] τ by the following relation:
 
:<math> t_{1/2} = \ln (2) \cdot \tau </math>
 
:<math> t_{1/2} = \ln (2) \cdot \tau </math>
  
== Examples ==
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The constant <math>\lambda</math> can represent various specific physical quantities, depending on the process being described.
:{{main|Exponential decay#Applications and examples}}
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* In first-order [[chemical reaction]]s, <math>\lambda</math> is the [[reaction rate constant]].
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* In [[pharmacology]] (specifically [[pharmacokinetics]]), the half-life of a drug is defined as ''the time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacologic, physiologic, or radiologic activity.''<ref>Taken from Medical Subject Headings. Year introduced: 1974 (1971).</ref>
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* For [[Electronics|electronic]] filters such as an [[RC circuit]] (resistor-capacitor circuit) or an [[RL circuit]] (resistor-inductor circuit), <math>\lambda</math> is the reciprocal of the circuit's [[time constant]] <math>\tau</math>, which is the same as the mean lifetime mentioned above. For simple RC or RL circuits, <math>\lambda</math> equals <math>1/RC</math> or <math>R/L</math>, respectively. The symbol <math>\tau</math> is related to the circuit's [[cutoff frequency]] ''f''<sub>c</sub> by
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:<math>\tau = RC = \frac{1}{2 \pi f_c}</math>  or, equivalently,  <math>f_c = \frac{1}{2 \pi R C} = \frac{1}{2 \pi \tau}</math>.
  
The generalized constant <math>\lambda</math> can represent many different specific physical quantities, depending on what process is being described.
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==Experimental determination==
  
* In an [[RC circuit]] or [[RL circuit]], <math>\lambda</math> is the reciprocal of the circuit's [[time constant]] <math>\tau</math> (the symbol is the same as the mean lifetime, noted above; the two quantities happen to be equal). For simple RC and RL circuits, <math>\lambda</math> equals <math>RC</math> or <math>L/R</math>, respectively.
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The half-life of a process can be readily determined by experiment. Some methods do not require advance knowledge of the law governing the decay rate, whether it follows an exponential or other pattern of decay.
* In first-order [[chemical reaction]]s, <math>\lambda</math> is the [[reaction rate constant]].
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* In [[biology]] (specifically [[pharmacokinetics]]), from [[MeSH]]:  ''Half-Life: The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacologic, physiologic, or radiologic activity. Year introduced: 1974 (1971)''.
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Most appropriate to validate the concept of half-life for [[radioactive decay]], in particular when dealing with a small number of [[atom]]s, is to perform experiments and correct computer simulations. Validation of physics-math models consists of comparing the model's behavior with experimental observations of real physical systems or valid simulations (physical and/or computer simulations).<ref>See [http://www.madsci.org/posts/archives/Mar2003/1047912974.Ph.r.html link] to test the behavior of the last remaining atoms of a radioactive sample. Retrieved October 19, 2007.</ref>
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When studying radioactive decay, the exponential model does ''not'' apply for a small number of atoms (or a small number of atoms is not within the domain of validity of the formula or equation or table). Some model simulations use pennies or pieces of candy.<ref>[http://www.exploratorium.edu/snacks/radioactive_decay.html Radioactive Decay Model] Retrieved October 19, 2007.</ref><ref>[http://www.sciencenetlinks.com/lessons.cfm?DocID=178 Radioactive Decay] Retrieved October 19, 2007.</ref> A similar experiment is performed with isotopes that have very short half-lives.<ref>For example, see Figure 5 in the [http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/cassy_pa_hwz-e.htm link]. See how to write a computer program that simulates radioactive decay including the required [[random]]ness in the [http://astro.gmu.edu/classes/c80196/hw2.html link] and experience the behavior of the last atoms. Retrieved October 19, 2007.</ref>
  
 
== Decay by two or more processes ==
 
== Decay by two or more processes ==
Some quantities decay by two processes simultaneously (see [[Exponential decay#Decay by two or more processes]]). In a fashion similar to the previous section, we can calculate the new total half-life <math>T_{1/2}</math> and we'll find it to be:
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 +
Some quantities decay by two processes simultaneously. In a manner similar to that mentioned above, one can calculate the new total half-life (<math>T_{1/2}</math>) as follows:
  
 
:<math>T_{1/2} = \frac{\ln 2}{\lambda _1 + \lambda _2} \,</math>
 
:<math>T_{1/2} = \frac{\ln 2}{\lambda _1 + \lambda _2} \,</math>
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:<math>T_{1/2} = \frac{t _1 t _2}{t _1 + t_2} \,</math>
 
:<math>T_{1/2} = \frac{t _1 t _2}{t _1 + t_2} \,</math>
  
i.e., half their [[harmonic mean]].
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that is, half their [[harmonic mean]].
  
 
== Derivation ==
 
== Derivation ==
Quantities that are subject to exponential decay are commonly denoted by the symbol <math>N</math>. (This convention suggests a decaying ''number'' of discrete items.  This interpretation is valid in many, but not all, cases of exponential decay.) If the quantity is denoted by the symbol <math>N</math>, the value of <math>N</math> at a time <math>t</math> is given by the formula:
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 +
Quantities that are subject to exponential decay are commonly denoted by the symbol <math>N</math>. (This convention suggests a decaying ''number'' of discrete items, an interpretation that is valid in many, but not all, cases of exponential decay.) If the quantity is denoted by the symbol <math>N</math>, the value of <math>N</math> at a time <math>t</math> is given by the formula:
  
 
:<math>N(t) = N_0 e^{-\lambda t} \,</math>
 
:<math>N(t) = N_0 e^{-\lambda t} \,</math>
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where <math>N_0</math> is the initial value of <math>N</math> (at <math>t = 0</math>)
 
where <math>N_0</math> is the initial value of <math>N</math> (at <math>t = 0</math>)
  
When <math>t = 0</math>, the exponential is equal to 1, and <math>N(t)</math> is equal to <math>N_0</math>. As <math>t</math> approaches [[infinity]], the exponential approaches zero. In particular, there is a time <math>t_{1/2} \,</math> such that
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When <math>t = 0</math>, the exponential is equal to 1, and <math>N(t)</math> is equal to <math>N_0</math>. As <math>t</math> approaches [[infinity]], the exponential approaches zero. In particular, there is a time <math>t_{1/2} \,</math> such that
  
 
:<math>N(t_{1/2}) = N_0\cdot\frac{1}{2}. </math>
 
:<math>N(t_{1/2}) = N_0\cdot\frac{1}{2}. </math>
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: <math>t_{1/2} = \frac{\ln 2}{\lambda}. \,</math>
 
: <math>t_{1/2} = \frac{\ln 2}{\lambda}. \,</math>
  
==Experimental determination==
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== See also ==
The half-life of a process can be determined easily by experiment.  In fact, some methods do not require advance knowledge of the law governing the decay rate, be it exponential decay or another pattern.
 
  
Most appropriate to validate the concept of half-life for [[radioactive decay]], in particular when dealing with a small number of atoms, is to perform experiments and correct computer simulations. See in [http://www.madsci.org/posts/archives/Mar2003/1047912974.Ph.r.html] how to test the behavior of the last atoms. Validation of physics-math models consists in comparing the model's behavior with experimental observations of real physical systems or valid simulations (physical and/or computer). The references given here describe how to test the validity of the exponential formula for small number of atoms with simple simulations, experiments, and computer code.
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* [[Exponential decay]]
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* [[Mean lifetime]]
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* [[Radioactive decay]]
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* [[Rate equation]]
  
In radioactive decay, the exponential model does '''not''' apply for a small number of atoms (or a small number of atoms is not within the domain of validity of the formula or equation or table). The DIY experiments use pennies or [[M&M's]] candies. [http://www.exploratorium.edu/snacks/radioactive_decay.html], [http://www.sciencenetlinks.com/lessons.cfm?DocID=178].  A similar experiment is performed with isotopes of a very short half-life, for example, see Fig 5 in [http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/cassy_pa_hwz-e.htm]. See how to write a computer program that simulates radioactive decay including the required [[random]]ness in [http://astro.gmu.edu/classes/c80196/hw2.html] and experience the behavior of the last atoms.  Of particular note, atoms undergo radioactive decay in whole units, and so after enough half-lives the remaining original quantity becomes an actual zero rather than [[asymptote|asymptotically]] approaching zero as with [[continuous function|continuous]] systems.
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== Notes ==
 +
<references/>
  
== See also ==
+
== References ==
{{wiktionary|half-life}}
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* Emsley, John. ''Nature's Building Blocks: An A to Z Guide to the Elements''. Oxford: Oxford University Press, 2003 (original 2001). ISBN 0198503407
* [[Exponential decay]]
+
* Magill, Joseph, and Jean Galy. ''Radioactivity Radionuclides Radiation''. Berlin: Springer, 2004. ISBN 3540211160
* [[Mean lifetime]]
+
* Brown, G.I. ''Invisible Rays: A History of Radioactivity''. Sutton Publishing, 2002. ISBN 0750926678
* [[Elimination half-life]]
 
* For non-exponential decays, see half-life in the article [[Rate equation]]
 
  
== External links==
 
* Time constant [http://www.facstaff.bucknell.edu/mastascu/elessonshtml/SysDyn/SysDyn3TCBasic.htm]
 
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]

Latest revision as of 16:04, 3 August 2023

Uranium ore. The most abundant uranium isotope is 238U, with a half-life of 4.5 × 109 years.

If a sample of material decays at a certain rate over time, its half-life is defined as the time it takes for the sample to decay to half its initial amount. This concept originated when studying the exponential decay of radioactive isotopes, but it is applied to other phenomena as well, including those described by non-exponential decay. An exponential decay process, as exemplified by the decay of radioactive isotopes, for example, is simply one in which the number of atoms disintegrating per unit time is proportional to the total number of radioactive atoms present.

In the case of radioactive decay, each different radioactive isotope has a particular half-life that is unaffected by changes in the physical or chemical conditions of the surroundings. This property is the basis for radiometric dating of rocks and fossils. In pharmacology, the half-life of a drug (in a biological system) is the time it takes for the drug to lose half its pharmacologic activity.

Example of radioactive decay

Carbon-14 (14C) is a radioactive isotope that decays to produce the isotope nitrogen-14 (14N). The half-life of 14C is about 5,730 years. This means that if one starts with 10 grams of 14C, then 5 grams of the isotope will remain after 5,730 years, 2.5 grams will remain after another 5,730 years, and so forth.

Calculation of half-life

Number of
half-lives
elapsed
Fraction
remaining
As
power
of 2
0 1/1
1 1/2
2 1/4
3 1/8
4 1/16
5 1/32
6 1/64
7 1/128
... ...

The table at right shows the reduction of the quantity in terms of the number of half-lives elapsed.

It can be shown that, for exponential decay, the half-life obeys the following relation:

where

  • is the natural logarithm of 2, and
  • , called the decay constant, is a positive constant used to describe the rate of exponential decay.

In addition, the half-life is related to the mean lifetime τ by the following relation:

The constant can represent various specific physical quantities, depending on the process being described.

  • In first-order chemical reactions, is the reaction rate constant.
  • In pharmacology (specifically pharmacokinetics), the half-life of a drug is defined as the time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacologic, physiologic, or radiologic activity.[1]
  • For electronic filters such as an RC circuit (resistor-capacitor circuit) or an RL circuit (resistor-inductor circuit), is the reciprocal of the circuit's time constant , which is the same as the mean lifetime mentioned above. For simple RC or RL circuits, equals or , respectively. The symbol is related to the circuit's cutoff frequency fc by
or, equivalently, .

Experimental determination

The half-life of a process can be readily determined by experiment. Some methods do not require advance knowledge of the law governing the decay rate, whether it follows an exponential or other pattern of decay.

Most appropriate to validate the concept of half-life for radioactive decay, in particular when dealing with a small number of atoms, is to perform experiments and correct computer simulations. Validation of physics-math models consists of comparing the model's behavior with experimental observations of real physical systems or valid simulations (physical and/or computer simulations).[2]

When studying radioactive decay, the exponential model does not apply for a small number of atoms (or a small number of atoms is not within the domain of validity of the formula or equation or table). Some model simulations use pennies or pieces of candy.[3][4] A similar experiment is performed with isotopes that have very short half-lives.[5]

Decay by two or more processes

Some quantities decay by two processes simultaneously. In a manner similar to that mentioned above, one can calculate the new total half-life () as follows:

or, in terms of the two half-lives and

that is, half their harmonic mean.

Derivation

Quantities that are subject to exponential decay are commonly denoted by the symbol . (This convention suggests a decaying number of discrete items, an interpretation that is valid in many, but not all, cases of exponential decay.) If the quantity is denoted by the symbol , the value of at a time is given by the formula:

where is the initial value of (at )

When , the exponential is equal to 1, and is equal to . As approaches infinity, the exponential approaches zero. In particular, there is a time such that

Substituting into the formula above, we have

See also

Notes

  1. Taken from Medical Subject Headings. Year introduced: 1974 (1971).
  2. See link to test the behavior of the last remaining atoms of a radioactive sample. Retrieved October 19, 2007.
  3. Radioactive Decay Model Retrieved October 19, 2007.
  4. Radioactive Decay Retrieved October 19, 2007.
  5. For example, see Figure 5 in the link. See how to write a computer program that simulates radioactive decay including the required randomness in the link and experience the behavior of the last atoms. Retrieved October 19, 2007.

References
ISBN links support NWE through referral fees

  • Emsley, John. Nature's Building Blocks: An A to Z Guide to the Elements. Oxford: Oxford University Press, 2003 (original 2001). ISBN 0198503407
  • Magill, Joseph, and Jean Galy. Radioactivity Radionuclides Radiation. Berlin: Springer, 2004. ISBN 3540211160
  • Brown, G.I. Invisible Rays: A History of Radioactivity. Sutton Publishing, 2002. ISBN 0750926678

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