Changes

<math>A = A_0 e^{-(t-t_0)/\tau}</math>

<math>A = A_0 e^{-(t-t_0)/\tau}</math>

Where <math>A</math> is the amount at time t and <math>A_0</math> is the amout at time <math>t_0</math>. <math>\tau</math> is the mean lifetime, which is the reciprocal of <math>\lambda</math>.

Where <math>A</math> is the amount at time t and <math>A_0</math> is the amount at time <math>t_0</math>. <math>\tau</math> is the mean lifetime, which is the reciprocal of <math>\lambda</math>.

Mean lifetime is a constant for any given isotope, but half-life (<math>\tau</math> ln 2) is of more practical use.

Mean lifetime is a constant for any given isotope, but half-life (<math>\tau</math> ln 2) is of more practical use.

The [[creationist]] blog [[Answers in Genesis]] suggests that half-life varies over time.<ref>[http://www.answersingenesis.org/articles/am/v1/n1/creation-research Raising the Bar on Creation Research - by Don DeYoung, Ph.D.]</ref>

The [[creationist]] blog [[Answers in Genesis]] suggests that half-life varies over time.<ref>[http://www.answersingenesis.org/articles/am/v1/n1/creation-research Raising the Bar on Creation Research - by Don DeYoung, Ph.D.]</ref>

However, indirect observations can allow us to infer radioactive decay rates over much longer time-scales. For example we can measure gamma radiation rates at specific frequencies from distant [[supernovae]] and compare this to the rate expected for the mass of the star. This has given rates for supernoave as distant as 169,000 [[light year]]s which are consistent with those measured today. Thus it would seem decay rates have been the same for at least the past 169,000 years.<ref>http://www.talkorigins.org/indexcc/CF/CF210.html</ref>

However, indirect observations can allow us to infer radioactive decay rates over much longer time-scales. For example, we can measure gamma radiation rates at specific frequencies from distant [[supernovae]] and compare this to the rate expected for the mass of the star. This has given rates for supernoave as distant as 169,000 [[light year]]s which are consistent with those measured today. Thus it would seem decay rates have been the same for at least the past 169,000 years.<ref>http://www.talkorigins.org/indexcc/CF/CF210.html</ref>

More recent measurements have noted some slight cyclic changes in decay rates on a 33-day period.<ref>http://www.purdue.edu/newsroom/research/2010/100830FischbachJenkinsDec.html</ref> While no definite explanation has been found, it is notable that 33 days is also the rotation period of the [[Sun]]'s core. It is possible that [[neutrino]] emissions from the sun are affecting either the measuring equipment or the rate of decay (making it stimulated rather than spontaneous). However, since this variation is cyclic it does not have any significant effect on radiometric calculations. It might however be important when calculating doses in the field of [[radiotherapy]].  

More recent measurements have noted some slight cyclic changes in decay rates on a 33-day period.<ref>http://www.purdue.edu/newsroom/research/2010/100830FischbachJenkinsDec.html</ref> While no definite explanation has been found, it is notable that 33 days is also the rotation period of the [[Sun]]'s core. It is possible that [[neutrino]] emissions from the sun are affecting either the measuring equipment or the rate of decay (making it stimulated rather than spontaneous). However, since this variation is cyclic it does not have any significant effect on radiometric calculations. It might however be important when calculating doses in the field of [[radiotherapy]].