Ask A Scientist© Physics Archive

name         T K
status       student
age          30s

Question -   I've read that scientist's can tell how old the Earth or
fossil's are by
by carbon-dating or rate of radioactive decay. My question: is the rate of
decay a constant no matter what the current atmospheric conditions are,
in other words would the rate of decay be different on a different planet
due to differences in gravity, atmosphere,temperature or whatever differences
there are. Does the decay stop or slow down in the vacuum of space?
------------------------------------------------
Let me begin by clarifying one point.  The rate of decay of a radioactive 
element
is not a constant.  Instead, the decay process can be described by a first 
order
(exponential) rate constant.  This means that the rate of decay is 
proportional to
the amount of material present.
                      rate = - k * [X]
     where k is the rate constant and [X] is the concentration of the 
material, X.
The negative sign indicates that the amount of material is decreasing with 
time.

As a result, if we know the amount at any time and the rate constant we can
predict how much was present at any time in the past and how much will be 
present
at any time in the future.

                       [X]' = [X] exp (- k * t)
     where [X] is the initial amount present, [X]' is the amount that will 
be left
after time, t.

Radiocarbon dating depends on the assumption that the relative abundance of the
radioactive carbon isotope in the atmosphere has been constant throughout 
earth's
history.  This concentration is used to estimate the initial concentration in
living material.  Once something dies it no longer gets fresh radioactive 
carbon
from its environment and the radioactive carbon it has continues to decay.  We
measure the amount of radioactive carbon, [X]', and then can use the equation
above to calculate time, t, since the decay rate constant, k, is known.

Since radioactive carbon is constantly being produced in the upper 
atmosphere by
interaction of non-radioactive gases with sunlight, and we have no reason to
believe that the rate of production has changed over earth's history, the
assumption that the relative abundance in the atmosphere has not changed is is
probably good.  Radioactive carbon dating has been checked against other 
methods,
such as comparing the spacings in the growth rings of trees, where 
possible, and
found to be correct.

Greg Bradburn
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There are many methods of radioactive dating, of which carbon dating is
only one.  It turns out that carbon dating doesn't work very well on old
things, because carbon-14's half life is too short.  It can be used
reliably only for artifacts younger than about 40,000 years.

There are several mechanisms of radioactive decay.  The most common ones
are alpha and beta decay, in which a nucleus gives off a helium nucleus and
an electron, respectively.  These mechanisms are completely insensitive to
the environment:  the rate of decay is constant and invariant.

There is one type of radioactive decay that is a little bit sensitive to
the environment.  This is electron capture, in which a nucleus absorbs an
electron.  This is the reverse of beta decay.  The process will go a bit
faster if the probability of the nucleus encountering an electron is
higher, as can happen if the chemical environment changes or the pressure
increases.  The change in rate is small, however, and tremendous pressure
changes are required to alter the rate measureably.  Anyhow, I don't think
that any nuclei used for radioactive dating decay by the electron capture
process (I could be wrong - I'm not a geologist).  At any rate, the most
important and widely-used radioactive dating methods use nuclei that decay
by the alpha or beta process.


Richard Barrans Jr., Ph.D.
================================================================

Hello,

The rate of decay (i.e., half-life) is characteristic of a radioactive 
element and "for all practical purposes" it is unaffected by temperature, 
pressure, atmospheric conditions, vacuum, space, etc.  I hope that someone 
else would comment on this question for "non practical purposes", i.e., 
small influences that other forces in nature may have on radioactive decay.

In radioactive dating, one compares the ratio of carbon 14 to carbon 
12.  This ratio in the earth's atmosphere is known.  When a living being 
dies and ceases to take in air, the ratio of carbon 14 to carbon 12 in its 
body begins to fall because of carbon 14 decay.
While carbon 14 decay is independent of the environment it is in, if we are 
going to use it for dating, we need to have  a reference. On the Earth, 
atmosphere is the reference because its ratio of C14/C12 is almost constant 
over the time scales for which carbon dating is appropriate.  Since C14's 
half life is over 5000 years, we can probably estimate age in samples in 
the 1000-100,000 year-old range using this technique. Older specimen would 
probably have too few C14 left in them to give us accurate estimates.   We 
could then use other elements or techniques.

Hope this helps you.
AK


Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
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