Radioactives Missing From The Earth
Radioactives Missing From The Earth
The 84 elements found on Earth occur as 339
isotopes
. Only 269 of these are
stable, and the other 70 are radioactive. An additional 1650
radioactive isotopes have been created in nuclear reactors and in
particle accelerators.
The following is a table of all 29 known radioactive isotopes that
have a half-life of one million years or more, and that are not
being continually produced
by natural nuclear
reactions. It has been sorted in order of half-life. For each
isotope, the table shows whether it is one of the ones found on Earth.
Isotope
|
Half-Life
(megayears)
|
Found on
Earth?
|
|
|
|
Vanadium 50
|
6,000,000,000
|
yes
|
Neodymium 144
|
2,400,000,000
|
yes
|
Hafnium 174
|
2,000,000,000
|
yes
|
Platinum 192
|
1,000,000,000
|
yes
|
Indium 115
|
600,000,000
|
yes
|
Gadolinium 152
|
110,000,000
|
yes
|
Tellurium 123
|
12,000,000
|
yes
|
Platinum 190
|
690,000
|
yes
|
Lanthanum 138
|
112,000
|
yes
|
Samarium 147
|
106,000
|
yes
|
Rubidium 87
|
48,800
|
yes
|
Rhenium 187
|
43,000
|
yes
|
Lutetium 176
|
35,000
|
yes
|
Thorium 232
|
14,000
|
yes
|
Uranium 238
|
4,470
|
yes
|
Potassium 40
|
1,250
|
yes
|
Uranium 235
|
704
|
yes
|
Samarium 146
|
103
|
no
|
Plutonium 244
|
82
|
by extreme effort
|
Curium 247
|
16
|
no
|
Lead 205
|
15
|
no
|
Hafnium 182
|
9
|
no
|
Palladium 107
|
7
|
no
|
Cesium 135
|
3
|
no
|
Technetium 97
|
3
|
no
|
Gadolinium 150
|
2
|
no
|
Zirconium 93
|
2
|
no
|
Technetium 98
|
2
|
no
|
Dysprosium 154
|
1
|
no
|
The thing to notice is that this list falls naturally into two
halves. Short-lived radioactives are suspiciously absent from the
Earth. If we had carried this list all the way down to 1,000 year
half-lives, the block of
no
's would be 37 long instead of 10
long.
The most obvious explanation for the above is that all these
elements were
present when the Earth was
formed
, but by now the short-lived ones have decayed away. This
explanation is
compatible
with the age
scientists accept for the Earth.
Of course, nothing about this list really proves that the Earth is
old. But the list is exactly what we would expect if the Earth is old,
and it is a very puzzling list if the Earth is young.
Footnotes:
- The list is of isotopes not being continually produced on
Earth.
I left out four isotopes because of this rule.
- Manganese 53 and Beryllium 10 are produced by cosmic-ray radiation
hitting dust in the upper atmosphere.
- Uranium 236 is produced in uranium ores by neutrons from other
radioactives.
- Iodine 129 is produced from Tellurium 130 by cosmic-ray muons.
Radioactives with half-lives shorter than one million years are
also produced: for example,
Carbon 14
(half life 5730 years).
The missing isotopes could have been present when the Earth was
formed.
It is reasonable to ask if they are missing because they
were somehow never created in the first place. The answer is that they
are not particularly difficult to produce "artificially", and current
scientific theories about stars and supernovas say that these elements
should have been produced in fairly large quantities. For example,
Technetium 97 is in the
no
list above, but it has been detected
in stars. The scientific theory about stars shows how they
manufacture Technetium 97 - and how Supernova 1987a
manufactured Cobalt-56
.
The list is compatible with the age scientists accept for the
earth.
That age is 4.55 billion years. For most practical
purposes, a radioactive material is no longer present after 10 or 20
of its half-lives. This is because 2
10
is about a thousand,
and 2
20
is about a million. So, after 20 half-lives, only
one millionth of the original amount remains.
Uranium 235's half life is 704 million years, so 4.55 billion years
is just a bit over six half-lives. It's reasonable for Uranium 235 to
still be around in small quantities after that amount of time. And,
in fact, it makes up about one percent of the Uranium now on Earth.
The amounts of Uranium 235 and Uranium 238 would have been about
equal, 4.55 billion years ago.
Finding Plutonium 244.
Its half life is 82 million years,
so 4.55 billion years is 55 half lives. You might reasonably ask how
come Plutonium 244 isn't listed as
no
. The answer is that
someone made a very serious effort to find it: their article is
referenced below. Eighty five kilograms of molybdenum ore were
chemically concentrated, and then the lot was tediously run through a
mass spectrometer. The amount of Plutonium 244 they found,
10
-14
grams, was so small that it would have averaged one
single radioactive decay every six years. Clearly, they could not have
detected this Plutonium 244 with a geiger counter. However, 55 half
lives ago, it would have been about one kilogram of plutonium
metal. That's believable in 85 kilograms of metal ore.
Samarium 146's half life is 103 million years, so 4.55 billion
years is 44 half lives. This means that Samarium 146 could be 200
billion times rarer than Uranium 235, but could be a thousand times
commoner than Plutonium 244. I predict that if anyone tries very very
hard to find Samarium 146, they will succeed. Curium 247, at almost
300 half lives, is completely out of the question.
In short, the cutoff point in the list is consistent with 4.55
billion years.
References:
The Age Of The Earth
, pages 80
and 376-387. However, I have used a more recent measurement of
Samarium 146's half life: see
WebElements
and
Brookhaven National
Labs
.
Detection of Plutonium-244 in Nature
, Hoffman et al,
Nature
234,132-134 (19 November 1971)
Last modified: 30 July 2000
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