Plutonium-244

Main article: Isotopes of plutonium
Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (a)		 Fission products of 235U by yield[2]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra№ 4–6 † 155Euþ
244Cm 241Puƒ 250Cf 227Ac№ 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Pu 243Cmƒ 29–97 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351

No fission products
have a half-life
in the range of
100–210k years…

241Am 251Cfƒ[4] 430–900
226Ra№ 247Bk 1.3k–1.6k
240Pu 229Th 246Cm 243Am 4.7k–7.4k
245Cmƒ 250Cm 8.3k–8.5k
239Puƒ 24.1k
230Th№ 231Pa№ 32k–76k
236Npƒ 233Uƒ 234U№ 150k–250k ‡ 99Tc₡ 126Sn
248Cm 242Pu 327k–375k 79Se₡
1.53M 93Zr
237Np 2.1M–6.5M 135Cs₡ 107Pd
236U 247Cmƒ 15M–24M 129I₡
244Pu№ 80M

...nor beyond 15.7M years[5]

232Th№ 238U№ 235Uƒ№ 0.7G–14.1G

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
m  metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4a–97a: Medium-lived fission product
‡  over 200ka: Long-lived fission product

Plutonium-244 (244Pu) is an isotope of plutonium that has a half-life of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any actinide except for the three naturally abundant ones uranium-235 (704 million years), uranium-238, and thorium-232. It is also longer than any other radioactive isotopes except samarium-146 (103 million years), potassium-40 (1248 million years), and a large number of isotopes with half-lives longer than the age of the universe, such as lutetium-176 (38×109 years).

Accurate measurements, beginning in the early 1970s, have detected primordial plutonium-244.[6] The amount of 244Pu in the pre-Solar nebula (4.57×109 years ago) was estimated as 0.008 of amount of 238U.[7] As the age of the Earth is about 57 half-lives of 244Pu, the amount of plutonium-244 left should be very small; Hoffman et al. estimated its content in the rare-earth mineral bastnasite as c244=1.0×10−18 g/g, which corresponded to the content in the Earth crust as low as 3×10−25 g/g[6] (i.e. the total mass of plutonium-244 in the Earth crust is about 9 g). Since plutonium-244 cannot be easily produced by natural neutron capture in the low neutron activity environment of uranium ores (see below), its presence cannot plausibly be explained by any other means than creation by r-process nucleosynthesis in supernovae. Plutonium-244 is thus the shortest-lived and heaviest primordial isotope yet detected or theoretically predicted.

However, the detection of primordial 244Pu in 1971 is not confirmed by recent, more sensitive measurements[7] using the method of accelerator mass spectrometry. This work shows that the abundance of 244Pu on Earth was overestimated. In this study, no traces of plutonium-244 in the samples of bastnasite (taken from the same mine as in the early study) were observed, and only the upper limit on this content has been obtained: c244 < 0.15×10−18 g/g, which is 370 (or less) atoms per gram of the sample, at least 7 times lower than the abundance measured by Hoffman et al..

Unlike plutonium-238, plutonium-239, plutonium-240, plutonium-241, and plutonium-242, plutonium-244 is not produced in quantity by the nuclear fuel cycle, because further neutron capture on plutonium-242 produces plutonium-243 which has a short halflife (5 hours) and quickly beta decays to americium-243 before having much opportunity to further capture neutrons in any but very high neutron flux environments. However, a nuclear weapon explosion can produce some plutonium-244 by rapid successive neutron capture.

References

  1. ↑ Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three element gap of instability after polonium (84) where no isotopes have half-lives of at least four years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1600 years, thus merits the element's inclusion here.
  2. ↑ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  3. ↑ Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics 71 (2): 299. doi:10.1016/0029-5582(65)90719-4.
    "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 y. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
  4. ↑ This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
  5. ↑ Excluding those "classically stable" isotopes with half-lives significantly in excess of 232Th, e.g. while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion.
  6. ↑ 6.0 6.1 D. C. Hoffman, F. O. Lawrence, J. L. Mewherter, F. M. Rourke: "Detection of Plutonium-244 in Nature", in: Nature 1971, 234, 132–134; doi:10.1038/234132a0.
  7. ↑ 7.0 7.1 J. Lachner et al. Attempt to detect primordial 244Pu on Earth. Phys. Rev. C 85 (2012) 015801. doi:10.1103/PhysRevC.85.015801