Calcium-48

Infobox isotope

alternate_names =
symbol =Ca
mass_number =48
mass = 47.952534(4)
num_neutrons = 28
num_protons = 20
abundance =0.187%
halflife = (4.3^+3.8_-2.5)E+19 a
background =#f99
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Calcium-48 is a rare isotope of calcium containing 20 protons and 28 neutrons. It makes up 0.187% of natural calcium by mole fraction. [cite web| url = http://physics.nist.gov/PhysRefData/Compositions/index.html| title = Atomic Weights and Isotopic Compositions| accessdate = 2006-10-27| last = Coursey| first = J. S.| coauthors = D. J. Schwab; R. A. Dragoset| year = 2005| month = February| work = NIST Physical Reference Data] Although it is unusually neutron-rich for such a light nucleus, the only radioactive decay pathway open to it is the extremely rare process of double beta decay. Its half-life is about 4.3×10¹⁹ years,cite journal | last = Balysh | first = A. | coauthors = "et al." | year = 1996 | title = Double Beta Decay of ⁴⁸Ca | journal = Physical Review Letters | volume = 77 | pages = 5186–5189 | doi = 10.1103/PhysRevLett.77.5186 ] so for all practical purposes it can be treated as stable. One factor contributing to this unusual stability is that 20 and 28 are both magic numbers, making ⁴⁸Ca a "doubly magic" nucleus.

Since ⁴⁸Ca is both stable and neutron-rich, it is a valuable starting material for the production of new nuclei in particle accelerators, both by fragmentation [cite journal | last = Notani | first = M. | coauthors = "et al." | year = 2002 | title = New neutron-rich isotopes, ³⁴Ne, ³⁷Na and ⁴³Si, produced by fragmentation of a 64A MeV ⁴⁸Ca beam | journal = Physics Letters B | volume = 542 | pages = 49–54 | doi = 10.1016/S0370-2693(02)02337-7 ] and by fusion reactions with other nuclei, for example in the recent production of ununoctium. [cite journal | last = Oganessian | first = Yu. Ts. | coauthors = "et al." | year = 2006 | month = October | title = Synthesis of the isotopes of elements 118 and 116 in the ²⁴⁹Cf and ²⁴⁵Cm+⁴⁸Ca fusion reactions | journal = Physical Review C | volume = 74 | pages = 044602 | doi = 10.1103/PhysRevC.74.044602 ] Heavier nuclei generally require a greater fraction of neutrons for maximum stability, so neutron-rich starting materials are necessary.

⁴⁸Ca is the lightest nucleus known to undergo double beta decay and the only one simple enough to be analyzed with the "sd" shell model. It also releases more energy (4.27 MeV) than any other double beta decay candidate. These properties make it an interesting probe of nuclear structure models and a promising candidate in the ongoing search for neutrinoless double beta decay.

References