element. The isotopes of tin have atomic masses of 112, 114, 115, 116, 117, 118, 119, 120, 122, and 124. 120Sn makes up almost a third of all tin; 118Sn, and 116Sn are also common, while 115Sn is the least common stable isotope. The isotopes with even mass numbers have no nuclear spin, while those with odd mass numbers have a spin of 1/2. Tin is among the easiest elements to detect and analyze by NMR spectroscopy which relies on molecular weight and its chemical shifts are referenced against . The large number of stable isotopes is thought to be a direct result of tin having the atomic number 50, a "magic number" in nuclear physics. Of the stable isotopes Tin-115 has a high capture cross section for fast neutron energies at 30 Barns. Two other isotopes Tin-117 ranks next with a cross section of 2.3 Barn while isotope Tin-119 has a slightly smaller cross section of 2.2 Barn. Before these cross sections were well known it was proposed to use Tin-Lead solder as a reactor coolant for fast reactors because of its low melting point. Current studies are for Lead or Lead-Bismuth reactor coolants because both heavy metals are nearly transparent to fast neutrons with very low capture cross sections. In order to use a Tin or Tin-Lead coolant the Tin would first have to go through isotopes separation to remove the 115, 117 and 119 isotopes from the material. Combined these three isotopes make up about 17% of the entire mass of natural Tin but represent nearly all of the capture cross section. Of the remaining seven isotopes Tin-112 has a capture cross section of 1 Barn. The other six isotopes forming 82.7% of all Tin have capture cross sections of 0.3 Barn or less making them effectively transparent to neutrons like Lead and Bismuth. Tin has 31 unstable isotopes, ranging in mass number from 99 to 139. The unstable tin isotopes have a half-life of less than a year except 126Sn which has a half-life of 230,000 years. 100Sn and 132Sn are two of the few nuclides with a "doubly magic" nucleus which despite being unstable, as they have very uneven neutron–proton ratios, are the endpoints beyond which tin isotopes lighter than 100Sn and heavier than 132Sn are much less stable. Another 30 metastable isomers have been identified for tin isotopes between 111 and 131, the most