ratio of xenon-136 to xenon-135 (or its decay products) can give hints as to the power history of a given reactor and the absence of xenon-136 is a "fingerprint" for nuclear explosions, as xenon-135 is not produced directly but as a product of successive beta decays and thus it can't absorb any neutrons in a nuclear explosion which occurs in fractions of a second. The stable isotope xenon-132 has a fission product yield of over 4% in the thermal neutron fission of which means that stable or nearly stable xenon isotopes have a higher mass fraction in spent nuclear fuel (which is about 3% fission products) than it does in air. However, there is as of 2022 no commercial effort to extract xenon from spent fuel during nuclear reprocessing. Isotopes Naturally occurring xenon is composed of seven stable isotopes: 126Xe, 128–132Xe, and 134Xe. The isotopes 126Xe and 134Xe are predicted by theory to undergo double beta decay, but this has never been observed so they are considered stable. In addition, more than 40 unstable isotopes have been studied. The longest lived of these isotopes are the primordial 124Xe, which undergoes double electron capture with a half-life of , and 136Xe, which undergoes double beta decay with a half-life of . 129Xe is produced by beta decay of 129I, which has a half-life of 16 million years. 131mXe, 133Xe, 133mXe, and 135Xe are some of the fission products of 235U and 239Pu, and are used to detect and monitor nuclear explosions. Nuclear spin Nuclei of two of the stable isotopes of xenon, 129Xe and 131Xe, have non-zero intrinsic angular momenta (nuclear spins, suitable for nuclear magnetic resonance). The nuclear spins can be aligned beyond ordinary polarization levels by means of circularly polarized light and rubidium vapor. The resulting spin polarization of xenon nuclei can surpass 50% of its maximum possible value, greatly exceeding the thermal equilibrium value dictated by paramagnetic statistics (typically 0.001% of the maximum value at room temperature, even in the strongest magnets). Such non-equilibrium alignment of spins is a temporary condition, and is called hyperpolarization. The process of hyperpolarizing the xenon is called optical pumping (although the process is different from pumping a laser). Because a 129Xe nucleus has a spin of 1/2, and therefore a zero electric quadrupole moment, the 129Xe nucleus does not experience any quadrupolar interactions during collisions with other atoms, and the hyperpolarization persists