Titanium-44 gets a lifetime
Stan Woosley Roland Diehl
This article is reproduced from Physics World, July 1998.
New laboratory measurements have probed what happens inside an exploding supernova. Three independent research teams have provided new and more accurate measurements of the half-life of 44Ti, a radioactive isotope of titanium produced in supernovae. Previous measurements had suggested a half-life of about 50 years, but with a large variation in values that was not well understood. Moreover, this half-life could not explain the large amount of 44Ti thought to exist in the remnant of the Casseopeia(Cas) A supernova, which exploded in our galaxy about 320 years ago. The improved measurements suggest that the half-life of 44Ti is longer, about 60 years, in better agreement with models of supernova nucleosynthesis.
According to the modern theory of stellar nucleosynthesis, almost all of the elements heavier than hydrogen and helium were produced by nuclear fusion in stars, and then spread around the universe by supernovae, stellar winds, planetary nebulae and so on. One of the interesting predictions of this theory is that some elements were not produced as they exist today, but as radioactive precursors. For example, the most abundant isotope of iron, 56Fe, originates from an unstable isotope of nickel, 56Ni. This isotope is produced in supernovae and decays first to cobalt and then to iron.
Heavy elements that contain a large number of protons are stabilized through an excess of neutrons: 56Fe, for example, has 26 protons and 30 neutrons. But supernovae generally contain material with equal numbers of neutrons and protons. Nuclei formed from nuclear burning are often unstable, later decaying to a stable isotope with excess neutrons. Indeed, it is this radioactive decay of 56Ni that makes a supernova shine for months after its explosion.
44Ti is one of the most interesting radioactive precursors. It is composed of equal numbers of neutrons and protons, while its stable product, 44Ca, has 24 neutrons and 20 protons (i.e. two protons turn to neutrons during the decay). The isotope is interesting for the study of nucleosynthesis for three reasons. First, its half-life is comparable with the expected interval between supernovae in our galaxy, roughly 30–100 years.
Second, during its decay to 44Ca it emits gamma-rays of 67.9, 78.4 and 1157 keV, which can be detected by gamma-ray telescopes operating above the Earth’s atmosphere. Finally, the synthesis of 44Ti requires exceptionally high temperatures, at least 5´ 109 K. This implies that it is only created in the very deepest layers ejected from a supernova, near the so-called "mass cut" that separates the remnant from the ejected supernova.
In 1994 the COMPTEL gamma-ray telescope situated on board the Compton Gamma-Ray Observatory first reported 1157keV gamma-rays from Casseopeia A, which is about 11 000 light-years away. This initially surprised astrophysicists, because the accepted half-life suggested that too little 44Ti would be left in the supernova for the instrument to detect. The initial gamma-ray flux reported by the COMPTEL team was quite uncertain, however, because it was difficult to separate the signal from the large noise levels in the instrument produced by particle bombardment in the Earth’s radiation belts. But the instrument’s imaging capability means that COMPTEL can extract weak signals from such noisy background data, and this observation is now widely accepted.
In 1997 Anatoli Iyudin from the Max Planck Institute and his COMPTEL colleagues derived a flux of 4.2(± 0.9)´ 10–5 for gamma-rays at 1157 keV. Non-imaging gamma-ray instruments have not yet detected gamma-rays due to 44Ti decay, but their sensitivities are lower.
The measured flux can be translated into the amount of 44Ti produced in CasA. However, this derived figure is very sensitive to the half-life (figure a). Previous half-lives ranged from 46 to 67 years, even though the uncertainty in the measurements was only about 2–3 years. Indeed, the half-life is very difficult to measure because only small amounts of the material can be produced, and its radioactivity level is very low. Some methods measure the radioactivity of a sample, while others measure the slow fall in the decay curve.
The gamma-rays detected from CasA have stimulated a burst of new laboratory efforts. A US–Canada collaboration, using the National Superconducting Cyclotron Laboratory at Michigan State University, has improved the radioactivity method by using a mixed radioactive beam of 44Ti and 22Na (J Görres et al. 1998 Phys. Rev. Lett. 80 2554). The half-life of 22Na is well known, so systematic uncertainties can be eliminated by measuring the activity of 44Ti relative to that for 22Na. The approach appears convincing, and agrees with several careful measurements from the decay curve.
One of these measurements was made by Irshad Ahmad and colleagues, who have combined independent measurements from three laboratories – the Argonne National Lab in the US, the Hebrew University in Jerusalem, and the Consiglio Nazionale Ricerche in Torino, Italy (Phys. Rev. Lett. 1998 80 2550). Meanwhile, Eric Norman and colleagues at the Lawrence Berkeley National Lab in California combined measurements over several time periods (Phys. Rev. C 1998 57 2010). All three results suggest a half-life of 60± 1 years, which should finally settle the issue. This longer half-life suggests that CasA has ejected about 2´ 10–4 solar masses of 44Ti, which is less than previously thought (figure b).
So what are the implications for our theory of nucleosynthesis in supernovae? The result suggests that there is still a large amount of material to be ejected, but not incredibly so. Standard calculations of elemental production in supernovae suggest that about 0.1–2.3´ 10–4 solar masses should be ejected, with a typical value of around 0.3´ 10–4. Larger values generally correspond to bigger stars (greater than 30 solar masses) and greater explosion energies. Other calculations suggest that slightly more 44Ti is produced due to different nuclear reaction rates and alternative ways of determining the mass cut. Asymmetries in the explosion mechanism could also enhance the synthesis of 44Ti, suggesting that a half-life of 60 years is consistent with the range of estimates from the supernovae models.
Perhaps the most interesting point is that CasA produced any 44Ti at all. Since this isotope is only created in the deepest layers of the supernova, its ejection implies that all of the overlying material must have escaped. In all models to date, the material just outside of 44Ti is very rich in 56Ni, suggesting that the supernova would have created at least 0.1 solar masses of 56Ni. Yet the decay to 56Fe releases so much energy that CasA should have been exceptionally bright, roughly 10 000 times brighter than was actually observed.
Although this seems incredible, it may be reasonable. CasA is far away in the disk of the galaxy, and dust along this line could have reduced the amount of light reaching Earth by a factor of 100. If CasA was also embedded in a dusty shell during the explosion – as suggested from X-ray images of the remnant – the light received could be reduced by another factor of 100. Finally, we know that the 44Ti escaped from the exploding star, which means we can put limits on the mass of the object that still lurks at the heart of CasA, most probably a neutron star. So why haven’t we seen this object yet, and when are we likely to see it? These are heady implications indeed for something as apparently simple as the half-life of an unstable nucleus.