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Abundances of Isotopes from nuclear line spectroscopy Janina Fiehl General facts about nuclear gamma line spectroscopy • The natural gamma radiation in the cosmos covers several orders of magnitudes in terms of their energy- roundabout 70 keV to 8 MeV. • Generally speaking the observed gamma particles have their origin in electron- positron annihilation or in the decay of radioactive elements. • The first gamma line from space was measured in the so-called HEAO-C1 experiment, which detected a gamma-ray line of the element 26Al in 1982. • Gamma spectroscopy has many advantages compared with optical measurements, especially when detecting super nova related data: the decays are not affected by the heat- or mass density, and while optical methods are affected by the shell structure of supernovae (several months can pass until the core area becomes visible), gamma spectroscopy is not. • In addition, the gamma radiation crosses clouds of interstellar gas and dust much easier than other kinds of electromagnetic radiation. Where do heavy elements in the universe come from? • Generally speaking, all elements that have a mass number higher than 4 must be generated in stars. The fusion process works well (exothermic) up to a mass number of roundabout 60. • Heavier elements can only be generated with the help of neutron currents, whereby we differentiate between the so-called R- (rapid) and S- (slow-) process. • Heavy elements can resign from starts in supernova explosions or with the help of convection- they are then carried away by solar winds. • These elements can only be used for gamma line spectroscopy once they have left the starts, as the star itself is opaque for the gamma radiation from within. Which fundamental information can we gain from gamma line spectroscopy? • • • • • The position at which certain elements are created helps to identify the active regions in the galaxy as well as the objects that can be found there. Elements that are generated at different times in the life of stars: Their detection and relative occurrence can help us to test star models. Instable elements with a long lifetime give information about the element‘s average production in certain parts of the galaxy. One example of such an element is 26Al (with a half life of τ= 1.04 106 years). Instable elements with a short lifetime (shorter than the average production rate) can help us to improve our understanding of time-critical events such as supernovae. As an example, 44Ti (τ= 87years) is used for this purpose. The shape of the line spectrum of the gamma particles can provide information concerning the circumstellar and interstellar matter as the gamma particles are slowed down by the interaction with such matter after they have been ejected. Furthermore, the Doppler broadening of the lines provides information concerning the rotational speed of different areas of the galaxy. Which elements and isotopes are suitable for gamma line spectroscopy? Source: Diehl, http://www.mpe.mpg.de/gamma/science/lines/ A short overview of previous and existing experiments • The first nuclear gamma ray line from space was detected with the experiment HEAO-C1 (26Al) in 1982. • Several balloon and satellite experiments followed. • In 1987, the SMM (solar maximum mission) satellite was able to observe the supernova 1987A. • The Compton Gamma ray observatory (1991 until 2000) launched by NASA , sensitive in regions from 0.1 to 10 MEV with E/DE= 10 performed extensive sky surveys, and was able to observe the supernova Cassiopeia A in 1994. • The INTEGRAL mission (ESA), launched in 2002, covers an energy range of 0.02- 8 MEV with E/DE=600 (at 2 MeV) with its spectrometer SPI. A short overview of previous and existing experiments SPI Payload module aboard INTEGRAL. SPI is a highresolution Germanium spectrometer. Source: Official MPE/MGG homepage Important elements in gamma line spectroscopy: 26Al Source: ESA official homepage Important elements in gamma line spectroscopy: 26Al Source: Diehl, Radioactive 26Al from massive stars in the Galaxy Important elements in gamma line spectroscopy: 26Al • The picture on the previous page shows at the left the line profiles obtained at different locations in the universe; a clear line shift in the top and bottom panels compared to the center panel (l = 0) is visible. Detailed modeling shows that the shift is fully consistent with the Doppler shift of the line energy (1809 keV) expected from the galactic rotation. • The map on the right shows this expectation based on modeling the Galactic rotation curve and a 3-dimensional distribution of 26Al sources. The dominant sources of 26Al emission are massive stars. This observation shows that 26Al emission is actually global, as we see the entire amount of it throughout our Galaxy and co-rotate with the Galaxy. • From this observation a present-day equilibrium mass of 2.8 +/- 0.8 solar masses of 26Al can be determined. In addition, this observation allows to independently estimate the frequency of core-collapse supernovae in our Galaxy to be 1.9 +/- 1.1 events per century. Important elements in gamma line spectroscopy: 26Al Source: Limongi, The Nucleosynthesis of 26Al and 60Fe in solar metallicity stars extending in mass from 11 to 120 solar units: the hydrostatic and explosive contributions Important elements in gamma line spectroscopy: 60Fe Source: Wang, SPI observations of the diffuse 60Fe emission in the Galaxy The SPI instrument on board INTEGRAL has observed the 60Fe lines (at 1173 and 1333 keV) from the inner galaxy. This picture shows the combined 60Fe signal from the two lines. The line flux is 3.7 ± 1.1 x 105 cm-2 s-1 per line. Important elements in gamma spectroscopy: 60Fe • The origin of the iron line is believed to be corecollapse supernovae which enrich the interstellar medium with isotopes such as 60Fe. From SPI measurements of the 26Al line the gamma ray flux ratio 60Fe/26Al = 0.148±0.06 is derived. 60Fe/26Al gamma flux ratio in different experiments and theoretical models Source: Wang, SPI observations of the diffuse 60Fe emission in the Galaxy. Upper hatched region: Prantzos 2004, solid line from Timmes 1995, lower hatched region Limongi 2006. Important elements in gamma line spectroscopy: 44Ti (Titanium) Source: Renaud, The Signature of 44Ti in Cassiopeia A Revealed by IBIS/ISGRI on INTEGRAL With a lifetime of 87 years, 44Ti emits two gamma-ray lines at 67.9 keV and 78.4 keV (these two lines can be seen above) and its remnant 44Sc decays at 1157 keV ,observable with SPI and IBIS onboard INTEGRAL. Important elements in gamma line spectroscopy: 44Ti (Titanium) • Due to its short decay time, 44Ti is supposed to be an ideal element to study the inner region of supernovae and their early remnants. • So, far, the element 44Ti has only been detected in one supernova event, which is Cassiopeia A (1997).It was detected with the help of the 1.16 Mev line of its remnant, 44Sc (Scandium) which decays to 44Ca. The flux measurements for this gamma particle are consistent with approx. 2.5 10-5 cm-2 s-1 • This is a surprising result as it was expected that galactic supernova should create a detectable line flux of approx. 10-4 cm-2 s-1, 2-3 supernova events in each century and a yield of 10-4 sun masses per event assumed. • It is not yet clear why Cas A is the only source of 44Ti that has so far been detected, but it clearly shows our understanding for supernova kinematics is rather incomplete. Important elements in gamma line spectroscopy: 44Ti (Titanium) Source: Diehl, http://www.mpe.mpg.de/gamma/science/lines/44Ti/44Ti_science.html Important elements in gamma line spectroscopy: 44Ti (Titanium) • Another interesting fact is the ejection speed of the 44Ti from Cas A- it is 430 (± 240) km s-1 • This is a small speed in comparison to the outer shells of exploding stars, but, but at the same time, a gamma line of Iron with a speed of more than 7000 km s-1 is supposed to have been detected. • An explanation might be an asymmetric explosion of the object. Electron-positron annihilation radiation and possible dark matter detection Sky map of the 511 keV gamma-ray line emission produced by electron-positron annihilation. Contour levels indicate intensity levels of 10-2, 10-3, and 10-4 photons cm-2 s-1 sr-1 (from the centre outwards). Electron-positron annihilation radiation and possible dark matter detection • • • • • The result seen on the previous page is unexpected, as one would expect the most annihilation radiation in the galactic disk. The reason is that possible sources, such as stars that produce radioactive material, supernovae, pulsars and others mainly live in the galactic disk (spiral arms). The line profile of the annihilation line suggests two distinctive components: a broad component from annihilation in a warm, neutral medium (several thousand Kelvin) in which positronium generation trough charge exchange dominates, and a narrow component from a warm, but ionized medium, in which the coulomb losses dominate and the positronium is formed radiatively. Another interesting fact is the disk’s asymmetry. It is hoped that its shape might deliver important facts that help to explain the radiation from the galactic center (one suggestion is a group of many binary star systems). Finally, it is suggested that this signal might be causes by dark matter which is trapped in the galaxy’s gravitational field, and decays to positrons. Electron-positron annihilation radiation and possible dark matter detection Electron-positron annihilation radiation and possible dark matter detection The distribution of low-mass X-ray binaries (LMXBs) observed with the imager IBIS above 20 keV (lower image) is remarkably similar to that of the positron annihilation line. The resemblance of the two distributions suggests that hard X-ray emitting LMXBs may be a major source of positrons in our galaxy. Source: Weidenspointer, An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays • Thank you very much for your attention! • Any questions?