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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?
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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
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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
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