1. Institute for Structure and Nuclear Astrophysics
Nuclear Science Laboratory
Figure 3: 60Fe/ Fe concentration in ocean crust
sample vs. age of sample in millions of years.
References -- Figure 1: http://astronomyonline.org/Stars/HighMassEvolution.asp
Figure 2: Jose, Iliadis. Nuclear Astrophysics: the unfinished quest for the origin of the
elements.
Figure 3: Knie, et al. 60Fe Anomaly in a Deep-Sea Manganese Crust and Implications for a
Nearby Supernova Source.
AMS RESULTS:
Sample Concentration
Blank Sample:
MSU:
AMS Test:
VERA Fe-2:
VERA Fe-3:
VERA Fe-4:
3
21
Part 2: Activity Measurement & Outlook
The second measurement that must be done is the Activity of the sample.
The decay scheme for 60Fe is illustrated in the figure on the right. For our
measurement, we are pursuing the direct decay measurement with the
VERA samples. This decay, from the excited state to the ground state in
60Co will either be by Internal Conversion (~98% of the time) or by Gamma
Ray emission (~2% of the time). With our VERA sample, we will be able to
measure the gamma rays produced using two HPGe Planar Detectors.
These three spectra are the
energy deposited (y axis)
vs. the position in the
detectors (x axis). The 1st
and 2nd are of the VERA
sample, “Fe-2.” The 2nd
has a shield in front of the
detector, blocking out
most of the 60Ni
contaminant. The 3rd
spectra, with the shield is
the “MSU” sample. Right
is the concentrations of
each sample. These were
as expected.
Part 1: Accelerator Mass Spectrometry (AMS)
The AMS facility at Notre Dame uses of the FN tandem accelerator and the Browne-Buechner Spectrograph (see
diagram). After the magnet, there are two detectors: a Parallel Grid Avalanche Counter to detect the position of
the beam and an Ionization Chamber which detects the energy loss of the beam. Both detectors are filled with
isobutene. Filling the gap between the magnet poles with Nitrogen allows us to separate stable isobars,
specifically 60Ni and 60Fe.
Figure 2: Layers of burning sequences in a massive,
before (left) and during (right) a SN explosion. Note
60Fe position before explosion; it is located in a layer
that will be expelled.
Figure 1: Life cycle of a Massive Star.
We therefore see evidence of
60Fe in 3 key places:
1. Galactic gamma-ray activity
from 60Fe’s decay from 60Co to
stable 60Ni.
2. Enrichment of its stable
daughter product, 60Ni, in
meteorites formed during the
early Solar System.
3. Ferromanganese ocean crust
samples, suggesting a
supernova about 3 million
years ago that deposited its
ejecta on the Earth.
The half life of 60Fe, which is in
dispute, can serve as important
constraints for early Solar
System and stellar evolution
models.
Motivation
The long lived radioisotope 60Fe is only
produced in AGB stars and in Core-
collapse Supernovae. It is found in the
carbon shell burning region of a massive
star, which is expelled from the star
during the supernova explosion.
60Fe Samples
Two separate sets of samples were used. The first set was produced at Michigan
State University (MSU) and the second set is from the Paul Scherrer Institute
(PSI) beam dump.
The MSU set was made in 2009. This set included two samples: “AMS Test” and
“MSU Sample”.
The PSI beam dump set consisted of 3 samples, all part of a dilution series,
called “Fe-2”, “Fe-3”, and “Fe-4”.
How to find Half life of long lived radioisotopes
For isotopes with long half lives, we count the number of atoms of interest ( )
and measure the activity of the sample ( ) once. Then from the following
equation, we can calculate the half life:
and the decay constant, and is the half life of the sample..
Measurement of the Half Life of the long lived radioisotope 60Fe
1 University of Notre Dame, Notre Dame, Indiana. 2 Michigan State University, Michigan. 3 Argonne National Laboratory, Illinois. 4 Vienna Environmental Research
Laboratory, Austria. 5 Hebrew University of Jerusalem, Israel. 6 The Australian National University, Australia. 7 Joint Institute of Astrophysics.
K. Ostdiek1,6, S. Austin2,7, W. Bauder1,7, M. Bowers1,7, P. Collon1,7, J. Green3, W. Kutschera4, W. Lu1,7, M. Paul5, D. Robertson1,7, A. Wallner6
This research is funded by a grant
though NSF, #Phy1068192.
Left: Two planar HPGE Detectors surrounded by a
lead castle, and attached to an automatic LN2 filling
system.
Below: VERA Sample evaporated and dripped onto
Mylar sheet.
Right: Spectrum of the
VERA Sample, run for 48
hours (Purple). Blue =
Background run. Note the
peak from the 60mCo decay
at 58.6 keV. The other peak
around 63.5 keV is a
background peak.
Measurement & Analysis
Ongoing!