This document summarizes an experiment measuring the half-life of 60Fe using gamma ray spectroscopy and accelerator mass spectrometry (AMS). Previous measurements of the 60Fe half-life ranged from 1.5-2.6 million years. The experiment aims to more precisely measure the half-life to help understand sites of 60Fe production like massive stars and supernovae. Gamma detectors measured the decay of 60Fe in ocean crust samples. AMS was used to determine the concentration of 60Fe in samples and yielded a preliminary half-life of (2.33 ± 0.26) million years. Upgrades to the AMS facility may help improve precision. The results will provide insight into nuclear astrophysics and early solar system formation

1. Measuring the Half Life of 60
Fe for
Stellar and Early Solar System Models
Karen M. Ostdiek
Nuclear Science Laboratory, University of Notre Dame
Notre Dame, Indiana U.S.A.
August 25, 2014

2. Motivation from Nuclear Astrophysics & Evidence of 60
Fe
Production Sites for 60Fe
Weak s - process, successive
neutron captures on 58Fe.
Likely sites include Massive
AGB stars and Core Collapse
Supernovae (CCSNe).
Knie, K., et al. Phys. Rev Letters. 93. (2004).
Life Cycle of Massive Star.
Evidence of 60Fe
in Galaxy through γ ray decay of
60Fe.
in Ocean Crust samples,
corresponding to 3 Myrs ago.
in Meteorite Inclusions, as an
enrichment of 60Ni.

3. Motivation from Previous Measurements
Roy and Kohman (1957)- 3·105 years, factor of 3 uncertainity.
Kutschera, et al. (1984)- (1.49 ± 0.27) · 106 years.
Rugel, et al. (2009)- (2.62 ± 0.04) · 106 years.
Technique for half lives of long-lived radioisotopes:
Activity, dN/dt → by Gamma Ray Spectroscopy
Number of Atoms of Interest, No → by AMS
dN
dt
= −λ · No = −
ln2
t1/2
· No

4. Activity Measurement - Decay Scheme for 60
Fe
60Fe
0+
2+
60Co
5+
2.62 Myr
10.5 min
1925.28 days
60Ni
2+
4+
0+
2+
1173 keV
1332 keV
58 keV
Grow-inDecay
Direct Decay
Full Decay Scheme including “Direct” and “Grow-in” Decays.

5. Activity Measurement - Decay Scheme for 60
Fe
60Fe
0+
2+
60Co
5+
2.62 Myr
10.5 min
1925.28 days
58 keV
Zoomed in on Direct Decay
Isomeric Decay of 60m
Co = Internal Conversion.

6. Activity Measurement - Decay Scheme for 60
Fe
60Fe
0+
2+
60Co
5+
2.62 Myr
10.5 min
1925.28 days
58 keV
Zoomed in on Direct Decay
Isomeric Decay of 60m
Co = Internal Conversion.
VERA “Fe-1” sample
evaporated to point source
(13% of original 100 mL).
Contains ∼ 1.4 · 1014 60Fe
atoms.
Emits γ ray of 58 keV ∼ 2% of
the time.

7. Activity Measurement - HPGe Planar Detector Setup
Two HPGe Planar Detectors (courtesy of ANL)
In Head-to-head geometry.
Total Efficiencies for both Detectors near 58 keV = 9 - 14 %
(depending on spacing between, using 241Am source).

8. Activity Measurement - Results
50 55 60 65 70
0
50
100
150
200
250
Energy in keV
Counts
1 day real time Runs
60Fe Sample, Fe 1
Background
The background peak at 64 keV - from 238
U decay chain
→ (decay of 234
Th to 234
Pa)

9. AMS @ ND - Facilities

10. AMS @ ND - Spectra - 95.3 MeV at 9+ Charge State
“Fe-2” Sample, Full Beam,
Expected Concentration: 2 · 10−7
“Fe-2” Sample, Shield In.
Background Run on Fe powder, ∼ 10−12
,
Shield In, Contaminations.
Measured 60Fe/56Fe values:
"VERA Fe-4"→ (1.49 ± 0.04) · 10−9
“VERA Fe-3"→ (1.52 ± 0.03) · 10−8
"VERA Fe-2"→ (1.31 ± 0.03) · 10−7
“VERA Fe-1” → not measured yet.

11. AMS @ ND - Low Energy Injection Upgrade
Proposed Layout for new Low Energy Injection.
Re-positions SNICS source, adds 45 degree Electrostatic Analyzer,
adds 90 degree Analyzing Magnet, adds off - set Faraday Cups

12. Activity and AMS Preliminary Results & Outlook
AMS
Have also explored 10 + charge state - too many
contaminants.
Attempted 11 + charge state, 113 MeV - Promising
Higher Energies using Second Carbon Stripper?
Experiment to double check concentrations this Fall &
measure Fe-1 Sample.
Activity
Second Sample from VERA.
58 days of data taking on Fe-1 Sample.
Using assumed # of 60Fe (∼ 1.4 · 1014),
Half Life - (2.33 ± 0.26) · 106 years - PRELIMINARY.

13. Thank You!!!
U. of Notre Dame
Philippe Collon
Tyler Anderson
William Bauder
Matthew Bowers
Wenting Lu
Daniel Robertson
Michael Skulski
Sam Austin - MSU
John Greene - ANL
Walter Kutschera - VERA
Martin Martschini - VERA
Michael Paul - Racah Institute
Peter Steier - VERA
Toni Wallner - ANU