The Joint Institute for Nuclear Astrophysics
Activation
Activation Measurement
Measurement of
of the
the
19
20
19F(n,
20
=25 keV
kT
F(n,γγ)) F
F Cross
Cross Section
Section at
at kT=25
kT=25
keV
Forschungszentrum Karlsruhe
in der Helmholtz-Gemeinschaft
Ethan Uberseder1,2, Michael Heil1, Franz Käppeler1, Joachim Görres2, and Michael Wiescher2
1Forschungszentrum
2University
Karlsruhe, Institut für Kernphysik, Karlsruhe, Germany
of Notre Dame, Dept. of Physics, Notre Dame, IN, USA
Cyclic
Cyclic Activation
Activation
Fluorine
Fluorine in
in Asymptotic
Asymptotic Giant
Giant Branch
Branch (AGB)
(AGB)
Stars
Stars
20FF (t
The
=11 s)
The short
short half-life
half-life of
of 20
(t1/2
1/2=11 s)
required
required the
the employment
employment of
of the
the cyclic
cyclic
activation
activation technique.
technique. The
The sample
sample was
was
irradiated
irradiated for
for 30
30 ss intervals
intervals in
in aa quasiquasistellar
stellar neutron
neutron spectrum
spectrum approximating
approximating
aa thermal
distribution
thermal distribution at
at kT=25
kT=25 keV.
keV.
The
The neutron
neutron spectrum
spectrum was
was produced
produced
via
via the
the 77Li(p,n)
Li(p,n)77Be
Be reaction
reaction at
at EEpp=1911
=1911
keV.
Following
keV. Following each
each irradiation
irradiation phase,
phase,
20
the
the 20FF decay
decay line
line at
at 1634
1634 keV
keV was
was
counted
counted for
for an
an equal
equal period
period using
using aa
HPGe
HPGe detector.
detector. An
An automatic
automatic beamstop
beamstop
blocked
blocked the
the proton
proton beam
beam during
during the
the
counting
counting phase,
phase, and
and the
the data
data acquiacquisition
sition system
system was
was gated
gated to
to prevent
prevent the
the
recording
recording of
of gamma
gamma events
events throughout
throughout
the
the beam-on
beam-on period.
period. AA pneumatic
pneumatic slide
slide
transported
the
samples
transported the samples alternately
alternately
between
between the
the two
two positions.
positions.
19FF overabundance
The
The observation
observation of
of an
an 19
overabundance at
at the
the surface
surface
of
AGBs
has
led
to
much
research
concerning
of AGBs has led to much research concerning the
the
nucleosynthetic
nucleosynthetic path
path of
of fluorine
fluorine in
in these
these stars.
stars. Presently,
Presently,
fluorine
fluorine production
production has
has been
been attributed
attributed to
to the
the region
region of
of
thermal
thermal instability
instability in
in AGBs
AGBs characterized
characterized by
by intermittent
intermittent
periods
periods of
of HH and
and He
He shell
shell burning.
burning. During
During this
this
evolutionary
evolutionary phase,
phase, (α,n)
(α,n) reactions
reactions generate
generate aa high
high
neutron
neutron flux
flux within
within the
the He
He intershell.
intershell. The
The liberated
liberated
neutrons
neutrons are
are necessary
necessary to
to initiate
initiate the
the proposed
proposed reaction
reaction
19F,
sequence
sequence that
that results
results in
in the
the production
production of
of 19
F, though
though
19
19F(n,γ)
20FF acts
F(n,γ)20
acts to
to destroy
destroy fluorine
fluorine prior
prior to
to being
being
dredged
dredged up
up to
to the
the surface.
surface. To
To quantify
quantify this
this effect,
effect, aa
precise
precise neutron
neutron capture
capture cross
cross section
section isis needed
needed for
for use
use
in
in current
current stellar
stellar models.
models.
14N(n,p)14C(α,γ)18O
14N(α,γ)18F(β+ν)18O
18O(p,α)15N(α,γ)19F
19F(n,γ)20F
19F(α,p)22Ne
Top Left: Neutron spectrum produced by 7Li(p,n)7Be at Ep = 1911 keV
Top Right: Gamma spectrum obtained from the HPGe detector
Above: Experimental setup for the cyclic activation
Preliminary Maxwellian Averaged Cross Sections [mbarn]
kT [keV] =
19F
DURATION (a)
Above: Visualization of the He intershell during AGB thermal pulses
5
10
15
20
25
30
40
50
60
80
100
Present
1.24
4.13
4.61
4.26
Bao et al.
2.0
6.7
7.8
7.4
3.76 ± 0.19
3.30
2.57
2.06
1.69
1.24
0.97
6.6
5.8 ± 1.2
4.6
3.8
3.2
2.5
2.2
•• Present
Present results
results show
show aa 43%
43% reduction
reduction in
in the
the MACS
MACS at
at kT
kT =
= 25
25 KeV
KeV and
and an
an improvement
improvement in
in the
the
uncertainty
uncertainty by
by aa factor
factor of
of four
four when
when compared
compared with
with the
the value
value given
given in
in the
the Bao
Bao compilation.
compilation.
19F(n,γ)
20FF indicates
19FF throughout
•• Such
Such aa reduced
reduced cross
cross section
section for
for 19
F(n,γ)20
indicates increased
increased survival
survival of
of 19
throughout the
the
thermal
pulses
of
AGB
stars.
Detailed
stellar
model
calculations
are
needed
to
precisely
thermal pulses of AGB stars. Detailed stellar model calculations are needed to precisely quantify
quantify the
the
effect
effect of
of the
the present
present cross
cross section
section on
on AGB
AGB surface
surface abundances.
abundances.
This project is funded by the NSF through grant
PHY0216783, and the Universities of JINA.