Michael Heil
GSI Darmstadt
Neutron-induced reactions
Or
How can one measure neutron capture cross sections in the
keV range on small scale facilities?
Outline
How can one measure neutron capture cross sections in the
keV range on small scale facilities?
• Summary of s-process nucleosynthesis and neutron capture data needs
• Production of neutrons (small vs. large scale facilities)
• Experimental methods and techniques
• Time-of-flight method with illustrative examples from FZK
• Activation method with illustrative examples from FZK
• Current challenges and possible contributions/solutions from FRANZ
Introduction: The s process
s process:
• responsible for nucleosynthesis of about half of
the heavy elements
• best understood nucleosynthesis process
• stellar sites are known
• advanced stellar models
σ
A
NA  constant
For the s process,
neutron capture cross
section measurements
are mainly needed.
Branchings
Classical analysis:
fβ 
λβ
λβ  λn

σ
σ
λ n  n n  σv
 NZ 1
Z+1
 NZ 1, A 1
A
(n,g)
A+1
Z+1
(b-)
(b-)
(n,g)
A-1
(n,g)
A
A+1
Branchings can be used to determine
• neutron density
• temperature
• mass density
• convection time scales
in the interior of stars
One needs the cross section of involved stable and the branch point nuclei.
Experimental challenge: Measure (n,g) of unstable isotopes
Nuclear data needs for the main s-process
Nuclear data need for the s-process
• reliable neutron capture cross section measurements
• stellar enhancement factors (SEF) and
• stellar b-decay rates are important
Terrestrial b-decay rates or cross sections
are “easy” to measure but in stellar plasma
additional effects have to be considered:
• nuclei are ionized
• equilibrium of ground state and excited
states due to hot photon bath
SEF
faster b-decay
gs
This can lead to drastically modified stellar b-decay rates.
Theoretical support needed!
Energy range of neutron capture cross
section measurements for the s process
In stars, the neutron energy distribution can be described
by a Maxwell-Boltzmann distribution:
Stellar neutron capture rate
σv 
8
π μ

1
kT 3/2

 E 
σ(E)

E

exp

  dE

 kT 
0
Typical neutron energy
distribution for kT=25 keV
We need to measure the cross sections in the range 1 keV – 500 keV
s-process sites
Two components were identified and connected to stellar sites:
Main s-process 90<A<210
TP-AGB stars 1-3 M⊙
shell H-burning
0.9·108 K
kT=8 keV
107-108 cm-3
13C(a,n)
He-flash
3-3.5·108 K
kT=25 keV
1010-1011 cm-3
22Ne(a,n)
Weak s-process A<90
massive stars > 8 M⊙
core He-burning
shell C-burning
3-3.5·108 K
~1·109 K
kT=25 keV
kT=90 keV
106 cm-3
1011-1012 cm-3
22Ne(a,n)
How to measure neutron capture cross
sections?
Neutron production:
•
e-
linear accelerators
(Geel, Oak Ridge)
• Spallation neutron sources
(Los Alamos, CERN)
• Van de Graaff / Tandem / RFQ
(Karlsruhe, Demokritos, Frankfurt ...)
Methods:
• Direct measurements (n,g)
- ToF method
- Activation method
• Indirect methods
- Inverse measurements (g,n)
- Coulomb dissociation
- Transfer reactions, e.g. (d,p)
The Time-of-Flight (ToF) method
neutron
production
pulsed beam,
short pulse
flight path length s
target
good timing
detector properties
start signal
Energy of neutron which caused the event:
stop signal
v 
s
t
E
1
2
mv
2
ToF-experiments in Karlsruhe
Neutron production: 7Li(p,n) reaction at energies above threshold (>1881 keV)
Pulse width: ~0.7 ns
42 BaF2 scintillators form a closed shell with
inner diameter of 20cm and thickness of 15cm
Detector efficiency e > 95% for capture events
Average current: 2 μA
Frequency: 250 kHz
10B
lead
+ araldite
n
6LiCO
3
n
n
sample
Collimated
neutron beam
Pulsed proton
Beam
n
Time resolution: ~ 600 ps
Energy resolution:
14% at 662 keV,
7% at 2.5 MeV
lead
n
7 Li-Target
77 cm flight path
Detection principle
Detection of prompt g-rays after neutron capture.
We need to measure g-rays after neutron capture
AX
+ n  A+1X + Q
Characteristic line at
Q
 gi
if detector has 100% efficiency
Sum energy spectra and corrections
Example 143Nd
Background from
scattered neutrons
and isotopic
impurities!
143Nd
143Nd
Measured background
with C sample
143Nd
Example 143Nd
143Nd
sample ladder
142Nd
208Pb/C
144Nd
143Nd
145Nd
197Au
146Nd
148Nd
Empty
144Nd
Measure background
from isotopes by using
samples with different
enrichment.
ToF spectra
No background for
early times
Cross section results
• Cross sections in the energy
range from 1 to 200 keV
• Cross sections with an
accuracy of ~2%
180Tam:
the world rarest isotope
Sample: world supply of
enriched tantalum, consisting of
150 mg oxide powder with a
180Tam content of only 5.5%.
Result: 1465 mb at kT=30keV,
Much smaller than theoretical predictions.
180Tam
Wisshak et al., Phys. Rev. Lett. 87 (2001) 251102
can be produced in the s process!
Activation experiments
Neutron production: 7Li(p,n) reaction at a proton energy of 1911 keV
H. Beer, F. Käppeler et al., Phys. Rev. C21, 534 (1980)
Induced activity can be measured after
irradiation with HPGe detectors.
HPGe
Gold foils for flux determination.
Activation sources
3H(p,n)
18O(p,n)
18O(p,n)
reaction
At Ep=2582 keV
Käppeler et al.
Phys. Rev. C35,936–941 (1987)
Heil et al.
Phys. Rev. C 71, 025803 (2005)
Advantages and disadvantages of the
activation technique

Only possible when product nucleus is radioactive

High sensitivity -> small sample masses [e.g. 28 ng for 147Pm(n,g)]

Use of natural samples possible, no enriched sample necessary

Direct capture component included


Measurement of radioactive samples possible due to excellent energy
resolution of HPGe detectors
So far only MACS at a thermal energy of kT=25, 5, and 52 keV possible
Example: 60Fe(n,g) by activation
The production of 60Fe in core collapse supernovae
depends strongly on the uncertain 59Fe(n,g) and
60Fe(n,g) cross section.
60Fe:
t1/2= 1.5(3) Ma
Detection of 60Fe with
INTEGRAL or RHESSI
The detection of the ratio 60Fe/26Al in our
galaxy can be used to test stellar models
60Fe/26Al
= 0.11 ± 0.03
Harris et al, A&A 433 (2005) L49
Activation of 60Fe
1325
1205
1027
27 %
61Fe
298 38 %
1205
6 min
Sample: 7.8·1015 atoms ~ 800 ng
60Fe
sample irradiated 40 times
for 15 min, then activity
counted for 10 min
1027
61Co
70 mm
sample
Result: <s>=10.2 (2.9sys) (1.4stat) mb
Example –
147Pm
Analyze combined branching
fβ 
λβ
λβ  λ n
σ  N 148Sm

σ  N 150Sm
solve for ln to obtain neutron density
λ n  n n  σv
A
147Pm
sample mass: 28 ng
147Pm
activation results
147Nd
147Pm
148Pm
mbarn
mbarn
mbarn
nn
108 cm-3
550±150
985±250
1410±350
4.1±0.6
544±90
1290±470
2970±500
2 . 7  0 .30
544±90
709±100
1014±175
 0 . 38
 0 . 60
4 . 94  0 .50
Wisshak et al. 1993
measured with
28 ng
Bao et al. 2000
Reifarth et al. 2003
Reifarth et al.,
Astrophysical Journal,
582 (2003) 1251
Summary: neutron
capture cross sections
• Light elements have small cross
sections and are difficult to measure,
but they are very abundant in stars.
Therefore, they can change the
neutron balance.
Most important neutron poisons:
12C(n,g)13C, 16O(n,g)17O, 22Ne(n,g)23Ne, 23Na(n,g)24Na, ….
• Neutron capture on medium mass nuclei are important for the s-process in
massive stars. Since these are the progenitors of supernovae explosions the
s-process determines the composition before the explosion.
• The reaction path around neutron magic nuclei is especially sensitive to
model parameters. Therefore, the neutron capture cross section of neutron
magic nuclei can constrain stellar models.
• Neutron capture measurements on unstable branch points are most
challenging.
The Frankfurt neutron source at the SternGerlach-Zentrum (FRANZ)
Neutron beam
for activation
neutron flux:
1·1012 s-1
2 mA proton beam
250 kHz
< 1ns pulse width
neutron flux: 4·107 s-1 cm-2
Design by Prof. Ratzinger,
Prof. Schempp, O. Meusel
and P. C. Chau
Factor of ~1000
higher than at FZK!!!
The Frankfurt neutron source at the SternGerlach-Zentrum (FRANZ)
Neutron beam
for activation
neutron flux:
1·1012 s-1
2 mA proton beam
250 kHz
< 1ns pulse width
neutron flux: 4·107 s-1 cm-2
Design by Prof. Ratzinger,
Prof. Schempp, O. Meusel
and P. C. Chau
Factor of ~1000
higher than at FZK!!!
Experimental program at FRANZ
63Ni
The Frankfurt neutron source will provide the highest neutron
flux in the astrophysically relevant keV region (1 – 500 keV)
worldwide.
79Se
81Kr
85Kr
147Nd
147Pm
148Pm
Neutron capture measurements of small cross sections:
• Big Bang nucleosynthesis: 1H(n,g)
• Neutron poisons for the s-process: 12C(n,g), 16O(n,g), 22Ne(n,g).
• ToF measurements of medium mass nuclei for the
weak s-process.
Neutron capture measurements with small sample masses:
• Radio-isotopes for g-ray astronomy 59Fe(n,g) and 60Fe(n,g)
• Branch point nuclei, e.g. 85Kr(n,g), 95Zr(n,g), 147Pm(n,g),
154Eu(n,g), 155Eu(n,g), 153Gd(n,g), 185W(n,g)
151Sm
154Eu
155Eu
153Gd
160Tb
163Ho
170Tm
171Tm
179Ta
185W
204Tl
Production of radioactive samples
So far, milli-gram samples are necessary to perform neutron capture
experiments on radioactive isotopes.
Problems:
• Activity of the samples:
Assume 500 mg 85Kr:
Ig=0.43 %, Eg = 514 keV:
30 GBq
• Availability of the samples
We need an experimental setup which allows to measure
neutron capture cross sections of nano-gram samples
We need a possibility to produce isotopically “pure” nanogram samples
Possible future experimental setup
Sample by ion implantation
of radioactive beams
Neutron production
via 7Li(p,n)
100
g
5.5
En (keV)
prompt flash
Neutron
beam
Proton beam
Proton
accelerator
g
g
(n,g)
on sample
4p BaF2
0 10
4 cm flight path
for high neutron flux
other reactions
39
4p BaF2 detector for
efficient g-ray detection
Reifarth et al. NIM A 524 (2004) 215–226
TOF (ns)
Sample production
radioactive
ions
To perform neutron capture experiments
on radioactive isotopes one needs samples with about 1015 atoms:
With FAIR and other upcoming RIB facilities (Spiral2, RIA, Eurisol)
intensities of >1010 ions/s are reached for a wide variety of isotopes.
Implantation of selected isotopes in thin carbon foils:
• beam intensity ≥ 1010 1/s (8.64·1014 1/day)
• beam size Ø < 2 cm
• high purity (<10% contaminant beam)
• thin backings (<1 mg/cm2 carbon backings)
-> low energy radioactive beam (< 5 MeV/u)
Expected production intensities:
• 6·109 for 59Fe
• 3·1010 for 85Kr
5 MeV/u 59Fe ions in carbon
Production rates at FAIR
k n o w n n u c le i
r-p ro c e s s p a th
Z
N
K.-H. Schmidt
Example 85Kr
• No experimental data available, theoretical calculations at 30 keV:
123 mb, 67 mb, 25 mb, 150 mb: Uncertain by a factor of 6
• Beam time of 2 days:
– 85Kr beam of 3.25·1010 1/s (> 5.6·1015 atoms in two days, 800 ng)
– Neutron flux of 1·108 neutrons/s/cm2
carbon
– Neutron capture cross section of 100 mb
collection of > 35 000 counts in 1 week
background from backing: 125 000
Activity of target:
50 kBq
Ig=0.43 %, Eg = 514 keV
85Kr
This setup would also allow measurements of very small (n,g) cross
sections (weak s-process, neutron poisons)
Summary
• Although the s-process is the best known nucleosynthesis process it is
still an exciting research field
– Many accurate cross section measurements allow to test advanced
stellar models in detail
– New neutron capture processes such as LEPP are discussed
• FRANZ and other neutron sources (e.g. short flight path at n_ToF) with
increased neutron fluxes will open completely new possibilities.
• There are many exciting experiments waiting to be performed and many
problems to be solved!