Neutron-induced%20reactions

1
Neutron-induced reactions

• Michael Heil
• GSI Darmstadt

Or How can one measure neutron capture cross
sections in the keV range on small scale
facilities?
2
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

3
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

For the s process, neutron capture cross section
measurements are mainly needed.
4
Branchings
Classical analysis

• 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
5
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
• This can lead to drastically modified stellar
  b-decay rates.
• Theoretical support needed!

SEF
faster b-decay
gs
6
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
Typical neutron energy distribution for kT25 keV
We need to measure the cross sections in the
range 1 keV 500 keV
7
s-process sites
Two components were identified and connected to
stellar sites
Main s-process 90ltAlt210
Weak s-process Alt90
TP-AGB stars 1-3 M?
massive stars gt 8 M?
core He-burning shell C-burning
3-3.5108 K 1109 K kT25 keV
kT90 keV 106 cm-3 1011-1012
cm-3 22Ne(a,n)
shell H-burning He-flash 0.9108
K 3-3.5108 K kT8 keV
kT25 keV 107-108 cm-3
1010-1011 cm-3 13C(a,n)
22Ne(a,n)
8
How to measure neutron capture cross sections?

• Methods
• Direct measurements (n,?)
• - ToF method
• - Activation method
• Indirect methods
• - Inverse measurements (?,n)
• - Coulomb dissociation
• - Transfer reactions, e.g. (d,p)

• Neutron production
• e- linear accelerators
• (Geel, Oak Ridge)
• Spallation neutron sources
• (Los Alamos, CERN)
• Van de Graaff / Tandem / RFQ
• (Karlsruhe, Demokritos, Frankfurt ...)

9
The Time-of-Flight (ToF) method
neutron production
flight path length s
pulsed beam, short pulse
target
detector
good timing properties
start signal
stop signal
Energy of neutron which caused the event
10
ToF-experiments in Karlsruhe
Neutron production 7Li(p,n) reaction at energies
above threshold (gt1881 keV)
Pulse width 0.7 ns Average current 2
µA Frequency 250 kHz

• 42 BaF2 scintillators form a closed shell with
• inner diameter of 20cm and thickness of 15cm
• Detector efficiency e gt 95 for capture events
  

• Time resolution 600 ps
• Energy resolution
• 14 at 662 keV,
• 7 at 2.5 MeV

11
Detection principle
Detection of prompt g-rays after neutron capture.
We need to measure g-rays after neutron capture
AX n ? A1X Q
if detector has 100 efficiency
Characteristic line at
12
Sum energy spectra and corrections
Example 143Nd
143Nd
Background from scattered neutrons and isotopic
impurities!
143Nd
Measured background with C sample
13
143Nd
Example 143Nd
143Nd
sample ladder 142Nd 208Pb/C 143Nd 145Nd 197Au 14
6Nd 148Nd Empty 144Nd
144Nd
Measure background from isotopes by using samples
with different enrichment.
14
ToF spectra
No background for early times
15
Cross section results

• Cross sections in the energy
• range from 1 to 200 keV
• Cross sections with an
• accuracy of 2

16
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 kT30keV, Much smaller than
theoretical predictions. 180Tam can be produced
in the s process!

• Wisshak et al., Phys. Rev. Lett. 87 (2001) 251102

17
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. Gold foils for
flux determination.
HPGe
18
Activation sources
3H(p,n)
18O(p,n)
18O(p,n) reaction At Ep2582 keV
Käppeler et al. Phys. Rev. C35,936941 (1987)
Heil et al. Phys. Rev. C 71, 025803 (2005)
19
Advantages and disadvantages of the activation
technique
?
Only possible when product nucleus is
radioactive High sensitivity -gt 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 kT25, 5, and 52 keV possible
?
?
?
?
?
20
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, AA 433 (2005) L49
21
Activation of 60Fe
Sample 7.81015 atoms 800 ng
27
1325
61Fe
6 min
298
38
60Fe sample irradiated 40 times for 15 min, then
activity counted for 10 min
1205
1205
1027
1027
61Co
Result ltsgt10.2 (2.9sys) (1.4stat) mb
22
Example 147Pm
Analyze combined branching
solve for ln to obtain neutron density
147Pm sample mass 28 ng
23
147Pm activation results
147Nd mbarn 147Pm mbarn 148Pm mbarn nn 108 cm-3
550150 985250 1410350 4.10.6 Wisshak et al. 1993
54490 1290470 2970500 Bao et al. 2000
54490 709100 1014175 Reifarth et al. 2003
measured with 28 ng
Reifarth et al., Astrophysical Journal, 582
(2003) 1251
24
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.

25
The Frankfurt neutron source at the
Stern-Gerlach-Zentrum (FRANZ)
26
The Frankfurt neutron source at the
Stern-Gerlach-Zentrum (FRANZ)
Neutron beam for activation
neutron flux 11012 s-1
2 mA proton beam 250 kHz lt 1ns pulse
width neutron flux 4107 s-1 cm-2
Design by Prof. Ratzinger, Prof. Schempp, O.
Meusel and P. C. Chau
Factor of 1000 higher than at FZK!!!
27
Experimental program at FRANZ
63Ni 79Se 81Kr 85Kr 147Nd 147Pm 148Pm 151Sm 154Eu
155Eu 153Gd 160Tb 163Ho 170Tm 171Tm
179Ta 185W 204Tl
The Frankfurt neutron source will provide the
highest neutron flux in the astrophysically
relevant keV region (1 500 keV) worldwide.

• 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)

28
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
• Ig0.43 , 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 nano-gram samples

29
Possible future experimental setup
Sample by ion implantation of radioactive beams
Neutron production via 7Li(p,n)
En (keV)
100
5.5
g
prompt flash
Neutron beam
Proton beam
Proton accelerator
g
g
other reactions
(n,g) on sample
4p BaF2
TOF (ns)
10
0
39
4 cm flight path for high neutron flux
4p BaF2 detector for efficient g-ray detection
Reifarth et al. NIM A 524 (2004) 215226
30
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 gt1010
  ions/s are reached for a wide variety of
  isotopes.
• Implantation of selected isotopes in thin carbon
  foils
• beam intensity 1010 1/s (8.641014 1/day)
• beam size Ø lt 2 cm
• high purity (lt10 contaminant beam)
• thin backings (lt1 mg/cm2 carbon backings)
• -gt low energy radioactive beam (lt 5 MeV/u)

• Expected production intensities
• 6109 for 59Fe
• 31010 for 85Kr

5 MeV/u 59Fe ions in carbon
31
Production rates at FAIR
K.-H. Schmidt
32
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 3.251010 1/s (gt 5.61015 atoms in
  two days, 800 ng)
• Neutron flux of 1108 neutrons/s/cm2
• Neutron capture cross section of 100 mb
• collection of gt 35 000 counts in 1 week
• background from backing 125 000

carbon
Activity of target 50 kBq Ig0.43 , Eg 514 keV
85Kr
This setup would also allow measurements of very
small (n,g) cross sections (weak s-process,
neutron poisons)
33
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!