Date post: | 01-Jan-2016 |
Category: |
Documents |
Upload: | marlene-cole |
View: | 222 times |
Download: | 0 times |
Neutron-induced reactions
Michael HeilGSI Darmstadt
Or
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
Outline
How can one measure neutron capture cross sections in the keV range on small scale facilities?
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
constantNσ AA
For the s process, neutron capture cross section measurements
are mainly needed.
Branchings
(n,)
Z+1
AA-1 A+1
A+1Z+1
(n,)
(n,)
(-) (-)
1A1,Z
1Z
nβ
ββ Nσ
Nσ
λλ
λf
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,) of unstable isotopes
Ann σvnλ
Classical analysis:
Nuclear data need for the s-process• reliable neutron capture cross section measurements• stellar enhancement factors (SEF) and• stellar -decay rates are important
Nuclear data needs for the main s-process
Terrestrial -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 -decay rates. Theoretical support needed!
gs
faster -decay
SEF
Energy range of neutron capture cross section measurements for the s process
03/2 dE
kT
EexpEσ(E)
kT
1
μπ
8σvStellar neutron capture rate
In stars, the neutron energy distribution can be described by a Maxwell-Boltzmann distribution:
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 Weak s-process A<90
TP-AGB stars 1-3 M⊙ 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(,n)
shell H-burning He-flash 0.9·108 K 3-3.5·108 K kT=8 keV kT=25 keV 107-108 cm-3 1010-1011 cm-3
13C(,n) 22Ne(,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,)
- ToF method
- Activation method
• Indirect methods
- Inverse measurements (,n)
- Coulomb dissociation
- Transfer reactions, e.g. (d,p)
The Time-of-Flight (ToF) method
start signal stop signal
neutron production
target
detector
flight path length s
t
sv Energy of neutron which caused the event: 2mv
2
1E
pulsed beam,short pulse
good timing properties
ToF-experiments in Karlsruhe
Neutron production: 7Li(p,n) reaction at energies above threshold (>1881 keV)
6LiCO3
samplesample
7 Li-Target
Collimatedneutron beam
10B + araldite
n
nn
nPulsed proton
Beam
n
lead
lea
d
77 cm flight path
42 BaF2 scintillators form a closed shell withinner diameter of 20cm and thickness of 15cmDetector efficiency > 95% for capture events
Pulse width: ~0.7 ns
Average current: 2 μA
Frequency: 250 kHz
Time resolution: ~ 600 ps Energy resolution: 14% at 662 keV, 7% at 2.5 MeV
Detection principle
AX + n A+1X + Q
iQ if detector has 100% efficiencyCharacteristic line at
Detection of prompt -rays after neutron capture.We need to measure -rays after neutron capture
Sum energy spectra and corrections
Example 143Nd
143Nd
143Nd
Measured background with C sample
Background from scattered neutrons
and isotopic impurities!
Example 143Nd 143Nd
143Nd
Measure background from isotopes by using samples with different
enrichment.
144Nd
sample ladder
142Nd208Pb/C143Nd145Nd197Au146Nd148NdEmpty144Nd
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
Wisshak et al., Phys. Rev. Lett. 87 (2001) 251102
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 can be produced in the s process!
Activation experiments
Induced activity can be measured after irradiation with HPGe detectors.
Gold foils for flux determination.
Neutron production: 7Li(p,n) reaction at a proton energy of 1911 keV
HPGe
H. Beer, F. Käppeler et al., Phys. Rev. C21, 534 (1980)
Activation sources
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)
18O(p,n)3H(p,n)
Only possible when product nucleus is radioactive
High sensitivity -> small sample masses [e.g. 28 ng for 147Pm(n,)]
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
Advantages and disadvantages of the activation technique
The production of 60Fe in core collapse supernovae depends strongly on the uncertain 59Fe(n,) and 60Fe(n,) cross section.
Detection of 60Fe with INTEGRAL or RHESSI
Example: 60Fe(n,) by activation
The detection of the ratio 60Fe/26Al in our galaxy can be used to test stellar models
60Fe: t1/2= 1.5(3) Ma
60Fe/26Al = 0.11 ± 0.03
Harris et al, A&A 433 (2005) L49
Activation of 60Fe
Sample: 7.8·1015 atoms ~ 800 ng
70 mm
sample
1205
61Co
1325
1205
61Fe6 min298 38 %
27 %
1027
1027
60Fe sample irradiated 40 times for 15 min, then activity
counted for 10 min
Result: <>=10.2 (2.9sys) (1.4stat) mb
Example – 147Pm
150Sm
148Sm
nβ
ββ Nσ
Nσ
λλ
λf
Ann σvnλ
solve for n to obtain neutron density
147Pm sample mass: 28 ng
Analyze combined branching
147Pm activation results
147Nd
mbarn
147Pm
mbarn
148Pm
mbarn
nn
108 cm-3
550±150 985±250 1410±350 4.1±0.6 Wisshak et al. 1993
544±90 1290±470 2970±500 Bao et al. 2000
544±90 709±100 1014±175 Reifarth et al. 2003
38.030.07.2
60.050.094.4
measured with 28 ng
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,)13C, 16O(n,)17O, 22Ne(n,)23Ne, 23Na(n,)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 Stern-Gerlach-Zentrum (FRANZ)
Design by Prof. Ratzinger, Prof. Schempp, O. Meusel and P. C. Chau
Neutron beam for activation
2 mA proton beam250 kHz< 1ns pulse widthneutron flux: 4·107 s-1 cm-2
neutron flux:1·1012 s-1
Factor of ~1000 higher than at FZK!!!
The Frankfurt neutron source at the Stern-Gerlach-Zentrum (FRANZ)
Design by Prof. Ratzinger, Prof. Schempp, O. Meusel and P. C. Chau
Neutron beam for activation
2 mA proton beam250 kHz< 1ns pulse widthneutron flux: 4·107 s-1 cm-2
neutron flux:1·1012 s-1
Factor of ~1000 higher than at FZK!!!
Experimental program at FRANZ
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,)• Neutron poisons for the s-process: 12C(n,), 16O(n,), 22Ne(n,).• ToF measurements of medium mass nuclei for the
weak s-process. Neutron capture measurements with small sample masses:• Radio-isotopes for -ray astronomy 59Fe(n,) and 60Fe(n,)• Branch point nuclei, e.g. 85Kr(n,), 95Zr(n,), 147Pm(n,),
154Eu(n,), 155Eu(n,), 153Gd(n,), 185W(n,)
63Ni79Se81Kr85Kr147Nd147Pm148Pm151Sm154Eu155Eu153Gd160Tb163Ho170Tm171Tm 179Ta185W204Tl
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: I=0.43 %, E = 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
Possible future experimental setup
4 BaF2
Proton beam Neutronbeam
TOF (ns)
En (keV)
100 39
100 5.5
(n,)on sample
other reactions
prompt flash
Sample by ion implantation of radioactive beams
Proton accelerator
Neutron production via 7Li(p,n)
4 cm flight pathfor high neutron flux
4 BaF2 detector for efficient -ray detection
Reifarth et al. NIM A 524 (2004) 215–226
Sample production
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)
5 MeV/u 59Fe ions in carbon
Expected production intensities: • 6·109 for 59Fe• 3·1010 for 85Kr
radioactive
ions
Production rates at FAIR
r-process pa th
Z
N
know n nucle i
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
– Neutron capture cross section of 100 mb
collection of > 35 000 counts in 1 week
background from backing: 125 000
Activity of target:50 kBqI=0.43 %, E = 514 keV
carbon
85Kr
This setup would also allow measurements of very small (n,) 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!