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Neutron-induced reactions

Neutron-induced reactions. Michael Heil GSI Darmstadt. 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?.

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Neutron-induced reactions

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  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 (n,g) Z+1 A+1 Z+1 (b-) (b-) (n,g) (n,g) A-1 A A+1 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 kT=25 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 90<A<210 Weak s-process A<90 TP-AGB stars 1-3 M⊙ massive stars > 8 M⊙ shell H-burning He-flash 0.9·108 K 3-3.5·108 K kT=8 keVkT=25 keV 107-108 cm-3 1010-1011 cm-3 13C(a,n) 22Ne(a,n) core He-burning shell C-burning 3-3.5·108 K ~1·109 K kT=25 keVkT=90 keV 106 cm-3 1011-1012 cm-3 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 10B + araldite lead n 6LiCO3 n sample n Pulsed proton Beam Collimatedneutron beam n n lead 7 Li-Target 77 cmflight path Neutron production: 7Li(p,n) reaction at energies above threshold (>1881 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 > 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  A+1X + Q Characteristic line at if detector has 100% efficiency

  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 146Nd 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 180Tamcontent of only 5.5%. Wisshak et al., Phys. Rev. Lett. 87 (2001) 251102 Result: 1465 mb at kT=30keV, Much smaller than theoretical predictions. 180Tam can be produced in the s process!

  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 Ep=2582 keV Käppeler et al. Phys. Rev. C35,936–941 (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 -> 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     

  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, A&A 433 (2005) L49

  21. Activation of 60Fe 70 mm sample 27 % Sample: 7.8·1015 atoms ~ 800 ng 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: <s>=10.2 (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 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) 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!!!

  26. The Frankfurt neutron source at the Stern-Gerlach-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!!!

  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 ofsmall 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 withsmall 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: • 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” nano-gram samples

  29. Possible future experimental setup Sample by ion implantation of radioactive beams Neutron production via 7Li(p,n) 100 5.5 En (keV) g prompt flash Neutron beam Proton beam Proton accelerator g g other reactions (n,g) on sample 4p BaF2 TOF (ns) 0 10 39 4 cm flight path for high neutron flux 4p BaF2 detector for efficient g-ray detection Reifarth et al. NIM A 524 (2004) 215–226

  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 >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

  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 3.25·1010 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 carbon 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)

  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!

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