Neutron-induced reactions Michael Heil GSI 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 For the s process, neutron capture cross section measurements are mainly needed.

Branchings (n,  ) Z+1 AA-1A+1 Z+1 (n,  ) (-)(-)(-)(-) 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 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 Stellar 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<210Weak s-process A<90 TP-AGB stars 1-3 M ⊙ massive stars > 8 M ⊙ core He-burning shell C-burning 3-3.5·10 8 K ~1·10 9 K kT=25 keV kT=90 keV 10 6 cm cm Ne( ,n) shell H-burning He-flash 0.9·10 8 K 3-3.5·10 8 K kT=8 keV kT=25 keV cm cm C( ,n) 22 Ne( ,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 signalstop signal neutron production target detector flight path length s Energy of neutron which caused the event: pulsed beam, short pulse good timing properties

ToF-experiments in Karlsruhe Neutron production: 7 Li(p,n) reaction at energies above threshold (>1881 keV) 6 LiCO 3 sample 7 Li-Target Collimated neutron beam 10 B + araldite n n n n Pulsed proton Beam n lead 77 cm flight path 42 BaF 2 scintillators form a closed shell with inner diameter of 20cm and thickness of 15cm Detector 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 A X + n  A+1 X + Q 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 143 Nd 143 Nd Measured background with C sample Background from scattered neutrons and isotopic impurities!

Example 143 Nd 143 Nd Measure background from isotopes by using samples with different enrichment. 144 Nd sample ladder 142 Nd 208 Pb/C 143 Nd 145 Nd 197 Au 146 Nd 148 Nd Empty 144 Nd

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%

180 Ta m : the world rarest isotope Wisshak et al., Phys. Rev. Lett. 87 (2001) Sample: world supply of enriched tantalum, consisting of 150 mg oxide powder with a 180 Ta m content of only 5.5%. Result: 1465 mb at kT=30keV, Much smaller than theoretical predictions. 180 Ta m 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: 7 Li(p,n) reaction at a proton energy of 1911 keV HPGe H. Beer, F. Käppeler et al., Phys. Rev. C21, 534 (1980)

Activation sources 18 O(p,n) reaction At E p =2582 keV Käppeler et al. Phys. Rev. C35,936–941 (1987) Heil et al. Phys. Rev. C 71, (2005) 18 O(p,n) 3 H(p,n)

Only possible when product nucleus is radioactive High sensitivity -> small sample masses [e.g. 28 ng for 147 Pm(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 60 Fe in core collapse supernovae depends strongly on the uncertain 59 Fe(n,  ) and 60 Fe(n,  ) cross section. Detection of 60 Fe with INTEGRAL or RHESSI Example: 60 Fe(n,  ) by activation The detection of the ratio 60 Fe/ 26 Al in our galaxy can be used to test stellar models 60 Fe: t 1/2 = 1.5(3) Ma 60 Fe/ 26 Al = 0.11 ± 0.03 Harris et al, A&A 433 (2005) L49

Activation of 60 Fe Sample: 7.8·10 15 atoms ~ 800 ng 70 mm sample Co Fe 6 min % 27 % Fe sample irradiated 40 times for 15 min, then activity counted for 10 min Result: =10.2 (2.9 sys ) (1.4 stat ) mb

Example – 147 Pm solve for n to obtain neutron density 147 Pm sample mass: 28 ng Analyze combined branching

147 Pm activation results 147 Nd mbarn 147 Pm mbarn 148 Pm mbarnn 10 8 cm ±150985± ±3504.1±0.6Wisshak et al ±901290± ±500Bao et al ±90709± ±175Reifarth et al 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: 12 C(n,  ) 13 C, 16 O(n,  ) 17 O, 22 Ne(n,  ) 23 Ne, 23 Na(n,  ) 24 Na, …. 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 beam 250 kHz < 1ns pulse width neutron flux: 4·10 7 s -1 cm -2 neutron flux: 1·10 12 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 beam 250 kHz < 1ns pulse width neutron flux: 4·10 7 s -1 cm -2 neutron flux: 1·10 12 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: 1 H(n,  ) Neutron poisons for the s-process: 12 C(n,  ), 16 O(n,  ), 22 Ne(n,  ). ToF measurements of medium mass nuclei for the weak s-process. Neutron capture measurements with small sample masses: Radio-isotopes for  -ray astronomy 59 Fe(n,  ) and 60 Fe(n,  ) Branch point nuclei, e.g. 85 Kr(n,  ), 95 Zr(n,  ), 147 Pm(n,  ), 154 Eu(n,  ), 155 Eu(n,  ), 153 Gd(n,  ), 185 W(n,  ) 63 Ni 79 Se 81 Kr 85 Kr 147 Nd 147 Pm 148 Pm 151 Sm 154 Eu 155 Eu 153 Gd 160 Tb 163 Ho 170 Tm 171 Tm 179 Ta 185 W 204 Tl

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 85 Kr: 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  BaF 2 Proton beam Neutron beam    TOF (ns) E n (keV) (n,  ) on sample other reactions prompt flash Sample by ion implantation of radioactive beams Proton accelerator Neutron production via 7 Li(p,n) 4 cm flight path for high neutron flux 4  BaF 2 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 atoms: With FAIR and other upcoming RIB facilities (Spiral2, RIA, Eurisol) intensities of >10 10 ions/s are reached for a wide variety of isotopes. Implantation of selected isotopes in thin carbon foils: beam intensity ≥ /s (8.64· /day) beam size Ø < 2 cm high purity (<10% contaminant beam) thin backings (<1 mg/cm 2 carbon backings) -> low energy radioactive beam (< 5 MeV/u) 5 MeV/u 59 Fe ions in carbon Expected production intensities: 6·10 9 for 59 Fe 3·10 10 for 85 Kr radioactive ions

Production rates at FAIR K.-H. Schmidt

Example 85 Kr 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: – 85 Kr beam of 3.25· /s (> 5.6·10 15 atoms in two days, 800 ng) –Neutron flux of 1·10 8 neutrons/s/cm 2 –Neutron capture cross section of 100 mb collection of > counts in 1 week background from backing: Activity of target: 50 kBq I  =0.43 %, E  = 514 keV carbon 85 Kr 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!