nuclear physics of the s process

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nuclear physics of the s process

• B2FH VI. Details of the s Process
• state of the art neutron capture and beta decay
• recent developments in detectors, methods,
  facilities
• outlook to future work

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cross section x abundance
2 processes Alt100 not saturated Agt100 flow
equilibrium
branchings indicated for Pb/Bi region
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r-process abundances
r-only isotopes predominantly r waiting
point concept
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stellar (n, g) cross sections
completeness 320 out of 400 cross
sections measured instead of 25
1.73 0.05 61.1 2.4
accuracy often 2 5 instead of factor 2 - 3
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MACS data _at_ kT30 keV
Oak Ridge Livermore Brookhaven CERN Dubna Geel Ge
nt Harwell Karlsruhe Los Alamos Notre
Dame Obninsk Rensselaer Tokyo
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status and requests
needed cross sections with uncertainties
between 1 and 5 for complete
set of isotopes from 12C to 210Po,
including unstable samples
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what about theory?
176Hf, 178Hf, 180Hf MACS uncertainties 1 - 2

exercise joined by 6 leading groups calculate
MACS of 174Hf and 182Hf prior to measurement
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s-process branchings MACS and ß-rates for
unstable isotopes
probing neutron density, temperature, pressure,
time scales !
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stellar enhancement of ß-rates

• ß- rates - decay of thermally populated excited
  states
• - bound beta decay
• EC - ionization versus capture from
  continuum

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ß-rates of unstable isotopes
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what determines quality of (n, ?) data?

• neutron source (energy range, flux, resolution)
• samples (available mass, purity, activity)
• detectors (resolution, efficiency,
  granularity)
• data acquisition (fast digitizers, off-line
  analyses)
• data analysis (simulations, R-matrix codes)
• methodology (TOF or activation)

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state of the art ?-ray detectors

• resolution compromised by efficiency, high
  resolution HPGe detectors
• crucial for activation measurements

• efficiency important for measurements of small
  cross sections and to
• compensate for sample mass or weak
  neutron neutron flux

• segmentation multi-detector arrays for higher
  efficiency, angular distributions,
• and background suppression, g
  calorimeters (FZK, LANL, n_TOF)

• sensitivity improved background suppression
  (C6D6 detectors)

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optimized C6D6 detectors
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g-ray calorimeters
42 independent modules in total 60 l of BaF2
ecasc gt 98 clear signatures good TOF resolution
detailed computer model for GEANTsimulations of
detector response to g-cascades from capture and
fission events and for evaluation of corrections
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data acquisition and analysis

• completeness data taking with flash-ADC for
  flexible and comprehensive
• off-line analysis (fast digitizers for
  recording full detector
• response, e.g. over 16 ms with 500 MS/s)

• simulations MCNP and GEANT are becoming
  standard tools for
• planning of experiments and necessarily -
  for the analysis
• of cross section measurements with complex
  detector arrays

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resolution sensitivity
151Sm(n,g)
recently at LANL 0.44 mg of 237Np smallest
sample ever used in TOF
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comparison of pulsed neutron sources
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activation technique at kT25 keV

• neutron production via 7Li(p,n)7Be reaction at
  Van de Graaff
• induced activity measured with HPGe detectors

• possible when product nucleus is radioactive
• very high sensitivity ? small samples small
  cross sections
• use of natural samples possible, no enriched
  sample necessary
• Direct Capture component included

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activation unique sensitivity
4-5 orders of magnitude higher flux than best
TOF facilities!
measurement of mbarn cross sections
measurements with ng samples, important for
cross sections of unstable isotopes
28 ng 147Pm
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activation versus TOF
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future strategies

• key cross sections need to be checked
• discrepancies different methods show
  inconsistent results
• uncertainties in historic data often
  underestimated or not recognized

• state of the art techniques mandatory
• facilities spallation sources DANCE, n_TOF,
  J-PARC,
• intense low energy accelerators
• detectors optimized and highly segmented 4p
  arrays
• DAQ fast digitizers for recording full
  information
• analysis GEANT and MCNP simulations

• complementary measurements
• redundancy measurement and data analysis
  repeated with different perspectives
• consistency combination of independent
  experiments (e.g. TOF and activation/AMS)
• sensitivity better accuracy, small and
  unstable samples

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unstable samples now and soon
s process
branch point status
63Ni 79Se 81Kr 85Kr 147Nd 147Pm 148Pm 151Sm 154E
u 155Eu 153Gd 160Tb 163Ho 170Tm 171Tm
179Ta 185W 204Tl
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summary

• present tools available for stable samples
• facilities, detectors,
  DAQ systems, analysis codes

near future all stable and some unstable
isotopes fluxes high enough
for most important unstable nuclei
challenges unstable isotopes ? MACS and ß
rates from s process to explosive
nucleosynthesis
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thank you
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how good are current data?
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the Frankfurt Neutron source at the SGZ
Ep 1.9 2.4 MeV, Dt 1 ns TOF mode 250 kHz,
2 mA or CW mode 175 MHz, 200 mA
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n_TOF measurements on Zr isotopes