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neutron reactions in astrophysics – status and perspectives. scenarios status and challenges new developments. I. Introductory remarks and present status II. Laboratory experiments and astrophysics III. Future options. big bang stellar He burning - PowerPoint PPT Presentation
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I. Introductory remarks and present status II. Laboratory experiments and astrophysicsIII. Future options
scenariosstatus and challengesnew developments
neutron reactions in astrophysics – status and perspectives
neutron capture scenarios
big bang
stellar He burning s process in TP-AGB and in massive stars
explosive nucleosynthesis p and r processes
neutron capture accounts for 75% of the stable isotopes, but only for about 0.005% of the total post BB abundances
Maxwellian averaged cross sections required
measure (En) by time of flight, 0.3 < En < 500 keV, determine average for stellar spectrum correct for SEF high accuracy, wide energy range
produce thermal spectrum in laboratory, measure stellar average directly by activation correct for SEF very high sensitivity
prompt -rays + TOF-method
detection of neutron capture events
* Moxon-Rae ~1% * PH-weighting ~20% * Ge, NaI < 1%
single ´s
all ´s * 4BaF2
~100%
(n,):
activation in quasi-stellar spectrum most sensitive * small cross sections, 1014 atoms! selective * natural samples or low enrichment
compilation of stellar (n,) cross sections
20 40 60 80 100 120NEUTRON NUMBER
1
10
100
1000
MA
XW
EL
LIA
N A
VE
RA
GE
D C
RO
SS S
EC
TIO
N
(mb)
BaBa
CdCd
CeCe
CrCr
DyDy ErEr
FeFe
GdGd
GeGe
HfHf
HgHgKrKr
MoMo NdNdNiNi
OsOs
PbPb
PdPd
PtPt
RuRuSeSe
SmSm
SnSn
SrSr
TeTe
WW
XeXe
YbYbZnZn
ZrZr
even-even nuclei
Bao & Käppeler 1987
Bao et al. 2000
Beer, Voss & Winters 1992
collect experimental data, renormalize, calculate MACS, recommend
based on educated choices by experienced experimentalist
complement by theory (SEF)
current update by Dillmann & Plag: KADONIShttp://nuclear-astrophysics.fzk.de/kadonis/
status of stellar (n,) cross sections
s process: = 1-3% p and r process: ~ 5%
what do we have?
nuclear input must be good enough that it doesn‘t punch through to calculated abundances!
what do we need?
beware:
discrepancies often larger thanuncertainties!!!
activation in quasi-stellar spectrum
- neutron source 7Li(p,n)7Be
- neutron flux 197Au(n,)198Au
- 15C detected via 5.3 MeV line
(t1/2=2.45
s)
most (n,) of unstable nuclei measured this way: 14C(n,)15C
neutron cone
Au/14C/Au
lithium
proton beam
half-life limits 0.1 s < t1/2 < 10 yr with -spec
no limit with AMS!
sample properties >1014 atoms impurities acceptable
activation in quasi-stellar spectrum
possible neutron sources:
7Li(p, n)7Be kT=25 keV 2·109 neutrons/s, 100
A
3H(p, n)3He 52 keV 1·108 “ “
18O(p, n)18F 5 keV 2·105 “ “higher beam currents needed for - activations at low energies - long-lived product nuclei - studies of double neutron captures
higher beam currents require new target technology!
complete info: (En) via TOF method
& folding with stellar spectrum larger samples *
limited sensitivity optimal efficiency
higher flux
limited selectivity enriched samples **
* not desirable and even excluded for unstable samples
** mandatory
>90% up to 10 MeV casc > 98%
E/E = 6% at 6 MeV clear signatures
t = 500 ps good TOF resolution
optimal efficiency : 4 BaF2 array
sample
Pb neutron target
p-beam
collimated n-beam
now also at Los Alamos and CERN
FZK
high neutron fluxes :spallation sources
PS213
n_TOF Collaboration
0.8 proton energy (GeV) 24 20 repetition rate (Hz) 0.4 250 pulse width (ns) 5 20 flight path (m) 185 200 average proton current (A) 2 20 neutrons per proton 760
since 1987
since 2001
wide neutron energy range from thermal to 250 MeV
still higher fluxes in future
J-PARC spallation source similar features than LANSCE, but 50 times more flux
LANSCE improved by factor of 10 – 20 by upgrade of LAMPF
n_TOF @CERN improved by factor of 100 by shorter flight path
Low energy proton accelerators with beam currents of up to 200 mA (Soreq Nucl. Research Center, Univ. of Frankfurt/M)
unstable samples: now and then
r and p process(n,) cross sections for a varietyof selected unstable isotopes(r : 60Fe, 106Ru, 126Sn, 182Hf...(p : 91,92Nb, 97,98Tc...)
for direct use in reaction networks
to derive rates of inverse
reactions
to test and assist statistical
models
63Ni79Se81Kr85Kr147Nd147Pm148Pm151Sm154Eu155Eu153Gd160Tb163Ho170Tm171Tm 179Ta185W204Tl
branch point status
s processfuture
+ 59Fe, 125Sn, 181Hf….
summary
• numerous remaining quests for s process (branchings, grains, massive stars) and many more for explosive nucleosynthesis
• present facilities and detectors suited for most stable isotopes
• new approaches required for radioactive samples
• spallation sources, new low energy accelerators, and RIB facilities promising, both for stellar and explosive nucleosynthesis
important for quantitative picture of galactic chemical evolution