A new approach to the 176 Lu puzzle clock or thermometer? an astrophysical quest and a nuclear...
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A new approach to the 176Lu puzzle
clock or thermometer? an astrophysical quest and a nuclear challenge
20 years of nuclear physics level schemes, cross sections, IR
finally the Torino solution
the s-process branching at 176Lu
152 154
151
Yb
176 179
Lu
Hf
p process
s process
r process
180
175
174 176
177 178
176 t1/2 = 36 Gyr !!
Audouze, Fowler, & Schramm identify 176Lu as a cosmic clock
(1972)
?
the clock is challenged by Richard Ward (1980)
life is never easy
lots of nuclear input: (1) of 176Lu under stellar temperatures (2) (n,) cross sections for s-process flow (3) isomeric ratio
152 154
151
Yb
176 179
Lu
Hf
p process
s process
r process
180
175
174 176
177 178
3.7 h
36 Gyrinduced transitionsby thermal photons?
(1) 176Lu decay
are isomer and ground state connected at high T ?
igs
mYb
176
Lu
Hf
175
174 176
177 178
3.7 h
36 Gyr
GAMS spectrometry at ILL Grenoble
first mediating level at 838 keV !
fn,eff
fn(nn, T)
Yb
176
Lu
Hf
175
174 176
177 178
3.7 h
36 Gyr
low mass AGB stars – the main s component in 1999
(2) stellar (n,(2) stellar (n,) cross sections) cross sections
• 40 BaF2 crystals 12 pentagons & 28 hexagons 15 cm crystal thickness
samplePb neutron target
p-beam
n-beam
(n,):TOF with total absorption calorimeter @ FZK
accurate (n,) cross sections at FZK
measured (En) by time of flight, 3 < En < 225 keV for all Yb, Lu, and Hf isotopes to ±1%, determined Maxwell-average for stellar spectrum
3.7 h (3) partial cross section to isomer(3) partial cross section to isomer
isomeric ratio = ( to isomer) / tot
activation in quasi-stellar spectrum
7Li(p,n)7Be
kT=25 keV
18O(p,n)18F
kT=5 keV
gamma spectroscopy with HPGe detector
isomeric ratio
spectrum after irradiation
176Lum 176Lug
improved nuclear physics input and
refined low mass AGB star model
level scheme of 176Lu + MACS to ± 1% for 174Yb, 176Yb 175Lu, 176Lu 176Hf, 177Hf, 178Hf… + IR(176Lu) @ kT= 5 keV kT=25 keV
branching factor fn (nn, T)
fn chosen for 6 differentneutron density situationsthroughout each thermal pulse covering a range 0.20 < fn < 0.92
3 1010 cm-3
3 109 cm-3
3 108 cm-3
s production of 176Lu and 176Hf during and between thermal
pulses
Yb
176
LuHf
175
174 176
177 178
h
Gyr
176Lu
176Hf 176Lu
176Hf
the main s component (in %)
1999 1999
176Lu 90 176Hf 113
ATOMIC MASSOV
ER
AB
UN
DA
NC
ES
NO
RM
AL
IZE
D T
O 15
1S
m
2006
104 96
after 5Gyr
96 97
summary
• the abundance ratio 176Lu/176Hf is determined by interplay of several nuclear physics features with the stellar environment decay rate, cross sections, isomers T(t) and nn(t)
• this interplay is so complex that the chance to obtain the correct answer simply by “ben trovare“ is negligible
• in a wider context this holds also for similar independent s-process branchings; hence these cases provide the most crucial test for stellar models of the AGB phase
Karlsruhe: C. Arlandini, H. Beer, S. Dababneh, M. Heil, N. Klay, R. Plag, R. Reifarth, G. Schatz, F. Voss, N. Winckler, K. WisshakGrenoble: H. Börner, C. Doll, F. Hoyler, B. Krusche, S. Robinson, K. SchreckenbachMunich: U. Mayerhofer, G. Hlawatsch, H. Lindner, T. von EgidyBasel: T. RauscherSofia: W. Andrejtscheff, P. PetkovObninsk: L. KazakovPrague: F. Becvar, M. KrtickaChicago: A. DavisBeijing: W. ZhaoTeramo: O. Straniero Torino: S. Bisterzo, M. Busso, R. Gallino