A new approach to the 176 Lu puzzle clock or thermometer? an astrophysical quest and a nuclear challenge 20 years of nuclear physics level schemes,

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