Lifetime measurement of the 6.791 MeV state in 15O
Naomi GalinskiSFU, Department of Physics, Burnaby BC
TRIUMF, Vancouver BC
CAWONAPS, 10 December 2010
Recipient of a DOC-FFORTE-fellowship of the Austrian Academy of Sciences at the Institute of
SFU
• Globular clusters:• Oldest known/visible objects
in our galaxy
• Compact groups of 100,000 - 1 million stars
• 1980: 16-20 Gyr
• Now: 10-15 Gyr
• Age of the universe:• WMAP 13.7±0.2 Gyr
• Globular clusters can’t be older than the universe L. Krauss and B. Chaboyer, Science 299, 65 (2003)
1) Primordial gas cloud
2) Globular clusters form first
3) Galactic disk forms
4) Globular clusters occupy galactic halo
Motivation
Age determination of globular clusters:
• Correlation between luminosity at MS turnoff point & age globular cluster
• CNO cycle dominates energy production at end of MS lifetime
• 14N(p, γ)15O is the slowest reaction
• Reaction rate uncertainty could change globular cluster age by 0.5-1 Gyr
Main sequence (MS) branch
(H->He)
Red giant branch(He->C)
MS turnoff point
Temperature
Lum
inos
ity
Motivation
• Need to know 14N(p, γ)15O reaction
rate at low (stellar) energies
• E0 30 keV (for T = 0.02 GK)
• Past experiments only go down
to ECM = 70 keV
• Energy below low-energy limit
of direct γ ray measurements
• Need to extrapolate down to
low energies using R-matrix
analysis of S-factor
Formicola et al., Phys. Let. B 591, 61-68 (2004)
The 14N(p, γ)15O reaction
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σ(E) =1
Eexp(−2πη)S(E)
S-factor of 14N(p,)15O reaction
Total S-factor of the 14N(p,)15O reaction with contributions of different transitions to states of
15O.Angulo et al., Nucl. Phys. A 690, 755-768 (2001)
•Largest remaining uncertainty in reaction rate is due to width, , of 6.791 MeV state
•This will constrain the R-matrix fit
•Obtain width from lifetime: = ℏ /
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R-matrix fits to the 14N(p,)15O 6.79 MeV transition.
Review article: Solar fusion cross sections II, the pp chain and CNO cycles, arXiv:1004.2318v3 [nucl-ex] 10 Oct 2010
Lifetime of 6.791 MeV state, τ [fs]
Confidence Level (%)
Measurement
Bertone et al. (2001)
90 DSAM, direct
Yamada et al. (2004)
> 0.42 90Coulomb excitation, indirect
Schürmann et al. (2008)
< 0.77 90 DSAM, direct
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1.60−0.72+0.75
• Results marginally disagree
• Only one group, Bertone et al., has claimed central value
• This value is not generally accepted
Previous measurements
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Eγ (ϑ , t) = Eγ01− β2(t)[ ]
1/ 2
1− β(t)cosθ[ ]
Simulated lineshapes for different lifetimes. These are fit to the data to determine the lifetime of the excited state.
• In DSAM an excited recoil populated by a reaction decays as it slows down in a heavy foil
• The Doppler shifted energy of rays emitted from a recoil traveling with reduced velocity (t)=v(t)/c is given by:
Doppler Shift Attenuation Method (DSAM)
3He + Au
Au
16O 15O
E(t)=max
E(t)=0
(0)=max
(t)=0
ejectile
16O15O
3He
• Lower limit of DSAM ~1 fs
• of 6.791 MeV state ~1 fs
• For accuracy need to know stopping powers
• Electronic stopping power better known
• Nuclear stopping not known so well
• Previous measurements low recoil velocity (≤0.0016)
• Nuclear stopping region
• 14N+p → γ+15O
• We had higher recoil velocity (≤0.055)
• Used inverse kinematic reaction
• 3He+16O → α+15O
DSAM and 6.791 MeV lifetime3He + Au
Au
15O E(t)=max
E(t)=0
(0)=max
(t)=0
TRIUMF ISAC II:
• Stable beam of 16O at 50 MeV (1st run) and 35 MeV (2nd run)
• 3He was implanted in a Au and Zr target foil.
• We used the Doppler shift lifetime (DSL) chamber, a target chamber specifically designed for DSAM experiments.
• The rays were detected using a Ge TIGRESS detector on a single mount
Experiment
Collimator
E Si detector(500 μm)
TIGRESSdetector at 0°
E Si detector(100 μm and 25 μm)
Implanted 3He(6×1017 atoms/cm2)
Au/Zr foil (25 μm)
Vacuum chamber
3He+16O → α+15O
16O
15O
Experimental setup
16O beam
Doppler Shift Lifetime chamber
ray spectrum
keV
Fig. Add back spectra of rays using the Zr foil
6791 keV 15O Doppler shifted
6176.3 keV 15O Doppler shifted
5239.9 keV 15O
5183 keV 15O Doppler shifted
Full energy peakSingle escape peakDouble escape peak
511 keV
937 keV 18F 3He(16O,p)
1369 keV 24Mg 12C(16O,4He)
Si detector particle ID spectrum
Fig. Si 2D spectrum from Zr foil. It is the energy deposited in the dE Si detector vs. the energy deposited in the E Si detector. Ejectiles can be identified this way.
dE [C
h]
E [Ch]
(15O)
3He (scat)p (18F)
Light charged Light charged particles from particles from
3He + He + 16O O →→ x + X x + XHeavier ejectiles
ray spectrum gated on
Figures: Doppler shifted 6.791 MeV ray peak for the 1st and 2nd experiment using either Au or Zr target foils.
Ungated spectrum
Spectrum gated on particles
Au foil 1st run
Au foil 2nd run
Zr foil 2nd run
Lifetime fit of 6.791 MeV state from the first experiment
Lifetime = fs
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1.7−0.5+0.7
PRELIMIN
ARY
PRELIMIN
ARY
Sky Sjue
• Analysis of 1st data set:
• Refine calibration of ray energy to get correct addback spectra
• Need to know centroid with precision within 1 keV
• Analysis of 2nd data set:
• Check GEANT4 simulation of kinematics of alpha particles
• Get lifetime of 6.791 MeV state from:
• lineshape analysis from Au foil data
• lineshape analysis from Zr foil data
• centroid shift analysis from Au and Zr data
Work in progress
Collaborators:
B. Davids1, S. Sjue1, T.K. Alexander, G.C. Ball1, R. Churchman1, D.S. Cross1,2, H. Dare3, M. Djongolov1, H. Al Falou1, P. Finlay4, J.S. Forster5,
A. Garnsworthy1, G. Hackman1, U. Hager1, D. Howell2, M. Jones6, R. Kanungo7, R. Kshetri1, K.G. Leach4, J.R. Leslie8, L. Martin1, J.N. Orce1, C. Pearson1, A.A. Phillips4, E. Rand4, S. Reeve1,2, G. Ruprecht1, M.A. Schumaker4, C. Svensson4, S. Triambak1, M. Walter1, S. Williams1, J.
Wong4
1TRIUMF, Vancouver, BC, Canada2Dept. of Phys., Simon Fraser University, Burnaby, BC, Canada
3Dept. of Phys., University of Surrey, Guildford, UK4Dept. of Phys., University of Guelph, Guelph, ON, Canada
5Dept. of Phys., Université de Montréal, QC, Canada6Dept. of Phys., University of Liverpool, Liverpool, UK
7Astr. and Phys. Dept., St. Mary’s University, Halifax, NS, Canada8Dept. of Phys., Queen’s University, Kingston, ON, CanadaReceipient of a DOC-FFORTE-fellowship of the Austrian Academy of Sciences
at the Institute of SFU
Simulation
Reaction kinematics
ejectile
15O recoil
Angular detection efficiency of the ray
detector
Beam and target characteristics
Intrinsic lineshape of high energy rays of the TIGRESS
detector
• Stopping power and straggling of recoil as a function of time in the target
Sky Sjue
Nuclear stopping: (<0.005)
• Collisions between atoms
• Large energy loss
• Changes direction of nuclei
Electronic stopping: (≥0.02)
• Long range collisions with e-
• Small energy transfer
• Small deflection of nuclei
Stopping mechanisms for recoils
• 14N(p,γ)15O • Direct kinematics• 15O has <0.0016• Nuclear stopping region
• 3He (16O,)15O* • Higher Q-value• Inverse kinematic reaction• 15O has <0.055• Electronic stopping region• Cleaner signal with coincidence detection of • We did previous measurements with 3He implanted foils
Reactions to measure lifetime
Globular cluster age uncertainties
• Age estimated to be between 10 - 15 Gyrs• Biggest uncertainties comes from deriving
distances to globular clusters
• Stellar evolution input parameters that can significantly affect age estimates:• Oxygen abundance [O/Fe]
• Treatment of convection within stars
• Helium abundance
• 14N+p → 15O+ reaction rate
• Helium diffusion
• Transformations from theoretical temps and luminosities to observed colors and magnitudes
• Biggest effect of the nuclear reactions is 14N+p → 15O+• Accounts for 0.5 - 1 Gyrs variation in ages
S factor -> luminosity -> cluster age
Degl’Innocenti et al., Phys. Let. B 590, 13-20 (2004)
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