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

€

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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