Docstoc

IMPRIVING THE RATE OF THE TRIPLE ALPHA PROCESS

Document Sample
IMPRIVING THE RATE OF THE TRIPLE ALPHA PROCESS Powered By Docstoc
					IMPROVING THE RATE OF THE TRIPLE ALPHA PROCESS

Clarisse Tur a, Alan Wuosmaa b , Sam Austin a , J. Lighthall b, S. Marley b , N. Goodman b , J. J. Bosb International Symposium on Nuclear Astrophysics – Nuclei In the Cosmos-IX June 25-30, 2006 CERN, Geneva, Switzerland
National Superconducting Cyclotron Laboratory (NSCL) / Michigan State University (MSU) a and Western Michigan University (WMU) b

What is the triple alpha experiment about?
 Goal: measure more precisely than has been done in the past the

internal pair emission branch of the 7.654 MeV state of 12C .  This will imply more precision on the knowledge of the rate of the fundamental triple- reaction that takes place in stars to form the element carbon. Current error on rate: +/- 10-12% ; goal: +/- 5.5 to 6%.  Do this by observing the e+e- decay pairs of the 0+ 7.65 MeV state of 0 12 C in coincidence with inelastically scattered protons at 135 from
12

C ( p, p')12C (7.65 MeV )

 Data will be taken using tandem accelerator at WMU – in the summer

of 2006. The incoming proton energy will be about 10.6 MeV.

Clarisse Tur – NIC9 – June 29, 2006

More on the triple alpha reaction
During the helium-burning stage in stars, three 4 He nuclei are fused into 12C in a resonant two-stage process called the triple-alpha process: STEP I:
4

He  4He 8Be( g.s.)
STEP II:

8

Be  4He 12C (7.65 MeV )

Then:
12

C (7.65 MeV )12C ( g.s.)  [( 3.21MeV ,  4.44 MeV )  (e  e  )]

A closer look at the triple alpha rate r3


The rate for the triple-  reaction can be approximately written as:

r3  rad exp( Q3 / kT )




Q3  ( M 12  3M  )c 2  Er ( Q3 is the energy released in the decay 12C (7.65 MeV ) 34 He ) rad     is experimentally determined as the product of 3
independently measured quantities:

Q3

given by:

rad

rad     

( rad is the radiative width of the 7.65 MeV state of carbon 12,  is   the total width taking into account the  ,  and e e decays of the Hoyle state )

Clarisse Tur – NIC9 – June 29, 2006

Current knowledge of r3
Parameter Value and Error Err % 2.7 % GOOD 9.2 % 6.4 % Expts. 8-1 3 2

rad /   / 

(4.12  0.11) 10 4
(6.74  0.62 ) 10 6

=>

ERROR ON 3-ALPHA RATE: ~11.6%



(60 .5  3.9) 10 6 eV

NEW : H. Crannell et al. - Based on analysis of existing electron scattering data – Nucl. Phy. A758 (2005) 399c

  (52 .0  1.4) 10 6 eV (2.7%) GOOD
And : Q3  (379 .38  0.20 ) 10 3 eV (6 measurements of Er) - GOOD: Contribution to fractional error on 3-alpha rate: 1.2%

=>

ERROR ON 3-ALPHA RATE: ~10.0%

Back to the triple alpha experiment
~10.6 MeV protons impinging on
carbon 12 target – inelastic scattering excites nuclei to 7.654 MeV state.
take advantage of strong resonance at incoming energy of ~10.6 MeV and 135 degrees in lab scattering angle. to reduce gamma ray background, require coincidence between scintillator tube and outer scintillator block. pair branch given by:

Improved version of Robertson et al, PRC 15, 1072 (77)

 # e  e  pairs(coincidence)   #7.654 MeVprotons( all )

Clarisse Tur – NIC9 – June 29, 2006

Accuracy in r3 which we want to achieve
Measure  / to +/5% (down from +/- 9.2%)
This experiment versus Robertson et al. (1977): - we won’t be statistics-limited (can run for a longer amount of time + 2 proton detectors instead of 1) - less pile-up, fewer chance coincidences (dc beam instead of a pulsed one, faster scintillator (BC404 versus NE102), faster electronics, greater granularity (outer block divided into 4 quadrants), can run at lower intensity for a longer amount of time) - less gamma background (thinner inner scintillator tube => 7% (Robertson et al.) versus 4.7% (us) according to simulations) - less target contaminants (better target purity) The accuracy that we can get will essentially only be limited by systematics (how well we can determine the gamma background for instance).

ERROR ON 3-ALPHA RATE: ~5.5 to 6%

=>

Clarisse Tur – NIC9 – June 29, 2006

Why is a better accuracy needed? SNII Nucleosynthesis A=16-40
Explosion of 15, 20 and 25 M star; average. Vary rate of 12C(,)16O;
Heger, Woosley, Boyes

All else same.
Production Factor “Same” PF for 1.2 x standard 12C(,)16O rate.

12C(,)16O

Multiplier (xBuchmann

1996)

Why is a better accuracy needed? Size of the iron core of an SNII
CORE-COLLAPSE SUPERNOVAE (SNII):
Fe Core Size (Solar Masses) 2.0 1.5 1.0 0.5

Even if R  r3 / r ,12 was determined with 10% accuracy, there would be an intrinsic uncertainty of 0.2 M sun on the core mass at onset of core-collapse (it takes an energy similar to the energy released in supernova explosion to dissociate that into nucleons!) => need better knowledge of both rates (right now, 10-12% accuracy on 3-alpha, 20% on the 12C ( ,  )16O ). Need a better knowledge of BOTH reaction rates. Knowing the 3-alpha rate to 6% is a first step in that direction.

25 M

Heger, Woosley, Boyes

1.0

1.5
12C(,)16O

2.0

Multiplier or 1/Triple alpha

Clarisse Tur – NIC9 – June 29, 2006

Why is a better accuracy needed? Asymptotic giant branch stars (AGB stars)
Following core H and He burning thermal flashes in He shell induce convective currents that generate (or carry?) C rich material in the inter-shell region. And then to the convective envelope. Convects to the stellar surface, blown off star by strong winds.

This 3rd dredge-up Brings 12C to stellar surface, contributes strongly to galactic nucleosynthesis

Karakas, et al. Clarisse Tur – NIC9 – June 29, 2006

AGB stars: Sensitivity to Nuclear Reaction Rates (F. Herwig, S. M. Austin)
ASYMPTOTIC GIANT BRANCH (AGB) STARS: Stronger pulses (or He flashes) are induced and hence the carbon enrichment of the stellar surface is increased by about a factor of two when either the 3-alpha rate is increased by its uncertainty or when the 14 N ( p,  )15O rate is decreased by its uncertainty. The latter has recently been determined to 5% (Lemut et al., arXiv:nucl-ex/0602012) => need better knowledge of 3-alpha rate.

Low 14N(p,) High 3

Herwig et al. Astro. Journal, 613: L73-L76 (2004) , and Herwig et al. Phys. Rev. C 73, 025802 (2006)

Simulations: Detector geometry

An acrylic tube + BC404 tube + 4 BC404 quadrants

Simulations: Stopping range of parent e+ and e-

11.64% of all e+ & e- exited detector (mainly through central hole. Those that stop in the detector, do so close to the center.

Clarisse Tur – NIC9 – June 29, 2006

Simulations: Comparing cascade gamma pairs to e+e- pairs from the Hoyle state

cut: (de > 0.) && (eOut > 0.)

3.4% of the 1M gamma pairs thrown make it through these minimum requirements.

Clarisse Tur – NIC9 – June 29, 2006

Summary and conclusion
 Want accuracy of 5% for the pair branch => accuracy of about 


 

6% for the rate of the 3-alpha process. Detector has been built at the NSCL and has already undergone extensive tests. The simulations give us confidence in the design of our detector and our measurement. Running in the summer of 2006, using WMU Tandem accelerator. Stay tuned!

Clarisse Tur – NIC9 – June 29, 2006


				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:5
posted:11/3/2009
language:English
pages:15