Laboratory Underground Nuclear Astrophysics Study of the BBN - - PowerPoint PPT Presentation

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Laboratory Underground Nuclear Astrophysics

Study of the BBN reaction D(4He,γ)6Li deep underground with LUNA Carlo Gustavino

For the LUNA collaboration

• Big Bang Nucleosynthesis
• Primordial abundance of light elements
• 6Li abundance problem
• The D(4He,γ)6Li measurement at LUNA
• Conclusions

Vulcano, 24-29 May 2010

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Big Bang Nucleosynthesis

The primordial abundance of light elements depends on:

• Barionic density ωb (measured by CMB experiments at the level of %)
• Standard Model (τn, ν, α..)
• Nuclear astrophysics (cross sections of interest for 20<E<1000 keV)

Comparison between abundance calculation and observation provides:

• Comparison between CMB and BBN results
• Understanding of post primordial production of light elements
• New physics

ωb(D)=0.021 ± 0.002 ωb(WMAP)=0.023 ± 0.001 Wonderful agreement between CMB and BBN determinations of ωb

7Li abundance predicted by BBN is

not compatible with ”Spite plateau”. Systematics in Astrophysics? Models? New physics?

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Good agreement between calculations and observations for D, 4He, 3He. Problems for 7Li, 6Li

Schematic BBN network Already measured by LUNA: P(D,γ)3He

3He(4He,g)7Be

(D, 7Li abundance) Next: D(4He,γ)6Li (6Li abundance) Spin off:

3He(D,p)4He

(3He abundance)

He

3

He

4

Be

7

Li

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H D p n

2 1 3 4 2 8 9 6 7 11 12 10

• 1. n p + e +
• 2. p + n D +
• 3. D + p He +
• 4. D + D He + n
• 5. D + D H + p
• 6. H + D He + n

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3 3 3 3 3 4 3 7 7 7 7 7 4 4 4 3 3 3 4 4

• 7. H + He Li +
• 8. He + n H + p
• 9. He + D He + p
• 10. He + He Be +
• 11. Li + p He + He
• 12. Be + n Li + p
• 13. He + D Li +

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e

Li

6 13

4 6

BBN reactions

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• D(4He,γ)6Li is the main reaction for the 6Li production
• Theoretical predictions for the S-factor differ by ~2 order of magnitude.
• The 6Li synthesis occurs mainly from 40 keV up to 400 keV, but NO DIRECT

MEASUREMENTS below 650 keV up to now.

• Indirect coulomb dissociation measurements have been done in the region of interest

(kiener 91, NIC2008, NPA2009). Not reliable because the nuclear part is dominant.

• The 6Li abundance in metal poor stars is very large (Asplund et al. 2006) compared to Big-

Bang Nucleosynthesis predictions (NACRE compilation). σ much larger and/or unforeseen 6Li sources older than the birth of the galaxy and/or new physics such as annihilation/decay of supersymmetric particles

D(4He,γ)6Li

LUNA BBN

The LUNA accelerator (Laboratory for Underground Nuclear Astrophysics) below the GRAN SASSO mountain offers a unique possibility of measuring the D(4He,γ)6Li cross section measurement FOR FIRST TIME. Need of a D(4He,γ)6Li direct measurement. Very difficult measurement because:

• Low reaction yield: σ~10 fbarn-pbarn at

Ecm=50-100 keV

• Beam Induced Background.

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Gran Sasso National Laboratory (LNGS)

LUNA

Cosmic background reduction: µ: 10-6 n: 10-3 γ: 10-2-10-5

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The LUNA (400 kV) accelerator

Voltage Range: 50-400 kV Output Current: 1 mA (@ 400 kV) Absolute Energy error: ±300 eV Beam energy spread: <100 eV Long term stability (1 h) : 5 eV Terminal Voltage ripple: 5 Vpp

• A. Formicola et al., NIMA 527 (2004) 471.

14N(p,γ)15O 3He(4He,γ)7Be 25Mg(p,γ)26Al 15N(p,γ)16O

D(4He,γ)6Li

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• α-beam (I~200 µA) on a D2 gas target: D(4He,γ)6Li
• High Purity Germanium detector to detect the 1,6 MeV gamma’s from D(4He,γ)6Li

NATURAL BACKGROUND REDUCTION

• 4π shield of lead to minimize the natural background
• N2 flushing to reduce the Radioactivity induced by Radon (Radon Box)

BEAM-INDUCED BACKGROUND

• Reduced volume for the gas target, to minimize the beam induced background (see later)
• 3He-beam on a D2 gas target, to measure the beam induced background (see later)
• Silicon detector faced the beam line as a monitor (see later)

D(4He,γ)6Li experimental approach

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Beam Induced Background origin d d

3He

α

d(α,α)d Rutherford scattering

d d n α

α beam Deuterium gas target d(d,n)3He reaction (n,n’γ) reaction on the surrounding materials (Pb, Ge, Cu). γ-ray background in the RoI for the D(α,γ)6Li DC transition (∼1.6 MeV)

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Ge Detector Silicon Detector Deuterium inlet Steel pipe LEAD Alpha beam Deuterium exhaust

D(4He,γ)6Li conceptual set-up

• Germanium detector close to the beam line
• Pipe to minimize the path of scattered deuterium and hence to minimize the

d(d,n)3He reaction yield

• Silicon detectors to monitor the neutron production detecting the protons

from the conjugate d(d,p)3H reaction

10 Silicon Detectors to detect D(D,p)3H protons Steel pipe to minimize D+D reactions yield Main Chamber Germanium Detector

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Experiment MC Simulation

D(D,p)3H protons detection with Si detector: Proton peak Energy and shape are well reproduced. Very good data/simulation agreement.

D(D,p)3H reaction (march 2009 test)

Silicon detectors

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Basic idea:

• Run with α-beam + deuterium target. Ge spectrum is mainly due to γ-lines due to (n,n’γ)

reactions due to the interaction of d(d,n)3He neutrons with the surrounding materials (Pb, Ge, Cu). In the Region of Interest (1580-1630 keV) is expected the γ-line due to the D(α,γ)6Li reaction.

• One run with 3He-beam + deuterium target. Same spectrum as before but the γ-line due to

the D(α,γ)6Li reaction.

Signal is obtained by subtracting the two spectra

Ge spectrum (november 2009 test)

Region of Interest Germanium Detector

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Statistical evidence as a function of time for several operating conditions (Eα=400 keV, Mukhamedzhanov S-factor). Sensitivity (inferred from the november 2009 test)

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• Very difficult measurement, no doubt.
• Even in the case of an upper limit for the astrophysical factor, the

measurement will give a solid experimental base to confirm/discard the existence of (unknown) mechanisms to produce the observed 6Li abundance (i.e. post primordial production or new physics).

• The neutron production has been minimized with the present set-up, at the

level of few neutron/second. However, a further study with DM people is in progress to prevent any possible interaction with the experiments at LNGS.

Conclusion Hope to see you next Vulcano workshop

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Extra Slides

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E(3he)=333 keV E(4he)=360 keV Similar proton spectra->similar neutron spectra

17 New measurement from LUNA needed!

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Impact on BBN Present status (including LUNA D(p, γ)3He but not 3He(4He, γ)7Be )

N.B. there is now a new D+D precision measurement (PRC 2006, LEONARD), therefore the relative uncertainty due to D+p and 3He+D is even higher!

NACRE Compilation, no direct measutrement Before LUNA Before LEONARD

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σ(E)=S(E)/E e–2πη

πη 2πη πη = 31.29 = 31.29 Ζ1Ζ2 √µ/Εcm µ= = m1m2

2 / (

(m1+m2)

Astrophysical Factor Gamow Factor

• Very low cross sections because of the coulomb barrier

UG experiments to reduce the background due to cosmic ray

N.B. differently from stars, in BBN we don’t have a fixed T (gamow peak), although there is a kinetic equilibrium

Why underground Measurements?

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Extrapol. Mesurements E S(E) Sub-Thr resonance Narrow resonance Non resonant process Tail of a broad resonance

• Very low cross sections
• Danger in extrapolating

UnderGround Measurements

Why underground Measurements?