LUNA results at BBN energies BBN is the result of the competition between the relevant nuclear...

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Transcript of LUNA results at BBN energies BBN is the result of the competition between the relevant nuclear...

  • Marciana Marina, Isola d'Elba June 23-27, 2014

    Laboratory Underground Nuclear Astrophysics

    LUNA results at BBN energies Carlo Gustavino

    INFN ROME

    • Nuclear Astrophysics • LUNA experiment • Big Bang Nucleosynthesis • 3He(4He,γ)7Be reaction (7Li abundance) • D(α,γ)6Li reaction (6Li abndance) • D(p, γ)3He reaction (D abundance) • Conclusion

  • 0

    = (KT)3/2

    1 8 πµ

    S(E) exp dE E KT

    EG E

    tunneling probability KT

  • event/month < Rlab < event/day

    ε ~ 10 % IP ~ mA ρ ~ µg/cm2

    Rlab= σ·ε·Ip·ρ·Nav/A

    pb < σ < nb

    Reaction rate in the laboratory

    E EG

    e E ESE

    − =

    )()(σ

    Beware of extrapolations!!

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  • Rlab > Bcosm+ Benv + Bbeam

    Experimental requirements

    Rlab= Reaction rate à High beam intensity, target density, high efficiency detectors Bcosm= Cosmic ray induced background à Underground Laboratory Benv= Natural radioactivity à Passive shielding (Lead, Copper), Anti-Radon Box and high purity detectors. Bbeam= Beam induced background à high purity targets, detector techniques (coincidence), analysis…

    4/28

  • Gran Sasso National Laboratory (LNGS)

    LUNA

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

    5/28

  • Cosmic ray induced background

    Radiation LNGS/Surface Muons 10-6

    neutrons 10-3

    40K 214Bi

    232Th

    Clear advantage of underground measurements for reaction with high Q-values. Concerning BBN: D(p,γ)3He (Q=5.5 MeV)

    6/28

  • Natural radioactivity background

    Passive shielding is more effective underground since the µ flux, that create secondary γs in the shield, is suppressed. Underground measurements are well suited also for low Q-value reactions. Concerning BBN: 3He(α,γ)7Be (Q=1586 keV) D(α,γ)6Li (Q=1473 keV)

    7/28

  • Voltage Range: 50-400 kV Output Current: 1 mA (@ 400 kV) Absolute Energy error: ±300 eV Beam energy spread:

  • BBN is the result of the competition between the relevant nuclear processes and the expansion rate of the early universe, governed by the Freidmann Equation:

    Big Bang Nucleosynthesis

    H 2 = 8π 3 Gρ

    H = Hubble constant

    G = Newton's gravitational constant

    ρ  = energy density (i.e. photons and neutrinos density in the early Universe)

    Therefore, the abundance of primordial isotopes ONLY depends on:

    •  Baryon density Ωb (BBN and CMB experiments) •  Particle Physics (τn, Neff, α..)

    •  Nuclear astrophysics, i.e. cross sections of BBN nuclear reactions

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  • -The BBN begins with the formation of Deuterium. -Nearly all the free neutrons end up bound in the most stable light element 4He. -  7Li and 6Li have small abundance

    because of the absence of stable nuclei with mass number 5 or 8.

    -  Direct observations are restricted to stable isotopes (D, 3He, 4He, 6Li, 7Li)

    Big Bang Nucleosynthesis

    BBN error budgets: 4He: Almost entirely due to Δτn (1) D: Mainly due to the D(p,γ)3He reaction (3) 3He: Mainly due to the 3He(d,p)4He reaction (9) 6Li: Mainly due D(α,γ)6Li reaction (13) 7Li: ..Many reactions of the BBN network (10, 9, 3,...)

    Big Bang Nucleosynthesis

    10/28

  • Determination of primordial abundances

    Direct Observations: •  Observation of a set of primitive objects (born when the universe was young) •  Extrapolate to zero metallicity: Fe/H, O/H, Si/H à 0 •  Systematics mainly due to post-primordial processes 4He: Observation in HII regions, quite large systematics. D: Observation of absorption lines in QSO. Accurate measurements. 3He: Solar System, very large systematics, not a powerful probe for BBN. 7Li: observation of metal poor stars absorption line (Spite plateau) 6Li: observation of metal poor stars absorption lines (controversial) 4He, D, 3He abundances measurements are broadly consistent with expectations. 7Li: Long standing “Lithium problem” 6Li: “Second Lithium problem”? A coherent theory (Cosmology, Astrophysics, Particle physics, Gravitation…) must provide the matching between theory Vs observations for the abundances of all the primordial light isotopes.

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  • Lithium observations 7Li primordial abundance: observation of the absorption lines at the surface of metal-poor stars in the halo of our Galaxy 6Li abundance : observation of the asymmetry of the 7Li absorption lines.

    Observed 7Li abundance is 2-4 times lower than foreseen: Well established ”7Li problem”.

    Observed 6Li abundance orders of magnitude higher than expected (Asplund2006). However the “Second Lithium problem” is debated, because:

    -Convective motions on the stellar surface can give an asymmetry of the absorption line, mimicking the presence of 6Li.

    -The BBN prediction for 6Li is affected by a v e r y l a r g e e r r o r, i n f a c t n o d i r e c t measurements of the D(4He,γ)6Li reaction have been done (before LUNA)

    The Lithium problem(s)

    Line strength: 7Li abundance Line Profile: 6Li/7Li ratio

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  • The Lithium problem(s)

    Line strength: 7Li abundance Line Profile: 6Li/7Li ratio

    Lithium observations The disagreement between the observed 7Li and 6Li abundance in metal poor stars with respect to BBN calculations is very large. Possible reasons are:

    • Systematics in the measured 7Li and 6Li abundances in the metal-poor stars.

    • Unknown processes before the birth of the galaxy, able to burn 7Li and synthetize 6Li.

    • New physics, e.g. sparticle annihilation/decay (Jedamzik2008), long lived negatively charged particles (Kusakabe2010)

    • …Nuclear physics, i.e. the lack of knowledge of the relevant nuclear reactions makes the BBN calculations not reliable.

    13/28

  • The 3He(4He,γ)7Be reaction is a key process to calculate the 7Li abundance. Four objectives for the 3He(4He,γ)7Be LUNA measurement: • Lowest uncertainty (4%). • Measurements well inside the BBN energy region, 90

  • Ecm=93 keV T=31,2 days

    Ecm=93 keV T=11,6 days

    3He(α,γ)7Be at LUNA: prompt γ and activation

    Signal+background bck

    Signal+background bck

    Prompt γs setup

    Delayed γs setup 15/28

  • 3He(α,γ)7Be

    Activation Prompt

    Rescaled Kajino (1987) Rescaled Descouvemont (2004)

    3He(α,γ)7Be at LUNA: result

    BBN

    D. Bemmerer et al., PRL 97, 122502 (2006) Gy. Gyurky et al. Phys.Rev, C 75, 035805 (2007) F. Confortola et al., Phys.Rev. C 75, 065803 (2007)

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  • The D(α,γ)6Li reaction dominates the 6Li production during BBN. Before LUNA, there was a lack of knowledge of the reaction rate in the BBN.

    D(α,γ)6Li: The 6Li/H abundance

    IN FACT: NO DIRECT MEASUREMENTS in the BBN energy region in literature (large uncertainty due to extrapolation) INDIRECT coulomb dissociation measurements (Kiener91, Hammache2010) strongly depends on the theoretical assumptions. FOR THE FIRST TIME, the D(4He,γ)6Li reaction has directly been studied inside the BBN energy region.

    17/28

  • d

    d

    α

    D(α,α)d Rutherford scattering α

    α beam Deuterium gas target

    3He d d

    n

    D(d,n)3He reaction Q=3.269 MeV

    Ecm(3He)= 0,820 MeV

    Ecm(n)=2.450 MeV

    (n,n’γ) reaction on the surrounding materials (lead, steel, copper and germanium) γ-ray background in the RoI for the D(α,γ)6Li DC transition (∼1.6 MeV)

    Beam Induced Background in the D(4He,γ)6Li measurement

    18/28

  • • Germanium detector close to the beam line to increase the detection efficiency. • Target length optimized. • Pipe to reduce the path of scattered deuterium, to minimize the D(d,n)3He reaction yield. • Si detector to monitor the neutron production through the detection of D(d,p)3H protons. • Lead, anti-radon Box to reduce and stabilize Natural Background. • Selected materials.

    D(4He,γ)6Li set-up

    19/28

  • Measurement method

    The measurement is performed into 2 steps:

    1.  Measurement with Eα=400 keV on D2 target. The Ge spectrum is mainly due to

    background induced by neutrons interacting with the surrounding materials (Pb, Ge, Cu). The D(α,γ)6Li signal is expected in a well defined RoI (1587-1625 keV).

    2.  Same as 1., but with Eα =280 keV. The Ge spectrum is essentially the same as before, while the gammas from the D(α,γ)6Li reaction are expected at 1550-1580 keV.

    D(α,γ)6Li Signal is obtained by subtracting the two spectra

    20/28

  • First D(α,γ)6Li cross section measurement at BBN energies

    D(α,γ)6Li: The 6Li/H abundance

    BBN

    PRL (accepted)

    S24(134 keV ) = (4.0±0.8(stat) ± 0.5(syst)) X 10−6 keV b S24(94 keV ) = (2