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

Laboratory Underground Nuclear Astrophysics Study of the BBN reaction D( 4 He , γ ) 6 Li deep underground with LUNA Carlo Gustavino For the LUNA collaboration •Big Bang Nucleosynthesis •Primordial abundance of light elements • 6 Li abundance problem •The D( 4 He , γ ) 6 Li measurement at LUNA •Conclusions Vulcano, 24-29 May 2010 1

Big Bang Nucleosynthesis The primordial abundance of light elements depends on: •Barionic density ω b (measured by CMB experiments at the level of %) •Standard Model ( τ n , 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 ω b (WMAP)=0.023 ± 0.001 ω b (D)=0.021 ± 0.002 7 Li abundance predicted by BBN is not compatible with ”Spite plateau”. Systematics in Astrophysics? Models? Wonderful agreement between CMB and 2 New physics? BBN determinations of ω b

Good agreement between BBN reactions calculations and observations for D, 4 He, 3 He. Schematic BBN network Problems for 7 Li , 6 Li 1. �� n ������� �� p + e + � 7 Be Already measured by LUNA: e 2. �� p + n �� D + � 3 P(D, γ ) 3 He 3. �� D + p �� He + � 12 3 4. �� D + D �� He + n 3 He( 4 He,g) 7 Be 3 5. �� D + D �� H + p 10 3 4 6 Li 7 Li 6. �� H + D �� He + n (D, 7 Li abundance) 13 Next: 11 9 4 He D( 4 He, γ ) 6 Li 3 He 7 ( 6 Li abundance) 8 3 4 6 Spin off: 7 3 4 7. �� H + He �� Li + � 2 3 p 3 D 3 H 8. � �� He + n �� H + p 3 He(D,p) 4 He 5 4 9. �� He + D �� He + p 3 ( 3 He abundance) 3 4 7 10. �� He + He � Be + � 2 7 4 4 1 11. �� Li + p �� He + He 7 7 12. �� Be + n ��� Li + p n 4 6 13. He + D �� Li + � 3

D( 4 He, γ ) 6 Li •D( 4 He , γ ) 6 Li is the main reaction for the 6 Li production •Theoretical predictions for the S-factor differ by ~2 order of magnitude. •The 6 Li 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 6 Li 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 6 Li sources older than the birth of the galaxy and/or new physics such as annihilation/decay of supersymmetric particles Need of a D( 4 He , γ ) 6 Li direct measurement. Very difficult measurement because: LUNA •Low reaction yield: σ ~10 fbarn-pbarn at BBN E cm =50-100 keV •Beam Induced Background. The LUNA accelerator (Laboratory for Underground Nuclear Astrophysics) below the GRAN SASSO mountain offers a unique possibility of measuring the D( 4 He , γ ) 6 Li cross section measurement FOR FIRST TIME. 4

Gran Sasso National Laboratory (LNGS) LUNA Cosmic background reduction: µ : 10 -6 n: 10 -3 γ : 10 -2 -10 -5 5

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. 14 N(p, γ ) 15 O 3 He( 4 He, γ ) 7 Be 25 Mg(p, γ ) 26 Al 15 N(p, γ ) 16 O D( 4 He, γ ) 6 Li 6

D( 4 He, γ ) 6 Li experimental approach • α -beam (I~200 µ A) on a D 2 gas target: D( 4 He , γ ) 6 Li •High Purity Germanium detector to detect the 1,6 MeV gamma’s from D( 4 He , γ ) 6 Li NATURAL BACKGROUND REDUCTION •4 π shield of lead to minimize the natural background •N 2 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) • 3 He-beam on a D 2 gas target, to measure the beam induced background (see later) •Silicon detector faced the beam line as a monitor (see later) 7

Beam Induced Background origin α α beam d α Deuterium d d( α , α )d Rutherford scattering gas target 3 He d d(d,n) 3 He reaction d n (n,n’ γ ) reaction on the surrounding materials (Pb, Ge, Cu). γ -ray background in the RoI for the D( α , γ ) 6 Li DC transition ( ∼ 1.6 MeV) 8

D( 4 He, γ ) 6 Li 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) 3 He reaction yield • Silicon detectors to monitor the neutron production detecting the protons from the conjugate d(d,p) 3 H reaction Silicon Detector Deuterium Alpha beam inlet Ge Detector Steel pipe LEAD Deuterium exhaust 9

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

D(D,p) 3 H reaction (march 2009 test) MC Simulation Silicon detectors Experiment D(D,p)3H protons detection with Si detector: Proton peak Energy and shape are well reproduced. Very good data/simulation agreement. 11

Ge spectrum (november 2009 test) Germanium Detector Region of Interest 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) 3 He 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 ) 6 Li reaction. • One run with 3 He-beam + deuterium target. Same spectrum as before but the γ -line due to the D( α , γ ) 6 Li reaction. Signal is obtained by subtracting the two spectra 12

Sensitivity (inferred from the november 2009 test) Statistical evidence as a function of time for several operating conditions (E α =400 keV, Mukhamedzhanov S-factor). 13

Conclusion • 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 6 Li 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. Hope to see you next Vulcano workshop 14

Extra Slides 15

E( 3 he)=333 keV E( 4 he)=360 keV Similar proton spectra->similar neutron spectra 16

New measurement from LUNA needed! 17

Impact on BBN Present status (including LUNA D(p, γ ) 3 He but not 3 He( 4 He, γ ) 7 Be ) Before LEONARD 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! Before LUNA NACRE Compilation, no direct measutrement 18

Why underground Measurements? Astrophysical Gamow Factor Factor σ (E)=S(E)/E e –2 πη πη 2 πη πη = 31.29 = 31.29 Ζ 1 Ζ 2 √ µ/ Ε cm = m 1 m 2 ( m 1 + m 2 ) µ= 2 / ( •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 19

Why underground Measurements? •Very low cross sections •Danger in extrapolating UnderGround Measurements S(E) Sub-Thr Extrapol. Mesurements resonance Tail of a broad Narrow resonance resonance Non resonant process E 20