Measurement of the cross section for the 4 He(α,n) 7 Be reaction as a possible solution to the cosmological lithium problem

Transcription

1 Measurement of the cross section for the 4 He(α,n) 7 Be reaction as a possibe soution to the cosmoogica ithium probem Takahiro KAWABATA Department of Physics, Kyoto University

2 Big Bang Nuceosynthesis (BBN) Primordia nuceosynthesis is considered to have taken pace from 1 sec to 2 min after the Big Bang. Bures, Noett & Turner (1999). Theoretica estimates of the primordia abundance can be compared with the observation. Primordia abundance is an important probe for the eary universe.

3 Primordia Abundances BBN theory has ony one free parameter baryon density. Baryon density is determined to be Ωb h 2 = (2.23±.2) 1 2 by the Panck sateite. BBN theory gives 1 1 ( Li H)BBN = ( ).62 three times arger than the observation. Cosmoogica Li probem

4 Possibe Soution to the Li probem ü Improvement of the stear modes. ü New physics beyond the standard BBN mode Cosmoogica variation of fundamenta constants. Decay of supersymmetric partices. ü Better understanding of cosmoogica nucear reaction rates. Main origin of 7 Li is EC decay of 7 Be. Enhancement of the destruction rate of 7 Be might sove the Li probem.

5 Primordia Li production/be destruction Main origin of 7 Li: EC decay of 7 Be 7 Be destruction T 9 ~.6.8 Most dominant 7 Be destruction channe: 7 Be(n,p) 7 Li Extensivey studied. No room to modify the known rate. Next dominant channe: 7 Be(n,α) 4 He No experimenta data at cosmoogica energies (T 9 ~.6.8). Both 7 Be and neutron are unstabe. New measurement is desired. M. S. Smith et a., Astrophys. J., Supp. Ser. 85, 219 (1993).

6 Previous Experiment C. H. King et a., Phys. Rev. C 16, 1712 (1977). Trick is needed because both 7 Be and neutron are unstabe. Inverse reaction 7 Be n Cosmoogica reaction 4 He 4 He Measure the Inverse Reaction He gas target 4 He Beam MeV 7 Be stopper 4 He 1 atm He + 11 µm Kapton 7 Be 5.8 cm Φ 12 µm A 478-keV γ-ray was measured off-ine. 478-keV γ-ray

7 Previous Resuts C. H. King et a., Phys. Rev. C 16, 1712 (1977). γ-ray Secptra Reaction Cross Section CM Energy in the inverse reaction E cm = MeV Measurements for the 4 He(α,n) 7 Be reaction must be done at ower energies. Higher than cosmoogica energies.

8 Experimenta Setup Experiment has been done at Neutron TOF Faciity in RCNP, Osaka. 1 atm He gas RCNP 12Φ Aamid 6 µm Beam AVF cycotron Coimator beam MeV 大型放射光施設 TOF path 13 m Neutron Counter Measured anges (Liq. Sci.) 2 2 mm 4He Aamid 6 µm Target 5 mm Liquid Scintiator (NE213)

9 Experimenta Resuts Measured cross sections were fit to obtain the tota cross section. Counts/Channe He(α,n) 7 Be E α = 39.3 MeV θ = 4 He + Ce Empty Ce 1/2-1 (E x = 429 kev) 3/2-1 (g.s.) Neutron Energy (MeV) dσ/dω (mb/sr) dσ/dω (mb/sr) He(α,n) 7 Be E α = MeV 3/2-1 (g.s.) 1/2-1 (.429) He(α,n) 7 Be E α = 38.9 MeV 3/2-1 (g.s.) θ c.m. (deg) He(α,n) 7 Be E α = 39.3 MeV 3/2-1 (g.s.) 1/2-1 (.429) He(α,n) 7 Be E α = 38.5 MeV 95% C.L. 3/2-1 (g.s.) θ c.m. (deg) Cross Section (mb) dσ dω = σ 4π L max L= 4 He(α,n) 7 Be A L P( cosθ) Present (Sum) Present (g.s., 3/2-1) Present (429 kev, 1/2-1) New Data! Previous 7 Be(n,α) 4 He Previous 7 Li(p,α) 4 He E α (MeV) Cross sections at ower energies were successfuy determined.

10 Comparison with compiations Measured cross sections were converted to those for the inverse reaction Be(n,α) 4 He Detaied baance Cross Section (mb) E c.m. (MeV) Wagoner ENDF/B-VII1 Present 7 Be gs (n,α) 4 He Present 7 Be*(n,α) 4 He Derived from 7 Li(p,α) 4 He σ n,α 4 He(α,n) 7 Be 7 Be(n,α) 4 He ( ) ( ) = 1+δ AB σ A + B C + D σ C + D A + B Ŝ A 2S A +1 ( )ŜCŜD k 2 CD ( 1+δ CD )ŜAŜB k 2 AB Estimation from 7 Li(p,α) 4 He ( ) = P n P σ p ( p,α ), P = kr F 2 E, R ( ) + G 2 ( E, R) Cross sections at ower energies: ü Systematicay smaer than the estimation by Wagoner widey used in the BBN cacuation, but cose to ENDF/B-VII.1 7 Be(n,α) 4 He does not contribute to destruct 7 Be.

11 Summary n Measurement of the 7 Be(n,α) 4 He reaction as a possibe soution for the cosmoogica 7 Li probem. q First measurement at the cosmoogica energy. q Successfuy measured down to E cm =.26 MeV. q Cross sections are much smaer than the previous estimation widey used in the BBN cacuation. q No enhancement in the 7 Be destruction mode.

12 Coaboration A bacheor thesis work by undergraduate students in Kyoto T. Furuno, M. Ichikawa,, M. Itoh, Y. Kanada-En'yo, T. Kawabata, A. Koshikawa, E. Miyawaki, M. Mizuno, T. Morimoto, M. Murata, T. Nanamura, R. Sawada, Y. Shikata, Y. Takahashi, D. Takechi, T. Takeda, M. Tsumura, S. Yoshida, and K. Watanabe Department of Physics, Kyoto University S. Kubono, S. Nishimura RIKEN N. Iwasa Department of Physics, Tohoku University

13 Observation of Primordia Li abundance ( ) ( ).16 obs Primordia Li abundance can be determined from the observation of meta-poor stars. Li H = S. G. Ryan et a., Astrophys. J. 6, L57 (2) Od New The observation shows a pateau in ithium verses metaicity.

14 Neutron Detection Puse shape discrimination was done to identify neutron events. V Fast-gated QDC Detection Efficiency t γ-ray event Sow-gated QDC neutron event Sow-gated QDC Neutron events were successfuy seected. 3 2 γ fash Neutron spectrum 2 1 Fast-gated QDC QDC fast-gated vs sow-gated Scint. ch TDC Scint. ch1

15 Comparison with compiations Measured cross sections were converted to those for the inverse reaction. σ n,α Detaied baance 4 He(α,n) 7 Be 7 Be(n,α) 4 He ( ) ( ) = 1+δ AB σ A + B C + D σ C + D A + B Ŝ A 2S A +1 ( )ŜCŜD k 2 CD ( 1+δ CD )ŜAŜB k 2 AB Estimation from 7 Li(p,α) 4 He ( ) = P n P σ p ( p,α ), P = kr F 2 E, R Cross sections at ower energies: ü Systematicay smaer than the estimation by Wagoner widey used in the BBN cacuation, but cose to ENDF/B-VII.1 7 Be(n,α) 4 He does not contribute to destruct 7 Be. ( ) + G 2 ( E, R)