A New Measurement of the E1 Component of

Transcription

1 A New Measurement of the E1 Component of the 12 C(α,γ) 16 O Reaction X. Tang Physics Division Argonne National Laboratory

2 Outline Why is the 12 C(α, γ) 16 O reaction important? The 12 C(α,γ) 16 O reaction (Previous efforts) The 16 N β-delayed α decay experiment at ANL Preliminary results

3 Carbon and Oxygen in the Universe α+α+α 12 C O(0.85%)+C(0.39%)+ 12 C + α 16 O The final C/O depends directly on the reaction rate ratio between 3α and 12 C(α,γ) 16 O. O(65%)+C(18%)+

4 Influences of the uncertainty in the 12 C(α,γ) 16 reaction rate in Nucleosynthesis 16 O Heger, Woosley, & Boyse (2002) S(300)=170 kevb

5 One of the most important nuclear reactions for astrophysicists Uncertainty in the 12 C(α,γ) 16 O reaction rate affects not only the nucleosynthesis but also the explosion itself. The determination of the ratio C/O produced in helium burning is a problem of paramount importance in Nuclear Astrophysics. W. Fowler, Nobel lecture, 1983 The fusion of 4 He and 12 C nuclei to 16 O is the most important nuclear reaction in the development of massive stars. NuPECC Long Range Plan 2004 We hope that will motivate experimentalists to undertake the difficult task of accurately measuring this rate.

6 12 C( 16 O The current status of 12 C(α,γ) 16 Difficulties in Direct Measurements (1974-?) 16 N β-delayed α decay

7 Level Scheme of 16 O 3 resonances 1 direct capture resonance (high lying) S factor resonance (sub threshold) resonance (sub threshold) E2 DC E1 E1 E2 Complicated reaction mechanism

8 Direct measurements E1 component S(E)=E*exp(2πη)*σ(E) 2πη=31.29*z a *z t *(m α /E α ) 2 1 barn=10-24 cm 2

9 By C. Rolf

10 16 N β-delayed α decay 16 N Lifetime = 7.13 s β-decay γ-ray ~100% 16 O* C+α threshold 16 O 12 C α Baye & Descouvemont predicted the interference peak in 1988!!! NPA458(1988)445 ~500

11 Interference peak S E1 x1.5 ~ 500 S E1 /1.5 Ec.m. (MeV) J. Humblet et al., Phys. Rev. C44, 2530(1991)

12 The best approach for S E1(300) Simultaneously fit 12 C(α,γ) 16 O, 12 C(α,α) 12 C, 16N(β) 16 N( Number of Levels: 2 + Background (1 - ) 2 + Background (3 - ) SE1(300)=80±20 kevb

13 SUM 16 N 12 C(α,γ) 16 O 12 C(α,α) 16 O(l=1) 12 C(α,α) 16 O(l=3) 16 N is the most sensitive constraint for SE1(300 kev) factor Azuma et al., PRC50(1994)1194

14 A Challenging Experiment Small alpha decay branch (1/100,000) requires intensive 16 N source w/o any radioactive contaminations. Huge beta background requires that the detector is insensitive to beta particles. Good linearity over the alpha energy range from 0.5 MeV to 2.5 MeV

15 The previous 16 N experiments Mainz ( ) 1974) Si(single) ) 35 μ Si (Eα>1.2MeV) TRIUMF ( ) 1997) Si(α+ 12 C) 11~16 μ Si+ 10 μg/cmμ 2 C Yale ( ) 1997) Si(β+α) 50 μ Si + 80μg/cm 2 TiN Seattle ( ) 1995) Si(α+ 12 C)? μ Si + 10~15 μg/cm 2 C + 250μg/cm 2 Au

16 Difficulties in the past experiments Si detectors are sensitive to the huge β background. A high energy tail in β- ray response function. Nonlinear response Radiation damage. Dead layer correction. Pulse height correction N α /N β ~ MeV β loses 3.3 kev in 10 μ Si!!! (pileup!!!)

17 TRIUMF :1 310:1 620:1 Singles Singles Coincid. 17,18 N subtracted 620:1

18 1.E+05 1.E+04 TRIUMF94 Seattle95 1.E+03 Counts N ( 1.E+02 1.E+01 1.E E cm (MeV) Yale: Z. Zhao et al., PRL 70(1993)2066 [S(E1,300keV)=95±6(stat)±28(sys) kev b] TRIUMF: R. Azuma et al., PRC 50(1994)1194 [S(E1,300keV)=79±16±14(sys) 14(sys) kev b] Systematic difference limits the precision of SE1.

19 Advantages of gas over solid-state Choose the thickness exactly as needed. This limits β sensitivity to minimum. No radiation damage Available with large areas Improved homogeneity No dead layer detectors Smaller pulse height defects Different technique, different systematic uncertainty

20 Study of the 16 N β-delayed α decay with a new technique 4 Ionization chambers Stepping motor, encoder 16 N beam T ½ =7.1 s Rotating wheel/cathode Rotating wheel, cathode

21 Twin ICs 12 C α Thin C foil 16 N E Grid Grid E attenuation cell wheel/cathode

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23 Angle dependence of the Frisch grid signal A G V G G 90 deg C t 0 deg A 90 deg A V G G G t 0 deg C Grid~E*(1-R(E)/D*cos(θ)) A

24 Production of 16 N beam d( 15 N, 16 N)p 22 o bending magnet 15 N ~100pnA D 2 16 N, I ~ 3x10 6 /s E=61.3±0.3 MeV Purity~70% Particle identification Particle identification 20 Ne 8+ Range (a.u.) 15 N 6,7+ 6+,7+ 16 O N 7+ Focal plan position 2 (a.u.) E 2 (a.u.)

25 Ti 1.3 mg/cm 2 Ti 1.3 mg/cm 2 Al 6.6 mg/cm 2 Only 5.5% 16 N used for decay measurements

26 Measure the α spectrum Left Ion Chamber pair Right Ion Chamber pair b2 b1 12 Wheel

27 Background from 210 Po in solder 115/hr 185/hr β background from a strong 22 Na source (10 5 /s) 160/hr 345/hr

28 Detector Calibration Grid(a.u.) Energy (a.u.) 90 deg 0 deg 10 B(n,α) 7 Li (for 150, 195 torr) Eα0= MeV Eα1= MeV 6 Li(n,α)t ( for 195 torr ) Eα2=2.056 MeV 150 torr 35 kev Energy (a.u.) Grid~E*(1-R(E)/D*cos(θ))

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30 Energy Calibration 10 B(n, α) 7 Li 6 Li(n, α)t 10 B-C- 6 LiF (10 μg/cm 2 ) α(1)=2.056 MeV α(2)=1.776 MeV α(3)=1.472 MeV 195 torr t(1) α(1) Grid (a.u.) 7 Li(2) 7 Li(3) α(2) α(3) Energy (a.u.) FWHM~35 kev Energy (a.u.)

31 Influence of geometry up 12 C down E_up α E_down anode grid grid anode wheel

32 Experiment Simulation

33 Irradiated target Background target Energy2 (a.u.) Energy1 (a.u.) Energy1 (a.u.) E(alpha) /E( 12 C) = 3

34 Irradiated target Background target Grid (a.u.) Energy (a.u.) Energy (a.u.) Grid vs. Energy for alphas

35 Irradiated target Background target Grid (a.u.) Energy (a.u.) Energy (a.u.) Grid vs. Energy for 12 Cs

36 Irradiated target Background target Energy 2 Energy 1 Energy 2 Energy 1 After cutting in grid vs. energy

37 + * Previous * + + France, Yale Thesis * Azuma et al., PRC50(1994)1194

38 Preliminary (150 torr only from Dec.2004)

39 R-Matrix Fitting Preliminary Different f-wave strength?

40 Azuma et al., PRC50(1994)1194 S E1 (300)=79±16(stat) ±14(sys) kev*b Relative decay branching ratio Uncertainty in Γγ(7.12MeV) Systematic differences between 4 sets of 12 C(α,γ) 16 Normalization of 12 C(α,γ) N subtraction 17 N subtraction Uncertainty in Energy resolution (±5keV)( Coincidence Eff. Possible noise events Energy Calibration 12 C( 16 O 16 O ±6 ±3 ±4 ±2 ±1 ±5 ± ±10

41 Ib(7.13)/Ib(9.63)=(4.8±0.4)/[(1.20±0.05)x10-3 ] 15 N

42 Outlook Direct Measurements: New Stuttgart & Karlsruhe direct measurements[3] Recoil Separator + gamma coincidence at Bochum & TRIUMF Underground laboratory Indirect Measurements: 12 C+a: new elastic scattering data from Notre Dame[1] 16 N: new ANL data (~1.6e5) 12 C+ 6 Li: transfer reaction + ANCs[2] 16 O coulomb break up: KVI 16 O(γ,α) 12 C: TUNL Red: On going Green: Finished Yellow: Under discussion [1] P. Tischhauser et al., Phys. Rev. Lett. 88, (2002) [2] C. Brune et al., Phys. Rev. Lett. 83, (1999) [3] Kunz et al., Phys. Rev. Lett. 86, (2001)

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44 Acknowledgement K. E. Rehm, I. Ahmad, J. Greene, A. A. Hecht, D. Henderson, R.V.F. Janssens, C. L. Jiang, E. F. Moore, M. Notani, R. C. Pardo, N. Patel, G. Savard, J. P. Schiffer, B. Shumrad, S. Sinha Argonne National Laboratory M. Paul Hebrew University, Jerusalem R. E. Segel, L. J. Jisonna Northwestern University Art Champagne University of North Carolina C. Brune Ohio University A. Wuosmaa Western Michigan University N. Scielzo (Fermi function)