Direct measurement of 12C + 4He fusion cross section at Ecm=1 Direct measurement of 12C + 4He fusion cross section at Ecm=1.5MeV at KUTL H.Yamaguchi K. Sagara, K. Fujita, T. Teranishi, M. taniguchi, S .Liu, S. Matsua, Maria T. Rosary, T. Mitsuzumi, M. Iwasaki Kyushu University Tandem accelerator Laboratory

Burning process in stars H-burning 4p → 4He via p-p chain & CNO cycle He-burning 3 4He → 12C C-burning O-burning Si-burning 4He+12C → 16O+g 12C/16O ratio affects widely further nuclear synthesis. 4He+12C → 16O+γ cross section has not been determined yet, in spite of 40 years efforts in the world. 4He+12C → 16O+γ experiment is very difficult. 4He + 12C → 16O + g world ~40 years +  g α 4α 3α C. Rolfs (Ruhr Univ.) goal Kyushu U. 17 years 2010 1970 1990

Why is 4He+12C→16O+γ experiment so difficult? At 0.3MeV 4He(12C,16O)g Cross Section is very small (~10-8 nb) due to Coulomb repulsion E1 E2 Cross section (S=const.) 10-5 0.7 0.3 2.4 Experiments Extrapolation Experiment stellar energy 0.3 Coulomb-barrier effect Really low-energy experiments near 0.3MeV are necessary to make reliable extrapolation.

Experimental methods for 4He+12C→16O+γ cross section      

γ S-factor has not been precisely determined yet. 4He+12C→16O+γ experiment with γ　detection → γ Stellar energy Cross section (S=const.) 10-5 No precise data at low energy due to ・low detection-efficiency of γ-rays ・huge Back Ground (BG) γ-rays Coulomb barrier effect S-factor has not been precisely determined yet.

Experimental methods for 4He+12C→16O+γ cross section   high detection efficiency (~ 40%: charge fraction) total S-factor can be measured    

Background (BG) reduction Cross section is very small Increase the yield Yield of 12C + 4He → 16O + γ Y(16O) = s・ N(12C)・N(4He )・ Detection Efficiency ・ Beam Time beam target detect ① ② ③ ・necessary components for Ecm=0.7MeV experiment high intensity 12C beam: ~ 10 pmA （Limit of our tandem accelerator） Thick windowless 4He gas target : ~20 Torr x 4 cm 　　　　　　　　　　　　 (Limit of DE in the target） - high detection efficiency (~40%) Cross section (S=const.) 10-5 0.7 0.3 2.4 ・Y(16O) ~ 5 counts/day at Ecm=0.7MeV → 1 month exp. extrapolate experiment at Ecm = 0.6 MeV →　10 month exp. at Ecm = 0.3 MeV →　7,000 year exp Background (BG) reduction N(16O)/N(12C) ~ 10-18 N(BG) / N(12C) < 10-19 Very hard to realize

Setup for 4He(12C,16O)g Experiment at Kyushu University Tandem Laboratory (KUTL) Tandem Accelerator buncher chopper Blow in windowless 4He gas target 12C beam RMS 12C Sputter ion source E-def D-mag Recoil Mass Separator (RMS) This is the setup for this experiment. Ion source produce the C beam. The beam is accelerated by the Tandem accelerator and injected to the windowless He gas target The recoil O is saparated from C beam by the Recoil Mass Separator, . The O is detected here. Using the beam chopper and buncher, we produce a pulsed beam, and measure Time Of Flight to reduce background. Tandem Long-time chopper D-mag Ecm = 2.4~0.7 MeV E(12C)=9.6~2.8 MeV E(16O)=7.2~2.1 MeV Final focal plane (mass separation) 16O Detector (Si-SSD)

Accel-decel operation of tandem accelerator ①Increase the 12C beam Accel-decel operation of tandem accelerator Y(16O) =　s・N(12C)・N(4He )・ Det.Efficiency ・ Beam Time accel-decel operation normal operation Al shorting bars for accel-decel operation At low acceleration voltage, focusing becomes weak, and beam transmission decreases. By alternative focus-defocus, focusing becomes strong, and beam transmission increases. ・10 times higher beam transmission is obtained by strong focusing. ・10 times more intense beam can be injected. Totally, beam intensity is ~100 times increased

②Increase the 4He gas target Windowless Gas Target ②Increase the 4He gas target Y(16O) =　s・N(12C)・N(4He )・ Det.Efficiency ・ Beam Time Blow-In Gas Target (BIGT) windowless & high confinement capability RMS TMP3 TMP4 MBP2 TMP2 TMP5 MBP1 beam TMP1 DP 1500 l/s 3000 l/s 330 l/s 520 l/s 350 l/s 24Torr beam 4.5cm SSD: beam monitor Differential pumping system (side view) center pressure: 24 Torr effective length: 3.98 ± 0.12 cm (measured by p+α elastic scattering) → target thickness is sufficient for our experiment (limited by energy loss of 12C beam) Thickest in the world

③Increase the 16O detection efficiency 12C + 4He → 16O +γ Recoil Mass Separator All the 16O recoils(±2°) in a charge state (~40%) are detected. Eject within 2° 4He windowless Gas target D mag 12C beam 16O detect 16O5+ E-def D mag 12C + 4He → 16O +γ yield has been increased Y(16O) =　s・N(12C)・N(4He )・ Detection Efficiency ・ Beam Time ① ② ③

BG reduction Background 12C are produced by Background reduction Goal: 　N(BG)/N(12C) < 10-19 N(16O)/N(12C) ~ 10-18 at 0.7MeV Background 12C are produced by multiple scattering charge exchange Background reduction ・Recoil Mass Separator 　　　　 background reduction ~10-11 ・ TOF with Pulsed beam ~10-2 ・Long-Time Chopper(RF deflector) ~10-3 RF-Deflector E-def D mag LTC D mag Another big problem is BG reduction. Our final goal is 10 (-19). By Recoil Mass Separator, BG are reduced to 10 (-11) level. By Time-Of-Flight technique with pulsed beam, BG are reduced to 10 (-2) . By Long-Time-Chopper, BG are reduced to 10 (-3) . I will explain Long-Time-Chopper soon. Totally, BG are reduced to 10(-16), at present. At present: N(BG)/N(16O) become 10-16

Long-Time Chopper(RF deflector) BG reduction Long-Time Chopper(RF deflector) pass only reaction products (16O) which are spread in time. f2=3×f1 V2=V1/9 f1=6.1MHz V1=±24.7kV V3=23.7kV reject BG + Pass　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 　　　　16O Flat-bottom voltage with LTC without LTC RF-Deflector LTC BG(12C) 16O5+ 500events Measurement of 4He(12C,16O)γ at Ecm = 2.4 MeV

4He(12C,16O)g at Ecm=2.4MeV experiment beam: 12C2+, frequency: 6.063MHz energy: 9.6MeV , intensity: ~35pnA target: 4He gas ~ 23.9 Torr x 3.98 cm observable: 16O5+ 7.2 ± 0.3 MeV abundance = 36.9 ± 2.1 % = efficiency 29hours data 941 counts 16O

4He(12C,16O)g at Ecm=2.4MeV experiment Ruhr univ. Our data 2.4MeV

4He(12C,16O)g at Ecm=1.5 MeV experiment target: 4He gas 15.0 Torr x 3.98 cm observable: 16O3+, 4.5 ± 0.3 MeV abundance = 40.9 ± 2.1 % = efficiency beam: 12C1+, frequency: 3.620MHz energy: 6.0MeV, intensity: 60pnA 95 hours data 16O 208 counts .

Cross Section and Stot-factor extrapolation Our exp. plan Stellar energy preliminary Kyushu U. Ruhr U. 1.5MeV Next experiment is Ecm=1.151.00.850.7 MeV

Further BG reduction is necessary 95 hours data 16O Ecm=2.4MeV σ～65nb 1.5MeV σ～0.7nb down to 0.7MeV Increased BG In order to go to low energy further BG Reduction is necessary!

further BG reduction 16O and 12C separation by Ionization chamber measure the ΔE (∝energy loss) by the ionization chamber (and E by the SSD) － ＋ cathode anode PR Gas 30Torr very thin foil (0.9μm) 16O,12C e－ Si-SSD ΔE E low energy ΔE of 16O is larger than 12C 16O 12C 4He E+DE DE BG reject We can separate 16O from BG (12C)

BG reduction by Ionization Chamber Huge 12C-BG will be eliminated using the ionization chamber. 95 hours data 16O 12C 4He E+DE DE BG reject 16O Ionization chamber will be available from October 2011.

Summary Direct measurement of 4He+12C 16O+γ cross section (total S-factor) is in progress at KUTL (Kyushu Univ. Tandem Lab.) Many new instruments and methods have been developed for this experiment. Ecm= 2.4 MeV experiment s= 64.6 nb, S-factor = 89.0 keV b Ecm= 1.5 MeV experiment s= 0.900 nb, S-factor = 26.6 keV b Now we are developing an ionization chamber. Experiments of 4He+12C 16O+γ at Ecm = 1.51.151.00.850.7MeV will be made in a few years. future plan , 2010 stellar energy Stellar energy

A rehearsal for extrapolation using R-matrix theory R.Kuntz, M.Fey, M.Jaeger, A.Mayer, W.Hammer Astrophysical J. 567. (2002) 643－650 Ecm［MeV］ 0.70 0.85 1.00 1.15 1.50 S-factor［keV b］ 70.0±7.0 50.0±5.0 45.0±4.5 35.0±3.5 30.0±3.0 Assumed data (±10%) 2＋ 1－ S(0.3MeV) extrapolated = 190±15 keV b Data from Ruhr university Assumed data Reliable theoretical curve will be necessary for extrapolation +  g 0.3