Published on December 4, 2007
Recoil Separator TechniquesJ.C. Blackmon, Physics Division, ORNL: Recoil Separator Techniques J.C. Blackmon, Physics Division, ORNL WF WF Target D QT QT QT FP DRS ORNL Recoil separator basics How do recoil separators compete? Why underground? What is a recoil separator?: Combination of magnetic and and electrostatic elements that spatially disperse charged reaction products by m/q What is a recoil separator? An alternate approach: An alternate approach Dipole magnet Some recoil separator properties: Some recoil separator properties High selectivity Good energy acceptance Modest angular acceptance Well-suited for inverse kinematics *apertures only Capture in Inverse Kinematics: Capture in Inverse Kinematics 50 (10-15 eV)cm2 - this measurement 74.3 (10-15 eV)cm2 - SRIM2003 Length = 20 cm 1019 atoms/cm2 What might be studied underground?: What might be studied underground? 14N(a,g) 18O(a,g) 22Ne(a,g) AGB stars ~ s process 14N(p,g) 17O(p,g) 17O(p,a) Red giants ~ CNO cycle (p,g) reactions: (p,g) reactions 17O(p,g)18F Oxygen ratios in presolar grains Galactic production of 17O Oxygen ratios in red giant atmostpheres Gamma rays from 18F decay in novae wgpg < 6 meV Dominate uncertainty for 1x108 K < T < 3x108 K Measure in inverse kinematics with a recoil separator? 17O(p,g)18F in inverse kinematics: 17O H2 17O(p,g)18F in inverse kinematics Daresbury Recoil Separator DE DE+E wg = 0.8 eV 680-keV resonance clean identification of reaction products much more difficult as beam energy decreases Beam rejection at low energies: Beam rejection at low energies 10-8 * 1 pmA 60 kHz 21Na(p,g) @ 220 keV/u (Bishop et al.) recoil-gamma coincidence High selectivity without Z identification (p,g) vs. inverse kinematics: (p,g) vs. inverse kinematics Energies < 200 keV/u gamma detection required in both cases no Z identification of heavy ion separator TOF can tag events of interest large recoil angle - transmission difficult poor beam suppression high FP count rate mA of HI beam vs. mA of protons It is difficult for inverse kinematics to compete with a high current proton accelerator underground. 12C(a,g)16O: 12C(a,g)16O Kunz et al. (01) Plaga et al. (87) Azuma et al. (94) SE1(300 keV) ~ SE2(300 keV) ~ 80 keVb limited by gamma backgrounds mA 4He 4 fusions/month Need s(300 keV) ~ 0.1 fb 4He(12C,g)16O with a recoil separator: 4He(12C,g)16O with a recoil separator 3x10-10 Ecm = 3.2 MeV How low in Ecm can this technique be pushed? 12C(a,g)16O vs. inverse kinematics: Ecm > 1.4 MeV recoil provides clear 16O tag Ecm < 1.4 MeV DE-E identification of recoil Z is lost Increasing recoil cone must be accepted Beam suppression is more difficult If 10-10 beam suppression & 1000 cosmics/day 10 recoil-gamma background events/day 12C(a,g) fusion rate underground probably 10 times > inverse kin. 12C(a,g)16O vs. inverse kinematics 12C(a,g)16O - My perspective: 12C(a,g)16O - My perspective Unique astrophysical importance Measurements in inverse kinematics will clearly improve our understanding Measurements in inverse kinematics will not measure the cross section near the Gamow window anytime soon (a,g) measurements above ground are limited by ambient backgrounds Measurements underground would clearly be a substantial improvement Issues: Level of beam induced background Robustness of solid carbon targets Would measuring 4He(12C,g)16O underground be more sensitive than 12C(a,g)16O? More robust/stable target, less background (13C) (a,g) on N=Z nuclei: (a,g) on N=Z nuclei Important for understanding supernova nucleosynthesis a-rich freeze-out, g-ray production (44Ti, 56Ni) Sparse experimental information, especially for heavier nuclei Statistical model calculations somewhat more uncertain due to low energy aN optical potentials. Rauscher et al. (00) Some of these reactions have significant target issues (stability under high beam currents) Measurement with a heavy ion beam on an alpha target could be easier and cleaner Conclusions: Conclusions It is difficult for recoil separator measurements of (p,g) reactions to compete with high-intensity proton beams for stable targets due to the very low energies. A compelling case can clearly be made for measuring these reactions underground. LUNA and other facilities have the capability to measure these reactions, but the list of interesting measurements is extensive, and the pace of measurements is slow. Improvements in our understanding of 12C(a,g)16O will be made through measurements in inverse kinematics above ground. However, these measurements are exponentially more difficult at low energies. Measurements at an underground facility are compelling and should be vigorously pursued. The capability to measure such (a,g) reactions at low energies currently does not exist anywhere. A strong case can be made for a new underground accelerator facility to address this important physics. mA beam of 4He High intensity heavy (A<40) ion beam & He jet target?