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GEM-MSTPC for direct measurements of astrophysical reaction rates

GEM-MSTPC for direct measurements of astrophysical reaction rates. H. Ishiyama 1 , K. Yamaguchi 2 , Y. Mizoi 3 , Y.X. Watanabe 1 , T. Hashimoto 4 , M.H. Tanaka 1 , H. Miyatake 1 , Y. Hirayama 1 , N. Imai 1 , Y. Fuchi 1 , S.C. Jeong 1 , T. Nomura 1 ,

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GEM-MSTPC for direct measurements of astrophysical reaction rates

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  1. GEM-MSTPC for direct measurements of astrophysical reaction rates H. Ishiyama1, K. Yamaguchi2, Y. Mizoi3, Y.X. Watanabe1, T. Hashimoto4, M.H. Tanaka1, H. Miyatake1, Y. Hirayama1, N. Imai1, Y. Fuchi1, S.C. Jeong1, T. Nomura1, S.K. Das3, T. Fukuda3, H. Makii5, S. Mistuoka5, I. Arai2, H. Yamaguchi4, S. Kubono4, Y. Wakabayashi4, S. Hayakawa4 1KEK, 2Univ. Tsukuba, 3Osaka Electro-Comm. Univ., 4CNS, 5JAEA,

  2. Typical event MSTPC(Multiple Sampling and Tracking Proportional Chamber) MSTPC ★ Active target for measurementsof (a, n) type reactions ★ 3D particle tracks & dE/dx He main gas (He + CO2 (10%)) Low gas pressure ( p = 120 torr) I = 103 pps I = 105 pps @TRIAC

  3. 8Li(a, n)11B reaction in r-process 8Li(a, n)11B Stability gap of A =8 Scenario of Supernovae explosion • Heavy elements are broken up to p and n. • a nucleus is produced by p and n @10GK • Seed nuclei around Fe are produced by p, n, and a @3GK (α-process) • Heavy elements are produced through neutron capture by seed nuclei @1GK (ｒ-process) Possible reactions to go beyond A = 8 1. a(a,a, g)12C 2. a(a,n,g)9Be 3.8Li(a,n)11B Neutrino driven wind in type Ⅱ SNe

  4. Measurement of 8Li(a, n)11B reaction (@RMS ⇒ TRIAC) Active target ~ MSTPC~ He gas target + 3D tracking detector Experimental setup T9=3 T9=2 T9=1 Excitation function of 8Li(a, n)11B reaction Our results (twice measured) @RMS (I = 103 pps) ★ Smaller cross sections than previous inclusive data ★ A resonance-like structure at Ecm = 0.85 MeV ★poor statistics below Ecm = 1.0 MeV Ex = 10.88 MeV in 12B Higher intensity 8Li beam of 105 pps is available @TRIAC

  5. Multi-Sampling and Tracking Proportional Chamber with Gas Electron Multiplier ~ GEM-MSTPC~ (gas gain) MSTPC I >104 pps ⇒ ★pulse height defect due to space chargegain limitation on anode wires ★ large switching noise at on/off of gating grid ★　slow signal due to positive ions ⇒GEM foil in order to avoid space charge gain limitation around wires and to get fast signals ☆ 400 mm GEM foil (THGEM) Thickness : 400 mm Hole Diameter : f 300 mm with rim, f 500 mm without rim Hole pitch : 700 mm ⇒　enough high gain(>103) in He + CO2 (10 %), p = 120 Torr C.K. Shalem, et al., NIM A558(06)468. ☆ 50 mm CERN standard GEM Thickness : 50 mm Hole Diameter : f 70 mm (external ), f 50 mm (internal ) Hole pitch : 150 mm ⇒ × Top GEM foil (THGEM) 100mm 700 mm 100mm Gas gain measurement 300 mm Bottom GEM Pad Gas gain > 103 GEM１ pad

  6. Time dependent gain instability occurs !!!! due to charging-up on insulator (rim) @ low injection rate of a-rays~ several 10 pps Gain instability occurs under higher injection rate (~several 10 k pps) due to discharge. HV ON HV OFF Original GEM THGEM with rim was modified to one without rim. rim 500 μm 400 μm 700 μm Gain shift disappeared ! HV ON Large leak current → decreasing effective voltage 500 μm insulator 400 μm 300 μm electrode

  7. For eliminating discharge….. ☆ 400 mm GEM foil (THGEM) Thickness : 400 mm Hole Diameter : f 500 mm without rim Hole pitch : 700 mm ☆ 400 mm GEM foil (THGEM) Thickness : 400 mm Hole Diameter : f 300 mm without rim Hole pitch : 700 mm Relatively high gain ( ~ factor of 2) at the same voltage applied to the GEM foil!!! ⇒　relatively low operation voltage Single GEM Several 10 k pps @ spot size of f a few cm (a rays from Am source) After decreasing the pulse height (~ 5 %) stable operation for a few days (single GEM)

  8. Ion feedback Top TOP + shield GEMs ＧＥＭ pad • Ions • accumulation in drift space • distorting electric field in D. S. • Ions generated by beam particles • <= strong electric field in D.S. • (1 - 2 kV/cm・atm) • Ions generated by gas multiplication process • (Ion feedback ,IF) • <= shield GEMs • Double GEM: IF ~ 6 % • Triple GEM: IF ~ 2 % @ 1 kV/ cm/atm Readout Pattern

  9. Ed = 1 kV/cm/atm ? Voltage applied to top plate ~ 3 kV (p = 120 Torr) ⇒ 19 kV/atm Measured drift velocity of electron Insulator (30 kV) He + CO2 (10%) p =120 Torr cover Top plate

  10. Careful treatment of GEM foil Oil free vacuum system Contamination by oil mist on surface of GEM electrode TMP Scroll pump Safekeeping Ar gas normal Diaphragm pump desiccator Pulse height of dE/dx signals

  11. Beam injection test (2009/12/10) Low-energy heavy-ion beam @TRIAC A/q = 6.5, E/u = 1.1 MeV/u, beam spot ~ f 10 mm I = several 100 – 100 kpps GEM-MSTPC He + CO2 (10%) gas, p = 120 Torr Double GEM configuration Ed, Et, EI = 1, 2, 4.7 kV/cm/atm Trigger: MCP (in front of GEM-MSTPC) Energy loss signal from each PAD ( ~ several 100 pps injection) Energy resolution (s) ~ 7 %

  12. Beam injection test I = 105 pps Energy loss signal from each PAD Injection rate & time dependence of averaged energy loss signal (normalized) I = 500 pps (normalized) Resolution of energy loss signal (ave.) 13 % @ I = 100 k pps 7 % @ I = 500 pps ~ 8 % OK? (s < 10 %) I = 100 k pps I = 500 pps I = 100 k pps I = 500 pps OK (allowed)

  13. Beam injection test I = 105 pps Drift time from each PAD (1 channel = 25 ns) Injection rate & time dependence of drift time (normalized) I = 500 pps (normalized) ~ 4 % I = 100 k pps I = 500 pps OK (allowed)

  14. Summary Thickness Hole diameter Rim gain shift (~100) gain shift (~104) stability ( ~ 105) remark 50 mm 70/50 mm no ----- ----- ------- *1 400 mm 300 mm yes +100% ----- ------- 400 mm 500 mm no a few % ~ -12 % no *2 400 mm 300 mm no < a few % ~ -5% yes *3 *1 : low gas gain ( several 10 / single), easily broken for discharge phenomena *2 : unstable behavior for about 105 pps beam ( discharge and/or aging (oil mist) ?) *3 : stable operation for about 105 pps beam , (double GEM configuration) For further improvement (rate capability, resolution of energy & position ) Triple GEM configuration ← beam test (Feb. 2010)

  15. Ion feedback Numerous ions feeding-back from GEM foil to drift space ⇒ distorting electric field Ion feedback ratio (IFB) = Nion/Nelectron < 1 % (goal) ⇒ Triple GEM configuration to prevent IFB Top (cathode) ion 1cm Drift field ; ~2 [ kV/cm/atm ] IFB ~ 2 % at present ! 400 μm GEM1 Status (GEM-MSTPC) ・ Gas gain in THGEM could be enough high (>103) in low pressure He –base gas. ・Gain instability due to the charging up disappeared after geometrical modification of THGEM. ・IFB was less than 2 % with triple GEM configuration. (goal < 1 %) Performance test for high rate capability is in progress. electron Pad (anode) 400μm GEM2 Et1 Et2 400 μm GEM3 Induction field ; ~5 [ kV/cm/atm ]

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