1. 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, S.K. Das 3, T. Fukuda 3, H. Makii 5, S. Mistuoka 5, I. Arai 2, H. Yamaguchi 4, S. Kubono 4, Y. Wakabayashi 4, S. Hayakawa 4 1 KEK, 2 Univ. Tsukuba, 3 Osaka Electro-Comm. Univ., 4 CNS, 5 JAEA,

2. MSTPC (Multiple Sampling and Tracking Proportional Chamber) MSTPC ★ Active target for measurements of ( , n) type reactions ★ 3D particle tracks & dE/dx He main gas (He + CO 2 (10%)) Low gas pressure ( p = 120 torr) I = 10 3 pps Typical event I = 10 5 pps @TRIAC

3. Stability gap of A =8 Scenario of Supernovae explosion 1.Heavy elements are broken up to p and n. 2.  nucleus is produced by p and n @10GK 3.Seed nuclei around Fe are produced by p, n, and a @3GK (α-process) 4.Heavy elements are produced through neutron capture by seed nuclei @1GK ( ｒ -process) 8 Li( , n) 11 B Possible reactions to go beyond A = 8 1.  ( ,  ) 12 C 2.  ( ,n,  ) 9 Be 3. 8 Li( ,n) 11 B Neutrino driven wind in type Ⅱ SNe 8 Li( , n) 11 B reaction in r-process

4. Measurement of 8 Li( , n) 11 B reaction (@RMS ⇒ TRIAC) Experimental setup Active target ~ MSTPC~ He gas target + 3D tracking detector T 9 =1 T 9 =2 T 9 =3 Ex = 10.88 MeV in 12 B Our results (twice measured) @RMS (I = 10 3 pps) ★ Smaller cross sections than previous inclusive data ★ A resonance-like structure at Ecm = 0.85 MeV ★ poor statistics below E cm = 1.0 MeV Excitation function of 8 Li( , n) 11 B reaction Higher intensity 8 Li beam of 10 5 pps is available @TRIAC

5. Multi-Sampling and Tracking Proportional Chamber with Gas Electron Multiplier ~ GEM-MSTPC~ (gas gain) GEM １ pad Top GEM Bottom Pad 100mm MSTPC I >10 4 pps ⇒ ★ pulse height defect due to space charge gain 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 Gas gain > 10 3 GEM foil (THGEM) Gas gain measurement ☆ 400  m GEM foil (THGEM) Thickness : 400  m Hole Diameter :  300  m with rim,  500  m without rim Hole pitch : 700  m ⇒ enough high gain (>10 3 ) in He + CO 2 (10 %), p = 120 Torr C.K. Shalem, et al., NIM A558(06)468. ☆ 50  m CERN standard GEM Thickness : 50  m Hole Diameter :  70  m (external ),  50  m (internal ) Hole pitch : 150  m ⇒ × 700  m 300  m

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

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

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

9. E d = 1 kV/cm/atm ? Voltage applied to top plate ~ 3 kV (p = 120 Torr) ⇒ 19 kV/atm Insulator (30 kV) cover Top plate Measured drift velocity of electron He + CO 2 (10%) p =120 Torr

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

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 ~  10 mm I = several 100 – 100 kpps GEM-MSTPC He + CO 2 (10%) gas, p = 120 Torr Double GEM configuration E d, E t, E I = 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 (  ) ~ 7 %

12. Beam injection test I = 10 5 pps I = 500 pps (normalized) I = 100 k pps I = 500 pps ~ 8 % Injection rate & time dependence of averaged energy loss signal (normalized) Energy loss signal from each PAD I = 100 k ppsI = 500 pps Resolution of energy loss signal (ave.) 13 % @ I = 100 k pps 7 % @ I = 500 pps OK (allowed) OK? (  < 10 %)

13. Beam injection test I = 10 5 pps Injection rate & time dependence of drift time (normalized) I = 500 pps (normalized) I = 100 k pps I = 500 pps ~ 4 % Drift time from each PAD (1 channel = 25 ns) OK (allowed)

14. Summary Thickness Hole diameter Rim gain shift (~10 0 ) gain shift (~10 4 ) stability ( ~ 10 5 ) remark 50  m 70/50  m no ----- ----- ------- *1 400  m 300  m yes +100% ----- ------- 400  m 500  m no a few % ~ -12 % no *2 400  m 300  m no < a few % ~ -5% yes *3 *1 : low gas gain ( several 10 / single), easily broken for discharge phenomena *2 : unstable behavior for about 10 5 pps beam ( discharge and/or aging (oil mist) ?) *3 : stable operation for about 10 5 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) = N ion /N electron < 1 % (goal) ⇒ Triple GEM configuration to prevent IFB Top (cathode) Pad (anode) 400 μm GEM1 ion electron Drift field ; ~2 [ kV/cm/atm ] 1cm 400 μm GEM2 400 μm GEM3 Et1 Et2 Induction field ; ~5 [ kV/cm/atm ] IFB ~ 2 % at present ! Status (GEM-MSTPC) ・ Gas gain in THGEM could be enough high (>10 3 ) 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.