Charge-state distribution measurements using gas charge stripper toward

Transcription

1 Charge-state distribution measurements using gas charge stripper toward 238 U and 136Xe acceleration at RIKEN RIBF Charge strippers at RIKEN RIBF Gas charge stripper - differential pumping system - offline pressure test - charge-state distribution measurements Results Summary RIKEN Nishina Center Accelerator Group H. Kuboki

2 Collaborators H. Okuno, S. Yokouchi, H. Hasebe, T. Kishida, N. Fukunishi, O. Kamigaito, A. Goto, M. Kase, and Y. Yano RIKEN Nishina Center

3 RIBF (RI-Beam Factory)

4 RRC RILAC (RIKEN Ring Cyclotron) (RIKEN Linear Accelerator) frc (fixed-frequency Ring Cyclotron) IRC (Intermediate-stage Ring Cyclotron) SRC (Superconducting Ring Cyclotron)

5 RIBF (RI-Beam Factory)

6 Charge strippers at RIKEN RIBF 238U and 136 Xe beams at 345 MeV/u 1 linear accelerator, 4 cyclotrons and 2 charge stripping sections U beam acceleration BigRIPS Ion Source RILAC RRC 1st charge stripper MeV/u frc IRC 2nd charge stripper SRC MeV/u Serious problems on 1st charge stripper (carbon foil 300 µg/cm2) Problems short lifetime 12~24 hours@ U beam14 pna (0.5 W ) energy spread E/E=0.55% (4σ) (uniformity of thickness) (0.33% without charge stripper) beyond the acceptance of rebuncher Requirements: Toughness and uniform thickness

7 Gas charge stripper long lifetime and uniform thickness lower charge state than solid materials Estimation of charge states in gas at equilibrium by empirical formulae 53+ : H.D. Betz et al., Phys. Lett. 22, 643 (1966). 63+ : R.O. Sayer, Rev. Phys. App. 12, 1543 (1977). 59+ : P. Strehl, Handbook of Accelerator and Engineering, World Scientific Press. 62+ : G. Schiwietz and P. Grande, NIM B , 125 (2001). All formulae are extrapolation from the data at low energy < 1 MeV/u Actual charge states are unknown Need to measure charge-state distributions

8 Gas charge stripper A gas charge stripper developed using a gas target system [1] [1] H. Kuboki et al., Phys. Rev. ST Accel. Beams, 13, (2010). Gas target chamber Mechanical Booster Pumps T. Kishida et al. NIM A438, 70 (1999).

9 Differential pumping system[1] Gas is injected into stage 1 14 cm Cell length (stage 1) : 14cm

10 Differential pumping system[1] Gas is injected into stage 1 Orifice : 10 mmφ 8 mmφ 6 mmφ 6 mmφ 14 cm Cell length (stage 1) : 14cm

11 Offline pressure test N2 gas

12 Offline pressure test N2 gas N2 7.7kPa (1.3 mg/cm2) was achieved under the vacuum interlock limit (4 10-3Pa) [1].

13 Thickness calibration[1] Si detector (Canberra RF30*30 500EB) Alpha source 241Am Alpha particle (energy is deposited in gas) MCA energy spectrum Thickness vs P1

14 Thickness calibration[1] Si detector Alpha source 241Am Alpha particle (energy is deposited in gas) MCA energy spectrum Thickness vs P1

15 Experimental setup Mar., Jun., and Oct Beam from RILAC RRC Gas charge stripper Energy at RRC exit : U35+ or Xe MeV/u Faraday cup F41 frc m

16 Setup of charge distribution measurement Gas charge stripper Beam from RRC U35+ or Xe20+ Faraday cup D16 Each charge state (q) was selected by bending magnets Faraday cup F41 fraction f= IF41/q (pna) ID16/qini (pna) qini : 35 for U and 20 for Xe

17 µg/cm N2 39 µg/cm Ar 32 µg/cm CO Charge states 62 Most probable charge state Fraction U charge state distribution[1] N2 Ar CO Thickness (µg/cm2) U56+ is not accepted by frc acceleration. (acceptable charge state is 71+ or higher) Eq. charge N2 Ar CO Eq. thickness (µg/cm2)

18 µg/cm N2 27 µg/cm Ar CO µg/cm Charge states 44 Most probable charge state Fraction Xe charge state distribution[1] N2 Ar CO Thickness (µg/cm2) Acceptable with frc (lowest charge state is 41.) Successful acceleration with gas stripper Eq. charge N2 Ar CO Eq. thickness (µg/cm2)

19 Summary Serious problems with the carbon foil stripper at RIBF. 1) short lifetime, 2) ununiform thickness. Gas stripper: - long lifetime - uniform thickness - low charge states A differential pumping system with N2 gas thickness up to 1.3 mg/cm2 Charge-state distributions in the gas stripper with U and Xe beams at 11 MeV/u U Xe N2 Ar CO2 Eq. charge Eq. thickness (µg/cm2) Eq. charge Eq. thickness U56+ are not acceptable without a major remodeling of frc. Xe41+ are acceptable for acceleration by frc.