Development of Polarized Proton Targets for Reactions with Radioactive Ion Beams

Transcription

1 Development of Polarized Proton Targets for Reactions with Radioactive Ion Beams J. P. Urrego-Blanco, C.R. Bingham, B. van den Brandt, A. Galindo-Uribarri, P. Hautle, J. A. Konter, E. Padilla-Rodal, P. Schmelzbach

2 Nuclear Physics with Radioactive Ion Beams Nuclear Physics studies far away from stability include - Single-particle properties - Spin-orbit interaction - Evolution of tensor interaction with isospin - Transfer reactions - Exotic distribution of nuclear matter - Interplay between structure and reactions - Energy generation in stars - Nucleosynthesis Complementary experiments with RIBs are done in two types of facilities: Fragmentation (fast beams: RIKEN, GSI, MSU) ) and ISOL facilities (post- accelerated beams: HRIBF, GANIL, TRIUMF)

3 Study of Elastic Scattering Resonances E p =2.2MeV An example with a stable target, normal kinematics: 7 Be: 6 Li(p,p) 6 Li Resonance parameters for the dominant E cm =1.56 MeV state in 7 Be Analysis of dσ/dω and A y Skill et al. (1995) Analysis of dσ/dω only McCray (1963) E R [MeV] 1.56± Γ [MeV] 0.4 ± E p =2.2MeV Γ p [MeV] 0.19± Excitation functions and angular distributions of cross sections and analyzing powers are required in order to unambiguously extract resonance parameters. M. Skill et al. M. Skill et al. NPA 581(1995)93 NPA 581(1995)93 RIBs for relevant experiments for the hot CNO cycle, element production rates in supernovae and, nuclear structure include 11 C, 7 Be, 17 F, 18 F

4 Transfer Reactions 118 Sn 117 ( d, t) Sn Cross Sections and Analyzing Powers J π =3/2 + J π =5/2 + Experiment E/A=6MeV E x =1.18MeV [ it 11 ( θ )] j = l + 1 l 2 = [ it ( θ )] 1 l j = l 2 dσ/dω [mb/sr] it 11 Spectroscopic tool for the j- assignment of single particle states. Information on the spin-orbit interaction. θ CM [deg] θ cm [deg] Sensitivity of the analyzing S. E. Vigdor and W. Haeberli, power increases at low Nucl.. Phys. A253 (1975) 55. energies. Transfer reactions with polarized targets will be the ultimate test of single particle behavior near newly accessible magic nuclei, e.g. 132 Sn and soon 78 Ni

5 A Polarized Proton Target for RIBs The Polarized Targets RIB intensities are typically 6-7 orders of magnitude smaller than those of SIBs! High density of scattering centers are required - Polystyrene foils doped with TEMPO - DNP scheme - Thickness: mg/cm2, ideal for the study of resonances of interest in astrophysics and for transfer reactions - Max. Polarization: 30% -Operation at T~225mK (limits polarization) and B<2.5T - Suitable for operation at intermediate (dynamic mode) and low energies (frozen spin mode) Thickness [μm] Spin velocity: 2000rpm spinning time: 45s Spin Coating Tests Thin Film Production PS Concentration (PS/Toluol) [%]

6 The Target System NMR system (Pol. registration) He Gas admixture Protons are polarized in a cryogenic environment BUT materials around the target should be kept to a minimum schematic Array of Detectors Beam line vacuum Target Chamber (Superfluid He leak tight) Si 3 N 4 Windows Teflon Frame Microwave System (~70GHz) Cryogenics (Dilution Refrigerator) T ~ 250mK Magnet B~2.5T PS Target doped with TEMPO Polarization [%] T=1.4K B=3.5T Vacuum Time [min] B. van den Brandt et al., NIM A381 (1996) 219. Cooling via superfluid 4 He film works!!! 4 He admixture

7 The Target System Proof of Principle at PSI Proof of principle with a Stable Beam -p( 12 C,p) reaction at 38MeV using the Phillips injector at PSI under similar conditions of RIBs at a typical ISOL facility (Intensities between 10 4 and 10 7 pps). - Target has been successfully operated in frozen spin mode at field intensities between T. - Depolarization rate under beam irradiation is directly related to heating by the beam. - Maximum polarization attainable is limited by base temperature in the target cell. Beam Beam flux monitor Energy losses in Si 3 N 4 windows: 520keV for 12 C at 38MeV 33keV for 1MeV protons

8 The Target System ~150K ~50K ~0.8K Detection systems are essential components of a polarized target system -Time correlated Temperature NMR Charged particle detectors allow the determination of polarization observables. - For low energy experiments, Silicon detectors can be operated at 4K. - For high energies, detection systems can be placed outside thin thermal shields. ~0.25K 4.2K

9 Counts Effects of the Magnetic Field in Spectra B-Field Effects p( 12 C, 12 38MeV, θ Lab =15 B=0T ~14mg/cm 2 Polystyrene Target doped with TEMPO B=0.4T B=0.8T E [11.5keV/Ch] Detection angle in the laboratory system does not correspond to the emission angle of the recoil. Interpretation of spectra from a polarized target require a precise determination of the recoils angular distribution at zero field (including particle-id). This information can then be used to calculate the trajectories at different fields. Geant4 simulations are in progress using data recently taken at HRIBF.

10 Operation in Frozen Spin Mode 150 m s =+1/2 120 m s =-1/2 Counts Recoil Spectra 12 C + 38MeV 6.5mg/cm 2 Polystyrene Target doped with TEMPO 8%<P<19% Measuring Time: 15min Energy [MeV] (B=0.8T, T=225mK) Beam Intensity = 2*10 6 pps Spectra normalized according to the beam intensity and taken under identical conditions show the effect of the spin of the target in the cross section for elastic scattering. This target allows the determination of spin dependent observables in low and intermediate nuclear reactions involving polarized protons.

11 Summary and Outlook Polarization might have a strong impact in the study of nuclei far f from stability. Among others, spin dependent observables are useful tools in resonance elastic scattering and transfer reactions. A prototype polarized proton target for reactions with heavy ions s at low and intermediate energies has been developed as a collaboration between PSI, UTK and ORNL. Initial tests have been performed in frozen spin mode through the reaction p( 12 C, 12 C)p at 38MeV, using the thick target technique at RIB-like intensities. Parameters of Operation: - Polystyrene foils doped with TEMPO - DNP scheme - Max. Polarization: 30% - Polarization Build-up time: ~4h - Thickness: mg/cm 2 - Operation at T~225mK (limits maximum polarization) - B=2.5T (dynamic mode) Future developments Future developments include interpreting spectra taken with the polarized target, improving reliability of windows and, improving the temperature inside the target cell.

12 Collaborators PSI: B. van den Brandt P. Hautle J. A. Konter P. A. Schmelzbach UTK ORNL: C. R. Bingham A. Galindo-Uribarri E. Padilla-Rodal (UNAM) Additional Information: - J. P. Urrego-Blanco et al., NIM B241 (2005) J. P. Urrego-Blanco et al., NIM B261 (2007) A. Galindo-Uribarri and J. P. Urrego-Blanco, Rev. Mex. Fís. F S53 (2007) 35.

13 RESERVE

14 Effect of the 4 He Film in the Target Cell p( 12 C, 12 38MeV θ Lab =15 14mg/cm 2 Polystyrene Target doped with TEMPO ~0.3mm superfluid 4 He film Counts Empty chamber 4 He in chamber E [11.5keV/Ch] - The target cell with 500 nm thick Si 3 N 4 windows is leak tight for superfluid Helium. - Superfluid film has little impact on the proton recoil distribution. Only the distribution of alpha particles originated in the C-C collisions is significantly affected. Energy losses in Si 3 N 4 windows: 520keV for 12 C at 38MeV 33keV for 1MeV protons

15 Polarization [%] Operation in Frozen Spin Mode p Target Polarization History Counts m s =+1/2 m s =-1/2 Recoil Spectra 12 C + 38MeV 6.5mg/cm 2 Polystyrene Target doped with TEMPO 8%<P<19% Beam Off Beam On ΔT [mk] m s =+1/2 120 m 2.2x10 6 s =-1/ Time [h] t [min] 1.9x x x x x Beam Intensity [pps] Energy [MeV] Measuring Time: 15min (B=0.8T, T=225mK) Beam Intensity = 2*10 6 pps Spectra normalized according to the beam intensity and taken under identical conditions show the effect of the spin of the target in the cross section for elastic scattering. This target allows the determination of spin dependent observables in low and intermediate nuclear reactions involving polarized protons.