B. Schüller

The beam handling system at the bonn isochronous cyclotron

Abstract The lay-out and the ion-optical structure of the beam handling system at the Bonn Isochronous Cyclotron is described. The following beam preparation modes are possible with two double monochromator systems: (1) double dispersive with an extremely high momentum resolution of 30 000; (2) double dispersive with an adjustable dispersion matching with a magnetic spectrograph; (3) nondispersive, nearly isochronous, variable momentum resolution up to 8000, adjustable time of flight resolution below 0.5 ns; (4) achromatic with a transmission of 100%. The practical experiences in operating the system and achieving the design performances are discussed. The results of rigorous test measurements are given.

12C + p → 13N(T = 32) resonance at 14.23 MeV

Abstract The excitation functions of the elastic scattering 12 C(p, p) 12 C and the reactions 12 C(p, p′) 12 C (4.44 MeV) and 12 C(p, α) 9 B g.s. have been measured at θ = 175° in the region of the first T = 3 2 state in 13 N with high energy resolution. A straightforward resonance analysis of the elastic scattering data yields Γ = 1.10 ± 0.09 keV, Γ p = 210 ± 11 eV and Γ p Γ = 0.191 ± 0.017 . The results are compared with shell-model predictions. The additionally measured angular distributions of the elastic scattering 12 C(p, p) 12 C above and below the resonance are compared with optical model calculations. The combination of the present results for Γ p and Γ p / Γ with previous results for Γ γ / Γ p and Γ γ Γ p / Γ yields Γ γ = 27.0 ± 2.2 eV as a new determination of the 13 N (T = 3 2 , 15.07 MeV → T = 1 2 , 0 MeV ) γ-decay width. The comparison with the analogous γ-decay in 13 C is discussed with respect to the charge symmetry of corresponding ΔT = 1 γ-transitions in mirror nuclei.

The α + α → 8Be double resonance at the 16.6 and 16.9 MeV levels

Abstract The elastic scattering 4 He(α, α) has been investigated in the energy range 32.6–35.4 MeV. The level parameters of the overlapping 2 + states in 8 Be at 16.6 and 16.9 MeV were obtained with a double resonance formalism. In R -matrix theory the level energies and widths are 16623 ± 3 keV, 107.7 ± 0.5 keV and 16925 ± 3 keV, 74.4 ± 0.4 keV, respectively. In complex-eigenvalue theory the resonance energies and widths are 16630 ± 3 keV, 108.5 ± 0.5 keV and 16918 ± 3 keV, 73.6 ± 0.4 keV, respectively. The relation between the parameter sets is discussed. The present results are compared with previous investigations where 8 Be is formed as a product nucleus. The isospin mixing of the doublet and the deduced charge-dependent matrix element are discussed with regard to theoretical predictions.

The 16O + p → 17F (T = 32) resonances at Ep = 11.26,12.71 and 14.57 MeV

Abstract Excitation functions for 16 O+p reactions have been measured with high energy resolution in the region of the first, second and seventh T = 3 2 resonances in 17 F at extreme backward angles. The observed resonance shapes have been analyzed with a single-level resonance formula taking the off-resonance spin-flip amplitude into account. The resonance parameters of the 17 F first T = 3 2 state studied with special emphasis are E x = 11193.3 ± 2.3 keV, Γ = 200 ± 40 eV and Γ p 0 = 19 ± 3 eV. This result and other results are compared with previous studies and theoretical predictions. The comparison with data of the mirror nucleus 17 O is discussed with respect to the observed charge asymmetry of the isospin-forbidden particle decay widths.

Optical model analysis of the elastic scattering of 45 MeV 3He particles on 1p shell nuclei

Abstract Angular distributions of elastically scattered 3He particles from the 1p shell nuclei 6Li, 7Li, 9Be, 10B, 11B, 12C and 16O were measured by using a 45 MeV 3He beam. The absolute differential cross section data obtained were analysed in terms of the standard optical model. Using the fact that the rms radius of the 1p shell nuclei is nearly constant a simple folding model allows finding an optical potential family whose real potential depth varies linearly with the target mass. The best fits exhibit a normalized volume integral of about 400 MeV · fm3 for the real potential. From the best-fit parameters a mean parameter set is deduced which varies smoothly with the target mass AT. The average parameter set compares well with the systematics found for heavier nuclei.

Ultra-high resolution measuring of beam momentum distributions

Abstract A simple and direct measuring method has been developed in order to determine precisely the momentum spread of highly resolved particle beams. An extremely high analyzing power of more than 100 000 can be achieved with simple deflecting magnets by an arrangement of extremely narrow slits. In order to discriminate against slit-scattered particles a semiconductor detector with a good energy resolution is used as beam current meter behind the analyzing slit. A homogeneous scanning of the beam to be analyzed is achieved with steering units. The method can be applied to all kinds of charged particle beams. It has been successfully used to measure the momentum spread of 3.5 × 10 −5 of a 14 MeV proton beam. Further applications and modifications are discussed.

The 9Be+α → 13C(T = 32) resonance at 6.44 MeV

Abstract The first T = 3 2 level in 13C has been studied as an isospin-forbidden resonance in the 9Be(α, n0), 9Be(α, n1) and 9Be(α, α0) reactions at an incident energy of 6443.5±2.0 keV, corresponding to an excitation energy of 15108.9±1.5 keV in 13C. Total and partial decay widths were deduced as Γ = 5.49±0.25 keV and Γ α0 /Γ ≧ 0.017 . The resonance parameters are discussed along the lines of similar investigations in 17O and 21Ne and with respect to the isobaric mass multiplet equation. Combining the present value for Γ with previous measurements of Γγ0/Γ yields the ground-state γ-decay width Γγ0 = 22.8±1.9 eV, which compares well with the most precise value until now known from inelastic electron scattering data. The weighted combination of all available information on the ground-state-γ-decay of the first T = 3 2 states in 13C and 13N leads to B( 13 C )/B( 13 N )−1 = −0.15±0.07 for the reduced M1 transition strengths. The charge asymmetry of the reduced M1 transition strengths is discussed with respect to a charge-dependent mixing in the initial and final states and a possible isotensor component of the M1 transition operator.

Microcomputer controlled energy variation of an isochronous cyclotron and beam handling system

Abstract An automatic 3 MeV variation of a 30 MeV α-beam was performed in 300 steps at the Bonn Isochronous Cyclotron. A microcomputer in connection with a control unit adjusted the cyclotron and the monochromator system parameters. This combination allowed a direct and precise measurement of broad excitation functions. Using fast magnetic field stabilization an energy-shift was accomplished to an accuracy of better than 1 part in 105 within 20 s. The stability of the beam position on the target was about 0.05 mm.

A fast precise magnetic field control

Abstract The field of an analysing magnet with a time constant of 5 s is adjusted within 9 s to better than 10−5 for a relative change of the nominal value up to 10−3. A final accuracy of 3 × 10−6 is achieved within 30 s later. A more sensitive mode allows a magnetic field stabilization to better than 1.5 × 10−7.

The polarized proton and deuteron beam at the Bonn isochronous cyclotron

Abstract The present state of the polarized proton and deuteron source at the Bonn cyclotron is described. At the end of the source, which is of the atomic beam type, typical ion beam intensities are 2 μA for protons and 3 μ A for deutrons. The overall transmission from the source to the first stopper after extraction from thecyclotron is 3%. Target currents with an energy resolution E / ΔE = 500 are 20 nA for deuterons and 10 nA for protons. For the proton beam a polarization P = −0.71 was measured. For the deuteron beam a pure vector polarization P z = −0.47 or various mixtures of vector and tensor polarization are obtained.