Transverse Profile Measurement using Beam Induced Fluorescence (BIF)

Non-intercepting Beam Induced Fluorescence (BIF) monitors measure transversal beam profiles by observation of fluorescence light originating from excited residual gas molecules. Thus they are an alternative to conventional intercepting devices. Single photon counting is performed using an image intensified digital CCD camera. We investigated the BIF process in the energy range of 7.7 keV/u to 750 MeV/u in residual nitrogen. Experiments at low beam energies were performed at a Marx-accelerator (NDCX) at Berkeley Lab [1] whereas mid and high energy experiments were carried out at GSI accelerators [2, 3]. Especially in the vicinity of targets the neutron-generated radiation level limits the monitor’s signal to background ratio. Therefore the radiation background was investigated for different ion species and particle energies. Background simulations using a Monte Carlo transport code are compared to experimental data taken with scintillators, thermo luminescence detectors and the BIF monitor. Alternative image intensifier techniques are presented as well as shielding concepts. Furthermore the dynamics of ionized nitrogen molecules in the electric field of intense ion beams is discussed. THE BIF METHOD AND APPLICATION When beam ions collide with residual gas molecules, some molecules are ionized remaining in an excited state with a certain probability. In a N2-dominated residual gas composition, a strong fluorescence at 390 nm < λ < 470 nm (blue), of about 60 ns lifetime, is generated by a transition band to the N+2 electronic ground state (B2Σ+u (v ) → X2Σ+g (v )+γ, for vibrational levels v) [4]. ’Single-photon counting’ was performed with a commercial image intensifier [5], equipped with a double MicroChannel Plate (MCP) for up to 10-fold photo-electron amplification. Green light from a P46 phosphor screen of 300 ns decay time is taper-coupled to a digital CCD camera with a IEEE-1394a interface [6]. The device is mounted on a fused silica viewport at a distance of 20 cm from the beam axis. Remote controlled CCTV lenses with focal distances of 8 to 25 mm, lead to typical resolutions of 100-500 μm/pixel. Beam profiles were recorded on a single shot basis. To select specific transitions, 10 nm narrow band interference filters were installed in the optical path. A more detailed description of the experimental setup can be found in [7, 8, 9, 11]. Work supported by EU, project FP6-CARE-HIPPI † [email protected] Figure 1: Beam profiles of a 10μA 5.4 MeV/u Ni beam in 10mbar nitrogen, recorded with spectral filters [9]. The N+2 profile @ 391nm shows a σ of 1.1mm whereas N2 profile @ 337nm has a σ of 2mm. This paper will focus on issues related to the challenging beam parameters of the FAIR-facility [3] like energies well above 100 MeV/u in considerable loss induced radiation environments and E-field induced profile falsifications for intense and strongly focused beams. During the last years the BIF method was applied successfully at the GSI heavy ion LINAC for various ion species and energies between 5 and 11.4 MeV/u [7, 8, 9]. An additional setup was installed behind the heavy ion synchrotron SIS-18 in a high energy beam transfer line (HEBT) close to a dump. Due to the beam energy between 60 and 750 MeV/u this location allowed to determine the radiation impact on the detector performance. In addition this part of the beam pipe was separated by vacuum windows so that residual gas densities from base-pressure 10 mbar up to atmospheric pressure could be applied. Systematic investigation of profile falsifications have shown that beam profile width remains constant up to nitrogen pressures of about 1 mbar and also, that N2 transitions lead to increased profile width ≥40% compared to ionic N+2 transitions [9], see Fig.1. Cross sections for heavy ion induced transitions in N+2 are predominant compared to electron induced transitions. Unlike transitions in neutral working gases (N2) which show enlarged beam profiles due to the secondary electron halo [10]. Although the contribution of N2 transitions is ≤ 20% and in the near UV, it should be suppressed by optical filters and discriminated against the desirable N+2 transitions at (391, 428, 470nm) [9]. For typical beam parameters at GSI LINAC and high energy beam transfer lines, profiles recorded with the BIF-monitor complied with SEM-grid (Secondary Electron Monitor) measurements within 10% [11], see Fig.2. Figure 2: Beam profiles recorded with the BIF-Monitor and a SEM-Grid agree within 10%. 1.5 μs pulse of 2·10 Xe at 200 MeV/u in HEBT line [11]. RADIATION IMPACT ON THE MONITOR In the vicinity of production targets like the p̄-target, the Super Fragment Separator-target (SFRS), or solid targets for plasmaphysics, a considerable amount of beam ions generates radiation which cannot be avoided. Our experimental area in a SIS-18 HEBT line is located just 2.1 m from the beam dump (Fe). Since all beam particles are stopped in the dump, the generated dose is comparable to fixed target experiments [12, 13]. Therefore radiation impact on the BIF-monitor was investigated in a realistic environment. During the first campaign a specific scaling of signal amplitude and background level with the beam energy was recognized [11]. Recent measurements for slowly extracted uranium ions of complementary energies are in good agreement with the 2005 data, although an Intensified CCD-camera (ICCD) with a different response characteristic and reproduction scale had to be used, see Fig.3. However, the signal amplitude scales with the Bethe Bloch law, whereas the background level scales with ∝E. With Li Li thermoluminescence dosimeters ≥83% of the total dose was determined as neutrons [14]. A semi-empirical neutron production yield for heavy ion projectiles ≥5 MeV/u in thick heavy metal targets estimates neutrons per incident projectile, where NT is the neutron number of the target and EP is the incident projectile energy in MeV/u, see Eq.1 [13].