Considerations for Primary keV Beams

A low-energy beam can be affected by the monitor in various ways: particles can be scattered, change their trajectories in the presence of an electric field or lose a considerable fraction of their energy and eventually be stopped in the mesh and foil assembly. These effects can cause beam loss, but also introduce strong distortions to the image of the real beam profile.

Range in Matter and Scattering

At low energies, the range of antiprotons and protons in matter differs. Particles of negative charge are subject to lower stopping power than their positive counterparts due to polarization of target electrons. As a result, the penetration depth for antiprotons is slightly larger than for protons at the corresponding energy. The phenomenon is known as Barkas effect and has been extensively studied for low energy particles in the last years [218–220]. The difference in proton and antiproton stopping powers is shown in Fig. 6.2. 1 0 20 40 60 80 100 120 140

Data for protons Data for antiprotons

Stopping power [keV/

m m] Energy [keV] 1000 100 10

Figure 6.2: Stopping power of protons [57] and antiprotons [219] in aluminium.

The stopping power S can be used to calculate the continuous slowing-down ap-

6.3 Design Considerations down to rest: RCSDA(E) = Z E 0 dE0 S(E0), (6.1)

whereE is the particle energy. For example, the CSDA range for protons in aluminium

is 3µm at 300 keV and 0.3µm at 20 keV [57], whereas for antiprotons it is about 4.5µm

at 300 keV and approximately 0.8 µm at 20 keV. Due to multiple scattering effects,

however, the penetration depth of particles is lower and for protons drops to 2.8µm at

300 keV and 0.2µm at 20 keV [57]. Furthermore, even thinner layers are expected to

disturb the beam and lead to its loss. This is qualitatively shown in Fig. 6.3.

a) b)

Y

X Z

300 keV antiproton beam 20 keV antiproton beam

200 nm thick Al foil negative charge

positive charge neutral particle

Figure 6.3: Qualitative simulations of 300 keV (a) and 20 keV (b) antiprotons interac- tion with a 200 nm thick aluminium foil. See text for details.

The influence of a thin aluminium foil on low energy antiprotons was simulated with Geant4 [244, 245]. A pencil-like beam of antiprotons impinges on an aluminium disk, 50 mm in diameter and 200 nm thick, at normal incidence; the resulting trajectories of primary and secondary particles are shown in Fig. 6.3. No secondary eV electron emis- sion is modelled in Geant4, but scattering and annihilation can be observed. Because 300 keV antiprotons are not stopped in the target material, their behaviour is similar to protons at the corresponding energy. However, annihilation introduces a significant change in the picture for 20 keV antiprotons. Not only does the low energy beam blow up, but also annihilation products emerge from the foil in random directions. It can be concluded that even a 200 nm foil makes the SEM a destructive beam monitor. It can be used in beam transfer lines and for first turn diagnostics, but needs to be optimised for minimal image distortion due to electric fields and annihilation products noise.

Perturbation in an Electric Field

A simple model of the SEM assembly was studied with SIMION [247]. The foil and mesh were approximated by two parallel circular disks of 50 mm diameter and separated by 5 mm. In addition, the foil was clamped with a thin ring at its edges and the whole object was on a negative potential of up to –10 kV. The grounded mesh surface was made transparent to the beam. Approximately 50 mm away from the foil, a simplified MCP and phosphor assembly was located with 0 volts and 2 kV applied to the front and the back of the microchannel plate, respectively, and 5 kV applied to the scintillating screen. Finally, electric fields generated by the monitor in its surroundings were computed.

The resulting field map was used for particle tracking, performed also in SIMION, to study the influence of the monitor on the primary beams. A point source of particles was located at a distance of about 20 cm from the centre of the foil. Particles of either

positive or negative charge and various energies were emitted in a small angle of 1◦

around the beam axis and their trajectories were recorded. The results are shown in Fig. 6.4.

For 300 keV particles and the highest voltages applied to the monitor, see Fig. 6.4a, the beams are only slightly perturbed. Their centres hit the foil 0.6 mm away from the reference trajectory and the shift direction depends on the charge of the particles. Positive protons are attracted by the foil, whereas negative antiprotons are repelled, thus the latter follow a slightly longer path. The influence of the monitor on 20 keV beams is much stronger and results in a shift of the proton beam centre by almost 8 mm, whereas 20 keV antiprotons do not hit the foil at all, see Fig. 6.4b. With the foil voltage decreased from –10 kV to –5 kV, see Fig. 6.4c, the distortion of initial trajec- tories is reduced, but still reaches about 4 mm and 5 mm for protons and antiprotons, respectively.

In order to minimise the influence of the electric fields of the SEM on the primary beams, additional shielding was included in the model. As shown in Fig. 6.5, the foil is surrounded by grounded walls with an exit window in the back to let the high energy beams go through unaffected. In the low energy mode, the back is covered by either a mesh or a foil. Also the electric fields around the MCP and phosphor assembly are screened by additional grounded walls. Furthermore, they protect the back of the assembly from the direct beam hit.

In the case of –10 kV applied to foil, a 20 keV proton beam centre is deflected by 0.8 mm, but antiprotons are still too strongly repelled and a significant fraction of the

6.3 Design Considerations

foil _mesh

foil _mesh

a) EBEAM = 300 keV, VFOIL = -10 kV, VMCP = 2 kV, VPHOSPHOR = 5 kV

b) EBEAM = 20 keV, VFOIL = -10 kV, VMCP = 2 kV, VPHOSPHOR = 5 kV

c) EBEAM = 20 keV, VFOIL = -5 kV, VMCP = 2 kV, VPHOSPHOR = 5 kV

foil _mesh

MCP/phosphor

assembly proton beam_–

antiproton (H ) beam

Figure 6.4: SEM influence on the primary beams for various beam energies and foil voltages as modelled in Simion: a) 300 keV and –10 kV, b) 20 keV and –10 kV, and c) 20 keV and –5 kV. The MCP at 2 kV and phosphor at 5 kV were present in all simulations but are shown only in the last figure.

beam is bent away from the monitor, see Fig. 6.5a. With the voltage reduced to -5 kV, both beams reach the foil and their displacement is 0.5 mm and 0.9 mm for protons and antiprotons, respectively, see Fig. 6.5b.