Status report about the e-  identifier Study of shielding against the stray magnetic field at the downstream end of the spectrometer F. Huveneers, Gh. Grégoire, M. Cyamukungu September 10, 2003 The photomultipliers of the Cerenkov detector have to be shielded. Shielding is made of a large and thick iron slab provided with a central hole (diam. 50 cm) located around the beam axis. The z-position of (the upstream face of) this slab is presently located 20 cm downstream of the last correction coil of the spectrometer. The present study estimates the residual field at the off-axis position of the photomultipliers.

Active magnetic elements From R. Palmer,Version 5 Compute stray field in r-z space after the last correction coil with/without an iron shielding of variable thickness r z Current densities

3D view of spectrometer with magnetic shielding Correction coils Solenoid Soft iron shielding Matching Focus Coupling

Computations Field equations in the presence of ferromagnetic materials are solved with Poisson/Superfish

Field lines without shielding z (cm) r (cm) Beam axis diam. 50 cm Correction coil End of solenoid O

With a 5 cm thick shielding z (cm) r (cm) Particle flight path 5 cm thick soft iron diam. 50 cm 20 cm O

With 15 cm thick iron shield 10 cm thick soft iron z (cm) r (cm) Particle flight path diam. 50 cm 20 cm O

Field along beam axis End of correction coil Field inside solenoid Location of the e-  identifier – Look at next transparency for detailed view

Stray field downstream along beam axis Approximate z-position of photomultiplier axis 1 kGauss limit

Influence of the shielding Shown here: modifications of B z along beam axis (r=0)

Conclusions A soft iron shield ~15 cm thick with a central hole of ~50 cm diameter is needed to reduce to 1 kGauss the stray field at the PM positions. - For thinner shields, the saturation of iron is reached around the central hole. The subsequent field modifications affect: - the tracking of particles and the simulations both upstream and downstream - the construction of the containment vessel for the correction coil and the solenoid. They should be hopefully taken into account in close cooperation with the relevant MICE experts. - The residual field is further reduced by a specific mumetal shield around the photomultipliers - the magnetic field configuration of the solenoid

Work underway 1.Optimization of the shielding parameters (thickness, central opening, shield saturation …) 2. Generation of a C++ program to evaluate field components at any point downstream the last correction coil and the shielding 3. Implementation in Geant 4 + for downstream tracking We are waiting for comments and suggestions from the experts … (