The ALC spectrometer The ALC detector is embedded inside a 5 Tesla superconducting solenoid. The detector consists of a forward (FW) and backward (BW) rings, each of them made of 10 scintillation positron counters. ALC detector and its exploded view as implemented in the simulation program. Positron counter EJ-204A (120x28x5 mm 3 ) BCF-92 (Ø 1mm) 2x SSPM 0701BGAmplifier (gain ~ 20, bw ~ 70 MHz) Photographs of the upgraded ALC spectrometer (left), of the detector module (middle right) and of the positron counter (bottom right) and a design view of the ALC detector (upper right). The detector system of the Avoided Level Crossing  SR spectrometer, installed at PSI, Switzerland, was completly rebuild in One of the main motivations for the upgrade was to simplify the dependence of the measured asymmetry on the applied external magnetic field. To achieve this goal, we have simulated the ALC detector using the numerical Monte Carlo package Geant4, and subsequently used this simulation to optimise the ALC design. The upgraded ALC detector (utilising Geiger-mode avalanche photodiodes instead of PMTs) is described in detail in a different poster of this conference. Geant4 simulation of the upgraded ALC spectrometer The project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no.: RII3-CT K. Sedlak 1, A. Stoykov 1,2, R. Scheuermann 1, T. Shiroka 1 Comparison of the simulations and measurements Geant 4 simulations (stars) are compared with data for two different copper sample thicknesses in presence and absence of a 1mm thick lead tube inside the forward detector mimicking a cryostat wall. Penetration depth (a) and oscillations of the beam- spot size (b) depend on the initial muon momentum. The same is true for the asymmetry. Hit multiplicities (M F, M B ) are constructed as the ratii of all individual forward (backward) counters over the total forward (backward) counts corrected for multiple hits that happened within ~80ns time window. Measured M F, M B are surprisingly well described by the simulation. Energy deposits of all particles (decay e +, e -, γ,  ) in a positron counter are summed up. Only hits above the energy threshold E thr (normally set to 0.5 MeV) are accepted. 1 Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 2 Joint Institute for Nuclear Research, Dubna, Russia Left: The different muon beam spatial distribution does not seem to influence the asymmetry significantly. Right: The initial muon spin polarisation, however, plays an important role. How does the initial muon beam influence the asymmetry? Threshold on the energy deposited in the positron counters Time-integral operation mode: Number of counts in backward, N B, and forward, N F, detectors is recorded for a predefined time interval. The counts are corrected for multiple hits by vetoing hits within ~80ns after any accepted hit. Subsequently the asymmetry is calculated as Conclusions The simulation of the ALC detector based on the Geant4 package reproduces the shapes of the experimentally measured quantities, and correctly predicts their dependence on the sample thickness. Even though the absolute normalisation of the simulations is not perfect, the Geant4 predictions turned out to be a very useful tool for the ALC detector upgrade optimisation. The differences between simulations and measurements can be due to imperfections in the description of the detector components, of muon beam parameters and of the approximation of the field map of the ALC magnet used in the simulation program, or due to effects not included in the simulation. B = 0 T B = 2 TB = 4 T Gaussian beam... muons distributed around the z-axis according to the Gaussian distribution Homogeneous beam... muons homogeneusly distributed in space inside the beam pipe Point-like beam... muons generated on the z-axis with x=y=0. TURTLE beam... beam generated by TURTLE using the setting of the present πE3 beam-line.