1. PROPOSAL FOR A MUON VETO SYSTEM BEHIND THE HADRONIC CALORIMETER (MUV-3) Problems and requirements:  Expected total rate for 9.25 < R < 120 cm: 12.1 MHz: - 3.9 MHz from decays before GTK3 - 5.7 MHz from K + decay (see presentation by S. Gallorini, TDAQ meeting, 3 February 2010)  Level 0 trigger rate must not exceed 1 MHz veto all triggers associated with muons crossing MUV-3  Random vetoing of good events must be kept as low as possible: MUV-3 time resolution must be as good as possible (random veto probability at 12.1 MHz = 1.2 % for 1 ns)  1 ns time resolution excludes light collection and readout using wave-length shifting fibers I. Mannelli, C. Biino, L. DiLella, N. Doble, R. Fantechi, F. Marchetto, V. Obraztsov HAC-MUV Working Group 23 March 2010

2. Proposal (Italo Mannelli): Counters consisting of a 2.5 cm thick scintillator plate facing a 2” PMT Air light guide Scintillator plate dimensions 15.8 x 15.8 cm 2 Distance between scintillator and PMT window 16 cm Only direct light is collected by the PMT Reuse as much as possible NA48 AKL hardware:  PMTs with housing and magnetic shielding  Constant Fraction Discriminators  High Voltage Power Supplies and (possibly) associated slow control  Cables (?)  Scintillator (suitably reshaped and reassembled)

3. Original sketches by Italo Mannelli Only direct light is collected by the PMT (all scintillator faces but one, and all walls, are blackened)

4. Results from a simple Monte Carlo simulation of scintillation light  Scintillator tile dimensions 158 x 158 x 25 mm  Distance between scintillator exit face and PMT cathode 160 mm  PMT quantum efficiency 10%  Expect ~170 photoelectrons (p.e.) for normal incidence in tile center; ~110 photoelectrons (p.e.) for normal incidence near tile corner.

5. MUV-3 counters are assembled into mechanical units (“boxes”), each consisting of 4 x 4 counters → 16 boxes in total For each of the four boxes near the beam pipe one counter has a Special shape to accommodate the beam pipe

6. Progress in mechanical design Box with 4 x 4 counter matrix Wall of 4 x 4 boxes → 256 counters (from A. Bianucci, INFN, Pisa)

7. Reusing the NA48 AKL PMTs Thin black plastic cylinder  -metal Black plastic cylinder and  -metal must be cut here for good light collection (no shadowing) but then the magnetic shielding may become too weak, especially for the PMTs near the beam pipe, close to the magnet bending the beam out of the SAC Should all counter walls be lined with  -metal plates? MUV-3 total thickness along beam 483 mm, including 50 mm for connectors and cables

8. A counter based on the proposed design was built ~30 years ago using NE102A scintillator and a 2” XP 2232B PMT B. Asman et al., NIM 187 (1981) 371 60photoelectrons were measured for normal incidence in center, dropping to ~52 for incidence near corner (compatible with the prediction for the proposed counter, given the different scintillator – PMT distance) Width of the timing distribution constant across the box, giving an inherent time resolution of 0.5 ns (rms) (with 70 m RG 213/U cable) Max. time shift from corner to centre 0.3 ns Use of 70m RG58/U cable worsens the time resolution by 15%

9. Čerenkov effect in the PMT window For the counter described in the 1981 NIM article a 2.8 ns earlier signal was observed, originating from particles traversing the PMT Measured most probable signal amplitude: ~15 photoelectrons (with scintillator removed and particle traversing the PMT) The same effect is expected for the proposed MUV-3 counters, but the Čerenkov signal should be earlier by only (16/38) x 2.8 ns ≈ 1.2 ns Turning the counter by 180º, so that particles first traverse the PMT and then the scintillator does not help because Čerenkov light is totally reflected at the PMT window exit (this happens for normal incidence in all transparent media with refractive index n > √2 ≈ 1.41) For borosilicate glass (PMT window ) n = 1.52 – 1.56 Discriminator threshold must be adjusted above signal level from Čerenkov light in PMT window

10. WORK TO DO TO VALIDATE THE PROPOSED MUV-3 SYSTEM Build one or two prototypes and study their performance on test beams - Signal amplitude and time resolution as a function of the beam particle impact point - Effect of Čerenkov light from particles traversing the PMT - Optimization of the scintillator type and thickness - Effectiveness of magnetic shielding, especially for the stray field produced by the magnet bending the beam away from the SAX Some tests are in preparation at IHEP (Protvino) using PMTs from the NA48 AKL The proposed construction scheme can also be applied to Anti-0, a system of counters to be installed immediately after the last collimator to veto   produced by halo muons interacting near the end of the collimator