K0 beam line for the E391a experiment May 27, 2005 Takao Inagaki (KEK)

Location

Collimation scheme

KL spectrum and n andγprofiles

Telescope for the profile measurement

Profiles　(meas. vs MC*)

Sizable vacuum effect

Detectors for energy spectra SLSD (+ lead glass) Cerberus

γspectra measured by SLSD+lead glass

Neutron and γ spectra by Cerberus

C4: suppressor of thermal neutron flux

Profiles and spectra of the halo Measured by Cerberus

Decay vertex distribution for 2γsample at trigger bias

How are the halo neutrons generated? Scattering at C2 + Hadronic Shower at C6 Main source of lower-energy tail events ~ 25 % events Multiple Scattering Main source of higher-energy edge events

Summary The profiles and spectra including the absorber effects were rather well reproduced by a MC simulation using GEANT(G-FULKA) code. Beam-line design becomes science from art. Further improvement: ・　suppress the second step scattering 　 ・　decrease n/KL ratio

Supplement

How are the halo neutrons generated? It consists of two kinds of source. From first look of e391 data, we have to reduce halo!!!

How are the halo neutrons generated? Multiple Scattering, 1st scattering is in the Pb absorber. No. of event is rather large. ~ 20 % of halo events Scattering at Vacuum window.

Summary for halo-neutrons Source of halo neutrons: 1. Multiple scattering typical case (a) C2C6halo (b) Pb abs  C6  halo (20%) 2. Hadronic shower at C6 (ex, C2C6shower) ( 25 %) Possible way: 1. Position of trimming collimator relative to Pb abs. 2. Active veto-counter at C6. Test of New Collimation Scheme…