**(1) New mask system** (transparencies incuding ones presented at LCWS00,
21 pages of
pdf ,352kB)
Sugimoto showed detailed studies on various geometries of beam pipe
at IP in terms of secondary photons entering into the CDC by JIM for B=3Tesla.
The IP beam pipe system consists of
two cylindrical parts(r=1 - 1.8cm of 0.5mm thick Be and r=7.3cm of 2mm thick Al)
and a conical one ( Be, smooth change of thickness from 0.5m to 2mm )
connecting between the two. First configulation is a fixed conical angle of
0.15 radian for the conical part, where the radius(r) of Be-cylindrical pipe
changes from 1.0 and 1.8cm with the half length(z) of z = (r+0.2)/tan(0.15) cm.
Number of the photons varies from 135(r=1cm) to 2(r=1.8cm) per bunch crossing.
The second configulation is the r=1cm Be pipe, where the half length varies from
2cm(cosine=0.894) to 7.94cm(cosine=0.992) and the Al pipe begins at 56.8cm from
IP. The photon number varies from 163(z=7.94cm) to 12(z=2cm) per bunch crossing.
So, we can control the photon number at an order of maginitude by changing
length of the Be pipe.
There is also another modifcation for better performance of forward tracker to
measure accolinearity angles of Bhabha events, which has been considered in
TESLA-TDR.

He demonstrarted a performance of the pair monitor located at 4m from IP (B=3T). The azimuthal angles in r= 6 - 8cm clearly showed asymmetrical distribution. He pointed out that the similar angular dependence can be seen anywhere along the beam line since the maximum radius must be positioned at phi=0 and 180 deg. for continous energy distribution of pairs.

Finally, he presented pre-talk for LCWS00, entitled as "New JLC Mask System at B=3T". He has studied pair background hits in detectors (CDC, VTX and endcap- Calorimeter) with various configulations of mask system for cases of B=2(L*=2m) and 3Tesla (L*=2 and 4.3m). He found that the least neutron background in CDC was observed at the mask system of B=3T and L*=4.3m, which is 0.1n/BX( into CDC). For this configulation (B=3T,L*=4.3m), if no compensation coil is necessary for supercondcting QC1 etc. , the dead angle can decrease from 0.2 to 0.1 radian and the minimum veto-angle might also be 22 mrad. For more details, please see his transparencies and the proceedings.

**(2) Others**

The next meeting will be on 15 November (Wednesday), 2000 13:30 - 15:30 at 3 gokan, 425.