Plan for calibration of the TOF array

K. Kainz and W. J. Llope

This page was last updated on September 8, 1998.

Phase 1: wall coverage

The first stage of the TOF beam sweep shall be devoted to casting the secondary beam upon all slats of the array, in order to aid in determining the relative offsets.

This is the current plan to cover the TOF slats:

• Only positives will be sent down the beamline.
• The beam momentum will be initially set to 4 GeV.
• The sweeper field will be set to 3 T, and the analyzer set to its full field (16.25 kG). Were the sweeper field set higher, the 4 GeV beam would likely be deflected out of the cave.
• Over the course of Phase 1, the sweeper field will be brought down to zero in 4 (or more) steps. At each sweeper step, the analyzer will be stepped down from its full field to zero.
• TOF data will be taken at each sweeper-analyzer setting.
• By the time both the sweeper and analyzer reach zero, TOF holes 56 to 190 should be covered, as shown in the plot below.


[Postscript version]

The table below shows the range of slats that may be covered by varying the analyzer field, for each beam polarity and sweeper field.

sweeper field (fraction of 6 T)	beam polarity	TOF holes covered with analyzer variation
1/2	positive	142-190
3/8	positive	125-169
1/4	positive	107-152
1/8	positive	82-135
0	positive	56-117

• With the sweeper kept off, the analyzer polarity would be reversed. The time estimate for this process is two hours.
• The remainder of the wall would be swept by stepping the analyzer field from zero to negative full field (-16.25 kG). The plot below shows how far the beam could be deflected onto the beam-left slats, for beam momentum of 2, 3, and 4 GeV.


[Postscript version]

• With the beam momentum at 4 GeV, the beam could reach TOF hole #15.
• If the beam momentum were decreased to 2 GeV, hole #2 could be reached.

About the plots shown above:

• The beam trajectory at the target was taken to be the same as it was during the heavy-ion runs (target at (-8,0,253), angle at 3.16 degrees)).
• The error bars in the above plot represent the spread in the beam-particle distribution among the neighboring slats. In determining this width, a 10 percent spread in the 4 GeV beam was presumed, and a gaussian was fit to the beam particle slat-hit distribution; the length of the error bar is the full width at half maximum.
• In general, the width of the slat-hit distribution tends to be lower when the beam is deflected less.
• Not all of the distributions could be fit to gaussians very well, but what is shown here should give a decent impression as to how many "analyzer steps" might be necessary to sufficiently cover the TOF slats in a given vicinity. For the extreme beam-right slats, it may be sufficient to take two (or three) runs at each sweeper-field setting shown: analyzer at full field, at half field, and at zero field. Once the sweeper gets close to 1/4 full field, though, it may be necessary to run five or six different analyzer settings, ranging from full analyzer to analyzer off.
• Alternately, the sweeper magnet could be brought down in smaller steps. Also, perhaps more data could be taken at each sweeper-analyzer setting; the online histograms could be used to determine when the slats in a given region have been sufficiently covered.

Phase 2: clean DDC tracks to associate with TOF hits

The goal of this stage of the TOF sweep is not to cover the wall, but to collect a clean sample of DDC tracks along with TOF hits. This would be used in the development of software to extrapolate the DDC tracks through the complicated fringe fields to the TOF wall.

During this stage, beamline preparation would proceed along these lines:

• The sweeper would remain off.
• The analyzer would be kept at negative polarity (left over from the end of Phase 1), and set at full (negative) field.
• Beam momentum would be decreased to 3 GeV. Again, only positive beam would be used.
• The D10 field would be adjusted, in an attempt to improve the DDC and TOF coverage.
• Some higher values of the beam momentum (up to 15 GeV) may be desirable here.
• When sufficient data is taken for negative analyzer polarity, the analyzer would then be switched back to positive polarity, and operated at full (positive) field.
• For the remainder of Phase 2, about 5 or 6 different settings of the beam momentum, ranging from 2 to 15 GeV, would be used. The D10 field would again be adjusted so as to improve coverage of the DDC and TOF.

TOF coverage is expected to be similar to that observed during the 1997 proton run.