The 'a' on page-1

1. The definition of the symmetry

The steering on the primary targets is done by the PCR operations team. The position of the beam at the target is monitored by a system of split foils, called TBIU, inside the target box. From the relative difference between left and right and between top and bottom, symmetries are calculated for the horizontal (SH) and vertical (SV) steering, respectively. The overall position at the target is monitored by the combined symmetry Sposition:

Sposition = sqrt( (SH2+SV2) / 2)

The overall symmetry is shown in percent.

This system has two major limitations:

  1. Even if the steering is very bad in one of the planes, the overall symmetry can still be as good as 71 percent.
  2. The angle of the incident beam is not monitored at all

To remedy these inconveniences, a new symmetry Scombined has been defined:

Scombined = Starget x SBSM


Starget = sqrt(SH x SV)


SBSM = sqrt(BH x BV)

where BH and BV are symmetries measured in a split foil, called BSM, located approximately 30 metres upstream of the target. If both symmetries are good, both the position and the angle at the target must be correct. The square root has been maintained to keep symmetry figures similar to the ones we usually had with the old formula.

2. What about the 'a' on page-1

The BSM monitor may be out of the beam or out of order. In principle this could be checked by a direct reading from the SPS page-1 program, but this turns out to be quite heavy in terms of computer and network load. On the other hand, all the intensities in all relevant monitors are anyway available through one single block read. Therefore the following strategy has been adopted:

3. What if there is no 'a' ?

If there is no 'a', one should always try to understand why not. There are several possibilities:

  1. The monitor is out of the beam.
  2. The monitor is broken. In that case the combined symmetry can not be measured until the monitor has been repaired.
  3. Somebody (usually an EA physicist) is doing a position scan with the BSM in order to measure the width and shape of the angular distribution of the primary beam. During such a scan, the 'a' will first disappear and the symmetry is good (the beam does not hit the monitor at all -> Sposition is used). Subsequently the 'a' will come back, the symmetry will be low, increase to normal and go down again. Then the 'a' will disappear again (the beam misses the monitor on the other side) and finally, the program puts the monitor back to its nominal position and the 'a' should come back with a reasonable symmetry.
  4. The angular steering is bad. In this case the position at the target may be OK (good symmetry, no 'a'), but the angle may be sufficiently bad to miss the BSM monitor.

4. Other typical cases

There are some other confusing cases, related to the symmetry on page-1:

  1. The multiplicity is low (0 or 1), but the symmetry is good. In this case, it is likely that a quadrupole is off, the beam is very big, i.e. particles hit every foil, but rarely the target. This may, of course, also occur when the target head has been taken out or if a very short target is used.
  2. There is no intensity. In those cases the symmetry calculation may (by chance) still give a reasonable value

Adapted from L. Gatignon