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:
- Even if the steering is very bad in one of the planes, the overall
symmetry can still be as good as 71 percent.
- The angle of the incident beam is not monitored at all
To remedy these inconveniences, a new symmetry Scombined has been
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:
- The page-1 program checks whether the intensity readings in the BSM foils
are above a given threshold
- If they are ok, the BSM symmetries are used, the new formula
Scombined is used for the symmetry on page-1 and the little 'a' is
shown in front of the symmetry indication.
- If the BSM sees no or too little intensity, the old formula
Sposition is used, the angle is therefore not included and the 'a'
is not shown on page-1
3. What if there is no 'a' ?
If there is no 'a', one should always try to understand why not. There are
- The monitor is out of the beam.
- The BSMs should move
automatically - without WOBBLING the position should be
- On T2, with no wobbling, the beam
should be centered on BTV 2309 in front of T2. Select 'all' in TZ systems BTV
(the screen is not an active element).
- The 'a' is generated by Eugenia's
program. Can be checked with SW from Lars Jensen :
- path /user/ljensen/dev/sem_sps/new_sem/appl
- programs semintensity,
- The monitor is broken. In that case the combined symmetry can not be
measured until the monitor has been repaired.
- 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.
- 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:
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
- There is no intensity. In those cases the symmetry calculation may
(by chance) still give a reasonable value
Adapted from L. Gatignon