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 defined:
Scombined = Starget x SBSM
where
Starget = sqrt(SH x SV)
and
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
several possibilities:
-
The monitor is out of the beam. Ask the PCR to check and put it back
-
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.
Clearly the latter case is a most worrying one. Therefore one
should always follow up the absence of the 'a'.
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 used.
- There is no intensity. In those cases the symmetry calculation may
(by chance) still give a reasonable value
Last updated on 2 May 1997 by
Lau Gatignon