Memorandum: Optimization of the E166 spectrometer

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Nov 15, 2013 (3 years and 8 months ago)

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Memorandum: Optimization of the E166 spectrometer


Ties Behnke, SLAC and DESY, May 1, 2003


Introduction

In the memorandum dated May 1, 2003 I presented some considerations about the
background in the double bend spectrometer configuration discussed. The m
ain
conclusion was that while the signal from converted photons is fairly independent from
the exact geometry of the beam line chosen, background photons present at the position
of the re
-
conversion target play a much more important role than previously as
sumed.


Peter since has proposed a new detector setup which removes the need for a sweeping
magnet, and shortens the distance between the reconversion target and the detector face
by a factor of two.


In this brief note I describe work done to further op
timize the double bend spectrometer,
and a first look at the new geometry proposed by Peter.


Optimisation of the Double Bend Spectrometer

The main problem identified in the current version of the double bend spectrometer are
photons which through multipl
e scattering and conversions in the walls of the beampipe
arrive at the reconversion target. Only a moderate fraction of these photons interact in the
re
-
conversion target, thus creating a photon background BEHIND the re
-
conversion
target, which has an ene
rgy spectrum rather similar to the one of the photons created in
the target by conversion from the incoming positrons. This point is illustrated in
Figure
1

and
Figure
2
.


A suppression of the background c
an be achieved by increasing
the angle of the double
bend sp
ectrometer magnets. In a previous study this has been investigated. There it was
actually found that the Signal to background ratio decreases slightly, if one increases the
bend angle from 60 to 9
0 degree. However this study was done under the assumption that
the transverse offset between the beamline and the detector was fixed at 25 cm, and that
no additional collimation was present between the primary target and the re
-
conversion
target. The situ
ation has been re
-
assessed with the following changes:



An increase of the distance between the beamline and the detector is allowed



The collimation, based on the studies reported in the memorandum dated May 1,
is optimized between the primary and the re
-
c
onversion target.

In addition focusing is present between the two magnets, as proposed by Yuri, to increase
the positron transmission efficiency.



Ties Behnke: Optimization of the E166 positron beam line


2


Figure
1
: Particle composition (top left), photon
energy spectrum (top right) a
nd positron energy
spectrum (bottom left) behind the re
-
conversion
target, for a "beam" of only positrons hitting the
target.


Figure
2
: Particle composition (top left), photon
energy spectrum (top right) and positron energy
spect
rum (bottom left) for a beam of only photons
hitting the reconversion target.


Under these changed conditions a 90
-
90 degree bend configuration actually performs
significantly better in terms of background that the 60
-
60 configuration, while still
maintai
ning an acceptable transmission efficiency. The layout of the beamline is shown
in
Figure
3
. It corresponds to the setup #4 discussed by Peter in his message from May 6,
2003.


For this beamline the performance is summarized in
Figure
4
. This beamline has a
positron transmission efficiency of 2.2%, and a signal to background ratio of 19.9 (this
should be compared to the performance of the 60/60 beamline: efficiency 4.3%, signal to
background close to 3). It

should be noted that the efficiency quoted does not take the de
-
focussing effects of the bending magnets into account. As shown by Yuri this might
reduce the transmission efficiency by a factor of 2. On the other hand the focusing
elements used are not re
ally optimized for the setup. I would not be surprised if a proper
optimization could not recover this factor of 2 in transmission efficiency.


The beam spot at the position of the reconversion target is shown in
Figure
5
.


In sum
mary it seems reasonable to assume that we can construct a beamline which has a
transmission efficiency of at least 1%, probably better, and has a signal to background
ratio of at least 10 (probably better). This should reduce the problem of the photon
bac
kground behind the reconversion target significantly.


Ties Behnke: Optimization of the E166 positron beam line


3


Figure
3
: Beamline layout corresponding to setup #4.

Reconversion
target

Analyzer block

Primary target

det
ector

Ties Behnke: Optimization of the E166 positron beam line


4


Figure
4
: Performance plot for beamline #4. top left: primary particle composition behind

the
primary target, top right: particle composition at the re
-
conversion target; bottom left: photon
energy spectrum at the reconversion target, bottom left: positron energy spectrum at the
reconversion target.


Ties Behnke: Optimization of the E166 positron beam line


5


Figure
5
: Beam
spot at the primary target (left) and the reconversion target (right). Shown is a
scatter plot of the two transverse beam dimensions.


Background studies for the detector setup

The configuration proposed by Peter has been studied in view of the background
performance. The plots shown are the same as were shown in the previous memorandum
(May 1, 2003), and show the composition of the signal seen at the detector for the
different sources. From this plot it is apparent that the number of background photons, as

expected, is significantly reduced to a point where they probably are acceptable.
Compared to the earlier setup the number of particles which have scattered in the iron
and are seen in the detector is significantly increased. The nature and importance of
these
particles needs to be studied in more detail. However from the studies presented by Peter
it seems that they do not reduce the analyzing power significantly, so they probably are
no problem.




Ties Behnke: Optimization of the E166 positron beam line


6


Figure
6
: Background composi
tion for beamline
setup#4, at the position of the detector. Shown is a
scatter plot of the energy of particles entereing the
detector, as a function of their scattering history. 0:
background, 1: beampipe, 2: collimator, 3: Analysis
iron, 4: reconversion t
arget, 5: anything else, 6: true
signal.


Figure
7
: Projection of
Figure
6

on the y
-
axis. Codes
are as described in
Figure
6
.



Conclusion

The detector setup #4 proposed by Peter

combined with a 90
-
90 beamline with an
tranverse offset increased to 45 cm seems to offer the promise of reasonable performance
combined with manageable background.