CAST FRC-D 2006-17
24 November 2006
FRC report
November 2006
K. Zioutas
CAST-spokesperson
Status of the CAST Magnet (Cryogenic) & the associated cryoplant
The CAST magnet is continuing to perform within expected cryogenic performance
parameters. Nominal temperature of 1.8K is maintained during data-taking and
background data taking by the associated cryoplant. The addition of (heated) “cold
windows” and the increasing He pressure in the beam pipe are causing an additional
heat load into the cryogenic system; however this is still within the capacity of the
cryo system to deal with.
As the main cryogenic compressor for the system has now reached 40,000 operating
hours, a major overhaul has been scheduled during February 2007. This is required to
maintain nominal performance and to comply with AT-ECR maintenance contract
C-169. The main Leybold Helium pump for the cryo system is continuing to perform
well, however it is operating at close to maximum capacity and is also scheduled for
routine maintenance during the 2006-7 shutdown. It has now run for 19,920 hours and
will also require a major overhaul at 40,000 hours.
The known leak from the magnet’s MFB liquid helium reservoir continues to be
successfully pumped via the isolation vacuum pumps and show’s no sign of
increasing. With respect to the next phase of the experiment, we will continue to
operate the cryoplant with its normal operational efficiency of over 99%. We will
however need to take great care to minimise the additional heat load from new
equipment due to be installed within the magnet crystat.
GRID measurements
As it has been explained in past reports, CAST performs periodically the so-called
GRID measurements with the help of the team of geometers at CERN. These consist
in the independent measurement of the position of the magnet in a set of reference
coordinates (GRID) previously defined to cover reasonably all range of movements.
These measurements are intended to detect any drift in the pointing ability of the
system with respect to the initial calibration values measured in 2002, the ones which
are used by the tracking software to determine the real absolute direction in which the
magnet is pointing at any time.
The latest measurements were performed during April and October 2006. In April the
system was found substantially unchanged with respect to the 2004 GRID, since only
a small shift of 0.3 mm vertical and 0.4 mm horizontal was observed. However, there
has always been a shift in comparison to the reference values of the grid of 2002, the
ones used for tracking. This shift was of the order of ~1.3 mm in the vertical axis (due
to the procedure of resetting the level value) and ~1.5 mm in the horizontal axis (due
to the extra freedom that was introduced into the system in June 2003 because of the
mechanical problems with the lifting screws). Although this shift was within our
acceptance of 1 arcmin, we have been able to correct for it by readjusting the motor
encoders, as shown in Figure 1. Furthermore we have implemented a new checking
system based on laser sensors that allows us to immediately detect potential drifting of
the order of few arcsec in both directions.
In conclusion, the tracking system has been performing well within our requirements
during last year’s operation.
Figure 1: Comparison between present and past measurements. The pointing precision was
similar to that of 2004 (up, left). Introducing a proper offset in both vertical and horizontal
axis (up, right) we were able to correct the drift compared with that of 2002 (down, left-right),
which defines our tracking accuracy. The deviations were within our acceptance of 1 arcmin.
Solar Filming
Twice a year, it is possible to directly observe the Sun through a window in the
experimental area and thus perform an optical crosscheck of the tracking system, the
solar filming. For this purpose, a camera is being aligned with the magnet axis and
additional software is applied to consider refraction of photons in the atmosphere. The
solar filming had been repeatedly performed during the past. Thus for the 2003 and
2004 data taking phase it was confirmed that the magnet was pointing to the Sun
while taking data with the accuracy required. Until March 2005, a webcam in
combination with a small telescope was used to film the Sun. In order to improve the
resolution and the alignment, the system was enhanced to provide a precision with
matches the desired accuracy of the solar tracking (0.02◦). Therefore, the new filming
system consisted of an ST-7 CCD camera and better optics with 200 mm focal length.
Furthermore, the concept used to align the filming setup with the optical axis of the
magnet was changed. Thus a higher accuracy of the measurements and a better quality
of the images was achieved.
Already first tests in spring 2005 showed that the CAST magnet was pointing to the
center of the Sun with an improved precision of the optical crosscheck, namely
O(0.03◦). A further enhanced setup was used in fall 2005. Not only was it made to be
more rigid and thus damping vibrations but also to be more flexible and like this
easier to align with the magnet axis. With this latest setup a precision of 0.02◦ could
be achieved at best and thus the accuracy of the tracking system could be reached.
Within this precision, the magnet is pointing to the solar core from which most axions
are expected to emerge. For March 2006 the analysis of the solar filming leads to a
similar result with an even slightly improved accuracy of 0.015◦: within the desired
precision the CAST magnet is pointing to the center of the Sun.
In October 2006, only pictures with parts of the solar disk could be obtained due to
bad weather conditions and trees preventing a full view of the sun. This latest filming
data is to be analyzed now.
Error! Objects cannot be created from editing field codes.
Figure 2: Superimposition of ~75 pictures taken during filming. For reference the crosshairs,
the pointers and the circle, within which a full view of the Sun is possible (i.e. no disturbances
due to disk of pointers or sphere occur), are added in the picture.
Slow control system
The CAST Slow Control provides data logging and continuous monitoring. The
software is based on National Instrument's LabView7.1. It is composed by one digital
data acquisition card and four analogue data acquisition cards (three with 24-bit
resolution and one with 16-bit resolution). The analogue cards are being used to
monitor the gas system (pressures, magnet temperature, etc.) and the main parameters
of the detectors, while a digital card is used for the monitoring of the interlock system.
In case of interlock alarms or when the pressures or the temperature are out of the
normal range, notifications are automatically sent to several CERN GSM and email
recipients. The data and status are accessible on-line on a web site to the whole CERN
domain and by a remote desktop connection. The system is fully operational since end
of July 2003, though the continuous improvements added also new functionalities
intended to make the slow control more complete in the monitoring of the experiment.
Present Status of the gas system
The gas system is one of the novel features of the CAST experiment for its upgrade
into the Phase II. The progress during 2006 was the natural evolution following a
period where the most relevant tests to investigate the behaviour of the experiment
with gaseous 4He in the magnet cold bores were done, such as quench and
thermoacoustic oscillations tests, using a preliminary gas system, and finally the
design, construction and commissioning of the final 4He system that was put in use for
the present data taking period.
The progress done in 2006 can be summarized in 3 different subjects:

Operation of the 4He gas system.
The 4He gas system, for the beginning of the phase II data taking, was put into
operation in the middle of November 2005. The objective of this system is to
exploit the available range of the gas density for the search of solar axions up to a
mass of ~0.43 eV (with 16.4 mbar at 1.8 K), before the final 3He gas system is put
into operation. Complemented with the experimental data taking runs, the
operation of this system also serves as basis for studying effects related with the
presence of the gas in the magnet cold bores that can serve as guidelines for the
design of the final 3He gas system.
The operation of this system allows the filling of the gas in very accurate steps of
density into the magnet cold bore meeting the requirements of CAST.
Throughout the year of 2006 the operation of the gas system was done smoothly
without problems by trained shifters following an established protocol. The
operation was only interrupted in the end of the 1Q2006 due to a scheduled
intervention for the Micromegas line. Taking advantage of this, it was installed a
new cold pressure transducer directly in the cold bore, in order to confirm the
non-existence of thermo-acoustic oscillations, and also new calibrated
thermometry around the cold X-ray window flanges on the MFB side. Due to the
successful operation of the 4He gas system, CAST scanned 141 discrete density
steps (15-Nov-06) reaching the value of 11.789 mbar at 1.8K (0.362 eV axion
rest mass).
 Studies and tests related with the 4He gas system operation.
Followed by the installation of a new miniature cryogenic pressure transducer,
associated with a low noise power supply and readout system, a series of tests
were done with diverse density fillings in the cold bore. After careful analysis of
the data it was confirmed the non-existence of thermoacoustic oscillations with
amplitude higher than 0.1% of the pressure setting, which is the background noise
of the pressure transducer. This meets the CAST specifications and reveals that
the dampers installed in the pipes for this purpose are working correctly.
A series of Computational Fluid Dynamics simulations were done to investigate
the effects of heat transport from hot regions to the cold bore due to convectional
heat transfer. These revealed a small density decrease in the very ends of the
magnetic length, due to heat transport from X-ray window region. These results
will be integrated in the evaluation of coherent conversion efficiency of axion-tophoton.
Measurements were done to investigate the composition of the frozen gases being
accumulating on the vacuum side of the windows during cyclic deposition by
cryopumping and outgassing due to warming up of the regions, associated with
the vertical movement of the magnet; these effects were enormously reduced by
additional pumping capacity and periodic bakeout.
Due to the convectional heat transfer on the gas inside the cold bores extremities,
a temperature drop of the window flange was observed with increasing gas
pressures; this phenomenon is now under study to estimate future implications
when the upgrade to 3He gas system is made.

Technical design of the 3He gas system.
The technical design of the future 3He gas system was finalized and the TDR was
written [document: CERN-SPSC-2006-029]. This complete document condenses
the constraints for the design and requirements of CAST, it describes the
functions of the system, specifications of the main components and typical modes
of operation; it also presents the project planning and cost estimate.
The technical design review was held in the middle of October with a panel of
internal and external referees covering the main technical areas. This panel is in
preparation of recommendations to be taken in consideration in the design of the
system. Due to the complexity of the 3He gas system, an intervention inside the
cryostat will be necessary. This includes modification of main gas lines,
introduction of cryogenic valves and cold pressure sensors. A study of the
integration of main components was started.
Objectives for the year 2007
The activities in the year 2007 will be governed by the construction, commissioning
and operation of the final 3He gas system. The construction of the 3He system will
imply a major intervention on the CAST experimental area that will start in the
beginning of 2007, beforehand the components selected and procured. Collaboration
with CERN experts from different technical fields will be held closely, such as
mechanical integration, vacuum and cryogenics, instrumentation, controls and safety,
etc.
Figure 3: Schematics of the future 3He gas system
The intervention will last the first quarter of the year and will include integration of
components inside and outside the cryostat and collaborating closely with experts
from different fields.
Figure 4: Design studies of the integration of the new components inside the cryostat
The commissioning of the system will be done in the second quarter and will include
tests of the several metering modes. The data taking period is due to last all second
half of 2007, and a request for extension through 2008 and 2009 was submitted to
SPSC recently.
Status of the new Micromegas/optic beamline
Description of the line
The realisation of the new line consisting on a new detector coupled to an x-ray optic
with an adapted shielding has been finalised. The aim of this new line is to profit from
the increase of sensitivity gained by the coupling of a Micromegas detector to the Xrays optics that reduced the spot on the detector from the width of the magnet bore to
a few mm leading to a significant reduction of the background. The new smaller
detector will profit from higher efficiency due to the use of a heavier gas (Xe-based
mixture). The shielding will protect
the Micromegas from a fraction of
the cosmic rays and natural
radioactivity.
The complete line was brought to
the PANTER x-ray test facility
(Munich) for characterisation tests
and calibration end of August.
Some preliminary results are
described in the following sections.
LLNL X-ray concentrator
for
the
new
Micromegas/optic beamline
LLNL completed fabrication of the
X-ray concentrator in August 2006.
Figure 5: LLNL concentrator located inside vacuum pipe and
manual adjusters.
The completed optic with its mounting fixture is shown in Figure 5. The optic
consists of 14 concentric polycarbonate layers, each in the shape of a truncated cone.
The inner surfaces of the cones are coated with approximately 350 Å of iridium to
ensure high photon reflectivity in the 1–10 keV energy range. The four large
adjusters allow accurate positioning of the concentrator’s optical axis with the
centerline of the magnet bore.
Once the optic was finished, the optic was calibrated at the PANTER X-ray test
facility located in Munich and operated by MPE. The measurement campaign
included characterization of the point spread function (PSF) and effective area (i.e.,
the throughput).
Unfortunately,
the
measured
performance of the optic did not match
that predicted from simulations. The
half-power diameter—a measure of the
PSF—was 250% larger than expected.
Capturing the flux in this larger focal
spot would require a larger detector
area, and hence, lead to a higher
background rate.
This non-optimal
performance would have been
acceptable, had it been the only issue.
Unfortunately,
the
throughput
(effective area) was also sub-optimal.
Figure 6 compares the measured
effective area versus energy with the
predicted values. The significant loss
in throughput precluded the use of this
optic in CAST.
Figure 6: Effective area versus energy for the LLNL
concentrator. The solid dots are the measurements, the solid
red curve the predicted performance. Geometric errors and a
putative contamination layer result in reductions that appear to
fit the data.
Once it was decided that the optic could not be used in the experiment, destructive
tests were performed to discover the origin of the problems. We have discovered
three distinct factors which have degraded the performance. This first has to do with
the roughness introduced during the deposition of the iridium on the polycarbonate.
This is the predominant reason for the increase in the spot size. The other two factors
reduce the throughput. The first of these factors has its origin in the methods used to
fabricate the substrates. We refer to these problems as geometric errors. The second
factor is the apparent presence of a contaminant on top of the iridium coating. This
preferentially absorbs low-energy photons. The relative effective of each factor is
shown in Figure 6.
LLNL is currently finishing tests to verify the origin of the errors and determine
methods to correct them. We are optimistic that we complete this work by the
beginning of 2007, and can modify our optical fabrication to ensure significant
improvements in a second concentrator. Our hopes are to have an optic ready for
calibration in the first half of 2007, with installation at CAST to follow immediately
after.
CAST data taking protocol for phase II
Due to the low statistics of the CAST data taking runs in phase II (only about 1.6
hours of data per density step in 4He phase, and even less next year in the 3He phase),
problematic events of moderate statistical significance may sometimes appear, caused
either by pure statistical departures, unknown or difficult-to-control systematic
effects, or eventually by a real axion signal. The course of action after such
occurrences must be carefully planned a priori, because the practice of allocating
extra exposure time on-the-fly for those density steps may lead to an inefficient use of
the available time or even to an intolerably long running time to finish the full CAST
scanning programme.
Therefore, in order to make an optimal use of the available exposure time in the
search for axions during the density scanning of phase II, a data taking protocol has
been investigated. This protocol aims to regulate in a strict and objective way the
sequence of actions after this kind of occurrences. In particular, such a protocol will
allow us:
 to follow a consistent strategy all throughout the scanning of axion masses.
 to be more efficient in the day-to-day operation of the experiment, leaving less
room for subjective appreciations on run coordination issues.
 to optimize the discovery potential of the experiment for a given fixed
available exposure time.
The scheme being studied has two main ingredients: a trigger condition to mark an
occurrence worthwhile of being further explored, and therefore to perform one or
more repetition of the density step in question; and a second condition to stop the
sequence of repetitions and resume the scanning (once the occurrence is considered to
be washed out statistically). Several ways to define these two conditions have been
considered, studying by means of simulations how much extra time they require and
especially how they affect the overall discovery potential of the experiment.
Optimization arguments as well as other practical considerations constrain the
freedom in the definition of both conditions. Those insights have been already very
useful for the coordination of CAST during the last months, although discussions are
still ongoing within the collaboration to finally define the protocol to be eventually
used. It should be soon ready and at work for the remaining scanning of CAST.
DETECTORS
TPC
The Time Projection Chamber (TPC) of CAST is operating continuously from the
beginning of CAST second phase. The TPC is a wire chamber having Ar-CH4 gases
as mixture in the active volume that makes use of the ALICE Front-end Digital Card
prototype electronics.
Experimental setup
The TPC is installed in one side of the magnet of the CAST experiment covering both
magnet bores. The TPC look for axions coming from the Sun that would convert into
photons in the magnetic field due to the Primakoff effect. The TPC was operating in
CAST first phase and its performance has been improved for the second phase.
Differential pumping
The development of two new differential windows made of 4 μm polypropylene foil
allowed to improve the TPC-leak rate towards the magnet achieved for the first phase
of CAST.
2004 LEAK RATE (TPC+diff.)
Ф(Ar) = 1.46 x 10-7 mbar.l/sec
Ф(CH4) = 4.6 x 10-8 mbar.l/sec
2005 LEAK RATE (TPC+diff.)
Ф(Ar) = 2.9 x 10-8 mbar.l/sec
Ф(CH4) = 1.3 x 10-9 mbar.l/sec
IMPROVEMENT RATIOS COMPARED
WITHOUT DIFFERENTIAL PUMPING
α (Ar) ~ 690
β (CH4) ~ 700 TPC ratio
The system achieved a decrease by a factor of 690 the leak rate for Ar and by a factor
of 700 for CH4 related to the leak rate of the TPC detector. Second phase setup
improves a factor 5 for Ar and a factor 35 for CH4 related to the best leak rate
measured in 2004.
This improvement has become crucial in CAST second phase, due to the existence of
cold windows keeping the He gas needed to restore coherence for the axion-to-photon
conversion. Higher leak rates would produce atomic layer deposition on the cold
windows inside the magnet, and therefore reducing the efficiency of the experiment.
Low voltage
A new quadrupolar power supply has been adapted to the preamplifiers of the TPC.
The new device, a TENMA 72-6905 has low ripple and noise (<2mVrms) being the
line regulation minor than 5mVolts. The power supply is equipped with and overload
and reverse polarity protection allowing the TPC DAQ to decrease the noise levels of
2004; this made the operation of the preamplifiers safer avoiding the loose of channels
due to voltage spikes.
Passive Shield
The installation of a passive shield that was already installed in 2004 has been
reproduced for the second phase of CAST and the final shield configuration for the
TPC detector is composed of several layers of material in order to reduce the
experimental background. The final setup of the shield is composed of 225 mm of
polyethylene, 1 mm of cadmium, 25 mm of lead and a 5 mm thick copper box
covering the TPC detector. A flush of nitrogen inside the copper box creates an over
pressure which helps, together with the plastic bag sealing the whole structure, to
decrease radon contamination.
See below (Figure 7) the comparison between 2005 and 2006 TPC background levels.
2005 BCKG
without shielding
2005 BCKG
First pieces
2005 BCKG
Copper box
+
Front part
2005 BCKG
No Radon Shielded
2006 BCKG
Radon Shielded
Figure 7: Comparison between the different levels of background that the TPC has related
with the different steps of the shielding installation. Black line shows the 2005 background
spectrum of the TPC without shielding; green line shows how the background changes as
soon as the first pieces of shielding are installed; blue line shows the background obtained
with the copper box and the front part of the shielding and pink line shows the background
spectrum once the shielding is completed. The difference between pink line and the wide
black line is due to the active shielding against Radon by flushing with Nitrogen gas at 250
l/hour the volume of the TPC.
Efficiency
In order to restore the coherence condition for the probability function of an axion-tophoton conversion, CAST needs to keep Helium gas in the magnet bores. To do this,
CAST has installed cold windows to keep the gas in the field region of the magnet.
The windows are made of 14 μm of polypropylene together with a stainless steal
strong back. These windows reduce the efficiency of the experiment, and each
detector has a new efficiency curve that includes the absorption of the 14 μm
polypropylene layer and the strong back area. See below the comparison of
efficiencies for TPC across the CAST experiment history (Figure 8)
Efficiency evolution
1
0.9
0.8
0.7
Efficiency
0.6
0.5
0.4
2003 (61.78%)
0.3
Panter (62.33%)
2004 (59.04%)
0.2
2005-2006 (52.1%)
0.1
0
0
500
1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500
Energy (eV)
Figure 8: Efficiency curves for TPC for CAST Phase I & II.
Data taking
The TPC detector is operating during the second phase of CAST since November
2005. The first set of pressure steps were taking from November 22nd till December
14th 2005. During this period TPC had noise problems with the old power supplies of
the preamplifiers and some of them stopped working due to spikes of the low voltage.
1.50E-04
1.40E-04
counts/keV/cm2/sec
1.30E-04
Background [2-10] keV
1.20E-04
Tracking [2-10] keV
1.10E-04
1.00E-04
9.00E-05
8.00E-05
7.00E-05
6.00E-05
5.00E-05
0
10
20
30
40
50
60
70
80
90
100
Pressure Setting
Figure 9: Evolution of the total number of counts in the range from 2 to 10 keV of the
energy spectrum of the TPC 2nd Phase data for all the pressure settings taken from November
22nd 2005 till September 9th 2006. The blue vertical line indicates the exchange of noisy
preamplifiers. Red vertical line is shows the moment at which Radon shielding was
improved.
This problem was solved for the next period of data taking that took place from
January 19th of 2006 till February 2nd of 2006. From April 29th till May 14th the third
data taking period of CAST second phase took place and an improvement of the
shielding against Radon was installed, lowering thus the level of the background in
the analog spectra. In Figure 9 it is shown the total number of counts per keV per cm2
and per sec of the TPC during all the trackings of CAST phase II.
5.59E-05
counts/keV/cm2/sec
3.59E-05
1.59E-05
-4.10E-06 0
-2.41E-05
10
20
30
Substracted
40
50
60
70
80
90
100
Lineal (Substracted)
-4.41E-05
Pressure Setting
Figure 10: Subtraction of tracking over background in the range from 2 to 10 keV for all
the pressure settings taken in the period November 22nd 2005 till September 9th 2006.
1.80E-04
1.60E-04
1.40E-04
BCKG in P
counts/keV/cm2/sec
1.20E-04
TRACKING as BCKG
1.00E-04
weighted all
8.00E-05
6.00E-05
4.00E-05
2.00E-05
0.00E+00
0
2
4
6
8
10
12
14
Energy (keV)
Figure 11:
TPC - background over the experimental area is position-independent due to
the shielding..
MM
Micromegas operation and preliminary analysis of the CAST second phase data.
During the first two runs of the CAST second phase in November-December 2005
and in January-February 2006 the Micromegas detector was installed in identical
setup as in 2004. Data was taken continuously and the operation was very good and
stable. The achieved level of background was about 4 × 10-5 counts/keV/cm2/s. The
detector was dismounted during the winter shutdown and during installation for the
third period of data taking at the end of April and incident forced the replacement of
the detector. The design of this new detector was identical to the one used for the first
runs except for an improved feature: the amplification copper mesh was coated with
gold with the purpose of stopping the 8 keV photons coming from copper
fluorescence that dominated the background. This detector has been operating very
reliably and the only drawback is the presence of a few dead strips causing the strip
signal to be slightly degraded with respect to the detector used at the beginning of the
year. However, the gold coating has had the expected beneficial effect on the level of
background and the copper peak at 8 keV has almost disappeared as it can be seen in
Figure 12a. During the data taking the detector showed a remarkable stability as seen
in Figure 12b and was operational for more than 98 % of the data taking period.
(a)
(b)
Figure 12: (a) Preliminary background spectra for the 2006 data, (b) Stability plot : mean
value of the calibration as a function of time
Preliminary results of the Micromegas detector at the PANTER x-ray facility
During the tests at the PANTER X-ray facility, the Micromegas line was mounted on
the beam line as shown in Figure 13. The aim was to go realise a complete test of the
system and to go through the alignment procedure.
Figure 13: The Micromegas beam line at the PANTER X-ray facility
The detector was tested with photon beams of varying energy using the standard gas
(Ar -5% Isobutane). Figure 14 shows the linear behaviour observed as a function of
energy. A clustering
Energy scanning
350
300
ADC bins
250
200
150
100
50
0
0
1
2
3
4
Energy (keV)
5
6
7
8
Figure 14: Response of the detector as a function of energy.
analysis in the strips was carried out adapting the analysis code used for the actual
detector at CAST. Different tests were performed with the detector alone or with the
detector coupled to the telescope. Figure 15 shows the spatial distribution and the
multiplicity obtained with a 6.4 keV beam on the detector. The multiplicity
distribution of the strips follows the expected pattern with an approximated symmetric
charge collected between X and Y strips. Figure 16 shows the Micromegas window
and some X-ray finger measurements during the alignment of the telescope. A new
gas mixture (Xe+He+Isobutane) was tested with successful results. Figure 17 shows
the energy resolution for a photon beam of 8 keV. This result can be improved by
optimising the drift and mesh voltages. It is expected that next year a complete set of
calibration tests will be repeated for the detector and the detector coupled to the new
optics at PANTER .
.
Figure 15:
Response of the detector to a 6.4 keV photon beam. On the left the spatial
distribution of the hits and on the right the strip multiplicity are shown.
Figure 16:
Response of the detector to a 8 keV photon beam. On the left the spatial
distribution of the hits and on the right the strip multiplicity are shown.
Figure 17: Energy resolution for a 8 keV photon beam using the Xe based mixture.
CCD
The pn-CCD detector was operated during the 2005/2006 data taking period in the same
configuration as during the 2005 data taking phase. Both, the detector performance and the
background count rate were stable at a level of (8.69 ± 0.06)•10-5 counts /cm2 / sec / keV,
which corresponds to (0.248 ± 0.013) counts per tracking run.
In a recent intervention in October 2006, the vacuum system of the CCD detector and the Xray telescope has been extended and upgraded. The control software of the telescope system
has also been updated to a safer and more flexible system for the He 3 operation. This
includes the implementation of an interlock to and from the magnet vacuum system, full
remote access, and a more structured user interface.
Regular measurements with an X-ray source were made during the data taking period to
verify the alignment of the X-ray optics, based on reference measurements made in 2004 and
2005. These measurements have verified the stability of the spot position on the CCD chip
within the required accuracy. The performance of the X-ray telescope system during the
2005/2006 data taking period was as expected, reaching its maximum sensitivity.
The results obtained with CCD / XRTelescope are shown in the following Table and Figures.
Other CAST related information
During the last year three specialized axion workshops have taken place: at CERN, at the
University of Patras and at the IAS / Princeton. While the first 2 were organized by CAST, in
the 3rd one, 4 invited CAST members made presentation. Since some time there is ongoing
wide discussion trying to reconcile CAST and PVLAS result. Axions are at present in the
spotlight of a world wide activity. CAST seems to be a well recognized experiment also
outside CERN. In June 2007 there is scheduled the next axion training-workshop at the
University of Patras, which will be co-organized also with DESY and the EU-ILIAS network:
this time the lectures will cover axions and WIMPs.
CAST has submitted to SPSC/CERN an memorandum, providing the physics justification and
other detailed information, asking to extend its data taking period into 2008–2009. Nearly all
CAST collaborating institutes agreed on this request.