1. The MICROMEGAS detector in CAST
Theopisti Dafni
IKP/Technische Universität-Darmstadt,
Large TPC Workshop December 2004,
Paris

2. Introduction :Axions-CAST MICROMEGAS in CAST
Outline:
Introduction :Axions-CAST
MICROMEGAS in CAST
Description
Results from 2003 data
Improvements in 2004
Further steps
Paris, 2004

3. The strong CP problem: Peccei-Quinn solution:
Axions
PRIMAKOFF effect
The strong CP problem:
Theory redicts violation of CP in the strong interactions, that the experiment contradicts
Peccei-Quinn solution:
New symmetry; spontaneously broken (t scale fα) introduces axion
K. van Bibber et al.,1989
Axion: pseudoscalar, neutral, practically stable
-conversion probability:
Paris, 2004

4. Cern Axion Solar Telescope
X-ray
detector
Transverse magnetic field (B) L
TPC mM
±8°
CCD+Telescope
B = 9T
L = 9,26m
±40°
Paris, 2004

5. X-Ray Telescope (Focusing Device) + CCD
X-Ray Detectors:
Detectors
X-Ray Telescope (Focusing Device) + CCD
TPC
MICROMEGAS
Signal: X-rays while tracking (magnet pointing to the Sun) over background
Paris, 2004

6. Micromegas
In situ…
…and on the table
Paris, 2004

7. Micromegas The CAST Prototype Magnet
4µm polypropylene
Pumping group (required leak rate < 10-5 mbar l sec-1)
4µm aluminized polypropylene + strongback
Read-out:
1st: Mesh
2nd: Strips
Paris, 2004

8. CAST Prototype: FIRST time the x-y read-out used
Micromegas
CAST Prototype: FIRST time the x-y read-out used
~70 mm
~310 mm
Via of
~75 mm
192 charge collection strips for x
192 charge collection strips for y
Paris, 2004

9. Micromegas : diff.pumping
Zone C
Tube
P = 10-6 mbar
Zone A
Detector
P= 1 bar (Ar)
Zone B
In-between chamber
P = 10-2 mbar P1 P2 A B C
Gate valve
Micromegas : diff.pumping
4µm poly
4µm aluminized poly+ strongback
micromesh
Clean materials (low natural radioactivity) : mechanical pieces out of plexiglas
Paris, 2004

10. Mean value of 5.9keV peak vs time
Analysis 2003 data:
Gas: 95%Ar+5%Isobutane
Calibration 55Fe
Mean value of 5.9keV peak vs time
5.9 keV
~ 16% (FWHM)
Detector Stable
95Ar escape peak
Paris, 2004

11. Conditions based on pulse shape and strips’ information:
Analysis 2003 data:
Conditions based on pulse shape and strips’ information:
X-Y strips Multiplicity
Correlation between strip charge and Pulse charge
Pulse rise time information
Fiducial
Dead time :~ 4%
Software efficiency:
SetA 3keV: 94.5% 6keV: 92%
SetB 3keV: 96% 6keV: 97%
SetC 3keV: 86% keV: 85.5%
Paris, 2004

12. GOOD (Calibration) pulse
Analysis 2003 data:
Bkg pulses
Pulse info
GOOD (Calibration) pulse
Paris, 2004

13. Analysis 2003 data: In one look
Efficiency *
Time (h)
Bkg Level **
(cts/s/cm2/keV)
Tracking Level**
Set A
53%
431.4
6.03x10-5
43.8
6.26x10-5
Set B
54%
121
8.22x10-5
11.5
8.14x10-5
Set C
48.5%
251
7.96x10-5
21.8
7.67x10-5
*: This is the efficiency over the axion spectrum (hardware, software), including the change in the hardware efficiency mentioned.
**: Between 1-7 keV
Paris, 2004

14. Analysis 2003 data: Superimposing…
--Tracking (magnet aligned to the sun)
--Background (any other time)
Paris, 2004

15. Analysis 2003 data: Combined for the 3 sets
gαγγ(95% CL):1.51x10-10 GeV
Paris, 2004

16. ga (95% C.L.) < 1.1610-10 GeV-1 CAST RESULTS 2003
Combined upper limit for 2003 data for all 3 detectors
Submitted to PRL
hep-ex/
Phase I:
1 year with vacuum in the magnet bores: m < 2.310-2 eV (during 2003 and 2004)
Phase II
1 year with 4He gas pressure increased from mbar :m < 0.26 eV
1 year with 3He gas pressure increased from mbar : m < 0.83 eV
Paris, 2004

17. 2004 run: Improvements 2003 -- Very few dead strips (< 3 )
-- No cross talk between x-y strips
-- New electronic cards, new decoding
-- Completely new cabling
Preliminary
Efficiency
@ 6 keV ~88 %
@ 3 keV ~68 %
392 h
538.8 h
Efficiency
@ 6 keV ~93 %
@ 3 keV ~89 %
2.2× 10-5 counts keV-1cm-2 s-1
5 × 10-5 counts keV-1cm-2 s-1
Paris, 2004

18. Use of X-ray focusing optics
MICROMEGAS PHASE II :
Poly? mM
Shielding (factor ~ 4 in background)
(in collaboration with Zaragoza)
Improved detector efficiency.
--By using heavier gas: Xenon (to be studied in Saclay)
--Optimization of the gas (to be studied in Athens)
--Not geometrical factor in the window transmission due to strongback
Improved intrinsic background (“golden strips”)
--Shield the copper surfaces directly in contact with the detector (strips) with a thin layer of gold.
Use of X-ray focusing optics
Paris, 2004

19. MICROMEGAS PHASE II : Optics
Goal: Xrays focused on a diameter < 1 mm
In collaboration with Livermore, NASA and Uni Chicago
pump mM
Tube
VT3 gate
Cold window
Focusing Device
Paris, 2004

20. MICROMEGAS on a telescope
At the PANTER facility in Munich in 2002
Paris, 2004

21. MICROMEGAS on a telescope Tests in Munich
6.5 keV γ + Panter telescope
Beam of photons of different
energies available keV
X-ray detection threshold : 0.6 keV
Energy resolution 6 keV
Background (after off-line analysis):
3 × 10-6 counts keV-1cm-2 s-1
for 47% efficiency
Paris, 2004

22. CAST end of phase I last November
Conclusions
CAST end of phase I last November
MICROMEGAS has successfully operated during 2003 and 2004 looking for axions
2003 result presented data under analysis
Work on progress to face CAST phase II. Improvements on detector: telescope, shielding…
Paris, 2004

23. And yet…
Paris, 2004

24. This is the end, my friend
Paris, 2004

25. A focusing device for MICROMEGAS?
pump mM
Tube
VT3 gate
Cold window
Optics
Paris, 2004

26. X-ray conversion efficiency
Dead time :~ 4%
Software efficiency:
SetA 3keV: 94.5% 6keV 92%
SetB 3keV: 96% keV 97%
SetC 3keV: 86% keV 85.5%
Hardware efficiency: strongback transparency 95%
Paris, 2004

27. Micromegas : Background
Most of the present background comes from cosmics and X-ray fluorescence (mainly Cu)
Paris, 2004