1. The CAST Micromegas detectors for the detection of rare events
Esther Ferrer Ribas
9th February 2015
©Firat Yilmaz
Esther Ferrer Ribas
Instrumentation days on gaseous detectors, October 2016, Clermont Ferrand

2. CAST: CERN Axion Solar Telescope
Baseline search: solar axions
Why we are looking for axions?
Predicted by SM extensions, neutral, very light, low interacting cross-section.
Most elegant solution to explain the apparent symetry between matter and anti-matter in the strong interactions (CP violation).
Dark matter candidates
Axions couple to photons in the presence of a magnetic field in all models.

3. Search strategies Relic Axions Solar Axions Axions in the laboratory
Axions that are part of galactic dark matter halo:
Axion Haloscopes (ADMX)
Solar Axions
Emitted by the solar core.
Axion Helioscopes (CAST  IAXO)
Crystals
Axions in the laboratory
“Light shinning through wall” experiments
Vacuum birefringence experiments
(see Carlo Rizzo’s presentation)

4.  0.3 evts/hour with ga= 10-10 GeV-1 and A = 14 cm2
CAST Physics
Detection in CAST
Conversion of axions into photons via the inverse Primakoff effect in a strong magnetic field
Production in the Sun
Conversion of thermal photons into axions via Primakoff effect in the solar core
Expected number of photons:
 0.3 evts/hour with ga= GeV-1 and A = 14 cm2

5. Helioscopes
Brookhaven (a few hours of data): Lazarus et al. PRL 69 (92)
Tokyo Helioscope (SUMICO): 2.3 m long 4 T magnet
No Liq. He
B=4T, L=2.3m
268A persistent current
16 PIN photodiodes
Altazimuth:Horiz. 360°, vert.±28°
Inoue et al. Phys.Lett.B668:93-97,2008.
Presently running: CERN Axion Solar Telescope (CAST)

6. Rotating platform to follow the sun
Search for solar axions with an LHC dipole
LHC dipôle : L = 9 m, B = 9 T
Rotating platform to follow the sun
2 sunset detectors
2 Sunrise detectors
Signal: excess of X-rays when pointing towards the sun
Need for X-ray detectors:
Excellent background discrimination
(natural radioactivity and cosmic rays)
Radiopure components+shielding+offline discrimination

7. Originalities of CAST
Use of X-ray telescope  increase S/B noise sensitivity improved by a factor 150 by focusing a ø43 mm x-ray beam to ø3mm
Low background techniques shieldings, low radioactive materials, simulation and modeling of backgrounds….

8. CAST results 2003 – 2004 CAST phase I 2006 CAST phase II - 4He Run
CAST Coll., JCAP 0704(2007) 010.
CAST Coll., PRL (2005) 94,
CAST Coll., JCAP 0902 (2009) 008.
CAST Coll, PRL (2011)
CAST Coll., Phys. Rev. D92 (2015) no2,
2003 – 2004
CAST phase I
vacuum in the magnet bores
2006
CAST phase II - 4He Run
axion masses explored up to eV (160 P-steps)
2007
3He Gas system implementation
CAST phase II - 3He Run
axion masses explored up to 1.17 eV
bridging the dark matter limit
2012
Revisit 4He Run with improved detectors
Revisit vacuum phase with improved detectors
New data in the pipeline.
Analysis ongoing…
The axion has not been observed limit on the coupling constant
Best world-wide limit for a wide range of masses

9. CAST Micromegas detectors
Drift: transparent X-ray window coupled to a vacuum pipe
Use of mesh and strip signal
Kapton 2D readout signal

10. Background History of CAST micromegas detectors
Classical Unshielded
Shielded Microbulk
Shielding Upgrade/muon veto

11. CAST Micromegas detectors: 2003-2006
•Active area:
6x6 cm2 with 2D readout 384 strips pitch 350 µm
•Gas: Ar + 5%iC2H4 at atmospheric pressure.
•Conversion region: 3 cm (100 V/cm).
•Amplification gap: 50 μm (104 V/cm).
•X-ray window: 4 μm aluminized mylar
•GASSIPLEX electronics
Unshielded detector
P. Abbon et al., New J.Phys.9:170,2007, physics/
Materials: plexiglass, copper mesh, kapton readout plane

12. Shielding test April 2004 Background reduction ~3 Polyethèlene Copper
Lead
Background reduction ~3

13. Detectors after 2006: shielding + new Micromegas technology
Sunrise Side: shielded Micromegas
Sunset Side: 2 shielded Micromegas
New Micromegas technology: Microbulk
Shielding: copper+ lead+polyethylene

14. Microbulk technology Intrinsically radiopure (kapton and copper only)
Read-out plane and mesh ALL IN ONE
Micromesh
5µm copper
Kapton 50 µm
Read-out plane
S. Andriamonje et al., JINST 5 (2010) P02001
Intrinsically radiopure (kapton and copper only)
S. Cebrian et al., Astr . Part. 34 (2011) 354
Microbulk used in CAST but also in NTOF, PANDAX
J. Galan et al., JINST 11 (2016) no.04, P04024

15. Detectors after 2006 : shielding + Microbulk technology
Detector description
Active area: 6x6 cm2 with 2D readout 240 strips pitch 550 µm
Gas: Ar + 5%iC2H4 at 1.4 bar.
Conversion region: 3 cm (100 V/cm).
Amplification gap: 50 μm (104 V/cm).
X-ray window: 4 μm aluminized mylar
GASSIPLEX electronics
Shielding
0.5 cm of Copper
2.5 cm archeological Lead
Polyethylene bricks (variable thickness up to 10 cm)
Enclosed in plexiglass box flushed with nitrogen
S. Aune et al., JINST 9 (2014) 9 P01001.

16. Detectors after 2006 : results
Background rejection
Rejection of at least two orders of magnitude

17. Detectors after 2006 : results
Stability sunrise Detector
3 years !!!!

18. Detectors after 2006 : measurements in the underground laboratory of Canfranc
Different shielding configurations

19. Detectors after 2006 : measurements in the underground laboratory of Canfranc
Testing different materials

20. Detectors after 2006 : modelling the background
Simulated CAST area gamma flux
Production of background counts by different interactions

21. Detectors after 2006 : adding an active shielding

22. Sunrise detector 2014-2015: new design, new electronics, X-ray optics
Use the 10 years of experience to design an optimised detector
Couple to an optimised X-ray optics for solar axion search
Based on the X-ray optics for NUSTAR satellite
F. Aznar et al., JCAP 12 (2015)

23. Sunrise detector

24. Sunrise detector 2014-2015 X-ray telescope calibrated in Summer 2016
Data analysis on going

25. AFTER CAST: IAXO (International Axion Observatory)
JCAP 06 (2011) 013
CAST is established as a reference result in experimental axion physics CAST PRL 2004 most cited experimental paper in axion physics
No other technique can realistically improve CAST in such wide mass range.
Future helioscope generation: improve the sensibility by 1 order of magnitude
LOW
BACKGROUND
DETECTORS
X RAYS
OPTICS
MAGNET

26. IAXO International Axion Observatory
No technology challenge (built on CAST experience)
New dedicated superconducting magnet
Use of X-ray focalisation over ~m2 area
Low background detectors (improve bck by 1-2 orders of magnitude)
Goal: in terms of signal to background ratio 4-5 orders of magnitude more sensitive in than CAST, which means sensitivity to axion-photon couplings down to a few ×10−12 GeV−1
IAXO baseline detectors: Microbulk technology
Key elements: Radiopure components + shielding + Offline discrimination
E. Armengaud et al., "Conceptual Design of the International Axion Observatory", JINST 9 (2014) T05002.

27. IAXO Sensibility Exploration of very extended QCD axion region
IAXO complements
ADMX
Exploration of very extended QCD
axion region

28. IAXO Detectors
Micromegas detectors are the base-line for IAXO but additional tecnologies, Ingrid Micromegas, TES, MMC, CCD will also show their potential in the coming years
Improvements foreseen for Micromegas to achieve background levels of 10-7 – 10-8 c keV-1 cm-2 s-1
Optimised gas to reduce the fluorescences peaks
Improved shielding
Autotrigger electronics

29. CONCLUSIONS Succes of Micromegas detectors in CAST:
Development of a new Micromegas technology « Microbulk »
Intrinsically radiopure
Stable
High topological discrimination
Low background techniques (shielding, underground laboratories, background simulations…)
Collaborations: CERN MPGD/ U. Zaragoza/LLNL/IRFU
Solar axion search goes on: IAXO
IAXO can become a generic axion facility with discovery protential in the next decade
Exciting work in front us: join us!

30. BACK UP

31. Succes of Micromegas detectors in CAST
2002
2013
Succes of Micromegas detectors in CAST
Development of a new Micromegas technology « Microbulk » (kapton and copper) very radiopure
Collaborations:
CERN MPGD
U. Zaragoza
Low background techniques (shielding, underground laboratories, background simulations…)
Student network between Zaragoza-Irfu
First experience with Micromegas for rare event detection:
Now Micromegas detectors used in a wide variety of applications (dark-matter, double beta…)

32. Microbulk used in CAST but also in NTOF, PANDAX
Microbulk Micromegas
Read-out plane and mesh ALL IN ONE
Micromesh
5µm copper
Kapton 50 µm
Read-out plane
Intrinsically radiopure
(kapton and copper only)
S. Cebrian et al., Astr . Part. 34 (2011) 354
S. Andriamonje et al., JINST 5 (2010) P02001
Small gaps (< 50 µm)
Optimised for high pressure
D. Attié et al., JINST 9 (2014)C04013.
2D detectors ultra-thin: segmented mesh
Rare event detection, Neutron profiles
T. Geralis et al., PoS TIPP2014 (2014) 055
Microbulk used in CAST but also in NTOF, PANDAX
J. Galan et al., JINST 11 (2016) no.04, P04024

33. Micromegas family Mesh Readout plane TWO mechanical entities
STANDARD
1996
BULK
2003
INGRID
2005
MICROBULK
2006
Mesh
Readout plane
TWO mechanical entities
INTEGRATED:
ONE single entity
Type of mesh
Any
type
30 µm
Stainless steel
1 µm
Aluminium
5 µm
Copper
Advantages
Demontability
Large Surface
Robust
Industrial manufacturing process (PCB)
Excellent energy resolution
Single electron efficiency
Intrinsically Flexible
Low mass
Radiopure

34. Historical context MWPC TPC Micro Pattern Gaseous Detectors: MPGD
Multi-Wire Proportional Chamber
G. Charpak et al., 1968
TPC
Time Projection Chamber
D. R. Nygren et al., 1974
ANODE
CATHODE
200 µm
MSGC
Micro-Strip Gas Chamber
A. Oed, 1988
Micro Pattern
Gaseous Detectors: MPGD
GEM
Gas Electron Multiplier
F. Sauli, 1997
MICROMEGAS
MICRO-MEsh GAseous Structure
I. Giomataris et al., 1996
CLASSICAL
1996
BULK
2003
INGRID
2005
MICROBULK
2006
RESISTIVE ANODE

35. IAXO Collaboration
~80 authors

36. IAXO costs

37. IAXO timeline Construction ~ 2.5 years Integration + commissioning
TDR:
~ 18 months

38. Mini Charged Particles
The WISPs zoo
Weakly Interacting Sub-eV Particles
(WISPs)
Hidden Photons (HPs)
Mini Charged Particles
Chameleons
Axion Like particles
(ALPs)
Axions
Stringy axion
Arion
Majoron
gag and ma are two independent “phenomenological” parameters

39. Extending sensitivity to higher masses
Axion to photon conversion probability:
Vacuum:
Γ=0, mγ=0
with
Coherence condition: qL < π
For CAST phase I conditions (vacuum), coherence is lost for ma > 0.02 eV
With the presence of a buffer gas it can be restored for a narrow mass range:
1.0 dP
e.g. for 50 mbar Δma ~ 10-3 eV 39

40. IAXO proto-collaboration
Big effort to strengthen collaboration  large consortium involved in a number of funding applications, covering all TDR needs
Zaragoza (Spain)
CEA-Saclay (France)
Mainz (Germany)
Bonn (Germany)
Heidelberg (Germany)
INR-Moscow (Russia)
LLNL
Columbia
MIT
INAF-Milano (Italy)
DTU (Denmark)
RBI (Croatia)
DESY (Germany)
U. Hamburg (Germany)
U.Valencia (Spain)
U. Cartagena (Spapin)
INFN-Frascati, (Italy)
U. Barry
S. Carolina
+ …
CERN
CEA/Saclay (France)
New partners welcome…
(*) Only shown groups for which formal activity is ongoing or under discussion/preparation.
Potentiaal interest in more groups than shown
IDM2016, Sheffield
Igor G. Irastorza /
Universidad de Zaragoza

41. 2003: CCD+Xray telescope, TPC (2), MM
2006: CCD+Xray telecope, 3 MM,
2013: 3MM, Ingrid+Xray telescope
: 2MM, MM+Xray telescope (designed for axion search) , Ingrid+Xray telescope