Lowering the background level and the energy threshold of Micromegas x-ray detectors for axion searches

0.18 MB·English

by  F. J. Iguaz

Lowering the background level and the energy
threshold of Micromegas x-ray detectors for axion
searches
F.J. Iguaz∗, F. Aznar †, J.F. Castel, T. Dafni, J.A. Garcia, J.G. Garza, I.G. Irastorza,
I. Ortega‡, A. Rodríguez and A. Tomas§
LaboratoriodeFísicaNuclearyAstropartículas,UniversidaddeZaragoza,Spain.
5 E-mail: [email protected]
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S. Aune, E. Ferrer-Ribas, J. Galan, I. Giomataris, D. Jourde and T. Papaevangelou
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n Serviced‘Electronique,desDétecteursetd‘Informatique,CEA-Saclay,Gif-sur-Yvette,France
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J M. Davenport and T. Vafeiadis

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CERN,EuropeanOrganizationforParticlePhysicsandNuclearResearch,Geneva,Switzerland

]
M S.C. Yildiz
I DepartmentofPhysics,DogusUniversity,AcibademKadikoy,Istanbul,Turkey
.
h
Axion helioscopes search for solar axions by their conversion in x-rays in the presence of high
p
- magneticfields.Theuseoflowbackgroundx-raydetectorsisanessentialcomponentcontributing
o
r tothesensitivityofthesesearches. Inthiswork,wereviewtherecentadvancesonMicromegas
t
s detectors used in the CERN Axion Solar Telescope (CAST) and proposed for the future Inter-
a
[ nationalAxionObservatory(IAXO).TheactualsetupinCASThasachievedbackgroundlevels

1 below10−6 keV−1 cm−2 s−1,afactor100lowerthanthefirstgenerationofMicromegasdetec-
v
tors.Thisreductionisbasedonactiveandpassiveshieldingtechniques,theselectionofradiopure
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6 materials,offlinediscriminationtechniquesandthehighgranularityofthereadout. Wedescribe
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indetailthebackgroundmodelofthedetector,basedonitsoperationatCASTsiteandattheCan-
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0 francUndergroundLaboratory(LSC),aswellasonGeant4simulations.Thebestlevelscurrently
1. achievedatLSCarelowthan10−7keV−1cm−2s−1andshowgoodprospectsfortheapplication
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of this technology in IAXO. Finally, we present some ideas and results for reducing the energy
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1 thresholdofthesedetectorsbelow1keV,usinghigh-transparentwindows,autotriggerelectronics
v: andstudyingtheclustershapeatdifferentenergies. Asahighfluxofaxion-like-particlesisex-
i pectedinthisenergyrange,asub-keVthresholddetectorcouldenlargethephysicscaseofaxion
X
r helioscopes.
a
TechnologyandInstrumentationinParticlePhysics2014
2-6June,2014
Amsterdam,theNetherlands
∗Speaker.
†Presentaddress:CentroUniversitariodelaDefensa,UniversidaddeZaragoza,Zaragoza,Spain
‡Presentaddress:CERN,EuropeanOrganizationforParticlePhysicsandNuclearResearch.
§Presentaddress:HighEnergyPhysicsgroup,BlackettLaboratory,ImperialCollege,London,U.K.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
Micromegasx-raydetectorsforaxionsearches F.J.Iguaz
1. Micromegasforaxionsearches
Axion helioscopes [1] aim to detect solar axions through their conversion by the inverse Pri-
makoff effect into x-rays (1-10 keV) using strong magnetic fields. The CERN Axion Solar Tele-
scope(CAST)[2]isthemostpowerfulexampleandhassetthebestexclusionlimitsintheaxion-
photon coupling for a wide range of axion masses [3, 4]. Three of the four CAST magnet bores
areequippedwithmicrobulkMicromegas(MM)detectors[5,6]astheyhavealightandradiopure
materialbudget[7];showanexcellentenergyresolution(12%FWHMat5.9keV)[8];haveahigh
discriminationpowertoselectx-rays(point-likeevents)frommuonsandgammas;canbeshielded
byapplyingthesametechniquesusedinrareeventsexperiments.
X-raydetectorsareanimportantissueforthefutureInternationalAxionObservatory(IAXO)
[9, 10]. Its main goal is to improve the signal-to-noise ratio more than 104 with respect to CAST,
i.e. 1-2 orders of magnitude in sensitivity to the axion-photon coupling. From the different im-
provementsforeseen,lowbackgroundx-raysdetectorsmustreachvaluesin10−7-10−8 s−1 keV−1
cm−2. MM technology is a perfect candidate according to the latest level reached in CAST-MM
during2013[11]andcurrentlythelimitachievedattheCanfrancUndergroundLaboratory(LSC).
In section 2, we will briefly describe the actual CAST-MM background model (exposed in
detail in [6]). We will thus present the physical motivation and the R&D for sub-keV CAST-MM
detectorsinsection3and,wewillfinishwithsomeconclusionsandprospects.
2. BackgroundmodelofCAST-MMdetectors
Intable1thedifferentcontributionstobackgroundmodelofCAST-MMdetectorsaredetailed.
Theyhavebeenidentifiedeitherbyin-situmeasurementsatCAST,byadetector’sreplicainstalled
at LSC or by Geant4 simulations. The main contribution is caused by muons, while the origin of
actuallimitsetatLSCisunknown. Somehypothesisareneutronsorthe39Arisotope.
Contribution Level(s−1 keV−1 cm−2) Shieldingtechniqueapplied
Before After
Gammaflux 7×10−5 None? Fullcoverageby10cmleadshielding
Radon 8×10−7 None Nitrogenfluxinsidetheshielding
Cosmicmuons 2×10−6 6×10−7 95%coveragebyanactivemuonveto
Alcathode 5×10−7 None Replacementbyanultrapurecoppercathode
LSClimit 1.1×10−7 Neutrons? 39Ar? Others?
Table 1: The different contributions to background level in 2-7 keV range identified in CAST-MM back-
ground model and the different shielding techniques applied to completely or partially remove them. For
clarity,valuesaregivenwithouterrors. Thefullmeasurementswiththeassociatederrorsarein[6].
3. Loweringtheenergythreshold
New calculations of axion production at the Sun by the so-called “BCA processes” in non-
hadronicaxionmodels[12]pointouttoanaxionfluxpeakingatenergiesaround1keV.Thisfact
motivatestheuseofsub-keVdetectorsinIAXO.Withthisaim,thereisaR&Dlinebasedon:
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Micromegasx-raydetectorsforaxionsearches F.J.Iguaz
Window The x-rays coming from the magnet enter the conversion volume via a gas-tight window
made of 5 µm aluminized mylar foil. This foil is supported by a metallic squared-pattern
strong-back,towithstandthepressuredifferencetothemagnet’svacuumsystem. Thisfoilis
transparentdowntoenergiesof1.5keV.Otherpossiblematerialslikepolyimide-basedfoils
arebeingstudied,astheirtransparencyisenlargeddownto0.7keV.
Gas OthermixturesthanAr+2%isobutanehavebeenstudiedtoincreasetheactualoperationpoint
(8×103),i.e,todecreasetheenergythreshold. Avalueof3×104 isreachedinneon-based
mixturesbutthepressuremustbeincreasedtomaximizethephotonconversion[8].
Electronics The actual one is based on the AFTER chip [13], which provides time information of each
strip. This feature improves the signal-to-noise ratio and have reduced the energy threshold
downto450eV.AutotriggersystemslikeAGET[14]mayfurtherenhancethisreduction.
Analysis The CAST-MM detectors have been calibrated in an electron beam at the CAST Detector
Laboratory [15]. In this setup, the fluorescence lines of different target materials ranging
from 2.3 (gold) to 8.0 keV (copper) are used to calculate the integrated signal efficiency
[11]. Thisdataalsoprovidesinformationonevent’stopology,whichcanbeusedintheanal-
ysis. To illustrate this issue, the distribution of the cluster’s width in Z (left) and the cluster
differencebetweenX&Ywidths(right)areshownrespectivelyinfigure1. Clustersarewider
atlowenergiesbecausemostofthex-raysmaybeabsorbedinthefirstmilimetersjustafter
the window and will suffer more diffusion effects. Cluster differences also increase at low
energiesaschargefluctuationsbetweenthetwodetectorplanes(XY)aremoreimportant.
5 7
4.5 2.3 keV 2.3 keV
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Number of events (%) 123...23555 4568....5990 kkkkeeeeVVVV Number of events (%) 2345 4568....5990 kkkkeeeeVVVV
1
1
0.5
0 0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Sigma in z direction (mm) Sigma difference in X-Y plane (-)
Figure1: Distributionofthecluster’swidthinZ(left,insigmaunits)andthecluster’swidthdifferencein
XY(right)forthedifferentfluorescencelinesusedinthex-raycalibrationofCAST-M18.
4. Conclusionsandprospects
During more than a decade in CAST, Micromegas detectors have reduced their background
levelsbytwoordersofmagnitude,reachingavalueof 7×10−7 keV−1 cm−2 s−1. Mostofcontri-
butions have been identified and removed applying shielding techniques used in rare event exper-
iments. The best levels currently achieved at LSC (a factor 7 lower) show good prospects for the
applicationofthistechnologyinIAXO.Recentcalculationsofaxionfluxmotivatetheuseofsub-
keVdetectorsinCAST.WehavepresentedthemainopenR&DlinesonthisissueforCAST-MM.
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Micromegasx-raydetectorsforaxionsearches F.J.Iguaz
Acknowledgments
WethanktoourcolleaguesofCASTformanyyearsofcollaborativework,R.deOliveiraand
histeamatCERNforthemanufacturingofthemicrobulkreadoutsandtheLSCstafffortheirsup-
portduringtheuseoftheirfacilities. WeacknowledgethesupportfromtheEuropeanCommission
under the European Research Council T-REX Starting Grant ref. ERC-2009-StG-240054 of the
IDEAS program of the 7th EU Framework Program. We also the acknowledge support from the
Spanish Ministry MINECO under contracts ref. FPA2008-03456 and FPA2011-24058, as well as
under the CPAN project ref. CSD2007-00042 from the Consolider-Ingenio 2010 program. These
grants are partially funded by the European Regional Development funded (ERDF/FEDER). My
personalgratitudeisforthesupportfromtheJuandelaCiervaprogram.
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