High-resolution kaonic-atom x-ray spectroscopy with transition-edge-sensor microcalorimeters

0.49 MB·

by  S. Okada

JournalofLowTemperaturePhysicsmanuscriptNo.
(willbeinsertedbytheeditor)
S.Okada · D.A.Bennett · W.B.Doriese ·
J.W.Fowler · K.D.Irwin · S.Ishimoto ·
4 M.Sato · D.R. Schmidt · D.S.Swetz ·
1 H.Tatsuno · J.N.Ullom · S.Yamada
0
2
High-resolution kaonic-atom x-ray

b
spectroscopy with transition-edge-sensor
e
F
microcalorimeters

9
1
26.07.2013

]
t
e
d Abstract We are preparing for an ultra-high resolution x-ray spectroscopy of
- kaonic atoms using an x-ray spectrometerbased on an array of superconducting
s
n transition-edge-sensormicrocalorimetersdevelopedbyNIST.Theinstrumenthas
i excellentenergyresolutionsof2–3eV(FWHM)at6keVandalargecollecting
.
s areaofabout20mm2.Thiswillopennewdoortoinvestigatekaon-nucleusstrong
c
interactionandprovidenewaccuratecharged-kaonmassvalue.
i
s
y Keywords Kaonicatom,X-rayspectroscopy,Transition-edgesensor
h
p
[
1 Introduction

2
v Anynegativelychargedleptonsorhadrons(e.g., m −,p −,K−, p,S −)apartfrom
9
the conventional electrons can be bound by the Coulomb field of an atomic nu-
3
cleus.TheCoulomb-boundsystem,so-calledexoticatom,isessentiallyahydrogen-
0
8 likeatomexceptforitsshortlifetimeandthelargereducedmassresultinginun-
. usuallysmallorbitalradii.Inthecaseofahadronfeelingstronginteractionwith
1
theatomicnucleus,so-calledhadronicatom,becauseofbeingshort-range force,
0
theeffectsappearinthemosttightlyboundenergylevelbeingthemostperturbed
4
1
: S.Okada·M.Sato·S.Yamada
v RIKENNishinaCenter,RIKEN,Wako,351-0198,Japan
i E-mail:[email protected]
X
r D.A. Bennett · W.B. Doriese · J.W. Fowler · K.D. Irwin · D.R. Schmidt · D.S. Swetz ·
a J.N.Ullom
NationalInstituteofStandardsandTechnology,Boulder,CO80303,USA
S.Ishimoto
HighEnergyAcceleratorResearchOrganization,Ibaraki305-0801,Japan
H.Tatsuno
LaboratoriNazionalidiFrascati,INFN,I-00044Frascati,Italy
2
by the strong force as a shift from that given only by the electromagnetic in-
teraction,and a broadening due to absorption of the hadron by the nucleus. The
shiftandwidthcanbeexperimentallyextractedfromcharacteristicx-ray-emission
spectroscopyofthehadronic-atomx-raysfeedingthelow-lyingstate.Thisoffers
auniqueopportunitytoinvestigatethestronginteractionbetweenhadronandnu-
cleus(ornucleon)atthelowenergylimitwhichisunobtainableintypicalscatter-
ingexperiments.
Anti-kaonK,namelyK−orK0,isthelightesthadroncontainingastrangeva-
lencequark.Thelow-energyKNsystemhasattractedattentionasatestingground
for chiral SU(3) dynamics in low-energy QCD and the role of the interplay be-
tween spontaneous and explicit chiral symmetry breaking due to the relatively
largemassofthestrangequark.
Inthesimplestkaonicatom,i.e.,kaonichydrogen(K−p),the1s-atomic-state
shift and width deduced from the spectroscopy of 2p–1s transition of the K−p
atom x-rays at ∼6.5 keV are directly related to the real and imaginary parts of
the complex K−p S-wave scattering length. Therefore the data have been inten-
sivelycollectedsofar1,2,3.TheresultsindicatethatKN interactioninlowenergy
is strongly attractive, which leads to particular interests in the depth of the real
K−-nucleus potentialatzeroenergy inconnectionwithpossiblekaon condensa-
tion in astrophysical scenarios (e.g., neutron star) and the possible existence of
“deeply-boundnuclearK− states”.
Manyexperimentshavecollecteddataonavarietyoftargetsotherthankaonic
hydrogenatomsofar4.Despiteofthefactmostoftheoriesreproducethedata4,5,
therehasbeenaconflictinthepotentialdepthbetweenphenomenologicalpoten-
tials and potentials constructed from more fundamental approaches. The former
aretypically180MeVdeep,whereasthelatterarelessthan50MeVdeep.Apart
fromkaonic-atomexperiments,alotofexperimentalsearchforthedeeply-bound
nuclear K− states have been performed in a past decade; however, only a small
amount of information is available6,7, which is not sufficient to discriminatebe-
tweenavarietyofconflictinginterpretations.
Recently,thekaonic-heliumatom(K−-He)hasattractedinterestinconnection
withthetheoreticalpredictions8,9 thatalargeshiftandwidthofthe2plevelmay
appearneartheresonancebetweenatomicandnuclearpoles,whichiscloselyre-
lated to the existence of deeply bound nuclear K− states. A recent calculation9
showstheshiftandwidthasafunctionoftherealpart(U )ofthepotentialdepth
0
atacertaincoupledpotentialdepth(U =120MeV)withthedifferentvaluesfor
coupl
K−-3HeandK−-4HeatomsasshowninFig.1.Thecalculationwasbasedonthe
coupled-channelapproachbetweentheKN channelandtheSp decaychannel.A
largeshiftof|e |∼10eVandwidthof|G |∼20eVarepredictedforK−-4He
2p 2p
when the potential depth is at around ∼ 200 MeV, whereas most of theoretical
calculations predict smaller values, e.g., e ∼ -0.2 eV and G ∼ 2 eV8. Mea-
2p 2p
surementsof3d–2ptransitionsinK−-3He(6.2keV)andK−-4He(6.5keV)have
beenperformedsofarwiththeconventionalsemiconductorspectrometershaving
the FWHM energy resolution of typically ∼ 200 eV at 6 keV11,12,13, which is
insufficienttoseesuchasmallspectraleffectsduetothestronginteraction.
3
Experiments Theories
hift [eV]20 :: KK ----34 HHee K --4 He K --3 He 20
S10 10
el
v
e
L
p 0 0
2 E1
T2
@KEK-PS T3
(Japan) E2 E3
-10 -10
@DAΦNE
(Italy)
T1 ( Ucoupl = 120 MeV )
-20 -20
2006200720082009201020112012 0 100 200 300U0 [MeV]
Publication year
Fig.1 The2p-levelshiftofK−-3HeandK−-4Heatomsobtainedfromtherecentexperiments
(left)atKEK-PS(E111)andDAF NE(E212,E313)comparedwiththetheoreticalcalculations
(right).Arecentcalculationbytheuseofthecoupled-channelmodelasafunctionofthereal
part(U )ofthepotentialdepthatacoupledpotentialdepthofU =120MeV(T19),compared
0 coupl
withothercalculationsshowingshiftsbeingveryclosetozero(T24,T35).Experimentalresults
sofararenotaccurateenoughtodistinguishbetweendifferentmodels.
2 Aproposedexperiment
Aiming at a breakthrough to the current situation, we are planning to perform
ultra-high-resolutionx-rayspectroscopyofkaonicatomsusinganarrayofNIST’s
transition-edge-sensor (TES) microcalorimetershaving a FWHM energy resolu-
tion of 2 – 3 eV at 6 keV, being two order of magnitude improved resolution
compared with the conventional semiconductor detector. The TES is a thermal
sensor that measures an energy deposition by the increase of resistance of a su-
perconducting film biased within the superconducting-to-normal transition. The
detailedworkingprinciplesandtherecentprogressoftheNIST’sTESsystemare
describedinRefs.14,15.Apreliminaryresultofarecenttestmeasurementshowsan
excellentFWHMtimeresolutionof∼0.5m secaswell.Nowwearepreparingan-
othertestmeasurementatahadronbeamlinetoevaluateanin-beamperformance
ofthex-rayspectrometer.
We plan to use a NIST-designed 160 pixel TES array each having 350 m m
square collecting area per pixel, being ∼20 mm2 in total, together with a time-
divisionSQUIDmultiplexerreadoutsystem.Forthecryostat,weutilizeapulse-
tubeadiabaticdemagnetizationrefrigerator(ADR)developedbyNISTandHPD,
102DENALI16,whosebasetemperatureis50mK.Thesizeofthissystemis33
cm×22cm×66cmtallwhichisrelativelyportableandcompact.Theportability
isessentialforourexperimentconstrainedbyverylimitedandbusybeamtime.
TheexperimentwillbeperformedatakaonbeamlineatJ-PARChadronhall,
e.g.,K1.8BRbeamline17.Kaonsareproducedbybombardingtheproductiontar-
get (Ni, Ag or Pt) with the primary proton beam from J-PARC 50 GeV proton
synchrotron,andextractedthroughthekaonbeamline.Thekaonic-heliumatoms
arecreatedbystoppingkaonsinaheliumtarget.
Fig.2showsapossibleapossibleexperimentalsetup.Incidentkaonsextracted
with a momentum of ∼ 900 MeV/c are degraded in carbon degraders, counted
withbeamlinecounterstogetherwithLuciteCˇherenkovcountersforreducingthe
4
Cryostat for Liquid
3He & 4He target
Cross section (flont view)
Cryostat for x-ray
detector system crAyoDsRtat He tank
(TES microcalorimeter) (1.3 K)
Kaonic
x-rays
Kaon beam
Al shield
TES array Helium 3 & 4
(50 mK) target cell
Kaon beam (~ 100 ml, 1.3 K)
detectors
10 cm
50 cm Kaon beam
Fig.2 (Coloronline)Aschematicbirds-eyeview(left)andacrosssectionfrontview(right)ofa
possibleexperimentalsetuparoundtheexperimentaltarget.Asetofbeamdetectors,acryogenic
liquid-3Heand4Hetargetsystem,andaNIST’sTESx-rayspectrometersystemareshown.
V
e400
1
unts / 330500 TES microcalorimeter
o
C250
Fig.3 (Color online) Simulated
spectraofofthe3d–2ptransitions 200
of kaonic-4He atoms detected by
150
differentx-raydetectors:aconven- Silicon drift detector
X-rays from
tional silicon drift detector with a 100 Compton scattering
FWHM energy resolution of 190 50
eV at the energy, and a TES mi-
0
crocalorimeter with a FWHM en- 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7.0
ergyresolutionof5eV. Energy (keV)
pion contamination, tracked by a high-rate beamline drift chamber and stopped
insidetheliquid-heliumtarget.Toensurethatkaonsarestoppedinthetarget,an
anti-coincidence counter is installed just downstream side of the target. X-rays
emittedfrom the K−-He atoms are detectedby a TES spectrometerviewing the
targetfromaside.
Themaintriggerwillbedefinedbythebeamlinecounterstoidentifyincom-
ingkaonsandtheanti-coincidencecountertoensurestoppedkaonsinthetarget.
X-ray signals of TES are coincident with the trigger to reduce accidental back-
grounds.
Fig.3 is a result of a GEANT4 simulation where K−-4He 3d–2p x-rays are
measured by different two x-ray detectors. One is a typical conventional silicon
driftdetectorusedinthepreviousexperiments11,12,13 havinga400-m m-thickac-
tivelayerwith100mm2 collectingareaand190eV(FWHM)energyresolution.
The other is a TES spectrometer having 5-m m-thick Bi absorbers with 20 mm2
collectingareaand5eV(FWHM)energyresolution.
In theprevious experiment11, there has beena non-negligible effectof spec-
tral distortion due to Compton scattering in the liquid 4He target. As shown in
5
Fig.3,theeffectiscompletelyseparatedfromthedistinctx-raypeakmeasuredby
TES.Althoughtheotherexperiments12,13utilisedagaseousheliumtargetsoasto
reducetheeffect,wecannowusealiquidheliumtargettoefficientlystopkaons
withoutsufferingtheComptonscatteringproblem.
Alotoffluorescencex-rays(e.g.,FeKa 1 6.40keV,FeKa 2 6.39keVandMn
Kb 6.49 keV) and kaonic x-rays other than K−-He x-rays were possible serious
background sources in the past experiments. In reversal, these are rather useful
asgood calibrationlinesintheproposed measurementwithaTESspectrometer.
Thepreciseenergycalibrationisessentialinsuchahighresolutionmeasurement;
andthuswewillcarefullyselectourcalibrationsourcesofthefluorescencex-rays
whoseenergies,widthandsatellitepeaksarewellknown.Moreoverasimultane-
ous measurement of 3d–2p x-rays in K−-4He (6.5 keV) and K−-3He (6.2 keV)
atoms witha mixedliquid3He and4He targetis possible,whichwill drastically
reducesthesystematicerrorsofrelativex-rayenergiesandwidthsforbothatoms.
ReferringtheyieldofK−-4He3d–2p x-rays obtainedintheprevious experi-
ment11,theyieldwasroughly estimatedtobeabout 30eventsperdayassuming
thattheJ-PARCK1.8BRbeamlineisemployedwithanoperationalprotonbeam
power of 60 kW. Four-days data acquisitionwillgive about 120 eventsresulting
in a statisticalerror of 0.1 eV in x-ray energy, assuming 2 – 3 eV (FWHM) en-
ergy resolutionwithout background. Thus, we couldobtainmore thanone order
ofmagnitudeimprovedstatisticalerrorsofthex-rayenergyandthewidthvalues
comparedtothepreviousexperimentswithareasonabledataacquisitiontime.
As well as K−-He atoms, there is another theoretical suggestion of a pre-
cise measurement of 3d–2p x-rays in K−-6Li and K−-7Li (∼ 15 keV), whose
resulthasapossibilitytodiscriminatebetweendeep(phenomenological)orshal-
low(chiral)potentials18.Moreoverthereareotherremarkablestudiesusingultra-
high-resolutionkaonicx-rayspectroscopieswithTESspectrometersweintendto
performasfollows:
1. Charged kaon mass measurement:Kaonic-atom x-ray spectroscopyhas been
alsoutilizedas a tool for measuring the charged kaon mass.The latestvalue
isdeterminedbytheaverageofthesixmeasurementstobe493.677 ±0.013
MeV(S=2.4)19.Thisrelativelylargeerrormainlycomesfromtwodatawhose
centralvaluesdiffervastlyabout60keV;thusanewaccuratemeasurementhas
beeneagerlyawaited.Weaimtoimprovetheprecisionofthismassmeasure-
ment with a high-precision measurement of K−-12C 5–4 x-ray (∼ 10 keV).
Theprecisionisexpectedtobe∼2.5keVcorrespondingtotheaccuracyof∼
0.05eVforthex-rayenergy.
2. Study of K− multi-nucleon absorption process: Recent theoreticalstudies of
strong interaction effects in kaonic atoms suggest that analysing so-called
’lower’ and ’upper’ levels in the same atom could separate one-nucleon ab-
sorption from multinucleonprocesses20. For this study, direct measurements
ofupperlevelwidthsinadditiontolowerlevelwidthsareessentialformedium-
weightandheavykaonicatoms(100–450keVx-rays)whosecandidateswere
examined recently21. The measurements will be performed in future with a
gamma-rayTESspectrometersimilartowhatNISThasalreadyachieved,hav-
inga256pixelarrayresultingin5cm2collectingareaandanaveragedFWHM
energyresolutionof53eVat100keV15.
6
3 Conclusions
Wearepreparinganext-generationkaonic-atomx-rayspectroscopyusinganx-ray
spectrometerbasedonanarrayofsuperconductingTESmicrocalorimetersdevel-
opedbyNIST.Precisemeasurementsof3d–2px-raysinkaonic-heliumatoms(∼
6 keV) and kaonic-lithium atoms (∼ 15 keV) with two order of magnitude im-
proved energy resolution compared with the conventional silicon detectors will
enableustoseesmallspectraleffectsduetothestronginteraction.Thiswilladd
a fresh dimension to the study of the depth of the real K−-nucleus potential at
zeroenergyinconnectionwiththeexistenceofdeeply-bound nuclearK− states.
Additionally,weintendtoimprovethe precisionofthe chargedkaon massmea-
surementwithhigh-precisionmeasurementofK−-12C5–4x-ray(∼10keV),and
studytheK−multi-nucleonabsorptionprocessinfuturewithdirectmeasurements
of upper level widths in medium-weight and heavy kaonic atoms using a multi-
pixelTESspectrometerwithauseabledynamicrangeabove400keV.
Acknowledgements ThisworkwassupportedbyJSPSKAKENHIGrantNumber25105514
andbyIncentiveResearchGrantfromRIKEN.
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