research article a camoo crystal low temperature detector for … · 2019. 7. 31. · ac coupled...

Research ArticleA CaMoO4 Crystal Low Temperature Detector for the AMoRENeutrinoless Double Beta Decay Search

G B Kim123 S Choi2 F A Danevich4 A Fleischmann5 C S Kang13

H J Kim6 S R Kim13 Y D Kim17 Y H Kim138 V A Kornoukhov9 H J Lee13

J H Lee3 M K Lee3 S J Lee310 J H So13 and W S Yoon138

1 Institute for Basic Science Daejeon 305-811 Republic of Korea2Department of Physics and Astronomy Seoul National University Seoul 151-747 Republic of Korea3 Korea Research Institute of Standards and Science Daejeon 305-340 Republic of Korea4 Institute for Nuclear Research 03680 Kyiv Ukraine5 Kirchhoff-Institut fur Physik Universitat Heidelberg 69120 Heidelberg Germany6 Physics Department Kyungpook National University Daegu 702-701 Republic of Korea7Department of Physics Sejong University Seoul 143-747 Republic of Korea8Korea University of Science and Technology Daejeon 305-350 Republic of Korea9 Institute for Theoretical and Experimental Physics 117218 Moscow Russia10NASA Goddard Space Flight Center Greenbelt MD 20771 USA

Correspondence should be addressed to Y H Kim yhkibsrekr

Received 4 July 2014 Accepted 30 August 2014

Academic Editor Hiro Ejiri

Copyright copy 2015 G B Kim et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited Thepublication of this article was funded by SCOAP3

We report the development of a CaMoO4crystal low temperature detector for the AMoRE neutrinoless double beta decay

(0]120573120573) search experiment The prototype detector cell was composed of a 216 g CaMoO4crystal and a metallic magnetic

calorimeter An overground measurement demonstrated FWHM resolution of 6ndash11 keV for full absorption gamma peaks Pulseshape discrimination was clearly demonstrated in the phonon signals and 76 120590 of discrimination power was found for the 120572 and120573120574 separationThe phonon signals showed rise-times of about 1ms It is expected that the relatively fast rise-time will increase therejection efficiency of two-neutrino double beta decay pile-up events which can be one of the major background sources in 0]120573120573

searches

1 Introduction

Recent neutrino oscillation experiments have been unveil-ing the properties of neutrinos [1 2] Their experimentalevidences strongly suggest that neutrinos are massive andencounter flavor mixing of mass eigenvalues The mixingangles and the differences between the square masses havebeen estimated However those observations do not providea direct measurement of the absolute mass and do not answerthe question of whether neutrino is its own antiparticle(Majorana-type) or not (Dirac-type)

Search for neutrinoless double beta decay (0]120573120573) is a keyexperiment to reveal unanswered nature of neutrinos [3ndash7]The double beta decay (2]120573120573) that accompanies the simul-taneous emission of two electrons and two antineutrinos isa rare process that is an allowed transition in the standardmodel Another type of double beta decay that does notemit any neutrinos 0]120573120573 can occur if neutrino is massiveMajorana particles (ie it is its own antiparticle) In the 0]120573120573

process the full available energy of the decay is carried bythe two electrons and the recoiled daughter which has a verysmall amount of energy compared with that of the electrons

Hindawi Publishing CorporationAdvances in High Energy PhysicsVolume 2015 Article ID 817530 7 pageshttpdxdoiorg1011552015817530

2 Advances in High Energy Physics

Therefore while the electron sum energy spectrum in the2]120573120573 is continuous up to the available energy release (119876

120573120573)

in the 0]120573120573 the spectrum should have a sharp peak at 119876120573120573

[2]The observation of 0]120573120573 would clearly demonstrate

that neutrino is not Dirac-type but rather is Majorana-typeparticle In that case physical processes that do not conservelepton number would be allowed Moreover the absolutemass scale so-called ldquoeffectiveMajorana neutrinomassrdquo ⟨119898]⟩

can be estimated using the 0]120573120573 half life (1198790]12

)

(1198790]12

)minus1

= 1198660]10038161003816100381610038161003816

1198720]10038161003816100381610038161003816

2 ⟨119898]⟩2

1198982119890

(1)

where 119898119890is the electron mass 1198660] is the kinematic phase-

space factor calculable with reasonable precision and 1198720]

is the model dependent nuclear matrix element Here theMajorana mass is defined as

⟨119898]⟩ =10038161003816100381610038161003816sum1198802

119890119895119898119895

10038161003816100381610038161003816 (2)

where the 119898119895rsquos are the mass eigenstates of the neutrino and

119880119890119895rsquos are the elements of the mixingmatrix between the flavor

states and mass eigenstatesExperimentally themeasurement limit of half life is often

used as the sensitivity to probe the rare event [7] In a mea-surement with nonnegligible backgrounds the sensitivitybecomes

1198790]12

prop 120575120576radic119872119905

119887Δ119864 (3)

where 120575 is the concentration of 120573120573 isotope in the detector120576 is the detection efficiency 119872 is the detector mass 119905 is themeasurement time 119887 is the background rate per unit massand energy and Δ119864 is the energy resolution of the detectorin other words the region of interest (ROI) of the energywindow at the 119876

120573120573value However in a case of a zero-

background experiment that observes no event in ROI duringthe measurement time the sensitivity becomes proportionalto the detector mass and the measurement time

1198790]12

prop 120575120576119872119905 (4)

To increase the detection sensitivity it is essential tohave a detector with high concentration of the isotope ofinterest detection efficiency energy resolution and efficientbackground rejection capability as well as to minimize back-grounds from internal and external sources in the regionof interest High energy resolution and detection efficiencyexperiment can be realized with crystal detectors containingthe isotope of interest The detector performance of recentlydeveloped low temperature detectors (LTDs) that operateat sub-Kelvin temperatures can perfectly meet the require-ments by utilizing state-of-the-art detector technologies withextreme energy sensitivity such as neutron transmutationdoped (NTD) Ge thermistors superconducting transitionedge sensors (TESs) or metallic magnetic calorimeters(MMCs) [8]

The AMoRE (Advanced Mo-based Rare process Experi-ment) project is an experiment to search for 0]120573120573 of 100Mo[9] AMoRE uses CaMoO

4crystals as the absorber and

MMCs as the sensor [10 11] CaMoO4is a scintillating

crystal that has the highest light output at room and lowtemperatures among Mo-containing crystals (molybdates)[12 13]

Choosing of 100Mo as the source of 0]120573120573 is advantageousThe nucleus has a high119876 value of 303440(17) keV [14] that isabove the intensive 2615 keV gamma quanta from 208Tl decay(232Th family) The natural abundance of 100Mo is 98 [15]which is comparatively high Furthermore enriched 100Mocan be produced by centrifugation method in amount oftens of kilograms per year with a reasonable price Also thetheoretically estimated half life of 100Mo is relatively shorterthan that of other 0]120573120573 candidates [16 17] However 2]120573120573 of48Ca with 119876

120573120573= 4272 keV (despite rather low concentration

of the isotope asymp 02) can be an irremovable backgroundsource in the ROI of 100Mo The AMoRE collaborationhas successfully grown 40Ca100MoO

4crystals using 100Mo

enriched and 48Ca depleted materials Three of the doubly-enriched crystals with masses in the range of 02ndash04 kg weretested in a low background 4120587 veto system to determine theirinternal backgrounds [18]

The present experimental work aims to test the lowtemperature detection concept with a CaMoO

4crystal and

an MMC that is suitable for a high resolution experimentto search for 0]120573120573 of 100Mo A 216 g natural CaMoO

4

crystal with an MMC phonon sensor was employed inthis experiment which was performed in an overgroundmeasurement facility The energy resolution and linearityof the detector setup particle and randomly coincidingevents discrimination by pulse shape analysis for backgroundrejection in a 0]120573120573 experiment are discussed in this report

2 Experimental Details

The detector setup was structured in a cylindrical shape withcopper support details as shown in Figure 1 A CaMoO

4

crystal 4 cm in diameter and 4 cm height was mountedinside the copper structure using metal springs The massof the crystal was 216 g It was grown with natural Ca andMo elements at the Bogoroditsk plant in Russia A patternedgold film was evaporated on one side of the crystal to serveas a phonon collector An MMC device the primary sensorfor detecting the phonon signals absorbed in the gold filmwas placed on a semicircular copper plate over the crystalThe thermal connection between the gold film and theMMCwas made using annealed gold wires Details regarding themeasurement principle and the detector structure of theMMC device were presented in previous reports [19 20]

When a particle hits a dielectric material most of theenergy deposited into the absorber is converted into theform of phonons High energy phonons with frequenciesthat are close to the Debye frequency are generated initiallyHowever they quickly decay to lower frequency phononsvia anharmonic processes When their energy becomes20ndash50K they can travel ballistically in the crystal [21]

Advances in High Energy Physics 3

Copper holder

Metallic light reflector

MMC device

Annealed gold wires

SQUID

200nm thick

gold film

216 g natural CaMoO4 crystal

Figure 1 (Color online) low temperature detector setup with a 216 gCaMoO

4crystal and an MMC sensor

The major down-conversion processes of these athermalphonons are isotope scattering inelastic scatterings by impu-rities and lattice dislocations and inelastic scatterings at crys-tal surfaces [22]These excess phonons eventually change theequilibrium thermal phonon distribution thereby causingtemperature increase

In the detector setup with the CaMoO4crystal and the

gold phonon collector film the ballistic athermal phononscan hit the crystal and gold interface transmit into the goldfilm and transfer their energy to the electrons in the film[23] The electron temperature of the gold film increasesquickly via electron-electron scatterings This temperaturechange is measured by the MMC sensor that is thermallyconnected with the gold wires The size of the gold filmand number of gold wires were chosen based on a thermalmodel study that considered the efficient athermal heat flowprocess [11] Consequently the gold film had a diameter of2 cm a thickness of 200 nm and an additional gold patternof 200 nm thickness on top of the gold film to increase thelateral thermal conductivity of the gold film

The detector assembly was installed in a dilution refriger-ator in an overground laboratory at KRISS (Korea ResearchInstitute of Standards and Science) The refrigerator wassurrounded by a 10 cm thick lead shield (except the top sur-face) to reduce environmental gamma-ray background Thedetector with anMMCoperates well in the temperature rangeof 10ndash50mK The signal size increases at lower temperaturessince the MMC sensitivity enhanced and the heat capacitiesdecreased However the signals have slower rise and decaytimes at lower temperatures as thermal conductances becomepoorer Larger signal size improves the energy thresholdand baseline energy resolution of the detector The energyresolution of the detector measured for particle absorptionevents can be worse than the baseline resolution because ofany uncorrelated mechanism that affects the signal size andshape Examples of such mechanisms include temperaturefluctuations due to instrumental instability or frequent eventrates position dependence of signal shapes or scintillationprocesses that are associates with phonon generations in aninhomogeneous way Therefore larger signal size does not

0 50 100 150

0

005

01

015

02

Time (ms)

Sign

al (a

u)

Figure 2 (Color online) a typical signal of 26MeV gamma-rayfull absorption events at 40mK in DC (blue bigger) and AC (redsmaller) coupling

0 02 04 06 087

71

72

73

74

75

Pulse height (au)

Mea

n tim

e (m

s)Cosmic muons

120572

120573120574

Figure 3 A scatter plot of the mean-time and pulse height obtainedfrom the background measurement of 95 h in an overgroundlaboratory 120572 and 120573120574 (including cosmic muons) events are clearlyseparated in terms of their mean-time values

guarantee a better energy resolution at certain temperaturesAt the present experimental condition including the back-ground rate from cosmic muons and external gamma-rays40mK was selected as the main measurement temperatureAt this temperature about 1ms rise-time was obtained forthe 26MeV gamma line without degrading the energy reso-lution A typical signal of 26MeV gamma-ray full absorptionevents is shown in Figure 2 The rise-time of the DC coupledsignal is 11ms which is somewhat slower than that of earliermeasurements for which shorter gold wires were used [11]

3 Pulse Shape Analysis

A two dimensional scatter plot of the pulse heights andmean-times of signals obtained in a 95 h background measurementis shown in Figure 3 The pulse height is the differencebetween the maximum value and baseline level of a signalThe maximum value is found using a quadratic polynomial


fit to the region of the signal near the pulse maximum Thebaseline level is the average voltage value in the time regionbefore the signal rises The mean-time parameter is definedas

119905mean =sum11990510+119903

11990510minus119897

(V119905times (119905 minus 119905

10))

sum11990510+119903

11990510minus119897V119905

(5)

where V119905is the measured voltage value at time 119905 subtracting

the baseline level 11990510

is the time when it reaches 10 ofthe pulse height and 119897 and 119903 indicate the time length of thesignal toward left and right directions from 119905

10 respectively

to calculate the mean-time 119897 is set to reach the time at thebaseline level while 119903 is a free parameter that was selectedto achieve the most efficient particle discrimination Herethe 119903 value was set to not include the negative part of anAC coupled signal (see Figure 2) which was used for theenergy spectrum because it was recorded with finer digitizerresolution (ie bigger gain is used) than the DC coupledsignal

The pulse-shape discrimination (PSD) between 120572 and120573120574 particles can be realized with the mean-time as a pulseshape parameter due to the difference in the rise and decaytimes of the two types of events A similar tendency ofPSD was reported for other low temperature scintillatingdetectors [24]The separation into two groups of the events inthe energy region between 4 and 5MeV of alpha-equivalentenergy can be readily observed from the distribution ofmean-time values shown in Figure 4The energy of120572 inducedevents was determined as described in the following sectionAlthough the two peaks have noticeable right-hand tailstoward higher mean-time values normal Gaussian functionswere used to fit the distributions We interpret the right-hand tails as a result of signal pile-up A parameter ofdiscrimination power (DP) is defined as

DP =1205831minus 1205832

radic12059021+ 12059022

(6)

where 120583119894are the mean values and 120590

119894are the standard

deviations of the Gaussian distributions for 120572 and 120573120574 eventsDP was found to be 76

The averaged pulse shapes for the two groups of events arecompared in Figure 5 Templates of 120572 events were obtainedby averaging out the 232Th 120572-decay events pulse profileswith energy release in the crystal 4082 keV (due to thecontamination of the crystal by thorium) whereas the eventscaused by cosmic muons with the same pulse height wereselected for the template of 120573 induced eventsThe normalizedpulses of alpha and beta templates are aligned at the time oftheir maximum values as shown in Figure 5 Both the riseand decay times of the 120572 and 120573 signals are clearly differentThe signals induced by 120572 events have faster rise and fasterdecay than those of the 120573 events

According to scintillation measurements of a CaMoO4

crystal at 7ndash300K [25 26] the scintillation decay time ofthe CaMoO

4crystal reaches hundreds of 120583119904 at 7 K The

crystal also shows different light outputs for 120572 and 120573120574

71 72 73 74 750

20

40

60

80

100

120

Mean time (ms)

Cou

nts

120572

120573120574

Figure 4 Distribution of mean-time parameter in 4MeV lt 119864 lt

5MeV region of alpha-equivalent energy Discrimination power wasfound to be 76 from fitting each group of distribution with a normalGaussian function Right-hand side tail is noticeable for the twogroups toward higher mean-time value

0

02

04

06

08

1

Time (ms)

minus2 0 2 4

096

098

1

102

Time (ms)

minus10 0 10 20 30 40 50

Sign

al (a

u)

120573

120572

Figure 5 (Color online) averaged pulse shapes of 120572 induced signalsfrom 232Th decays (blue) and the corresponding (ie with thesame pulse height) muon-induced signals (red) The signals arenormalized for their pulse height and shifted in time to align themaximum point of signals

events [27] A slowly decaying scintillationmechanismwouldcause slow generation of phonon in the CaMoO

4crystal 120572

and 120573120574 particle events may have different fractions of slowcomponent for phonon generation This difference in slowphonon generation at mK temperatures may induce differentpulse shapes for 120572 and 120573120574 particle events

4 Energy Spectrum

Because athermal phonon absorption in the phonon collectorsignificantly contributes to the signal size [11] the signalshave some degrees of position dependence for their pulseheight and shape In this detector the signals with faster rise-times show bigger pulse heights for the same energy eventsThis effect can be observed in Figure 3 The distribution of


mean-time values and pulse heights for alpha and gamma-rayfull absorption lines has anticorrelated slopes This negativeslope more dominantly appears in the alpha signals which islikely because the full-energy 120574 peaks originate frommultipleCompton scatterings in the crystal that smear the positiondependence on the event location The pulse height is notan optimal parameter to obtain a high resolution In theoptimal filtering method [28] that is often adopted in highresolutionmicrocalorimeters the signals are assumed to haveone shape but with different amplitudes Thus the optimalfilteringmethod is not applicable to provide a high resolutionspectrum for the present signals

In the present analysis a new parameter LA (left area)was used as an amplitude parameter to reduce the positiondependence effect of the large crystal detector It is defined as

LA =

119905mean

sum11990510minus119897

V119905 (7)

where the variables are as defined in (5) LA is a partialintegration for the leading part of a signal Because theintegration range of the LA parameter is set by a shape-dependent parameter the mean-time this parameter is lessinfluenced by the pulse shape For instance the correlationcoefficient between the pulse height and mean-time wasminus062 for 4082 keV 120572 signals but it was 008 between the LAand mean-time

In a calibration run a thoriated tungsten rod was used asan external gamma-ray source The source was placed in thegap between the cryostat and the lead shield

When a linear energy calibration was applied to gamma-ray peaks deviations from the linear calibration of lessthan 04 were found for low-energy peaks A quadraticfunction with no constant term was used for the calibrationof electron-equivalent energy for 511 583 911 and 2615 keVpeaks for the spectrum shown in Figure 6The correspondingenergy resolutions of the peaks are listed in Table 1

Figure 7 shows the linearities of the electron and alphasignalsThe LA values of the electron and alpha peaks dividedby the linear calibration of the gamma-ray peaks are plottedin the upper figure The LAenergy ratios for the alpha peaksare about 6 larger than those for the gamma-ray peaksThe quadratic fit functions are shown as dotted and dashedlines for gamma and alpha peaks respectively The residualsin the lower figure indicate deviations from the quadraticfunctions for the two groups With the quadratic calibrationa very small deviation is expected near 3MeV for electronmeasurements It implies that this method can supply anaccurate energy calibration at the 119876

120573120573value of the 2]120573120573 of

100MoAlpha events can be separated from 120573120574 events using the

mean-time parameter The energy spectrum of alpha eventsis shown in Figure 8 An energy calibration for this spectrumwas performed with a quadratic function for the alpha peaksas discussed above These backgrounds were bulk eventsof alpha decays in the crystal mainly from radionuclidesof U and Th decay chains most of them are identified asshown in Figure 8 Because this crystal was developed toinvestigate the scintillation properties ofCaMoO

4 its internal

Table 1 Energy resolutions of gamma-ray full absorption peaks inthe calibration measurement

Energy (keV) FWHM (keV)511 72 plusmn 06583 65 plusmn 05911 68 plusmn 032615 109 plusmn 04

0 1000 2000 3000

103

102

101

100

208Tl

208Tl228Ac

228Ac583keV911keV

965969keV 2615keV

PP + 208Tl511keV

Energyee (keV)C

ount

s per

2 k

eV b

in

Figure 6 Energy spectrum measured with an external source over65 h

background was not exceptionally low The AMoRE col-laboration has developed 40Ca100MoO

4growing technology

with low radioimpurities Internal alpha activities of about80 120583Bqkg of 226Ra and 70120583Bqkg of 228Th were found in a196 g 40Ca100MoO

4[18]

Not only alpha signals can be rejected efficiently butalso 120573 decays of 212Bi 214Bi and 208Tl can be tagged andeliminated from the data by using information about asso-ciated alpha-emitting nuclides Random pile-ups of events(first of all from the 2]120573120573) could be a substantial sourceof background of LTD to search for 0]120573120573 due to the poortime resolution [29] Relatively fast response of the MMCamong the LTDs is a certain advantage to discriminate thebackground

5 Conclusion

LTDs made of crystal scintillators containing isotopes ofinterest have distinct advantages in 0]120573120573 searches Such LTDsmake it possible to provide a high detection efficiency tothe 0]120573120573 Taking the advantage of high resolution sensortechnologies these dielectric detectors in the heat (phonon)measurement can have comparable energy resolution tothose of HPGe detectors The comparison in heatlightmeasurement channels makes unambiguous separation ofalpha events from electron events As discussed in this reportpulse shape discrimination is also possible using only thephonon measurement This PSD capability of the phononsensor increases discrimination power for alpha backgroundsignals or can simplify the detector cell design by using onlyone phonon sensorwithout a photon sensor that is commonly


0 2000 4000 6000

1

102

104

106

108

LAe

nerg

y (a

u)

Energy (keV)

(a)

0 2000 4000 6000minus2

0

2

Energy (keV)

Resid

uals

times10minus3

(b)

Figure 7 (Color online) ratio of signal amplitude (LA) and energyin linear calibration for 120574 (circle blue) and 120572 (triangle red) peaks(a) Quadratic calibration lines (dotted and dashed lines) are shownfor 120574 and 120572 peaks respectively Deviations of the ratios from eachquadratic functions are shown in (b)

2000 3000 4000 5000 6000 7000

0

10

20

30

40

50

Cou

nts

147Sm

232Th

228Th227Th

210Po

218Po

238U

230Th

238U226Ra

223Ra

220Ra211Bi

224Ra

Energyae (keV)

Figure 8 Energy spectrum of bulk alpha events in a backgroundmeasurement

used for particle discrimination [30] and will reduce thenumber of measurement channels

In comparison with signals from PMTs or conventionalsemiconductor detectors phonon signals from a crystaldetector in the LTD concept are typically slow Even thoughthe energy resolution of these detectors does not sufferfrom the slow signal in the low activity environment ofan underground experiment random coincidence of events

leads to an unavoidable background because of the slow rise-time [29] Two consecutive electron events that occur in atime interval that is much shorter than the signal rise-timecan be regarded as a single event Such randomly coincidentevents particularly of 2]120573120573 are an unavoidable source ofbackgrounds in the 0]120573120573 taking into account that the 2]120573120573

event rate of 100Mo is expected to be about 10mBq in 1 kg40Ca100MoO

4

In the present experiment the phonon signals had rise-times of 11ms which is much faster than the rise-timesof LTDs with NTD Ge thermistors The fast rise-time pro-vides efficient rejection possibility for randomly coincidentevents [31] Moreover a photon detector composed of a 2inch Ge wafer and an MMC sensor showed a temperatureindependent rise-time of about 02mswith reasonable energyresolution [32] Simultaneousmeasurements with the photondetector will increase the discrimination power not just foralpha events but also for randomly coincident events

For the AMoRE project 40Ca100MoO4crystals will be

used as the detector material together withMMCs in phononand photon measurement setups We aim to reach a zerobackground with improved energy resolution of a few keVThe first stage experiment is expected to be constructed witha 10 kg prototype detector by 2016 We plan to perform alarge scale experiment with 200 kg 40Ca100MoO

4crystals

in the next 5-6 years The sensitivity of the experiment tothe effective Majorana neutrino mass is estimated to be inthe range of 20ndash50meV which corresponds to the invertedscheme of the neutrino mass

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This research was funded by Grant no IBS-R016-G1 andpartly supported by the National Research Foundation ofKorea Grant funded by the Korean Government (NRF-2011-220-C00006 and NRF-2013K2A5A3000039) F A Danevichwas supported in part by the Space Research Program of theNational Academy of Sciences of Ukraine

References

[1] J Beringer J F Arguin R M Barnett et al ldquoReview of particlephysicsrdquo Physical Review D vol 86 Article ID 010001 2012

[2] R N Mohapatra S Antusch K S Babu et al ldquoTheory ofneutrinos a white paperrdquo Reports on Progress in Physics vol 70no 11 pp 1757ndash1867 2007

[3] S R Elliott and P Vogel ldquoDouble beta decayrdquo Annual Review ofNuclear and Particle Science vol 52 pp 115ndash151 2002

[4] F T Avignone S R Elliott and J Engel ldquoDouble beta decayMajorana neutrinos and neutrino massrdquo Reviews of ModernPhysics vol 80 p 481 2008

[5] W Rodejohann ldquoNeutrino-less double beta decay and particlephysicsrdquo International Journal of Modern Physics E vol 20 no9 pp 1833ndash1930 2011


[6] A Giuliani and A Poves ldquoNeutrinoless double-beta decayrdquoAdvances in High Energy Physics vol 2012 Article ID 85701638 pages 2012

[7] J J Gomez-Cadenas J Martın-Albo MMezzetto F Monrabaland M Sorel ldquoThe search for neutrinoless double beta decayrdquoRivista del Nuovo Cimento vol 35 no 2 pp 29ndash98 2012

[8] C Enss Cryogenic Particle Detection vol 99 Springer 2005[9] H Bhang R S Boiko D M Chernyak et al ldquoAMoRE exper-

iment a search for neutrinoless double beta decay of 100Moisotope with 40Ca 100MoO

4cryogenic scintillation detectorrdquo

Journal of Physics Conference Series vol 375 no 4 Article ID042023 2012

[10] S J Lee J H Choi F A Danevich et al ldquoThe developmentof a cryogenic detector with CaMoO

4crystals for neutrinoless

double beta decay searchrdquo Astroparticle Physics vol 34 no 9pp 732ndash737 2011

[11] G B Kim S Choi Y S Jang et al ldquoThermal model and opti-mization of a large crystal detector using a metallic magneticcalorimeterrdquo Journal of Low Temperature Physics vol 176 no5-6 pp 637ndash643 2014

[12] H J Kim A N Annenkov R S Boiko et al ldquoNeutrino-lessdouble beta decay experiment using Ca

100MoO4scintillation

crystalsrdquo IEEE Transactions on Nuclear Science vol 57 no 3pp 1475ndash1480 2010

[13] S Pirro J W Beeman S Capelli M Pavan E Previtali and PGorla ldquoScintillating double-beta-decay bolometersrdquo Physics ofAtomic Nuclei vol 69 no 12 pp 2109ndash2116 2006

[14] S Rahaman V-V Elomaa T Eronen et al ldquoQ values of the76Ge and 100Mo double-beta decaysrdquo Physics Letters B NuclearElementary Particle and High-Energy Physics vol 662 no 2 pp111ndash116 2008

[15] M EWieser and J R D Laeter ldquoAbsolute isotopic compositionof molybdenum and the solar abundances of the p-processnuclides 9294Mordquo Physical Review C vol 75 Article ID 0558022007

[16] J Barea J Kotila and F Iachello ldquoLimits on neutrino massesfrom neutrinoless double-120573 decayrdquo Physical Review Letters vol109 no 4 Article ID 042501 2012

[17] J D Vergados H Ejiri and F Simkovic ldquoTheory of neutrinolessdouble-beta decayrdquoReports on Progress in Physics vol 75 no 10Article ID 106301 2012

[18] J H So H J Kim V V Alenkov et al ldquoScintillation prop-erties and internal background study of 40Ca100MoO

4crystal

scintillators for neutrino-less double beta decay searchrdquo IEEETransactions on Nuclear Science vol 59 no 5 pp 2214ndash22182012

[19] W S Yoon Y S Jang G B Kim et al ldquoHigh energy resolutioncryogenic alpha spectrometers using magnetic calorimetersrdquoJournal of Low Temperature Physics vol 167 no 3-4 pp 280ndash285 2012

[20] W S Yoon G B Kim H J Lee et al ldquoFabrication of metallicmagnetic calorimeter for radionuclide analysisrdquo Journal of LowTemperature Physics vol 176 no 5-6 pp 644ndash649 2014

[21] J P Wolfe Imaging Phonons Acoustic Wave Propagation inSolids Cambridge University Press 2005

[22] S W Leman ldquoInvited review article physics and Monte Carlotechniques as relevant to cryogenic phonon and ionizationreadout of Cryogenic Dark Matter Search radiation detectorsrdquoReview of Scientific Instruments vol 83 Article ID 091101 2012

[23] Y H Kim H Eguchi C Enss et al ldquoMeasurements andmodeling of the thermal properties of a calorimeter having

a sapphire absorberrdquo Nuclear Instruments and Methods inPhysics Research A Accelerators Spectrometers Detectors andAssociated Equipment vol 520 no 1ndash3 pp 208ndash211 2004

[24] C Arnaboldi C Brofferio O Cremonesi et al ldquoA noveltechnique of particle identification with bolometric detectorsrdquoAstroparticle Physics vol 34 no 11 pp 797ndash804 2011

[25] V B Mikhailik S Henry H Kraus and I Solskii ldquoTempera-ture dependence of CaMoO

4scintillation propertiesrdquo Nuclear

Instruments and Methods in Physics Research A vol 583 no 2-3 pp 350ndash355 2007

[26] V B Mikhailik and H Kraus ldquoPerformance of scintillationmaterials at cryogenic temperaturesrdquo Physica Status Solidi Bvol 247 no 7 pp 1583ndash1599 2010

[27] A N Annenkov O A Buzanov F A Danevich et al ldquoDevel-opment of CaMoO4 crystal scintillators for a double beta decayexperiment with 100Mordquo Nuclear Instruments and Methods inPhysics Research A vol 584 no 2-3 pp 334ndash345 2008

[28] Y N Yuryev Y S Jang S K Kim et al ldquoSignal processing incryogenic particle detectionrdquoNuclear Instruments and Methodsin Physics Research A Accelerators Spectrometers Detectors andAssociated Equipment vol 635 no 1 pp 82ndash85 2011

[29] D M Chernyak F A Danevich A Giuliani E Olivieri MTenconi and V I Tretyak ldquoRandom coincidence of 2]2120573decay events as a background source in bolometric 0]2120573 decayexperimentsrdquo European Physical Journal C vol 72 no 4 pp 1ndash6 2012

[30] DArtusa F Avignone III OAzzolini et al ldquoExploring the neu-trinoless double beta decay in the inverted neutrino hierarchywith bolometric detectorsrdquo httparxivorgabs14044469

[31] D M Chernyak F A Danevich A Giuliani et al ldquoRejectionof randomly coinciding events in ZnMoO TeX scintillatingbolometersrdquo The European Physical Journal C vol 74 p 29132014

[32] H J Lee C S Kang S R Kim et al ldquoDevelopment of ascintillation light detector for a cryogenic rare-event-searchexperimentrdquo submitted in Nuclear Instruments and Methods inPhysics Research Section A

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

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Atomic and Molecular Physics

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Advances in Condensed Matter Physics

OpticsInternational Journal of



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Therefore while the electron sum energy spectrum in the2]120573120573 is continuous up to the available energy release (119876

120573120573)

in the 0]120573120573 the spectrum should have a sharp peak at 119876120573120573

[2]The observation of 0]120573120573 would clearly demonstrate

that neutrino is not Dirac-type but rather is Majorana-typeparticle In that case physical processes that do not conservelepton number would be allowed Moreover the absolutemass scale so-called ldquoeffectiveMajorana neutrinomassrdquo ⟨119898]⟩

can be estimated using the 0]120573120573 half life (1198790]12

)

(1198790]12

)minus1

= 1198660]10038161003816100381610038161003816

1198720]10038161003816100381610038161003816

2 ⟨119898]⟩2

1198982119890

(1)

where 119898119890is the electron mass 1198660] is the kinematic phase-

space factor calculable with reasonable precision and 1198720]

is the model dependent nuclear matrix element Here theMajorana mass is defined as

⟨119898]⟩ =10038161003816100381610038161003816sum1198802

119890119895119898119895

10038161003816100381610038161003816 (2)

where the 119898119895rsquos are the mass eigenstates of the neutrino and

119880119890119895rsquos are the elements of the mixingmatrix between the flavor

states and mass eigenstatesExperimentally themeasurement limit of half life is often

used as the sensitivity to probe the rare event [7] In a mea-surement with nonnegligible backgrounds the sensitivitybecomes

1198790]12

prop 120575120576radic119872119905

119887Δ119864 (3)

where 120575 is the concentration of 120573120573 isotope in the detector120576 is the detection efficiency 119872 is the detector mass 119905 is themeasurement time 119887 is the background rate per unit massand energy and Δ119864 is the energy resolution of the detectorin other words the region of interest (ROI) of the energywindow at the 119876

120573120573value However in a case of a zero-

background experiment that observes no event in ROI duringthe measurement time the sensitivity becomes proportionalto the detector mass and the measurement time

1198790]12

prop 120575120576119872119905 (4)

To increase the detection sensitivity it is essential tohave a detector with high concentration of the isotope ofinterest detection efficiency energy resolution and efficientbackground rejection capability as well as to minimize back-grounds from internal and external sources in the regionof interest High energy resolution and detection efficiencyexperiment can be realized with crystal detectors containingthe isotope of interest The detector performance of recentlydeveloped low temperature detectors (LTDs) that operateat sub-Kelvin temperatures can perfectly meet the require-ments by utilizing state-of-the-art detector technologies withextreme energy sensitivity such as neutron transmutationdoped (NTD) Ge thermistors superconducting transitionedge sensors (TESs) or metallic magnetic calorimeters(MMCs) [8]

The AMoRE (Advanced Mo-based Rare process Experi-ment) project is an experiment to search for 0]120573120573 of 100Mo[9] AMoRE uses CaMoO

4crystals as the absorber and

MMCs as the sensor [10 11] CaMoO4is a scintillating

crystal that has the highest light output at room and lowtemperatures among Mo-containing crystals (molybdates)[12 13]

Choosing of 100Mo as the source of 0]120573120573 is advantageousThe nucleus has a high119876 value of 303440(17) keV [14] that isabove the intensive 2615 keV gamma quanta from 208Tl decay(232Th family) The natural abundance of 100Mo is 98 [15]which is comparatively high Furthermore enriched 100Mocan be produced by centrifugation method in amount oftens of kilograms per year with a reasonable price Also thetheoretically estimated half life of 100Mo is relatively shorterthan that of other 0]120573120573 candidates [16 17] However 2]120573120573 of48Ca with 119876

120573120573= 4272 keV (despite rather low concentration

of the isotope asymp 02) can be an irremovable backgroundsource in the ROI of 100Mo The AMoRE collaborationhas successfully grown 40Ca100MoO

4crystals using 100Mo

enriched and 48Ca depleted materials Three of the doubly-enriched crystals with masses in the range of 02ndash04 kg weretested in a low background 4120587 veto system to determine theirinternal backgrounds [18]

The present experimental work aims to test the lowtemperature detection concept with a CaMoO

4crystal and

an MMC that is suitable for a high resolution experimentto search for 0]120573120573 of 100Mo A 216 g natural CaMoO

4

crystal with an MMC phonon sensor was employed inthis experiment which was performed in an overgroundmeasurement facility The energy resolution and linearityof the detector setup particle and randomly coincidingevents discrimination by pulse shape analysis for backgroundrejection in a 0]120573120573 experiment are discussed in this report

2 Experimental Details

The detector setup was structured in a cylindrical shape withcopper support details as shown in Figure 1 A CaMoO

4

crystal 4 cm in diameter and 4 cm height was mountedinside the copper structure using metal springs The massof the crystal was 216 g It was grown with natural Ca andMo elements at the Bogoroditsk plant in Russia A patternedgold film was evaporated on one side of the crystal to serveas a phonon collector An MMC device the primary sensorfor detecting the phonon signals absorbed in the gold filmwas placed on a semicircular copper plate over the crystalThe thermal connection between the gold film and theMMCwas made using annealed gold wires Details regarding themeasurement principle and the detector structure of theMMC device were presented in previous reports [19 20]

When a particle hits a dielectric material most of theenergy deposited into the absorber is converted into theform of phonons High energy phonons with frequenciesthat are close to the Debye frequency are generated initiallyHowever they quickly decay to lower frequency phononsvia anharmonic processes When their energy becomes20ndash50K they can travel ballistically in the crystal [21]


Copper holder


MMC device

Annealed gold wires

SQUID

200nm thick

gold film








0 50 100 150

0

005

01

015

02

Time (ms)

Sign

al (a

u)


0 02 04 06 087

71

72

73

74

75

Pulse height (au)

Mea

n tim

e (m

s)Cosmic muons

120572

120573120574







119905mean =sum11990510+119903

11990510minus119897

(V119905times (119905 minus 119905

10))

sum11990510+119903

11990510minus119897V119905

(5)




10 respectively



DP =1205831minus 1205832

radic12059021+ 12059022

(6)









71 72 73 74 750

20

40

60

80

100

120

Mean time (ms)

Cou

nts

120572

120573120574



0

02

04

06

08

1

Time (ms)

minus2 0 2 4

096

098

1

102

Time (ms)

minus10 0 10 20 30 40 50

Sign

al (a

u)

120573

120572



4crystal 120572


4 Energy Spectrum





LA =

119905mean

sum11990510minus119897

V119905 (7)





120573120573value of the 2]120573120573 of



4 its internal



0 1000 2000 3000

103

102

101

100

208Tl

208Tl228Ac

228Ac583keV911keV

965969keV 2615keV

PP + 208Tl511keV

Energyee (keV)C

ount

s per

2 k

eV b

in



4growing technology


4[18]


5 Conclusion



0 2000 4000 6000

1

102

104

106

108

LAe

nerg

y (a

u)

Energy (keV)

(a)

0 2000 4000 6000minus2

0

2

Energy (keV)

Resid

uals

times10minus3

(b)


2000 3000 4000 5000 6000 7000

0

10

20

30

40

50

Cou

nts

147Sm

232Th

228Th227Th

210Po

218Po

238U

230Th

238U226Ra

223Ra

220Ra211Bi

224Ra

Energyae (keV)






4




4crystals




Acknowledgments


References


























4crystal
























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




Copper holder


MMC device

Annealed gold wires

SQUID

200nm thick

gold film








0 50 100 150

0

005

01

015

02

Time (ms)

Sign

al (a

u)


0 02 04 06 087

71

72

73

74

75

Pulse height (au)

Mea

n tim

e (m

s)Cosmic muons

120572

120573120574







119905mean =sum11990510+119903

11990510minus119897

(V119905times (119905 minus 119905

10))

sum11990510+119903

11990510minus119897V119905

(5)




10 respectively



DP =1205831minus 1205832

radic12059021+ 12059022

(6)









71 72 73 74 750

20

40

60

80

100

120

Mean time (ms)

Cou

nts

120572

120573120574



0

02

04

06

08

1

Time (ms)

minus2 0 2 4

096

098

1

102

Time (ms)

minus10 0 10 20 30 40 50

Sign

al (a

u)

120573

120572



4crystal 120572


4 Energy Spectrum





LA =

119905mean

sum11990510minus119897

V119905 (7)





120573120573value of the 2]120573120573 of



4 its internal



0 1000 2000 3000

103

102

101

100

208Tl

208Tl228Ac

228Ac583keV911keV

965969keV 2615keV

PP + 208Tl511keV

Energyee (keV)C

ount

s per

2 k

eV b

in



4growing technology


4[18]


5 Conclusion



0 2000 4000 6000

1

102

104

106

108

LAe

nerg

y (a

u)

Energy (keV)

(a)

0 2000 4000 6000minus2

0

2

Energy (keV)

Resid

uals

times10minus3

(b)


2000 3000 4000 5000 6000 7000

0

10

20

30

40

50

Cou

nts

147Sm

232Th

228Th227Th

210Po

218Po

238U

230Th

238U226Ra

223Ra

220Ra211Bi

224Ra

Energyae (keV)






4




4crystals




Acknowledgments


References


























4crystal
























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics





119905mean =sum11990510+119903

11990510minus119897

(V119905times (119905 minus 119905

10))

sum11990510+119903

11990510minus119897V119905

(5)




10 respectively



DP =1205831minus 1205832

radic12059021+ 12059022

(6)









71 72 73 74 750

20

40

60

80

100

120

Mean time (ms)

Cou

nts

120572

120573120574



0

02

04

06

08

1

Time (ms)

minus2 0 2 4

096

098

1

102

Time (ms)

minus10 0 10 20 30 40 50

Sign

al (a

u)

120573

120572



4crystal 120572


4 Energy Spectrum





LA =

119905mean

sum11990510minus119897

V119905 (7)





120573120573value of the 2]120573120573 of



4 its internal



0 1000 2000 3000

103

102

101

100

208Tl

208Tl228Ac

228Ac583keV911keV

965969keV 2615keV

PP + 208Tl511keV

Energyee (keV)C

ount

s per

2 k

eV b

in



4growing technology


4[18]


5 Conclusion



0 2000 4000 6000

1

102

104

106

108

LAe

nerg

y (a

u)

Energy (keV)

(a)

0 2000 4000 6000minus2

0

2

Energy (keV)

Resid

uals

times10minus3

(b)


2000 3000 4000 5000 6000 7000

0

10

20

30

40

50

Cou

nts

147Sm

232Th

228Th227Th

210Po

218Po

238U

230Th

238U226Ra

223Ra

220Ra211Bi

224Ra

Energyae (keV)






4




4crystals




Acknowledgments


References


























4crystal
























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics






LA =

119905mean

sum11990510minus119897

V119905 (7)





120573120573value of the 2]120573120573 of



4 its internal



0 1000 2000 3000

103

102

101

100

208Tl

208Tl228Ac

228Ac583keV911keV

965969keV 2615keV

PP + 208Tl511keV

Energyee (keV)C

ount

s per

2 k

eV b

in



4growing technology


4[18]


5 Conclusion



0 2000 4000 6000

1

102

104

106

108

LAe

nerg

y (a

u)

Energy (keV)

(a)

0 2000 4000 6000minus2

0

2

Energy (keV)

Resid

uals

times10minus3

(b)


2000 3000 4000 5000 6000 7000

0

10

20

30

40

50

Cou

nts

147Sm

232Th

228Th227Th

210Po

218Po

238U

230Th

238U226Ra

223Ra

220Ra211Bi

224Ra

Energyae (keV)






4




4crystals




Acknowledgments


References


























4crystal
























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




0 2000 4000 6000

1

102

104

106

108

LAe

nerg

y (a

u)

Energy (keV)

(a)

0 2000 4000 6000minus2

0

2

Energy (keV)

Resid

uals

times10minus3

(b)


2000 3000 4000 5000 6000 7000

0

10

20

30

40

50

Cou

nts

147Sm

232Th

228Th227Th

210Po

218Po

238U

230Th

238U226Ra

223Ra

220Ra211Bi

224Ra

Energyae (keV)






4




4crystals




Acknowledgments


References


























4crystal
























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics























4crystal
























FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics








FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics



research article a camoo crystal low temperature detector for … · 2019. 7. 31. · ac coupled...

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