A detector module with highly efficient surface-alpha event rejection operated in CRESST-II Phase 2

Abstract

(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image)

The cryogenic dark matter experiment CRESST-II aims at the direct detection of WIMPs via elastic scattering off nuclei in scintillating CaWO...... crystals. We present a new, highly improved, detector design installed in the current run of CRESST-II Phase 2 with an efficient active rejection of surface-alpha backgrounds. Using CaWO...... sticks instead of metal clamps to hold the target crystal, a detector housing with fully-scintillating inner surface could be realized. The presented detector (TUM40) provides an excellent threshold of ...... keV and a resolution of ...... keV (at 2.60 keV). With significantly reduced background levels, TUM40 sets stringent limits on the spin-independent WIMP-nucleon scattering cross section and probes a new region of parameter space for WIMP masses below 3 GeV/c....... In this paper, we discuss the novel detector design and the surface-alpha event rejection in detail.

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R. Strauss1,2,a, G. Angloher1, A. Bento3, C. Bucci4, L. Canonica4, A. Erb2,5, F. von Feilitzsch2, N. Ferreiro1,P. Gorla4, A. Gtlein2, D. Hauff1 , J. Jochum6, M. Kiefer1, H. Kluck8,9, H. Kraus7, J.-C. Lanfranchi2, J. Loebell6,A. Mnster2, F. Petricca1, W. Potzel2, F. Prbst1, F. Reindl1, S. Roth2, K. Rottler6, C. Sailer6 , K. Schffner4 ,J. Schieck8 , S. Scholl6 , S. Schnert2 , W. Seidel1 , M. von Sivers2,10 , M. Stanger2 , L. Stodolsky1 , C. Strandhagen6,A. Tanzke1 , M. Ufnger6 , A. Ulrich2 , I. Usherov6 , S. Wawoczny2 , M. Willers2 , M. Wstrich1 , A. Zller2

1 Max-Planck-Institut fr Physik, 80805 Munich, Germany

2 Physik-Department, Technische Universitt Mnchen, 85748 Garching, Germany

3 CIUC, Departamento de Fisica, Universidade de Coimbra, 3004 516 Coimbra, Portugal

4 INFN, Laboratori Nazionali del Gran Sasso, 67010 Assergi, Italy

5 Walther-Meiner-Institut fr Tieftemperaturforschung, 85748 Garching, Germany

6 Physikalisches Institut, Eberhard-Karls-Universitt Tbingen, 72076 Tbingen, Germany

7 Department of Physics, University of Oxford, Oxford OX1 3RH, UK

8 Institut fr Hochenergiephysik der sterreichischen Akademie der Wissenschaften, 1050 Wien, Austria

9 Atominstitut, Vienna University of Technology, 1020 Wien, Austria

10 Present address:Albert Einstein Center for Fundamental Physics, University of Bern, 3012 Bern, Switzerland Received: 8 October 2014 / Accepted: 20 July 2015 / Published online: 31 July 2015 The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract The cryogenic dark matter experiment CRESSTII aims at the direct detection of WIMPs via elastic scattering off nuclei in scintillating CaWO4 crystals. We present a new, highly improved, detector design installed in the current run of CRESST-II Phase 2 with an efcient active rejection of surface-alpha backgrounds. Using CaWO4 sticks instead of metal clamps to hold the target crystal, a detector housing with fully-scintillating inner surface could be realized. The presented detector (TUM40) provides an excellent threshold of 0.60 keV and a resolution of 0.090 keV

(at 2.60keV). With signicantly reduced background levels, TUM40 sets stringent limits on the spin-independent WIMP-nucleon scattering cross section and probes a new region of parameter space for WIMP masses below 3GeV/c2. In this paper, we discuss the novel detector design and the surface-alpha event rejection in detail.

1 Introduction

There is compelling evidence that a signicant fraction of the Universe is made of dark matter, suggesting new particles beyond the standard model [1]. Weakly interacting massive particles (WIMPs) [2] are well motivated candidates and might be detectable with earth-bound experiments.

a e-mail: mailto:[email protected]

Web End [email protected]

A variety of detectors were built during the last two decades searching for WIMP-induced nuclear recoils with different target materials and techniques [3]. While for higher WIMP masses (m [greaterorsimilar] 6 GeV/c2) the liquid xenon based

LUX [4] experiment reports the strongest upper limit for the elastic spin-independent WIMP-nucleon cross-section (7.6 1010 pb at m 33GeV/c2), the cryogenic exper

iments SuperCDMS [5] (4.1 GeV/c2 [lessorsimilar] m [lessorsimilar] 6.0 GeV/c2), CDMSlite [6] (3.0 GeV/c2 [lessorsimilar] m [lessorsimilar] 4.1 GeV/c2) and

CRESST-II Phase 2 [7] (m [lessorsimilar] 3.0 GeV/c2) are particularly sensitive to low-mass WIMPs.

Besides the long-standing claim for a WIMP detection by DAMA/LIBRA [8], during the last years, also the experiments CoGeNT [9], CDMS-Si [10] and CRESSTII [11] reported a signal excess which could possibly be interpreted as low-mass WIMPs. Under standard assumption of isoscalar WIMP-nucleon scattering [12] these interpretations, however, are incompatible with the strongest upper limits mentioned above. In addition, results from the XENON100 experiment [13] and a re-analysis of the CRESST-II commissioning-run data [14] disfavour a low-mass WIMP scenario for cross-sections accessible by the respective experiments.

Consequently, for CRESST-II Phase 2 a new detector design was developed providing a strongly reduced background level. First data from CRESST-II Phase 2 [7] acquired

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by a new detector module (called TUM40) presented here explore a new region of parameter space below 3GeV/c2

and strongly constrain the results from CRESST-II. It rules out the WIMP solution at m 11.6GeV/c2 (M2) and dis

favours the solution at m 25.3GeV/c2 (M1).

2 The CRESST-II experiment

2.1 Experimental basics

The Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) uses scintillating CaWO4 crystals as target material [11]. This unique multi-element approach makes CRESST-II sensitive to a wide range of WIMP masses m. Due to the A2-dependence of the coherent spin-independent WIMP-nucleon cross-section [2], the expected event rate is strongly enhanced for scatters off the heavy element W (A 184) which for the obtained threshold

is the dominant target nucleus for m [greaterorsimilar] 5GeV/c2; for lower WIMP masses, however, mostly O (A 16) and Ca

(A 40) scatters can be observed [7]. The scintillation light

output of CaWO4 depends strongly on the ionization strength of the interacting particle [15]. This effect is called quenching and is utilized in CRESST-II detectors for particle discrimination [16].

In a CRESST-II detector module, a cylindrical CaWO4 crystal of typically 300g in mass and 4cm in diameter and height is equipped with a W transition-edge-sensor (TES) to measure phonons induced by a particle interaction. It is referred to as the phonon detector. In addition, a light detector which consists of a 500m thick silicon on sapphire disc

of 4cm in diameter equipped with a W-TES is used as an absorber for the scintillation light. Both detectors are read out simultaneously with SQUIDs.

In the conventional detector design [11], the CaWO4 target crystal is held in place by bronze clamps covered by a thin lm of Al. The elasticity of bronze avoids stress due to different thermal contraction of the various detector components, which otherwise might induce phonon events (with no light signal associated) that could mimic recoils of heavy nuclei [17].

The phonon and light detectors are surrounded by a polymeric multilayer foil (commercially available under the label VM2002). It is highly reective improving the light collection. Furthermore, it is scintillating to establish an active veto against surface-alpha decays. Additional light produced by alpha particles hitting the foil is used to veto the corresponding nuclear recoils entering the target crystal.

2.2 CRESST-II Phase 2

In July 2013, a new dark matter run of CRESST-II with upgraded detectors was started (Phase 2). A total of 18 mod-

ules, corresponding to an overall target mass of 5kg of

CaWO4 were installed. Seventeen modules are fully operational, 11 of which are of the conventional detector design. To reduce the surface backgrounds observed in the previous run of CRESST-II [11] which are suspected to originate from a 210Po contamination of the bronze clamps, material selection and Rn-prevention methods were improved. For the latter, Rn-depleted air, supplied by the CUORE group [18] was used during assembly and mounting of the detectors.

In addition, six alternative detector modules of three different new designs (two each), which aimed at providing an efcient rejection of surface-alpha events were installed: (1) the target crystal is held by CaWO4 sticks (subject of this paper), (2) a smaller CaWO4 crystal, called carrier, carrying the TES is held by bronze clamps1 and is glued to the main target crystals (called carrier-type) and (3) a carrier-type detector is surrounded by a cup-shaped Si light detector (called beaker-type) [19]. For the two latter designs, background identication relies on phonon pulse-shape discrimination between the carrier and the main crystal.

The neutron shielding of the CRESST-II setup was augmented by an innermost layer of 3.5cm of polyethylene to further moderate neutrons. The rst (105) live-days of

CRESST-II Phase 2 were used for a low-mass WIMP analysis [7] and in the present paper for an investigation of the detector performance and backgrounds. Data recorded since January 2014 are still blinded for future analysis.

2.3 Illustration of CRESST-II data

The two-channel readout of CRESST-II detectors allows one to derive Ep and El, the measured energies in the phonon and light detector, respectively. Both channels are calibrated with gamma sources (typically 57Co). In the phonon detector, an energy Ep = 122 keV is assigned to the signal of a 122keV

gamma. The energy of the detected scintillation light of such an event is dened as El = 122 keVee, the so-called electron-

equivalent energy.2 Consequently, the relative light output, called light yield LY = El/Ep, is dened as 1 for 122keV

gammas.

The LY (and to a lesser certain extent also Ep) depends on the type of particle interaction. Due to (light) quenching, nuclear recoils have a reduced light output. This effect is quantied by Quenching Factors (QF). Since the total energy is shared between phonons and emitted photons, nuclear recoils have a slightly (O(5 %)) higher phonon signal com

pared to electron recoils of the same energy [7]. Considering this, the event-type independent total deposited energy3 E

1 The clamps are covered with scintillating Parylene.

2 The unit [keVee] is equivalent to [keV] in the calculations below.

3 In [16] the event-type independent total deposited energy is denoted as Er . The correction is performed, but not explicitly mentioned.

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can be expressed as

E = El + (1 )Ep = Ep(1 (1 LY )) (1)

for an event of a certain LY . The parameter represents the fraction of the total energy deposition escaping the crystal as scintillation light for an event at LY = 1. For the crystal

TUM40 a value of = 0.066 0.004 (stat.) was deter

mined in-situ [7] which is in agreement with independent measurements performed with other CaWO4 crystals, e.g.

using alpha-induced nuclear recoils for calibration [20].

CRESST-II data is usually displayed in the LY vs. E plane where event populations of different types of particle interactions appear as distinct horizontal bands. For CRESSTII detectors, the mean of the dominant electron recoil band LYe(E) is parametrized empirically by

LYe(E) = (p0 + p1E)(1 p2 exp(E/p3)), (2)

where the parameters p0, p1, p2 and p3 are derived by a t to the data4 [16]. The rst factor in Eq. 2 depends on the calibration of the detector. Due to the choice of the LY normalization the parameter p0 is usually very close to 1. The parameter p1 which accounts for linear corrections to the calibration is in most cases consistent with zero and therefore neglected in the following. The second factor in Eq. 2 describes the exponential reduction of the LY towards lower E which is known as non-proportionality effect [2123]. As phenomenologically explained by Birks law, the relative scintillation light output is reduced for low-energy electrons due to their increased local energy loss [15]. The reduction of the LY for a certain kind of particle interaction x = e, , O, Ca, W is described

by QFx which we dene as

QFx(E) =

LYx(E)

p0 . (3)

For electron recoils, this effect depends on the individual CaWO4 crystal used and is described by (see Eqs. 2 and 3):

QFe(E) = 1 p2 exp(E/p3).

QFx for alpha particles and nuclear recoils is, in general, energy dependent [16,23]. However, in the region-of-interest (ROI) for the dark matter search (E [lessorsimilar] 40 keV) constant QFs are a sufcient approximation5: QF = 22 %

[11], QFO = (11.20.5) %, QFCa = (5.940.49) % and

QFW = (1.720.21) % [16].

4 The values found for the crystal TUM40 are: p0 = 0.938, p1 = 4.6

105 keV1, p2 = 0.389 and p3 = 19.34keV.

5 The values of the QFs depend, to a certain extent, also on the individual crystal (e.g. due to different optical properties). In [16] a model is presented to account for this effect. The values given here are mean values measured in CRESST-II detectors.

Fig. 1 Schematic plot of light yield vs. energy. Separate horizontal event bands arise: of beta/gamma events at LY 1 (decreasing in LY

at energies [lessorsimilar]20keV, see Eq. 2), of (degraded) alpha events at LY 0.22

and of nuclear recoils off O, Ca and W at LY [lessorsimilar] 0.1. Possible surface

206Pb events appear at 103keV (at LY 0.01) and depending on

their origin above or below this energy (red arrows). If the corresponding alpha particles hit the scintillating housing, additional light is produced (veto) which shifts these events out of the nuclear-recoil bands (green arrows). The region of interest for dark matter search is indicated by a dashed black line. It includes the nuclear-recoil bands (O, Ca and W) and extends in energy from threshold (here: 0.6keV) to 40keV

Due to statistical uctuations of the phonon and light signals the event bands have a nite width which can be well described by a Gaussian [16]. The width (in LY ) of a certain recoil band is given by

x(E) =

1 E

2l + [parenleftbigg]d Eld E ph

2+ S1El + S2E2l, (4)

with l and ph being the baseline uctuation of phonon and light detector, respectively, S1 the deposited energy per detected photon (accounting for photon statistics) and S2 accounting for possible position dependencies in light production and detection (not relevant in the ROI).6

In Fig. 1, where the LY is plotted against the (event-type independent) total deposited energy E, a schematic view of the event bands (beta/gamma, alpha, O, Ca and W) is shown. Due to the nite resolution these populations overlap at lower energies. The ROI for WIMP search which includes all three nuclear-recoil bands (O, Ca and W) and extends in E from threshold (here: 0.6keV) to 40keV is depicted with dashed black lines.

6 For TUM40 these parameters are l = 269eV, ph = 91eV, S1 =

256eV and S2 0.

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2.4 Observed background in CRESST-II

During the previous dark matter run of CRESST-II, 730kg-days of exposure have been acquired by 8 detector modules with a total target mass of 2.4kg [11]. Several types of

backgrounds are identied, mainly originating from intrinsic contaminations of the CaWO4 crystals and from contaminations of the direct vicinity of the detectors. In the electron-recoil band a variety of beta-spectra and gamma-lines up to the MeV range are visible in typical CRESST-II detectors [22]. At low energies E < 100 keV the spectra of commercially available crystals are usually dominated by intrinsic contamination of 227Ac and 210Pb [24]. Their mean count

rate in the ROI ranges from 6 to 30/(kgkeVday). For crystals from in-house production at the Technische Universitt Mnchen (TUM) the background level could be reduced signicantly to 3.51/(kgkeVday) in the ROI (for TUM40) [25].Peaks originating from cosmogenic activation of different tungsten isotopes (electron-capture transitions), dominate the low-energy spectrum [25].

In the alpha band, several populations of events were observed. The distinct alpha peaks from intrinsic contamination by natural decay chains, by rare earth metals (e.g.

147Sm, 144Nd) and by the radioactive isotope 180W [26] have energies in the MeV range and are far off the ROI [25,27].However, if alpha emitters are embedded in the bulk of material surrounding the detectors (e.g. the bronze clamps) the corresponding alpha particles can lose part of their energy before being absorbed in the CaWO4 crystal. This population of degraded alphas can leak down to the ROI [11]. Recoiling nuclei from surface alpha contamination, either on the crystal or on surrounding materials, show up at very low LY comparable with that of W recoils. For CRESST-II detectors mainly

206Pb recoils from 210Po decays (see Fig. 2) are relevant:

210Po 206 Pb(103 keV) + (5.3 MeV). (5)

If the decay occurs sufciently close to surfaces the full energy of 103keV is detected. However, Pb-recoils from

Fig. 2 Illustration of 210Po decays occurring either in surrounding material or close to the surface of the target crystal [11]

210Po implanted in surrounding surfaces, deposit less energy in the phonon detector. In contrast, for Pb-events originating from the crystals surface part of the alpha energy is deposited additionally in the target crystal. The different classes of

206Pb recoils observed at low LY (QFPb 1.4%) are illus

trated as red arrows in Fig. 1.

3 The novel detector design

3.1 Fully-scintillating detector housing

In the previous dark matter run of CRESST-II, surface-alpha decays of 210Po caused the highest identied background contribution. A maximum-likelihood analysis attributed up to 25% of the excess events [11] to 206Pb recoils from a

contamination on the non-scintillating bronze clamps holding the CaWO4 crystal. Pb-recoils which occur at LY s as low as W-recoils can be shifted out of the nuclear-recoil region if the corresponding alpha produces additional light. This can be achieved by surrounding the crystal by scintillating material completely. It was reported earlier [11,28] that the polymeric foil surrounding the target crystal scintillates sufciently to establish such a veto. However, all attempts to cover the bronze clamps with scintillating material (e.g. plastic scintillator) have failed. Thermo-mechanical stress in the clamp-plastic bilayer can relax and cause false signals in the phonon detector. Since pulse-shape differences are not sufcient to fully discriminate such events and no scintillation light is produced, at low energies, such events can mimic recoils of heavy nuclei.

The new detector design presented here uses a different approach to avoid any non-scintillating surface inside the detector housing and, thus, to reach an efcient surface-event rejection. CaWO4 sticks, penetrating the polymeric foil through a set of tightly tting holes and spring-loaded by pure bronze clamps on the outside of the Cu-housing, hold the CaWO4 crystal in place. Thereby, elasticity of the system is maintained down to mK temperatures while solely scintillating material is in line-of-sight of the CaWO4 target crystal.

Figure 3 shows a sketch of the novel detector design. A 249g block-shaped (rectangular) CaWO4 crystal of 32mm edge length and 40mm height is held by eight CaWO4 sticks from the side (diameter 2.5mm, length 7.5mm) and one CaWO4 stick from the bottom (diameter 4.0mm, length8.2mm). The CaWO4 crystal and the sticks are produced at the TUM (see Sect. 3.2). The side sticks are pressed onto the crystal by bronze clamps and the bottom stick is xed in the copper support structure. The sticks have a polished spherical end which provides a point-like contact with the target crystal. The remaining surface is covered with polymeric foil

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light detector (with TES)

block-shaped target crystal

reflective and

scintillating housing

CaWO4 sticks(with holding clamps)

Fig. 3 Schematic view of the novel fully-scintillating detector module. A block-shaped target crystal is held by CaWO4 sticks. The bottom stick is xed in the copper support structure and the eight side sticks are held by bronze clamps from outside the holder. The positions of the sticks are indicated by white circles. Together with the scintillating polymeric foil an active veto against all surface events is realized

(VM2002). A conventional silicon on sapphire light detector is mounted onto the novel detector holder.

With this design, each nuclear recoil from alpha decays occurring on any surface in the holder is vetoed by additional scintillation light from the alpha. Two different cases have to be considered:

1. 210Po decays on the surface of the target crystal deposit (in the phonon detector) the full energy of the 206Pb recoil

(103keV) plus potentially part of the energy of the corresponding alpha particle depending on the implantation depth.7 In the light detector, the scintillation light of the Pb-recoil and of part of the alpha energy, as well as the additional light produced in the foil or CaWO4 are measured. The energy corresponding to the additional light detected is called Eveto. The LY of this kind of surface event LYs can be expressed as

LYs = p0QFPb +

Eveto + p0QF(E 103 keV)

E , (6)

with QFPb 1.4% [11].

2. 210Po decays on the surface of the polymeric foil or the CaWO4 sticks induce Pb-recoils in the CaWO4 crystal which deposit 103keV or less. In the light channel, the light of the Pb-recoil plus the scintillation light from foil or stick are detected. This gives a LY of

LYs = p0QFPb +

Eveto

E . (7)

Typical values of Eveto for 5MeV alphas are 40keVee

for the foil and 1000keVee for the CaWO4 sticks. In Fig.

1 the mean of the band of vetoed surface events is illustrated (green arrows), clearly separated from the nuclear-

7 Decays of 222Rn and its daughters impact the respective nuclei into the CaWO4 material. Consequently, the 210Po nuclei end up at different (implantation) depths of O(100nm) with respect to the surface.

recoil bands by additional light produced in the foil (yellow arrow).

In case a Pb-event is vetoed by the corresponding alpha interacting in a CaWO4 stick, Eveto is sufciently high to shift it to far above the electron-recoil band. In addition, a certain fraction (1 %) of the energy deposited by an alpha

in a stick is visible in the phonon channel (see Sect. 4.1). This effect is neglected in Eqs. 6 and 7.

3.2 CaWO4 crystals produced at TUM

CaWO4 crystals produced at the TUM are operated in the second phase of CRESST-II. A Czochralski growth furnace was set up with the intention to improve the radiopurity, the optical properties and to secure the supply of CaWO4 single crystals for CRESST and future rare-event searches [29]. The crystals, which can be grown in a reproducible way and of sufcient size (diameter >35mm, height [greaterorsimilar]40mm) since 2012, are further machined at the crystal lab of the TUM.

Several techniques to investigate and improve the optical and scintillation properties have been developed [30]. First measurements, in which in-house produced CaWO4 crystals were operated as cryogenic detectors, were carried out at the CRESST test cryostat (located underground at the LNGS in Italy) to investigate radiopurity and light output [31]. Using these experimental data, dedicated studies of the intrinsic alpha contamination [27,31] gave the rst indication that the radiopurity (total internal alpha background 3mBq/kg) is

improved by a factor of 210 with respect to commercially available crystals. Concerning scintillation and optical properties, the performance of crystals from external suppliers could not yet be accomplished. With a standard cylindrical CRESST-size crystal (TUM27), of which all sides are optically polished except the side facing the light detector, the fraction L of the total deposited energy detected as light (for a beta/gamma event) is L1.1 % [31]. This compares with

a maximum of L2.4 % achieved with the best commercial

crystals [20]. Within these studies, the inuence of roughening a larger part of the surface was measured. The light output of TUM27 was increased by 20% to L1.3%, when the lat

eral surface was roughened in addition, while no inuence on the phonon properties was observed.

The block-shaped crystal TUM40, which is supported by CaWO4 sticks, was operated in several measurement campaigns at the test cryostat at LNGS before being installed into the CRESST setup. Even though the quality of the CaWO4 material is comparable to that of TUM27 (same raw materials and growth procedure) a higher amount of detected light (L1.6%) [31] was measured with this novel detector.

Roughening ve surfaces instead of only one did not significantly change L, however, reduced position dependencies of the light signal detected [31]. Problems with stress relaxations occurred whenever CaWO4 sticks were in contact with

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CaWO4 carrier crystal with TES

block-shaped CaWO4 target crystal

polished spots

Fig. 4 The nal state of the CaWO4 crystal TUM40, as used in the experiment. The crystal was produced in-house at the TUM. A CaWO4 carrier (10 20 2mm3) with the W-TES is glued onto the polished

side of the main crystal. The areas where the CaWO4 sticks touch (7

7mm2) remain polished while the rest of the crystal is roughened

roughened surfaces [31]. Therefore, the very spots (7mm

7mm) where the sticks touch the crystal are left polished while the rest of the surface is roughened. The side facing the light detector is completely roughened while the opposite one is polished (in contact with bottom stick). A picture of the crystal TUM40 is shown in Fig. 4.

The TES is evaporated onto a separate CaWO4 carrier (10

20 2mm3) which is then glued onto the main absorber.

This avoids having to expose the main crystal to very high temperatures (up to 600C) during the evaporation process of the TES which degrades the scintillation light output [32].

The W-TES has a transition temperature of Tc = 20.2 mK.

The carrier is glued onto the polished surface which is opposite to the light detector. The bond wires used to electrically connect the W-TES are fed through small slits in the surrounding polymeric foil [31].

4 Results

4.1 Detector performance

After a commissioning period of 2weeks, TUM40 and

the light detector were operated under stable conditions in the CRESST-II setup. Before starting the dark matter run, a gamma-calibration campaign with a 57Co source was performed. The 122keV gamma-line is used for calibrating the heater pulses which are periodically injected onto the TESs. Heater pulses ensure the calibration down to lowest energies and the long-term stability of the operation point (see, e.g., [11]).

The use of the CaWO4 sticks does not inuence the detector performance, and no signs of microphonic noise are observed. Pulses from the phonon channel are comparable to the ones of conventional detector modules.8 An excellent res-

8 Within the model for cryogenic particle detectors [33] decay times of thermal and non-thermal signal components of the particle pulses are t = 91.6ms and n = 21.1ms, respectively.

Fig. 5 Low-energy spectrum acquired with the module TUM40 and an exposure of 29kg-days in CRESST-II Phase 2. The prominent peaks at11.27keV and 2.60keV originate from cosmogenic activation of 182W

(see [25] for details). Left inset t to the 2.60keV line (red line). A resolution of = (0.0900.010) keV is achieved [7]. Right inset the

energy-dependent trigger efciency of TUM40 which is measured with injected heater pulses (grey dots) is shown. A t with an error function (red line) yields an energy threshold (50 % efciency) of Eth (0.603

0.02)keV [7]

olution of = (0.0900.010) keV (at 2.60keV) is achieved

with TUM40 as shown in Fig. 5. In the low-energy spectrum recorded in CRESST-II Phase 2 from an exposure of 29kg-days, peaks from cosmogenic activation of tungsten isotopes are visible (see [25]). The trigger efciency of the phonon detector was probed with injected heater pulses (grey dots in Fig. 5, inset). A t with an error function (red line) gives a trigger threshold (50 % efciency) of Eth (0.6030.02)keV

[7].

At an early stage of the CRESST experiment, events originating from mechanical stress relaxations were observed. They were caused by a tight and rigid clamping of crystals [17]. Dedicated studies [31] and rst data of CRESST-II Phase 2 show that by using exible bronze clamps mechanical stress on the stick-crystal interfaces is mitigated sufciently with the novel detector design.

Particle events in the CaWO4 sticks themselves produce scintillation light measured in the light detector. Due to the much smaller size of the sticks compared to the bulk crystal the relative light output of stick events is higher by a factor of up to three. To a certain extent, the induced phonon signal is detected in the TES of the main crystal through the stick-crystal interface. Since that connection is point-like only a degraded phonon signal from particle events in the sticks is expected. A dedicated measurement was performed at the test cryostat to quantify this so-called phonon-quenching effect at such interfaces [31]. A 147Sm alpha source was placed at the CaWO4-stick (bottom) to calibrate the energy scale of particle events. A degradation of the phonon signal from events in the sticks by about two orders of magnitude (by a factor of 966) was found. Due to the increased rela-

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tive light output, the population of beta/gamma and alpha events in the CaWO4-sticks exhibits light yields of 2030 and 46, respectively, far off the bulk crystals recoil bands.These event classes can be well separated from the ROI by LY (e.g. beta/gamma events with 7.7 C.L. at E = 10 keV

[31]). In addition, stick events can be discriminated by pulse shape (the thermal component t [33] is enhanced) at energies [greaterorsimilar]10keV. However, nuclear recoils occurring on the sticks surfaces and the corresponding alpha particle hitting a non-scintillating surface (e.g. the bronze clamps outside the housing) might leak into the ROI at smallest energies [lessorsimilar]1keV. Considering the area of unvetoed surfaces, only 0.5 0.2 events are expected from this background com

pared to O(100) events of leakage from the beta/gamma

band [25]. For the next-generation CRESST detectors, an additional scintillation veto is planned.

4.2 Surface backgrounds

The main purpose of the novel detector design was to reduce signicantly backgrounds related to surface-alpha decays. In CRESST-II a mean rate of 0.05/[kgday] from degraded

alphas and Pb recoils was observed in the ROI (1240keV)

of the eight detector modules operated [11].

First data of TUM40 from the CRESST-II Phase 2 are shown as a LY-energy plot in Fig. 6. Events in the populated beta/gamma band at LY 1 are indicated by black dots.

Within the blue lines, namely the 90% upper bound of O recoils and the 90% lower bound for W recoils, at LY

0.1 the nuclear recoil region is shown. The region where 80 % of all vetoed surface events are expected is depicted in shaded green. Therein, 12 events appear at recoil energies of

103keV which can be identied clearly as Pb-recoils from

210Po decays. They are vetoed by the additional light signal produced by the alpha interacting in the polymeric foil. In addition, these events can be tagged by a different light-pulse shape (illustrated by black squares in Fig. 6). This is possible, since the scintillation-time constant of the foil ( 100 ns)

is fast compared to the one of CaWO4 ( 500 s) [34]. The

events are found at a mean LY of 0.36 which corresponds

to a mean additional light energy of Eveto 37 keVee (in

units of electron-equivalent energy). Taking into account that for TUM40 1.6% of the deposited energy in the CaWO4

crystal is detected as scintillation light (see Sect. 3.2), the actual amount of light energy detected from a 5.3MeV alpha impinging the foil is 0.59keV.

The vetoed events at 103keV must originate from 210Po

decays close to surfaces of either the crystal or the surrounding foil. Only the recoiling 206Pb-nucleus is detected in the phonon channel (the alpha interacts in the foil). Consequently, for geometrical reasons a similar number of events where only the corresponding alpha is detected in the crystal should be observed. In the alpha band, a peak arises at

Fig. 6 Light yield (LY) vs. energy (E) of TUM40 data with an exposure of 29kg-days. The populated beta/gamma band is visible at LY 1. At LY 0.1, the 90% upper bound of O recoils and the

90% lower bound for W recoils are shown (full blue lines). The region where 80% of all vetoed surface events are expected is depicted in shaded green. 12 vetoed Pb-recoils (black squares) are observed at E 103keV which can in addition be tagged by pulse shape. The 5

vetoed events at energies between 130keV and 300keV are Pb-recoils from the crystals surface where also part of the energy of the corresponding alpha is absorbed. The areas within red lines at LY 0.22

(LY 0.01) indicate the reference region in which degraded alpha

(206Pb recoil) events would be expected

a energy of 5.3MeV. Nine events are identied as alphas

from 210Po [25] which is consistent with the 12 Pb recoils observed.

In the green band of vetoed events additional 5 events are observed between 130keV and 300keV. Most probably those originate from shallow implanted 210Po decays in the surface of the crystal. Surface events vetoed by the CaWO4 sticks are expected to lie at LY 10 which is far off the ROI. Due to

limited statistics such events could not be veried.

In Fig. 6 two reference regions are dened (within red lines): (1) for degraded alpha events in the alpha band at LY 0.22 from 40300keV, (2) for 206Pb recoils at LY

0.01 from 40107keV. With 29kg-days of exposure both reference regions are free of events (the event at E 85keV

in reference region 1 is identied as a Pb-recoil and can be rejected). For comparison, assuming the background level to be as observed in the previous run of CRESST-II, in this exposure, 8.12.8 degraded alphas and 6.92.6 Pb-recoils

would be expected. This proves the high efciency of the active surface veto resulting from the new detector design.

5 Conclusions

The CaWO4 crystal TUM40 operated in the novel detector module has reached signicantly reduced background levels. By supporting the target crystal with CaWO4 sticks a phonon detector with an excellent performance has been realized. A resolution of = (0.0900.010) keV (at 2.60keV)

123

352 Page 8 of 8 Eur. Phys. J. C (2015) 75 :352

and a trigger threshold of 0.60keV were achieved [7].

Using a CaWO4 crystal grown at the TUM [29] the intrinsic background level could be signicantly reduced: the average rate of low-energy events (140keV) amounts to3.51/(kgkeVday) [25]. The surface-event veto due to the fully-scintillating housing rejects backgrounds from surface-alpha decays with high efciency. With 29kg-days of exposure from the ongoing run of CRESST-II Phase 2 no events related to degraded alphas or Pb recoils are observed, while these were the dominant identied background sources during the previous run of CRESST-II.

Using the rst data of TUM40, a low-mass WIMP analysis was performed. Limits on the spin-independent WIMP-nucleon cross section were achieved and a new region of parameter space for WIMP masses below 3GeV/c2 was probed [7].

The performance of TUM40 demonstrates that the phonon-light technique using CaWO4 as target material has great potential for WIMP searches.

Acknowledgments This research was supported by the DFG cluster of excellence: Origin and Structure of the Universe, the DFG Transregio 27: Neutrinos and Beyond, the Helmholtz Alliance for Astroparticle Phyiscs, the Maier-Leibnitz- Laboratorium (Garching), the Science & Technology Facilities Council (UK) and by the BMBF: Project 05A11WOC EURECA-XENON. We are grateful to LNGS for their generous support of CRESST, in particular to Marco Guetti for his constant assistance.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Web End =http://creativecomm http://creativecommons.org/licenses/by/4.0/

Web End =ons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Funded by SCOAP3.

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