The DAMIC dark matter experiment
A. Aguilar-Arevalo, D. Amidei, X. Bertou, D. Bole, M. Butner, G. Cancelo, A. Castañeda Vázquez, A. E. Chavarria, J. R. T. de Mello Neto, S. Dixon, J. C. D'Olivo, J. Estrada, G. Fernandez Moroni, K. P. Hernández Torres, F. Izraelevitch, A. Kavner, B. Kilminster, I. Lawson, J. Liao, M. López, J. Molina, G. Moreno-Granados, J. Pena, P. Privitera, Y. Sarkis, V. Scarpine, T. Schwarz, M. Sofo Haro, J. Tiffenberg, D. Torres Machado, F. Trillaud, X. You, J. Zhou
TThe DAMIC dark matter experiment
J.R.T. de Mello Neto f ∗ on behalf of the DAMIC CollaborationA. Aguilar-Arevalo a , D. Amidei b , X. Bertou c , D. Bole b , M. Butner d , j , G. Cancelo d ,A. Castañeda Vázquez a , A.E. Chavarria e , S. Dixon e , J.C. D’Olivo a , J. Estrada d ,G. Fernandez Moroni d , K.P. Hernández Torres a , F. Izraelevitch d , A. Kavner b ,B. Kilminster g , I. Lawson h , J. Liao g , M. López i , J. Molina i , G. Moreno-Granados a ,J. Pena e , P. Privitera e , Y. Sarkis a , V. Scarpine d , T. Schwarz b , M. Sofo Haro c ,J. Tiffenberg d , D. Torres Machado f , F. Trillaud a , X. You f and J. Zhou e a Universidad Nacional Autónoma de México, México D.F., México b University of Michigan, Department of Physics, Ann Arbor, MI, United States c Centro Atómico Bariloche - Instituto Balseiro, CNEA/CONICET, Argentina d Fermi National Accelerator Laboratory, Batavia, IL, United States e Kavli Institute for Cosmological Physics and The Enrico Fermi Institute, The University ofChicago, Chicago, IL, United States f Universidade Federal do Rio de Janeiro, Instituto de Física, Rio de Janeiro, RJ, Brazil g Universität Zürich Physik Institut, Zurich, Switzerland h SNOLAB, Lively, ON, Canada i Facultad de Ingeniería - Universidad Nacional de Asunción, Paraguay j Northern Illinois University, DeKalb, IL, United StatesE-mail: [email protected]
The DAMIC (Dark Matter in CCDs) experiment uses high resistivity, scientific grade CCDs tosearch for dark matter. The CCD’s low electronic noise allows an unprecedently low energythreshold of a few tens of eV that make it possible to detect silicon recoils resulting from in-teractions of low mass WIMPs. In addition the CCD’s high spatial resolution and the excellentenergy response results in very effective background identification techniques. The experimenthas a unique sensitivity to dark matter particles with masses below 10 GeV/c . Previous resultshave demonstrated the potential of this technology, motivating the construction of DAMIC100, a100 grams silicon target detector currently being installed at SNOLAB. In this contribution, themode of operation and unique imaging capabilities of the CCDs, and how they may be exploitedto characterize and suppress backgrounds will be discussed, as well as physics results after oneyear of data taking. The 34th International Cosmic Ray Conference,30 July- 6 August, 2015The Hague, The Netherlands ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://icrc2015.nl a r X i v : . [ phy s i c s . i n s - d e t ] O c t he DAMIC dark matter experiment J.R.T. de Mello Neto f
1. Introduction
A well established body of evidence from astrophysics and cosmology supports the existenceof cold dark matter as the major component of the material content of the universe. The leadingcandidate for this dark matter is a hypothetical weakly interacting massive particle (WIMP) [1,2]. WIMPs could produce keV-energy nuclear recoils when scattering elastically off target nucleiin the detector. Minimal supersymmetric extensions to the standard model favor particles above50 GeV/c , while other models relating dark matter with the baryon asymmetry prefer massesaround 5 GeV/c [3, 4, 5]. Several experiments have reported statistically significant evidence ofWIMPs scattering on light nuclear targets [7, 6].The DAMIC (Dark Matter in CCDs) experiment uses the bulk silicon of scientific-gradecharge-coupled devices (CCDs) as the target for coherent WIMP-nucleus elastic scattering. Dueto the low readout noise of the CCDs and the relatively low mass of the silicon nucleus, CCDsare ideal instruments for the identification of the nuclear recoils with keV-scale energies and lowerfrom WIMPs with masses < 10 GeV/c .The first DAMIC measurements were performed in a shallow underground site at Fermilab us-ing several 1-gram CCD detectors developed for the Dark Energy Survey (DES) camera (DECam)[8]. With 21g-days DAMIC produced the best upper limits on the cross-section for WIMPs below4 GeV/c [9]. DAMIC is now located in SNOLAB laboratory 2 km below the surface in the ValeCreighton Mine near Sudbury, Ontario, Canada.
2. The DAMIC detectors
The DAMIC CCDs feature a three-phase polysilicon gate structure with a buried p-channel.The CCDs are typically 8 or 16 Mpixels, with pixel size of 15 µ m × µ m, with a total surfacearea of tens of cm . The CCDs are 675 µ m thick, for a mass up to 5.2 g. A high-resistivity (10-20k Ω cm) n-type silicon allows for a low donor density in the substrate ( ∼ cm − ), which leads tofully depleted operation at low values of the applied bias voltage ( ∼
40 V for a 675 µ m-thick CCD).Fig. 1 shows a cross-sectional diagram of a CCD pixel, together with a sketch depicting the WIMPdetection principle. The substrate voltage also controls the level of lateral diffusion of the chargecarriers as they drift the thickness of the CCD. The lateral spread (width) of the charge recorded onthe CCD x - y plane may be used to reconstruct the z -coordinate of a point-like interaction [10].Long exposures are taken in DAMIC ( ∼ e − pix − day − at the operating temperatureof ∼
140 K) contributes negligibly to the noise. During readout, the charge held at the CCD gatesis measured by shifting charge row-by-row and column-by-column via phased potential wells toa low capacitance output gate. The inefficiency of charge transfer from pixel to pixel is as lowas 10 − . The readout noise for the charge collected in a pixel is ∼ e − which corresponds to anuncertainty of ∼ Fe source, with fluorescence X-rays from a Kapton target exposed to the Fe source and with α s from Am were performed. As shown in fig. 2, the detectors presentan excellent linearity and energy resolution (55 eV RMS at 5.9 KeV) for electron-induced ioniza-2 he DAMIC dark matter experiment
J.R.T. de Mello Neto f Buriedp channel3-phaseCCD structurePoly gateelectrodesn —— (10 k Ω -cm)Photo-sensitivevolume(200_300 µ m)Transparentrear window Biasvoltagexy Figure 1 . Cross-sectional diagram of the CCD described in this work.
2. FULLY-DEPLETED CCD PHYSICS AND OPERATION
Figure 1 shows a cross-sectional diagram of the fully-depleted, back-illuminated CCD. A conventionally-processed,three-phase CCD is fabricated on a high-resistivity, n-type silicon substrate. We have fabricated CCD’s on both100 mm and 150 mm diameter high-resistivity silicon substrates. The resistivity of 100 mm wafers is as high as10,000–12,000 -cm, while the initial work on 150 mm wafers has been on 4,000–8,000 -cm silicon.The thickness of the CCD results in improved near-infrared sensitivity when compared to conventional thinnedCCD’s. This is due to the strong dependence of absorption length on wavelength at photon energies approachingthe silicon bandgap. Figure 2 shows measured quantum e ciency (QE) versus wavelength for a fully-depleted,back-illuminated CCD operated at C. The QE is especially high at near-infrared wavelengths. The CCDshown in Figure 2 has a two-layer anti-reflection (AR) coating tuned for good red response. It consists of 60 nmof indium tin oxide (ITO) and 100 nm of silicon dioxide (SiO ).Thick, fully-depleted CCD’s also greatly reduce the problem of “fringing” at near-infrared wavelengths. Fringing occurs when the absorption depth of the incident light exceeds the CCD thickness. Multiple reflectionsresult in fringing patterns that are especially a problem in 10–20 µ m thick CCD’s used in spectrographs.A unique feature of the CCD shown in Figure 1 is the use of a substrate bias to fully deplete the substrate.For a thick CCD fabricated on high-resistivity silicon the channel potential is to first order independent of thesubstrate bias. This is because for typical substrate thicknesses and doping densities considered here onlya small fraction of the electric field lines from the depleted channel terminate in the fully-depleted substrate.Hence the vertical clock levels can be set to optimize operating features such as well capacity and CTE whilethe substrate bias is used to deplete the substrate.The substrate bias also plays a role in the point-spread function of the CCD. For light absorbed near theback surface of the CCD the lateral charge spreading during transit of the photogenerated charges through thefully-depleted substrate to the CCD collection wells is described by an rms standard deviation given by
1, 6 od kTq y D ( V sub V J ) (1)where k is Boltzmann’s constant, T is absolute temperature, q is the electron charge, y D is the thickness ofthe depleted substrate, V sub is the applied substrate bias voltage, and V J is an average potential near theCCD potential wells due to the channel potentials. V sub V J is the voltage drop across the region wherethe photogenerated holes are drifted by the electric field. This result is a simplified asymptotic form that isindependent of the substrate doping and is valid for high electric fields in the substrate. Therefore in this casethe PSF is directly proportional to y D , T , and 1/ ( V sub V J ). The PSF for a CCD of this type can beimproved by reducing the substrate thickness and operating the CCD at high substrate bias. PSF measurementsare described in more detail in Section 5. z (500 μ m) (a) A CCD pixel (cid:1)(cid:1)(cid:2) (cid:3)(cid:4) (cid:5)(cid:3)(cid:4) (cid:3)(cid:4) (cid:3)(cid:4)(cid:3)(cid:4) (cid:3)(cid:4)(cid:3)(cid:4) (cid:3)(cid:4) (cid:3)(cid:4)(cid:3)(cid:4) (cid:1)(cid:2) (cid:3)(cid:4)(cid:3)(cid:5)(cid:6)(cid:7)(cid:8)(cid:9)(cid:10)(cid:8)(cid:4)(cid:3)(cid:11) (cid:6)(cid:4)(cid:7)(cid:8)(cid:9) (cid:9)(cid:12)(cid:5)(cid:4)(cid:3)(cid:13)(cid:7)(cid:7)(cid:3)(cid:5)(cid:8)(cid:14)(cid:4) (cid:10)(cid:11)(cid:12)(cid:8)(cid:13)(cid:8)(cid:14)(cid:15)(cid:16)(cid:8)(cid:9)(cid:17)(cid:18)(cid:15)(cid:4)(cid:10)(cid:16)(cid:18)(cid:10)(cid:17)(cid:15)(cid:15)(cid:8)(cid:13)(cid:4)(cid:14)(cid:19) (cid:3)(cid:4) z xy WIMP (b) WIMP detection in a CCD
Figure 1. a) Cross-sectional diagram of a 15 µ m ⇥ µ m pixel in a fully depleted, back-illuminated CCD.The thickness of the gate structure and the backside ohmic contact are µ m. The transparent rear window,essential for astronomy applications, has been eliminated in the DAMIC CCDs. b) Dark matter detection ina CCD. A WIMP scatters with a silicon nucleus producing ionization in the CCD bulk. The charge carriersare then drifted along the z -direction and collected at the CCD gates. image Energy measured by pixel / keV image α e μ X-ray?n, WIMP? Diffusionlimited p i x e l s Front Back (a) Portion of a DAMIC image
Energy measured by pixel / keV α X-ray absorption pe from Si fluorescence X-ray absorption (b) Emission of Si fluorescence X-ray
Figure 2. a) 50 ⇥
50 pixel portion of a CCD image, taken when the detector was at ground level. Differentkinds of particles are recognizable (see text). For better contrast, only pixels with deposited energy > Fesource. The 1.7 keV cluster is a photoelectron (pe) from the absorption of a Si fluorescence X-ray, emittedfollowing photoelectric absorption of the incident 5.9 keV Mn K a X-ray in a nearby site. ing of the 3-phase gates (“parallel clocks”), while higher frequency clocks (“serial clocks”) movethe charge of the last row horizontally to a charge-to-voltage amplifier (“output node”). The in-efficiency of charge transfer from pixel to pixel is as low as 10 and the readout noise for thecharge collected in a pixel is ⇠ e [2]. Since on average 3.6 eV is required to ionize an electronin silicon, the readout noise corresponds to an uncertainty of ⇠ Figure 1: a) Cross-sectional diagram of a 15 µ m × µ m pixel in a fully depleted, back-illuminatedCCD. The thickness of the gate structure and the backside ohmic contact are ≤ µ m. The transparent rearwindow has been eliminated in the DAMIC CCDs. b) Dark matter detection in a CCD. A WIMP scatterswith a silicon nucleus in the active region, producing ionization from the nuclear recoil which drifts alongthe z-direction and is collected at the CCD gates. Energy calibration using X-rays
Energy / keV R e c o n s t r u c t e d e n e r g y / k e V Calibration data to X-ray lines
C OAl-K Si-K Ca-K Fe Am Energy / keV V a r ( E ) / k e V -3 -2 Energy resolution from X-ray lines F a n o = . E [keV] Al Si Esc K ⇥ Esc K K K ⇥ Fe
15 Semin´ario CBPF April 15, 2015
Figure 2: a) Reconstructed energy of an X-ray line compared to is true energy. The labeled K α markers arefluorescence lines from elements in the Kapton target and other materials in the CCD setup. The Fe and
Am markers are X-rays emitted by the radioactive sources. Linearity in the measurement of ionizationenergy is demonstrated from 0.3 kev to 60 keV. b) Variance of the X-ray lines as a function of energy. Theeffective Fano factor is 0.16, typical for a CCD [11]. tion, as measured with X-ray sources [10]. The ionization efficiency of nuclear recoils is signifi-cantly different than that of electrons. Previous measurements have been done down to energies of3-4 keV r [12, 13] in agreement with Lindhard theory [14]. From this, DAMIC’s nominal 50 eV ee threshold corresponds to ∼ r .The total charge and shape of each hit is extracted using dedicated image analysis tools. Infig. 3 a sample of tracks recorded during a short exposure at sea level to a Cf source is shown.Clusters from different types of particles may be observed. Low energy electrons and nuclearrecoils, whose physical track length is <15 µ m, produce ”diffusion limited” clusters, where the3 he DAMIC dark matter experiment J.R.T. de Mello Neto f Buriedp channel3-phaseCCD structurePoly gateelectrodesn —— (10 k Ω -cm)Photo-sensitivevolume(200_300 µ m)Transparentrear window Biasvoltagexy Figure 1 . Cross-sectional diagram of the CCD described in this work.
2. FULLY-DEPLETED CCD PHYSICS AND OPERATION
Figure 1 shows a cross-sectional diagram of the fully-depleted, back-illuminated CCD. A conventionally-processed,three-phase CCD is fabricated on a high-resistivity, n-type silicon substrate. We have fabricated CCD’s on both100 mm and 150 mm diameter high-resistivity silicon substrates. The resistivity of 100 mm wafers is as high as10,000–12,000 -cm, while the initial work on 150 mm wafers has been on 4,000–8,000 -cm silicon.The thickness of the CCD results in improved near-infrared sensitivity when compared to conventional thinnedCCD’s. This is due to the strong dependence of absorption length on wavelength at photon energies approachingthe silicon bandgap. Figure 2 shows measured quantum e ciency (QE) versus wavelength for a fully-depleted,back-illuminated CCD operated at C. The QE is especially high at near-infrared wavelengths. The CCDshown in Figure 2 has a two-layer anti-reflection (AR) coating tuned for good red response. It consists of 60 nmof indium tin oxide (ITO) and 100 nm of silicon dioxide (SiO ).Thick, fully-depleted CCD’s also greatly reduce the problem of “fringing” at near-infrared wavelengths. Fringing occurs when the absorption depth of the incident light exceeds the CCD thickness. Multiple reflectionsresult in fringing patterns that are especially a problem in 10–20 µ m thick CCD’s used in spectrographs.A unique feature of the CCD shown in Figure 1 is the use of a substrate bias to fully deplete the substrate.For a thick CCD fabricated on high-resistivity silicon the channel potential is to first order independent of thesubstrate bias. This is because for typical substrate thicknesses and doping densities considered here onlya small fraction of the electric field lines from the depleted channel terminate in the fully-depleted substrate.Hence the vertical clock levels can be set to optimize operating features such as well capacity and CTE whilethe substrate bias is used to deplete the substrate.The substrate bias also plays a role in the point-spread function of the CCD. For light absorbed near theback surface of the CCD the lateral charge spreading during transit of the photogenerated charges through thefully-depleted substrate to the CCD collection wells is described by an rms standard deviation given by
1, 6 od kTq y D ( V sub V J ) (1)where k is Boltzmann’s constant, T is absolute temperature, q is the electron charge, y D is the thickness ofthe depleted substrate, V sub is the applied substrate bias voltage, and V J is an average potential near theCCD potential wells due to the channel potentials. V sub V J is the voltage drop across the region wherethe photogenerated holes are drifted by the electric field. This result is a simplified asymptotic form that isindependent of the substrate doping and is valid for high electric fields in the substrate. Therefore in this casethe PSF is directly proportional to y D , T , and 1/ ( V sub V J ). The PSF for a CCD of this type can beimproved by reducing the substrate thickness and operating the CCD at high substrate bias. PSF measurementsare described in more detail in Section 5. z (500 μ m) (a) A CCD pixel (cid:1)(cid:1)(cid:2) (cid:3)(cid:4) (cid:5)(cid:3)(cid:4) (cid:3)(cid:4) (cid:3)(cid:4)(cid:3)(cid:4) (cid:3)(cid:4)(cid:3)(cid:4) (cid:3)(cid:4) (cid:3)(cid:4)(cid:3)(cid:4) (cid:1)(cid:2) (cid:3)(cid:4)(cid:3)(cid:5)(cid:6)(cid:7)(cid:8)(cid:9)(cid:10)(cid:8)(cid:4)(cid:3)(cid:11) (cid:6)(cid:4)(cid:7)(cid:8)(cid:9) (cid:9)(cid:12)(cid:5)(cid:4)(cid:3)(cid:13)(cid:7)(cid:7)(cid:3)(cid:5)(cid:8)(cid:14)(cid:4) (cid:10)(cid:11)(cid:12)(cid:8)(cid:13)(cid:8)(cid:14)(cid:15)(cid:16)(cid:8)(cid:9)(cid:17)(cid:18)(cid:15)(cid:4)(cid:10)(cid:16)(cid:18)(cid:10)(cid:17)(cid:15)(cid:15)(cid:8)(cid:13)(cid:4)(cid:14)(cid:19) (cid:3)(cid:4) z xy WIMP (b) WIMP detection in a CCD
Figure 1. a) Cross-sectional diagram of a 15 µ m ⇥ µ m pixel in a fully depleted, back-illuminated CCD.The thickness of the gate structure and the backside ohmic contact are µ m. The transparent rear window,essential for astronomy applications, has been eliminated in the DAMIC CCDs. b) Dark matter detection ina CCD. A WIMP scatters with a silicon nucleus producing ionization in the CCD bulk. The charge carriersare then drifted along the z -direction and collected at the CCD gates. image Energy measured by pixel / keV image α e μ X-ray?n, WIMP? Diffusionlimited p i x e l s Front Back (a) Portion of a DAMIC image
Energy measured by pixel / keV α X-ray absorption pe from Si fluorescence X-ray absorption (b) Emission of Si fluorescence X-ray
Figure 2. a) 50 ⇥
50 pixel portion of a CCD image, taken when the detector was at ground level. Differentkinds of particles are recognizable (see text). For better contrast, only pixels with deposited energy > Fesource. The 1.7 keV cluster is a photoelectron (pe) from the absorption of a Si fluorescence X-ray, emittedfollowing photoelectric absorption of the incident 5.9 keV Mn K a X-ray in a nearby site. ing of the 3-phase gates (“parallel clocks”), while higher frequency clocks (“serial clocks”) movethe charge of the last row horizontally to a charge-to-voltage amplifier (“output node”). The in-efficiency of charge transfer from pixel to pixel is as low as 10 and the readout noise for thecharge collected in a pixel is ⇠ e [2]. Since on average 3.6 eV is required to ionize an electronin silicon, the readout noise corresponds to an uncertainty of ⇠ Figure 3: a) 50 ×
50 pixel segment of a DAMIC image exposed to a
Cf source when the detector was atground level. Only pixels with deposited energy > ee are colored. b) Event with two nearby clustersdetected after illuminating the CCD with a Fe source. The 1.7 keV cluster is a photoelectron (pe) fromthe absorption of a Si fluorescence X-ray, emitted following photoelectric absorption of the incident 5.9 keVMn K α X-ray in a nearby site. spatial extent of the cluster is dominated by charge diffusion. Higher energy electrons ( e ), fromeither Compton scattering or β decay, lead to extended tracks. α particles in the bulk or fromthe back of the CCD produce large round structures due to the plasma effect [16]. Cosmic muons( µ ) pierce through the CCD, leaving a straight track. The orientation of the track is immediatelyevident from its width, the end-point of the track that is on the back of the CCD is much wider thanthe end-point at the front due to charge diffusion.
3. The DAMIC experiment at SNOLAB
Fig. 4 shows the infrastructure already installed in SNOLAB. A packaged CCD (2k × µ m-thick) is shown in fig 4a. The device is epoxied to a high-purity silicon support piece. TheKapton signal flex cable bring the signals from the CCDs up to the vacuum interface board (VIB).The cable is also glued to the silicon support. A copper bar facilitates the handling of the packagedCCD and its insertion into a slot of an electropolished copper box (fig 4b). The box is cooled to ∼
140 K inside a copper vacuum vessel ( ∼ − mbar). An 18 cm-thick lead block hanging fromthe vessel-flange shields the CCDs from radiation produced by the VIB, also located inside thevessel (fig 4c). The CCDs are connected to the VIB through Kapton flex cables, which run alongthe side of the lead block. The processed signals then proceed to the data acquisition electronicboards. The vacuum vessel is inserted in a lead castle (fig 4b) with 21 cm thickness to shield theCCDs from ambient γ -rays. The innermost inch of lead comes from an ancient Spanish galleon4 he DAMIC dark matter experiment J.R.T. de Mello Neto f CCDSi supportCopperbarKaptonsignal cable PolyethyleneLeada)b) c) d)
Figure 3. a) A packaged DAMIC CCD. b) The copper box housing the CCDs. c) Components of the DAMICsetup, ready to be inserted in the vacuum vessel. d) The vessel inside the lead castle, during installation ofthe polyethylene shield. (ITO) coating deposited on the backside after thinning the CCD to 500 µ m . The third CCD wasoptimized for DAMIC by maximizing its mass (the CCD is un-thinned, 675 µ m-thick) and mini-mizing radioactive contamination (the ITO layer containing b -radioactive In is eliminated). The500- µ m CCDs are inserted in adjacent slots of the copper box, with copper plates above and below.The 675- µ m CCD is in a lower slot of the box, separated from the other CCDs by ⇠ ⇥
4k pixels and 675 µ m thickness. Clusters of energy deposits are found in the acquired images with the following procedure. First,the pedestal of each pixel is calculated as its median value over the set of images. The pedestals arethen subtracted from every pixel value in all images. Hot pixels or defects are identified as recurrentpatterns over many images, and eliminated (“masked”) from the analysis ( >
95% of the pixels weredeemed good). Pixel clusters are selected as any group of adjacent pixels with signals greaterthan four times the RMS of the white noise in the image. The resulting clusters are consideredcandidates for particle interactions. Relevant variables (e.g. the total energy by summing over allpixel signals) are calculated for each cluster. For the studies presented in this paper, we required thecluster energy to be > Figure 4: a) A packaged DAMIC CCD. b) The copper box housing the CCDs. c) Components of the DAMICsetup, ready to be inserted in the vacuum vessel. d) The vessel inside the lead castle, during installation ofthe polyethylene shield. and has negligible
Pb content, strongly suppressing the background from bremsstrahlung γ sproduced by Bi decays in the outer lead shield. A 42 cm-thick polyethylene shielding is used tomoderate and absorb environmental neutrons.
4. Measurements of radioactive contamination
The ultimate sensitivity of the experiment is determined by the rate of the radioactive back-ground that mimics the nuclear recoil signal from the WIMPS. The SNOLAB underground labo-ratory has low intrinsic background due to its 6000 m.w.e. overburden. Dedicated screening andselection of detector shielding materials, as well as radon-suppression methods, are extensivelyemployed to decrease the background from radioactive decays in the surrounding environment.The measurement of the intrinsic contamination of the detector is fundamental. For silicon-basedexperiments the cosmogenic isotope Si, which could be present in the active target, is particularlyrelevant since its β decay spectrum extends to the lowest energies and may become an irreduciblebackground. The analysis methods used to establish the contamination levels exploit the uniquespatial resolution of the CCDs.The identification of α -induced clusters is the first step in establishing limits on uranium andthorium contamination [15]. Radiogenic α s lose most of their energy by ionization, creating adense column of electron-hole pairs that satisfy the plasma condition [16]. For interactions deepin the substrate, the charge carriers diffuse laterally and lead to round clusters of hundreds of5 he DAMIC dark matter experiment J.R.T. de Mello Neto f Cluster
Cluster Δ t = 35 days(x o , y o )E = 114.5 keV E = 328.0 keV Figure 8.
Candidate b decay sequence found in data. The first cluster was detected in an image taken on2014/08/05 and deposited 114.5 keV of energy. A second cluster, with energy 328.0 keV, was observed inan image taken 35 days later. Both tracks appear to originate from the same point (yellow star) in the CCD x - y plane. decays or by emission of a g -ray in 4% of the decays. Po is itself radioactive and decays by a emission. The possible contamination from Po in the CCD has been discussed in Section 3.2.The intermediate nuclei, P and
Bi, are expected to remain in the same lattice site as theirparent nuclei and throughout their lifetimes. Therefore, the b s produced by each decay pair shouldoriginate from the same pixel (out of 8 ⇥ ) on the x - y plane of the CCD. Through a search forelectron-like tracks starting from the same spatial position, individual Si – P and
Pb –
Bidecay sequences can be selected with high efficiency. We performed this search with the lowestbackground data set (Table 1) in the 675 µ m CCD. Given the background level ( ⇠
10 electrons perday in a CCD), the number of accidental coincidences among uncorrelated tracks are small forperiods of time comparable to the half-lives of P and
Bi. A candidate decay sequence foundin the data is shown in Figure 8 to illustrate the search strategy. b decay sequences The first step in the search for decay sequences is to find the end-points of the b tracks. Theprocedure is illustrated in Figure 9. First, we find the pixel with the maximum signal in the cluster,and we use it as a seed point. Then, for every pixel of the cluster we compute the length of theshortest path to the seed point, where the path is taken only along pixels that are included in thecluster. We refer to this as the “distance” from the seed point. The pixel with the greatest distanceis taken as the first end-point of the track. Finally, we recompute the distance of every pixel fromthe first end-point, and take the pixel with the largest distance as the second end-point of the cluster.To find a b decay sequence, we calculate the distance from the end-points of every b clusterin an image to the end-points of every b cluster in later images. Thus, for every pair of clusters wehave four distances corresponding to each end-point combination. The minimum of these distancesis defined as the “cluster distance.” The pair is considered a candidate for a decay sequence if thecluster distance is smaller than 20 pixels and the clusters have at least one pixel in common. Werefer to the cluster in the earlier (later) image as the “first” (“second”) cluster.To reduce the number of accidental pairs, we impose additional criteria on the energy of theclusters and their time separation. For the Si – P sequence search, we require the energy of thefirst cluster to be <
230 keV and the energy of the second cluster to be < Pb –
Bi sequence search, we require the energy of the first cluster to be in the range 30–65 keV,– 12 –
Figure 5:
Candidate β decay sequence found in data. The first cluster had 114.5 keV of energy. A secondcluster, with energy 328.0 keV, was observed in an image taken 35 days later. Both tracks appear to originatefrom the same point (yellow star) in the CCD x - y plane. micrometers in diameter, whereas α particles that strike the front of the CCD lead to mostly verticalclusters according to a phenomenon known as “blooming” [11]. Simple criteria are sufficientto efficiently select and classify α s. Spectroscopy of plasma α s can be used to establish limitson Pb,
U and
Th contamination in the bulk of the CCD. In special DAMIC runs, witha dynamic range optimized for α energies, four plasma α s whose energies are consistent with Po were observed. One of them cannot be
Po, as it coincides spatially with two higherenergy α s recorded in different CCD exposures, and is therefore likely part of a decay sequence.When interpreting the other three as bulk contamination of Po (or
Pb), an upper limit of <37 kg − d − (95% CL) is derived. In the U chain, the isotopes U, Th and
Ra decay byemission of α s with energies 4.7-4.8 MeV. Since the isotopes’ lifetimes are much longer than theCCD exposure time, their decays are expected to be uncorrelated. No plasma α s were observed inthe 4.5-5.0 MeV energy range, and an upper limit on the U contamination of < 5 kg − d − (95%CL) is correspondingly derived (secular equilibrium of the isotopes with U was assumed). Asimilar analysis results in a upper limit of < 15 kg − d − (95% CL) on Th contamination in theCCD bulk [15].A search for decay sequences of two β tracks was performed to identify radioactive contami-nation from Si and
Pb and their daughters. Si leads to the following decay sequence: Si −→ P + β − with τ / =
150 y , Q − value =
227 keV P −→ S + β − with τ / =
14 d , Q − value = .
71 MeVA total of 13 candidate pairs were observed in the data. With detailed Monte Carlo simulationsthe overall efficiency for detection of Si – P decay sequences in the data set was determinedto be ε Si = . + − kg − d − (95% CI) for Si in the CCD bulk [15]. With asimilar procedure the upper limit on the
Pb decay rate in the CCD bulk has been deduced as <
33 kg − d − (95% CL). 6 he DAMIC dark matter experiment J.R.T. de Mello Neto f Alexis A. Aguilar-Arévalo (ICN-UNAM) CIPANP2015 May 22, 2015 Vail, Colorado DM search, 2014 data ● Data used: - 36 days with 3 CCDs: 2 x 500 µ m (2.2 g), & 1 x 675 µ m (2.9 g) - 7 additional days with the 675 µ m CCD Exposure: ~0.3 kg·d
Fit method: Unbinned Likelihood- Lindhard ionization efficiency(k=0.15)- v = 220 km/s - v esc = 544 km/s- v E = 232 km/s - ρ = 0.3 GeV/cm Best fit: m
WIMP = 26 ± 46 GeV σ WIMP = (7 ± 16)x10 -4 pb c bg = (67± 13) drumin(-logL) = -396.5Null hypothesis: c bg = (74± 5) dru min (-logL) = -396.1 DM signal model:
Effiiciency as in prev slide
Figure 6:
Cross section exclusion limit at 90% CL for the DAMIC 2014 results (solid black) compared toDAMIC 2012 (dashed black) [9], CRESST 2014 (solid green) [18] , CDMSlite 2013 (solid red) [17].
5. Dark matter search
The data acquired in 2014 came from two CCDs 500 µ m thick and 2.2 g exposed for 36 daysand another CCD 675 µ m thick and 2.9 g exposed for 7 days. We assumed a local WIMP densityof 0.3 GeV/cm , dispersion velocity for the halo of 220 km/s, earth velocity of 232 km/s and aescape velocity of 544 km/s. The Lindhard model was used to obtain recoil energies as discussedabove. The data analysis proceeded with a two-dimensional gaussian fit to each hit in the images.The noise was used to set the signal threshold and simulation was used to estimate the efficiencydown to the threshold. Based on this efficiency, the total exposure was calculated as ∼ .
6. Conclusions
We have shown that DAMIC is producing high quality science and it is a leading experimentat low WIMP mass. The CCD detectors with their unique imaging capabilities allow us to measureinternal contamination of silicon in a unique way. Stringent 95% CL upper limits on the presence ofradioactive contaminants in the silicon bulk were placed. The dark matter search will be performedwith an upgraded experiment, DAMIC100, a low background detector with 100 g of sensitive mass,consisting of 18 CCDs, each of them with 16 Mpix, 675 µ m thickness and 5.5 g. The measuredlevels of radioactive contamination are already low enough for its successful operation. We arecurrently testing the new CCDs. The baseline plan is to operate the DAMIC-100 experiment forone year to collect approximately 30 kg-day exposure by the end of 2016.7 he DAMIC dark matter experiment J.R.T. de Mello Neto f References [1] G. Jungman, M. Kamionkowski and K. Griest,
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