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A. Introduction Astrophysical evidence on a variety of distance scales shows that a large fraction (> 96%) of the mass-energy density of the universe is a combination of “dark matter” and “dark energy”. A large fraction of the dark matter must be non-baryonic -- not constituted of the protons and neutrons that make up ``ordinary'' matter. A compelling explanation for the missing mass is the existence of Weakly Interacting Massive Particles (WIMPs). These particles are also well motivated by particle physics theories beyond the Standard Model, and thus the terrestrial discovery of WIMPs would have enormous intellectual merit [1,2,3]. With a typical mass of about 100 GeV and moving at speeds relative to the Earth of about 220 km/s, WIMPs only deposit several tens of keV when scattering with nuclei. Because they only interact through the weak force and gravity, WIMPs very rarely scatter with ordinary matter. Experimental techniques that combine low radioactivity, low energy thresholds, efficient discrimination against electron recoil backgrounds, and scalability to large detector masses can test models in which the dominant form of dark matter is WIMPs. The best current limit on the spin-independent WIMP-nucleon cross-section is 9 x 10-44 cm2 for a 100 GeV WIMP, set by the XENON10 experiment [4]. New results are expected in fall 2007 from the CDMS-II experiment, likely probing even smaller WIMP-nucleon cross-sections [5]. A cost-effective and simple experimental design would be to use an ultra-pure fiducial target of 100 kilograms up to 100 tonnes of noble liquid, with no electric field. This detector concept is dubbed CLEAN, standing for Cryogenic Low Energy Astrophysics with Noble liquids [6,7,8,9]. Considerations of self-shielding and radioactive isotope contamination lead us to consider liquid neon (LNe) and liquid argon (LAr). The same cryogenic equipment may be used to contain and cool both LNe and LAr, and because the same wavelength shifters and photomultiplier tubes can also be used, a single apparatus could be used to perform low-background experiments with both liquids. At the 100 tonne scale operating with LNe, the detector may also be used to measure the p-p solar neutrino flux to 1% precision. Several properties of LAr and LNe make this overall approach attractive:

• LAr and LNe scintillate strongly in the vacuum ultraviolet and are transparent to their own scintillation light, allowing for event detection with a low energy threshold.

• LAr and LNe are dense enough (1.4 and 1.2 g cm-3, respectively) to allow efficient self-shielding against gamma rays and fast neutrons.

• Position reconstruction in a LAr or LNe detector can be achieved using photomultiplier hit pattern and timing distributions [7,8,9] allowing fiducialization of an ultra-pure target volume. This allows the efficient rejection of gamma rays and fast neutrons, which predominantly deposit their energy at large detector radii.

• Pulse shape discrimination (PSD) is possible because both LAr and LNe have two distinct mechanisms for the emission of scintillation light. These two scintillation channels are populated differently for electron recoils than for nuclear recoils, allowing these two types of events to be

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distinguished on an event by event basis. This discrimination method was used in the ZEPLIN-I single-phase LXe experiment [10], and it was pointed out by McKinsey and Coakley [6] that the much longer triplet time constant in LNe should allow superior PSD, to be verified experimentally. Following this observation, Boulay and Hime recognized the possibility that the same advantage could apply to LAr, potentially allowing PSD at the sub-ppb levels of electronic recoil contamination needed to perform a competitive dark matter experiment [11]. In the intervening years, R&D effort has quantified the PSD efficiency, in both LNe and LAr, at the energies relevant to dark matter detection (see section III).

• The ability to exchange LAr with LNe, with different sensitivities to dark matter and fast neutrons, would allow both event populations to be distinguished and characterized, just as Ge and Si are used for this purpose in the CDMS II experiment [12].

• Argon and neon are relatively inexpensive detector materials. The price of natural argon is very small ($1000/ton), and the current price of neon is about $90,000/ton. By comparison, the current price of xenon is about $1,500,000 per ton. In the long term, when materials costs become significant, argon and neon offer the possibility of multi-tonne detectors at a reasonable cost.

Here we propose to construct and test a small-scale (100 kg fiducial) detector, Mini-CLEAN, which can be taken underground and surrounded with shielding to perform a dark matter search. Mini-CLEAN would thus serve as the first major step towards tonne scale and multi-tonne scale detectors using LAr and LNe in single-phase operation. While Ar and Ne are expected to exhibit a smaller WIMP scattering rate compared to a Xe experiment with similar exposure (see Figure 1, left), they do allow much stronger discrimination against electron recoil events. We project that when suitably shielded and operated in a deep underground laboratory, Mini-CLEAN will have a sensitivity to spin-independent WIMP-nucleon scattering of 2 x 10-45 cm2 for a 100 GeV WIMP (see Figure 1, right).

Figure 1. Left: WIMP elastic scattering spectra in several different noble liquids, assuming a WIMP-nucleon scattering cross-section of 1 x 10-44 cm2 for a 100 GeV

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WIMP. Right: Dark matter cross-section limits for various past experiments, as well as the projected sensitivity of Mini-CLEAN. The top three curves represent spin-independent limits from ZEPLIN-II (green line) [13], CDMS (magenta line) [12], and XENON10 (red line) [4], while the blue line represents the sensitivity projected for Mini-CLEAN when operated with LAr. This limit assumes a 2-year run with 100 kg fiducial mass, and discrimination efficiency as predicted by the detailed Micro-CLEAN measurements of the scintillation time dependence for electron and nuclear recoils (see Section III). Also shown are several dark matter parameter space predictions assuming supersymmetry: L. Roszkowski et al. [14] in light blue, J. Ellis et al. [15] in pink, and Baltz and Gondolo [16] in green. The difference between the proposed design of Mini-CLEAN and the designs proposed by the WARP [17] and ArDM [18] collaborations is that Mini-CLEAN will not employ an electric field to extract ionization charge. This will allow efficient collection of scintillation light and thereby optimize PSD efficiency, crucial for achieving a low energy threshold and good WIMP sensitivity. We note that light collection efficiencies in two-phase LAr experiments are projected to be much smaller than in Mini-CLEAN; for example the ArDM collaboration [19] projects a global light collection efficiency of 2% to 5% (0.8 to 2.0 photoelectrons/keV, given the LAr scintillation yield of 40,000 photons/MeV), a factor of at least 3 times less than the 6 photoelectrons/keV projected for Mini-CLEAN. Since pulse-shape discrimination efficiency improves exponentially with scintillation signal yield, a factor of 3 less light collection results in considerably less effective pulse shape discrimination efficiency at a given nuclear recoil energy. In addition, avoiding the use of an electric field allows a simple design, scalable to the tonne-scale and beyond, without extremely high voltages and event pile-up issues related to finite electron drift speed and 39Ar background. Finally, designs using charge collection are not suited to exchanging the target with LNe because LNe has a very low electron drift speed. As noted in the recent DMSAG report [20], in the event of a statistically significant WIMP signal, it is crucial that there be multiple experiments using different detector technologies and detector materials. This will allow the A2 dependence of the signal rate to be measured and the recoil spectra to be compared to check the consistency of the model. For this reason, multiple approaches to tonne-scale WIMP detection need to be developed, and it is our opinion that LAr and LNe are two of the most promising and cost-effective target materials for reaching the largest WIMP detector masses. The DMSAG report strongly favors the development of noble liquid technologies, including the single phase LAr/LNe technology. Recommendation #4 of the DMSAG report, regarding noble liquid detector technologies, states:

a) The sub-panel supports the development of one two-phase xenon-based detector at the 100 kg scale and above.

b) The sub-panel supports the development of detectors using liquid argon and/or liquid neon technology. WARP and miniCLEAN/DEAP represent two quite different technologies in their application to liquid argon. Both of these techniques should be explored to discover which has greater potential.

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Additionally, in Recommendation #8, DMSAG specifically endorses the funding of the next stage of the CLEAN Argon/Neon detector development, which is represented by this proposal. The Mini-CLEAN collaboration has grown significantly during the past two years, now boasting 12 institutions in the U.S. and Canada with a breadth of experience from the Sudbury Neutrino Observatory, SuperKamiokande, KamLAND, Majorana, and XENON10 collaborations. U.S. institutions presently receive funds from across agencies and offices, including the DOE NP and HEP Offices of Science and the NSF. The project is managed through an Advisory Panel consisting of the institutional PI’s and presently chaired by Andrew Hime. The technical program is organized via a suite of working groups and working group leaders including, analysis, backgrounds, calibrations, engineering, research and development, and simulations. Each working group convenes a conference call on a weekly basis. Face-to-face engineering and full collaboration meetings have occurred quarterly during the past year.

B. Project Description Mini-CLEAN Apparatus The Mini-CLEAN detector will consist of a spherical vessel filled with purified LNe or LAr at a temperature of 27 K or 87 K respectively. The center of the vessel will be viewed by 92 photomultiplier tubes (PMTs) immersed in the liquid. In the center of the spherical vessel will be mounted a soccer-ball-shaped array of acrylic plates. Tetraphenyl butadiene (TPB), a wavelength shifting fluor, will be evaporated onto the inward-pointing surface of each plate. Light from each wavelength shifter plate will be transported to the nearest PMT via a 20-cm-long acrylic light guide. The wavelength shifter plates would enclose a central LAr mass of 360 kg. A conceptual sketch of Mini-CLEAN is shown below in Figure 2. Ionizing radiation events within the wavelength shifter plate array will cause scintillation in the vacuum ultraviolet (80 nm in LNe or 125 nm in LAr). The ultraviolet scintillation light will be absorbed by the wavelength shifter and re-emitted at a wavelength of 440 nm. The photon-to-photon conversion efficiency is about 100% for LAr scintillation [21], and about 130% for LNe scintillation [22]. The blue light will then be detected by the PMTs. For electron-like events in both LAr and LNe, we project a signal of about 6 photoelectrons/keV (compared to 4.9 photoelectrons/keV already achieved in Micro-CLEAN). We expect carefully chosen PMTs to function well even though they are immersed in LAr or LNe. PMTs designed for low temperature use have been operated successfully while immersed in LAr (87 K) by several groups worldwide [23,24]. Recently, a test at LNe temperature (27 K) was performed by the McKinsey group at Yale University on a 20-cm diameter R5912-02MOD PMT from Hamamatsu Corp. The same model of PMT has operated for 12 months while immersed in LAr during recent tests with the Micro-CLEAN detector, and several months of testing immersed in LNe have recently begun. With a light

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detection efficiency that remains essentially unchanged despite cooling, and a gain that remains acceptably high, this model of PMT can be used while immersed in either LAr or LNe.

Figure 2. Left: A cutout view of the proposed Mini-CLEAN design. A stainless steel vessel contains 92 photomultipliers viewing an active volume of cryogenic liquid. The wavelength shifting film is coated on the inside surface of acrylic plates, which fit together to form a 92-sided expanded dodecahedron pattern. Acrylic light guides transport the light to the photomultipliers. The central vessel is surrounded by a copper infra-red shield and a vacuum can for thermal insulation. Right: The Mini-CLEAN detector immersed in a 6-m diameter, 6-m tall water shield. The central stainless steel vessel would be constructed of low-radioactivity stainless steel. Stainless steel tubes with flanges on the ends would be welded to the spherical stainless steel vessel, and a PMT assembly would go in each tube. The diameter of the wavelength-shifting sphere would be 80 cm. The diameter of the stainless steel sphere would be about 160 cm without the stainless steel tubes, or 200 cm including them.

The central scintillation vessel will be contained in a large vacuum can for thermal insulation (see Figure 2), also made from low-radioactivity stainless steel. The diameter of the vacuum can will be about 220 centimeters. The first stages of two pulse tube refrigerators will be thermally attached to a copper can, surrounding the central scintillation vessel to minimize the infrared heat load. Near both the top and bottom of the vessel will be ports for introducing and removing liquid. Cooling power will be provided by the second stages of the pulse tube refrigerators, which will continuously

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liquefy neon or argon to fill the spherical vessel. Liquid that passes out of the vessel will be vaporized and passed through a purification system, then re-liquefied. Attached to the scintillation vessel will be a bellows assembly connected to a linear feedthrough at room temperature. This will allow a radioactive source to be moved within the LAr or LNe volume. This capability will be useful for the overall calibration of the experiment, including testing position reconstruction. In order to minimize radon backgrounds, it is important that the wavelength shifter not come in contact with air for any extended period of time. Fortunately, it is well known that Rn-free (< 1 microBq/m3) nitrogen gas can be produced by passing it through activated charcoal [25]. By continuously purging the central stainless steel vessel with Rn-free nitrogen gas during assembly, Rn daughter plate-out can be kept small while the wavelength-shifting plates are within the stainless steel vessel. Each wavelength shifter module will be individually cleaned and coated within a purged glove box before plugging it in to the central vessel, thereby minimizing its exposure to room air. The electronics for Mini-CLEAN will consist of 92 channels of waveform recorders. The proposed devices are 8-channel VME modules from CAEN. They record 12-bits of FADC at 250 MHz. The record length is 1.25 million samples (5 ms), easily covering the desired PSD range of 100 µs. Hardware zero suppression provides convenient compression of the recorded data size and allows for high rate data transfer, useful for runs with certain calibration sources. Triggering is provided by counting PMTs over threshold coincident in a narrow time window (100 ns). This is accomplished with external trigger electronics. Other required components include 92 channels of high voltage and a data acquisition computer with a bridge to the VME crate. When LAr is being used as the scintillation medium, we will pass it through a room temperature getter to remove all non-noble gas impurities. This is proven and existing technology, and very similar to the approach taken by the various 2-phase LXe experiments, which require long electron drift lengths and typically have very stringent requirements on liquid purity. Because neon has a low binding energy on a variety of surfaces, we expect to be able to rid it of all contaminants by passing it through cooled charcoal. The residence time for any species on a charcoal surface is proportional to exp(Eb/kT). All contaminants of concern have higher binding energies to charcoal than Ne, and at LNe temperature, Eb >> kT for each of these. In practice, the variation of residence times for contaminants on charcoal surfaces is reflected in a variation in adsorption constant, which can be determined by measuring the amount of time that it takes each kind of impurity atom to break through a tube filled with charcoal as impurity-doped neon gas is flowed through it. In the summer of 2005, adsorption coefficients in neon-saturated charcoal were measured as a function of temperature using a small cryogenic system at Yale. Impurities studied were H2, N2, Ar and Kr. It was found that at liquid nitrogen temperature (77 K) all tested impurities have adsorption coefficients greater than 103 liters per gram of

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charcoal. This result, recently published in Nuclear Instruments and Methods [26], implies that all impurities except helium (which is benign) can be removed from neon by passing it through cooled charcoal. In order to reduce the gamma and fast neutron background from radioactivity in the surrounding cavern, we plan to situate the detector at the center of a 6 meter tall, 6 meter diameter water tank. By instrumenting the tank with a modest number of photomultipliers, the water tank may also serve as a muon veto. The shielding tank, as well as associated infrastructure such as water purification system, platform on top of the tank, electrical power, and data connectivity, would be provided by the host underground laboratory, either Homestake or SNOLAB (see attached letters of support). Research and Development Initial studies of LAr and LNe scintillation have taken place using small prototypes operated by members of the CLEAN/DEAP collaboration. A prototype dubbed Micro-CLEAN is currently in operation in the McKinsey lab at Yale. Micro-CLEAN is a 4 kg LAr/LNe volume surrounded by TPB wavelength shifter viewed by two 20-cm PMTs immersed in the cryogenic liquid. The Micro-CLEAN detector is shown in photograph and schematic in Fig 3. The goals for this detector are to measure scintillation yields and scintillation time dependence in LAr and LNe for both electronic and nuclear recoils. In addition, a 7-kg prototype dubbed DEAP-1 will be run at SNOLAB by the end of 2007. The goals for this detector are to measure PSD in LAr, study backgrounds from surface contamination, and to perform an initial dark matter search with LAr.

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Figure 3: Left: Schematic of Micro-CLEAN. The active region contains 4 kg of LAr or LNe. Right: Photograph of the Micro-CLEAN PMT assembly. Scintillation yield: Liquid argon produces about 40,000 photons per MeV for electron-like events [27]. In recent months, we have used Micro-CLEAN to test the magnitude of scintillation signal that is possible using photomultipliers immersed in LAr. Calibration with 122 keV gamma rays gives a signal yield of 4.9 photoelectrons/keV, as shown in Figure 4 (left).

Figure 4. Left: Energy calibration of Micro-CLEAN when filled with LAr. The signal yield at 122 keV is 600 photoelectrons, giving 4.9 photoelectrons/keV. Right: Simulated Co-57 spectrum in Micro-CLEAN. The features of the experimental data are well reproduced by Monte Carlo. We have built a GEANT4-based simulation that includes an extensive level of detector detail including estimates of the optical properties of the noble liquids, TPB wavelength shifter, acrylic light guides, and a full three-dimensional model of the PMTs. The simulation also uses scintillation timing light yields based on prototype data, as well as a data acquisition model that includes PMT timing and charge spectra. Figure 4 shows a comparison of the simulation’s prediction (right) of the response of the micro-CLEAN prototype to a 57Co source to actual data (left), and we see that the model reproduces the data with a good degree of accuracy. With further measurements of the optical properties of the materials planned for use in Mini-CLEAN, and with associated upgrades to the simulation, we will be able to use the simulation to develop analyses used to help reject backgrounds in advance of running the detector. Pulse shape discrimination studies: In recent months, we have performed measurements of the PSD capabilities of LAr using the Micro-CLEAN detector. We collected data sets of both nuclear recoils (scattered neutrons tagged with organic scintillator) and electronic recoils (from Compton scattering of 511 keV gammas; coincident 511 keV gamma rays are tagged with a NaI detector). Sample LAr scintillation traces from electronic and nuclear recoils are shown in Figure 5 (left). We find that nuclear recoils exhibit a different scintillation time dependence than electronic recoils, even at the low energies needed for WIMP searches (see Figure 5, right). By measuring Fprompt , the fraction of scintillation

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signal in the prompt component, nuclear recoil events can be efficiently discriminated from electronic recoil events. Figure 6 (left) shows a scatter plot of both nuclear recoil events and electronic recoil events. We do find that the mean Fprompt values for both nuclear and electronic recoils vary with energy, becoming more similar at low energy. This is presumably a consequence of the fact that dE/dx for nuclear recoils increases with energy, while dE/dx for electronic recoils decreases with energy. Based on these measurements, and assuming Gaussian statistics for the distribution of Fprompt we can predict the probability that an electronic recoil in LAr will be mistaken for a nuclear recoil, as a function of energy and signal yield. Our measurements predict an electronic recoil contamination less than one part in 109

at a nuclear recoil energy of 50 keV, given a signal yield of 6 pe/keV as we expect in Mini-CLEAN. In Figure 6 (right) are shown results from our measured discrimination to date.

Figure 5. Left: Sample LAr scintillation events induced by an electronic recoil (top) and a nuclear recoil (bottom). Right: The time dependence of LAr scintillation light for both electron-like events (top curve) and neutron-like events (bottom curve). Due to lack of statistics resulting from the need to carefully tag electron-like events to avoid neutron backgrounds in our above-ground laboratory, we have only been able to demonstrate electronic recoil contamination of 8 x 10-7 thus far in the range of 80 to 160 photoelectrons, corresponding to a nuclear recoil energy range of 50 to 100 keV in Mini-CLEAN. We expect to demonstrate the needed discrimination of 1 part in 1010 for a 1-ton LAr experiment with underground running of Mini-CLEAN and high-statistics calibration with gamma ray sources.

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Figure 6. Left: Fprompt scatter plot showing both tagged nuclear recoil events (blue, top band) and tagged electronic recoil events (red, bottom band). Right: Fprompt distribution for tagged nuclear and electronic recoils, showing a discrimination of better than 106 above 80 photoelectrons. It is possible that certain kinds of subdominant background events (for example, gamma rays scattering in the PMTs, or 39Ar betas going into the wavelength shifter) could mimic the nuclear recoil scintillation signal time dependence. Because many of these subdominant background events will cause light to be collected in a small number of PMTs, a requirement that the light be evenly distributed should allow these events to be vetoed. This is just one possible example of how the 92 channels in a Mini-CLEAN detector might be exploited to reduce backgrounds. Recent measurements at Yale University have also confirmed that PSD in LNe is highly effective for discrimination between electron-like and nuclear recoil events. The scintillation from both nuclear recoil events (produced using neutrons from a d-d generator) and Compton scattering events (produced with 511 keV gamma rays) was measured using an earlier LNe scintillation detector with a signal yield of 0.9 photoelectrons/keV [28]. The scintillation decay curves, normalized to the intensity of the prompt scintillation component, exhibit a slow decay component that has less than a third of the intensity in nuclear recoils than in electron-like events with the same total scintillation yield. Comparing the fraction of prompt (< 100 ns) scintillation light, Fprompt for Compton scattered electrons and nuclear recoils in a restricted but identical range of total scintillation light, we measure a gamma rejection efficiency of 99.7% at 40 photoelectrons, and, given the measured widths and means of the Fprompt distributions for electron and nuclear recoils, we project a gamma rejection efficiency of 99.999% at 70 photoelectrons. Liquid neon, unlike liquid argon, does not suffer from internal beta-decay backgrounds, and so the requirements on PSD efficiency in liquid neon are less stringent (see Section IV). Because the gamma rejection efficiency may depend on the energy of the events as well as the total detected scintillation signal, we aim to repeat these measurements at low

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recoil energies and higher light collection in the coming months using Micro-CLEAN. Nuclear recoil scintillation efficiency: Because ionization by heavy particles is suppressed by the so-called Lindhard factor [29] and the scintillation in both LAr and LNe is largely the result of ionization and subsequent recombination, the scintillation yield for nuclear recoils is less than that for electron recoils of the same energy. The ratio of these two quantities is known as the nuclear recoil scintillation efficiency. The scintillation yield from nuclear recoils is also suppressed because electronic excitations in dense tracks are more likely to be quenched through interactions with electrons or other electronic excitations. Knowledge of the nuclear recoil scintillation efficiency is crucial for projecting the sensitivity of any scintillation detector to WIMP dark matter. Using Micro-CLEAN and a 2.8 MeV d-d neutron generator, we have recently performed a measurement of the nuclear recoil scintillation efficiency of LAr. Neutrons emitted by the neutron generator are scattered in the active LAr and detected in coincidence with a BC501 organic scintillator module. Knowledge of the initial neutron energy, the mass of the Ar nucleus, and the scattering angle allows calculation of the energy deposited in the LAr. Comparison of the neutron scintillation yields at a variety of scattering angles with the scintillation yield from 122 keV gamma rays allows calculation of the nuclear recoil scintillation efficiency. The results of this measurement indicate a nuclear recoil scintillation efficiency of about 25% in the energy range relevant for WIMP detection, and this work is currently being prepared for publication.

Backgrounds Mitigation of radioactive background poses a significant challenge to the design of the experiment and the choice of detector construction materials. The Mini-CLEAN radioactivity budget is summarized in Table I. Radioactivity in the cavern rock produces gamma rays and fast neutrons that can be effectively attenuated in a water shield that is ~150 cm thick. The entire detector and shield must be housed deep underground in order to avoid high energy neutrons induced by cosmic-ray muon interactions in the surrounding rock and detector materials. Our studies [30] indicate that an overburden in excess of 4000 m.w.e. is required to reduce cosmic-ray induced activity to an acceptable level. With adequate shielding from external backgrounds, careful consideration must be given to the choice of detector construction materials to ensure that internal contamination is at an acceptable level. Of primary concern are fast neutrons produced by (α,n) interactions due to U and Th activity. Fast neutrons will scatter elastically and inelastically from target nuclei and produce a signal that is otherwise indistinguishable from a WIMP signal. The GERDA collaboration has identified several vendors of stainless steel that produce material with 238U and 232Th levels less than 1 ppb [31], and stainless steel at this level of purity serves as an appropriate material for the outer cryostat with only a minor contribution to

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neutron background. Based upon our experience from SNO, acrylic at <1 ppt 238U and 232Th is the material of choice to eliminate radioactivity close to the target volume. The PMT array, with 238U and 232Th at the level of 100 ppb and 175 ppb, respectively, is the dominant component of (α,n) background. Table I: Summary of the major components of the Mini-CLEAN detector and the projected intrinsic radioactivity used as input to the background model and simulations.

Component Material 238U / 232Th Natural-K

Light guides 480 kg Acrylic 480 / 480 ng 3 ppb PMT Sphere 60 kg SiO2 6.0 / 10.5 mg 100 ppm

12 kg B2O3 1.2 / 2.1 mg 100 ppm 1050 kg SS 1.05 / 1.05 mg 2 ppm

Outer Cryostat 1575 kg SS 1.58 / 1.58 mg 2 ppm 150 kg Cu 15 / 15 µg 10 ppb

In addition to fast neutrons, nuclei that recoil into the LAr target will also create a background to the WIMP signal. The primary concern is radon daughters that plate out on the wavelength shifter surface, followed by decays in which the recoiling nucleus enters the LAr while the alpha particle is absorbed into the wavelength shifter. This leaves three primary backgrounds for consideration, namely 39Ar in the central LAr target, radon daughters on the wavelength shifter, and fast neutrons produced in the PMT array. We discuss the total background rates producing signal in the central target and in a Region-of-Interest (ROI) defined by the energy interval of 12.5 to 25 keV electron equivalent [equal to 50 to 100 keV nuclear recoil energy, given a quenching factor of 25%] and a fiducial mass of 100 kg LAr defined by a fiducial radius of ~27 cm. In this ROI we expect a signal event rate of ~1/year in LAr assuming a 100 GeV WIMP with a spin-independent nucleon cross-section of 1 x 10-45 cm2. Background rates below this level [~8x10-4 events/keV/kg/year] are required to enable the desired sensitivity to WIMPs in the Mini-CLEAN detector. As described below, this can be achieved in the Mini-CLEAN detector proposed. 39Ar Background in the central target: Beta and gamma activity is dominated by the beta decay of 39Ar in the LAr target. 39Ar is a unique, first forbidden beta emitter with an end-point energy of 565 keV. At ~1 Bq per kg of natural argon [32], 1.1 x 1010 events per year will be produced within the total target mass of 360 kg. Within the energy and fiducial volume ROI this rate is reduced by a factor of ~100 to 1.1 x 108 events per year. For comparison, external gamma rays are dominated by 238U, 232Th, and 40K in the PMT sphere and yield event rates roughly three orders of magnitude smaller than that due to 39Ar activity in the central target. Our studies in Micro-CLEAN (section III) project that we will be able to reject 39Ar background at 1 part in 109 or better. Consequently, we expect the 39Ar background will be reduced to less than 0.1 event/year in our ROI.

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While we are optimistic that that such PSD is achievable based upon data from our prototype experiments and simulations, we plan to demonstrate this directly with the Mini-CLEAN detector operating in a shielded environment underground. It is worth noting that an underground water source has recently been identified by the WARP collaboration that holds promise for extracting large amounts of argon gas depleted in 39Ar [33]. Our collaborators at the University of South Dakota (USD) have identified an independent underground source. This potential source of 39Ar-depleted gas is being pursued in collaboration with LANL with the aim to produce enough depleted argon for the Mini-CLEAN experiment, though we do not assume this for purposes of estimating our WIMP sensitivity. USD and LANL are also developing a technique for depleting natural argon using a novel variation on the method of Laser Isotope Separation. Radon daughters on the central target inner surface: Radon daughters that plate out on the wavelength shifter surface present a significant hazard for a sensitive WIMP search. Decay rates of ~1/m2/day, characteristic of that achieved for detector components in SNO, would yield a total of ~367 recoil nuclei each year in the target volume. Of the recoiling nuclei that enter the liquid we estimate that only ~8 events will enter into the energy window relevant to a WIMP search. This background can be mitigated through fiducialization of the target volume. Simulations indicate that approximately 6 cm position resolution should be achievable at our energy threshold. In this case, less than 0.3 events/year would contaminate our analysis window utilizing a 100 kg fiducial target. Recent studies indicate that the accompanying alpha particle that is directed towards the wall will scintillate in the wavelength shifter. This has the effect of “boosting” the recoil signal out of the energy ROI with a further reduction of background, though this effect remains to be fully quantified and we do not require this to reach our target WIMP sensitivity. Fast Neutrons from the PMT Array: Fast neutrons are dominated by (α,n) interactions in the PMT glass and roughly 90% (10%) of the neutrons produced result from the B2O3 (SiO2) constituting the bulk of the glass. We predict ~42,000 neutrons are produced each year from the PMTs. Of these a total of 232 make their way to the LAr target where they deposit observable recoil energy, ~7 of which contaminate the energy window and fiducial volume ROI. For comparison, the total number of neutrons detected from the outer stainless steel cryostat is about a factor of 100 smaller than those from the PMTs. Those originating from the inner acrylic are negligible. Further reduction of the PMT neutron background can be made since the neutrons can be tagged effectively in LAr using the delayed coincidence between the prompt neutron recoil and the subsequent capture gamma rays. Using this tag, we predict that ~1.3 neutrons/year would persevere to contaminate our analysis ROI. This number is reduced by a further factor of six to ~0.2 events/year by the acrylic between the PMTs and the active LAr.

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Verification of Signal versus Background: Our simulations of detector radioactivity indicate that we can achieve an acceptable background level of ~0.5 events/year in our analysis ROI. A detailed understanding of the detector response to both signal and backgrounds will be developed through Monte Carlo simulations that are bench-marked or “trained” through a suite of calibration measurements with sources deployed both internal and external to the central target. With the Monte Carlo well bench-marked against calibration data, a number of in situ tests will be available using the LAr production data itself. For example, we can exploit events in the outer (26 < R < 40 cm) volume and within the same energy ROI to predict the background rates expected within our 100 kg target. With 39Ar removed via PSD, the contributions from surface radon daughters and neutrons could be determined separately using a combination of neutron tagging and radial profile. In this manner, we can effectively “blind” the data set within the fiducial volume and energy ROI relevant to a WIMP search. Unique to the Mini-CLEAN concept is the potential to exchange the target from LAr to LNe. This allows careful diagnosis of a potential WIMP signal versus an anomalous and unpredicted background. The expected signal in LNe is about 5 to 10 times smaller than in LAr for the same target mass and energy threshold, neutron and radon daughter backgrounds are very similar both in rate and how they are distributed in the detector. Consequently, exchange of the target in the same detector allows us to effectively perform a “beam-on/beam-off” experiment in the event that a WIMP signal is discovered. While LNe is easily purified of any significant internal radioactivity, PSD is required to mitigate external gamma activity. Our simulations indicate that this activity is dominated by radioactivity in the PMTs. For Mini-CLEAN operating with LNe we find that there are about 104 gammas/keV/year contaminating the energy ROI and 100 kg fiducial volume. Prototype experiments (see section III) indicate that PSD at the level of 1 in 105 in LNe can be expected for a 70 photoelectron or larger signal, and this is sufficient to overcome the external gamma background in Mini-CLEAN. This allows for the assessment of signal versus background upon interchange of LAr with LNe as noted above and also makes LNe a good candidate for a sensitive WIMP search on its own when scaled to large masses. Work Plan We propose to design and assemble the Mini-CLEAN detector, make basic measurements related to its capability for dark matter, and then perform a dark matter search in an underground facility. Since our collaboration has been designing this experiment for some time, and because we have considerable experience through the DEAP-1 and Micro-CLEAN prototypes, we are well-prepared to complete construction promptly. We are proceeding with design studies while the proposal is being reviewed, and estimate that nearly all design decisions will be complete by January 2008. At the

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time of this writing we are receiving quotations and firming up costs and delivery timelines. The subsystem requiring the most effort is the acrylic light guide system together with an assembly plan that minimizes exposure to radon. We anticipate the detector will be first assembled on the surface, in the high bay laboratory at Yale, for initial shakedown and characterization towards the end of 2008. Initial studies include: (A) measurement of light output and collection efficiency, using radioactive sources, (B) measurement of the nuclear quenching factor using the neutron generator, and (C) measurement of PSD capability. For the first time we will be able to measure (D) position resolution using light pattern reconstruction in a detector with 4π PMT coverage. In addition, we plan on a collimated source that will allow us to (E) measure or set limits on the attenuation and scattering lengths of liquid argon and liquid neon. We will also be able to use the neutron generator and radioactive sources to (F) develop the neutron capture tagging technique. These surface tests will also allow us to debug and finalize all major subsystems before moving underground. We anticipate moving the detector underground when the infrastructure, particularly the external water shielding, is ready and expect that this will occur in the first quarter of 2009. This infrastructure will be provided by the underground laboratory, and have received positive indications from both Homestake/DUSEL and SNOLAB (see attached letters), either of which would be suitable for the experiment. When underground, the emphasis of calibrations and detector studies will shift to background estimation, the most basic of which are PSD studies (C) with tagged electron and neutron recoils in the shielded environment. These calibrations, including a high rate calibration for 2-3 months to demonstrate 39Ar PSD at the desired level (G), will assist us in determining our ultimate sensitivity for the dark matter search. The most important and final stage of the work plan is to perform a search for dark matter sensitive to cross sections as low as ~2x 10-45 cm2 for a 100 GeV WIMP. This will require a 2-year exposure in an underground laboratory. We expect to initially run with liquid argon, as the sensitivity is expected to be higher, assuming we achieve our projected background suppression. Depending on the outcome, we would further characterize our backgrounds using target exchange to liquid neon. In the best case, of a sizeable signal, we could attempt to demonstrate the A2 dependence of the cross section that is characteristic of dark matter scattering off nuclei. Alternately, when the experiment becomes background limited, we would run with liquid neon as research and development towards ton or multi-ton detectors. Regardless of the outcome of the dark matter search, we envision underground operation of Mini-CLEAN as a valuable step towards development of a ton-scale detector, and our collaboration will pursue design studies in parallel. More recently a broader collaboration has been formed between the existing Mini-CLEAN collaboration in the U.S. and colleagues in Canada. Referred to as the DEAP/CLEAN collaboration, our goal is to achieve the most effective and

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rapid progress toward a tonne-scale detector using the single-phase liquid cryogen technology with the aim of reaching a WIMP-nucleon cross-section sensitivity of 10-46 cm2. We consider the design, construction, and operations of the Mini-CLEAN experiment a vital ingredient for understanding the technical challenges in engineering and fabricating a tonne scale detector. Besides testing the capabilities of LNe and LAr for p-p neutrino and WIMP detection, a Mini-CLEAN detector would have a wider impact in neutrino physics. For example, a LNe-based scintillation detector could be used to measure the cross-section for coherent neutrino-nucleus elastic scattering at a stopped-pion neutrino source, and set new limits on certain non-standard interactions [34]. A full-scale CLEAN detector would be highly sensitive to coherent scattering of supernova µ � and τ � neutrinos, with a capability to determine the binding energy of a neutron star in the event of a nearby supernova [35].

Management Plan An organizational and management structure for the Mini-CLEAN project has been defined that includes two Co-Spokespersons (Dan McKinsey at Yale and Andrew Hime at LANL). Group Leaders have been assigned the responsibility of overseeing the major subsystems as it concerns the detector design, fabrication, and commissioning plan. A Scientific Executive Committee has been formed, consisting of the institutional PI’s: Los Alamos National Laboratory (Dr. A. Hime-PI) will be responsible for the engineering and detector assembly plan, materials testing and QC, PMT procurement and testing, hardware in aid of radon-free assembly, and the fabrication of encapsulated calibration sources. Parallel efforts are underway with the University of South Dakota in pursuit of argon sources depleted in 39Ar. Dr. Hime will act as Co-Spokesperson for Mini-CLEAN. Yale University (Prof. D.N. McKinsey-PI) will design and construct the cryogenic liquification and purification systems, associated slow-control systems, and the central scintillation vessel. Yale will also build the low-radon nitrogen gas system, and develop the slow control software. Prof. McKinsey will act as Co-Spokesperson for Mini-CLEAN. Boston University (Prof. E. Kearns-PI) will design and build the pulse digitization and trigger electronics, as well as define data structures and a first-level data analysis framework. Some electronics will be provided from existing equipment and internal funds. The National Institute of Standards and Technology (Dr. K. Coakley-PI) will be responsible for developing and validating spatial and temporal statistical methods for event reconstruction and pulse shape discrimination.

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The University of North Carolina (Prof. R. Henning-PI) will be responsible for the design and construction of the radioactive source manipulation system. They will also provide counting facilities at the Kimballton mine for radioactive assay once these are commissioned. The University of South Dakota (Prof. D.-M. Mei-PI) will be responsible for the simulation of radioactive backgrounds in Mini-CLEAN. Parallel efforts are underway with Los Alamos National Laboratory in pursuit of argon sources depleted in 39Ar. The University of Texas (Prof. J. Klein-PI) will be responsible for the full detector simulation of Mini-CLEAN, including wavelength shifter, scintillator, PMTs, and data acquisition system, as well as the analysis tools to analyze both detector data and simulation events. The Texas group will permanently move to the University of Pennsylvania in the Fall of 2008, where help with the design of the electronics and DAQ system will be available. Testing of the optical properties of the detector materials and PMT response will be done at Texas but will continue at Penn. The University of New Mexico (Prof. Dinesh Loomba-PI) are collaborating closely with LANL in the measurement of PMT properties, characterization of light attenuation properties in acrylic, Monte Carlo simulation, and will aid in the fabrication of the acrylic light guide system. Massachusetts Institute of Technology (Prof. Joe Formaggio-PI) is developing a triggered neutron source for calibration and has expressed interest to aid in the instrumentation of the outer muon-veto. The University of Alberta (Prof. A. Hallin-PI) will develop methods to measure and minimize radon contamination on the wavelength-shifting plates and aid in the fabrication of the acrylic light guide system. Carleton University (Prof. K. Graham-PI) will take responsibility for designing appropriate external shielding for Mini-CLEAN. Queen's University (Prof. M.G. Boulay-PI) is developing the 7 kg, DEAP-1 experiment dedicated to LAr and for its deployment in SNOLAB. Queen's will contribute to studies of background reduction in Mini-CLEAN and to Monte-Carlo and analysis software. At present, the Mini-CLEAN collaboration consists of approximately 15 FTE level of effort, including scientific staff, undergraduate and graduate students, postdocs, technical and engineering staff. In addition to the institutional PI’s and commitments noted above, we have also been in direct collaboration with Fraser Duncan at SNOLAB and with Kevin Lesko at DUSEL regarding our siting requirements and underground infrastructure needs.

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Broader Impact Education is an integral component of experimental science. The Mini-CLEAN experiment will provide projects for many undergraduates, with subjects ranging from simple electronics, vacuum and cryogenics, data analysis, and Monte Carlo. The Mini-CLEAN experiment will also provide technical training for several graduate students and postdocs at the various collaborating institutions. At present, the existing collaboration includes 6 undergraduate students, 5 graduate students, and 8 postdocs, all working at various FTE levels on the project. The experience derived from the project, and the development of these young scientists in particular, will be of great value to the continued growth of the field as well as in the industrial applications it may foster. Research on LAr and LNe may have practical applications in radiation detection. In particular, the highly effective PSD that is exhibited by LAr and LNe may make them useful alternatives to organic scintillator for sensitive fast neutron detection, despite the kinematic disadvantage resulting from their large masses relative to the proton. Detection of fast neutrons in an intense gamma ray background is a common problem, especially for the trace detection of fissionable material. References [1] G. Jungman et al., Physics Reports 267, 195 (1996). [2] M. W. Goodman and E. Witten, Phys. Rev. D 31, 3059 (1985). [3] B. W. Lee and S. Weinberg, Phys. Rev. Lett. 39, 165 (1977). [4] J. Angle et al., astro-ph/0706.0039. [5] B. Cabrera, CDMS and SuperCDMS, SLAC Summer Institute, August 2, 2007, http://www-conf.slac.stanford.edu/ssi/2007/talks/Cabrera_080207.pdf [6] D. N. McKinsey and J. M. Doyle, J. Low Temp. Phys. 118, 153 (2000). [7] D. N. McKinsey and K. Coakley, Astroparticle Physics 22, 355 (2005). [8] K. J. Coakley and D. N. McKinsey, Nucl. Inst. and Meth. A 522, 504 (2004). [9] M. G. Boulay, A. Hime, J. Lidgard, nucl-ex/0410025. [10] G. J. Alner et al., Astropart. Phys 23, 444 (2005).

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[11] M. G. Boulay and A. Hime, Astroparticle Physics 25, 179 (2006). [12] D.S. Akerib et al., (CDMS Collaboration), Phys. Rev. D 73 , 011102 (2006). [13] G.J. Alner et al., (ZEPLIN-II Collaboration), astro-ph/0701858. [14] L. Roszkowski et al. arXiv:0705.2012. [15] J. Ellis et al., hep-ph/0502001. [16] Baltz and Gondolo, hep-ph/0407039. [17] P. Benetti et al., astro-ph/0701286. [18] A. Rubbia, J. Phys. Conf. Ser. 39 129 (2006), hep-ph/0510320. [19] Presentation by L. Kaufmann, The ArDM Project, at the 7th UCLA Symposium on Sources and Detection of Dark Matter and Dark Energy in the Universe, 2006, www.physics.ucla.edu/hep/dm06/talks/kaufmann.pdf [20] The Dark Matter Scientific Assessment Group, A Joint Sub-panel of HEPAP and AAAC Report on the Direct Detection and Study of Dark Matter (July 5, 2007); www.science.doe.gov/hep/DMSAGReportJuly18,2007.pdf [21] Presentation by L. Kaufmann, The ArDM Project, at TeV Particle Astrophysics,Venice, 2007. [22] D. N. McKinsey et al., Nucl. Inst. and Meth. B 132, 351 (1997). [23] P. Cennini et al., Nucl. Inst. and Meth. A 432, 240 (1999). [24] Prof. Stefan Schonert, presentation to the GERDA collaboration, February 2005. [25] Borexino Collaboration, Astroparticle Physics 18, 1 (2002). [26] M. K. Harrison, W. H. Lippincott, D. N.McKinsey, and J. A. Nikkel, Nucl. Inst. and Meth. A 570, 556 (2007). [27] S. Amerio et al., Nucl. Inst. and Meth. A 527, 329 (2004). [28] J. A. Nikkel, R. Hasty, W. H. Lippincott, and D. N. McKinsey, arXiv: astro-ph/0612108. [29] 3. J. Lindhard et al., Mat.-Fys. Medd 33, 14 (1963); Mat.-Fys. Medd. 33, 10 (1963).

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[30] D-M. Mei and A. Hime, Phys. Rev. D73, 053004 (2006). [31] Laura Baudis, private communication. [32] P. Benetti et al., astro-ph/0603131. [33] Presentation by C. Galbiati, Initial Results on Exploration of Deep Underground Sources of Argon, at Topics in Astroparticle and Underground Physics, Sendai, Japan, 2007. www.awa.tohoku.ac.jp/taup2007/slides/workshop11/roomA/10Underground%20Ar.pdf [34] K. Scholberg, Phys. Rev. D 73, 033005 (2006). [35] C. J. Horowitz et al., Phys. Rev. D 68, 23005 (2003). [36] E. Aprile et al., Physical Review D 72, 072006 (2005). [37] K. Ni et al., arXiv:0708.1976, accepted to Nuclear Instruments and Methods A.