XENON: A Liquid Xe Search Experiment at LNGS

Columbia University: Elena Aprile, Karl-Ludwig Giboni, Pawel Majewski, Kaixuan Ni, Bhartendu Singh, Masaki Yamashita Brown University: Richard Gaitskell, Peter Sorensen, Luiz DeViveiros University of Florida: Laura Baudis, David Day Lawrence Livermore National Laboratory: William Craig, Adam Bernstein, Chris Hagmann : Tom Shutt, John Kwong, Kirk McDonald Rice University: Uwe Oberlack, Omar Vargas : Daniel McKinsey, Richard Hasty, Hugh Lippincott

Contact person Elena Aprile ([email protected])

Letter of Intent

1 1. The physics case The goal of the XENON experimental program is direct detection of dark matter in the form of weakly interacting massive particles (WIMPs) with an array of ten identical liquid Xe detector modules each with a 100 kg fiducial mass (XENON100).Current theoretical models indicate that WIMP interaction rates probably lie between the current sensitivity of existing experiments which is equivalent to ~0.25 evts/kg/day and ~2 evts/100 kg/yr normalized for a Xe target. The best prospects for the unambiguous identification of a WIMP nuclear recoil signal lie in detectors that have negligible radioactive background competing with the dark matter signal. This can be achieved principally by using nuclear recoil discrimination in order to veto competing electron recoil events (associated with gamma and beta backgrounds), effective shielding, and through the operation of a large homogeneous detector volume with 3-D position resolution. The latter information can be used to select single hit events characteristic of a WIMP interaction while rejecting multiple hit events associated with backgrounds that propagate from the edge of the detector into the fiducial volume. Combined analyses of the latest observational data continue to provide compelling evidence for a significant cold dark matter component in the composition of the Universe. Reviews of the data can be found in [2-4] and also in a discussion of the latest direct detection results in Chardin [5] and the Drees and Gerbier article in the reviews section of the Particle Data Group [6]. The power of the background rejection has been fully demonstrated by experiments such as CDMS [7] and EDELWEISS [8]. The EDELWEISS experiment in the Frejus Tunnel in Europe has reported the best limit characterized by a sensitivity to 100 GeV WIMPs with a normalized cross section of ~1.5x10-42 cm2 based on an exposure of 32 kg-days with a single cryogenic Ge detector of mass 0.32 kg. In 2004 this experiment will upgrade to 8 kg of Ge detectors in a new cryostat. The CDMS II [7] experiment is expected to report a larger exposure using 6 Ge and Si detectors, with lower energy threshold, before the end of 2004, and make subsequent upgrades to 7 kg total mass. The sensitivity of the cryogenic detector based experiments for a dark matter search will ultimately be limited by the mass of available target given the huge challenges involved in scaling well beyond target masses of ~50 kg. The DAMA experiment has operated an array of 100 kg of NaI over a period of 7 years and has reported [9] the observation of an annual modulation signal (>6 ) which is attributed to WIMP interactions. The sensitivity of this experiment is limited by gamma backgrounds which will be reduced in the LIBRA [10] upgrade of the experiment, but these backgrounds are not expected to fall by more than a factor of a few. The dominant systematics (stability of cuts) involved with attempting limited gamma discrimination from NaI will not improve the sensitivity of the annual modulation technique. The Japanese XMASS-DM [11- 13] and Boulby Dark Matter ZEPLIN [14] programs are now operating prototype Xe detectors in underground sites of 2 kg and 6 kg respectively. Using discrimination based on primary scintillation light alone, the ZEPLIN I experiment has reported results within a factor of 2 of that of EDELWEISS. These experiments represent the first stage of Xe detector programs that will ultimately deploy detectors of the order of 1 tonne. The GENIUS-TF [15], GEDEON [16] and MAJORANA [17] experiments are all based on Ge detectors which do not have any intrinsic background discrimination. Their improvement in sensitivity will be based on aggressive reduction of external and internal radioactive sources. In order to achieve an increase in scattering rate sensitivity to ~10-46 cm2 a fiducial target mass on the order of 1 tonne will be required, with less than ~10 background events per year. An increase in target mass alone is not sufficient, unless the competing backgrounds are eliminated. Even the smallest background rate observed in all current experiments will increase with the mass and the exposure time and will be the true limiting factor of any kind of experiment. Efficient and redundant background rejection schemes are a key requirement for any WIMP experiment, along with the capability to sense nuclear recoil energy depositions as low as a few keV. For the XENON[18] experiment, the design goal of 99.5% background rejection efficiency is achieved by the simultaneous measurement of the ionization and scintillation signals produced in pure LXe by a WIMP recoil, down to a threshold of ~16 keV recoil energy (at which the detector is fully efficient for dark matter detection). Event localization in 3-D and the use of a LXe self-shield provide additional discrimination power. The XENON detector modules will require a gamma event rate within their fiducial volume below 2x10-3 events/keVee/kg/day to reach the

2 target sensitivity after application of the 99.5% background rejection. The key reasons for proposing LXe as WIMP target and detector are: • The high density (3 g/cm3), high atomic number (Z=54) permit a compact self-shielded detector. • Simultaneous measurement of the ionization and scintillation signals produced by WIMP interactions and the different amplitude and time response of these signals for electron and nuclear recoils provides powerful and efficient discrimination against background events. • It is available in large quantities at a reasonable cost (about $1k/kg). • It can be purified to achieve long distance drift of free ionization electrons. Additional processing can reduce the traces of radioactive elements such 85Kr, 42Ar, Ra to the low level required. • It contains appreciable even/odd isotopes, suitable for spin-indep./spin-dependent interactions. The modular approach (10 x XENON100) is preferred over a monolithic 1-tonne detector, to permit design optimization with a single module with a phased construction over a reasonable time scale of the full scale experiment. The modular approach is also preferred from an operational stand–point to maximize exposure times, and facilitate safe operation. Each module is a liquid xenon time projection chamber (LXeTPC) operated in dual phase (with both liquid and gas) and an active fiducial region on the order of 100 kg. An additional layer of LXe (of comparable mass to the inner volume) will be operated as a scintillator veto. Monte Carlo simulations, taking into account the dominant background radiation expected in the active volume have been carried out to optimize the geometry and shield thickness. The current module baseline is very similar to that in the original XENON proposal, shown in Figure 1. The backgrounds, purification, and screening sections of this proposal outline the radioactivity targets and methods that will be employed to achieve them. The figure below shows the sensitivity projected for XENON1T (1-tonne) experiment, in comparison to current WIMP searches, which are probing event rates at ~0.25 evts/kg/day. The CDMS and EDELWEISS experiments are expected to improve sensitivity by over an order of magnitude in 2004–5. In order to continue progress in dark matter sensitivity it will be important to have a liquid xenon experiment at the 100kg scale operational and taking science data by 2006. The operation of the XENON100 (100 kg fiducial) module for 3 months once it is in zero background mode (which would be achieved if the background after rejection is below 1x10-5 cts/keVee/kg/day) would permit setting a sensitivity limit at ~2x10–45 cm2. It is expected that longer periods of operation would be required prior to this successful run in order to establish optimal operating conditions, and to perform the necessary calibrations. This module would be a prototype for a larger array of modules for XENON1T which would be capable of collecting 20 events/year for WIMPs with a cross section of ~2x10-46 cm2 (with a negligible background).

Figure 0: (left) Current direct detection dark matter limits from experiments discussed in the text are shown, along with the projected sensitivity of XENON100 and XENON1T. (right) The original XENON100 prototype design from 2001 proposal.

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The size of the XENON100 detector is such that it would enable direct verification of the DAMA annual modulation signal, the former having comparable size and recoil threshold with the latter. Natural Xe is sensitive to both spin independent, and spin dependent (odd neutron) interactions of the WIMPs. Given that the XENON100 detector will be operated with no background contamination for a signal corresponding to a normalized cross section of 10-41–10-42 cm2 (>1 evt/kg/day) the dark matter recoil spectrum will be measured directly, at the same time as testing for its annual modulation. However, it must be noted that the modulation occurs at a few % level of the overall interaction rate, and so the direct search for nuclear recoil events is a much more powerful search tool in the early phases of the experiment. A test for annual modulation would require the stable operation of the system for a number of annual cycles. The XENON1T array would allow annual modulation search at an order of magnitude greater sensitivity than the current DAMA sensitivity. The current status of the project is the following: the first phase of R&D for a 10 kg prototype was approved by the NSF, with funding allocated for 2 years, starting September 2002. A request for continuation of the funding was submitted in October 2003. The proposal has been very favorably reviewed in December 2003. Funding was approved to support the operation the 10 kg prototype at the Soudan Underground Laboratory as the best way to optimize the design of the XENON100 module. The 10 kg prototype has been built by the collaboration and is currently being tested and calibrated with gamma rays, alpha partcles and at the Columbia Nevis Laboratory It will be improved by replacing the light readout with much lower activity PMTs and by implementing a LXe self-shield around the target, before moving it underground by the Summer of 2005. The XENON collaboration, led by Columbia University, currently includes groups at Brown, Princeton, Yale, Rice and University of Florida, as well as LLNL. We expect to grow, especially with interested groups abroad. With continued NSF, and possibly DOE support, we should be able to install and operate XENON100 at LNGS by the end of 2007.

2. The XENON-100 Detector

2.1 Baseline Design and Principle of Operation The proposed XENON100 detector is a dual phase Xe TPC, with the active volume defined by a 50 cm diameter CsI photocathode immersed in the liquid, at about 30 cm from the first of three wire grids defining a gas proportional scintillation region. An array of 85 two-inch diameter PMTs located above the wire grids, is used to detect both direct and proportional light. The baseline design uses Hamamatsu R9288 PMTs, as the 10 kg prototype, but with selected materials for low radioactivity (see Sect. 5). The TPC is enclosed in a leak-tight cylindrical structure made of polytetrafluoroethylene (PTFE) and oxygen free copper (OFHC). The PTFE is used as effective UV light reflector and as electrical insulator. The fraction of direct light heading downward will be efficiently detected with the CsI photocathode [23]. The whole structure is immersed in a bath of LXe, serving as active veto shield against background. The LXe for shielding is contained in a double wall vacuum cryostat, made of stainless steel and is cooled by a pulse tube refrigerator. An array of 64 PMTs are mounted on the walls of the cryostat, fully immersed in LXe to detect the direct scintillation light from the shield. With both target and shield Xe volumes kept at the same temperature and pressure, the thickness of the vessel enclosing the TPC can be minimized. To precisely measure the LXe level, with an accuracy of 0.1 mm required by the dual phase operation, we will use a commercially available system, based on ultrasound. The detector design is shown schematically in Figure 1. The amount of active Xe in the TPC is about 180 kg. With fiducial volume cuts applied for background reduction to the level required for a sensitive WIMP search, the active target is reduced to about 120 kg. Clearly, if the dominant radioactivity from the PMTs is reduced, we will recover a larger fraction of the active target for a WIMP search (see Sect.5) The estimated background rate and experiment sensitivity has been calculated based on 99.5% rejection efficiency of the TPC and on a LXe shield thickness of 5 cm.

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Figure 1: (left) Schematic of XENON100 detector module. Figure 2: (center) Time structure of signals. Figure 3: (right) Schematic of XENON100 TPC

The TPC background rejection capability is based on the differentiation of ionization and scintillation signals associated with nuclear recoils, such as produced by WIMP or neutron interactions, and electron recoils, such as produced by background beta and gamma-rays. The amount of ionization generated by a nuclear recoil is comparatively very small. The trick of the dual phase operation is to extract this small number of ionization electrons from the liquid to the gas, and to detect the proportional scintillation photons they produce. The proportional light emission along electron drift path makes even single electron detection easy with a simple PMT. The challenge in the dual phase operation is to simultaneously detect the much weaker direct scintillation light signals, down to an energy threshold less than 16 keV. The XENON TPC will operate with a field of 5 kV/cm across the 30 cm drift gap. To optimize the design, calculations of the electrostatic field in the liquid and of the proportional scintillation yield dependence on electron drift path in the gas and on extraction point from the liquid, have been done with programs Maxwell and Garfield [24-25]. The simulated structure is visualized in Figure 3. Field shaping rings, spaced by 1 cm, are mounted on the outer PTFE vessel. Results of the field calculations show that charging-up of the PTFE dielectric and ionization electrons loss are negligible in the proposed geometry. For the gas amplification region, detailed 2D and 3D simulations show that either wire grids or meshes give similar photon yield, within 2%, Scintillation photons are produced almost uniformly along the gas gap, regardless the electron extraction point location. Considerations of optical transparency, ease of fabrication and reliability will determine the choice between meshes and wires. An event in the XENON TPC will be characterized by three signals corresponding to detection of direct scintillation light, proportional light from ionization electrons and CsI photoelectrons. The pulses occur within the maximum drift time of 150 µs, for a saturated electron drift velocity of 2mm/us in LXe. As shown schematically in Figure 7 the timing and the amplitude of these three signals are distinct for a nuclear recoil and for an electron recoil. Since electron diffusion in LXe is small, the proportional scintillation pulse is produced in a small spot with the same X-Y coordinates as the interaction site, allowing 2D localization with an accuracy of 1 cm. With the more precise Z information from the drift time measurement, the 3D event localization provides an additional background discrimination via fiducial volume cuts.

2.2 High Voltage Systems for TPC and for PMTs The original HV design based on multiplying the HV directly on the TPC field shaping rings has been modified, due to concerns of radioactivity associated with large ceramic capacitors and glass encapsulated diodes, mounted near the sensitive volume. In consultation with engineers at LLNL, we have adopted a new design based on an external high voltage supply, connected to the detector field cage by a hermetic feed-through. Filtering and impedance limiting is done prior to the supply entry point in the detector. The high voltage feed-through will be custom-designed to meet the low radioactivity requirement. A resistive voltage divider made of 29 individual 1 GΩ resistors directly mounted on the shaping rings will provide the appropriate field levels. The total power dissipation by the divider chain is less than 1 watt.

5 The high voltage resistors will use a resistive material such as Ruthenium oxide (RuO2) deposited on a ceramic substrate. To reduce the radioactive background, different substrates will be characterized, such as standard alumina ceramic as well as silicon nitride (Si3N4) and quartz. RuO2 will also be screened for radioactivity although the mass involved is extremely small. To multiply the photoelectrons emitted from the photocathode, a PMT requires a high voltage difference between cathode and anode, and intermediate voltages for each dynode. Typically, the cathode is on ground and the anode at ~1 kV. The intermediate voltages are provided by a resistive divider in the base of the PMT. A coupling capacitor, finally, removes the DC component from the signal output. Since XENON is a low background cryogenic instrument we need to simultaneously minimize both power dissipation in the liquid and radio-impurities. If we operate our PMTs without the usual resistive voltage divider we can potentially remove almost all the power dissipation and much of the radio-impurities associated with this structure. We envision a scheme where all the PMTs share common low impedance voltages, i.e. all first dynodes are connected to the same potential etc. We thus have 11 HV lines entering the TPC volume, one for the cathode, and one for each of the 10 dynodes. The cathode and the steel enclosure of the PMT are at about –1 kV, and the dynodes from all the PMTs are connected to the respective fixed voltage. The last dynodes, however, will be resistively isolated from each other and capacitively bypassed to the local ground. The anode and thus the signal source is at ground potential. In this way we eleminate practically all the heat production. Additionally, we minimize the radioactive impurities contained in the ceramic substrates of resistors and capacitors. The voltages are generated by individual adjustable power supplies, outside of the detector cryostat. Within the detector the voltages are distributed with a copper-on-Kapton wiring harness. A further copper-on-Kapton strip-line wiring harness is connected to each PMT anode allowing us to move signals out of the active target without cross talk or signal distortion associated with long individual wires. Gain normalization and low noise post amplification will be provided for each PMT external to the detector set up.

2.3 PMTs Readout Electronics/DAQ The DAQ system will acquire full waveforms from the inner, outer shield and external muon veto PMTs. The total event length is determined by the maximum electron drift time of 150 µs. The primary light signal will have a characteristic width of <100 ns whereas the proportional light signals will have widths of a few µs determined by the electron drift in the gas. The proportional signal will also be extended if multiple scatters occur within the LXe volume. To maximize the information from the light pulse shape, a two fold digitization will be adopted. Since the simulations show that the hit pattern for the primary light will be a weak function of event X-Y position, some ganging of the PMTs is desirable to reduce ADC cost and event size. Fast ADCs at 4 ns/sample and with single photoelectron threshold, are assumed with 4-fold ganging. If tests show that 2 ns or 1 ns sampling is required, a higher degree of ganging will be implemented. Slow ADCs will record the inner PMTs signals at 100 ns/sample (as well as ganged signals from the outer Xe, and external muon veto PMTs). This configuration for the inner PMTs is required to maximize position resolution. In addition, it maximizes the effectiveness of subsequent rejection of multiple scatter events in software. The gain, sampling time and extended 12-bit resolution are optimized for proportional light detection. The baseline trigger design will use single photoelectron discriminators for all inner PMTs fed to 2-fold coincidence units. Since it will not be known, a priori, if the trigger has occurred on a primary, secondary, or tertiary pulse, a symmetric pre and post trigger time will be adopted for the ADC. Due to the width of the secondary signal the trigger coincidences will be based on a 2 µs window. Subsequent fast analysis in PC#1 and #2 will reject clearly spurious events. Additional studies on the operating detector will further enhance trigger capabilities, thus rejecting residual noise triggers. This will include enhanced sensitivity to proportional scintillation signals by requiring spatial correlation of the photon hits. The effect of trigger and event filtering on dark matter sensitivity (which is dominated by lowest energy events) will be studied closely. The trigger event rate for underground operation has been simulated and will be ~ 1 Hz when all the baseline radioactivity levels (discussed in Sec.5) are met. Higher rates (~ 30 Hz) can be accommodated. The total event size is ~ 3MB / event. The two PCI bus configuration (ADC to PC) will be able to handle

6 this data rate at the first level. Direct hardware vetoing from the outer LXe will reduce the total trigger rate in background running, however, some sampling of these events will be kept to provide continuous calibration of the detector during long-term operation. Additional software event rejection in PC #1 and #2, and event compression in PC #3 and #4, will reduce the event storage requirements to 10 GB/day. Direct analysis for dark matter signals will be performed on a much smaller fraction of this data, however, the remaining data will be required to monitor the overall response of the detector. The maximum event rates for external gamma and neutron source calibration runs will also need to be limited to avoid significant pile-up in the 0.64 ms event window.

2.4 PMTs Calibration The calibration of the target and shield PMTs is needed to equalize the gain and to correct for differences in the quantum efficiency. The R9288 PMT has 10 multiplication stages with a total amplification of typically 5 x106. For this calibration we intend to follow closely the method developed for the MEG experiment [26]. The inner chamber and the shield will be equipped with fast light sources, either a set of blue emitting LEDs as in the MEG experiment or alternatively an external light pulser and optical fibers introducing the light into the active volume. The width of the anode pulse is proportional to the square root of the number of photoelectrons emitted from the photocathode. Adjusting the amplification, the gain of all channels will be equalized. Repeating the calibration with different light levels will also establish the linearity of the light measurement. With equal gain, the PMTs still have differences in quantum efficiency, which show as differences in the amplitude value of the known light pulses. We will introduce an alpha source into the active volume with a motion feedthrough. Note that we do not need the drift field during this calibration. A detailed Monte Carlo simulation of the light propagation will predict the amount of light hitting each PMT from the alpha particles. The measured amplitude for each PMT can be corrected to achieve the same response for all PMTs. Once the PMTs system is calibrated and the linearity is established, we will apply the drift field and measure the energy response of the system with external gamma-ray and neutron sources.

2.5 PMTs Light Detection Simulation Simu;ations have been carried out to determine the primary light collection efficiency, which defines the detector’s energy threshold, as well as to estimate the X-Y position resolution from the proportional scintillation light of the secondary signal. The primary UV scintillation light is either detected directly by an array of PMTs in the gas above the liquid or it is converted to charge by a CsI photocathode operating in the liquid, resulting in the tertiary signal, after drift. Some light is absorbed by wires or meshes, by PTFE walls, or by liquid xenon. We assumed 90% transparency of the wires and 95% reflectivity for PTFE. The absorption length of liquid xenon is taken as 100 cm and the scattering length as 30 cm [27- 29]. A large fraction of UV photons from the liquid will undergo total reflection at the liquid-gas interface, due to the large change in index of refraction (1.61 vs 1.0).

Figure 4: (left) Total number of photoelectrons (p.e.) collected from PMTs (blue) and CsI (yellow). Figure 5: (right) Reconstructed x-y position of hits.

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The simulations show that the PMTs detect about 10% of direct light for most events in the sensitive volume. The bottom CsI photocathode detects between 30% and 70% of the light, depending on event location. The total light collection efficiency is usually more than 40%. We simulated the number of photoelectrons released on the PMT and CsI photocathodes, taking into account their respective quantum efficiencies of 20% and 30%, and the W value for photon generation in liquid xenon (23.7 eV). For uniformly distributed nuclear recoil events of 2 keV electron equivalent energy (keVee), Figure 4 shows the resulting distributions for the PMTs (blue) and CsI (yellow), as well as their sum (inset). About 5 p.e./keV is achieved for the proposed detector geometry. The secondary proportional light signal provides X-Y position information, from the relative number of photoelectrons collected by each of the PMTs. We assumed a drift field of 5 kV/cm in the LXe, a Xe gas pressure of 2 atm, and a gas gap of 4 mm between liquid and lower wire grid at a field of 13 kV/cm. The PMT array is 5 mm from the upper grid, i.e. ~10 mm from the proportional gap. With a simple center-of- gravity method, we are able to reconstruct positions with a resolution of 1.2 cm, for uniformly distributed events with 2 keVee. The addition of 24 outer PMTs in the current design improves X-Y localization for events near the walls of the chamber. Figure 5 shows the reconstructed position (blue) for such events, compared to the true position (red cross). The number of photoelectrons is indicated for each PMT; the large yellow and red circles mark the chamber walls of liquid and gaseous xenon, respectively.

3. Cryogenic System and Xe Purification

3.1 Cryogenics For XENON100 a total of about 120 liters of liquid xenon is needed for target and the shield. The thermal insulation of the detector cryostat has to be carefully designed to minimize the complexity and the cost of the cooling system. Due to the narrow temperature margin (3.4K) between the liquid and solid phase, temperature control during liquefaction is especially important. Overcooling can damage the PMTs and the wire grids. Liquefaction of Xe gas with LN2 is commonly used and is cheap and effective for small scale prototypes. For a large detector, which requires reliable long-term cooling in an underground environment, a mechanical refrigerator is safer and, in the long run, cheaper. We plan to use a Pulse Tube Refrigerator (PTR), similar to the one we are using on the 10 kg prototype. The estimated heat load on the XENON100 detector is 50 W, so that even the current PTR version can pre-cool the cryostat, condense the gas, and maintain the LXe temperature stable at the desired value. Optionally, pre-cooling can be accomplished with an LN2 flow through cooling coils on the cryostat. We estimate roughly 1 day to pre-cool the detector, and about 2 days for Xe liquefaction. The large Xe mass stabilizes the temperature and the pressure in the system. Even in the case of catastrophic system failure the thermal inertia will provide sufficient time to terminate the experiment in a controlled and safe fashion. An additional LN2 cooling system controlled by the chamber pressure might be activated in case of emergency. This system is battery operated.

3.2 Xe Purification, Recirculation and Monitoring The 30 cm depth of the XENON detector imposes severe purity requirements on the LXe, in order for electrons to drift freely from the event site to the liquid surface. Electronegative impurities must be well below 1 part per billion (ppb). Moreover, stable operation of the detector demands that this high purity be maintained over long periods of time. To fulfill these requirements, we will use a combination of getters and spark purifier for the Xe gas, and continuous recirculation after liquefaction.

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Figure 6: Purification and recirculation system

The challenge is in the choice of materials both in the construction of the gas system and in the detector system itself. Commercially available purifiers, such as Oxisorb filters, molecular sieves and high- temperature getters, are based on impurity absorption. With a combination of these components, several groups, including the Columbia group, have demonstrated the required purity level in large quantities of Xe. However Oxisorb and certain molecular sieves must be avoided because of the intrinsic U content and Rn, which lead to radioactive Rn daughters 210Pb, 210Bi, 210Po in the Xe. To complement the high temperature getters, we plan to use a spark purifier, which has been shown to be extremely effective for ultra high purity in xenon high pressure ionization chambers[30-31]. The technique is based on a continuous spark discharge between metal electrodes in a vessel filled with high density Xe. The metal is sputtered into a fine dust which acts as efficient, large surface area getter, even after the discharge has stopped. In addition, UV light from the spark breaks down large organic molecules, enhancing the purification process. The best results have been achieved with titanium electrodes. Impurities from the detector represent another challenge. The detector and shield vessels will be evacuated and out-gassed for an extended time, but the PMTs in the vessels limit the bake-out temperature to 70oC. In addition, LXe is a well solvent due to Van der Waals interactions with impurity molecules. Thus, once pure Xe is liquefied in the detector, the purity level could change. We therefore plan on implementing a continuous recirculation system. LXe will be removed, separately from the detector and shield, evaporated and pumped through the getters. The heat of evaporation is used to cool the re-purified xenon in a heat exchange, before re-introducing it in the vessels. Before detector filling, and the during operation, a purity monitor of the type developed for the ICARUS experiment [32] will be used to measure the electron lifetime. The monitor relies on the drift of electrons extracted from a photocathode by a Xe flash lamp (or UV laser) pulse. The measurement of the electrons induced signal yields directly the charge lost due to impurities, and thus electron lifetime. The method offers a quick and reliable lifetime measurement, in a broad range of values, even under noisy conditions.

3.3 Kr Removal System Liquid Xe, as a condensed noble gas, is readily purifiable for most radioactive impurities. The one notable exception is 85Kr, which is difficult to separate from Xe. It undergoes beta decay with a 690 keV end-point at 10.76 year half-life. Its abundance is such that Kr gas has an 85Kr decay rate of ≈1 Bq/cm3. A concentration of roughly 150 ppt (by volume) Kr in Xe is needed to meet our target of 2x10-3 events/kg/keVee/day. Xe is separated by air by distillation, a process that also removes Kr. However there appears to be no commercially available method to measure Kr levels < 10 ppb, achievable by mass spectrometry. The Princeton group, under a separate NSF grant awarded in Sept 2003, will develop a Kr separation system based on chromatographic adsorption in a charcoal column at room temperature. The method relies on the stronger adsorption (by a factor of ≈ 17) of Xe than Kr on charcoal. The goal of

9 the 2 year grant is design and construction of a system capable of cleaning 10 kg Xe to the ppt Kr level. Equally important, the system will also concentrates the removed Kr, allowing to measure the ppt level. Our strategy for managing Kr in the XENON experiment is two-fold. First, using the chromatographic system as a measurement tool, we will purchase from SpectraGas Xe which has been specially distilled for very low levels of Kr. We cannot obtain guaranteed Kr level, but believe that ≤ 200 ppt levels should be attainable. The $10/liter ($2000/kg) price reflects a small increase over market price for extra distillation steps. As a second strategy, we plan to construct a full-scale Kr purification system, to be located underground. This will serve as a backup if SpectraGas is unable to deliver low Kr levels, and will also allow convenient reprocessing of the Xe in the event of air leaks or operational errors which introduce air (and hence Kr) into the Xe. We will also continue to study use of a cryogenic distillation column as an alternative system. Other backgrounds, such as Rn daughters, will be removed with techniques used for the Borexino experiment [33].

4. Detector Shield and Background Simulations Figure 7 shows the position variation of the gamma background (scale in log(evts/keVee/kg/day)) due to inner detector PMTs, with the assumed activity baseline goal of 10 mBq/PMT. There are 85 2” PMTs in the TPC of radius 250 mm and depth 300 mm. A depth cut at -50 mm, and a radial cut at 200 mm, is required to reduce the average background in the remaining fiducial volume to below 2x10– 3 /keVee/kg/day. Similar Monte Carlo simulations were conducted for the other background components. Figure 8 shows the low energy spectra (evts/keVee/kg/day) of gamma events from the inner PMTs and the stainless steel (30 mBq/kg 60Co)cryostat. The outer shield PMTs contribute at a factor 10 lower than inner PMTs. The 85Kr background produces a flat continuum in this region at a level of 1 evts/keVee/kg/day/30 ppb Kr and will be reduced to be sub-dominant (see Sect. 4.3)). Also shown is the Monte Carlo spectrum for neutrons from U/Th ( ,n) from PMTs and stainless steel, plotted in units of evts/keVee/kg/day, where a quenching factor of 33% as average value from current results [34-36] is used.

Figure 7 (left) and Figure 8 (right) (see text for details)

The overall background event rate in the detector is contributed to by both internal and external sources of gammas and neutrons. For a WIMP of 100 GeV, the sensitivity goal for XENON detectors is a normalized cross-section per nucleon of 2x10–46 cm2, which has an interaction rate of ~2 events/100 kg/yr, and a low energy differential rate at the full detection threshold of 6 keVee of ~1.2x10–5 /keVee/kg/day. In order to build up sufficient statistics (100’s of events) for WIMP events at this cross-section, multiple detectors will be required. However, in the MC design of this first module, we use this sensitivity as the goal. The goal for the total gamma interaction rate, within the fiducial volume needs to be < 2x10-3 evts/keV/kg/day. This is based on a projected discrimination (electron vs. nuclear recoils) within the inner

10 Xe volume of at least 99.5% using the ratio of primary scintillation light to ionization yield. It should be noted that WIMP events are nuclear recoil interactions that occur at a single site within the Xe detector. The K/U/Th content of the PMTs contribute significant gamma activity in the fiducial volume. The inner PMTs will have direct line of sight to the upper levels of the inner Xe volume. It will be necessary to make a depth (Z) cut. Monte Carlo simulations of the PMT activity show that for a total K/U/Th of 10 mBq/2” PMTs (assuming 85 inner PMTs) the background will be sufficiently attenuated at a depth of 5 cm. The gamma contribution is reduced by the direct attenuation of the Xe and by a significant additional factor using a multiple scatter cut within the Xe. Based on the Monte Carlo results, accounting for all radioactive sources discussed in this section, we increased the TPC diameter from 40 cm (original proposal) to 50 cm. The outer LXe shield has been kept at 5 cm thickness. In addition, the detector has been designed with a minimum of dead material adjacent to the inner volume since this would adversely impact the effectiveness of the multiple hit cut. The activity from the 64 outer Xe veto PMTs was simulated using the same activity per PMT as that for the inner PMTs. The 5 cm LXe shield reduces the inner event rate, as does the multiple scatter cut. The outer PMTs are found to give a factor 10 smaller contribution to the event rate in the fiducial volume. A low radioactivity PMT development program with Hamamatsu will be required to achieve the baseline goal of PMT activity of 10 mBq/2” PMT (the assumed split in the Monte Carlo was K/U/Th 4/2/4 mBq). At present the XMASS collaboration reports [37] its lowest activity PMT (Hamamatsu R8778) with K/U/Th of 140/18/7 mBq/2” PMT with their stated goal of achieving a factor 10 improvement. Note that the higher K is less of a concern because it makes <5% contribution to gamma background relative to U/Th of the same activity. The replacement of the PMT glass with metal to minimize radioactive glass should permit the baseline levels to be achieved for our compact metal channel R9288. Note that the Monte Carlo studies show that PMTs with an activity background of a factor 5-10 higher than the XENON baseline, would still yield gamma interaction rate at the required level. However, a second factor relating to the neutron production from tubes needs also to be considered. The total event rate due to the PMTs in the Xe detector will be 0.5 Hz at the baseline activity. The outer pressure vessel of the XENON detector will be fabricated from stainless steel. The choice of stainless steel as opposed to another material, such as copper with lower intrinsic radioactivity, is a trade off based on cost of construction. Monte Carlo results show that steel at a level of activity of 300 mBq/kg (assumed dominated by 60Co) will allow us to meet the target gamma rate in the inner fiducial region, however, our baseline goal will be to achieve a factor 10 below this. Prior to construction, the stainless material will be surveyed using the screening facilities. Screening data from the Borexino Collaboration [38] show a level of <~30 mBq/kg was typical of most of the stainless steel samples counted. Surface beta emitter activity is not projected to be a problem since the Xe inner fiducial region is well separated from all surfaces by 2.5–5 cm of Xe. The electrical components that will be used within the detector and shield will be subject to screening to insure their contribution is subdominant. The Xe itself will be subject to continuous re-purification during operation. The external cavern gamma activity will be attenuated using a 22 cm Pb shield. The outer 5 cm layer of LXe relaxes the requirement of an ultra low 210Pb content in the Pb of the inner lining. Pb with an activity of 6 Bq/kg can be used for the inside, and 5 cm of Pb with standard low activity of 30 Bq/kg can be used in the remainder. The (α,n) neutrons, in the energy range 100 keV–8 MeV (“intermediate” energy), arising from U/Th ‘s in the rock can be attenuated to the desired level by 60 cm of polyethylene moderator. Although the underground site has not been selected, this projection assumes a representative flux of 4x10-6 /cm2/s. A significant contribution to the neutron background arises from the U/Th content within the shield due to the PMTs and stainless steel. For a total activity of the detector of 2 Bq U/Th (6 mBq x 149 PMTs + 10 mBq/kg x 100 kg Steel), and assuming a neutron production rate of 10-5 of this rate, the low energy neutron recoil spectrum (converted into /keVee) is simulated to give an interaction rate of 3x10-5 evts/keVee/kg/day at threshold. Unlike the gamma signal, the direct attenuation, or multiple hit veto, of the neutrons in the Xe gives a modest reduction (~4x) since the m.f.p. between scatters for 1 MeV neutrons is ~12 cm, and additionally the average energy loss per collision with Xe is only ~1/64th of the kinetic energy. In order to reach the goal, a further reduction of ~3x will be required. An additional path to vetoing the intermediate energy neutrons can be through their moderation after the initial recoil event

11 by a polyethylene layer directly outside the stainless pressure vessel and their subsequent captu on an additional Gd (or Cd) layer. The capture will generate ~8 MeV gamma showers that can be detected in the Xe and will be used as veto for the earlier single nuclear recoil within the Xe inner region. This process will take place in the <<320 µs window used for the DAQ event duration. As part of the first year of this program we intend to study in detail the likely neutron yields from U/Th decays in both PMT materials, and also the Stainless Steel. We will model possible veto strategies based on thermal neutron capture. As a baseline design we are assuming that 30 cm of the polyethylene shield will be placed within the Pb shield (with the remaining 30 cm outside the Pb) to optimize the identification of the rogue neutrons. We will also study whether it is necessary to further moderate the steel activity using a PTFE layer directly inside the pressure vessel at the outer edge of the 5 cm outer Xe veto. Cosmic ray muons will contribute electromagnetic backgrounds through direct interaction in the detector and surrounding shield, and also through the generation of neutrons and gammas in the Pb and surrounding rock. A 2” plastic scintillator veto completely surrounding the Pb/polyethylene shield will be able to tag muons entering the entire shield volume with an ~99.9% efficiency [39,40]. This will be sufficient to veto the signal due to neutrons (and gammas) generated by the muons in the shield to sub- dominant level for the XENON goal at underground depths >~3700 mwe. The high energy neutrons (10–2000 MeV) created in the rock by muons are not significantly attenuated by the moderator shield, but will generate high multiplicity events within the Pb shield. Depth is the most effective way to decrease the signal from these “punch-through” neutrons. Simulations show that the event rate in the XENON detector at threshold of nuclear recoils from these neutron events at a depth of ~3700 mwe (at LNGS, or NUSL-Homestake 4800 ft) will be ~1x10–5 /keVee/kg/day. For comparison the simulated neutron signal rate, which scales with the muon rate, plus a minor modification for change in average muon energy, will be 25x higher at Soudan, 45x higher at WIPP, and 25x lower at Sudbury. Thus sites such as LNGS, or Homestake 4800 ft will allow XENON to reach target sensitivity with the poly/Pb/plastic scintillator shield outlined above. At Sudbury the additional factor of 25x will further reduce the “punch-through” neutron component and relax the efficiency of plastic scintillator required to a level ~90%. In order to reach the target sensitivity at either Soudan, or WIPP, it would require a significant additional shield designed specifically to attenuate/veto the high energy neutrons, in addition to vetoing the direct interaction of muons discussed in the previous paragraph at a level >>99.9%.

5. Materials Screening To screen the detector construction materials with respect to their radio-purity we propose to initiate a collaboration with the Gran Sasso Underground Laboratory. We also plan to use the new SOLO screening facility in the Soudan Mine in Minnesota. We plan to screen all the detector components (mainly for the 238-U chain, 232-Th chain, 40-K, 60-Co), by gamma-ray spectroscopy using HPGe detectors. This technique can reach sensitivities down to mBq/kg, or 10-10 g/g levels for U and Th contaminations, which is sufficient for our needs. The Soudan SOLO facility is at 2080 m.w.e., where the muon flux is reduced by a factor of 104 with respect to the surface. It uses 2 Ge detectors which have been previously operated as double beta decay detectors. The detectors are shielded with about 40 cm of lead and the entire setup is flushed with N2. The background of one of the Ge detectors in the energy region 60-2700 keV is about 200 events/kg/day. In case higher sensitivities will be needed, the more sophisticated facility at LNGS could be used. Several of the currently most sensitive large-volume HPGe detectors are located in this facility, under about 3500 m.w.e of rock, which reduces the muon flux by a factor of 106 and the neutron flux by about 3 orders of magnitude. Strong ventilation with outside air reduces the Rn level to about 30 Bq/m3. The detectors are shielded by 25 cm of lead, 10 cm of OHFC and by neutron shields. The setups are continuously flushed with nitrogen gas. An airlock system allows to load samples into the shield without exposing the sample chamber to radon. The sample chambers are large and accommodate samples up to 15 l in volume. The background of one of the large (413 cm3 Ge detectors in the 60-2700 keV energy region is 30 events/kg/day.

12 The XENON detector samples will be shipped to Soudan and/or LNGS and measured for up to 20 days, depending on the efficiency and background of the Ge spectrometer, and on the sample size. The counting efficiencies will be determined by Monte Carlo simulations.

6. Environmental resources and forecast of emissions

The XENON experiment uses no hazardous chemicals. It will use electrical power. It may make use of a water shield, in which case an amount of water not to exceed 1000 m3 would be used. It will use liquid nitrogen and liquid xenon. The only emissions from the experiment are inert gases – nitrogen, and in the event of an accident, xenon, and clean water.

7. Milestones

Milestone Date Description of Major Tasks (taken from Project Timeline) (end of period) Year 1 (October 1, Completion of initial 2 year R&D Program 2004 ) Successful operation of 10 kg prototype at Nevis Lab Nuclear recoil discrimination measured Energy Threshold optimization completed Finalize choice of photodetector configuration for XENON100 (PMTs/ CsI / MCP / Gas Wire Readout / GEMs) Kr removal from Xe – demonstration of prototype system Q3 Low activity PMT Development / Selection completed

Improved, low activity 10 kg prototype Completed Q4 XENON-10kg running in Soudan mine Year 2 (October 1, XENON100 Gas Purification System procurement completed 2005) Q1 Designs for Feedthroughs / Level Detector / Slow control completed HV Systems Design Completed Q3 PMT procurement completed DAQ procurement completed Chamber/Cryostat Design completed Q4 Shield / Muon Veto procurement completed Year 3 (October 1, Gas Purification System operational 2006) Q1 Detector Construction completed

DAQ / Slow Control completed Q3 Detector Calibration above ground Q4 Kr reduction from Xe completed Low Background Shield / Muon Veto - Construction completed Year 4 (October 1, Commissioning of XENON100 in Gran Sasso 2007) Q1, Q2 Year 4 Q3 Physics run of XENON100 in Gran Sasso to Year 7 (October 1, 2010) Q2 Year 7 Q3, Q4 Decommissioning of the 100 kg module

REQUESTS TO THE LABORATORY 13

We will need a space of 6 m x 5 m x 5m (height) for the experimental room, which will include a small clean room. In addition, we will need a 3 x 3 m control room housing the computers and electronics, with internet link to the surface. Other requirements are a 1 ton crane and air conditioning of the experimental and computer rooms. The requests for electrical power amounts to 100 kW with a 3-phase line. We also need a UPS, namely 10 kW on an emergency generator. The time needed for installation, commissioning, running and decommissioning amounts to 6 months, 6 months, 3 years and 6 months, respectively. We expect to have about 5-7 people on site during installation and commissioning, 2-3 during running and 5 during decommissioning. Outside space: we request one office space in the new building with a 5 years occupancy time, and use of the library and seminar rooms. Infrastructure: we will need access to the clean room in the Hall di Montagio, access to the machine shop and to the low background counting facility. Requests to the Services of the laboratory: we will need nitrogen gas (1 bottle per week), and possibly high purity water for a water shield. We will need liquid nitrogen amounting to 100 liters/week. In a special event of a power failure, we would need liquid nitrogen amounting to 3 full dewars of 240 liters each to maintain the temperature of the liquid xenon.

SAFETY AND ENVIRONMENTAL ISSUES

We have reviewed the issues of the experiment being underground, in a seismic zone, close to a highway and in a national park. We believe our proposed experiment would not cause any hazards, since it will use only inert, nobel gases and liquids, and possibly clean water.

This experiment involves liquid xenon, a condensed noble gas operated at a temperature of about 160 K (- 110 C), mechnical equipment and electronics, and possibly a water shield. There are no hazardous materials or chemicals. The primary cooling is expected to be a mechanical cooler, though liquid nitrogen will also be used. The primary hazard in the experiment is failure of the cryogenic system containing liquid xenon. There will be redundant safety systems to ensure the integrity of this system, both for the safety of personnel and the detector, and to protect the high valuable ultra-pure liquid xenon. These safety systems are discussed in detail below. However we begin by noting that even in the case of a catastrophic failure of the experiment (which would probably require some external system to fall onto the experiment), the amount of liquid xenon released will be less that 300 Kg, or roughly 60 m3. This presents a comparable risk to the equivalent amount of liquid nitrogen, which is already present in greater quantities in Gran Sasso.

The safety systems to prevent this occurrence are as follows. The Xe gas is stored in stainless steel vessels, mounted in stainless steel dewars constantly kept at LN2 temperature. Once condensed in the detector and shield system, the liquid is kept at the operating temperature by a refrigerator, or in the case of a total power loss by a reserve of LN2. In case of an emergency, a pressure rise in the detector will trigger a valve opening the gas line for orderly recovery of the xenon. This recovery will initiate automatically, with no operator assistance required. If for any reason the automatic recovery should malfunction, the pressure in the target/shield vessel would rise until the break point of a rupture disc. Note that the break point will be chosen around 2.5 atm above the ambient pressure, much lower than the safety limit of the vessel or any components connected to it. The excess pressure will be vented, preferably into an exhaust line from where the xenon can still be recovered in gas form. After the break of the rupture disc the pressure will go rapidly to ambient pressure after the release of a few liters of gaseous xenon. If a

14 loss of cooling triggered the overpressure, the liquid xenon will slowly evaporate. We note that to evaporate moderate gas flow of 10 Standard Liter Per Minute (SPLM) about 90 Watts of constant heat supply are required. The total heat transfer of the XENON100 structure is limited to about 100 to 150 WattsUnder most forseable failure modes, evaporation of all the liquid will require many hours with no risk to personnel or adverse influence to the environment.

COLLABORATION MEMBERS

We have assembled an excellent team, with a well balanced mix of expertise. The geographic location of all universities involved in XENON is optimal: Columbia, Yale, Princeton and Brown are within a short trip by car or train. Direct flights of a few hours connect Columbia to Rice and University of Florida. LLNL is the only institution on the West Coast. We expect to expand the team with more US and foreign institutions. Weekly teleconferences will continue to provide the tight coordination between the institutions, insuring the success of the proposed program. Columbia University (Elena Aprile, PI) will manage and coordinate the effort within the collaboration. The PI will take full responsibility for the proposed work and time schedule. We summarize the responsibilities below. Columbia (Elena Aprile, PI) will be responsible for the design, construction and testing of the detector vacuum cryostat and cryogenics system. Columbia, with contributions from Princeton, will also be responsible for the design and fabrication of the TPC vessel, grids and CsI photocathode and PMTs system. This includes simulations for detector optimization. The design and fabrication of the spark purifier and the purity monitor will also be Columbia responsibility. Princeton (Tom Shutt, PI) will be responsible for the design, construction and testing of the Xe purification and circulation system, including the procurement of Xe gas with low Kr level. A Kr removal system to be used underground will also be developed and tested by Princeton. The cost for these systems and for the gas is included in the Columbia budget. K.McDonald intends to seek support from DOE for detector activity relevant to XENON. Brown (Richard Gaitskell, PI) will be responsible for the design and testing of the readout electronics/DAQ and data analysis software. The cost of a system based on commercially available units is included in the Columbia budget. Brown, with contributions from University of Florida, will also be responsible for Monte Carlo simulations. Brown has also designed and costed the shield needed for underground operation, assuming a deep site such as LNGS or SNO. R. Gaitskell also intends to seek support from DOE for the above effort on the XENON experiment. Lawrence Livermore National Laboratory (LLNL) (William Craig, PI) will be responsible for the design, fabrication and testing of the HV system for the TPC drift field and for the PMTs, including analog processing subsystems. The cost for these systems (both labor and material &supplies) have been included in the Columbia budget. In addition, travel cost has been explicetely included to allow Columbia staff to interact with the system design engineers (Norm Madden and Jacques Millaud) at LLNL through the build phase. If funding for the LLNL effort on these XENON subsystems is received from a subsequent DOE proposal, the Columbia funding for these subsystems will not be required from NSF. If DOE or other funding is not received for the LLNL effort, the design engineers at LLNL will advice the Columbia team and will be involved in overall system integration and test. The advisory role will be supported by internal LLNL funding. Rice (Uwe Oberlack, PI) will be responsible for the design, fabrication and test of the LED and radioactive sources calibration system required for detector and shield PMTs. Rice will also be responsible for the selection according to gain of the Hamamatsu PMTs, for their optimal distribution in detector and shield sections. Rice will also take primary responsibility for the development of software algorithms for event position reconstruction from PMTs information. Yale (Daniel McKinsey, PI) will be responsible for the design and development of the detector slow control system, including the precision liquid level meter. Yale will also take responsibility for the design

15 and set up required for a neutron calibration of both the 10 kg prototype and the 100kg module, making available a d-d neutron generator which will be purchased with startup funds. University of Florida (Laura Baudis, PI) will be responsible for all materials screening for radioactivity, using facilities at MPI , Gran Sasso Laboratory and Soudan. A measurement of the quenching factor in LXe , using the tandem facility available at the University of Florida is also proposed. Startup funds will be used to build a LXe system to support the operation of a detector, designed with advice from Columbia.

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