First limits on WIMP nuclear recoil signals in ZEPLIN-II: A two-phase xenon detector for dark matter detection

Abstract

Results are presented from the first underground data run of ZEPLIN-II, a 31 kg two-phase xenon detector developed to observe nuclear recoils from hypothetical weakly interacting massive dark matter particles. Discrimination between nuclear recoils and background electron recoils is afforded by recording both the scintillation and ionisation signals generated within the liquid xenon, with the ratio of these signals being different for the two classes of event. This ratio is calibrated for different incident species using an AmBe neutron source and 60Co γ-ray sources. From our first 31 live days of running ZEPLIN-II, the total exposure following the application of fiducial and stability cuts was 225 kg × days. A background population of radon progeny events was observed in this run, arising from radon emission in the gas purification getters, due to radon daughter ion decays on the surfaces of the walls of the chamber. An acceptance window, defined by the neutron calibration data, of 50% nuclear recoil acceptance between 5 keVee and 20 keVee, had an observed count of 29 events, with a summed expectation of 28.6 ± 4.3 γ-ray and radon progeny induced background events. These figures provide a 90% c.l. upper limit to the number of nuclear recoils of 10.4 events in this acceptance window, which converts to a WIMP nucleon spin-independent cross-section with a minimum of 6.6 × 10-7 pb following the inclusion of an energy-dependent, calibrated, efficiency. A second run is currently underway in which the radon progeny will be eliminated, thereby removing the background population, with a projected sensitivity of 2 × 10-7 pb for similar exposures as the first run

XAX: A multi-ton, multi-target detection system for dark matter, double beta decay and pp solar neutrinos

“…In the case of XAX, however, the benefits of double-phase detectors are also realized. S2 signals, obtained from the TPC operation, can provide 3-D position reconstruction accurately, as is the case in conventional double-phase detectors (such as ZEPLIN-II [1], XENON10 [2], and WARP [12]). What makes XAX unique is the possibility to impose 4-D (position and time) coincidence between the S1 and S2 signals for additional reduction of background.…”

“…This should be compared with the current cross section limits (2008) at the level 10 −7 -10 −8 pb [1,2], and theoretical expectations (from supersymmetry theory) extend to the level 10 −10 pb and below [16]. Thus, XAX could cover the most favored phase space predicted by supersymmetry models, if the neutralino is indeed the dark matter particle.…”

“…Radon has a lifetime of ∼4 days, and hence does not survive in the stored xenon gas, but can cause background problems in the Xe detectors [1,2,3] by contaminating the target or gas systems during assembly and transferring through deposition of its decay products ( 218 P o, 214 P o) on the detector walls where they further decay by alpha emission. The alphas themselves are in the MeV range, and hence do not result in low energy events if emitted into the liquid, but if emitted into the wall, the nucleus recoils into the liquid with (by momentum conservation) < 100 keV recoil energy and thus can produce a signal similar to that of a recoil Xe nucleus.…”

“…Detection of dark matter events is based on the wellestablished two-phase TPC (Time Projection Chamber), giving both primary (S1) and secondary (S2) scintillation signals for each event, from which nuclear recoils can be discriminated from gamma background with less than 1% overlap [1,2,3]. This method currently reaches a sensitivity of < 10 −7 pb as shown in Fig 7. The purpose of this paper is to demonstrate that a factor of 1000 improvement on the existing WIMP sensitivity can be achieved in XAX by the use of larger target masses with improved self-shielding and a new ultra-low background photon detection device QUPID surrounding each target volume in a 4π array for position sensitivity.…”

Preparation of neutron-activated xenon for liquid xenon detector calibration

“…In recent years, LXe has also attracted interest as a material for direct dark matter detection [5,6,7,8,9], in which hypothetical weakly interacting massive particles (WIMPs) can be detected from their elastic scattering from target xenon nuclei, producing a signal with energy of a few keV. LXe satisfies the requirements for a sensitive dark matter detection due to its scalability to large masses, good background discrimination and lowenergy threshold.…”