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Each chapter is driven by an unquenchable thirst for Zeppelin knowledge and pulls the reader deeper into the world of Led Zeppelin, where the fascination continues…. Search Search for: Search. Switch skin. Download Now. What do you think? Comments Our site Facebook. We see no evidence of A value of 0. The equivalent simulations searches.

An accu- underground. This was, however, a conservative estimate They are alumina based and their average contam- and the present data suggest much lower contami- ination was measured at ppb U, ppb Th nation, which can be confirmed independently. Polypropylene shield and veto detector in the veto scintillator or in the LXe target. The decay rate is 0. The neutron detection efficiency delayed tagging in 2.

Other contributed to the WIMP target a prime design items were screened and their locations chosen to consideration as well as background measurements minimise their impact: stainless steel parts e. A tonne of scintillator vacuum ports , indium seals, LN2 in the litre within the lead castle provides confirmation of the reservoir.

Simulation of bling, etc. However, the cessfully. How- evidence for such a significant rate, and we revised ever, owing to their large mass, the polypropylene the upper limit down to 0. Rarer types of electron recoil event this radiological contamination is entirely consis- tent with the interaction rate recorded in the veto 3. Furthermore, there are differences in energy A significant amount of potassium is involved spectrum and spatial distribution in ZEPLIN-III, in sensitising internal photomultiplier surfaces, as shown in Figure 2, which confirm these contri- namely the bialkali photocathodes.

Others will generate cient to trigger the veto even if the highest energy bremsstrahlung photons in the quartz. On the other hand, the photons are also produced in the quartz and can polypropylene generates 0.

The inate at low energies for interactions just above the castle walls and roof are made from lead smelted phototube. In the fiducial volume, some 10 mm into thin stainless steel containers to make 1. The HPGe spectra dow. This optical signal can both increase the scin- show only a vestigial amount of 60 Co. In any case, this effect should be carefully assessed in future noble liquid instruments where scintillation is detected by photomultipliers.

The 10 2 solid histograms are cumulative and represent interactions x0. The apparent electron recoils and yield no ionisation, by com- energies are 30—60 keVee for both histograms. A popula- tion of fast pulses is clearly visible, which we attribute to paring with the corresponding timing spectrum for Cherenkov photons generated in the phototube windows by the upper grid surface.

Inset: array time response for three this population are indeed faster than nuclear recoil events with 30 keVee reconstructed energy approximately scintillation. This Cherenkov emission may be dangerous when For this and other reasons optical cross-talk be- associated with low-energy electron recoils in the tween phototubes must be avoided, so that these fiducial volume. If the origin is 40 K, this would effects remain localised to a single channel and are have to involve a bremsstralhung photon also gen- therefore easier to identify.

However, around their envelopes than with the original ones, the x, y positions recovered from S1 and S2 in- since the upgrade photomultipliers are slightly nar- dependently are likely to be inconsistent for these rower.

However, there is no significant direct path events, leading to their probable rejection. This trend is indeed ob- served in our data before these events are removed 3. A sim- Multiple interactions where one vertex occurs in ilar calculation can be done for neutron interac- a region of the detector which is optically coupled to tions.

Assuming, very approximately, an expo- the phototube array but from which no charge can nential recoil spectrum with characteristic energy be extracted can lead to a particularly challenging E0 , then the spectrum of double-vertex events is background. This 4. Radioactivity neutrons was identified early on as a background mechanism in two-phase detectors. In ZEPLIN-III an initial as- The good agreement between data and simula- sessment during construction showed that efficient tions obtained for the spectrum of electron recoils rejection based on S1 light pattern and S1-S2 vertex at low energy confirms the radiological contami- consistency would be necessary [2].

In the first run nation values adopted for each component; these this background became perhaps the most challeng- are subsequently used to calculate neutron produc- ing, in particular because these populations can- tion rates in those materials.

Muon-induced show- not be reproduced accurately with calibration point ers, in particular photo-production in electromag- sources. This contribution is collection for the dead vertex is not known pre- not dominant in a detector of the size of ZEPLIN- cisely.

Consider Ref. The energy is reconstructed from the coin- practically none in lead. Yields in pure elemental cident S1 components, but there is only one S2. In the case of the photomultipliers this is done 10 -2 10 at sub-component level, since some materials are Integral rate right further away from the fiducial volume than oth- -3 Differential rate left 10 ers.

It is computa- Feedthroughs 0. The Boulby rock has 65 ppb U Figure 5: Simulated differential and integral rates of neutron and ppb Th assumed in secular equilibrium [29]. Neutrons are generated in the rock to a depth of 3 m and tallied initially on a test surface in the lab- The event rates discussed above are for single oratory. The simulated integral flux is adjusted at scatters in the 6. The Although the event Yearly rates of single scatters in 5—50 keVnr in can still be rejected from the multiple S2 pulses, it the fiducial volume and the component-level break- is often useful to establish the distribution of scat- down are given in Table I and shown in Figure 5.

This may provide additional The total of 3. We note that searches. Finally, identified by additional S2 pulses in two-phase sys- the detection threshold is likely to be a little higher tems although they will become MSSI events if the than 5 keVnr, so a value just below 0. However, the time- pected. This is very close to the original goal of the of-flight for radioactivity neutrons should not be ne- experiment.

The veto-coincident data analysed so glected as a valuable discriminant. Energy spectra and spa- though it should affect both S1 and S2 pulses. In tial distributions are presented for electron recoils this instance photon emission is delayed with re- in the fiducial volume.

The low energy rate is spect to the original optical stimulus depending on 0. In the veto detector installed around the WIMP target.

Whether this More recently, a WIMP search with CaF2 Eu scin- can be matched in the second run depends on the tillator at Kamioka Observatory found that the sen- optical performance of the photomultipliers, which sitivity below 10 keVee was limited by Cherenkov unfortunately is not ideal. This type assays and Monte Carlo simulations to determine of event may be difficult to identify if the light pat- their contribution to the radioactivity budget led tern is not too peaked in a single channel.

We note to good matches to the measured distributions. Similar studies for other experi- these degrade the discrimination power of two- ments, namely XENON [31], have been equally phase detectors. They pose a real challenge to successful, confirming that this methodology pro- current experiments and their mitigation should duces accurate results. The total rate of single elastic events. This will not limit power eventually observed is not hindered signifi- the sensitivity of the experiment.

Acknowledgements [21] M. Jones,2 G. Kalmus,6 A. Kobyakin,1 A. Kovalenko,1 V. Lebedenko,2 A. Lindote,5, 6 I. Liubarsky,2 M.

Lopes,5 R. Lyons,2 P. Majewski,6 A. Murphy,3 F. Neves,5, 2 S. Paling,6 J. Pinto da Cunha,5 R. Preece,6 J. Quenby,2 L. Reichhart,3 P. Scovell,3 C. Silva,5 V. Solovov,5 N. Smith,6 P. Smith,6 V. Stekhanov,1 T. Sumner,2 C. Thorne,2 L. Upper limits on signal rate are derived from the maximum empty patch in the data.

This is the tightest constraint on that explanation of the DAMA result yet presented using a xenon target. PACS numbers: Mc, Gx Dark matter in the form of weakly interacting massive atomic mass [6]. In such inelastic dark mat- tering against xenon nuclei in the 6.

The summed scintillation signal from the liquid is denoted by S1. As described in Ref. WIMPs with 0. Red triangles are data, fitted by a gamma distribution. The outlined histogram AmBe data and blue squares are Cs data.

Recoiling electrons and events is an adequate approximation to that for signal nuclei produce different proportions of the singlet and events. The power of the timing cut to reduce elec- is the zero-field light yield of nuclear recoils relative to tron recoil background increases with energy, as seen in that of electron recoils.

An energy range of 20—80 keV nu- Fig.