Physical Review D - Particles, Fields, Gravitation and Cosmology, 2009
The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses a 12 kg two-phase x... more The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses a 12 kg two-phase xenon time-projection chamber to search for the weakly interacting massive particles (WIMPs) that may account for the dark matter of our Galaxy. The detector measures both scintillation and ionization produced by radiation interacting in the liquid to differentiate between the nuclear recoils expected from WIMPs and the electron-recoil background signals down to $10 keV nuclear-recoil energy. An analysis of 847 kg Á days of data acquired between February 27, 2008, and May 20, 2008, has excluded a WIMPnucleon elastic scattering spin-independent cross section above 8:1 Â 10 À8 pb at 60 GeVc À2 with a 90% confidence limit. It has also demonstrated that the two-phase xenon technique is capable of better discrimination between electron and nuclear recoils at low-energy than previously achieved by other xenon-based experiments.
Results are presented from the first underground data run of ZEPLIN-II, a 31 kg two phase xenon d... more 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 60 Co 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 keV ee and 20 keV ee , 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.
We present the first measurements of the electroluminescence response to the emission of single e... more We present the first measurements of the electroluminescence response to the emission of single electrons in a two-phase noble gas detector. Single ionization electrons generated in liquid xenon are detected in a thin gas layer during the 31-day background run of the ZEPLIN-II experiment, a two-phase xenon detector for WIMP dark matter searches. Both the pressure dependence and magnitude of the single-electron response are in agreement with previous measurements of electroluminescence yield in xenon. We discuss different photoionization processes as possible cause for the sample of single electrons studied in this work. This observation may have implications for the design and operation of future large-scale two-phase systems.
Gaseous and liquid xenon particle detectors are being used in a number of applications including ... more Gaseous and liquid xenon particle detectors are being used in a number of applications including dark matter search and neutrino-less double beta decay experiments. Polytetrafluoroethylene (PTFE) is often used in these detectors both as electrical insulator and as a light reflector to improve the efficiency of detection of scintillation photons. However, xenon emits in the vacuum ultraviolet wavelength region (λ=175 nm) where the reflecting properties of PTFE are not sufficiently known. In this work we report on measurements of PTFE reflectance, including its angular distribution, for the xenon scintillation light. Various samples of PTFE, manufactured by different processes (extruded, expanded, skived and pressed) have been studied. The data were interpreted with a physical model comprising both specular and diffuse reflections. The reflectance obtained for these samples ranges from about 47% to 66% for VUV light. Fluoropolymers, namely ETFE, FEP and PFA were also measured.
ABSTRACT Computer-generated 3-D models of chromatin structures are presented and applied for DNA ... more ABSTRACT Computer-generated 3-D models of chromatin structures are presented and applied for DNA damage assessment from the indirect action of water radicals formed in a single spur.
Compact gamma cameras with a square-shaped monolithic scintillator crystal and an array of silico... more Compact gamma cameras with a square-shaped monolithic scintillator crystal and an array of silicon photomultipliers (SiPMs) are actively being developed for applications in areas such as small animal imaging, cancer diagnostics and radiotracer guided surgery. Statistical methods of position reconstruction, which are potentially superior to the traditional centroid method, require accurate knowledge of the spatial response of each photomultiplier. Using both Monte Carlo simulations and experimental data obtained with a camera prototype, we show that the spatial response of all photomultipliers (light response functions) can be parameterized with axially symmetric functions obtained iteratively from flood field irradiation data. The study was performed with a camera prototype equipped with a 30 x 30 x 2 mm3 LYSO crystal and an 8 x 8 array of SiPMs for 140 keV gamma rays. The simulations demonstrate that the images, reconstructed with the maximum likelihood method using the response ob...
Bubble formation in liquid xenon underneath a Thick Gaseous Electron Multiplier (THGEM) electrode... more Bubble formation in liquid xenon underneath a Thick Gaseous Electron Multiplier (THGEM) electrode immersed in liquid xenon was observed with a CCD camera. With voltage across the THGEM, the appearance of bubbles was correlated with that of electroluminescence signals induced by ionization electrons from alpha-particle tracks. This confirms recent indirect evidence that the observed photons are due to electroluminescence within a xenon vapor layer trapped under the electrode. The bubbles seem to emerge spontaneously due to heat flow from 300K into the liquid, or in a controlled manner, by locally boiling the liquid with resistive wires. Controlled bubble formation resulted in energy resolution of σ/E7.5% for ~6,000 ionization electrons. The phenomenon could pave ways towards the conception of large-volume 'local dual-phase' noble-liquid TPCs.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1994
The purification system and the method of impurity monitoring developed for a positron emission t... more The purification system and the method of impurity monitoring developed for a positron emission tomography (PET) scanner based on liquid xenon position sensitive detectors are described. The present results show that an attenuation length of the scintillation photons in the liquid of the order of 50 cm can be reached by using only commercial purifiers. Although our preliminary measurements show that a strong electron attachment exists in liquid xenon, the electron attenuation length of 2 cm achieved is sufficient for the PET detectors.
The history, present situation and prospects on electron multiplication and secondary scintillati... more The history, present situation and prospects on electron multiplication and secondary scintillation in liquid xenon and their application to radiation detectors are shortly discussed.
The attenuation length and refractive index of liquid xenon for its intrinsic scintillation light... more The attenuation length and refractive index of liquid xenon for its intrinsic scintillation light ðl ¼ 178 nmÞ have been measured in a single experiment. The value obtained for the attenuation length is 364718 mm: The refractive index is found to be
A large area avalanche photodiode (LAAPD) has been investigated as a possible photosensor for det... more A large area avalanche photodiode (LAAPD) has been investigated as a possible photosensor for detecting liquid xenon scintillation. The LAAPD proved to be operational at the temperature of liquid xenon (down to -105°C) with the dark current significantly reduced with respect to that at room temperature (less than 1 pA versus 30 nA at 20°C). The bias voltage needed to
IEEE International Conference on Dielectric Liquids, 2005. ICDL 2005., 2005
The measurement of the scintillation efficiency for xenon nuclear recoils relative to that for 12... more The measurement of the scintillation efficiency for xenon nuclear recoils relative to that for 122 keV γ-rays is reported in the energy range from 5 keV to 140 keV. Prior to those measurements, the performance of the chamber with respect to the light collection efficiency, energy, time and position resolutions was assessed using 122 keV and 511 keV γ-rays.
IEEE Symposium Conference Record Nuclear Science 2004., 2004
The design and performance of a 1.2 liter liquid xenon chamber equipped with 7 two-inch photomult... more The design and performance of a 1.2 liter liquid xenon chamber equipped with 7 two-inch photomultiplier tubes, with the purpose of studying the scintillation response of xenon to γ-rays and neutrons, is described. Measurements with γ-rays indicate a high VUV light collection efficiency resulting in ~5.5 photoelectrons per 1 keV of deposited energy. The energy resolution (FWHM) is 18% and 22%, for 122 keV and 511 keV γrays, respectively. An algorithm for the reconstruction of the scintillation coordinates in (x,y) plane was developed and tested. The position resolution is estimated to be σ=6.9 mm for 122 keV γ-rays.
Proceedings of 1999 IEEE 13th International Conference on Dielectric Liquids (ICDL'99) (Cat. No.99CH36213), 1999
Here, we report on the present state of development of a liquid xenon detector composed by multiw... more Here, we report on the present state of development of a liquid xenon detector composed by multiwire ionisation chambers, which has been developed for positron emission tomography (PET). It uses the fast scintillation signal for coincidence and subsequent trigger of the event processing. The charge signals induced on the wires are used for position determination along the radial and transaxial
Physical Review D - Particles, Fields, Gravitation and Cosmology, 2009
The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses a 12 kg two-phase x... more The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses a 12 kg two-phase xenon time-projection chamber to search for the weakly interacting massive particles (WIMPs) that may account for the dark matter of our Galaxy. The detector measures both scintillation and ionization produced by radiation interacting in the liquid to differentiate between the nuclear recoils expected from WIMPs and the electron-recoil background signals down to $10 keV nuclear-recoil energy. An analysis of 847 kg Á days of data acquired between February 27, 2008, and May 20, 2008, has excluded a WIMPnucleon elastic scattering spin-independent cross section above 8:1 Â 10 À8 pb at 60 GeVc À2 with a 90% confidence limit. It has also demonstrated that the two-phase xenon technique is capable of better discrimination between electron and nuclear recoils at low-energy than previously achieved by other xenon-based experiments.
Results are presented from the first underground data run of ZEPLIN-II, a 31 kg two phase xenon d... more 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 60 Co 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 keV ee and 20 keV ee , 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.
We present the first measurements of the electroluminescence response to the emission of single e... more We present the first measurements of the electroluminescence response to the emission of single electrons in a two-phase noble gas detector. Single ionization electrons generated in liquid xenon are detected in a thin gas layer during the 31-day background run of the ZEPLIN-II experiment, a two-phase xenon detector for WIMP dark matter searches. Both the pressure dependence and magnitude of the single-electron response are in agreement with previous measurements of electroluminescence yield in xenon. We discuss different photoionization processes as possible cause for the sample of single electrons studied in this work. This observation may have implications for the design and operation of future large-scale two-phase systems.
Gaseous and liquid xenon particle detectors are being used in a number of applications including ... more Gaseous and liquid xenon particle detectors are being used in a number of applications including dark matter search and neutrino-less double beta decay experiments. Polytetrafluoroethylene (PTFE) is often used in these detectors both as electrical insulator and as a light reflector to improve the efficiency of detection of scintillation photons. However, xenon emits in the vacuum ultraviolet wavelength region (λ=175 nm) where the reflecting properties of PTFE are not sufficiently known. In this work we report on measurements of PTFE reflectance, including its angular distribution, for the xenon scintillation light. Various samples of PTFE, manufactured by different processes (extruded, expanded, skived and pressed) have been studied. The data were interpreted with a physical model comprising both specular and diffuse reflections. The reflectance obtained for these samples ranges from about 47% to 66% for VUV light. Fluoropolymers, namely ETFE, FEP and PFA were also measured.
ABSTRACT Computer-generated 3-D models of chromatin structures are presented and applied for DNA ... more ABSTRACT Computer-generated 3-D models of chromatin structures are presented and applied for DNA damage assessment from the indirect action of water radicals formed in a single spur.
Compact gamma cameras with a square-shaped monolithic scintillator crystal and an array of silico... more Compact gamma cameras with a square-shaped monolithic scintillator crystal and an array of silicon photomultipliers (SiPMs) are actively being developed for applications in areas such as small animal imaging, cancer diagnostics and radiotracer guided surgery. Statistical methods of position reconstruction, which are potentially superior to the traditional centroid method, require accurate knowledge of the spatial response of each photomultiplier. Using both Monte Carlo simulations and experimental data obtained with a camera prototype, we show that the spatial response of all photomultipliers (light response functions) can be parameterized with axially symmetric functions obtained iteratively from flood field irradiation data. The study was performed with a camera prototype equipped with a 30 x 30 x 2 mm3 LYSO crystal and an 8 x 8 array of SiPMs for 140 keV gamma rays. The simulations demonstrate that the images, reconstructed with the maximum likelihood method using the response ob...
Bubble formation in liquid xenon underneath a Thick Gaseous Electron Multiplier (THGEM) electrode... more Bubble formation in liquid xenon underneath a Thick Gaseous Electron Multiplier (THGEM) electrode immersed in liquid xenon was observed with a CCD camera. With voltage across the THGEM, the appearance of bubbles was correlated with that of electroluminescence signals induced by ionization electrons from alpha-particle tracks. This confirms recent indirect evidence that the observed photons are due to electroluminescence within a xenon vapor layer trapped under the electrode. The bubbles seem to emerge spontaneously due to heat flow from 300K into the liquid, or in a controlled manner, by locally boiling the liquid with resistive wires. Controlled bubble formation resulted in energy resolution of σ/E7.5% for ~6,000 ionization electrons. The phenomenon could pave ways towards the conception of large-volume 'local dual-phase' noble-liquid TPCs.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1994
The purification system and the method of impurity monitoring developed for a positron emission t... more The purification system and the method of impurity monitoring developed for a positron emission tomography (PET) scanner based on liquid xenon position sensitive detectors are described. The present results show that an attenuation length of the scintillation photons in the liquid of the order of 50 cm can be reached by using only commercial purifiers. Although our preliminary measurements show that a strong electron attachment exists in liquid xenon, the electron attenuation length of 2 cm achieved is sufficient for the PET detectors.
The history, present situation and prospects on electron multiplication and secondary scintillati... more The history, present situation and prospects on electron multiplication and secondary scintillation in liquid xenon and their application to radiation detectors are shortly discussed.
The attenuation length and refractive index of liquid xenon for its intrinsic scintillation light... more The attenuation length and refractive index of liquid xenon for its intrinsic scintillation light ðl ¼ 178 nmÞ have been measured in a single experiment. The value obtained for the attenuation length is 364718 mm: The refractive index is found to be
A large area avalanche photodiode (LAAPD) has been investigated as a possible photosensor for det... more A large area avalanche photodiode (LAAPD) has been investigated as a possible photosensor for detecting liquid xenon scintillation. The LAAPD proved to be operational at the temperature of liquid xenon (down to -105°C) with the dark current significantly reduced with respect to that at room temperature (less than 1 pA versus 30 nA at 20°C). The bias voltage needed to
IEEE International Conference on Dielectric Liquids, 2005. ICDL 2005., 2005
The measurement of the scintillation efficiency for xenon nuclear recoils relative to that for 12... more The measurement of the scintillation efficiency for xenon nuclear recoils relative to that for 122 keV γ-rays is reported in the energy range from 5 keV to 140 keV. Prior to those measurements, the performance of the chamber with respect to the light collection efficiency, energy, time and position resolutions was assessed using 122 keV and 511 keV γ-rays.
IEEE Symposium Conference Record Nuclear Science 2004., 2004
The design and performance of a 1.2 liter liquid xenon chamber equipped with 7 two-inch photomult... more The design and performance of a 1.2 liter liquid xenon chamber equipped with 7 two-inch photomultiplier tubes, with the purpose of studying the scintillation response of xenon to γ-rays and neutrons, is described. Measurements with γ-rays indicate a high VUV light collection efficiency resulting in ~5.5 photoelectrons per 1 keV of deposited energy. The energy resolution (FWHM) is 18% and 22%, for 122 keV and 511 keV γrays, respectively. An algorithm for the reconstruction of the scintillation coordinates in (x,y) plane was developed and tested. The position resolution is estimated to be σ=6.9 mm for 122 keV γ-rays.
Proceedings of 1999 IEEE 13th International Conference on Dielectric Liquids (ICDL'99) (Cat. No.99CH36213), 1999
Here, we report on the present state of development of a liquid xenon detector composed by multiw... more Here, we report on the present state of development of a liquid xenon detector composed by multiwire ionisation chambers, which has been developed for positron emission tomography (PET). It uses the fast scintillation signal for coincidence and subsequent trigger of the event processing. The charge signals induced on the wires are used for position determination along the radial and transaxial
Chapter on Rare Gas Liquid Detectors Detectors from "Electronic Excitations in Liquefied Rare Gas... more Chapter on Rare Gas Liquid Detectors Detectors from "Electronic Excitations in Liquefied Rare Gases", Eds. W.F. Schmidt and E. Illenberger, ASP 2005.
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Papers by Vitaly Chepel