- Email: [email protected]
The ZEPLIN III dark matter project
New Astronomy Reviews 49 (2005) 277–281 www.elsevier.com/locate/newastrev
The ZEPLIN III dark matter project T.J. Sumner Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BZ, UK on behalf of the ZEPLIN Collaboration Available online 14 March 2005
Abstract ZEPLIN III is an advanced two-phase xenon detector designed to search for weakly interacting massive particles (WIMPs), such as the neutralino, which make up the dark matter halo of the Galaxy. It has an active target mass of 6 kg and uses two signal channels to discriminate between different particle species interacting in the detector. This allows nuclear recoil signatures of WIMPs to be separated from the more numerous background of events generated by photons. By pushing the performance of this type of detector to its limits and incorporating a neutron veto, it will be shown that ZEPLIN III should reach a limiting sensitivity to a coherent elastic WIMP-nucleon scattering cross-section of 10 8 pb. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Dark matter: Galactic; Wimps; Detectors: Xenon
1. Introduction The ZEPLIN Collaboration1 is developing a series of detectors exploring scalable xenon technology with a view to reaching tonne-scale instruments. Each is designed to have a competitive sensitivity to WIMP-nucleon elastic scattering on the world scene at the time of its deployment. E-mail address: [email protected]. URL: http://astro.imperial.ac.uk. 1 Edinburgh University, Imperial College London, ITEP, LIP Coimbra, Rochester University, Rutherford Appleton Laboratory, Sheffield University, Texas A&M, UCLA.
There are currently three progenitors envisaged in this sequence before embarking upon the final stage leading to a tonne of active xenon. ZEPLIN I has already been deployed and operated in the underground laboratory at Boulby in the UK, and is producing competitive upper limits (Smith et al., 2004; Alner et al., 2004). It uses a single liquid phase scintillation technique with pulse shape discrimination to look for events with shorter timeconstants than expected from electron recoils, which constitute the dominant background. ZEPLIN II is a two-phase detector using two signal channels, both using photo-detectors to provide a higher level of discrimination. The initial
1387-6473/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2005.01.018
278
T.J. Sumner / New Astronomy Reviews 49 (2005) 277–281
interaction occurs in the liquid phase and an immediate flash of light is seen, whose amplitude depends on the amount of excitation produced plus a contribution from fast recombination of some fraction of the simultaneous ionisation produced. Any ionisation (electrons) which survives recombination is then drifted in an applied electric field to the surface of the liquid where a sufficiently high field is applied to extract it into the gas phase. Once in the gas phase the electrons experience another electric field which is sufficiently high to induce electroluminescence from exciting the gas itself. This is seen as a second flash of light by the same photo-detectors. The ratio of the secondary to primary light flashes is a measure of the relative amounts of escaping ionisation and excitation/recombination. This ratio depends on the nature of the interacting species and allows for a high level of discrimination between nuclear and electron recoil (Benetti et al., 1993). In ZEPLIN II, the electric fields to be used are such that electron recoils should produce both primaries and secondaries, whereas nuclear recoils are only likely to produce primaries. The photo-detectors are placed in the gas phase. ZEPLIN III on the other hand is designed to operate in a much higher field regime and the intention is to have both primary and secondary signals for electron and nuclear recoils. In addition, the photo-detectors are placed in the liquid phase to maximise the sensitivity to the primary scintillation, which ultimately sets the detector energy threshold, even though the trigger will be derived from the secondary signal. This is
because the secondary signal is always larger than the primary. Hence, the ZEPLIN programme has three progressively more demanding (and more sensitive) detectors whose collective experience feeds into a future project using a 1 tonne target.
2. ZEPLIN III Fig. 1 shows, on the left, a cross-sectional view of the target central volume of the ZEPLIN III detector, and on the right a cut-away representation of the whole target chamber. An array of 31 closepacked PMTs immersed in the liquid xenon is used to view the scintillation light from the liquid and the subsequent electroluminescent emission from the gas phase. Placing the PMTs in the liquid removes two interfaces between media with large mismatches in their refractive index. Using a thin slab geometry improves the zero and single reflection (total internal reflection from the liquid surface) light collection for primary scintillation. These two features result in a very low energy threshold for ZEPLIN III. The electric fields (drift field, extraction field and electroluminescence field) are produced by just one wire grid plane and the top electrode surface. This keeps the fiducial volume and its immediate surroundings as free from solid surfaces as possible, to avoid spurious feedback from surface effects, such as photoelectric emission in the presence of intense UV photon fluxes. The liquid surface is 3.5 cm above the grid plane and the gas gap is 0.5 cm. The fiducial volume is defined by using 3-d event recon-
Fig. 1. A cross-section cut through the target volume and a cut-away representation of the target chamber of the ZEPLIN III detector.
T.J. Sumner / New Astronomy Reviews 49 (2005) 277–281
struction from the primary to secondary timing to give depth and from the secondary hit pattern in the PMT array to give the radial position. The size of the fiducial volume depends on the quality of the position reconstruction (Davidge, 2004), and simulations indicate a 6 kg fiducial mass. Half a centimetre below the grid plane is another wire plane with a potential matched to that on the PMT photocathodes. This serves two purposes. Firstly, it isolates the electric field at the PMT photocathodes from the other applied fields. Secondly, it allows us to generate a reverse field region between this lower grid and the main grid from where no events can generate a secondary pulse. Most of the events occurring in this region will be due to X-rays from the PMTs, and being able to discount them immediately from the data avoids their contributing significantly to the dead-time. The central technology concepts and requirements for ZEPLIN III have already been proven using high-field two-phase xenon prototypes in the laboratory (Akimov, 2003; Howard et al., 2001). These tests have included operation of PMTs in liquid xenon at high pressure, high-field two phase operation, in which single electron extraction from the liquid can be seen, and discrimination between a-particles and c-rays. Fig. 2 shows
Fig. 2. A typical signal from our two-phase prototype from an a-particle interaction.
279
an example of a two-phase signal from an alpha particle interaction. Alphas are inherently hard to detect through their ionisation signal, and this plot illustrates how easy it becomes in a high-field twophase xenon system. It is also worth noting that a c-ray interaction with the same visible primary energy shows a secondary which is even larger by a factor of 50!
3. Performance simulations Extensive Monte Carlo simulations have been carried out to asses the performance of ZEPLIN III (Davidge, 2004; Dawson et al., 2001; Araujo, 2003). These simulations have addressed light collection from the liquid and gas phases, position reconstruction, and internally generated c-ray and neutron backgrounds. A simulated data set has been produced, which has then been analysed in a similar way to that proposed for real data. A fully implemented GEANT4 toolkit has been assembled. Fig. 3 shows the latest results from the light collection simulations. As seen in lefthand panel the average light collection efficiency from the fiducial region (5 < Z < 35 mm) is 3.5 phe/keV, and is almost independent of depth. The PMTs being used here have quite high measured quantum efficiencies of 30% Araujo et al., 2004. The predicted mean sensitivity to ionisation electrons crossing the gas phase, with an applied potential difference of 40 kV from the main grid to the top electrode, is 28 phe/electron. The position sensitivity obtained by using the gas luminescence signal has been studied by using a Monte Carlo simulation on a fine positional grid to derive templates of very good statistical quality for the hit patterns expected in each photomultiplier. A simulated data set was then produced for the expected hit patterns from single electrons escaping from the liquid surface at random positions. This data set was then analysed to reconstruct the positions using a minimum v2 technique matching to the template distributions. Fig. 4 shows the result for single electrons with a cut applied at 10 detected photoelectrons. The reconstruction is good, even for single electrons, over the central fiducial region.
280
T.J. Sumner / New Astronomy Reviews 49 (2005) 277–281
Fig. 3. Results of the light collection simulation.
Fig. 4. Results of the position reconstruction simulation on the left. The reconstructed position is on the vertical axis and the actual position on the horizontal axis. Results of the discrimination simulation on the right.
4. Background simulations Simulations of the photon background expected from the measured U and Th in the PMTs have been done by initiating photons from their locations and tracking interactions in the xenon. The re-
sult is shown in Fig. 5. At low energies the rate is dominated by Compton scattering being flat at 10 counts/keV/kg. A veto system included in the simulation lowers the rate by a factor 3. The efficiency of rejection of the background depends on the ratio of secondary to primary signals and their
T.J. Sumner / New Astronomy Reviews 49 (2005) 277–281
281
Fig. 5. The simulated c-ray background (including a veto system). On the right is the simulated neutron background with the total event rate as a function of energy.
spread, both of which depend on the particle species. Fig. 4 shows, on the right-hand side, a scatter plot of primary and secondary values for c-rays and nuclear recoils (assumed to be a-like with a quenching factor of 0.2). The distributions are well separated and applying a non-linear cut between them yields c-ray rejection factors of 104–105. The horizontal line segments follow the 105 boundary. In the lowest energy primary bins this also significant rejection of nuclear recoil signal. A factor of 104 allows a limiting 1-year experiment sensitivity of 10 8 pb. However, reaching that limiting sensitivity requires suppression of the (a-n) neutron background caused by U and Th in the PMTs and shown in Fig. 5. The single-site elastic scattering component limits the sensitivity of ZEPLIN III to 10 7 pb. A n-veto will remove these events. References Akimov, D.Yu., et al., 2003. Liquid xenon for WIMP searches: measurement with a two-phase prototype.
In: Spooner, N.J.C., Kudryavtsev, V. (Eds.), The Identification of Dark Matter IV. World Scientific, pp. 371–376. Alner, G.J., et al., 2004. First limits on nuclear recoil events from the ZEPLIN I Galactic Dark Matter Detector, (submitted). Araujo, H., 2003. ZepIII: A (GEANT4) simulation tool for ZEPLIN 3. Internal Report, Imperial College, London, p. 215. Araujo, H.M., et al., 2004. Low-temperature study of 35 photomultiplier tubes for the ZEPLIN III experiment. Nucl. Instrum. Methods A 521, 407–415. Benetti, P., et al., 1993. Detection of energy deposition down to the keV region using liquid xenon scintillation. Nucl. Instrum. Methods A 327, 203. Davidge, D.C.R., 2004. Development of a two phase liquid xenon dark matter detector. PhD Thesis, Imperial College London, University of London. Dawson, J., et al., 2001. Monte Carlo studies of ZEPLIN III. Nucl. Phys. B 110, 109–111. Howard, A.S., et al., 2001. Measurements with a twophase xenon prototype dark matter detector. In: Spooner, N.J.C., Kudryavtsev, V. (Eds.), The Identification of Dark Matter III. World Scientific, pp. 457– 462. Smith, N.J.T., et al., 2004. This Conference.
The ZEPLIN III dark matter project T.J. Sumner Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BZ, UK on behalf of the ZEPLIN Collaboration Available online 14 March 2005
Abstract ZEPLIN III is an advanced two-phase xenon detector designed to search for weakly interacting massive particles (WIMPs), such as the neutralino, which make up the dark matter halo of the Galaxy. It has an active target mass of 6 kg and uses two signal channels to discriminate between different particle species interacting in the detector. This allows nuclear recoil signatures of WIMPs to be separated from the more numerous background of events generated by photons. By pushing the performance of this type of detector to its limits and incorporating a neutron veto, it will be shown that ZEPLIN III should reach a limiting sensitivity to a coherent elastic WIMP-nucleon scattering cross-section of 10 8 pb. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Dark matter: Galactic; Wimps; Detectors: Xenon
1. Introduction The ZEPLIN Collaboration1 is developing a series of detectors exploring scalable xenon technology with a view to reaching tonne-scale instruments. Each is designed to have a competitive sensitivity to WIMP-nucleon elastic scattering on the world scene at the time of its deployment. E-mail address: [email protected]. URL: http://astro.imperial.ac.uk. 1 Edinburgh University, Imperial College London, ITEP, LIP Coimbra, Rochester University, Rutherford Appleton Laboratory, Sheffield University, Texas A&M, UCLA.
There are currently three progenitors envisaged in this sequence before embarking upon the final stage leading to a tonne of active xenon. ZEPLIN I has already been deployed and operated in the underground laboratory at Boulby in the UK, and is producing competitive upper limits (Smith et al., 2004; Alner et al., 2004). It uses a single liquid phase scintillation technique with pulse shape discrimination to look for events with shorter timeconstants than expected from electron recoils, which constitute the dominant background. ZEPLIN II is a two-phase detector using two signal channels, both using photo-detectors to provide a higher level of discrimination. The initial
1387-6473/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2005.01.018
278
T.J. Sumner / New Astronomy Reviews 49 (2005) 277–281
interaction occurs in the liquid phase and an immediate flash of light is seen, whose amplitude depends on the amount of excitation produced plus a contribution from fast recombination of some fraction of the simultaneous ionisation produced. Any ionisation (electrons) which survives recombination is then drifted in an applied electric field to the surface of the liquid where a sufficiently high field is applied to extract it into the gas phase. Once in the gas phase the electrons experience another electric field which is sufficiently high to induce electroluminescence from exciting the gas itself. This is seen as a second flash of light by the same photo-detectors. The ratio of the secondary to primary light flashes is a measure of the relative amounts of escaping ionisation and excitation/recombination. This ratio depends on the nature of the interacting species and allows for a high level of discrimination between nuclear and electron recoil (Benetti et al., 1993). In ZEPLIN II, the electric fields to be used are such that electron recoils should produce both primaries and secondaries, whereas nuclear recoils are only likely to produce primaries. The photo-detectors are placed in the gas phase. ZEPLIN III on the other hand is designed to operate in a much higher field regime and the intention is to have both primary and secondary signals for electron and nuclear recoils. In addition, the photo-detectors are placed in the liquid phase to maximise the sensitivity to the primary scintillation, which ultimately sets the detector energy threshold, even though the trigger will be derived from the secondary signal. This is
because the secondary signal is always larger than the primary. Hence, the ZEPLIN programme has three progressively more demanding (and more sensitive) detectors whose collective experience feeds into a future project using a 1 tonne target.
2. ZEPLIN III Fig. 1 shows, on the left, a cross-sectional view of the target central volume of the ZEPLIN III detector, and on the right a cut-away representation of the whole target chamber. An array of 31 closepacked PMTs immersed in the liquid xenon is used to view the scintillation light from the liquid and the subsequent electroluminescent emission from the gas phase. Placing the PMTs in the liquid removes two interfaces between media with large mismatches in their refractive index. Using a thin slab geometry improves the zero and single reflection (total internal reflection from the liquid surface) light collection for primary scintillation. These two features result in a very low energy threshold for ZEPLIN III. The electric fields (drift field, extraction field and electroluminescence field) are produced by just one wire grid plane and the top electrode surface. This keeps the fiducial volume and its immediate surroundings as free from solid surfaces as possible, to avoid spurious feedback from surface effects, such as photoelectric emission in the presence of intense UV photon fluxes. The liquid surface is 3.5 cm above the grid plane and the gas gap is 0.5 cm. The fiducial volume is defined by using 3-d event recon-
Fig. 1. A cross-section cut through the target volume and a cut-away representation of the target chamber of the ZEPLIN III detector.
T.J. Sumner / New Astronomy Reviews 49 (2005) 277–281
struction from the primary to secondary timing to give depth and from the secondary hit pattern in the PMT array to give the radial position. The size of the fiducial volume depends on the quality of the position reconstruction (Davidge, 2004), and simulations indicate a 6 kg fiducial mass. Half a centimetre below the grid plane is another wire plane with a potential matched to that on the PMT photocathodes. This serves two purposes. Firstly, it isolates the electric field at the PMT photocathodes from the other applied fields. Secondly, it allows us to generate a reverse field region between this lower grid and the main grid from where no events can generate a secondary pulse. Most of the events occurring in this region will be due to X-rays from the PMTs, and being able to discount them immediately from the data avoids their contributing significantly to the dead-time. The central technology concepts and requirements for ZEPLIN III have already been proven using high-field two-phase xenon prototypes in the laboratory (Akimov, 2003; Howard et al., 2001). These tests have included operation of PMTs in liquid xenon at high pressure, high-field two phase operation, in which single electron extraction from the liquid can be seen, and discrimination between a-particles and c-rays. Fig. 2 shows
Fig. 2. A typical signal from our two-phase prototype from an a-particle interaction.
279
an example of a two-phase signal from an alpha particle interaction. Alphas are inherently hard to detect through their ionisation signal, and this plot illustrates how easy it becomes in a high-field twophase xenon system. It is also worth noting that a c-ray interaction with the same visible primary energy shows a secondary which is even larger by a factor of 50!
3. Performance simulations Extensive Monte Carlo simulations have been carried out to asses the performance of ZEPLIN III (Davidge, 2004; Dawson et al., 2001; Araujo, 2003). These simulations have addressed light collection from the liquid and gas phases, position reconstruction, and internally generated c-ray and neutron backgrounds. A simulated data set has been produced, which has then been analysed in a similar way to that proposed for real data. A fully implemented GEANT4 toolkit has been assembled. Fig. 3 shows the latest results from the light collection simulations. As seen in lefthand panel the average light collection efficiency from the fiducial region (5 < Z < 35 mm) is 3.5 phe/keV, and is almost independent of depth. The PMTs being used here have quite high measured quantum efficiencies of 30% Araujo et al., 2004. The predicted mean sensitivity to ionisation electrons crossing the gas phase, with an applied potential difference of 40 kV from the main grid to the top electrode, is 28 phe/electron. The position sensitivity obtained by using the gas luminescence signal has been studied by using a Monte Carlo simulation on a fine positional grid to derive templates of very good statistical quality for the hit patterns expected in each photomultiplier. A simulated data set was then produced for the expected hit patterns from single electrons escaping from the liquid surface at random positions. This data set was then analysed to reconstruct the positions using a minimum v2 technique matching to the template distributions. Fig. 4 shows the result for single electrons with a cut applied at 10 detected photoelectrons. The reconstruction is good, even for single electrons, over the central fiducial region.
280
T.J. Sumner / New Astronomy Reviews 49 (2005) 277–281
Fig. 3. Results of the light collection simulation.
Fig. 4. Results of the position reconstruction simulation on the left. The reconstructed position is on the vertical axis and the actual position on the horizontal axis. Results of the discrimination simulation on the right.
4. Background simulations Simulations of the photon background expected from the measured U and Th in the PMTs have been done by initiating photons from their locations and tracking interactions in the xenon. The re-
sult is shown in Fig. 5. At low energies the rate is dominated by Compton scattering being flat at 10 counts/keV/kg. A veto system included in the simulation lowers the rate by a factor 3. The efficiency of rejection of the background depends on the ratio of secondary to primary signals and their
T.J. Sumner / New Astronomy Reviews 49 (2005) 277–281
281
Fig. 5. The simulated c-ray background (including a veto system). On the right is the simulated neutron background with the total event rate as a function of energy.
spread, both of which depend on the particle species. Fig. 4 shows, on the right-hand side, a scatter plot of primary and secondary values for c-rays and nuclear recoils (assumed to be a-like with a quenching factor of 0.2). The distributions are well separated and applying a non-linear cut between them yields c-ray rejection factors of 104–105. The horizontal line segments follow the 105 boundary. In the lowest energy primary bins this also significant rejection of nuclear recoil signal. A factor of 104 allows a limiting 1-year experiment sensitivity of 10 8 pb. However, reaching that limiting sensitivity requires suppression of the (a-n) neutron background caused by U and Th in the PMTs and shown in Fig. 5. The single-site elastic scattering component limits the sensitivity of ZEPLIN III to 10 7 pb. A n-veto will remove these events. References Akimov, D.Yu., et al., 2003. Liquid xenon for WIMP searches: measurement with a two-phase prototype.
In: Spooner, N.J.C., Kudryavtsev, V. (Eds.), The Identification of Dark Matter IV. World Scientific, pp. 371–376. Alner, G.J., et al., 2004. First limits on nuclear recoil events from the ZEPLIN I Galactic Dark Matter Detector, (submitted). Araujo, H., 2003. ZepIII: A (GEANT4) simulation tool for ZEPLIN 3. Internal Report, Imperial College, London, p. 215. Araujo, H.M., et al., 2004. Low-temperature study of 35 photomultiplier tubes for the ZEPLIN III experiment. Nucl. Instrum. Methods A 521, 407–415. Benetti, P., et al., 1993. Detection of energy deposition down to the keV region using liquid xenon scintillation. Nucl. Instrum. Methods A 327, 203. Davidge, D.C.R., 2004. Development of a two phase liquid xenon dark matter detector. PhD Thesis, Imperial College London, University of London. Dawson, J., et al., 2001. Monte Carlo studies of ZEPLIN III. Nucl. Phys. B 110, 109–111. Howard, A.S., et al., 2001. Measurements with a twophase xenon prototype dark matter detector. In: Spooner, N.J.C., Kudryavtsev, V. (Eds.), The Identification of Dark Matter III. World Scientific, pp. 457– 462. Smith, N.J.T., et al., 2004. This Conference.