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Cryogenic Low Energy Astrophysics with Neon
Nuclear Physics B (Proc. Suppl.) 138 (2005) 106–107 www.elsevierphysics.com
Cryogenic Low Energy Astrophysics with Neon A. Hime∗a a
Physics Division, Los Alamos National Laboratory MS H803, P-23, Los Alamos, NM 87545, USA
The unique properties of liquid neon make possible a large fiducial target with ultra-low background and energy threshold for the simultaneous detection of low-energy solar neutrinos and WIMP dark matter.
1. INTRODUCTION With evidence for active solar neutrino flavor transformation[1] and the ultimate resolution of the long standing solar neutrino problem, future generation experiments aim to measure the low-energy solar neutrinos in real time and with high precision. In addition, the possible existence of new, Weakly Interacting Massive Particles (WIMPs) could provide the missing cold dark matter in the Universe. Low-energy solar neutrinos, the dominant pp-flux in particular, could be detected via the recoil electron emitted in elastic scattering interactions. The direct detection of WIMPS relies on a sensitive and low-energy threshold measure of the small recoil energy imparted to the target nucleus[2]. The detection of both low-energy solar neutrinos and WIMP dark matter requires a detector with large target mass, ultra-low radioactivity, and low energy threshold. It would be ideal to realize a technology capable of the simultaneous detection and discrimination of both low-energy neutrinos and dark matter in an environment of negligible radioactivity. Such a possibility might exist based upon a large fiducial mass of liquid neon. 2. PROPERTIES OF LIQUID NEON The original concept[3] of a CLEAN (Cryogenic Low Energy Astrophysics with Neon) has undergone some revision recently[4] as new ideas have emerged. The properties of liquid neon of∗ This
research is supported by Los Alamos National Laboratory-Directed Research and Development funds.
0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.11.026
fer a potentially unique opportunity to realize a truly dual-purpose detector of low-energy neutrinos and WIMP dark matter: 1. Like all of the liquified noble gases, liquid neon scintillates brightly in the extreme ultra-violet (EUV), providing a large signal for ionizing events of about 15000 photons per MeV. 2. Noble liquids do not absorb their own scintillation light. The light results from the decay of excimers and is of lower energy that that required to excite the groundstate atom. Consequently, the scintillation light can be detected from the large fiducial volume required in the detection of rare events. 3. Liquid neon is relatively dense and can thus serve as a self-shielding medium. Together with the long absorption length for the scintillation light, this allows for a detector design utilizing large target mass confined to a fiducial volume of ultra-low radioactivity. 4. Neon contains no long-lived radioactive isotopes and requires depth in excess of 5000 m.w.e. to avoid production of cosmogenic activity. In addition, the 27K boiling point of liquid neon should allow for simple purification from 39 Ar and 85 Kr using conventional cold-traps and getters. 5. Scintillation light is produced with two distinct singlet (about 10 nano-seconds) and
A. Hime / Nuclear Physics B (Proc. Suppl.) 138 (2005) 106–107
triplet (2 to 3 micro-seconds) states. The occupation of the two states is different for electronic (neutrino scattering) and nuclear recoil events, allowing for an effective discrimination between low-energy solar neutrinos and WIMPS. 3. DETECTOR DESIGN AND DEVELOPMENT With these salient features in mind, a detailed Monte Carlo code has been developed to study the design constraints for a CLEAN detector. The code accurately simulates the light production, propagation, and ultimate detection in a spherical array of commercially available photomultiplier (PMT) tubes as a function of the size of the detector. All relevant radioactive contaminants, which we assume to be dominated by the PMT array and support structure, are included in the simulation. The position of events can be reconstructed based upon the relative position and timing of the hit PMTs. By defining a fiducial mass of some 40 tons in a detector radius of about 300 cm we find that the low-energy threshold is dominated by the leakage or miss-reconstruction of low-energy X-rays and gamma-rays original to the PMT glass. Nonetheless, thresholds as low as about 12 keV appear achievable and the basic concept for discriminating between solar neutrinos and WIMP recoils appears in tact. Indeed, a detector can be conceived that would be capable of a 1% measurement of the pp solar neutrino flux that would simultaneously provide a limit on the spin-independent cross-section for 100 GeV WIMP as low as 10−46 cm2 . Detailed design of a CLEAN detector based upon these initial studies relies on an accurate measure of certain liquid neon parameters. In particular, the following research and development is presently deemed necessary to bring a complete design to fruition: 1. While liquid neon is transparent to its own scintillation light care must be taken to remove impurities such as oxygen and nitrogen to ensure an adequate absorption length. A gas purification system is under construction to study the purification
107
capability of neon using conventional charcoal cold traps. The same system will be used to study the purification capability for radioactive contaminants such as 39 Ar and 85 Kr. 2. Critical to the ability to extract low-energy WIMP recoil events from the solar neutrino background is pulse-shape discrimination based on the relative amplitudes of singlet and triplet light components. A 10 to 15 kg liquid neon test-bench is under construction to study the difference in light output for electrons and neutrons and to measure more accurately the salient neon light properties. 3. Since the neon light is produced in the EUV, wavelength shifters must be employed to shift the light into the wavelength regime detectable by conventional PMTs. A teststand to study PMT properties both at room temperature and in liquid neon is presently under construction. 4. CONCLUSIONS We have briefly described the basic concepts and program underway to realize liquid neon as a truly dual-purpose detector of low-energy solar neutrinos and WIMP dark matter. REFERENCES 1. R.G.H. Robertson, in these proceedings. 2. W. Seidel, in these proceedings. 3. D.N. McKinsey and J.M. Doyle, J. low Temp. Phys. 118 (2000) 153. 4. D.N. McKinsey and K.J. Coakley, astroph/0402007.
Cryogenic Low Energy Astrophysics with Neon A. Hime∗a a
Physics Division, Los Alamos National Laboratory MS H803, P-23, Los Alamos, NM 87545, USA
The unique properties of liquid neon make possible a large fiducial target with ultra-low background and energy threshold for the simultaneous detection of low-energy solar neutrinos and WIMP dark matter.
1. INTRODUCTION With evidence for active solar neutrino flavor transformation[1] and the ultimate resolution of the long standing solar neutrino problem, future generation experiments aim to measure the low-energy solar neutrinos in real time and with high precision. In addition, the possible existence of new, Weakly Interacting Massive Particles (WIMPs) could provide the missing cold dark matter in the Universe. Low-energy solar neutrinos, the dominant pp-flux in particular, could be detected via the recoil electron emitted in elastic scattering interactions. The direct detection of WIMPS relies on a sensitive and low-energy threshold measure of the small recoil energy imparted to the target nucleus[2]. The detection of both low-energy solar neutrinos and WIMP dark matter requires a detector with large target mass, ultra-low radioactivity, and low energy threshold. It would be ideal to realize a technology capable of the simultaneous detection and discrimination of both low-energy neutrinos and dark matter in an environment of negligible radioactivity. Such a possibility might exist based upon a large fiducial mass of liquid neon. 2. PROPERTIES OF LIQUID NEON The original concept[3] of a CLEAN (Cryogenic Low Energy Astrophysics with Neon) has undergone some revision recently[4] as new ideas have emerged. The properties of liquid neon of∗ This
research is supported by Los Alamos National Laboratory-Directed Research and Development funds.
0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.11.026
fer a potentially unique opportunity to realize a truly dual-purpose detector of low-energy neutrinos and WIMP dark matter: 1. Like all of the liquified noble gases, liquid neon scintillates brightly in the extreme ultra-violet (EUV), providing a large signal for ionizing events of about 15000 photons per MeV. 2. Noble liquids do not absorb their own scintillation light. The light results from the decay of excimers and is of lower energy that that required to excite the groundstate atom. Consequently, the scintillation light can be detected from the large fiducial volume required in the detection of rare events. 3. Liquid neon is relatively dense and can thus serve as a self-shielding medium. Together with the long absorption length for the scintillation light, this allows for a detector design utilizing large target mass confined to a fiducial volume of ultra-low radioactivity. 4. Neon contains no long-lived radioactive isotopes and requires depth in excess of 5000 m.w.e. to avoid production of cosmogenic activity. In addition, the 27K boiling point of liquid neon should allow for simple purification from 39 Ar and 85 Kr using conventional cold-traps and getters. 5. Scintillation light is produced with two distinct singlet (about 10 nano-seconds) and
A. Hime / Nuclear Physics B (Proc. Suppl.) 138 (2005) 106–107
triplet (2 to 3 micro-seconds) states. The occupation of the two states is different for electronic (neutrino scattering) and nuclear recoil events, allowing for an effective discrimination between low-energy solar neutrinos and WIMPS. 3. DETECTOR DESIGN AND DEVELOPMENT With these salient features in mind, a detailed Monte Carlo code has been developed to study the design constraints for a CLEAN detector. The code accurately simulates the light production, propagation, and ultimate detection in a spherical array of commercially available photomultiplier (PMT) tubes as a function of the size of the detector. All relevant radioactive contaminants, which we assume to be dominated by the PMT array and support structure, are included in the simulation. The position of events can be reconstructed based upon the relative position and timing of the hit PMTs. By defining a fiducial mass of some 40 tons in a detector radius of about 300 cm we find that the low-energy threshold is dominated by the leakage or miss-reconstruction of low-energy X-rays and gamma-rays original to the PMT glass. Nonetheless, thresholds as low as about 12 keV appear achievable and the basic concept for discriminating between solar neutrinos and WIMP recoils appears in tact. Indeed, a detector can be conceived that would be capable of a 1% measurement of the pp solar neutrino flux that would simultaneously provide a limit on the spin-independent cross-section for 100 GeV WIMP as low as 10−46 cm2 . Detailed design of a CLEAN detector based upon these initial studies relies on an accurate measure of certain liquid neon parameters. In particular, the following research and development is presently deemed necessary to bring a complete design to fruition: 1. While liquid neon is transparent to its own scintillation light care must be taken to remove impurities such as oxygen and nitrogen to ensure an adequate absorption length. A gas purification system is under construction to study the purification
107
capability of neon using conventional charcoal cold traps. The same system will be used to study the purification capability for radioactive contaminants such as 39 Ar and 85 Kr. 2. Critical to the ability to extract low-energy WIMP recoil events from the solar neutrino background is pulse-shape discrimination based on the relative amplitudes of singlet and triplet light components. A 10 to 15 kg liquid neon test-bench is under construction to study the difference in light output for electrons and neutrons and to measure more accurately the salient neon light properties. 3. Since the neon light is produced in the EUV, wavelength shifters must be employed to shift the light into the wavelength regime detectable by conventional PMTs. A teststand to study PMT properties both at room temperature and in liquid neon is presently under construction. 4. CONCLUSIONS We have briefly described the basic concepts and program underway to realize liquid neon as a truly dual-purpose detector of low-energy solar neutrinos and WIMP dark matter. REFERENCES 1. R.G.H. Robertson, in these proceedings. 2. W. Seidel, in these proceedings. 3. D.N. McKinsey and J.M. Doyle, J. low Temp. Phys. 118 (2000) 153. 4. D.N. McKinsey and K.J. Coakley, astroph/0402007.