DETECTOR RESPONSE TO LOW ENERGY NUCLEAR RECOILS D Ann Barker and D.-M. Mei University of South Dakota
11/11/11 D. Barker AARM Minneapolis, MN 2 Low Energy WIMP Induced Nuclear Recoils WIMP WIMP Elastic scattering Conservation of Momentum WIMP velocity ~ 240 km/s Nucleus WIMP mass determines recoil energy 100 GeV mass, E r 20 kev 10 GeV mass, E r 2 kev Recoiling Nucleus How do we detect such a small energy?
11/11/11 D. Barker AARM Minneapolis, MN 3 Detection Techniques Solid State Pure ionization: CoGeNT, CDEX Ionization collecting electron-hole pairs No discrimination between e-recoil and n-recoil Ionization and scintillation: DAMA/LIBRA Ionization scintillation light Limited e/n recoil discrimination with pulse shapes Ionization and heat (Bolometer): CDMS, EDELWEISS Ionization collecting electron-hole pairs Temperature changes (heat) measured Excellent e/n recoil discrimination with ionization yield per total energy Ionization/scintillation and heat: CRESST Ionization scintillation light Temperature changes (heat) measured Good e/n recoil discrimination with ionization yield per total energy
11/11/11 D. Barker AARM Minneapolis, MN 4 Detection Techniques Nobel Liquids Ionization + scintillation single phase argon Ionization scintillation light Timing difference between the single-state and triple-state allows an excellent pulse shape discrimination for n-recoil against e-recoil Ionization + scintillation dual phase Xe/Ar Ionization scintillation light (signal 1, S 1 ) Electrons drifted to gas phase (signal 2, S 2 ) Discrimination provided by ratio S 2 /S 1 Key: Ionization
11/11/11 D. Barker AARM Minneapolis, MN 5 Ionization Efficiency Definition: the fraction of energy deposition that ionizes the orbital electrons of the surrounding atoms It is usually calculated using electronic stopping power and nuclear stopping power Ionization efficiency:! η = de $ # & " dx %! ν = de $ # & " dx % elec nucl ε = η η +ν
11/11/11 D. Barker AARM Minneapolis, MN 6 Ionization Efficiency & Visible Energy Pure ionization detectors E r : nuclear recoil energy E v : visible energy Ionization and scintillation detectors ζ: scintillation efficiency E r = E v ε E r = E v ε ζ
11/11/11 D. Barker AARM Minneapolis, MN 7 Nuclear Recoil Energy & WIMPs Nuclear recoil energy must be measured as accurate as possible WIMPs with 100 GeV Ionization efficiency is critical WIMPs with 10 GeV
11/11/11 D. Barker AARM Minneapolis, MN 8 Understanding Physics Processes Behind Ionization Efficiency Outgoing WIMP Recoiling Nucleus Ion WIMP Electronic Stopping Power Nuclear Stopping Power Non-Ionizing Nuclear Electronic
11/11/11 D. Barker AARM Minneapolis, MN 9 New Definition of Ionization Efficiency Correct for nuclear portion contributing to signal ε = η + ( c ν ) η +ν Nuclear Component Larger than traditionally calculated
11/11/11 D. Barker AARM Minneapolis, MN 10 Calculated the Ionization Efficiency for Germanium
11/11/11 D. Barker AARM Minneapolis, MN 11 Comparison Between Models
11/11/11 D. Barker AARM Minneapolis, MN 12 Comparison Between Models
11/11/11 D. Barker AARM Minneapolis, MN 13 Verification of the Model How to accurately measure the low energy nuclear recoils? Thermal neutrons only for energy below 1 kev Elastic scattering with TOF, difficult to set up Inelastic scattering of 72 Ge(n,n,e) E0 Transition Can access low energy Monte Carlo Recoil Energy 692 kev
11/11/11 D. Barker AARM Minneapolis, MN 14 Scintillators are more complicated A combination of ionization efficiency and scintillation efficiency How to decouple the convolution? Without electric field o Calculate ε as accurately as possible o Birk s law ζ = 1 1+ kb de dx kb: Birk s constant E r = E v ε ζ ζ = 1 1+ kb de ρe D dx Ionization must be calculated correctly With electric field kb: Birk s constant, E D : electric field
11/11/11 D. Barker AARM Minneapolis, MN 15 Total Scintillation Efficiency D.-M. Mei et. al. Astroparticle Physics, 30 (2008) 12-17
11/11/11 D. Barker AARM Minneapolis, MN 16 Total Scintillation Efficiency D.-M. Mei et. al. Astroparticle Physics, 30 (2008) 12-17
11/11/11 D. Barker AARM Minneapolis, MN 17 Total Scintillation Efficiency D.-M. Mei et. al. Astroparticle Physics, 30 (2008) 12-17
11/11/11 D. Barker AARM Minneapolis, MN 18 Thank You!
11/11/11 D. Barker AARM Minneapolis, MN 19 Sources D.-M. Mei et. al. Astroparticle Physics, 30 (2008) 12-17 C.E. Aalseth et. al. (CoGeNT), arxiv:1106.0650v2 (2011) Z. Ahmed et. al. (CDMS), arxiv:1011.2482v3 (2011) E. Simon et. al. Nucl. Instrum. Methods Phys. Res. A 507 (2003) 643-656 Y. Messous et. al. Astrophys. 3 (1995) 361-366 C. Chasman et. al. Phys. Rev. Lett., 21 1430 (1968) K.W. Jones et. al. Phys. Rev. C, 4 125 (1971) T. Shutt et. al. Phys. Rev. Lett. 69 3425 (1992)