Xe-based detectors: recent work at Coimbra C.A.N.Conde, A.D. Stauffer, T.H.V.T.Dias, F.P.Santos, F.I.G.M.Borges, L.M.N.Távora, R.M.C. da Silva, J.Barata, P.N.B.Neves, J.M.Escada, L.P.M.M.Carita, S.do Carmo, A.Trindade, J.Mariquito, P.J.B.M.Rachinhas Workshop on Xenon-Based Detectors Nov 2009, Berkeley

Energy resolution degradation: drift electric-field effects Tertiary-Scintillation Gas Proportional Scintillation Counter New detectors developed at Coimbra: Electron diffusion in Xe, vs Xe-CH 4 & Xe-CF 4 Multi-Grid HP Gas Proportional Scintillation Counter Electroluminescence fluctuations in Xe, vs Xe-CH 4 & Xe-CF 4 Summary The Gridded Gas Proportional Ionization Counter Discontinuities in energy resolution & linearity of Xe detectors 2/21 Detector gas filling: Xe vs Xe-CH 4 & Xe-CF 4

Discontinuities in energy resolution Xenon filled detectors exhibit sudden increases in energy resolution whenever a new Xe atomic-shell becomes available for photoionization Monte Carlo 3/21 M L F and w-value are discontinuous Xe MC simulation experimental

Discontinuities in Fano factor and w-value Monte Carlo w  ph w-value and Fano factor F are E xr dependent Reflect photoionization XS  ph F  ph 4/21 ■ MC □ experimental

Discontinuities in energy linearity & w =E xr /n Mean number n of primary (sub-ionization) electrons produced in Xe as a function of absorbed x-ray energy E xr Monte Carlo 5/21 Is n proportional to E xr ?

Energy resolution and Energy linearity E xr > 4782 eV distributions shift to lower n: discontinuity in w & linearity. distributions broaden: discontinuity in F & R int E xr < 4782 eV (L 3 ): M shell vacancy; M-photoelectron (~3500 eV) dominates; E xr > 4782 eV (L 3 ): inner vacancy (L); photoelectron (few eV); various Auger electrons (30eV to ~4000eV). Xe L 3 binding energy = 4782 eV Monte Carlo 6/21 n

Energy resolution degradation: drift electric-field effects Tertiary-Scintillation Gas Proportional Scintillation Counter New detectors developed at Coimbra: Electron diffusion in Xe, vs Xe-CH4 & Xe-CF4 Multi-Grid HP Gas Proportional Scintillation Counter Electroluminescence fluctuations in Xe, vs Xe-CH 4 & Xe-CF 4 The Gridded Gas Proportional Ionization Counter Discontinuities in energy resolution & linearity of Xe detectors Research topics Detector gas filling: Xe vs Xe-CH 4 & Xe-CF 4

High E 0 : Photoelectrons carry most of the photon energy E 0 are scattered mostly forward. have long trajectories in the gas Long trajectories in the gas: energy gain/loss from the drift field is not negligible. Deposited energy is higher (or lower) than E 0. Energy resolution degradation: drift electric-field effects 7/21

60 keV x rays 200 keV x rays Intrinsic curve : accounts for fluctuations in # of primary (sub-ionization) electrons (F Xe =0.17; w Xe =E 0 /n=21.5 eV). Distributions (PENELOPE): for E/p=0.1 to 0.8 Vcm -1 Torr -1 : Spreads Г vary with drift field (  drift field photon energy Energy resolution degradation: drift electric-field effects Drift field effects: Fluctuations increase with 8/21

Energy resolution degradation: drift electric-field effects Tertiary-Scintillation Gas Proportional Scintillation Counter New detectors developed at Coimbra: Electron diffusion in Xe, vs Xe-CH4 & Xe-CF4 Multi-Grid HP Gas Proportional Scintillation Counter Electroluminescence fluctuations in Xe, vs Xe-CH 4 & Xe-CF 4 The Gridded Gas Proportional Ionization Counter Discontinuities in energy resolution & linearity of Xe detectors Research topics Detector gas filling: Xe vs Xe-CH 4 & Xe-CF 4

 ion  exc Electron scattering cross sections in Xe and CH 4 9/21

10/21 Electron scattering cross sections in Xe and CF 4

Electron drift velocities in Xe, Xe-CH 4 and Xe-CF 4 Addition of CH 4 or CF 4 to Xe increases drift velocity Monte Carlo 11/21

where Addition of CH 4 or CF 4 to Xe increases drift velocity decreases longitudinal and transverse electron diffusion 12/21 Monte Carlo Electron diffusion in Xe, Xe-CH 4 and Xe-CF 4

Monte Carlo Addition of CH 4 or CF 4 to Xe increases drift velocity decreases longitudinal and transverse electron diffusion where 13/21 Electron diffusion in Xe, Xe-CH 4 and Xe-CF 4

Energy resolution degradation: drift electric-field effects Tertiary-Scintillation Gas Proportional Scintillation Counter New detectors developed at Coimbra: Electron diffusion in Xe, vs Xe-CH4 & Xe-CF4 Multi-Grid HP Gas Proportional Scintillation Counter Electroluminescence fluctuations in Xe, vs Xe-CH 4 & Xe-CF 4 The Gridded Gas Proportional Ionization Counter Discontinuities in energy resolution & linearity of Xe detectors Research topics Detector gas filling: Xe vs Xe-CH 4 & Xe-CF 4

Electroluminescence fluctuations in Xe vs Xe-CH 4, Xe-CF 4 decreases EL (n. of excitations, i.e. sc.photons, produced per electron in sc. gap) increases EL fluctuations (CF 4 has catastrophic effect …) The addition of CH 4 or CF 4 to Xe Monte Carlo 14/21 5 cm drift, 1 atm ↔ 5 mm, 10 atm

Monte Carlo 15/21 Electroluminescence fluctuations in Xe vs Xe-CH 4, Xe-CF 4

Energy resolution degradation: drift electric-field effects Tertiary-Scintillation Gas Proportional Scintillation Counter New detectors developed at Coimbra: Electron diffusion in Xe, vs Xe-CH4 & Xe-CF4 Multi-Grid HP Gas Proportional Scintillation Counter Electroluminescence fluctuations in Xe, vs Xe-CH 4 & Xe-CF 4 The Gridded Gas Proportional Ionization Counter Discontinuities in energy resolution & linearity of Xe detectors Research topics Detector gas filling: Xe vs Xe-CH 4 & Xe-CF 4

Tertiary-Scintillation Gas Proportional Scintillation Counter TS-GPSC prototype 16/21

TS-GPSC Results Best results obtained for scintillation electric fields just above Xe ionization threshold voltage across GEM-structure below charge multiplication. Typical spectrum 109 Cd source 17/21 R G FWHM 8.2%

Energy resolution degradation: drift electric-field effects Tertiary-Scintillation Gas Proportional Scintillation Counter New detectors developed at Coimbra: Electron diffusion in Xe, vs Xe-CH4 & Xe-CF4 Multi-Grid HP Gas Proportional Scintillation Counter Electroluminescence fluctuations in Xe, vs Xe-CH 4 & Xe-CF 4 The Gridded Gas Proportional Ionization Counter Discontinuities in energy resolution & linearity of Xe detectors Research topics Detector gas filling: Xe vs Xe-CH 4 & Xe-CF 4

Multigrid High Pressure Xe GPSC (Indicated voltages are ideal values) 18/21 primary electrons are produced in the absorption/drift region primary electrons produce secondary scintillation VUV photons along gap between G 1 and G 2 VUV photons release electrons from CsI photocathode at backplane of detector electrons are collected at G 4 giving the detector signal

Multigrid High Pressure Xe GPSC – Experimental results Pulse amplitude vs G 3 -G 4 potential barrier (  V 34 ) Charge gain vs E/p in scintillation gap 19/21 V3<V4V3>V4 5.4 bar 5 bar 3 bar 1.5 bar E/p (Vcm -1 Torr -1 ) Gain

Energy resolution degradation: drift electric-field effects Tertiary-Scintillation Gas Proportional Scintillation Counter New detectors developed at Coimbra: Electron diffusion in Xe, vs Xe-CH4 & Xe-CF4 Multi-Grid HP Gas Proportional Scintillation Counter Electroluminescence fluctuations in Xe, vs Xe-CH 4 & Xe-CF 4 The Gridded Gas Proportional Ionization Counter Discontinuities in energy resolution & linearity of Xe detectors Research topics Detector gas filling: Xe vs Xe-CH 4 & Xe-CF 4

The gridded GPIC: definition of multiplication volume Grid around the anode: ideal to define multiplication volume However, grid diameter too small, unfeasible at 1 atm. Solution: planar microstructure where PIC conventional anode is hemmed in by a close second anode. 20/21

The gridded GPIC - Experimental keV 21/21 R M M R

Experimental results with gridded GPIC

At the atomic absorption edges, an electric-field triggered discontinuity may become noticeable as the ejected photoelectron tends to have much lower energy after a new atomic shell becomes photoionizable than before. However this non-linearity is only about 4% of the intrinsic non-linearity. Energy resolution degradation: drift electric-field discontinuities at atomic edges Intrinsic discontinuity

Drift velocities for electrons in Xe and Xe-CH 4 Monte Carlo Addition of CH 4 or CF 4 to Xe increases drift velocity