Development of wavelength shifters Development of wavelength - PowerPoint PPT Presentation

Development of wavelength shifters Development of wavelength shifters for the ArDM argon dark matter for the ArDM argon dark matter detector detector detector detector Konstantinos Mavrokoridis Konstantinos Mavrokoridis Konstantinos Mavrokoridis Konstantinos Mavrokoridis k.mavrokoridis@shef.ac.uk IOP HEPP conference, Lancaster, 02.04.08

The ArDM collaboration The ArDM collaboration A B d A. Badertscher, A. Baeztner, R. Chandrasekharan, L. Kaufmann, L. Knecht, M. Laffranchi, t h A B t R Ch d kh L K f L K ht M L ff hi A. Marchionni, G. Natterer, P. Otiougova, A. Rubbia (contact person), J. Ulbricht ETH Zurich, Switzerland C. Amsler, V. Boccone, S. Horikawa, C. Regenfus, J. Rochet C A l V B S H ik C R f J R h t Zurich University, Switzerland A. Bueno, M.C. Carmona-Benitez, J. Lozano, A.J. Melgarejo S. Navas, A.G. Ruiz University of Granada, Spain U i it f G d S i M. Daniel, M. Del Prado, L. Romero CIEMAT, Spain P. Mijakowski, P. Przewlocki, E. Rondio Soltan Institute Warszawa, Poland H. Chagani, P. Lightfoot, P. Majewski, K. Mavrokoridis, N. Spooner University of Sheffield, UK K. Mavrokoridis (Sheffield) ArDM &WLS development 02.04.08 2/17

Talk outline Talk outline WIMP detection principle The ArDM project h j TPB -wavelength shifter (WLS) Wavelengh shifters (WLS) deposition methods Argon scintillation properties Experiments to optimize WLS Experiments to optimize WLS Installation of WLS in ArDM target K. Mavrokoridis (Sheffield) ArDM &WLS development 02.04.08 3/17

WIMP detection principle WIMP detection principle K. Mavrokoridis (Sheffield) ArDM &WLS development 02.04.08 4/17

The ArDM project The ArDM project The The Argon Argon Dark g Dark Matter Matter (ArDM) (ArDM) is ( ( ) ) is a 1 1- -ton ton two two phase phase liquid/gaseous liquid/gaseous argon argon scintillation/ionisation scintillation/ionisation WIMP WIMP detector detector. . Two-stage LEM for electron multiplication and readout to measure ionization charge Greinacher chain: supplies the right Voltages to the field shaper rings and the cathode up to 500 kV Field shaping rings Cathode grid 14 PMTs below the cathode to detect the scintillation light K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 05/17

TPB - wavelength shifter TPB - wavelength shifter Tetraphenyl butadiene (TPB) is an aromatic compound which fluoresces when its pi orbital electrons are excited, either by UV radiation or ionization. TPB powder has been used which can absorb the 128 nm light and reemit it in the visible wavelength (~430 nm) which is within the g ( ) sensitivity range of the PMTs. TPB powder can be deposited to PMT window/reflectors using evaporation, spraying and doping. K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 06/17

TPB deposition methods: TPB deposition methods: Evaporation Evaporation Evaporation Evaporation TPB evaporated samples on 3M foil (specular reflector) K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 07/17

TPB deposition methods: TPB deposition methods: Spraying Spraying Spraying Spraying TPB TPB sprayed samples on TTX cloth (diffuse reflector) d l TTX l th (diff fl t ) K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 08/17

Argon scintillation Argon scintillation Singlet Singlet ~128 nm 128 Triplet Processes induced by charged particles in Argon The The argon argon excimers excimers are are created created in in three three nearly nearly degenerate spin states, two singlets ( degenerate spin states, two singlets ( 1 Σ − − and and 1 Σ Σ + ) ) and a triplet ( and a triplet ( 3 Σ + ) p p ( ( ) [M.Suzuki et al. NIM (1982) 565] . ) [ ( ) ] K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 09/17

Gas argon scintillation Gas argon scintillation T2 T1 Scintillation light from argon has two distinct decay times - a slow component, T2 (triplet), and a fast component, T1 (singlet). The slow component, T2, can be used as a measure of the purity of the Argon. The purest gas argon has a T2 of about 3200 ns. K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 10/17

The gas argon apparatus 1 (a) The gas argon apparatus 1 (a) This apparatus consisted of a sealed PVC tube containing a PMT. 3M foil (multilayer pol mer film polymer film, which is a specular reflector) and TTX ( PTFE cloth, which is a diffuse hich is a spec lar reflector) and TTX ( PTFE cloth hich is a diff se reflector) coated with TPB (wavelength shifter that shifts 128 nm to 420 nm) were placed around the interior walls of the tube. An alpha source was placed within the tube at a fixed distance from the PMT. Measurements were taken for a range of thicknesses of TPB from 0.2 mg/cm 2 to 4.0 mg/cm 2 deposited both via evaporation and spraying. Additionally the distance of the alpha source from the PMT was altered in order to investigate the consequence of the l h f th PMT lt d i d t i ti t th f th solid angle on the light collection of the shifted light. We recorded the effect of each separation on the number of photoelectrons collected at the PMT by plotting the slow component decay time (T2) against light collection for increasing distance d. p y ( ) g g g K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 11/17

The gas argon apparatus 1 (b) The gas argon apparatus 1 (b) K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 12/17

The gas argon apparatus 1 (c) -Results The gas argon apparatus 1 (c) -Results mg/cm 2 The The best best wavelengh wavelengh- -shifter shifter found found to to be be 0 0. .2 2 mg/cm TPB TPB thickness TPB thi k TPB thi k thickness evaporated evaporated on t d t d on TTX TTX TTX TTX cloth! cloth! l th! l th! K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 13/17

GAr apparatus 2 -PMT window coating GAr apparatus 2 -PMT window coating The second gas argon apparatus was constructed based on the actual geometry of the full scale ArDM target. The experiment consisted of a sealed PVC tube containing a 3" PMT which was coated with TPB powder of various PMT which was coated with TPB powder of various thickness and with various deposition methods. The sides and bottom of the PVC tube were covered with 3M foil reflector which was coated with 1 mg/cm2 TPB powder. An alpha source was positioned 10 cm away from the PMT window. We recorded the effect of various PMT window coatings on the number of photoelectrons collected at the PMT by plotting the slow component decay time against the total light collection. h l li h ll i K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 14/17

PMT window coating results PMT window coating results The The best best wavelength wavelength shifter shifter will will be be the the one one at at which which at at a given given purity g purity (slow p p y ( y (slow component) component) the p p ) ) the scintillation scintillation pulse pulse p has has more more area! area! K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 15/17

Installing the WLS in ArDM target Installing the WLS in ArDM target Reflectivity @430nm Reflectivity @430nm ~97% y @ y @ ~97% Shifting eff. 128 nm Shifting eff. 128 nm → 430nm 430nm >97% >97% TPB powder evaporated on TTX cloth K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 16/17

Summary & Conclusions Summary & Conclusions The ArDM project: a 1-ton two phase liquid/gaseous argon scintillation/ionisation WIMP detector . The primary VUV scintillation light of argon (wavelength 128 nm) needs to be Th i VUV i ill i li h f ( l h 128 ) d b shifted to visible light in order to match the sensitivity range of the PMTs. TPB powder wavelength shifter was used TPB powder wavelength shifter was used. TPB powder deposition methods were: evaporation, spraying and doping. Two gas argon apparatus were constructed in order to optimize the WLS. The best coated reflector was TTX cloth with 0.2 mg/cm 2 TPB deposited by evaporation. The best coating of PMT window found to be 0.5 mg/cm 2 TPB d deposited by evaporation. it d b ti K. Mavrokoridis (Sheffield) ArDM & WLS development 02.04.08 17/17