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Signal and Background in LENS LONU-LENS Mini-Workshop on Low-Energy Solar Neutrinos & LENS Blacksburg, VA, October 1

Signal and Background in LENS LONU-LENS Mini-Workshop on Low-Energy Solar Neutrinos & LENS Blacksburg, VA, October 14 Christian Grieb Virginia Tech. LENS-Indium: Signal. CC -c apture in 115 In to excited isomeric level in 115 Sn. Tag: Delayed emission of (e/ )+ 

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Signal and Background in LENS LONU-LENS Mini-Workshop on Low-Energy Solar Neutrinos & LENS Blacksburg, VA, October 1

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  1. Signal and Background in LENS LONU-LENS Mini-Workshop on Low-Energy Solar Neutrinos & LENS Blacksburg, VA, October 14 Christian Grieb Virginia Tech

  2. LENS-Indium: Signal CC -capture in 115In to excited isomeric level in 115Sn Tag: Delayed emission of (e/)+  Threshold: 114 keV  pp-’s 115In abundance: ~ 96% CC-capture: Faithful reproduction of  spectrum • Background Challenge: • Indium-target is radioactive! (t = 6x1014 y) • 115In β-spectrum overlaps pp-signal • Basic background discriminator: • Time/space coincidence tag • Tag energy: E-tag = Eβmax+116 keV • Requires good spatial & energy resolution • 7Be, CNO & LENS-Cal signals • not affected by Indium-Bgd!

  3. LENS Expected Result: Low Energy Solar -Spectrum >98% Flux <2MeV Signal (t = 4.76 µs) • LENS-Sol Signal • = • SSM(low CNO) + LMA • x • Detection Efficiency e • Rate: pp 40 pp ev. /y /t In • 2000 pp ev./ 5y/10t In  ±2.5% • Design Specification: S/N ≥ 3 pp:e = 64% 7Be:e = 85% pep:e = 90% Access to pp  spectral shapefor the first time

  4. Indium --Background Structure – Space / Time coincidence β1 (Emax< 2 keV) (b = 1.2x10-6)* E() -114 keV 115In =4.76s 115In e/ 116 keV β0 + n (BS) (Emax = 499 keV)  498 keV  497 keV 115Sn 115Sn *Cattadori et al: 2003 Signal Signal Signature: Prompt e- ( ) followed by low energy (e-/) ( ) and Compton-scattered  ( ) ->time/space coincidence -> tag fixed energy 613keV ->compton scattered shower Background: Random time and space coincidence between two -decays ( ); Extended shower ( ) can be created by: a) 498 keV  from decay to excited state; b) Bremsstrahlungs -rays created by ; c) Random coincidence (~10 ns) of more -decays; Or any combination of a), b) and c). Background

  5. Indium Radioactivity Background Background categories n 115In –decays in (quasi) prompt coincidence produce a tag: Basic tag candidate: Shower near vertex (Nhit ≥ 3) - chance coincident with 115In β in vertex Type A: A1 = β + BS (Etot = 498 keV) (x1) A2 =  (498 keV)(x1) Type B: 2 β-decays (x10-8) Type C: 3 β-decays (x10-16) Type D: 4 β-decays(x10-24) Strong suppression via energy Suppression via tag topology Example Type C: 3 βs in quasi-prompt (~10ns) coincidence

  6. Indium Background Rates Valid Tag: Time and Space coincidence: Two events within 10s inside the same volume (cubic cell 7.5cm) Spatial Resolution: assume cubes 7.5 x 7.5 x 7.5 cm3 Energy Resolution: E/E 4.3% at 613 keV (900 pe / MeV) Only for A1:  energy > 450keV and has to create at least one Bremsstrahlungs  > 40keV Separation of A1 Background and Signal Tag: E=115 keV or E/E > 5

  7. N BS  spectrum for E+E  450 keV after production of at least 1 BS  with E  40keV E [keV] Indium Beta Spectrum & Bremsstrahlung N N - spectrum for E+E  450 keV after production of at least 1 BS  with E  40keV 115In --spectrum 8.610-3 9.010-3 E [MeV] E [keV] Type A1 Background: Consider only events where E+E  450 keV and at least one BS  with E  40keV is produced  reduce computing time by 9.010-3 x 8.610-3 = 7.7 10-5

  8. Hit Multiplicity (“Nhit”) n Example: Event with Nhit = 3 Nhit distribution for -event tags Demand ≥3 Hits in tag shower Nhit n Nhit distribution for In-Background Classify all events according to Nhit Optimize Cut Parameters individually for each class Nhit

  9. β1 (Emax< 2 keV) (b = 1.2x10-6)* 115In β0 + n (BS) (Emax = 499 keV)  498 keV 115Sn *Cattadori et al: 2003 Tag Energy Total Energy deposit in Tag A1 Bkgd A2 Bkgd -Events (pp) N year-1 t In-1 keV-1 B Bkgd C Bkgd D Bkgd Powerful energy Separation for A1 & A2 Background Not so powerful for B,C,D Background Etag [keV]

  10. Rates after Tag Energy Cut Type A1 / A2 Background: ok Type B / C: still problematic

  11. Shower Radius Shower radius R for  event tags is defined by propagation of 497 keV  in the scintillator Shower radius for A2 Bkgd is naturally identical (498 keV ) For B, C Bkgd random coincidence rate scales with allowed shower volume Tag Shower Radius distribution N B Bkgd -Events (pp) A2 Bkgd C Bkgd R [mm] R

  12. N BS  spectrum for E+E  450 keV after production of at least 1 BS  with E  40keV E [keV] Hit Separation Identification of remaining Type B Background Most likely case: one of the two s produces a low energy BS  ray. The second  decay (2) occurs randomly and is separated from the shower produced by 1. Type B scales with the allowed volume, i.e. x3 N Hit Separation Distribution 1:Emission of low E BS  ray 2: ”lone ”, randomly Coincident within <10ns x x

  13. Signal and In-Background Rates • Signal / Background ~3 with pp- event detection efficiency 64% • Remember: only pp- events affected by Indium Background, 7Be, pep and CNO Background-free

  14. pp-Signal and In-Background revisited N Shower Radius R Total Energy deposit in Tag Black: pp  events Blue: A1 Bgd Green: A2 Bkgd Red: C Bkgd N year-1 t In-1 2keV-1 R [mm] N Hit Separation x E [keV] x [mm]

  15. LENS Design Figures of Merit *Pmt’s on three sides only LENS is a feasible detector, 125t of liquid scintillator and ~13300 photomultiplier channels for ~2000 pp- events in 5 years with full spectroscopic information plus 7Be, pep and CNO

  16. Additional Slides

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