nerix PMT Calibration and Neutron Generator Simulation Haley Pawlow July 31, 2014 Columbia University REU, XENON

nerix PMT Calibration and Neutron Generator Simulation Haley Pawlow July 31, 2014 Columbia University REU, XENON

Dark Matter XENON nerix Project 1-> PMT Calibration Project 2-> Neutron Generator Simulation 07/31/14 Haley Pawlow, XENON 2

Fritz Zwicky 1933-> Using Virial Theorem to analyze Coma cluster motion, found the visible mass was too small to explain high velocities of galaxies Missing mass could be dark matter Coma cluster 07/31/14 Haley Pawlow, XENON 3

Vera Rubin Ø 1970-> Rubin measured rotational velocity of spiral galaxy as a function of radius Ø Used Newtonian Mechanics to compute mass Ø Velocity doesn t obey 07/31/14 Haley Pawlow, XENON 4

Bullet Cluster Ø Two hot x-ray gas clouds (red) collide like a shock wave, producing a bullet shape Ø Dark matter (blue) passes right through each other with no interaction aside from gravitational force 07/31/14 Haley Pawlow, XENON 5

Weakly Interacting Massive Particles (WIMPs) Ø Leading dark matter candidate because it agrees nicely with super symmetry Ø Non-baryonic matter Ø Interact with weak force Ø Neutral in charge and color Ø nonrelativistic Ø Stable or have lifetimes comparable to age of Universe 07/31/14 Haley Pawlow, XENON 6

Detecting WIMPs Indirect Detection Ø Observe particles or gamma rays resulting from WIMP annihilation or decay Direct Detection Ø Look for signal from WIMP nuclear recoils Ø Leaders are Dual-Phase, liquid noble gas detectors Fermi Gamma Ray Telescope LHC (Production) XENON100 07/31/14 Haley Pawlow, XENON 7

Dark Matter XENON nerix Project 1-> PMT Calibration Project 2-> Neutron Generator 07/31/14 Haley Pawlow, XENON 8

Direct Detection Ø Simultaneous measurement of ionization and scintillation signals enables XENON to discern between nuclear and electronic recoils 07/31/14 Haley Pawlow, XENON 9

XENON100 Ø 161kg of LXe Ø Optimize design for low energy threshold Ø Purify Xenon Ø Optimize shielding (underground in Gran Sasso, Italy) 07/31/14 Haley Pawlow, XENON 10

Time Projection Chamber (TPC) Ø WIMP scatters with Lxe nucleus, causing a nuclear recoil where atom collides into other atoms Ø Atoms can either be ionized, releasing electrons, or excited, emitting photons which are detected by the photomultiplier tubes (scintillation->s1) Ø Remaining electrons accelerated across electric field to top gas, producing secondary light (ionization->s2) 07/31/14 Haley Pawlow, XENON 11

Measuring S1 and S2 Ø light yield (s1) = total photons/energy Ø charge yield (s2) = # of electrons/energy Ø S2/S1 differentiates between electronic(gamma/beta) and nuclear recoils Ø Leff is largest systematic error in reported LXe WIMP searches 07/31/14 Haley Pawlow, XENON 12

Position of event Ø S2/S1 differentiates between electronic(gamma/beta) and nuclear recoils Ø Using drift time and Voltage, calculate distance (z direction) Ø PMTs record x and y coordinate of event (hit pattern) Ø Reconstruct position of event 07/31/14 Haley Pawlow, XENON 13

Dark Matter XENON nerix Project 1-> PMT Calibration Project 2-> Neutron Generator 07/31/14 Haley Pawlow, XENON 14

nerix: How can we calibrate XENON? Ø Analyze how different particles, like neutrons and gamma rays, interact with LXe atoms Ø Measure s1 and s2 to distinguish electromagnetic background from WIMPs in XENON Ø Need to convert from measured scintillation in Photomultiplier tubes (PMTs) to energy 07/31/14 Haley Pawlow, XENON 15

nerix Experimental setup Ø Neutron generator emits monochromatic neutrons that elastically scatter with atoms inside LXe. Ø Two organic liquid scintillators detect neutrons at fixed signal Ø Measure light and charge yield 07/31/14 Haley Pawlow, XENON 16

nerix Detector Ø Same Dual-phase, TPC concept as XENON100 Ø nerix has 4 top PMTs and 1 bottom PMT Ø XENON100 has 98 top PMTs and 78 bottom PMTs 07/31/14 Haley Pawlow, XENON 17

Dark Matter XENON nerix Project 1-> PMT Calibration Project 2-> Neutron Generator 07/31/14 Haley Pawlow, XENON 18

PMTs Ø Incident photon emits a single photoelectron (PE) from photocathode Ø PE attracted to dynode by applied electric field Ø PE accelerates, releasing more e- s, which are all attracted to second dynode Ø Dynodes multiply amount of charge via photoelectric effect Ø PMT Gain = # of e produced/photoelectron 07/31/14 Haley Pawlow, XENON 19

Why do we need to calibrate Ø PMTs give us a voltage signal, we need to convert to photoelectrons using the gain Ø With the gain we can measure s1 and s2 (light and charge signals) PMTs? Bottom PMT array for XENON100 07/31/14 Haley Pawlow, XENON 20

Gaussian Distribution of PMTs Frequency 800 600 400 200 0 Channel 1, PMT 1 at 700 V 1000 Mean = 9.50e+05 +/- 1.72e+04, Λ = 0.045 10 0 500 1000 1500 2000 Charge[e] spe Signal Noise Difference between S/N 3 Ø Number of PE is a poisson distribution: 90% of signal is noise, 10% of signal from 1 PE. This way contribution of multiple PE is minimized Ø 1 st peak around zero is noise, second peak from one photoelectron 07/31/14 Haley Pawlow, XENON 21

Gaussian Distribution of PMTs Frequency 400 350 300 250 200 150 100 Channel 1, PMT 1 at 700 V Mean = 9.50e+05 +/- 1.72e+04, Signal Noise spe = 0.045 Difference between S/N Ø Λ = 0.045, the 1-PE peak is 4.5% of signal Ø Agrees with Poisson distribution 50 400 600 800 1000 1200 1400 1600 1800 2000 Charge[e] 3 10 07/31/14 Haley Pawlow, XENON 22

PMT Calibration Ø 4 top PMTs and 1 bottom PMT, 4 channels each Ø PMT Gain = # of electrons produced/pe Ø PMT gain increases exponentially, seen as line in log linear plot 07/31/14 Haley Pawlow, XENON 23

Dark Matter XENON nerix Project 1-> PMT Calibration Project 2-> Neutron Generator 07/31/14 Haley Pawlow, XENON 24

Neutron Generator 1. Heat filament and release deuterium gas 2. As cathode heats up, electrons extracted to the grid 3. Ionized deuterium gas ions accelerate toward titanium deuteride target 4. Ions collide with target and are either stopped or produce neutrons 5. Neutrons are mono-energetic, minimizing spread in neutron energy at a 90 degree angle 5 1 3 4 2 07/31/14 Haley Pawlow, XENON 25

Why modify Neutron Generator holder? Ø Increase voltage 40kV->100kV Ø With a higher voltage we get greater neutron flux Ø Faster data collection 07/31/14 Haley Pawlow, XENON 26

Electrical Breakdown across HV cable Ø Need to avoid electrical discharge at HV cable connection to neutron generator Ø Analyze electrical breakdown (dielectric strength) for different dielectric materials (Teflon, oil, and air) 07/31/14 Haley Pawlow, XENON 27

Electrical Breakdown in Air E(r)[V/mm] Air 14000 12000 10000 8000 6000 4000 50 kv 80kV 100kV 150kV Inner radius Outer radius Breakdown Voltage 2000 0 0 1 2 3 4 5 6 7 8 9 10 radius [mm] At the HV cable radius, the electric field exceeds electrical breakdown for 100kV 07/31/14 Haley Pawlow, XENON 28

Electrical Breakdown in Oil E(r)[V/mm] 25000 20000 15000 10000 Mineral Oil 50 kv 80kV 100kV 150kV Inner radius Outer radius Breakdown Voltage 5000 0 0 1 2 3 4 5 6 7 8 9 10 radius [mm] At the HV cable radius, the electric field exceeds electrical breakdown for 100kV 07/31/14 Haley Pawlow, XENON 29

Electrical Breakdown in Teflon E(r)[V/mm] 120 100 80 60 40 3 10 Teflon 50 kv 80kV 100kV 150kV Inner radius Outer radius Breakdown Voltage 20 0 0 1 2 3 4 5 6 7 8 9 10 radius [mm] At the HV cable radius, the electric field is well below electrical breakdown for 100kV 07/31/14 Haley Pawlow, XENON 30

GEANT4 Monte Carlo Simulation Ø Optimize ratio between Teflon and oil to produce least amount of neutron scatters Teflon cut radius = radius of mineral oil *Image from GEANT4 simulation 07/31/14 Haley Pawlow, XENON 31

Example: Teflon/oil = 1.5743 52% of neutrons scatter 07/31/14 Haley Pawlow, XENON 32

Conclusions Teflon/Oil Scattered/Total neutrons 1.57 0.522 0.5 0.522 0.19 0.520 Ø Largest Teflon cut produces least amount of neutron scatters Ø Amount of scattering is negligible +/- 0.4% Ø Use maximum amount of Teflon in holder design to keep neutron generator fixed 07/31/14 Haley Pawlow, XENON 33

SolidWorks Model for Neutron Generator Holder Ø Increase holder tube diameter from 2->3 Ø Maximize use of Teflon to minimize neutron scatters 07/31/14 Haley Pawlow, XENON 34

SolidWorks Neutron Generator Holder Model ü HV cable surrounded with Teflon to avoid electrical breakdown ü Neutron Generator firmly secured in carrier with Teflon/oil ~1.5 07/31/14 Haley Pawlow, XENON 35

Next Steps for nerix Ø Measure nuclear and electronic recoils by varying electric fields across chamber Ø More precise measurement of gamma rays at low energy with Cs 137 Compton scatters 07/31/14 Haley Pawlow, XENON 36

ROOT/C++ GEANT4 SolidWorks Graduate School Takeaways Dark Matter, XENON, nerix Brookhaven National Lab Particle Physics Lectures Research Environment 7/31/2014 Haley Pawlow, XENON 37

Acknowledgements Elena Aprile for giving me this opportunity Antonio Melgarejo, Marc Weber, Luke Goetzke, and Matt Anthony for their support and guidance The XENON team at Nevis for being so inviting John Parsons and Amy Garwood for organizing the REU program NSF REU program for their generous funding 07/31/14 Haley Pawlow, XENON 38

Questions???

Appendix 08/31/14 Haley Pawlow, XENON 40

Measuring # of photons from PMT Gain = # of electrons/photoelectron emitted PMT Quantum Efficiency = 35% (# photons/pe) Number of photons from PMT Light Detection Efficiency Total # photons emmited (type, Energy, E field) 08/31/14 Haley Pawlow, XENON 41

Relative scintillation efficiency of nuclear recoils (Leff) Leff is largest systematic error in reported LXe WIMP searches 08/31/14 Haley Pawlow, XENON 42

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Monte Carlo Simulation of ideal Teflon/Oil ratio 08/31/14 Haley Pawlow, XENON 46

Teflon/oil= 0.50 52 % of neutrons scatter 08/31/14 Haley Pawlow, XENON 47

Teflon/oil = 0.191 52% of neutrons scatter 08/31/14 Haley Pawlow, XENON 48