Low Background Experiments and Material Assay. Tessa Johnson NSSC Summer School July 2016

Transcription

1 Low Background Experiments and Material Assay Tessa Johnson NSSC Summer School July 2016

2 Outline How do we detect particles? Some interesting questions relating to particle physics How can particle detection solve them? Experiments trying to answer these questions Backgrounds to particle physics experiments What backgrounds exist to sensitive experiments? How these backgrounds are mitigated Low background assay techniques

3 How do we detect particles? Can a particle ionize a material? Geiger Counter: Inert gas Ionized particle amplifies inside of the cavity Detector clicks, or displays voltage Applied voltage

4 How do we detect particles? Can a particle ionize a material? Scintillation A + e A + e e e A + e A + A + e A + Phonons Ways to detect ionization e e Free Charge e e e *Fast moving particles can also be detected by Cherenkov radiation

5 What questions can we answer by detecting particles? What is the bulk of our universe made of? Is lepton number a conserved quantity? What is the absolute mass of the neutrino?

6 What is our universe made of? Non- luminous matter? Fritz Zwicky 1933: Fritz Zwicky measures a discrepancy in calculations of galaxy cluster mass Vera Rubin Outer stars are rotating much faster than expected!

7 What is our universe made of? WMAP CMB heat map Pink = x- ray image Blue = gravitational lensing map Cosmological evidence of dark matter cosmic microwave background anisotropies Astrophysical evidence of dark matter Bullet cluster; mass distribution and baryons separated What is the dark matter (85% of matter!) made of? Weakly Interacting Massive Particle (WIMP) is a favored Candidate 7

8 Experiments looking for WIMPS Noble Liquid Detectors WIMP WIMP Bubble Chamber Detectors Recoil Semiconductor Detectors Cryogenic Thermometer Detectors Scintillation Crystal Detectors CDMS CRESST

9 Is lepton number conserved? Or rather is the neutrino its own antiparticle? This could explain the matter/antimatter asymmetry in the universe! =? vs. P. Dirac E. Majorana + m =0 i/@ + m c =0

10 Is lepton number conserved? Or rather is the neutrino its own antiparticle? This could explain the matter/antimatter asymmetry in the universe! 2 0 *Measuring this process would also allow measurement of the absolute neutrino mass!

11 Experiments looking for 0 EXO- 200 GERDA SNO+ CUORE

12 More about neutrinos! Neutrinos are notoriously difficult to detect. And have displayed some interesting properties! We would like to know more about: Oscillation properties Mass hierarchy Absolute mass CP violating phase? Sterile neutrinos?

13 Neutrino Oscillation Experiments Deep Underground Neutrino Experiment (DUNE) is the next biggest thing should measure mass hierarchy and CP violating phase

14 Neutrino Absolute Mass Experiments Neutrinoless double beta decay could measure absolute mass KATRIN Experiment

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16 Backgrounds to particle physics experiments Cosmic Rays Mostly protons, some αs, small component e # and heavy nuclei Interactions in the atmosphere: p + N X + π + s p + N X + K + s π s and K s decay to produce μ - μ are highly ionizing and have very little stopping power!

17 How to reduce cosmic ray related backgrounds? Go Underground! Boulby SNOLAB Gran Sasso Soudan JINPING Sanford WIPP Kamioka CanFranc

18 How to reduce cosmic ray related backgrounds? Go Underground! Gran Sasso Laboratory DarkSide- 50 Xenon1T OPERA Borexino DAMA/LIBRA CRESST CUORE GERDA LVD Limestone coverage of ~1300 m (3800 m.w.e.) DarkSide- 50 Muon flux measured to be >1x10 6 % decrease from muon flux at sea level

19 Backgrounds to particle physics experiments γ Rays from natural radioactivity 238 U t 1/2 = 4. 5e9 yr 232 Th t 1/2 = 14e9 yr Long- lived radioisotopes exist in trace amounts all over the environment! Sometimes they exist in secular equilibrium meaning all daughter isotopes in equal parts α s and β s are stopped by material, but γ s can travel far! 40 K t 1/2 = 1. 3e9 yr

20 How to reduce natural radioactivity related backgrounds? Build a big shield! Water shield: Attenuation of water to a 2.6 MeV gamma ( 208 Tl) ~ 2 m Lead shield: Attenuation of lead to a 2.6 MeV gamma ( 208 Tl) ~ 2 cm Majorana 0υββ Experiment Xenon- 1T Water Tank

21 How to reduce natural radioactivity related backgrounds? Choose radiopure materials for the detector! More on this later!

22 Backgrounds to particle physics experiments Neutrons Elastic neutron scatter: Inelastic neutron scatter: n n n γ Recoil (Z, A) (Z, A+1)* Causes a nucleus to recoil Creates an ionizing track Neutron is captured into nucleus Excited nucleus decays, emitting gammas Sometimes left as a radioactive isotope

23 Sources of Neutrons Cosmogenic: Radiogenic: Spontaneous fission: μ # n 235 U (Z,A) (Z+2, A+3)* n Alphas are emitted in the 238 U and 232 Th chains! n n

24 How to reduce neutron related backgrounds? Active Vetos! Muon Veto: Water Cherenkov Detector Neutron Capture Veto: DarkSide- 50 Scintillating Muon Veto VETO PANELS Panels LZ Schematic 10 B+n! (1775keV) + 7 Li 10 B+n! (1471keV) + 7 Li (6.4%) (93.6%) EXO Li! 7 Li + (478keV)

25 How to reduce neutron related backgrounds? Choose radiopure materials for the detector! More on this later!

26 Backgrounds to particle physics experiments Radon Backgrounds Rn is a noble gas easily separated from parent material Can easily enter a liquid or gas stream From 222 Rn to 210 Pb is only a 4 day half- life can have many α s, β s, γ s from daughters 210 Pb can Plate out on surfaces, causing a longer- lived backgrounds

27 Backgrounds to particle physics experiments Radon Backgrounds use as calibration? 214 Bi - > 214 Po has a short half- live (164 us) Can be used for counting total internal radon background, or even for calibration! Event viewer from EXO- 200

28 How to reduce radon related backgrounds? Suppress radon in your experiment s environment! Sanford Laboratory (where LUX/LZ lives) *Filtering by carbon absorption Choose radiopure materials for the detector! More on this later!

29 Choosing low background materials An important part of a low background experiment! Different assay techniques exist choose the one that works best for the material in question Passive gamma analysis Neutron activation analysis (NAA) Inductively- coupled plasma mass spectroscopy (ICP- MS) Radon emanation system Beta cage

30 Passive Gamma Analysis (in HPGe detector) Leave materials in a clean, shielded detector for a long time Backgrounds from the environment and detector itself can mask the measurement of U, Th, K Use of underground facilities Radiopure materials in detector itself Environment purged of Rn or flushed with nitrogen Use of ancient or low radioactivity lead (no cosmogenically activated isotopes) Sometimes Monte Carlo is required to fit spectra Low Background Counting Facitlity, Sanford Underground Research Facility

31 Ancient lead Shipwreck BC CUORE 0υββ experiment Ancient lead from shipwrecks used in many low background experiments!

32 Neutron Activation Analysis Irradiate materials in a neutron flux, count γ rays from products in a γ- counter 238 U(n, γ) 239 U (t 1/2 =23.5 m) - > 239 Np (γ s at 103, 106, 228, 278 kev) (t 1/2 =2.35 d) 232 Th(n, γ) 233 Th (t 1/2 =21.8m) - > 233 Pa (γ s at 300, 312 kev) (t 1/2 =27 d) 41 K(n, γ) 42 K (γ at 1524 kev) (t 1/2 =12.4 h) - - get 40 K from natural abundance Not good for materials that irradiate to something radioactive!

33 Inductively- Coupled Plasma Mass Spectrometry (ICPMS) Fragments of a material s surface are ionized and analyzed with a mass spectrometer particle beam Material sample Plasma Mass spectrometer

34 Radon Emanation Material samples are placed in a vial and allowed to outgas the radon component Decaying radon daughters are detected with a pin diode Outgassed radon enters a gas flow *Photos taken from a Xenon collaboration presentation

35 Beta Cage Directly measures β or α emissions from a thin film of material Important for experiments with materials close to the active volume, such as CMDS or CUORE Filled with a noble gas One of the CDMS Detectors

36 Conclusions: Particle physicists are trying to answer some big questions by detecting rare particle interactions Ultra- low backgrounds are required to reach interesting sensitivities There are some different techniques available; the use of the material and composition of the material itself guide determine what method is best