Low  Background  Experiments  and  Material  Assay

Tessa  JohnsonNSSC  Summer  School

July  2016

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

How  do  we  detect  particles?Can  a  particle  ionize  a  material?

Geiger  Counter:

Applied  voltage

Inert  gas

• Ionized  particle  amplifies  inside  of  the  cavity

• Detector  “clicks,”  or  displays  voltage

How  do  we  detect  particles?Can  a  particle  ionize  a  material?

Scintillation

Phonons

Ways  to  detect  

ionization

Free  Charge

e�e�e�

e� e�

e�

e�e�e�

e�

e�A+

A+

A+A+ A+

A+

*Fast  moving  particles  can  also  be  detected  by  Cherenkov  radiation  

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?

What  is  our  universe  made  of?• Non-­‐luminous  matter?

1933:  Fritz  Zwicky  measures  a  discrepancy  in  calculations  of  galaxy  cluster  mass    

Fritz  Zwicky

Vera  Rubin

Outer  stars  are  rotating  much  faster  than  expected!

What  is  our  universe  made  of?

7

Cosmological  evidence  of  dark  matter  – cosmic  microwave  background  anisotropies

Astrophysical  evidence  of  dark  matter  – Bullet  cluster;  mass  distribution  and  baryons  separated  

Pink =  x-­‐ray  imageBlue =  gravitational  lensing  map

WMAP  CMB  heat  map

What  is  the  dark  matter  (85%  of  matter!)  made  of?    Weakly  Interacting  Massive  Particle  (WIMP)  is  a  favored  Candidate

Experiments  looking  for  WIMPS

Recoil

WIMP WIMP

Noble  Liquid  Detectors

Bubble  Chamber  Detectors

Cryogenic  Thermometer  DetectorsSemiconductor  Detectors

CRESSTCDMS

Scintillation  Crystal  Detectors

Is  lepton  number  conserved?Or  rather  – is  the  neutrino  its  own  antiparticle?    This  could  explain  the  matter/antimatter  asymmetry  in  the  universe!

⌫̄ = ⌫?

E.  MajoranaP.  Dirac

vs.

�i/@ +m = 0 �i/@ +m c = 0

Is  lepton  number  conserved?Or  rather  – is  the  neutrino  its  own  antiparticle?    This  could  explain  the  matter/antimatter  asymmetry  in  the  universe!

*Measuring  this  process  would  also  allow  measurement  of  the  absolute  neutrino  mass!

0⌫��2⌫��

Experiments  looking  for 0⌫��

EXO-­‐200

CUORE

GERDA

SNO+

More  about  neutrinos!

• Oscillation  properties• Mass  hierarchy  • Absolute  mass• CP  violating  phase?• Sterile  neutrinos?

Neutrinos  are  notoriously  difficult  to  detect….  And  have  displayed  some  interesting  properties!

We  would  like  to  know  more  about:

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

Neutrino  Absolute  Mass  ExperimentsNeutrinoless double  beta  decay  could  measure  absolute  mass

KATRIN  Experiment

Backgrounds  to  particle  physics  experimentsCosmic  Rays

Mostly  protons,  some  𝜶s,  small  component  𝒆# and  heavy  nuclei

Interactions  in  the  atmosphere:𝑝 + 𝑁 → 𝑋 +  𝜋+𝑠𝑝 + 𝑁 → 𝑋 +𝐾+𝑠

𝝅’s  and  𝑲’s  decay  to  produce  𝝁 -­‐ 𝝁 are  highly  ionizing  and  have  very  little  stopping  power!  

How  to  reduce  cosmic  ray  related  backgrounds?Go  Underground!

SNOLAB

Sanford

Boulby

JINPING

Kamioka

Gran  Sasso

CanFranc

Soudan

WIPP

How  to  reduce  cosmic  ray  related  backgrounds?Go  Underground!

DarkSide-­‐50Limestone  coverage  of  ~1300  m(3800  m.w.e.)

• DarkSide-­‐50• Xenon1T• OPERA• Borexino• DAMA/LIBRA• CRESST• CUORE• GERDA• LVD

Muon  flux  measured  to  be>1x106%  decrease  from  muon  

flux  at  sea  level

Gran  Sasso Laboratory

Backgrounds  to  particle  physics  experiments𝛾 Rays  from  natural  radioactivity

238U𝒕𝟏/𝟐 = 𝟒.𝟓𝐞𝟗 yr

40K𝒕𝟏/𝟐 = 𝟏.𝟑𝒆𝟗  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!

232Th𝒕𝟏/𝟐 = 𝟏𝟒𝒆𝟗  yr

How  to  reduce  natural  radioactivity  related  backgrounds?Build  a  big  shield!

Attenuation  of  water  to  a  2.6  MeV  gamma  (208Tl)  ~  2  m

Lead  shield:Water  shield:

Majorana  𝟎𝝊𝜷𝜷 Experiment

Xenon-­‐1T  Water  Tank

Attenuation  of  lead  to  a  2.6  MeV  gamma  (208Tl)  ~  2  cm

How  to  reduce  natural  radioactivity  related  backgrounds?Choose  radiopurematerials  for  the  detector!

More  on  this  later!

Backgrounds  to  particle  physics  experimentsNeutrons

(Z,  A) (Z,  A+1)*

𝛄

Recoil

Elastic  neutron  scatter:

• 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

Inelastic  neutron  scatter:

n nn

Sources  of  NeutronsCosmogenic: Radiogenic: Spontaneous  fission:

𝝁#

(Z,A)        (Z+2,  A+3)*

n

n

Alphas  are  emitted  in  the  238U  and  232Th  chains!

235U

n

n

How  to  reduce  neutron  related  backgrounds?Active  Vetos!

Neutron  Capture  Veto:Muon  Veto:

VETO  PANELSScintillating  Muon  Veto  

Panels

EXO-­‐200

10B+ n ! ↵(1775keV) +7 Li10B+ n ! ↵(1471keV) +7 Li⇤

7Li⇤ !7 Li + �(478keV)

(6.4%)

(93.6%)

DarkSide-­‐50Water  Cherenkov  Detector

LZ  Schematic

How  to  reduce  neutron  related  backgrounds?Choose  radiopurematerials  for  the  detector!

More  on  this  later!

Backgrounds  to  particle  physics  experimentsRadon  Backgrounds

• Rn  is  a  noble  gas  – easily  separated  from  parent  material

• Can  easily  enter  a  liquid  or  gas  stream

• From  222Rn  to  210Pb  is  only  a  4  day  half-­‐life  – can  have  many  𝛼’s,  𝛽’s,  𝛾’s  from  daughters

• 210Pb  can  “Plate  out”  on  surfaces,  causing  a  longer-­‐lived  backgrounds

Backgrounds  to  particle  physics  experimentsRadon  Backgrounds  – use  as  calibration?

• 214Bi  -­‐>  214Po  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

How  to  reduce  radon  related  backgrounds?

Choose  radiopurematerials  for  the  detector!More  on  this  later!

Suppress  radon  in  your  experiment’s  environment!

Sanford  Laboratory(where  LUX/LZ  lives)

*Filtering  by  carbon  absorption

Choosing  low  background  materials

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

An  important  part  of  a  low  background  experiment!

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• Radiopurematerials  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

Ancient  lead

Shipwreck  50-­‐20  BCCUORE  𝟎𝝊𝜷𝜷 experiment

Ancient  lead  from  shipwrecks  used  in  many  low  background  experiments!

Neutron  Activation  Analysis

• Irradiate  materials  in  a  neutron  flux,  count  𝛾 rays  from  products  in  a  𝛾-­‐counter• 238U(n, 𝛾)239U  (t1/2=23.5  m)  -­‐>  239Np  (𝛾’s  at  103,  106,  228,  278  keV)  (t1/2=2.35  d)• 232Th(n, 𝛾)233Th  (t1/2=21.8m)  -­‐>  233Pa  (𝛾’s  at  300,  312  keV)  (t1/2=27  d)• 41K(n, 𝛾)42K  (𝛾 at  1524  keV)  (t1/2=12.4  h)    -­‐-­‐ get  40K  from  natural  abundance

Not  good  for  materials  that  irradiate  to  something  radioactive!

Inductively-­‐Coupled  Plasma  Mass  Spectrometry(ICPMS)

Material  sample

particle  beam

PlasmaMass  spectrometer

Fragments  of  a  material’s  surface  are  ionized  and  analyzed  with  a  mass  spectrometer

Radon  EmanationMaterial  samples  are  placed  in  a  vial  and  allowed  to  outgas  the  radon  component

Outgassed  radon  enters  a  gas  flow

Decaying  radon  daughters  are  detected  with  a  pin  diode

*Photos   taken  from  a  Xenon  collaboration  presentation

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

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