Abdel Bachri, Southern Arkansas University
In collaboration with Perry Grant, Martin Hawron,
Clayton Martin, and Azriel Goldschmidt
physica scripta, 2014 under review
Arkansas Space Grant Consortium
Hot Springs, AR
April 7, 2014
Abdel Bachri, Southern Arkansas University
2 double-beta decay  2  :
Nucleus (A, Z)  Nucleus (A, Z+2) + e  + e + e  + e
Allowed in the Standard Model
(conserves Lepton #)
Maria Goeppert-Mayer
typical T1/2=1018-1021 years
0 double-beta decay  0  :
Nucleus (A, Z)  Nucleus (A, Z+2) + e + e
T1/2
Wendell. H Furry
1022  1023 years
Golden plated channel:
a) 2 electrons
b) E1+ E2=Q = 2.458 MeV
Experiments currently taking data:
 The neutrinoless double beta decay detection would give
further insight to the nature of the neutrino
 Are neutrinos and anti-neutrinos the same? (i.e. Majorana)
neutrinos participate in β+ decays while antineutrinos participate in β- decays
 Indicate lepton number violation, which is an important
requirement to explain the current matter anti-matter
asymmetry in the universe.
 SM of particle physics is incomplete?
 Can the mass range of the neutrino
be refined? (current range between .002 and 2.2 eV)
 What will further knowledge of the
neutrino reveal about the nature of
the universe? DM

0 is the key experiment for neutrino physics
Neutrino Experiment with a Xenon Time Projection Chamber
Canfranc Underground Laboratory (LSC)
provides 2520 w.m.e. of natural shielding to help
reduce external radiation interference.
But the TPC will
be constructed
from materials
with inherent
radioactive
impurities that
could affect the
accuracy of
measurements.
In particular, the beta decays of Bi-214 and
Tl-208 emit gamma rays in the problematic
energy range.
All calculations for alpha and neutron flux depend
directly on the mass of the materials involved in TPC.
TPC dimensions
•Inner Radius:
•Length of Cylinder:
•Cylinder Thickness:
•End-Cap Thickness:
•Total Mass of Ti:
•Total Mass of PTFE:
•PTFE Thickness:
52.4 cm
104.8 cm
1.5 cm
0.75 cm
490 kg
151.82 kg
1.0 cm
Detects neutrinoless
double beta decay (0νββ)
electrons.
Measures energy released
by interaction with xenon.
Unfortunately, many forms of ionizing
radiation can cause a signal in the TPC,
such as cosmic rays, thermal neutrons,
and gamma rays.
Photomultiplier tubes
register the light emitted
during scintillation.
A tracking system will
look for the signature
electron paths of 0νββ
1)
2)
Radioactive contamination of detector materials (Long-lived
radioactive isotopes ): Time Projection Chamber (Xenon housing),
readout plane, PMTs etc. Careful selection of radiopure material is
necessary to reduce this background
Radioactive contamination of laboratory walls. Background can be
discriminated via shielding
3)
Radioactive contamination of shielding itself
4)
Cosmic rays: high energy muons
causing ionization signals with
active xenon volume. Consider
operating in an underground site
Importance of Energy resolution and background suppression
Due to its large half-life, an optimal background identification is
mandatory in order to reject events whose energy falls inside the
energy window, to the greatest extent possible.
5)
6)
238U
and 232Th Decay Chains
214Bi,
a member of the 238U
decay chain, undergoes
beta decay to 214Po emitting
a 2.447 MeV photon.
Concentration of 0.03 ppb in
ultra-pure titanium.
208Tl,
a member of the
232Th decay chain
undergoes beta decay to
208Pb emitting a 2.615 MeV
photon.
Concentration of 0.2 ppb in
ultra-pure titanium.
The radioactive decay of U-236 and thorium (Th-232) via Alpha (α):
n p
p
X
n  p 4
p 2
Y  He
4
2
2
(α, n) reaction
4
2
< 10 MeV
He2  npp X  npp2Y  n
 Low energy neutron output via α-n reactions should be
determined using an alpha bombardment of materials
 Gamma ray production was then calculated from the
given neutron flux
Prompt gamma rays occur when the excited
nucleus sheds energy to return to its ground state.
Delayed gamma rays occur after beta decay, when
the nucleus is still excited and returns to its ground
energy state.
 Alpha particles can be absorbed by low Z elements
 Once the alpha is absorbed neutrons can be
emitted/captured and produce problematic gammas
 Teflon is used to line the inner chamber because it reflects the
ultra- violet light produced during scintillation of Xenon
 Teflon produced from carbon, fluorine, Oxygen
 All significant contributors to alpha-neutron reactions
We calculate a flux of neutrons expected from the
natural decay of uranium and thorium within the
materials of the TPC.
Time Projection Chamber
Photo-Multiplier
Tubes
Incoming gamma ray
Interaction with Xe
Electron tracks
It’s neutrinoless
double beta decay!!
Electroluminescence signals
Main interactions of photons (-rays,
and X-rays) in xenon volume .
For a given incident photon energy, certain modes of
interaction are more likely than the others.
For instance, at 0.01 MeV, Photoelectric absorption is
roughly 1,000 times more likely to occur than Compton
scattering.
The landscape near the region of interest of interest.
A small quantity of U or Th will create a significant background.
The decay spectra will overlap the endpoint of 0Nββ
Gamma Interference Scenarios
d 1 2 2
=  rc P( E , ) 2 [ P( E ,  )  P( E ,  ) 1  1  cos  ]
d 2
Gives the differential cross section for Compton scattering,
Where Eγ is the incident photon energy,
θ is the photon scattering angle,
α = fine structure constant,
rc = Compton radius,
d  me c 2
= 2 d
dEs
Es
And, Finally
P( E , ) =
1
1
E
me c
2
(1  cos  )
Gives a relationship between the solid angle and the scattered
photon energy, where Es is the scattered photon energy.
d d 
d me c 2
 =

dEs = 
 2 dEs d
d  dEs
d  Es
Giving the cross section for a range of scattered photons resulting from the
Compton scattering of a given incidental photon.
2
Differential Compton Cross Section ddEs (cm )
Differential Compton Cross Section vs Scattered Photon Energy
4.00E-030
In 136Xe medium at 20 bar pressure,
3.50E-030
2.615 MeV Initial Photon
2.447 MeV Initial Photon
3.00E-030
over the energy range of concern,
2.50E-030
d
dE s
2.00E-030
1.50E-030
1.00E-030
5.00E-031
0.0
0.5
1.0
1.5
2.0
Scattered Photon Energy (MeV)
The upper limit values
for the rate of
occurrence of each
culprit scenario
constituting a gamma
ray background events
2.5
3.0
follows an inverse exponential trend
• Determine contamination levels in materials
• Calculate production of α-particles that can
undergo α-n reactions
• Calculate number of neutrons produced via α-n
• Determine flux of neutrons
• Identify problematic gamma rays.
•All calculations for alpha and neutron flux depend
directly on the mass of the materials involved in
TPC.
Examination of region of interest: ±100 keV from the Q-value
Using 1 event per year as a maximum allowable background
Calculated the flux of neutrons required to produce maximum background for a specific gamma
R = N F R = rate of events, N = number of atoms, σ = Cross section, F = flux
Display of the gamma
rays from neutron
capture of the
isotopes of Xe, Ti, and
the other TPC
elements. Prompt
gamma rays occur
when the excited
nucleus sheds energy
to return to its
ground state. Delayed
gamma rays occur
after beta decay, when
the nucleus is still
excited and returns to
its ground energy
state.
 Different gamma rays cataloged , the rates and probabilities of






the most
problematic ones were identified for those within ROI
An upper limit of approx 100 neutrons are expected to be
produced per year from the materials of the TPC
We expect few important gamma event caused by radiation
from the materials to occur.
Most problematic gamma ray can be reduced to less than one
event per year by shielding the TPC with a 1 m water shield.
Thermal Neutron Capture will not be a major source of
background for the 100 kg xenon TPC
Future studies should include fast neutrons caused by
spallation of cosmic muons.
Funded by the US Department of Energy (DOE) and the Arkansas
Space Grant Consortium.
Thanks to Azriel Goldschmidt LBNL for insight and guidance.
INBRE 2010
Detection of Oνββ decay
 Must use an isotope that is energetically forbidden to decay
through single beta decay or the singles will dominate the
results of any experiment.
 This experiment uses xenon 136:
136Xe
→ 136Ba + 2e-
 Has relatively high Q-value of 2458 KeV
Oνββ Signal: 2 electrons, E1+ E2= Q
Experimental signatures:
• Two e- from same place
at the same time
• Daughter (Z+2,A) nuclei
appears
• The sum of e- kinetic
energy equals to Q
Contamination
Contamination values were measured by several
sources and used to calculate alpha production.
•U-238 in Ti:
3.00 g×10-11
•Th-232 in Ti:
20.00 g×10-11
•U-238 in PTFE:
1.00 g×10-11
•Th-232 in PTFE:
0.54 g×10-11
Yield Values
Neutron yield values were measured by
bombardment of a target material by a beam of
6.5 MeV alpha particles (neutron/106 α)
•Oxygen:
0.132
•Carbon:
0.252
•Fluorine:
17.95
Final neutron yields were calculated to be
Prototype Xenon Time Projection Chamber
1 kg 136Xe gas at 10-20 bar pressure
~ 9 liters volume gas
Testing in anticipation of a full scale,
100 kg xenon mass TPC capable of
detecting neutrinoless double beta
decay
Will determine plausibility of high
pressure xenon gas TPC.
Under construction and soon to be
ready for preliminary testing
M. Hawron, Southern Arkansas University
 First line of shielding is the Earth,
the detector will be located deep
underground to limit the number of
cosmic ray muons and high energy
neutrons from muon spallation.
 Passive shielding: Radiopure TPC
 Active Veto Shielding
Canfranc Underground Laboratory
Proposed 100 Kg HPXe-136 TPC
Main Parameters
n ~ 10-6 n/cm2  ~ 2 x 10-2 g/cm2 s
 ~ 2 x 10-7 m/cm2 s
Current Progress:
Prototype detector nearing completion at Lawrence Berkeley
National Laboratory
Once completed, tested, calibrated, this detector will explore the
energy resolution capabilities of 136Xe.
Prototype Time Projection Chamber
at LBNL (1 kg Xenon at 20 atm)
Event topology – TPC can track events
that occur with it.
PMT Plane
P. Grant, Southern Arkansas University
NEXT Collaboration: Neutrino Experiment with Xenon
TPC. Funded 100kg 136Xe TPC to be developed and
built at the Canfranc Underground Laboratory
Shielding from low energy
neutrons that occur from
(α, n) reactions and
naturally occurring fission
in the rocks around the
detector is required
Interest is in the low energy
neutron
Canfranc Underground Laboratory
Main Parameters
n ~ 10-6 n/cm2  ~ 2 x 10-2 g/cm2 s
 ~ 2 x 10-7 m/cm2 s
 Neutrons: Have no charge and do not interact with matter via the
electromagnetic force, they are a Baryon and being such is acted on by the
strong nuclear force only, hence difficult to identify within detectors and pose a
real problem as background for any dark matter or Oνββ experiment.
 While one can minimize the internal backgrounds by choosing radiopure
components, there will always be an external background, which comes mainly
from the laboratory walls, but also from underground muons and neutron
activation.
 The radioactive decay of U-236 and thorium (Th-232) via Alpha (α):
n p
p
X
n  p 4
p 2
Y  He
4
2
2
(α, n) reaction
4
2
< 10 MeV
He2  npp X  npp2Y  n
 Even minute quantities of U or Th will constitute a significant background.
 On the Earth’s surface, most neutrons arise from the hadronic component of
cosmic-rays. Muons spallation give rise to secondary neutrons in shallow
underground laboratories significantly contributing the total neutron flux. In
deep underground laboratories, however, the neutron flux is over beared by
(α; n) reactions and fission neutrons from surrounding rocks
P. Grant, Southern Arkansas University
INBRE 2010
Thermal Neutron
Capture
The captured neutron excites the nucleus and through the production of
prompt gamma rays or delayed gamma rays from beta decay returns the
nucleus to it stable energy level.
Thermal Neutrons are captured by atoms in all the materials making up the TPC
Considers titanium (cp-1, 484.9 kg) or stainless steel (316, 429.9 kg) pressure vessel
100 kg 136Xe enriched to 80%, and 151.8 kg of Teflon (PTFE) that will line the
inside of the pressure vessel
INBRE 2010
Gamma rays produce from Neutron Capture in stable isotopes of Xe and TPC
Isotope
Xe-124
Xe-124
Xe-124
Xe-124
Xe-128
Xe-128
Xe-128
Xe-128
Xe-128
Xe-128
Xe-128
Xe-129
Xe-129
Type Energy (Kev) Cross section (b)
delayed
111.3
2.70E-03
delayed
141.4
9.10E-04
prompt
223.7
5.00E-04
prompt
335.46
5.40E-03
delayed
39.578
6.90E-04
delayed
196.56
4.20E-04
prompt
278.56
2.50E-03
prompt
282.05
3.90E-03
prompt
318.18
4.60E-03
prompt
321.7
1.10E-03
prompt
403.1
1.06E-02
prompt
470.09
1.40E-02
Prompt
510.33
3.30E-01
PGAA K₀
6.20E-05
2.10E-05
1.20E-05
1.20E-04
1.60E-05
9.70E-06
5.00E-08
9.00E-05
1.06E -4
2.50E-05
2.40E-04
3.27E-04
7.62E-03
Half Life
56.9 s
56.9 s
8.88 d
8.88 d
All thermal neutron capture data was obtained from the LBNL isotopes project
Over 3500 problematic gamma rays catalogued
Includes all naturally occurring isotopes of the elements that make up the
building materials
P. Grant, Southern Arkansas University
INBRE 2010
Examination of region of interest: ±100 keV from the Q-value
Using 1 event per year as a maximum allowable background
Calculated the flux of neutrons required to produce maximum background for a specific gamma
R = N F R = rate of events, N = number of atoms, σ = Cross section, F = flux
Masses of materials
used
Titanium = 484.9 kg
Stainless = 429.9 kg
PTFE = 151.8 kg
80% 136Xe = 100 kg
Used natural
abundances in the
calculations except
with xenon in which
80 kg is 136Xe and the
remaining mass is
distributed at natural
abundance for the
other isotopes
Calculating neutron water shield to reduce the natural low energy neutron flux from the
rocks in the underground laboratory to levels that avoid contamination of the results.
1
I
n = ln( )
x
I0
μn= neutron attenuation coefficient of water, .1 cm-1
Χ = thickness of the water shield
I = desired flux
This neutron flux from the rock at
I0 = natural flux
Canfranc Laboratory that was used here
is 3.82 x 10-6 cm-2s-1as reported for the
IGEX-DM dark matter experiment
A 94 cm water shield is needed to
reduce the natural neutron flux at
Canfranc to less than
3.07 X 10-10 cm-2s-1
P. Grant, Southern Arkansas University
INBRE 2010
Conclusion
Most problematic gamma ray can be reduce to less than one event per year by shielding the
TPC with a 1 m water shield.
Means a low cost shield can negate the effects from thermal neutron capture
But what about neutron flux produced inside the water shield from the materials making
up the TPC.
Calculated by another team member to be 4.45 x 10-11 cm-2s-1
Adds a total of .15 events per year of the most problematic gamma
Σ
Of the conducted research boils down to
Thermal Neutron Capture will not be a major source
of background for the 100 kg xenon TPC
P. Grant, Southern Arkansas University
INBRE 2010
Further Research
As more materials are selected to make up all the components the thermal
neutron capture will also have to be evaluated.
Investigate the gammas above the Q-value
They may deposit only part of their energy in the TPC.
P. Grant, Southern Arkansas University
INBRE 2010
Further Research
As more materials are selected to make up all the components the thermal neutron capture will
also have to be evaluated.
Investigate the gammas above the Q-value
They may deposit only part of their energy in the TPC.
Background from Low Energy Neutrons in a High
Pressure Xenon Time Projection Chamber for Neutrinoless
Double Beta Decay
What is Neutrinoless Double Beta Decay?
M  A, Z   D  A, Z  2  2e
Why do physicist attempt to look for this extremely rare event?
It would show directly that the electron
neutrino is its own antiparticle (i.e. Majorana).
Indicate lepton number
violation, which is an important
requirement to explain the
current matter antimatter
asymmetry in the universe.
Allow for the absolute
mass of the neutrino and
the neutrino mass
hierarchy to be
determined
Why do physicist attempt to look for this extremely rare event?
It would show directly that the electron
neutrino is its own antiparticle (i.e. Majorana).
Indicate lepton number
violation, which is an important
requirement to explain the
current matter antimatter
asymmetry in the universe.
Allow for the absolute
mass of the neutrino and
the neutrino mass
hierarchy to be
determined
Detection of O-νββ decay
Must use an isotope that is energetically forbidden to decay through single beta decay or the
singles will dominate the results of any experiment.
This experiment uses xenon 136
Naturally occurring concentration of
136Xe
Easy to enrich to higher concentrations
Has relatively high Q-value of 2480 keV
is 8.9 percent
Eliminating or reducing background radiation levels that could
contaminate results.
Place detector deep underground – shields the detector from muons and high energy neutrons
from muon spallation.
Event topology – TPC can track events that occur with it.
Muon Veto for extremely high energy muons that penetrate deep into the earths surface.
Shielding from naturally occurring gamma sources i.e. uranium 238, thorium 232
Selecting low activity
materials for detector
construction
Shielding from low energy
neutrons that occur from
(α_n) reactions and naturally
occurring fission in the rocks
around the detector
Current Situation:
Prototype detector nearing completion at Lawrence Berkeley National Lab
Once completed, tested, calibrated, this detector will explore the energy resolution capabilities
of 136Xe.
NEXT Collaboration: Neutrino Experiment with Xenon TPC
Funded 100kg 136Xe TPC to be developed and built at the Canfranc Underground Laboratory
2500 w.m.e. depth
Gamma rays produce from Neutron Capture in stable isotopes of Xe and TPC
Isotope
Xe-124
Xe-124
Xe-124
Xe-124
Xe-128
Xe-128
Xe-128
Xe-128
Xe-128
Xe-128
Xe-128
Xe-129
Xe-129
Type Energy (Kev) Cross section (b)
delayed
111.3
2.70E-03
delayed
141.4
9.10E-04
prompt
223.7
5.00E-04
prompt
335.46
5.40E-03
delayed
39.578
6.90E-04
delayed
196.56
4.20E-04
prompt
278.56
2.50E-03
prompt
282.05
3.90E-03
prompt
318.18
4.60E-03
prompt
321.7
1.10E-03
prompt
403.1
1.06E-02
prompt
470.09
1.40E-02
Prompt
510.33
3.30E-01
PGAA K₀
6.20E-05
2.10E-05
1.20E-05
1.20E-04
1.60E-05
9.70E-06
5.00E-08
9.00E-05
1.06E -4
2.50E-05
2.40E-04
3.27E-04
7.62E-03
Half Life
56.9 s
56.9 s
8.88 d
8.88 d
All thermal neutron capture data was obtained from the LBNL isotopes project
3500 gamma rays catalogued
Includes all naturally occurring isotopes of the elements that make up the building materials
Examination of region of interest: ±100 keV from the Q-value
Using 1 event per year as a maximum allowable background
Calculated the flux of neutrons required to produce maximum background for a specific gamma
R = N F
R = rate of events, N = number of atoms, σ = Cross section, F = flux
Masses of materials
used
Titanium = 484.9 kg
Stainless = 429.9 kg
PTFE = 151.8 kg
80% 136Xe = 100 kg
Used natural
abundances in the
calculations except
with xenon in which
80 kg is 136Xe and the
remaining mass is
distributed at natural
abundance for the
other isotopes
Calculating neutron water shield to reduce the natural low energy neutron flux from the rocks
in the underground laboratory to levels that avoid contamination of the results.
1
I
n = ln( )
x I0
μn= neutron attenuation coefficient of water, .1 cm-1
Χ = thickness of the water shield
I = desired flux
I0 = natural flux
This neutron flux from the rock at
Canfranc Laboratory that was used here is
3.82 x 10-6 cm-2s-1as reported for the
IGEX-DM dark matter experiment
A 94 cm water shield is needed to
reduce the natural neutron flux at
Canfranc to less than
3.07 X 10-10 cm-2s-1
Using the Research findings
Most problematic gamma ray can be reduce to less than one event per year by shielding the TPC
with a 1 m water shield.
Means a low cost shield can negate the effects from thermal neutron capture
But what about neutron flux produced inside the water shield from the materials making
up the TPC.
Calculated by another team member to be 4.45 x 10-11 cm-2s-1
Adds a total of .15 events per year of the most problematic gamma
Σ
Of the conducted research boils down to
Thermal Neutron Capture will not be a major source of
background for the 100 kg xenon TPC
Double Beta Decay
Two neutrino Double
Beta Decay has been
well documented first
observed in 1987
Neutrinoless Double Beta Decay
This supports idea of the Majorana nature of
Neutrinos
 Decay Energy Spectrum
2 neutrino
0 neutrino
Region of
Interest
Attenuation (μm) is the loss of intensity of a beam travelling through a
medium. μm has units of cm2/g
Attenuation length (λ) is the length required for 63% of the intensity to
drop. λ has units of cm.
Intensity of the beam at a given length is equivalent to the probability of a
single particle passing through said length without attenuation.
μm varies with the medium in question.
λ varies with the medium in question, and the density of the medium.
Attenuation length is given by
=
1

1
m 
and the probability of a
single particle passing through the medium is given by
M. Hawron, Southern Arkansas University
P = 1  e x / 
d 1 2 2
=  rc P( E , ) 2 [ P( E ,  )  P( E ,  ) 1  1  cos  ]
d 2
Gives the differential cross section for Compton scattering,
Where Eγ is the incident photon energy,
θ is the photon scattering angle,
α = fine structure constant,
rc = Compton radius,
d  me c 2
= 2 d
dEs
Es
And, Finally
P( E , ) =
1
1
E
me c
2
(1  cos  )
Gives a relationship between the solid angle and the scattered
photon energy, where Es is the scattered photon energy.
d d 
d me c 2
 =

dEs = 
 2 dEs d
d  dEs
d  Es
Giving the cross section for a range of scattered photons resulting from the
Compton scattering of a given incidental photon.