Abdel Bachri, Southern Arkansas University

In collaboration with Perry Grant, Martin Hawron, Clayton Martin, and Azriel

Goldschmidt

physica scripta, 2014 under review

High Pressure Xenon TPC Radiation Background for Neutrinoless Double Beta Decay Experiment

Arkansas Space Grant ConsortiumHot Springs, AR

April 7, 2014Abdel Bachri, Southern Arkansas University

Double bb decay

e eNucleus (A, Z) Nucleus (

2 double-bet

A, Z+2) +

a decay 2

+ + +

:

e e

bb

Allowed in the Standard Model (conserves Lepton #)

Maria Goeppert-Mayer

Proposed in 1935 and observed in 1986 in several isotopes:

typical T1/2=1018-1021 years

Neutrinoless bb decay

Nucleus (A, Z) N

0 double-beta deca

ucleus (A, Z+2) +

y 0 :

+e e

bb

Wendell. H Furry

Proposed in 1938 by Furry. The rarest decay in nature, yet to be observed. Experiments currently taking data: COBRA, CUORICINO , DCBA, EXO, GERDAMOON, KamLAND-Zen, MAJORANA, NEXT, SNO+, SuperNEMO

Golden plated channel:

a) 2 electronsb) Eb1+ Eb2=Qbb = 2.458 MeV

22 231/2 10 10 yearsT

Importance of bb StudyThe neutrinoless double beta decay detection would

give further insight to the nature of the neutrinoAre 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

0bb is the key experiment for neutrino physics

“NEXT”- Collaboration136Xe → 136Ba + 2e-

Neutrino Experiment with a Xenon Time Projection ChamberCanfranc 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: 52.4 cm• Length of Cylinder: 104.8 cm• Cylinder Thickness: 1.5 cm• End-Cap Thickness: 0.75 cm• Total Mass of Ti: 490 kg• Total Mass of PTFE: 151.82 kg• PTFE Thickness: 1.0 cm

High Pressure Xenon Time Projection Chamber (TPC)

Detects neutrinoless double beta decay (0νββ) electrons.

Measures energy released by interaction with xenon.

Photomultiplier tubes register the light emitted during scintillation.

A tracking system will look for the signature electron paths of 0νββ

Unfortunately, many forms of ionizing radiation can cause a signal in the TPC, such as cosmic rays, thermal neutrons, and gamma rays.

Types of Background Anticipated1) 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

2) 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

5) Importance of Energy resolution and background suppression

6) 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.

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.

238U and 232Th Decay Chains

The radioactive decay of U-236 and thorium (Th-232) via Alpha (α): (α, n) reaction

Low Energy Neutron Background

4 22 2

n p n pp pHe X Y n

4 4 22 2

n p n pp pX Y He

< 10 MeV

Why are α-n reactions important?Low energy neutron output via α-n reactions should

be determined using an alpha bombardment of materialsGamma 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-Neutron (α-n) ReactionsAlpha particles can be absorbed by low Z

elementsOnce the alpha is absorbed neutrons can be

emitted/captured and produce problematic gammasTeflon is used to line the inner chamber because it

reflects the ultra- violet light produced during scintillation of XenonTeflon 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.

Gamma Ray Background Interference

Incoming gamma ray

Electron tracks

Interaction with Xe

Time Projection Chamber

Electroluminescence signals

Photo-Multiplier Tubes

It’s neutrinoless double beta decay!!

Photon – Matter Interactions in XenonMain 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

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,

Calculations

2

2e

s ss s

m cd d ddE dE dd dE d E

= =

2 2 2 11 ( , ) [ ( , ) ( , ) 1 cos ]2 c

d r P E P E P Ed =

2

2e

s s

m cd ddE E

=

2

1( , )1 (1 cos )

e

P EEm c

=

Gives a relationship between the solid angle and the scattered photon energy, where Es is the scattered photon energy.

And, Finally

Giving the cross section for a range of scattered photons resulting from the Compton scattering of a given incidental photon.

Problematic Gamma Rays

0.0 0.5 1.0 1.5 2.0 2.5 3.0

5.00E-031

1.00E-030

1.50E-030

2.00E-030

2.50E-030

3.00E-030

3.50E-030

4.00E-030

2.615 MeV Initial Photon2.447 MeV Initial Photon

Diffe

rent

ial C

ompt

on C

ross

Sec

tion

d

dEs (c

m2 )

Scattered Photon Energy (MeV)

Differential Compton Cross Section vs Scattered Photon Energy

In 136Xe medium at 20 bar pressure,

over the energy range of concern,

follows an inverse exponential trend

s

ddE

The upper limit values for the rate of occurrence of each culprit scenario constituting a gamma ray background events

Neutron Induced Gamma Background

• 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-valueUsing 1 event per year as a maximum allowable backgroundCalculated 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.

Conclusion & OutlookDifferent gamma rays cataloged , the rates and probabilities

of the mostproblematic ones were identified for those within ROIAn upper limit of approx 100 neutrons are expected to be

produced per year from the materials of the TPCWe 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.

Acknowledgements

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 decayMust 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

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 QbbOνββ Signal: 2 electrons, Eb1+

Eb2= Qbb

ContaminationContamination 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

α – particle production rate

Yield ValuesNeutron 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: 17.95Final 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

Experimental Considerations: Shielding

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 TPCActive Veto ShieldingMain Parametersn ~ 10-6 n/cm2 ~ 2 x 10-2 g/cm2 s ~ 2 x 10-7 m/cm2 s

Canfranc Underground LaboratoryProposed 100 Kg HPXe-136 TPC

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.

NEXT Collaboration: Neutrino Experiment with Xenon

TPC. Funded 100kg 136Xe TPC to be developed and built at the Canfranc Underground Laboratory

Prototype Time Projection Chamber at LBNL (1 kg Xenon at 20 atm)

Event topology – TPC can track eventsthat occur with it.

PMT Plane

P. Grant, Southern Arkansas University

Experimental Considerations: Shielding

Main Parametersn ~ 10-6 n/cm2 ~ 2 x 10-2 g/cm2 s ~ 2 x 10-7 m/cm2 s

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

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):

(α, n) reaction

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

Low Energy Neutron Background

4 22 2

n p n pp pHe X Y n

4 4 22 2

n p n pp pX Y He

< 10 MeV

P. Grant, Southern Arkansas University INBRE 2010

Thermal Neutron Capture

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

Low Energy Neutron Background

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.

INBRE 2010

Isotope Type Energy (Kev) Cross section (b) PGAA K

₀

Half LifeXe-124 delayed 111.3 2.70E-03 6.20E-05 56.9 sXe-124 delayed 141.4 9.10E-04 2.10E-05 56.9 sXe-124 prompt 223.7 5.00E-04 1.20E-05Xe-124 prompt 335.46 5.40E-03 1.20E-04Xe-128 delayed 39.578 6.90E-04 1.60E-05 8.88 dXe-128 delayed 196.56 4.20E-04 9.70E-06 8.88 dXe-128 prompt 278.56 2.50E-03 5.00E-08Xe-128 prompt 282.05 3.90E-03 9.00E-05Xe-128 prompt 318.18 4.60E-03 1.06E -4Xe-128 prompt 321.7 1.10E-03 2.50E-05Xe-128 prompt 403.1 1.06E-02 2.40E-04Xe-129 prompt 470.09 1.40E-02 3.27E-04Xe-129 Prompt 510.33 3.30E-01 7.62E-03

Gamma rays produce from Neutron Capture in stable isotopes of Xe and TPC

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 materialsP. Grant, Southern Arkansas University INBRE 2010

Examination of region of interest: ±100 keV from the Q-valueUsing 1 event per year as a maximum allowable backgroundCalculated 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 kgStainless = 429.9 kgPTFE = 151.8 kg80% 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.

0

1 ln( )nI

x I =

μn= neutron attenuation coefficient of water, .1 cm-1 Χ = thickness of the water shield I = desired fluxI0 = 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

Water shield to limit neutron background

P. Grant, Southern Arkansas University INBRE 2010

ConclusionMost 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

Thermal Neutron Capture will not be a major source of background for the 100 kg xenon TPC

Σ Of the conducted research boils down to

P. Grant, Southern Arkansas University INBRE 2010

Further ResearchAs 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 DecayWhat is Neutrinoless Double Beta Decay?

, , 2  2M A Z D A Z e

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 decayMust 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 is 8.9 percent

Easy to enrich to higher concentrations

Has relatively high Q-value of 2480 keV

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 LabOnce completed, tested, calibrated, this detector will explore the energy resolution capabilities of 136Xe.

NEXT Collaboration: Neutrino Experiment with Xenon TPCFunded 100kg 136Xe TPC to be developed and built at the Canfranc Underground Laboratory2500 w.m.e. depth

Isotope Type Energy (Kev) Cross section (b) PGAA K

₀

Half LifeXe-124 delayed 111.3 2.70E-03 6.20E-05 56.9 sXe-124 delayed 141.4 9.10E-04 2.10E-05 56.9 sXe-124 prompt 223.7 5.00E-04 1.20E-05Xe-124 prompt 335.46 5.40E-03 1.20E-04Xe-128 delayed 39.578 6.90E-04 1.60E-05 8.88 dXe-128 delayed 196.56 4.20E-04 9.70E-06 8.88 dXe-128 prompt 278.56 2.50E-03 5.00E-08Xe-128 prompt 282.05 3.90E-03 9.00E-05Xe-128 prompt 318.18 4.60E-03 1.06E -4Xe-128 prompt 321.7 1.10E-03 2.50E-05Xe-128 prompt 403.1 1.06E-02 2.40E-04Xe-129 prompt 470.09 1.40E-02 3.27E-04Xe-129 Prompt 510.33 3.30E-01 7.62E-03

Gamma rays produce from Neutron Capture in stable isotopes of Xe and TPC

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-valueUsing 1 event per year as a maximum allowable backgroundCalculated 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 kgStainless = 429.9 kgPTFE = 151.8 kg80% 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.

0

1 ln( )nI

x I =

μn= neutron attenuation coefficient of water, .1 cm-1 Χ = thickness of the water shield I = desired fluxI0 = 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 findingsMost 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

Thermal Neutron Capture will not be a major source of background for the 100 kg xenon TPC

Σ Of the conducted research boils down to

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

bb 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 and the probability of a

single particle passing through the medium is given by

Calculations

1 1

m

=

/1 xP e =

M. Hawron, Southern Arkansas University

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,

Calculations cont.

2

2e

s ss s

m cd d ddE dE dd dE d E

= =

2 2 2 11 ( , ) [ ( , ) ( , ) 1 cos ]2 c

d r P E P E P Ed =

2

2e

s s

m cd ddE E

=

2

1( , )1 (1 cos )

e

P EEm c

=

Gives a relationship between the solid angle and the scattered photon energy, where Es is the scattered photon energy.

And, Finally

Giving the cross section for a range of scattered photons resulting from the Compton scattering of a given incidental photon.