A R EAL T IME L IQUID X ENON P URITY M ONITOR
{ D YLAN T EMPLES , D R . M ICHELLE D OLINSKI }
N EUTRINOS & D OUBLE -B ETA D ECAYS
2n0 → 2p+ + 2e− + 2ν̄e
R ESULTS
L IQUID X ENON -136
• Neutrinos are abundant in the universe, yet rarely interact
with matter, and are difficult to detect.
• The mass relationships of the different varieties of neutrinos is known, but the absolute mass of each is unknown.
• It has not been determined whether neutrinos are Majorana
particles (meaning they are their own antiparticle).
• Some atoms (such as 136 Xe) are capable of underoging double beta decay (2νββ),
(2)
• This is the only decay mode allowed for 136 Xe, with a halflife ∼ 1021 years [1].
•
136
Xe can only decay through double beta decay due to energy restrictions.
• Energetic, charged particles travelling through liquid
xenon causes photon emission (scintillation) and electron
emission (ionization).
• The number of particles emitted is proportional to the energy of the charged particle.
• Energies calculated from these two channels can be combined to determine an absolute energy scale for the energy
deposited in the LXe.
2n0 → 2p+ + 2e−
(3)
• Observation of this decay would confirm that neutrinos are
their own antiparticle, and allows the neutrino mass to be
calculated (depending on the decay mechanism).
• Current measurements show the 0νββ half-life of 136 Xe, if
it exists, is > 1025 years [3].
• Easily adaptible design for various standard plumbing
sizes.
• Purity monitor components selected are easily available
and low-cost.
• Linear density of shaping rings for ideal collection efficiency is around 2 rings/cm [4].
• Drift cell length can be tuned to be sensitive to various lifetime ranges. A drift length similar to EXO-200’s is used to
achieve similar lifetimes.
• 3D models created in S OLID W ORKS and manufactured out
of ABS plastic using a 3D printer
Figure 4: A rendering of the assembled shaping ring array and housing,
created using S OLDI W ORKS.
EXO-200
Figure 2: Feynman diagrams of the 2νββ and 0νββ decays [2].
• Nuclei capable of undergoing 2νββ could theoretically undergo a neutrinoless double beta decay mode, 0νββ, which
requires the exchange of a Majorana neutrino,
D REXEL U NIVERSITY D EPT. OF P HYSICS
Figure 1: Diagram of EXO-200 time projection chamber configuration
showing collection mechanisms for ionization and scintillation channels.
• The EXO-200 detector is searching for the 0νββ signal from
136
Xe,
136
Xe →136 Ba + 2e−
(1)
• Conflat vacuum system gaskets were selected to be the
shaping rings. The smallest available size was selected to
minimize the amount of LXe needed.
• A resistive voltage divider is required to provide each ring
with the voltage necessary to create a linearly decreasing
electric potential.
• High-tolerance, SMT resistors were selected to create this
divider inside the shaping rings.
• An electrostaic simulation of the detector geometry was run
in COMSOL M ULTIPHYSICS to determine exact potential
and collection efficiency.
• A 2D cross-section of the detector was used due to cylindrical symmetry.
Figure 5: Results of COMSOL silmulation showing electric potential
(top) and field lines (bottom).
• As electrons drift through the LXe they can be captured by
electronegative impurities, such as O2 and N2 .
• Energy resolution depends on the number of electrons captured from a decay event.
• Maintaining high energy resolution is important to distinguish the signal of 0νββ from that of 2νββ.
• Since the design was finalized, the only free parameter to
effect collection efficiency is photocathode diameter.
• Various photocathode diameters of interest were simulated,
and collection efficiencies determined, by counting field
lines that terminate on the anode.
• The results of this were fit to a cubic polynomial, shown in
Figure 6.
• The diameter can be adjusted by limiting the spot size illuminated by the UV LED. Efficiencies above 80% can be
achieved by limiting the spot size to less than a 1 cm diameter.
T HEORY OF O PERATION
• LXe purity can be directly monitored for electronegative
impurities by knowing precise electron lifetimes.
• These lifetimes can be used as a position-dependent correction factor when calculating deposited energy in EXO-200.
• Using the photoelectric effect, a UV LED can be used to liberate a known number of electrons, N0 from a photocathode.
• Shaping rings create a uniform electric field to guide electrons to the anode.
• To create this field, a linearly decreasing electric potential is
required across the drift region
• By attaching each evenly spaced shaping ring with resistors
of the same value, each ring gets the correct voltage.
• This gives the number of electrons that were not attenuated
as they crossed the drift region, Ne .
• The electron lifetime can be caluclated by knowing the parameters which define the drift region, and how electrons
are attenuated.
Figure 6: Collection efficiencies for various photocathode diameters are
plotted. Cubic fit shown in green, and residuals are shown below.
Figure 3: Schematic representation of purity monitor design
O UTLOOK
• The attenuation can be described by
Ne = N0 e
−t/τ
(4)
where τ is the lifetime.
• The time of flight, t, is a function of drift length, l, and electron drift speed vβ ,
~
t = l/vβ (E)
(5)
• Work has begun on the development of a data acquisition
system.
• A housing for the anode and associated components must
be designed.
• The completedpurity monitor system must be evaluated in
a vacuum system to establish a baseline before moving on
to LXe testing.
R EFERENCES
[1] N. Ackerman et al. Observation of Two-Neutrino Double-Beta Decay in
136 Xe with EXO-200. Rev. Mod. Phys., 80:481-516, 2008.
[2] F. Avignone, S. Elliot & J. Engel Double Beta Decay, Majorana Neutrinos, and
Neutrino Mass. Phys. Rev. Lett., 109 032505, Nov 2007.
[3] M. Auger et al. Search for Neutrinoless Double-Beta Decay in
EXO-200. Phys. Rev. Lett., 109 032505, Jul 2012.
136 Xe
with
[4] M. Jewell UV LED Based Purity Monitor for nEXO. Drexel University Senior
Thesis, May 2014.