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High-pressure Xenon Gas TPC for 0- -Decay
Search with Minimal Use of Photomultipliers
David Nygren
[email protected]
Physics Division, Lawrence Berkeley National Laboratory &
Fysikum, Stockholm University
1. Introduction
A search for 0-  decay in 136Xe requires a large sensitive mass, perhaps several
hundred kg to attain sensitivities appropriate to our knowledge of neutrino masses [1].
At the same time, superb energy resolution is desired as well as best possible particle
tracking to identify true events and reject backgrounds. In other words, a HPXe TPC
design for 0-  search must accomplish four purposes:
1. Support large active masses with a seamless, fully active fiducial surface.
2. Measurement of the primary ionization with near-intrinsic resolution.
3. Track daughter electrons to determine event topology and spatial origin.
4. Efficient detection of the weak primary scintillation to define a robust t0.
These challenges can be met with a high-pressure xenon gas electroluminescent (EL)
TPC. High-pressure xenon (HPXe) density is defined here to be  ≈ 0.05 g/cm3,
approximately 10 bars at normal temperature.1 A cylindrical TPC with volume given
by R = 50 cm radius and length L = 200 cm is assumed to illustrate feasibility,
determine channel counts, and define R&D needs. This generic TPC could be
“symmetric” or “single-ended”. The generic TPC holds an active mass of ~150 kg in
all cases. Present candidate approaches to realize a large-scale system appear to be:
1. Symmetric “conventional”. This HPXe TPC design was evaluated in [2,3],
and would employ dense arrays of PMTs in each readout plane. The PMT
arrays detect primary scintillation for the t0, secondary scintillation from EL
to measure energy, and also provide the tracking function. A symmetric
design is necessary to obtain the separated function capability (see below).
The walls are lined with highly reflective Teflon. An obvious criticism of this
concept is that the number of specialized, costly, PMTs needed to realize this
configuration is too large for interesting values of active mass. The number of
PMTs in a honeycomb array is N ~0.7 x (R/r)2, where the factor point 0.7
represents a “fill” factor. With an assumption that the PMT diameter r = 3 cm,
N ≈ 750 PMTs, ~1500 for both readout planes. In addition to high costs,
perhaps unacceptably high radioactive backgrounds originate from the PMT
arrays, which are in close proximity to the active volume. Because electron
tracks in xenon are highly convoluted, another criticism is that tracking with
PMTs and center-of-gravity algorithms will display inadequate performance.
Two-track resolution requires at least three pixels separation, and a 3 cm
diameter PMT is much too large compared to probable track overlap.
1
It is conceivable that an optimum HPXe density might be closer to  ≈ 0.1 g/cm3.
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2. Symmetric “WLS bars”. This TPC concept employs a large circumferential
array of wavelength-shifting scintillator (WLS) bars to detect both primary
scintillation and intense secondary scintillation, but the tracking function is
performed with other devices. A penalty must be paid in scintillation photon
detection efficiency, since a substantial fraction of the WLS light is not
captured. The optical sensitivity determines the minimum detectable primary
scintillation and hence the energy threshold. Presumably this is still OK, as
argued below. The basic geometry of concept 2 is sketched in figures 1 and 2.
Diameter: ~100 cm
Figure 1. End view of the cylindrical array of WLS bars. To reduce reflective losses,
the WLS bars and PMTs must be inside the clean xenon volume. The diameter of the
WLS bars here is ~5 cm and the number of WLS bars is 56. It may turn out that the
maximum possible PMT diameter is ~3 cm, and the number of WLS bars is ~100.
EL region A
EL region B
Tracking is
done here
Figure 2. Symmetric TPC concept 2 has a circumferential array of WLS bars for
detection of primary and secondary scintillation. PMTs (in red) are attached to each
end of the WLS bars. The event, shown as a wiggly track, generates primary
scintillation recorded by all WLS bars as a short pulse (~20 ns). Subsequently, the
track drifts into the EL region and generates a long secondary scintillation. The total
length between planes is 200 cm (vertical and horizontal scales differ).
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3. Asymmetric “single-ended”. This TPC concept maintains a separated
function design, but with only one anode plane to generate the EL. The
primary and secondary scintillation is detected by a single array of PMTs
behind the cathode plane. The PMT array is similar to concept 1 but less
dense. The choice for the density of PMTs can be made to realize a desired
energy threshold. The walls of the cylinder are lined with a highly
reflecting but non-specular material such as Teflon to maximize photon
detection efficiency, as in concept 1. Other devices provide the tracking
function, as in concept 2. This concept offers some simplifications and
reduced channel count. However, for the same mass and density, concept 3
requires a maximum drift distance twice as large as either of the
symmetric TPC concepts. This increases the HV needed as well as
diffusion.
Performance Considerations
Primary scintillation and t0
A robust t0 is needed to place an event in z, the drift direction. At the Q-value of 2480
keV, the total number of primary scintillation photons is approximately 3x104. A
number far above noise must be detected to provide a robust t0. The primary
scintillation is fast, occurring within ~20 ns. The primary scintillation detection
system must not introduce noise at a significant level relative to the number of
detected primary scintillation photons. Only a PMT-based detection appears capable
of providing both the needed sensitivity and very low noise at the single photoelectron
level.
Photon detection efficiency
An important performance parameter is the overall photon detection efficiency  . It
is reasonable to place a goal for  at 1%. In that case, about 300 photons would be
detected at the Q-value. If a primary scintillation threshold of ~10 photoelectrons is
assumed, most of the 2-  spectrum would be accessible.
For concept 1, I needed to maximize , since the EL production from a wire grid is
limited relative to that of parallel meshes (see below). I argued that the photon
detection efficiency  might be as high as  = 3%, but the estimate was very rough,
with an uncertainty factor of two.
For concept 2, the WLS bars subtend roughly half the total solid angle. The product
of solid angle (1/2), reflectivity loss (1/2), WLS loss (1/6), and QE of PMTs (1/4)
suggests  ≈ 1% for concept 2, and the number of photoelectrons at the Q-value is
~300. But the uncertainty is also probably a factor of 2.
For concept 3, the  calculation is quite elastic, since the value of  is a choice. If the
choice, for example, is  = 0.3%, then the number of photoelectrons at the Q-value is
~90. It appears that for any of the concepts, enough primary scintillation can be
detected to obtain an adequately robust t0.
However, the photon detection efficiency also enters in the energy resolution.
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Energy resolution
I have discussed the advantages of electroluminescence (EL) for best possible energy
resolution in previous papers [2,3], with a more detailed elaboration in [3].
The expectation for good energy resolution is based on a favorable Fano factor F =
0.15  0.02 in HPXe and low excess noise in EL. In the decay 136Xe  136Ba, whether
or not neutrinos are emitted, the Q-value energy of 2480 keV is released. The energy
resolution for 0-  mode is E/E = 2.35 x (FW/Q)-1/2. Together with WI = 1.13 x
W0 = 24.8 eV (this value includes the impact of recombination in HPXe), the total
number of primary ionization electrons liberated is NI = 2.48  106  24.8 = 1 x 105.
The intrinsic ionization fluctuation is only ~124 electrons rms. At the Q-value, the
intrinsic energy resolution goal is:
E/E = 2.9  10-3 FWHM.
(1)
To preserve this intrinsic resolution is a challenge. The detection process must
introduce minimal degradation. Resolutions of this level have not been achieved at
MeV energies in xenon.
To include the impact of noise and fluctuations in the detection process, a factor G is
introduced, with the reasonable assumption that that these effects are uncorrelated
with the intrinsic ionization fluctuations. The energy resolution becomes
E/E = 2.35((F + G)WI/Q)1/2
(2)
The goal is make G < F, or even much less than F, if possible. I showed in [3] that to
meet the minimum goal, G = F, each primary ionization electron must lead to at least
10 detected secondary scintillation (EL) photons. As the total number of primary
ionization electrons is ~1 x 105, the number of detected photons npe must be at least 1
x 106 at the Q-value. If G equals F, then
Q/Q = 4.2 x 10-3 FWHM.
(3)
In summary, for any concept, at least one million photons must be detected at the Qvalue to achieve the energy resolution goal. The corresponding dynamic range
requirement on signal processing is less demanding than one might imagine at first,
because the secondary signal is spread out over many tens of s.
Photon generation
There appear to be two options:
1. A plane of wires, similar to a standard MWPC, but with thicker wires;
2. Two parallel metallic meshes, separated by a small gap of a few mm.
Wire Plane
In [3], the wire plane configuration was evaluated and found to produce about 330
photons per electron. If  = 3%, as estimated for concept 1, this is an adequate source
of EL for concept 1.
Parallel Mesh
For concepts 2 and 3, to compensate for the lower photon detection efficiency , the
more conventional, uniform-field, parallel-mesh configuration to generate EL must be
used. Under constant, uniform E/p and total voltage V, the total EL gain  is the
number of photons generated per electron:
 = V/VPH
(4)
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where VPH is the average potential needed to generate one photon at the given E/p
value. An empirical formula, describing the EL gain  in xenon gas is given in [5]:
 = 140(E/p –0.83)p (UV photons/e-cm) x
E/p is in units of kV/(cm-bar), x is gap width in cm, and p is in bars.
(5)
From (5), VPH is about 9 volts for E/p of 4 in these units. As the first excitation level
of xenon is 8.32 eV, this indicates pleasingly high conversion efficiency in the
optimum E/p range. But good energy resolution can be obtained only in the range 3 <
E/p < 6.6 [6].
It seems prudent to assume that the number of photons needed in concept 2 is a factor
of ~3 greater than in concept 1. Hence, a gain of
 ≈1 x 103
(6)
is desired for concept 2. This corresponds, using (5) for E/p = 3 kV/(cm-bar), to a
gap width of ~3 mm at 10 bars. The HV needed across the gap is ~10 kV. The HV is
large and the gap small, so considerable care will be required to grade the field at the
edges.
For concept 3, with still lower  = 0.3%, the gain needed is even larger,
 ≈3 x 103
This corresponds for E/p = 3 kV/(cm-bar) to 30 kV and a 10 mm EL gap.
(7)
Tracking function
A large EL gap width is not necessarily benign, since the electrons spend more time
making light, smearing out the track details somewhat. To gain perspective on this, I
recommend reading the paper about the Italian-Dutch Beppo-SAX satellite, which
deployed a 5 bar EL TPC in space, quite successfully [7].
In [2,3], in addition to the energy measurement, the PMT arrays provided the tracking
function with an Anger camera concept, as in [7,8]. However, multiple scattering in
xenon is strong, leading to highly convoluted event topologies. The PMT diameter of
5 cm in [3] offers essentially no track-pair resolution since track segments are likely
to overlap in z well within such a distance. Most likely, simulations will show a clear
advantage in segmentation of tracking elements on a scale of 1 cm.
Diffusion during drift also influences the desired scale of detector segmentation. An
electric field in the range of E/p ≈ 0.066 V/cm-Torr, or 500 (1000) V/cm at  = 0.05
(0.1) g/cm3 may be near optimum. From  = (2kTx/eE)1/2 where T is electron
temperature, k is Boltzmann’s constant, x is drift distance, e is the electron charge, the
transverse diffusion for x = 100 cm and E = 1000 V/cm is
xy ≈ 7 mm rms
(7)
This is large but acceptable. The drift velocity  is approximately  ≈1 mm/s for
E/p = 0.066 V/cm-Torr. In the high-field EL region the drift velocity increases by
roughly a factor of 2 – 3 [7].
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The total event length for both electron tracks is approximately 300 mm, and the
ionization density along a track is about 333/mm. Consider first, a small segment of a
track parallel to z, as the ionization density along z for a track parallel to z is
independent of diffusion.
For concept 2, a 1 mm track segment of ne = 333 electrons drifts into the 3 mm EL
region and produces a burst of ne x  = 333 x 1000 ≈ 3.33 x 105 EL photons. As the
drift velocity increases within the gap, the photon burst lasts for ~1.5 s. In other
words, each electron resides in the EL zone for about 1.5 s, and the density within
the EL zone corresponds to a ~1 mm segment of track (for the track parallel to z).
A detection element of 1 cm2 at a distance of 1 cm from the EL region will subtend a
solid angle fraction of ~ 0.04. About 13,000 photons/s will impinge on such a
detection element, a fairly large signal. Which detection technique is optimum for the
task?
1. Gas-filled photodetectors
It may be possible to exploit gas-filled photon-sensitive avalanche detectors such as
those based on CsI or CsI-TMAE photocathodes. However, sealing and operating an
array of these seems to be quite daunting, especially if the pressure inside is not the
same as the ambient. Most gas-filled detectors don’t work as well at higher than
atmospheric pressure, and photon-sensitive gas detectors generally work better at
pressures much lower than 1 bar. I would rate this option as difficult, primarily due to
challenges of perfect sealing.
2. Silicon p-i-n photodiode
A silicon p-i-n photodiode of 1 cm2 at a distance of 1 cm from the EL plane with an
assumed 25% QE will generate a photocurrent I ≈ 0.5 na from this burst. This signal
will be useful only if detector dark current and electronic noise are two orders of
magnitude smaller. Current commercial devices from Hamamatsu specifications
indicate dark currents of ~4 na/cm2; these devices would need to be cooled to
approximately -30 to reach the 10 pa range, an impossible temperature for a large
HPXe TPC. Silicon p-i-n photodiodes of this type are hence inappropriate. Gain is
needed.
3. Avalanche photodiode (APD)
These devices exist in large format (Advanced Photonix) and have been used in EXO,
in direct contact with LXe. The gain is about 100 - 200, leading to signals of ~20 na.
But the dark currents at room temperature are much larger, in the range of 400 na.
These devices cannot be used for the tracking function.
4. Small diameter WLS bars
With an assumption of 1 cm2 exposure, a small diameter WLS bar, maybe 4 - 6 mm
diameter, would convert the signal of 1.3 x104 UV photons/s/mm of track to a signal
of ~1 x 103 visible photons at the end of the bars. This signal could be easily detected
by multi-Geiger cell silicon PMTs (SiPM). The Barcelona group’s involvement with
T2K provides considerable experience with the Hamamatsu Multi-Pixel Photon
Counters, a commercial SiPM device. The WLS bars could be oriented in a
conventional x-y plane, with a larger or more tightly packed second layer, so as to
yield a signal of similar quality to the top layer. Since the signals are large, it is even
conceivable to use a three-layer x-u-v configuration to minimize ambiguities in
reconstruction; but this complexity may not be necessary, since pulse height
information helps to resolve x-y tracking ambiguities. The construction of the small
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diameter WLS bars would be similar to the larger circumferential WLS bars of
concept 2, except that instead of PMTs, SiPMs would be attached at both ends. It is,
likely that WLS bars inside quartz tubes would be completely immune to large overpressure and swelling from xenon intrusion. Radioactivity should be very low. The
signals of interest for tracking are always presumed to be significantly larger than the
SiPM noise, which is in the range of 1 MHz but primarily at 1 photoelectron level; a
threshold at a level corresponding to 4 – 6 photoelectrons should be adequate to reject
almost all noise.
5. Pixels based on SiPM
As suggested by JJ, it is possible to construct a readout plane of 1 cm2 pixels using the
SiPMs with some optimized optical coupling, probably using a wavelength shifter.
With assumptions of photon detection efficiency of 0.3 and effective coverage of the
0.04 solid angle of 0.1, the 1 mm track segment produces a detected photon flux of
~400/s, a comfortable level for these devices. The main issue is cost and system
complexity due to the large number of these devices, 104/m2.
Signals
The PMTs must be operated at a sufficiently high gain m that single pe detection is
very efficient: m ≈ 1 x 107. An average single photoelectron pulse in 100 ohms is
about 5 mV under these conditions. Some of this gain can come from a preamp that
stretches the pulse as part of the shaping process. This requirement is necessary both
for efficient detection of the primary scintillation and for precise measurement of
energy through photoelectron counting.
With electron drift velocity  ≈ 1 mm/s in pure xenon, an event near the Q-value
may span from about 10 s up to several 10’s or perhaps 100 s in drift time. The 1
million pe’s, for a  decay event near the Q-value, are spread out in time and
distributed over 200 PMTs in the WLS array. A PMT will record about 5,000
pe/event, at a rate of about 50 – 500/s, not more than ~50 per 10 ns. Upward
fluctuations will of course occur, but even then, the instantaneous signal level for
energy measurement is thus fairly modest.
PMTs naturally vary in quantum efficiency, and in gain. Fortunately, it is not
necessary for all PMTs have identical a priori quantum efficiency, nor does the
primary calibration goal require knowledge of absolute quantum efficiency. It is
sufficient to determine the relative quantum efficiencies  within the ensemble of
PMTs and the single photoelectron pulse height of each PMT. The total energy Q is
Q = So ab qab /sa a(rb)
(8)
So is an overall conversion constant, qab is the signal measured in PMT “a” in time
slice “b”, sa is the average single photoelectron signal for PMT “a”, and a(r) is the
efficiency for PMT “a” to detect a photon originating at radius r (at the opposite
readout plane) from the TPC axis of symmetry. Because the radius where EL photons
originate depends on the particular topology and origin of each event, r is a function
of time-slice “b” that is determined from tracking for each event. It is important that
PMTs display reasonably similar , since a weak PMT with low  contributes a larger
variance per detected photon. Allowing a maximum spread of even 10% in  will
introduce a small additional variance in Q. Increasing the total number of EL photons
slightly compensates for this tiny contribution to energy resolution.
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The contribution of errors in a and sa to Q resolution is rather forgiving. With the
reasonable assumption that all PMTs in the ensemble contribute similarly,
Q/Q  N-1/2((s/s)2 + (/)2)1/2
(9)
If the number of PMTs in each readout plane is N ≈200, and, if the average errors in
sa and  are ~2%, impact on Q/Q is less than 1 x10-3. It should not be difficult to
calibrate  and sa for each PMT to better than 2% accuracy.
Electronics and Signal Processing
The natural signal processing solution here is continuous waveform sampling and
digitization for each PMT/SiPM. Even though the basic PMT and SiPM signals are
fast, on the order of a few ns, there is a severe cost in power dissipation to sample at
200+ MHz rates, and no scientific benefit to be obtained by preserving this highfrequency domain. It seems likely that a 12-bit sampling rate at 5 – 10 MHz for the
PMTs of the circumferential WLS array of concept 2 or the cathode PMT array of
concept 3 will be adequate; an engineering analysis may show that an even slower
digitization rate loses no significant accuracy/precision for the energy measurement.
Every single photoelectron pulse from the PMTs must be detected and transmitted for
the t0 function, and for PMT calibration. The expected noise rate for 25 mm diameter
PMTs should be in the range of 200 Hz. High quality signal shaping must be
employed that matches the relatively slow sampling rates. In addition to linearity and
dynamic range, the baseline behavior for extended signals must be stable and well
understood to maintain accuracy of integration.
For the tracking function with SiPMs, with either SiPM pixels or small-diameter
WLS bars, the requirement for precision/accuracy of signal reconstruction is at least
one order of magnitude less stringent. Waveform sampling at perhaps 1 – 2 MHz with
8-bit resolution may be entirely adequate.
It seems quite likely that all ADCs for the waveform sampling should be inside the
pressurized volume, with serialized readout through optical fiber. If all data is
serialized without zero-suppression, the number of fibers that must penetrate the
pressure vessel is not small. Digitization of 200 PMTs at 12 bits and 10 MHz
generates 24 gbits/s. Worse, digitization of 104 SiPMs at 8 bits and 2 MHz generates
160 gbits/s. This is undesirable.
The further question, clearly, to be resolved is whether a digital threshold is imposed
to suppress zeroes before transmission. Roughly speaking, the imposition of zerosuppression will reduce the PMT data rate transport by four orders of magnitude, and
by five orders of magnitude if the tracking function is done with SiPM pixels. The
reward is hence great, and the cost is modest. Some additional circuit complexity is
required, but not much, and very conventional.
The basic datum is a string of ADC values that are continuously above threshold; as
soon as a value below threshold is found, a string termininates. Such strings will vary
hugely in length. Each string must be time-stamped to permit subsequent time
correlations that indicate the occurrence of an event. A time-stamped string is called a
“Hit”. In this scenario, the number of bits/s to be transported is ~4 Mbits/s. Buffers of
Hits can be assembled from many channels and transmitted at comfortable rates
through relatively low-power circuitry in a small number of fibers. Once received at
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the exterior DAQ, the Hits must be re-assembled into one time-ordered stream to
permit triggering and event building.
With or without zero-suppression, the gain of the PMT is first adjusted such that the
mean of the single photoelectron pulses is well above electronic noise, and is recorded
with adequate resolution (e.g., a typical pulse height of ~16 counts above baseline).
The digital threshold is placed in the “valley” of the single photoelectron amplitude
spectrum. For the energy measurement, it is essential to measure the mean of the
single photoelectron pulse height spectrum to an accuracy of about 1 – 2% rms for
each PMT. This can happen automatically, since the PMTs generate such single
photoelectron pulses continuously. It is even better to use a synchronized LED at low
luminous levels to measure the actual photon-generated spectrum without
contamination from other processes inside the PMT that distort the single
photoelectron spectrum slightly. It should be easy to mount a blue or UV LED in the
anode plane to illuminate the cathode or annular PMT arrays.
Conversely, an electronic noise pulse may cause only a single sample to reach above
threshold, whereas a true photoelectron pulse will generally lead to several samples
rising above threshold. It is reasonable to require that at least three samples above
threshold occur, so that electronic noise can be rejected. Waveform capture may
include some pre- and post-history to facilitate baseline management.
Coherent noise
The digital threshold approach introduces a very useful non-linearity in the data flow.
Experience shows that an analog sum approach for such a large number of inputs is
prone to serious difficulties due to the presence of small but coherent noise sources.
Such coherent noise can arise from power supplies, radio signals, and other electrical
or electronic systems nearby. Such coherent noise may be imperceptible in single
channel performance, but, adding coherently, coherent noise may easily produce large
and perhaps unmanageable signals in a wide fan-in. Coherent noise is often unseen
until the complete system is assembled, when it is difficult to develop
countermeasures. A per-channel digital threshold completely eliminates coherent
noise, since the threshold is by construction much higher than per-channel noise.
Time resolution
To define a t0 trigger signal it is necessary to process the Hits from all PMTs in the
circumferential WLS array to sense a coincidence in time. The time resolution
needed to recognize the event t0 is modest, about 20 - 30 ns, since the primary
scintillation decay constants in HPXe are 3 and 27 ns. Signal shaping of ~400 ns
permits a modest digitization rate of 5 – 10 MHz while ensuring accurate capture of
single pe signals with 4 – 10 over-threshold samples. Since signal/noise will be
excellent, and the pulse shape for single photoelectron pulses well understood,
reconstruction of pulse time for an isolated single photoelectron pulse will likely be
better than 1/20 of the sampling interval. The t0 time resolution, from the detection of
an ensemble of primary scintillation photons is likely to be significantly better than 10
ns.
In the reconstruction of event topology and energy, the time resolution requirement is
even more modest since the time span as each electron arrives at and passes through
the ELzone is about 1.5 s. The digitizer for the PMTs should be a high quality 12-bit
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ADC. The ADC will capture the small single pe signals characteristic of primary
scintillation as well as larger secondary scintillation for energy measurement. The Hit
data are next time-sorted to permit the triggering and event-building functions to
occur efficiently. An event trigger keeps all waveforms within the maximum time
span of interest, plus some “buffer” time zones to see if some strange energy deposit
had occurred earlier/later.
Tracking –  events
Since about 333 electrons/mm contribute to track imaging at  = 0.05 g/cm3, purely
statistical effects affecting spatial resolution are much less than 1 mm rms, even with
the large maximum diffusion (7), xy ≈ 7 mm rms. A natural WLS dimension, or
spacing, seems to be 5 mm. For a TPC diameter of 100 cm, each dimension of
tracking will hence require ~200 WLS bars. The number of channels would double if
SiPMs are placed at both ends, which should be considered for both redundancy and
resolution reasons. For an x-y readout, about 800 channels would be needed per end,
although it might be reasonable to sum the analog signals from both ends of a bar
prior to signal processing. With that assumption as well, the tracking function needs
400 x 2 = 800 channels. These channels should display essentially zero noise, since
the threshold can be placed far above the single pe level.
Signal transport
An average single photoelectron pulse may generate ~10 bytes, including time stamp.
If each PMT produces noise pulses at 200 Hz, the PMT arrays of either concept 2 or 3
produces a continuous flow of ~3 Mbits/s. The tracking WLS array should produce a
much smaller total data rate, so it should be reasonable to expect that the total data
rate for the detector is ~4 Mbits/s with zero-suppression.
The sensitive fiducial volume can be shielded adequately by placing the electronics
package in the dome-shaped endcap. The ability of the electronics to withstand high
pressures needs to be demonstrated.
Noise counts and  decay
The maximum noise rate that the PMT is allowed to have can be deduced from the
maximum acceptable impact on energy resolution. PMT noise adds both an offset and
an additional variance to the signal. For double beta-decay, the typical true event may
occupy the detector for an interval of 10 – 100 s and produce 1 x 106 detected pe’s at
the Q-value. If the PMT noise is statistically well behaved, then an offset of 0.1%, or
1,000 additional pe noise “counts” to be subtracted appears tolerable, almost
negligible. The incremental variance for an increase in the total number to 1.001 x
106 photoelectrons is truly insignificant. This in turn requires that the total noise rate
for each readout plane be less than 1 x 107 Hz, or ~100 kHz per PMT. Since the noise
rate of a modern low radioactivity 25 mm diameter PMT is typically much less than 1
kHz, double beta-decay search criteria are likely to be very well satisfied.
The DAQ can be organized to take random samples of each ADC stream to monitor
the stability of baseline and noise. Many other calibration tasks need to be defined
and requirements determined. The realization of the intrinsic energy resolution in
xenon depends on the quality of the signal-processing chain.
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Perspective
The application of electroluminescence in a large high-pressure xenon gas TPC to
realize unprecedented energy resolution has been revisited with a focus on WLS bars
and/or SiPMs in addition to PMTs. The use of a conventional parallel gap solves the
requirement for high EL signal gain, perhaps up to 3000 in HPXe. A very attractive
reduction in PMT count is achieved compared with concept 1, and tracking is
expected to be far better with tracking elements of ~10 mm scale than with the 50 mm
diameter PMTs of [3]. Concept 1 has served mainly now as a springboard to realize
improved tracking and cost reductions as envisaged in concepts 2 and 3.
The primary concerns for these HPXe TPC concepts for the 0-  decay search are
to verify that, with an annular array of large WLS bars or with a single-ended cathode
array of PMTs, the primary scintillation detection efficiency is adequate and that the
energy resolution goals can be met. Further requirements are the demonstrations that a
large HV can be supported across the EL gap at 10 bars, and to show that the tracking
performance with WLS bars or SiPM pixels meets expectations.
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