Characterizing Electron Background with the Cryogenic Dark Matter Search Beta Cage

1
A PREDECESSOR TO A SCREENER OF ULTRA-LOW-LEVEL
RADIATION THE PROTOTYPE
BETA CAGE
K. Poinar, D.S. Akerib, D.R. Grant, R.W Schnee,
T. Shutt Case Western Reserve University
Z. Ahmed, S.R. Golwala California Institute of
Technology
Funded by Case Support of Undergraduate
Research and Creative Endeavors (SOURCE), and the
9th annual DNP Conference Experience for
Undergraduates
The beta cage is a proposed multi-wire
proportional chamber that will be the most
sensitive device available to screen low-energy
(200 keV or less) betas emitted at rates as low
as 10-5 counts keV-1 cm-2 day-1 (of order 10-4
Bq/m2). The beta cage has potential use in
carbon or tritium dating, with 3H/1H sensitivity
of 10-20 and 14C/12C sensitivity of 10-18. Its
design and construction were motivated by the
Cryogenic Dark Matter Search, whose sensitivity
to the dark matter candidate WIMPs is currently
limited by low-energy beta contamination. The
prototype chamber is built to assess the accuracy
of isotope identification by reconstruction of
the beta energy spectrum. The prototype beta
cage is a 40 cm x 40cm x 20cm frame containing
two regions (upper and lower) of wire planes,
contained within a chamber of noble gas. To
reduce background, the chamber contains only
enough mass to stop the betas of interest within
the volume. Samples are placed beneath the grid
emitted betas produce a shower of secondary
electrons, which the high-voltage anode wires
multiply and collect. Their readouts allow
discrimination of its events from background and
a subsequent determination of the beta source.
Signal collection and DAQ
CDMS beta background
Applications of the beta cage
Direct detection of dark matter has become an
experimental priority because of its implications
in cosmology, astrophysics, and high-energy
particle physics. Cosmological data indicate that
the universe is made of 4 baryons, 23
non-baryonic dark matter, and 73 dark energy.
The mass and properties of Weakly Interactive
Massive Particles (WIMPs) make them a generic
candidate for this dark matter as well as the
favored theoretical lightest supersymmetric
particle. The search for WIMPs thus represents a
convergence of independent arguments from
cosmology and particle physics, with implications
for both. this beta background is
critical to attaining this extended sensitivity.
The CDMS experiment distinguishes electronic
recoil events (caused by gamma rays and betas)
from nuclear recoil events (caused by neutrons
and WIMPs) by sensing the charge each event
imparts to charge collection plate electronic
recoil events have significantly higher charge
yield per energy than do nuclear recoil events.
Beta events pose a problem because they impart a
lesser amount of charge to the collection plates
because of their tendency to happen very close to
the detectors surface. For example, beta events
near the negative-biased surface diffuse a
significant number of electrons to the negative
plate, which causes less charge to be collected
at the positive plate. Similarly, events on the
positive surface diffuse holes to the positive
plate to cause reduced ionization yield. Thus,
the set of beta events droops into the nuclear
recoil band. The risetime of the phonon signal
allows the elimination of beta particles they
have a significantly faster phonon pulse than
most nuclear recoils, so timing cuts eliminate
99.99 of betas. The cuts also limit, though,
the observable signal region, as a fraction of
nuclear recoils also happen on timescales below
the cut.
Accurate measurements of the level of beta
activity of a sample will allow for inexpensive
and quick screening of test samples. Techniques
that produced passing samples can be applied to
fabricate full detectors for use in the CDMS
experiment, while samples that fail will give
feedback to improve production and handling
techniques. The chamber would be potentially
applicable to liquid noble experiments with 40K
x-ray backgrounds in their photomultiplier tubes
alpha particles originating from various radon
daughters appear to limit other experiments. The
full-size beta cage would be the worlds most
sensitive detector of all non-penetrating
radiation.
Three data channels are read from each MWPC
(trigger and bulk), resulting in only six total
readout channels. To reduce ambient gamma
backgrounds that penetrate the chamber and cause
ionization, the bulk channels are read only when
the trigger region registers a signal. The
energy of the particle is given by the time delay
between the readings (100-500 µs). Position in
the xy-plane is coarse in the prototype chamber,
given by only three regions (fiducial, veto x,
veto y). In the full-size chamber, data will be
read from all 200 wires in each plane, giving 5mm
x 5mm xy-resolution.
Vacuum chamber and argon gas
WIMPs can be detected via elastic scattering
from atomic nuclei. These events happen with
very low frequency, and thus detection must take
place underground to shield from the cosmic ray
flux. The Cryogenic Dark Matter Search (CDMS)
has developed technology to detect such rare
scatters, and is on track to extend its
sensitivity by two to three orders of magnitude.
Beta electrons from traces of radioactive
isotopes present in the thin films on the
detector surfaces mimic WIMP signals, and this
low-energy electron background (5-100 keV) limits
the experiments sensitivity, so reducing
There are many possible applications outside of
the physics field as well. The beta cage has
potential use in carbon or tritium dating, where
its sensitivity would make it potentially
competitive with accelerator mass spectrometers.
The beta cages isotope sensitivity could have
applications in groundwater contamination
analysis, radioactive environment sampling,
medical exposure assessment, sediment dating, and
bioremediation studies. The design and
construction of the smaller prototype chamber is
a cost-effective way to test the feasibility and
plan the production of
Readout
Channels w Bulk Fiducial Anode w Trigger Fiducial
Anode w Bulk Veto Andoe w Trigger Veto Anode w
Bulk Veto Cathode (crossed) w Trigger Veto
Cathode (crossed)
Argons size and chemical properties make it the
standard gas for use in drift chambers it
provides a desirable amount of amplification near
the anode wires. Noble gases are used because
their limited degrees of freedom cause a tendency
to ionize when struck with energy. However,
electron excitation rather than liberation would
create a photon avalanche that would overwhelm
the electron avalanche. The photons would
continually ionize the chamber by freeing
photoelectrons from its walls, making the beta
cage a discharge chamber that, instead of
amplifying
High voltage (2500-2800 V) is supplied to the 25
µm wires over four channels one each for the
drift field shapers, the trigger MWPC anode, the
bulk MWPC anode, and the bulk MWPC cathodes.
Thus full freedom to adjust voltages to optimize
gains and stability is allotted. A low-pass
filter eliminates 20 kHz noise from the
transformers in the high voltage unit the
filters are homemade in a NIM format box. Bias
resistors prevent crosstalk between readout
channels that share the same high voltage, and
blocking capacitors before the data acquisition
eliminate the voltage offset that the signals (3
mV) sit on. For cleanliness, this circuitry is
located outside of the chamber, in the NIM box
with the filters.
Top view of the prototype beta cage. Blue
indicates the UHMWPE frame, each plane of which
will hold 80 wires spaced 5mm apart, electrically
connected via the green PCB tracks. The planes
are separated vertically by 5mm. The purple
cells indicate the x- and y- fiducial regions,
which are 35 cm across. The signals from the
wires of each purple region are ganged together
the AND of the x- and y- regions makes the
fiducial (inner) volume, and the sum of the
remaining regions constitutes the veto (outer)
volume.
External side view of the prototype beta cage.
The blue regions are the trigger (bottom) and
bulk (top) MWPCs, which consist of three stacked
planes
the full-size chamber. The prototype reads data
for only six channels, which reflects savings in
electronics and data acquisition when compared to
the full-size beta cages seventy-two channels.
The prototype chamber is significantly smaller
than the full-size chamber (100cm x 100cm x 40cm)
and also is not subject to the full-size
chambers high radiopurity standards. Its gas
(P10) is commercially available, whereas the
full-size chamber will require complex gas
handling to mix neon and methane, and recycle the
neon. The prototype chamber will allow for some
testing with neon. The prototypes primary
purpose is to test the functionality of the wire
chamber to identify a beta-emitting isotope based
on its energy spectrum.
(5mm apart) over which cathode, anode, and
cathode wires are strung. The 18 orange lines
are the copper drift field shapers, which are 1mm
thick square planar rings. They are kept at
increasing potentials (via a series of voltage
dividers) and isolated by 9mm thick UHMWPE
spacers (gray).

• Detection of Betas in the Multi-Wire Proportional
  Chamber
• g The sample is placed in the bottom of the
  chamber.
• g A beta emitted from the sample passes through
  the trigger region and ranges out in the bulk
  region, creating secondary electrons by ionizing
  argon atoms along its path.
• g Secondary electrons in the trigger region drift
  to the high voltage (2500-2800 V) trigger anode
  wire, where the electric field is greatest.
• g Amplification of order 105 occurs, producing
  the electron avalanche and registering a signal
  that activates the data acquisition system.
• g The chambers internal electric field causes
  the secondary electrons in the bulk drift region
  to move upward with a speed of 1cm/ µs, toward
  the bulk MWPC.
• g The larger field near the bulk anode causes the
  electrons to accelerate, avalanche, and produce a
  signal as before.
• g Time delay between trigger and bulk signals
  shows how far the secondary electrons drifted and
  thus how far the beta traveled. Very short
  delays (less than 1 µs) indicate betas that
  escaped the chamber. These signals will not be
  analyzed.
• g The wire signal is proportional to the amount
  of ionization the beta caused, and thus its
  initial energy. The amount of charge collected
  by the ADC will allow energy reconstruction.

Data acquisition NIM logic setup. The trigger
signal, after 105 gain at the anode and 10x
external amplification, is 30 mV/keV, enough to
activate the NIM-level discriminator. The logic
setup generates a gate, which activates the ADC
to begin reading the bulk channels. (The ADCs
busy output vetoes any new trigger signals that
may come during data collection.) Bulk channels
have gain of only 104, and so after 10x external
amplification their magnitudes are 3 mV/keV. The
ADC integrates the charge in the full-size
chamber the waveform will be digitized for better
background rejection. The ADCs 50O input
impedance converts the amplified 3 mV/keV peak
height bulk MWPC signals to a peak current of 60
µA/keV. With 12 ns pulse decay time due to
capacitance of the cables and wire planes, the
total charge is 0.7 pC/keV. The ADC calibration
is 4 counts/pC (3 counts/keV) with 800 pC maximum
range (1.1 MeV).
pulses, would generate a constant signal. A
methane quench is used to prevent this
overrunning of photons. Photons are absorbed now
by the methane molecules, which form neutral
hydrogen and organic molecules. P10 (90 argon,
10 methane) is the chamber gas.
Cathodes
Anode
The vacuum chamber shown from below. Three of
the NW-50 ports are used for gas handling P10
is flowed into the chamber, and the flow rate
out is observed with a homemade Erlenmeyer
bubbler. The third gas port attaches to a
pressure meter and a bellows valve for vacuum
pump ac- cess to the chamber. The remaining five
ports contain SHV feed-throughs to deliver high
voltage to the chamber wires and to read
Internal side view of the prototype beta cage.
The trigger and bulk MWPCs are shown the
full-size chamber will have an additional veto
MWPC located below the The drift field shapers
are visible as a series of dashes on the
sides. The pink region represents the outer
vacuum chamber, and the outer gray region is
extra lead shielding to surround the full -
size chamber, and possibly the prototype as
well).
30
Monte Carlo simulations (in MCNP) show the
isotropic range of 156 keV electrons, which
represents the maximum energy of 14C decay. The
20 cm of argon in the trigger region and drift
volume above the sample will contain 99 of 156
keV electrons thus the vast majority of the
decays from 14C will be contained in the
chamber. 14C and 109Cd (which has an endpoint of
84 keV) will be used to calibrate the prototype
chamber and test its ability to reconstruct
energy spectra to identify isotopes.
The electron recoil band (top) is distinguished
from the nuclear recoil band (bottom) based on
its higher ionization yield per recoil energy.
WIMP signals occur in the nuclear recoil band.
signals from them. Each feed-through contains 2
or 3 SHV connectors, enough to pass up to ten
separate high voltage channels to the beta cage.