Transcript g - Physics - Case Western Reserve University

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.
CDMS beta background
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% nonbaryonic 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.
Applications of the beta cage
Multi-wire proportional chamber
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 world’s most sensitive detector of all non-penetrating radiation.
WIMPs can be detected via elastic
There are many possible applications
scattering from atomic nuclei.
These
outside of the physics field as well. The beta
events happen with very low frequency,
cage has potential use in carbon or tritium
and thus detection must take place
dating, where its sensitivity would make it
underground to shield from the cosmic ray
potentially competitive with accelerator mass
flux. The Cryogenic Dark Matter Search
spectrometers. The beta cage’s isotope
(CDMS) has developed technology to
sensitivity could have applications in
detect such rare scatters, and is on track
groundwater
contamination
analysis,
to extend its sensitivity by two to three
radioactive environment sampling, medical
orders of magnitude. Beta electrons from
exposure assessment, sediment dating, and
Beta-emitting
and
electron-capture
isotopes.
Those
in
bold
can
be
detected
by
inductively
traces of radioactive isotopes present in
bioremediation studies.
coupled
plasma
mass
spectrometry
(ICP-MS)
with
sensitivity
greater
than
1ppb
(its
current
the thin films on the detector surfaces
standard). The remaining 12 beta-emitting isotopes cannot presently be detected. Depending
mimic WIMP signals, and this low-energy
The design and construction of the smaller
on ICP-MS sensitivity, all 21 beta-emitting isotopes listed may be undetectable.
electron background (5-100 keV) limits
prototype chamber is a cost-effective way to
the experiment’s sensitivity, so reducing
test the feasibility and plan the production of
the full-size chamber. The prototype reads data for only six channels, which reflects
this beta background is critical to attaining this extended sensitivity.
savings in electronics and data acquisition when compared to the full-size beta
cage’s seventy-two channels. The prototype chamber is significantly smaller than the
The CDMS experiment distinguishes electronic recoil events (caused by gamma
full-size chamber (100cm x 100cm x 40cm) and also is not subject to the full-size
rays and betas) from nuclear recoil events (caused by neutrons and WIMPs) by
chamber’s high radiopurity standards. Its gas (P10) is commercially available,
sensing the charge each event imparts to charge collection plate – electronic
whereas the full-size chamber will require complex gas handling to mix neon and
recoil events have significantly higher charge yield per energy than do nuclear
methane, and recycle the neon. The prototype chamber will allow for some testing
recoil events. Beta events pose a problem because they impart a lesser amount
with neon. The prototype’s primary purpose is to test the functionality of the wire
of charge to the collection plates because of their tendency to happen very close
chamber to identify a beta-emitting isotope based on its energy spectrum.
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
Cathodes
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.
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.
Red: 133Ba gamma calibration
Signal
Black: 133Ba beta calibration
region
Blue: 252Cf neutron calibration
Note the ionization yield droop of the electron
recoil band into the WIMP signal region
The multi-wire proportional chamber
(MWPC) makes use of an electric field
localized inside a drift volume to detect
particles. The beta contamination will be
low enough levels that ambient gamma
rays will be the limiting background of the
full-size chamber. Most gamma rays will
pass through the chamber gas without
interaction, but greater mass of gas will
cause a greater interaction rate. The size
of the beta cage must therefore be the smallest possible that
would stop beta particles within its volume. Simulations in
MCNP show that a 40cm x 40cm x 20cm argon drift region
will fully contain 99% of 156 keV electrons, which represents
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 yfiducial regions, which are 35 cm
across. The signals from the wires
of each purple region are ganged
together; the AND of the x- and yregions makes the fiducial (inner)
volume, and the sum of the
remaining regions constitutes the
veto (outer) volume.
Anode
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 endpoint of 14C, a planned calibration
source of the prototype chamber.
Simulations of the electric field within the
drift chamber have been done in Maxwell
3D. They have confirmed that field edge
effects are small enough to be contained in
a defined veto region, that the grounded
vacuum chamber surrounding the MWPCs
does not affect the drift chamber field, that
no areas contain high enough field to cause
gas breakdown, and that the wire planes will cause sufficient
avalanche gain and allow the drifting electrons to be collected.
Except for very thin planes through the center of the wires, all
field lines from the drift chamber terminate on the bulk anode.
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
Maxthe outer vacuum
well 3D
chamber, and
simulations
the outer gray
show the potential
region is extra
in the chamber due to the
lead shielding
wire planes. A 5mm x 15 mm
to surround
unit cell is shown – other unit cells
the full - size
border on the long faces. From left to
chamber, and
right are the parallel cathode, anode, and
possibly the
crossed cathode. The cathodes are grounded;
prototype
the anode is held at high voltage (2500-2800 V).
as well).
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
(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 chamber’s 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.
Signal collection and DAQ
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.
Readout Channels
w Bulk Fiducial Anode
w Bulk Veto Andoe
w Bulk Veto Cathode (crossed)
Vacuum chamber and argon gas
Argon’s 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
External electronics setup. The low-pass filter uses 1 GΩ and 0.001 μF components to eliminate ionize the chamber by
noise from the power supply; values for Rbias and Cbl are 1 GΩ and 100 pF. The blocking
freeing photoelectrons from
capacitor eliminates the DC high voltage and passes the signal; the bias resistors isolate the
signals from one another. SHV feedthroughs in NW-50 ports connect circuitry to the chamber. its walls, making the beta
cage a discharge chamber
that, instead of amplifying
Data acquisition NIM logic setup.
pulses, would generate a constant signal. A methane quench is
The trigger signal, after 105 gain at
used to prevent this overrunning of photons. Photons are
the anode and 10x external
absorbed now by the methane molecules, which form neutral
amplification, is 30 mV/keV,
hydrogen and organic molecules. P10 (90% argon, 10% methane)
enough to activate the NIM-level
is the chamber gas.
discriminator. The logic setup
w Trigger Fiducial Anode
w Trigger Veto Anode
w Trigger Veto Cathode (crossed)
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.
generates a gate, which activates
the ADC to begin reading the bulk
channels. (The ADC’s 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
ADC’s 50Ω 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).
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 access to the chamber. The
remaining five ports contain
SHV feed-throughs to
deliver high voltage to the
chamber wires and to read
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.
30”