A P REDECESSOR TO A S CREENER OF U LTRA- L OW- L EVEL R ADIATION: THE P ROTOTYPE.
APREDECESSOR TO A SCREENER OF ULTRA-LOW-LEVEL RADIATION: THE PROTOTYPE
Beta-emitting and electron-capture isotopes. Those in bold can be detected by inductively coupled plasma mass spectrometry (ICP-MS) with sensitivity greater than 1ppb (its current standard). The remaining 12 beta-emitting isotopes cannot presently be detected. Depending on ICP-MS sensitivity, all 21 beta-emitting isotopes listed may be undetectable.
External electronics setup. The low-pass filter uses 1 GΩ and 0.001 μF components to eliminate noise from the power supply; values for Rbias and Cbl are 1 GΩ and 100 pF. The blocking 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.
Multi-wire proportional chamber
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
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
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
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.
show the potential
in the chamber due to the
wire planes. A 5mm x 15 mm
unit cell is shown – other unit cells
border on the long faces. From left to
right are the parallel cathode, anode, and
crossed cathode. The cathodes are grounded;
the anode is held at high voltage (2500-2800 V).
Red:133Ba gamma calibration
Black:133Ba beta calibration
Blue:252Cf neutron calibration
Note the ionization yield droop of the electron
recoil band into the WIMP signal region
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 world’s 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.
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 experiment’s sensitivity, so reducing
Vacuum chamber and argon gas
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 cage’s 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
wBulk Fiducial Anode wTrigger Fiducial Anode
wBulk Veto Andoe wTrigger Veto Anode
w Bulk Veto Cathode (crossed) wTrigger Veto Cathode (crossed)
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 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 triggerMWPC anode, the bulk MWPC anode, and thebulk MWPC cathodes. Thus full freedom toadjust 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 cage’s 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 chamber’s 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 prototype’s 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
gThe 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.
gSecondary 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.
gThe 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.
gTime 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.
gThe 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 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).
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.
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
third gas port
attaches to a
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
the outer vacuum
the outer gray
region is extra
the full - size
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.