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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”