Sheffield DRIFT-CYGNUS Group
The DRIFT (Directional Recoil Identification from Tracks) programme is a UK-US joint effort at the UK’s Boulby deep underground site that develops and runs detectors designed to determine the direction of the recoil tracks expected from WIMP dark matter particles. CYGNUS is an international cooperation of directional dark matter groups aiming towards construction of a tonne-scale directional detector. DRIFT is the first, largest and longest running directional dark matter effort comprising 6 US and UK institutes. The CYGNUS consortium includes 5 current generation experiments world-wide (DRIFT, MIMAC, DM-TPC, D3 and NEWAGE) with ~150 scientists from 20 institutions across 8 countries. The motivation is that DRIFT-CYGNUS experiments could determine definitively that signal recoil events from WIMPs are of non-terrestrial (galactic) origin and correlated with our galactic motion. The technology is hence in principle very powerful at proving the existence of galactic dark matter. The Sheffield group has played a key role in the field, pioneering many of the concepts for directionality co-founding DRIFT and leading formation of CYGNUS with a first joint meeting in Boulby in 2007. For DRIFT Sheffield has contributed in core areas, conceiving, designing and building many of the DRIFT-II detector components, including the data acquisition systems, slow control, data-pipelines, much electronics, vessel, internal detector field cages and supports, gas handling and vacuum systems, shielding and underground site. The UK has led major data analysis and simulations efforts.
|Group Activities||Group Members|
Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK
Contact: Neil Spooner (firstname.lastname@example.org, tel. 0114 2224422)
Recent Papers and Drafts
A list of papers is given at the end of these pages. Below are links to our CYGNUS meetings that contain detailed information on progress for DRIFT and all the other directional efforts.
Radon backgrounds in the DRIFT-II directional dark matter experiments (2013) arXiv1307.5525
High Precision Measurements of Carbon Disulfide Negative Ion Mobility and Diffusion (2013) arXiv:1301.7145v2
Radon mitigation in the DRIFT-II directional dark matter gas time projection chambers (2013) in preparation here
Astronomy and Geophysics article - Searching for a Directional Dark Matter Signature at Boulby Mine (2013) here
Spin-dependent limits from the DRIFT-IId directional dark matter detector (2012) arXiv:1010.3027
Determination of the direction in galactic coordinates of nuclear recoils induced by the elastic scattering of Weakly Interacting Massive Particles (WIMPs) in a low pressure gas, holds real prospect for yielding a definitive signal for the Dark Matter, known to comprise ~90% of the mass of our Galaxy. This possibility arises thanks to the motion of our Solar System towards the constellation Cygnus. This means that in principle there is a clear directional recoil signal in such an experiment, sometimes termed a WIMP telescope, that includes a daily sidereal modulation in the Earth’s frame that can not be mimicked by any terrestrial background, including neutrons or even neutrinos.
In recent years this powerful new approach to dark matter searches has become more recognised and so the directional WIMP field has rapidly expanded, with now 5 experiments operating worldwide (DRIFT, MIMAC, DM-TPC, D3 and NEWAGE). Nevertheless, our latest experiment at the UK’s Boulby Underground Laboratory, DRIFT-IId, maintains a strong world leadership position producing results now competitive even with non-directional efforts in the SD WIMP regime. Furthermore, there is the new possibility now to extend the energy threshold sensitivity of directional technology below 20 keV to allow for the first time start of directional searches for very low mass WIMPs (<20 GeV). The interest in this arises partly from continuing observation in multiple conventional non-directional detectors of events consistent with discovery of low mass WIMPs. Initial work by us suggests that only 1-5 m3 of directional detector suitably upgraded could be sufficient to study if the claimed events are of truly galactic origin. Regardless of the low mass WIMP issue it is widely recognised that observation of a directional, clearly non-terrestrial, signal is vital for the field and preparations are needed to allow timely implementation.
Funding and Support Background
DRIFT has been consistently funded by the US NSF and originally PPARC in the UK. Recent UK STFC support has come through the particle physics consolidated grants for staff. The programme is now strongly backed by the DMUK group of nine institutes representing the UK’s dark matter community. It is also strongly supported by the CYGNUS international consortium aiming to build a tonne-scale experiment. The programme is in line with the STFC roadmap and PAAP strategy [1,2] and with recommendations of the STFC Tovey dark matter review, adopted by Science Board, that re-confirms the importance of seeking a directional signature for WIMP dark matter . The US strongly supports UK efforts on directionality and now assists directly with infrastructure funds to STFC for DRIFT at Boulby. The Boulby Laboratory itself, with manager Dr. Sean Paling, is now funded by the STFC-Futures line, and the Boulby mine company, CPL, are strongly supportive, offering us substantial independent infrastructure funds and expertise and in-kind resources. CPL are in line with a DRIFT-CYGNUS objective to proceed to build and install DRIFT-III at Boulby. This is a 24 m3 next generation directional experiment. The Figure below show the intended new laboratory to be built at Boulby for DRIFT-III.
WIMP Searches and the Case for Seeking a Directional Galactic Signature
Precision measurements from two very different fields, cosmology and particle physics, are now allowing us to build a consistent picture of our Universe for the first time. At the Large Hadron Collider (LHC), the recent discovery of a Higgs Boson like particle is consistent with theories to explain particle mass. Searches for new particles predicted by Supersymmetry (SUSY) and other theories are advancing towards unifying our picture of the fundamental forces. Meanwhile, a multitude of cosmological observations has accurately determined the mass-energy content of the Universe whereby 4% is baryonic, the rest comprising 73% dark energy and 23% dark matter, both as-yet of undetermined composition. Work to determine the nature of dark energy is still in its infancy. However, for dark matter, thanks to the convergence of many astrophysical observations from local to cosmological scales with particle physics models built over many years, there is strong evidence to suggest that Weakly Interacting Massive Particles (WIMPs), produced in the early Universe, are the answer. Conclusive detection of WIMPs has been identified as among the most important scientific targets for the 21st century . Success would initiate a new type of astrophysics and have profound impact on our understanding of fundamental particles, conclusively linking work at the LHC to cosmology and the nature of our own galaxy.
The LHC detectors, or successors, may feasibly observe SUSY-like particles consistent with WIMPs. However, particle lifetimes can not be measured sufficiently in these experiments to prove consistency with dark matter. An alternative route comes from observations that dark matter gravitationally collects in a variety of astrophysical settings. Thus, by searching for WIMP-WIMP annihilation products, such as from accumulations in the Sun, they may be observable indirectly. Unfortunately, such observations must be disentangled from complex astrophysical processes that may give similar signals. A less ambiguous approach, currently the focus of major investment by 20 collaborations world-wide, is to search directly for the nuclear recoils from elastic scattering of WIMPs off the target nuclei of earth-based experiments . Whilst this is more robust than the alternatives, there exists a big challenge from background radiations that can swamp or mimic the dark matter nuclear recoil events, expected with 10-100 keV energy. Given the significance to science of a detection claim, the evidence to support discovery must be exceptional. Thus for direct searches it remains absolutely crucial that a clear signal distinct from backgrounds be observed.
To deal with these backgrounds efforts have for many years been directed at technology able to reduce the gamma component, for instance through detection of two simultaneous discrimination parameters such as scintillation and ionization in liquid noble gases (e.g. XENON ), or phonons and ionization in bolometers (e.g. CDMS ). Perfecting this gamma rejection has driven direct search efforts because gammas are the first dominant intrinsic background. Unfortunately, as sensitivity to WIMP cross sections has lowered, it has become clear that this approach has not properly addressed the most dangerous, though lower rate, background from ~MeV neutrons, from rare radon-related backgrounds and even from solar neutrinos. Unlike gammas, all these backgrounds will produce nuclear recoils indistinguishable from WIMPs, able to fake WIMPs regardless of the gamma rejection efficiency. This is exacerbated because neutron backgrounds are difficult to model and so can not even be accurately subtracted. For instance, muon-induced neutron rates in heavy elements, like lead and copper, are not well understood, nor are fission neutrons in 235U, 238U and 232Th, or (α, n) reactions from light elements typical of detector materials (e.g. Li, F, Na, etc.) .
The flow of new direct WIMP search results in recent years dramatically highlights this situation (see Fig. 1). In experiments such as CoGeNT [10,11], CRESST , Edelweiss , and most significantly CDMS  announced in April 2013, events are observed above predicted backgrounds and claimed either as due to WIMPs or as unexplained recoil-like background. In either case there is a clear, urgent need now for improved signal identification, to eliminate the possibility that neutron or other recoil events are responsible, to understand why results are in potential contradiction with the limits set by other experiments, such as XENON-100 , or indeed to confirm definitively that WIMPs have been observed. An alternative approach to address the problems observed is to use our motion through the galaxy to identify a non-ambiguous WIMP signature, clearly separated from all terrestrial backgrounds. This is feasible because WIMP velocities on Earth arise primarily from the rotation of the solar system about the galactic centre (see Fig. 2a) . An experiment using this would aim to observe not just that recoils consistent with WIMP interactions are present, but also that such events are of truly galactic origin and so must be due to WIMPs. The UK since 1990 has pioneered studies of such techniques . One possibility is to use the predicted small annual modulation in the event rate expected due to the Earth’s orbital motion around the Sun. However, as highlighted by the controversy over the DAMA experiment’s claimed observation, it is hard to exclude the possibility that natural seasonal variations will mimic that signal [16,17].
Fortunately, a much stronger signal exists if a sophisticated detector could be built to observe the direction of nuclear recoils. This would allow observation of the direction dependence of the WIMP stream, coming from (l = 90°, b = 0°) in galactic coordinates, roughly in the direction of the constellation Cygnus . Colloquially we have a WIMP “wind” blowing from Cygnus that produces a modulation of both the WIMP interaction rate and the WIMP velocity direction (see Fig. 2b). At 45° latitude (Boulby is at 52°) the directions would oscillate from downwards to southwards. This sidereal “day-night” modulation is large, with a forward-back asymmetry of at least 1:100 and impossible to be mimick by any terrestrial background since these must follow the Earth’s rotation, isotropic in galactic coordinates . If there are unforeseen solar day/night effects on the directionality of such backgrounds, these would quickly go out of phase with the sidereal cycle. We thus have a new, clean, discovery parameter, simply a WIMP signal pointing at Cygnus (see Fig. 2c). Such an observation is considered by many to be a necessary condition for proof of dark matter.
Fig. 2: (a) Solar system velocity in the galaxy.
Fig. 2: (b) WIMP directional change due to Earth’s rotation.
Fig. 2: (c) Simulation of WIMP velocity directions in galacatic coordinates and resulting recoil directions.
The DRIFT Directional WIMP Search and Power of TPC Detectors
In a solid the nuclear recoil tracks expected from WIMPs are only 10-1000 nm and so virtually unobservable. Meanwhile, theoretical studies by us have shown that an important requirement of directional detectors is the ability to determine the forward-back (or head-tail) sense of the track, found to be typically ten times more critical than simple track orientation information . First theoretical detector studies by Sheffield and Nottingham confirmed that such a head-tail is likely observable in a gas type detector . These factors point strongly to use of low pressure gas Time Projection Chamber (TPC) technology, in which WIMP-induced recoils are extended to mm length. It is from this concept that the Directional Recoil Identification From Tracks (DRIFT) project was founded. Whilst the gas TPC idea implies a large detector volume is needed, this apparent disadvantage is outweighed by the simplicity of technology and remarkable particle identification power feasible, that makes the sensitivity per unit mass high (see Fig. 3). To date, no DRIFT detectors have required any gamma shielding. No cryogenics, complex engineering or specialist underground infrastructure is needed and detectors can have simple elongated shapes, for instance well suited to an underground site like Boulby. The identification power, and basis for directional sensitivity, comes from 4 separate measurement parameters:
- total ionisation,
- particle range,
- dE/dx track topology
- track orientation in galactic coordinates
Fig. 3: Simulation comparing range and dE/dx difference between nuclear recoil and electron recoil of similar energies.
Analysis then allows extraction of the WIMP interaction point, initial recoil direction in galactic coordinates, and track sense. Simulations show that angular resolution, even with ~mm position resolution, of 20-80 deg (20-100 keV) is feasible so that even a 1 m3 detector can reach a proton axial cross section background free discovery at 10-2 pb and exclusion at 4 x 10-4 pb, depending on target choice and WIMP mass . Furthermore, the isotropy in galactic coordinates of all backgrounds means that WIMP identification is feasible even with significant neutron background or can be used as a new means to suppress that background (see Fig. 4).
Fig. 4: Typical simulation of 50 WIMP and 50 background neutrons - likelihood analysis shows good WIMP directionality remains (for refs. see text).
Based on the above the UK-US DRIFT project has built a series of 1 m3 prototype demonstrators, DRIFT-I and DRIFT-IIa,b,c,d, aimed at perfecting the directional sensitivity, background rejection and engineering necessary to allow design of larger scale experiments, ultimately to tonne-scale. The last years have seen major progress on key issues, but simultaneously first significant directional WIMP physics has been possible. Five key recent results are:
- First demonstration of head-tail direction sensitivity, observed in 1 m3 with <0.7mm diffusion .
- First directional dark matter run with CS2-CF4 gas resulting in ~2 pb sensitivity to spin-dependent (SD) WIMP-proton interactions from 19F (Fig. 1). This is x1000 more sensitive than the limits set by its closest directional sensitive competitor, and comparable then to the best non-directional SD limits .
- New radon mitigation techniques pioneered including vetos, material screening and texturised thin film cathode. The result now is that our dominant background from Radon Progeny Recoils is reduced by >500 over DRIFT-IIa to <1 event per week in DRIFT-IId, even prior to fiducialisation cuts .
- Discovery and implementation of powerful means to fully fiducialise detectors using minority carrier transport in the gas. This is a major result for the field and means DRIFT is no long background limited but severely volume limited and thus requires scale-up in order to make progress .
- Discovery using fine-grain readout that directionality is likely feasible for low mass WIMPs .
World-wide Directional Experiments
An exciting, but confused picture is emerging for direct searches. Regarding non-directional work focussed on high mass WIMPs, notable are the achievements by XENON-100 of spin independent limits at 2 x 10-45 cm2 (55 GeV/c2)  and by COUPP at 3 x 10-39 cm2 (60 GeV/c2) for SD WIMP-proton . This contrasts with the low mass WIMP regime where there are now numerous, possibly conflicting, positive detections (Fig. 1). Partly provoked by this, and the arguments above, many new directional TPC experiments have emerged using different readout and gas schemes, so that DRIFT is no longer unique:
- DM-TPC (US-UK) - parallel woven mesh chambers plus a CCD camera read-out ,
- NEWAGE (Japan) - micro-dots associated with a flash ADC read-out ,
- MIMAC (France) - pixelized micromegas with a self-triggered specially-developed readout ,
- D3 (Hawaii) – ATLAS-based pixel readout with electron focusing .
These groups are already coordinating towards a common scale-up experiment CYGNUS, summarized in our collective publication . DM-TPC, NEWAGE and MIMAC are now operating small chambers underground (at WIPP, Kamiokande and Modane). The CCD-based R&D detector of DRIFT, using GEM stages, has also achieved good results see below . All these experiments have emphasized high readout resolution. However, it is worth noting that fine resolution at the readout plane is pointless if track charge distribution is dominated by diffusion in the gas. DRIFT has this under control by unique use of negative ion CS2. This yields diffusion <1.5 mm per m, x5 lower than CF4, allowing x5 greater active volume per readout plane. As detailed below, this and other remarkable properties of CS2 are a game-change in the field. We note of particular relevance to the readout with GEMs planned here the exceptional tolerance to sparking, allowing high gain (>1000-10,000) at <40 Torr where CF4 is typically restricted to >75 Torr.
- Fidicualisation to >90% of target volume by minority carrier timing with CS2 with O2. This holds prospect for complete rejection of all radon progeny backgrounds, unavailable to other techniques.
- Diffusion <1.5 mm per m, x5 lower than CF4, allowing x5 greater active volume per readout plane.
- Very slow ion drift time, ~1 ms per m, allowing exceptional timing resolution in the z-dimension, equivalent to <200 μm, and lower cost electronics as ADCs of only ~500kHz are required.
- Exceptional tolerance to impurities, to 1% air (cf ppm in CF4) and minimal detector aging.
- Exceptional tolerance to sparking, allowing high gain (>1000-10,000 depending on technique) even at <40 Torr (CF4 typically is restricted to >75 Torr). Particularly important for low mass WIMPs.
- Ability to use multiple additive WIMP target gases including CF4, Xe, CO2 yet maintain –ve ion gas advantages even at 95% additive, e.g. CS2 (5%)+CF4 (95%) is proven to work.
- Proven to work in all readout schemes, with low threshold - micromegas, GEMS, CCDs and wires.
- Low cost and proven safety, to ISO1800, due to industry use and our underground work experience.
The DRIFT-CYGNUS Directional Programme Results and History
Here we describe key R&D progress and UK contributions. DRIFT was started by Spooner with Snowden-Ifft in 1999 and the UK has since led contributions to all aspects. In 2007 we also instigated the CYGNUS global cooperation towards an eventual tonne-scale experiment. Work started with DRIFT-I  but most critical results have come from the 1 m3 DRIFT-IIa-d devices. A prime aim has been to verify components for a next stage 24m3 DRIFT-III at Boulby and to support this we have just built a new 1 m3 detector, DRIFT-IIe. With DRIFT-IId this increases our operating volume to 2 m3.(Previous web pages describing earlier work in more detail can be found here).
First engineering runs of DRIFT I at Boulby occurred during 2002/3, showing stable operation for around 9 months. The collaboration was comfortable that these tests confirmed basic operation of what was a new technology in the field - a low pressure, negative ion TPC underground. A significant result was the realization underground that the rejection efficiency for gammas was as predicted so that Pb gamma shielding for DRIFT I would not be needed. Gamma rejection works in quite a different way to that of other technologies and is difficult to quantify in a meaningful way for comparison - DRIFT aims to be a zero background detector. A reasonable guide would be rejection >1 in 105 at threshold in DRIFT I. During 2003 (after around a year of operation) a series of issues developed in the detector that took considerable effort to identify and correct. These were compounded by faults in the SLAC-built data acquisition boards and in the MWPC protection electronics that were eventually found not to be adequate. The result was a higher instance of wire breakage than should have resulted and hence delays in diagnosis because of the time needed to repair broken wires before the next test. By early 2004 the causes of the behavior had been successfully identified and corrected and DRIFT I was operational again, taking data. Three key issues were identified: (i) ageing of the Lucite MWPC structure, (ii) gas quality changes due to a small vacuum leak which also compounded the MWPC ageing effect, and (iii) a fatigued high voltage feed through that resulted in discharges. The Lucite ageing, identified as due to slow stress release over 18 months, resulted in a slight warping of the detector and hence wire breakdown, resulting then in wire breakage because of the electronics protection circuits. The warping was successfully corrected by retrofitting tensioning battens. The vacuum leak was successfully traced to one of the DAQ soldered feedthroughs, which was replaced. The HV feed breakdown fault was finally identified optically and replaced. Various other improvements to the detector were also instigated including: new lower noise DAQ cables, improved detector trolley apparatus to help with MWPC operations, re-configured slow control to improve monitoring and safety, new hard-wired pressure cut to ensure switch off of high voltage in case of a leak, new HHV filter to reduce signal noise. Considerable progress was made in data analysis and simulation codes. The later focused on potential sensitivity of the technology to different halo models. Unshielded data from the first operational period was sufficient to allow preliminary identification of neutrons from the cavern walls in agreement with Fluka and GEANT 4 simulations of the expected neutron rate.
Below is a schematic showing the operational principle of CS2 in a negative ion time projection chamber.
Following collection of around 2 weeks unshielded but vetoed data the neutron shield (8 tons of poly-pellets) was successfully installed. All detector refurbishments made in 2004 continued to function well including the tensioning battens retro-fitted to correct the slow fatigue warping. This had greatly strengthened confidence in the DRIFT II design, then on-going. However, despite progress to identify and correct faults in the Temple-SLAC DAQ this continued to cause issues. In particular, the veto channels (needed to reject stray low energy side alphas) were found to be malfunctioning; with continued variations in noise and board malfunctions. Meanwhile, with a much superior design thanks to the lessons from DRIFT I, construction of the first DRIFT II modules had gone very well, with the first units already being successfully commissioned on the surface. DRIFT II uses much simpler, lower threshold and commercial-based DAQ. Given this the collaboration decided to focus resources on rapid deployment of DRIFT II, using DRIFT I now as a test-bed for DRIFT II components, including the new DAQ. DRIFT I was subsequently de-commissioned and now resides in the Science Museum.
The design of DRIFT II, including engineering drawings, DAQ and gas systems, was completed by the end of 2003. Space constraints in the JIF area eventually ruled out a vessel significantly larger than DRIFT I. Instead DRIFT II was envisaged as an array of detectors of 1 m3 active volume, starting with DRIFT IIa. Meanwhile, the slow fatigue issues revealed in the DRIFT I MWPCs, due to use of low background Lucite, also indicated improved design of the MWPCs, to make them more robust and easier to deal with. DRIFT II was thus based on a simpler and more robust vessel and detector construction but designed also for duplication for a possible modular array. Specific detector design details for DRIFT II modules are as follows:
- MWPCs - vertical, modular, double sided, interchangeable with Lucite frame design
- 3D readout (DRIFT I was 2D) with position resolution Δx=2mm, Δy=0.2mm, Δz=0.1mm
- Threshold possible down to 250 NIPs
- Grouped readout DAQ
- 60 cm CH neutron shielding (no gamma shielding)
- Up to 100 kV cathode voltage
Full technical details of the DRIFT-II design are given in . Briefly it comprises a 1.5 m3 steel vacuum vessel in which is contained the target gas, typically 30 Torr CS2 + 10 Torr CF4
For DRIFT-IIa-d UK groups designed and built the vessel, field cages, all daq, slow and gas control, neutron shielding, radon mitigation strategy, site, ran operations and selected analysis and simulations.
|Fig. 5: (a) schematic of DRIFT-II (b) DRIFT-II|
Fig. 5: (c) DRIFT-II at Boulby with and without Shielding.
From this first version, run at Boulby until 2007 with a central cathode of 20 μm SS wires, two key results emerged [36, 37]. Firstly, the phenomenal electron rejection of an NITPC arises because WIMP interactions have short tracks with high ionization density relative to background electrons [38, 39]. Low energy alphas can still trigger the detector but are rejected on the basis of their measured range . DRIFT-IIa was first to demonstrate this use of dE/dx measurement, tuned so the detector essentially never triggers on the large background of Compton recoil electrons yet retains high efficiency for nuclear recoils . Analysis of 252Cf neutron exposures found agreement with GEANT4 simulations to 5% and with cuts in place and threshold of 1000 Negative Ion Pairs (NIPs) (47.2 keV S recoil), a nuclear recoil detection efficiency of ~60% was achieved with a gamma rejection factor, measured using 137Cs, of >8×10-6 [41, 39]. Secondly, data from shielded dark matter runs revealed a large population of recoils at ~100 keV. Analysis showed these to arise from 222Rn decays whose progeny attach electrostatically to the central cathode wires (see Fig. 6). In 37% of subsequent radon progeny alpha decays the alpha buries itself in the 20 μm cathode wire, becoming invisible to the detector but producing a WIMP-like recoiling Pb nucleus . Development of mitigation against these radon progeny recoils (RPRs), here confirmed as the dominant background, has been a prime basis of subsequent upgrades.
Fig. 6: Schematic showing how radon progeny recoil (RPR) is formed on cathode wire and seen when alpha is lost inside the wire.
For this upgrade all components of DRIFT-IIa were tested for Rn out-gassing using a purpose-built Rn detector at Sheffield, then replaced with non-emanating alternatives. This yielded a 10-fold decrease in Rn inside the detector . However, measurements at various gas flow rates showed that RPRs also come from 210Pb plated on the cathode wires. A process of nitric acid wash was developed at Boulby to tackle this. These combined efforts achieved for DRIFT-IIb a total RPR reduction of x30 over DRIFT-IIa.
This duplicate detector was built for Occidental, to allow one detector to continue underground, and one above ground with easy access for tests. DRIFT-IIc work focussed on directional recoil experiments using a 252Cf source to quantify the powerful directional head-tail signatures . Simulations by Sheffield first indicated that this should be possible in CS2 . Observation of S recoils in DRIFT-IIc confirmed this for the first time experimentally (see Fig. 7a). These tests also first quantified the directional range-based signature. In DRIFT’s 40 torr gas typical heavy (F and S) recoils from WIMPs are 1-2 mm long, so observing this signature is broken into two parts. When Cygnus lies near the x axis the Δx component should, on average, be bigger than when Cygnus lies near the z axis. Using DRIFT-IIc,d at thresholds relevant to dark matter the collaboration has been able to observe this range “oscillation” signal with neutrons  (Fig. 7b). It is weaker than head-tail but combined with that should allow a detection of WIMPs with only a few 10s of events . Recent Sheffield analysis developed to interpolate grid pulses has now also allowed first production of full reconstructed 3D tracks (see Fig. 7c).
Fig. 7: (a) Head-tail directionality observed in DRIFT-IIc from S-recoils induced by neutrons
Fig 7: (b) Recent Sheffield analysis showing directionality observed in DRIFT-IId by asymmetry in x and z orientation of recoil tracks. The blue points are from data taken with reduced electronics with filtering done in software.
Fig 7: (c) 3D track reconstruction by Sheffield analysis (NB: x-y direction errors under development) (see text for refs).
DRIFT-IIdIn 2008/9 work to develop new gas mixtures started with aim to open sensitive to SD WIMP-proton interactions using CF4 (19F has ground state spin-parity ½+ from an un-paired proton). The flexibility to change target material on short timescales is a major advantage uniquely opening for DRIFT the possibility, exploited in this PRD, to play a crucial role in disentangling the claimed detections of WIMPs. Key results here were confirmation that CS2 operates well with many target gas additives, notably Xe, He, CO2 and CF4, yet retains the low diffusion, negative ion transport phenomenon [44, 45]. Recently CS2 has been found to operate even with 95% added CF4 . DRIFT-IIb was then upgraded with this and a new UK-designed daq and further Rn mitigation to produce DRIFT-IId. Fig. 8 shows results of 47.4 live days from 139 g of a 30 Torr CS2 – 10 Torr CF4 gas with data passing analysis cuts shown in black. These events are due to remaining RPRs from the cathode wires. They suffer maximum diffusion, over 50 cm, and so typically have larger widths than WIMP recoils, as demonstrated by the red points from a 252Cf neutron calibration. This basic z-fiducialisation allows an acceptance signal window, shown in tan, to be drawn to avoid background events but keep recoil sensitivity. Despite the small mass and acceptance efficiency the SD limit from this analysis is comparable with other experiments, but with directionality (see Fig. 1) .
Fig. 8: (a) DRIFT-IId data showing RPR events (black) and neutrons (red) plus selection widow used for limits, (b) limits set by DRIFT-IId from recent blind analysis prior to fiducialisation with directional capability in place (for refs. see text).
DRIFT-II Radon Mitigation and with Z-fiducialization
Despite success of basic z-fidcualisation and first Rn mitigation efforts by us, the primary background in DRIFT has remained central cathode RPRs. However, 2012/13 has seen major R&D advances to solve this issue, with wide implications. The UK has worked on all aspects, introducing new Rn scrub techniques, new RPR analysis, on underground fabrication of new thin cathodes. Sheffield designed and built the new triple-gas control manifold for safe use of minority carrier O2 used for the new z-fiducialisation.
Radon and gas Purity Apparatus at Sheffield
UK R&D here has focussed on new technology to reduce the intrinsic Rn content in the TPC gas. In addition to design and construction of the target gas mix and control manifolds for DRIFT-IId and e (see Fig. 9), Sheffield has already built 3 dedicated gas test stands (GTS) (see Fig. 10). These are:
- a facility to measure low-level Rn emanation from detector components (at <1 mBq/m3),
- a gas recirculation test stand with inter-changeable home-built filters to study the effectiveness of Rn reduction schemes in different gas mixes, and
- a large multi-target gas recirculation rig with residual gas analyser (RGA) and cartridges for testing absorption of electro-negative and other impurities. Progress in this area is detailed further in WP2.
|Fig. 9: Sheffield-built WIMP target gas mix and control system at Boulby, including modification for O2 safe mixing.|
Fig. 10: (a) Sheffield gas rig GTS-1 for radon emanation studies.
Fig. 10 (b) Sheffield rig GTS-2 for radon filtration tests.
Fig. 10: (c) Sheffield rig GTS-3 for gas impurity tests and filtering with CS2 (see text for refs).
Recent Radon Emanation Results
|Sample||Rn emanation (atoms/s)||Scaling & Notes|
|Black HV cables||0.1069±0.0134||None, full set tested|
|Rubber bungs (old)||0.0333±0.0027||x2 and x0.718. ½ number of bungs.|
|Aluminized Mylar||0.0076±0.0046||x2. Sample was ½ cathode area.|
|FEP ribbon cables||0.00±0.02||None|
|PTFE signals cables||0.00±0.02||None|
|Total||0.258±0.034||GPCC implied rate 0.0277 ± 0.017|
|Sample||Rn emanation (atoms/s)||Scaling & Notes|
|White HV cables||0.0053±0.0019||None, full set tested|
|20 silicone bungs||0.0129±0.0015||x2 and x0.718. ½ number of bungs.|
|Aluminized Mylar||0.0076±0.0046||x2. Sample was ½ cathode area.|
|FEP ribbon cables||0.00±0.02||None|
|PTFE signals cables||0.00±0.02||None|
|Total||0.136±0.031||GPCC implied rate 0.051 ± 0.013|
Radon Reduction and Gas Purity Tests
The stringent background specifications for scale-up, for all WIMP masses, plus the finite efficiency of any rejection scheme, means it remains vital to reduce intrinsic levels. A multi-pronged strategy has been developed by us, using the 3 specialist gas test stands (GTS1-3). This has included leading UK work on measurement of Rn emanation from detector parts as above and replacement by alternatives. Together with acid etching of MWPCs, improved analysis cuts and use of ultra-thin, alpha-transparent central cathodes, this strategy has so far reduced rates by ~x500 compared to DRIFT-IIa. However, despite this, it is clear that active absorption of Rn, that will always emanate from detector components, is needed for scale-up. Note the issue is not Rn in the gas supply, which can be controlled by ageing, but emanation from materials in the detector. The core technical challenge here is to establish a Rn filtration set-up through which the low pressure WIMP target gas mixtures can be passed that does not itself adversely disturb the target gas mix or introduce new impurities. Initial work at Sheffield on various getters has indicated a feasible route, for instance using ATC carbon. The Fig. 10 (d) and (e) below shows first results of Rn absorption in ATC carbon using rig GTS-2 with an internal calibrated Rn source. Efficient Rn absorption is seen when gas flow is diverted through the filter. Conversely using rig GTS-3, when real target gas CS2 is passed through the filter although there is absorption it is not severe and saturates to a constant level.
Fig. 10: (d) Data from the Sheffield radon trap rig GTS-2 showing effect on Rn circulation vs. time when an ATC carbon filter is introduced, and then removed, (e) data from the Sheffield CS2 gas recirculating rig GTS-3 showing the effect on CS2 pressure of passing the gas through the same ATC filter, pressure drops but is then stable (see text for refs).
Textured Thin `Film Cathode
As described above radon contamination is the primary background in DRIFT, specifically RPRs from Rn daughters like 218Po plated on the central cathode wires and undergoing an alpha decay (see Fig. 6). In this case, the resulting 214Pb daughter either enters the detector volume or the 20 μm wire, while the accompanying alpha does the opposite. If the alpha enters the detector, the event is tagged and vetoed but in ~37% of decays the alpha is lost in the wire (its range is 12 μm), and only the 214Pb recoil (RPR) is observed. A further solution developed by DRIFT to mitigate this is to use an ultra-thin cathode such that fewer alphas are absorbed. Such a cathode, comprising 0.9 μm aluminised film , was successfully developed by UNM and installed on DRIFT-IId. Data from this gave a further factor ~30 drop in RPR rate (see below).
(The above plots show the reduction in RPR backgrounds achieved with the thin film cathode. Remaining events are discussed below).
More importantly an upgrade version has been texturised such that the longest linear dimension is considerably shorter than the alpha range in the material, thereby making the film essentially completely transparent to alphas (see Fig. 11). Latest data from DRIFT-IId with this is now consistent with complete elimination of RPRs at the measuring sensitivity achievable.
Fig. 11: (a) Schematic of texturised cathode showing how alphas from Rn progeny recoils have path length always greater than maximum film dimension and so always escape and can be vetoed
Fig. 11: (b) Photo of thin film installation and of texturized thin film manufactured at UNM
The DRIFT MWPCs have edge guard regions in x and y planes to veto in-coming backgrounds parallel to the MWPCs, but complete detector fiducialisation requires also a means to localise events in the perpendicular z-direction, particularly to veto events (RPRs) from material in the central cathode. Achieving this has been a major goal for the field, made hard because detection of ionisation there (+ve ions) is difficult as no avalanche gain is feasible on the cathode at -30 kV. New research has now confirmed a startling alternative that yields absolute positioning of all events in the volume . The technique, minority carrier drift with oxygen, was installed on DRIFT-IId in June 2013, resulting in zero background operation for the first time. The O2 additive (~1%) works such that in addition to the normal CS2 –ve ions produced by particle interactions, one or more minority molecule species are also produced that travel at faster velocity. This results in extra pulses arriving prior to the main pulse, with time separation proportional to the z-drift distance traversed by the ions between interaction point and readout plane (see Fig. 12a). Fig. 12b shows part of example data in DRIFT-IId from a calibration alpha depositing charge in a track directed away from the MWPC. Minority pulses are seen prior to main peaks but with time separation increasing for charge deposited further away from the MWPC. This allows absolute position reconstruction. Recent DRIFT-IId data indicates complete event by event fiducialisation with >90% acceptance .
Fig. 12: (a) Typical recoil charge pulse with O2 minority carrier pre-pulses that allow absoulte z-location to be determined by time, (b) example alpha track with progression of minority peaks vs. z-position seen to increase on RHS, (c) typical S-recoil with pre-pulse allowing positioning to a few cm (see text for refs).
Given the DRIFT-II success the collaboration has proceeded with deign of the 24 m3 DRIFT-III (see below) and in parallel built a new 1 m3 detector, DRIFT-IIe, to test all major components (see Figs 13, 14). Sensitivity predictions for DRIFT-II and DRIFT-III are shown in Fig. 1.
For DRIFT-III UK effort has been key to the Boulby lab design and shielding but notably to new lower-noise electronics, and the complex gas mix and recirculation system required to handle multiple target gases including O2. For DRIFT-IIe UK groups designed and built the vessel, multiplex electronics, slow and gas control, shielding, site, and has responsibility for operations.
The following photos show the design, construction and installation of DRIFT-IIe for Boulby started in mid 2013.
Fig. 13: (a) The DRIFT-IIe vessel shipping to Boulby and installation following vacuum tests at Sheffield.
Fig. 13 (b) The DRIFT-IIe high voltage feeds from UNM.
Fig. 13 (c) Design and installation of neutron shielding for DRIFT-IIe and DRIFT-IId (2013).
Fig. 13 (d) DRIFT-IIe design and construction of field cages and MWPC.
Fig. 13 (e) DRIFT-IIe construction at Oxy
Fig. 13 (f) DRIFT-IIe gas supply system from Sheffield
The DRIFT-III design includes several new innovations currently being optimised in DRIFT-IIe: (i) segmented vessel to allow ease of scalability suitable for Boulby tunnels, (ii) 2 x 2 m MWPCs with single plane of alternating grid-anode wires viewing 2 back-to-back drift volumes of 50 cm, (iii) texturised central cathode and fiducialisation with O2, (iv) all channels separately instrumented with low noise electronics, (v) provision of intrinsic Rn scrub and gas recirculation. Aspects (iv) and (v) are closely matched to the two aims and workpakages of this PRD, WP1 and WP2. DRIFT-IIe will run simultaneously with DRIFT-IId using the new Sheffield gas systems.
Fig. 14: Schematics of DRIFT-III.
A core aspect of DRIFT-III will be lower noise electronics. In preparation for this DRIFT-IIe will be used to trial three new electronics concepts to allow optimisation for x5 reduced noise with scalable cost : (a) Colorado State University (CSU) daq – based on trans-impedance amplifier with “accumulator” board for digitizing, (b) Oxford University daq (Prof. Hans Kraus) – based on Edelweiss electronics, and (c) Sheffield University Multiplex daq with BNL front end (see Fig. 15).
Fig. 15: Sheffield Multiplex DAQ under development and (bottom right) Edelweiss electronics from Oxford University under test on DRIFT-IId.
Low Mass WIMP Directional R&D – Optical CCD Readout R&D
Along with the fiducialisation result above a very important new advance in DRIFT comes from low threshold directional studies with UNM using optical readout. It is this that reveals the possibility of directional sensitivity below 20 GeV. MWPCs have so far been used in DRIFT because m2 areas are easily feasible at low cost and the ~1mm position resolution, though quite poor, is well matched to the ~mm gas diffusion over 1 m. However, finer resolution TPCs using optical CCDs or xy strips with GEMs are a useful tool for studying the physics of directionality.
Sheffield pioneered use of CCDs for this a decade ago  and later with Edinburgh built early xy strip readouts. More importantly, Sheffield has contributed charge transport simulations and designs for optical R&D and, with Leverhulme funding, has built a new optical TPC with CMOS camera and a novel electron focussing double Thick GEM (see Fig. 16).
Fig. 16: Sheffield CMOS optical TPC vessel showing top quartz window. Fig. 16: (a) Sheffield latest Thick GEM from CERN and (b) GARFIELD++ simulation of this.
From the UNM work Fig. 17 shows example tracks from the UNM detector using 3 CERN GEMs (hole pitch ~140 μm) and a back-illuminated 10 e- noise CCD with 100 Torr CF4. Full details are in . Clear head-tail is evident for high energy events (Fig. 17b) as also observed by DM-TPC  but for the first time this is seen even at low energy (10 keVee in Fig.17c) as measured by a simple skewness parameter calculated by projecting the track pixels along the major axis. Moreover, electrons of similar energy (12 keVee example in Fig. 17d) show very different dE/dx topology, easily rejected if the S/N is sufficient to reveal the full track details. A core advance here is discovery of very high light gain with CS2 using GEMs (~x200,000).
Using these preliminary results, the interest is to determine an optimum target gas and pressure to maximise the event rate above a directional energy threshold sufficient to allow directional detection of low mass WIMPs, such as the CDMS result at 8.6 GeV/c2 . Firstly, Fig. 18 shows the event rate vs. target mass and energy thresholds expected for the CDMS example. Note the A2 dependence means the highest event rate is not when the target and WIMP masses are well-matched. Now, for directional detection we need to know the directional threshold not just the energy threshold. A core aim of this PRD, is to experimentally determine this better and optimize it. The new CCD data already shows that at least ~600 μm is feasible, but it will depend on the chosen recoiling target, the medium that its recoiling in and the pressure. For instance, lowering the pressure will lower the directional energy threshold needed to give a 600 μm track. Since the recoil energy spectrum is exponentially falling with energy, a slight decrease in threshold can give a large increase to the total rate. But lowering the pressure will also decrease the rate because of the reduction in target material. Fig. 19 shows, for examples CO2, CF4, and CS2 and Eth = E (600 μm), the effect on the event rate of these competing issues, for a fixed volume rather than a fixed target mass. It can be seen that the event rate turns over at some pressure for a given target as expected (the peaks correspond to an energy threshold of ~5 keVee). More importantly it indicates that even with this directional threshold volumes of a few m3, like DRIFT-II, are sufficient for sensitivity at the CDMS, or similar, level (see Fig. 20). Thanks to UNM Prof. Dinesh Loomba for these plots Fig. 18-20.
Low Mass WIMP Directional R&D – XY-Strip Readout R&D
The small-scale optical readouts above are well suited to physics studies, but the cost and internal backgrounds from the multiple cameras and optics required for a full experiment mean that charge-based readout is more appropriate. MWPCs have so far been chosen for DRIFT rather than xy strips with GEMs or Micromegas. This is because MWPCs are easily capable of m2 areas at low cost, have resolution well matched to the gas diffusion (~mm), and have lower intrinsic background because of the minimal materials used (simply 20 μm wires). However, discovery of the new minority carrier fiducialisation technique and the new requirements for low mass WIMPs now changes matters. Firstly, the latter can clearly benefit from the improved position resolutions of xy strips (~200 μm), even though this means reducing diffusion by increasing field voltage or reducing drift distance. Secondly, although the readout planes still need minimal background from materials, the fiducialisation now relaxes this requirement. Meanwhile, micro-strip and GEM technology has also progressed rapidly recently, achieving deployment at the m2 scale . Furthermore, new measurements have anyway shown that very low background is feasible (e.g. <9.3 μBq/cm2 232Th, < 13.9 μBq/cm2 235U ). Based on this work is progressing on a new scheme for directional detectors using xy strip readout with CS2+O2. Fig. 21 shows a 10 x 10 cm 200 μm pitch strip readout from UNM that can be combined the thick GEM gain stage we have built for tests of this concept.
|Fig. 21: UNM xy-strip readout and test alpha event.|
The CYGNUS Global Directional Collaboration (tonne-scale)
Regardless of the possible low mass WIMP signals at relatively high SD cross section an eventual tonne-scale directional experiment will be needed to study halo WIMP velocity distributions and reach SI cross sections below non-directional experiments. Such a device could occupy a volume similar to MINOS or ~1/10th the NOVA neutrino experiment. Nevertheless, recognising that construction would require a global effort, the various directional groups undertook formation in 2007 of a new cooperation to unite experimental and theoretical efforts towards a joint 1 tonne design we call CYGNUS. A concept document for this was published in 2010 . This concludes that the cost driver is not the volume itself, as underground excavation is cheap, but the readout electronics cost per channel. The group includes all 5 main TPC efforts world-wide, DRIFT, MIMAC, DM-TPC, NEWAGE and D3, with ~150 scientists from 20 institutes and 8 countries. In subsequent CYGNUS meetings at MIT (2009), Stockholme (2010), Modane (2011), Chicago (2012) and Toyama (2013)  work has progressed on a set of agreed workpackages, well aligned with this PRD: WP1 target gas physics, WP2 directional sensitivity, WP3 background rejection, WP4 readout techniques, WP5 vessel and shielding, WP6: application to other physics.CYGNUS2007
Wider Impact of DRIFT-CYGNUS
The sensitivity of the detector concepts of DRIFT-CYGNUS using a low pressure gas time projection chamber technology has multiple and wide benefits in non-academic areas requiring ultra-low background and fine-grain particle identification, including in industry. For instance, background rejection is one of the most important problems encountered in fast neutron detection. The proposed advances that are focused on producing very accurate tracking of nuclear recoils provide a route to enabling improved neutron source localization. This opens a broad range of applications in homeland security and nuclear safety. The directional tracking capability is also relevant to the new field of muon tomography. The commonality here is due to the remarkable particle identification and directional tracking capacity made feasible with very low background in our TPC detectors. Some routes to impact are detailed as follows:
- in medicine by improving prospects for background reduction in new instrumentation for PET/SPECT scanners. Here the fine grain strip readout proposed can greatly aid development of charge readout, providing a new concept to improve the speed of patient scans, and hence lower patient does, and also lower the cost relative to conventional scintillator devices.
- in homeland security where a new breed of directional fast neutron detector using the new TPC technology holds prospect for uses in locating illicit nuclear material transit, for instance at ports and transport hubs. A further application is in assay of scrap steel material arriving at steel plants where contamination by fission material is a large problem.
- in radio-assay of rare isotopes for forensics and environmental science. The concepts proposed allows for development of a new type of assay detector whereby samples are inserted into a low pressure, low background, TPC such that emanated alphas, low energy x-rays or betas from contamination can be tracked in 3D and monitored against time. Rare isotopes such as 218Po can be monitored. The capability to detect radon at very low levels is also of interest in environmental studies, for instance as a means of detecting rock movement underground.
- in the nuclear industry. As for (3) such a device would open possibility for assay of rock samples required in the study of propagation of radio-nuclide from nuclear waste. This is required in research of techniques for storage of radioactive waste. The low background assay capability of the techniques proposed also has a role here.
- the TPC technology also opens new areas of muon tomography for mining. Here the concept is to use the accurate ionization charge tracking as a means to determine the angular distribution and intensity of muons in an underground setting. This can be translated into a density map of the overburden. This has applications in mining where there is interest to do in-situ mapping for mineral exploitation and also to map water movement that may be a danger to workings. Further geo-physics applications of muon tomography are also feasible with our technology, for instance in the study of volcanoes and of earth movement as part of land slip predictions.
- in the climate change field through application in carbon sequestration. As with (5) the concept here is of new technology to map overburden using muon tomography. Here the application is to allow monitoring of stored CO2 underground as a means to verifying safety of storage for climate change mitigation. TPCs are attractive particularly if the gas purity issues addressed above can be implemented.
- in the wider field of detectors there are numerous potential applications for improved charge readout schemes that can benefit from our proposed thick GEM and strip readout advances.
- in wider industry there is a broad range of potential further routes to impact from several spin-out applications form the technology. Examples are: i) muon and neutron cosmic tracking and detection, ii) x-ray assay, iii) assay of alpha particles from radon and other radioactive sources, iv) high sensitivity metrology of neutrons, v) BiPo coincidence studies from 222Rn and 220Rn, vi) fundamental physics of particle interactions in gases, vii) studies of cosmic cascade in matter, and (viii) studies of transport of radio-nuclides through materials.
- Finally, the high interest in dark matter research, linked to study of particles and underground science, allow societal impact to the public and to schools through lectures and popular dissemination.
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Sheffield DRIFT PhD thesisT. Lawson, PhD thesis, University of Sheffield (2006)
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