Superfluid Helium Could Reveal Lightweight WIMPs


Gravitational lensing of background galaxies by galaxy cluster Cl 0024+17 strongly suggests the presence of dark matter. Despite such astronomical evidence, physicists are yet to spot dark matter in the laboratory. (Courtesy: NASA, ESA, M J Jee and H Ford/Johns Hopkins University)

A new detector sensitive to low-mass dark matter particles too light for current experiments to see has been proposed by physicists in the US. Built around a bath of superfluid helium-4, the device would use field ionization to spot single helium ions ejected from the superfluid’s surface by colliding weakly interacting massive particles (WIMPs). The existence of dark matter has been inferred from unexpectedly high stellar and galactic velocities since the early 20th century, and recent observations of gravitational waves have only strengthened the case by ruling out some competing modified-gravity models. Despite the efforts of dozens of experimental collaborations worldwide, dark matter particles have not been detected directly. However, WIMPs are still the dark matter candidate favoured by most physicists.

Unexplored regions

Dark matter surveys conducted until now have focused largely on high-mass particles, and are relatively insensitive to candidates lighter than 10 GeV/c2, or about ten times the mass of the proton. Some recent theories have proposed WIMPs with masses below this threshold, so with a view to filling this observational gap, Humphrey Maris, George Seidel and Derek Stein at Brown University conceived a detector model that could extend the lower mass limit by three or four orders of magnitude. The team decided on 4He for the detector mass since it receives more energy per collision than heavier targets, and the low internal radioactivity minimizes false positive results. When dark matter particles interact with the target, recoiling helium atoms are expected to trigger phonons and rotons – quasiparticle excitations – which, in superfluid 4He, can propagate without scattering. When these excitations reach the surface of the superfluid, helium atoms are expelled by quantum evaporation. A similar technique was developed a decade ago by Maris, Seidel and colleagues at Brown University for the HERON neutrino detector. In that experiment, evaporated helium atoms were deposited on a silicon wafer calorimeter suspended above the superfluid, causing a measurable increase in temperature. “This worked fine if a large amount of energy was deposited in the liquid thereby producing many rotons and many atoms,” explains Maris. “But the method was inadequate for the detection of the small number of atoms that would be evaporated if the energy deposit was by a dark matter particle with, for example, a mass of 1 MeV.”

Single-atom sensitivity

The novelty of the new approach lies in the device’s sensitivity to individual atoms. This makes the minimum detectable transferable kinetic energy (the energy imparted to a helium nucleus by a dark matter collision) equal to the binding energy of a helium atom to the liquid. Since no existing large-area calorimeter could be sensitive to such tiny energies, individual helium atoms ejected at low speed can only be detected if they are first accelerated significantly. The trick proposed by the team at Brown University is to have evaporated atoms pass near to arrays of positively charged, sharp metal tips. Strong local electric fields ionize the helium, and the resulting positive ions are accelerated toward a cathode at energies within the range detectable by current calorimeters. “The addition of the field ionization opens up the possibility of detecting energy deposits into the helium that are smaller by a factor of about 10,000 than in the previous work that we did. This will make it possible to detect dark matter in a mass range far below what has been previously achieved,” Maris told physicsworld.com. Assuming the Standard Halo Model of dark matter distribution – in which the galaxy is permeated uniformly by WIMPs of a single type, and the local galactic escape velocity is the maximum particle speed allowed – the researchers expect such single-atom sensitivity to translate to a detectable dark matter particle mass of 0.6 MeV/c2, or less than a thousandth the mass of a proton. A modified scheme that could achieve even greater sensitivity has also been presented by the group. Instead of employing bulk helium as the detector mass, a solid crystalline target could be used, which would also be susceptible to phonons initiated by colliding WIMPs. A helium film coating the crystal would exhibit the same excitation-induced quantum evaporation effect but with a lower phonon energy threshold. By lining an ultrapure target crystal with a few monolayers of caesium (to which 4He binds especially weakly), an atomically thin film of helium could further lower the WIMP mass sensitivity by orders of magnitude.

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Tanzanian Helium Discovery May Be Double Initial Estimate

Helium resources discovered in Tanzania last year may be almost twice as large as first thought, helping address possible future shortages of the gas used in medical scanners and nuclear energy when production begins, possibly as early as 2020, the exploration company said. Researchers working with Lisbon-based Helium One Ltd. calculated the Rukwa gas field in southwestern Tanzania could contain as much as 98.9 billion cubic feet of the resource, Chief Executive Officer Thomas Abraham-James said in an interview. In June 2016, researchers from the universities of Oxford and Durham in England said the “probable resources” of the “game-changer” discovery were 54 billion cubic feet, seven times the world’s total annual consumption of the inert gas. “Once we complete the exploration, define the gas and the asset has matured, we will be confident about the nature of the gas facility,” Abraham-James said by phone. The company intends to convert the sources to reserves next year and then “can determine the likely size of the production scenario,” he said. The inert gas is critical to technology such as MRI scanners, missile-guidance systems and industrial-leak detection systems, and known reserves are quickly running out. Tanzania’s resources are the first instance of helium being found by direct exploration rather than during a search for natural gas, according to Jon Gluyas, professor of geo-energy at Durham University, who was part of the first discovery team. He said the size of the Tanzanian find won’t be confirmed until test wells are drilled, although the world’s limited production sites mean development will impact global supply.

Global Supply

The U.S. is the world’s leading supplier of the gas, although it’s due to shut its Federal Helium Reserve in 2021, putting global distribution “under intense strain,” the U.S. Subcommittee on Energy and Minerals said in June as it pushed for legislation to allow for the extraction of the gas on federal lands. Qatar, Algeria and Russia are the other main producers, but the Middle Eastern country’s recent diplomatic standoff with its neighbors has raised concern over whether it can be a stable supplier, the subcommittee said. Helium One will need more financing to complete exploration and begin drilling, and will seek to refine and export the gas as Tanzanian demand for it is minimal, Abraham-James said. The U.S.’s total helium reserves and resources were estimated at 744 billion cubic feet, including 153 billion measured reserves, in December 2006, according to a U.S. Geological Survey in January. It put the rest of the world’s helium resources at about 1.13 trillion cubic feet. The Tanzania discovery “has a long way to go before its potential can be assessed,” said Maura D. Garvey, director of market research at Intelligas Consulting in Dedham, Massachusetts. “There are many questions as to whether the helium will be economically recoverable.” Wit a current “slight oversupply” of the gas in the market at the moment, “the potential volume and global impact need to be addressed for when the production can come on stream,” she said by email.

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Helium Play Would Need Better Royalty Rates

A better royalty rate in Alberta for helium is needed before the City of Medicine Hat and a private company will begin niche drilling programs in this province, officials tell the News. Both entities are currently drilling in Saskatchewan, and say the elusive but valuable gas is under southeast Alberta as well. However, pending the outcome of talks with the province on a new framework for extraction costs, they won’t greenlight drilling in southeast Alberta. “There’s more clarity (in Saskatchewan) that we’re looking for in Alberta,” Jeff Vogt, CEO of the Weil Group Canada, said this week as plans to process production in Medicine Hat were announced. It would receive helium from Weil’s leases in Saskatchewan, while talks to accept any city production from three proposed wells in that province are ongoing. Also in the works are discussions with the Alberta government to update its framework. “Saskatchewan has for 60 years had a program for helium for helium’s sake,” said Vogt. “We, along with Medicine Hat, have sought out that clarification (from Alberta), and I think that’s coming.” Royalties are essentially the price paid by companies extracting natural resources that are considered public property. The Alberta government updated its royalty schedule in 2016 — an election promise of the New Democrats — but the headline items were oil and natural gas rates. There is no history of helium exploration in Alberta and the more obscure commodity is lumped in with petroleum extraction that can be charged up to 30 per cent. Saskatchewan charges a 5 per cent rate on volumes of the inert gas that’s produced in the provinces. Officials with Alberta Energy Ministry say the government is committed to diversifying our energy sector and adding “value-added jobs.” “Given the recent interest in potential helium production, Alberta Energy officials are currently analyzing what the appropriate royalty for helium would be and will be making a recommendation to government soon,” said spokesperson Mike McKinnon in an email to the News. The News has learned that meetings took place in late March — about six months before the city’s helium strategy was made public between city administrators, the Medicine Hat Chamber of Commerce and provincial representatives. Mayor Ted Clugston said this week that changing the province rates would spur more drilling. “Five per cent of something is better than 30 per cent of nothing,” said Clugston. This week the Weil announced it was close to approving a new helium “CryoHub” that would gather, cool and liquify the gas for export out of Western Canada. The plant can be supplied with proven reserves at this point, said Vogt. However, it is “scalable” and could expand to accommodate more production from a bigger drilling program. “Medicine Hat is (geographically) central to these prospects,” said Vogt. “We’ve taken a look at several sites (in Alberta) and have some land assembled and have done the work to prove our reserves. We would do more with greater clarity.” The municipal energy exploration wing said this fall it will drill three helium-specific wells in Saskatchewan, and has an undisclosed number of prospects in Alberta as well. Success is based on finding dome-like formations of non-porous rock that capture volumes of the extremely light gas as it travels from greater depths. The city has said a substantial find would boost city revenue and also draw industry, such as Weil’s proposed plant and related industries. Talks between the two are “advanced” about processing. “They have there business case and we have our,” said Brand Maynes, the manager of the city’s petroleum exploration division, of talks with Weil. “We still need to talk about how it might look … but a lot of tough discussions have already happened.” Helium, which is highly sought after for industrial and high-tech manufacturing, super computers and other uses, is worth about 100 times more than natural gas but is found in much smaller amounts. Only 6 billion cubic feet of helium is produced and traded each year around the globe. The same volume of natural gas is produced each day from wells in Alberta alone.

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Largest Helium Facility Comes To Canada

The Weil Group Canada, a subsidiary of Weil Group Resources headquartered in Richmond, Virginia will be launching the first ever helium (He) liquefaction facility in Canada. The explorative company which sets out to seek, develop and produce untapped energy, has identified nearly two billion cubic feet of proven reserves of He and is moving to commission a central He liquefaction facility called the CryoHub™ in the city of Medicine Hat, Alberta within the next 20 months. The company has been in discussion with officials from Medicine Hat, to locate its new CryoHub facility in the city and to develop He resources in the region. The proposed facility would be a central hub to receive He production from the company’s other current projects which are located in Alberta and Saskatchewan. Weil was the first company in 60 years to bring commercial grade He on stream in Canada, when it began operations in its Mankota, Saskatchewan He purification facility in early 2016. Weil has been active in Canada since 2012 and has continued extensive exploration and development in search of the increasingly rare element.

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China Advances HTGR Technology


China’s State Nuclear Power Technology Corp. announced it has completed the installation of a high-temperature gas-cooled reactor consisting of two 250-MW high-temperature reactor pebble-bed modules. Courtesy: China Nuclear Engineering and Construction Group

China’s State Nuclear Power Technology Corp. (SNPTC) has completed the installation of its high-temperature gas-cooled reactor (HTGR) project, the joint venture told the International Atomic Energy Agency (IAEA). The reactor uses helium as a coolant instead of water. After the helium is heated to 750C (1,382F), it is sent to a steam generator where it heats water until it becomes high-temperature steam. That steam then flows into a steam turbine to generate electricity. The process uses a graphite-moderated nuclear reactor and a once-through uranium fuel cycle. SNPTC’s project, which consists of two 250-MW high-temperature reactor pebble-bed modules (HTR-PM, Figure 4), is located in Shandong province. Tests at the project are expected to end in April 2018, at which time the reactor will go into commercial operation. “The success of this project will establish a milestone for the nuclear industry. It will pave the way for others,” said Mikhail Chudakov, IAEA deputy director general for nuclear energy, as reported by World Nuclear News. Construction began on the reactor on December 9, 2012. The project is a joint venture of China Nuclear Engineering and Construction Group (CNEC) and Tsinghua University, which teamed up in 2003 “to shoulder the special task of advancing the national science and technology project including scientific research of the HTR, the engineering research and demonstrative project investment construction,” according to CNEC. The technology was praised during a roundtable discussion held at the IAEA General Conference. “Unlike typical reactors, high-temperature reactors are particularly suitable to generate high-temperature process heat in addition to electricity. High-temperature heat from advanced nuclear reactors may be able to have a direct role in climate change mitigation as an alternative energy source for industrial processes,” an IAEA release says. CNEC highlighted some potential uses of HTGR technology. “HTR produces the superheated steam at [571C], which can be applied in not only high-efficiency power generation, but also petrochemical industry and heavy oil thermal recovery with the implementation of heat and power co-generation,” according to the group. Looking to the future, the development of very-high-temperature reactors (VHTRs) could expand the potential uses of the technology even further. “The steam outlet temperature can reach up to [1,000C] to form the clean heat source, which can be applied in steel making, coal gasification [and] liquefaction and thermochemical hydrogen production. VHTR with its extensive potential is expected to become one of [the] major high-temperature heat source suppliers in the hydrogen era with great prospect,” according to CNEC. CNEC is also currently working with Saudi Arabia on the early stages of an HTGR desalination joint venture. A memorandum of understanding (MOU) was signed August 24, 2017, by Zu Bin, deputy general manager of CNEC, and Prince Turki bin Saud bin Mohammed Al-Saud, chairman of the King Abdulaziz City for Science and Technology and chairman of the board of directors of Saudi Technology Development and Investment Co. “According to the MOU, the two parties will work together to carry out [a] feasibility study on developing seawater desalination projects using [HTGR] and start negotiations and preparations for the establishment of an HTGR desalination joint venture, so as to deepen the two countries’ cooperation in HTGR projects and pave the way for tapping into [the] nuclear desalination market of Saudi Arabia,” according to CNEC.

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