Helium-3 spin echo Helium-3 can be used to do
spin echo experiments of surface dynamics, which are underway at the Surface Physics Group at
the Cavendish Laboratory in Cambridge and in the Chemistry Department at
Swansea University.
Neutron detection Helium-3 is an important isotope in instrumentation for
neutron detection. It has a high absorption cross section for thermal
neutron beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction :n + 3He → 3H + 1H + 0.764 MeV into charged particles
tritium ions (T, 3H) and
hydrogen ions, or protons (p, 1H) which then are detected by creating a charge cloud in the stopping gas of a
proportional counter or a
Geiger–Müller tube. Furthermore, the absorption process is strongly
spin-dependent, which allows a
spin-polarized helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in
neutron polarization analysis, a technique which probes for magnetic properties of matter. The United States
Department of Homeland Security had hoped to deploy detectors to spot smuggled plutonium in shipping containers by their neutron emissions, but the worldwide shortage of helium-3 following the drawdown in nuclear weapons production since the
Cold War has to some extent prevented this. As of 2012, DHS determined the commercial supply of
boron-10 would support converting its neutron detection infrastructure to that technology.
Cryogenics Helium-3 refrigerators are devices used in experimental physics for obtaining temperatures down to about 0.2
kelvin. By
evaporative cooling of helium-4, a
1-K pot liquefies a small amount of helium-3 in a small vessel called a helium-3 pot. Evaporative cooling at low pressure of the liquid helium-3, usually driven by
adsorption since, due to its high price, the helium-3 is usually contained in a closed system to avoid losses, cools the helium-3 pot to a fraction of a kelvin. A
dilution refrigerator uses a mixture of helium-3 and helium-4 to reach
cryogenic temperatures as low as a few thousandths of a kelvin. Helium-3 the primary coolant for
superconducting quantum computing.
Nuclear magnetic resonance Helium-3 nuclei have an intrinsic
nuclear spin of
ħ, and a relatively high
gyromagnetic ratio. Because of this, it is possible to use
Nuclear magnetic resonance (NMR) to observe helium-3. This analytical technique, usually called 3He-NMR, can be used to identify helium-containing compounds. It is however limited by the low abundance of helium-3 in comparison to helium-4, which is itself not NMR-active. Helium-3 can be
hyperpolarized using non-equilibrium means such as spin-exchange optical pumping. During this process,
circularly polarized infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in an
alkali metal, such as
caesium or
rubidium inside a sealed glass vessel. The
angular momentum is transferred from the alkali metal electrons to the noble gas nuclei through collisions. In essence, this process effectively aligns the nuclear spins with the magnetic field in order to enhance the NMR signal. The hyperpolarized gas may then be stored at pressures of 10 atm, for up to 100 hours. Following inhalation, gas mixtures containing the hyperpolarized helium-3 gas can be imaged with an MRI scanner to produce anatomical and functional images of lung ventilation. This technique is also able to produce images of the airway tree, locate unventilated defects, measure the
alveolar oxygen partial pressure, and measure the
ventilation/perfusion ratio. This technique may be critical for the diagnosis and treatment management of chronic respiratory diseases such as
chronic obstructive pulmonary disease (COPD),
emphysema,
cystic fibrosis, and
asthma. Because a helium atom, or even
two helium atoms, can be encased in
fullerene-like cages, the NMR spectroscopy of this element can be a sensitive probe for changes of the carbon framework around it. Using
carbon-13 NMR to analyze fullerenes themselves is complicated by so many subtle differences among the carbons in anything but the simplest, highly symmetric structures.
Radio energy absorber for tokamak plasma experiments Both MIT's
Alcator C-Mod tokamak and the
Joint European Torus (JET) have experimented with adding a little helium-3 to a H–D plasma to increase the absorption of radio-frequency (RF) energy to heat the hydrogen and deuterium ions, a "three-ion" effect.
Nuclear fuel {{chem2|^{3}He}} can be produced by the low temperature fusion of → {{chem2|^{3}He}} + γ + 4.98 MeV. If the fusion temperature is below that for the helium nuclei to fuse, the reaction produces a high energy alpha particle which quickly acquires an electron producing a stable light helium ion which can be utilized directly as a source of electricity without producing dangerous neutrons. increases rapidly with temperature until it maximizes and then gradually drops off. The DT rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy. {{chem2|^{3}He}} can be used in fusion reactions by either of the reactions {{chem2|^{2}H + ^{3}He -> ^{4}He + ^{1}p}} + 18.3
MeV, or {{chem2|^{3}He + ^{3}He -> ^{4}He + 2 ^{1}p}} + 12.86 MeV. The conventional
deuterium +
tritium ("
D–T") fusion process produces energetic neutrons which render reactor components
radioactive with
activation products. The appeal of helium-3 fusion stems from the
aneutronic nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy by-product, the
proton, can be contained by means of electric and magnetic fields. The momentum energy of this proton (created in the fusion process) will interact with the containing electromagnetic field, resulting in direct net electricity generation. Because of the higher
Coulomb barrier, the temperatures required for {{chem2|^{2}H + ^{3}He}} fusion are much higher than those of conventional
D–T fusion. Moreover, since both reactants need to be mixed together to fuse, reactions between nuclei of the same reactant will occur, and the D–D reaction ({{chem2|^{2}H + ^{2}H}}) does produce a
neutron. Reaction rates vary with temperature, but the D–{{chem2|^{3}He}} reaction rate is never greater than 3.56 times the D–D reaction rate (see graph). Therefore, fusion using D–{{chem2|^{3}He}} fuel at the right temperature and a D-lean fuel mixture, can produce a much lower neutron flux than D–T fusion, but is not clean, negating some of its main attraction. The second possibility, fusing {{chem2|^{3}He}} with itself ({{chem2|^{3}He + ^{3}He}}), requires even higher temperatures (since now both reactants have a +2 charge), and thus is even more difficult than the D-{{chem2|^{3}He}} reaction. It offers a theoretical reaction that produces no neutrons; the charged protons produced can be contained in electric and magnetic fields, which in turn directly generates electricity. {{chem2|^{3}He + ^{3}He}} fusion is feasible as demonstrated in the laboratory and has immense advantages, but commercial viability is many years in the future. The amounts of helium-3 needed as a replacement for
conventional fuels are substantial by comparison to amounts currently available. The total amount of energy produced in the {{chem2|^{2}D + ^{3}He}} reaction is 18.4 M
eV, which corresponds to some 493
megawatt-hours (4.93×108 W·h) per three
grams (one
mole) of {{chem2|^{3}He}}. If the total amount of energy could be converted to electrical power with 100% efficiency (a physical impossibility), it would correspond to about 30 minutes of output of a gigawatt electrical plant per mole of {{chem2|^{3}He}}. Thus, a year's production (at 6 grams for each operation hour) would require 52.5 kilograms of helium-3. The amount of fuel needed for large-scale applications can also be put in terms of total consumption: electricity consumption by 107 million U.S. households in 2001 totaled 1,140 billion kW·h (). Again assuming 100% conversion efficiency, 6.7
tonnes per year of helium-3 would be required for that segment of the energy demand of the United States, 15 to 20 tonnes per year given a more realistic end-to-end conversion efficiency. A second-generation approach to controlled
fusion power involves combining helium-3 and
deuterium, {{chem2|^{2}D}}. This reaction produces an
alpha particle and a high-energy
proton. The most important potential advantage of this fusion reaction for power production as well as other applications lies in its compatibility with the use of
electrostatic fields to control fuel
ions and the fusion protons. High speed protons, as positively charged particles, can have their kinetic energy converted directly into
electricity, through use of
solid-state conversion materials as well as other techniques. Potential conversion efficiencies of 70% may be possible, as there is no need to convert proton energy to heat in order to drive a
turbine-powered
electrical generator.
He-3 power plants There have been many claims about the capabilities of helium-3 power plants. According to proponents, fusion power plants operating on
deuterium and helium-3 would offer lower capital and
operating costs than their competitors due to less technical complexity, higher conversion efficiency, smaller size, the absence of radioactive fuel, no air or water
pollution, and only low-level
radioactive waste disposal requirements. Recent estimates suggest that about $6 billion in
investment capital will be required to develop and construct the first helium-3 fusion
power plant. Financial break even at today's wholesale
electricity prices (5 US cents per
kilowatt-hour) would occur after five 1-
gigawatt plants were on line, replacing old conventional plants or meeting new demand. The reality is not so clear-cut. The most advanced fusion programs in the world are
inertial confinement fusion (such as
National Ignition Facility) and
magnetic confinement fusion (such as
ITER and
Wendelstein 7-X). In the case of the former, there is no solid roadmap to power generation. In the case of the latter, commercial power generation is not expected until around 2050. In both cases, the type of fusion discussed is the simplest: D–T fusion. The reason for this is the very low
Coulomb barrier for this reaction; for D+3He, the barrier is much higher, and it is even higher for 3He–3He. The immense cost of reactors like
ITER and
National Ignition Facility are largely due to their immense size, yet to scale up to higher plasma temperatures would require reactors far larger still. The 14.7 MeV proton and 3.6 MeV alpha particle from D–3He fusion, plus the higher conversion efficiency, means that more electricity is obtained per kilogram than with
D–T fusion (17.6 MeV), but not that much more. As a further downside, the rates of reaction for
helium-3 fusion reactions are not particularly high, requiring a reactor that is larger still or more reactors to produce the same amount of electricity.
Alternatives to He-3 To attempt to work around this problem of massively large power plants that may not even be economical with D–T fusion, let alone the far more challenging D–3He fusion, a number of other reactors have been proposed – the
Fusor,
Polywell,
Focus fusion, and many more, though many of these concepts have fundamental problems with achieving a net energy gain, and generally attempt to achieve fusion in thermal disequilibrium, something that could potentially prove impossible, and consequently, these long-shot programs tend to have trouble garnering funding despite their low budgets. Unlike the "big" and "hot" fusion systems, if such systems worked, they could scale to the higher barrier
aneutronic fuels, and so their proponents tend to promote
p-B fusion, which requires no exotic fuel such as helium-3. == See also ==