Stable
isotope
3
He
4
He
Relative
atomic mass
3.016 029 32
4.002 603 2541
Mole
fraction
0.000 002
0.999 998
Helium isotopes in geochronology
3
He is a product of radioactive decay of 3H (half-life 12.32 years). The relative variations in the
isotope-amount ratio n(3He)/n(3H) can be interpreted in terms of elapsed time. This has been
especially useful in aquatic systems including oceans, lakes, and aquifers that received large
inputs of 3H from precipitation following thermonuclear bomb test periods. 3H-3He dating
provides elapsed time since a water mass became isolated from the atmosphere, in the time range
from the mid-1950s to the present. Such studies are important for establishing the sustainability
of groundwater resources in shallow aquifers [17-21].
4
He is a product of radioactive decay in the uranium and thorium decay series. As a
4
result, He concentrations can be used to estimate the relative ages of minerals and groundwater.
In closed systems (systems that do not exchange matter with their surroundings), relative
variations in the mole ratio n(4He)/n(U) can be interpreted in terms of elapsed time, although
other processes can alter the distribution of helium, which is highly mobile in terrestrial
environments [17-21].
4
He concentrations commonly increase along groundwater flow paths through cumulative
release from aquifer materials. This rate of accumulation can be used to estimate the time since
groundwater was recharged at the surface. The 4He accumulation method of groundwater dating
typically can be used in deeper aquifers where groundwater is relatively old and the 3H-3He
method cannot be used because of the relatively short half-life of 12.32 years for 3H [17-21].
Helium isotopes in industry
3
He has a large absorption cross section for neutrons, which makes it especially useful for
radioactivity detection. In this application, neutrons produced by radioactive decay of elements,
such as uranium and plutonium, enter the detector where the reaction 3He (n, p) 3H produces 1H
and 3H atoms. This induces further collisions and release of electrons, which interact with
charged surfaces to generate an electric current. Large amounts of 3 He are used to produce
neutron detectors in portal monitors for detecting illicit radioactive materials at ports, border
crossings, and airports (Figure 1), but unfortunately, the isotope 3He is rare and there is a need to
incorporate alternative gases for use in neutron detectors [22, 23]. 3He neutron detectors are also
used in devices that determine proportions of water, oil, and gas in wells drilled for energy
production [22, 23]. Other important uses of 3He include lasers, gyroscopes used for missile
stability and guidance, and cryogenic research (ultra-low temperature, less than 1 K) [22, 23].
The global supply of 3He available for research and practical applications has become
severely limited in recent years such that prices have increased substantially and some uses have
been curtailed. A major source of 3 He is recovered from nuclear weapons containing 3H when
the warheads are reconditioned or dismantled. 3He accumulates in such devices as a radiogenic
product of 3H decay. The annual supply of new 3He has decreased with reductions in nuclear
arsenals [22, 23]. In July 2016, there was a report of a recent discovery of a world-class helium
gas field in the Tanzanian East Africa Rift Valley with a calculated resource of 54 billion cubic
feet (BCf) in part of the valley. This can be compared to an annual global consumption of 8 BCf.
Fig. 1: Radiation detectors are installed in many areas to screen people, vehicles, and cargo for
radioactive materials. 3He detectors are sensitive to thermal neutrons and can be used to detect
isotopes of uranium and plutonium that might be used in nuclear weapons along with other
sources that produce neutrons by radioactive decay. (Image Source: U.S. Government
Accountability Office) [24].
Helium isotopes in medicine
3
He is used in magnetic resonance imaging (MRI) of the lungs [22, 23].
Glossary
absorption cross section – the probability of the absorption of an incident particle by a target
nucleus. [return]
atomic number (Z) – The number of protons in the nucleus of an atom.
electron – elementary particle of matter with a negative electric charge and a rest mass of about
9.109 × 10–31 kg. [return]
element (chemical element) – a species of atoms; all atoms with the same number of protons in
the atomic nucleus. A pure chemical substance composed of atoms with the same number of
protons in the atomic nucleus [703]. [return]
gamma rays (gamma radiation) – a stream of high-energy electromagnetic radiation given off
by an atomic nucleus undergoing radioactive decay. The energies of gamma rays are higher
than those of X-rays; thus, gamma rays have greater penetrating power.
half-life (radioactive) – the time interval that it takes for the total number of atoms of any
radioactive isotope to decay and leave only one-half of the original number of atoms. [return]
isotope – one of two or more species of atoms of a given element (having the same number of
protons in the nucleus) with different atomic masses (different number of neutrons in the
nucleus). The atom can either be a stable isotope or a radioactive isotope.
isotope-amount ratio (r) – amount (symbol n) of an isotope divided by the amount of another
isotope of the chemical element in the same system [706]. [return]
magnetic resonance imaging – medical imaging technique that uses strong magnetic fields to
form images of the body. [return]
neutron – an elementary particle with no net charge and a rest mass of about 1.675 × 10–27 kg,
slightly more than that of the proton. All atoms contain neutrons in their nucleus except for
protium (1H). [return]
proton – an elementary particle having a rest mass of about 1.673 × 10–27 kg, slightly less than
that of a neutron, and a positive electric charge equal and opposite to that of the electron. The
number of protons in the nucleus of an atom is the atomic number.
radioactive decay – the process by which unstable (or radioactive) isotopes lose energy by
emitting alpha particles (helium nuclei), beta particles (positive or negative electrons), gamma
radiation, neutrons or protons to reach a final stable energy state. [return]
radioactive isotope (radioisotope) – an atom for which radioactive decay has been
experimentally measured (also see half-life).
radiogenic – produced by the decay of a radioactive isotope, but which itself may or may not be
radioactive. [return]
stable isotope – an atom for which no radioactive decay has ever been experimentally measured.
thermal neutron – a neutron not bound to an atomic nucleus in thermal equilibrium with its
surroundings, thus, having relatively low kinetic energy. [return]
thermonuclear bomb – uses the energy from a primary fission reaction to compress and ignite a
secondary nuclear DT fusion reaction. [return]
X-rays – electromagnetic radiation with a wavelength ranging from 0.01 to 10 nanometers—
shorter than those of UV rays and typically longer than those of gamma rays.
References
17.
M. Ozima, and Podosek, F.A. Noble Gas Geochemistry: 2nd Edition. Cambridge
University Press (2002).
18.
D. K. Solomon. In Environmental Tracers in Subsurface Hydrology, pp. 425. Kluwer
Academic Publishers, Boston (2000).
19.
D. K. Solomon, and Cook, P.G. In Environmental Tracers in Subsurface Hydrology, pp.
397. Kluwer Academic Publishers, Boston (2000).
20.
P. Schlosser, Stute, M., Dörr, H., Sonntag, C., and Münnich, K.O. Earth and Planetary
Science Letters. 89, 353 (1988).
21.
B. A. Mamyrin, and Tolstikhin, I.N. Helium isotopes in nature. Elsevier, New York
(1984).
22.
D. Kramer. Physics Today. 63 (6), 22 (2010).
23.
G. V. Jean. Advancing Hidden Nuclear Material Detection. National Defense Industrial
Association. 2014 Feb. 28.
http://www.nationaldefensemagazine.org/archive/2010/December/Pages/AdvancingHiddenNucle
arMaterialDetection.aspx
703. I. U. o. P. a. A. Chemistry. Compendium of Chemical Terminology, 2nd ed. (the "Gold
Book"). Blackwell Scientific Publications, Oxford (1997).
706. Coplen. Rapid Communications in Mass Spectrometry. 25 (2011).