13.3 Artificial transmutation and particle accelerators

Artificial Transmutation

Process and Historical Context

Artificial transmutation is the process of changing one element into another by bombarding nuclei with high-energy particles. Before this technique existed, there was no way to deliberately alter an element's identity.

Ernest Rutherford performed the first artificial transmutation in 1919, transforming nitrogen into oxygen by hitting nitrogen atoms with alpha particles. The reaction looked like this:

$^{14}_{7}\text{N} + ^{4}_{2}\text{He} \rightarrow ^{17}_{8}\text{O} + ^{1}_{1}\text{H}$

The core challenge in transmutation is overcoming the Coulomb barrier, the electrostatic repulsion between two positively charged nuclei. Since both the projectile and the target nucleus carry positive charges, they naturally repel each other. The incoming particle needs enough kinetic energy to get close enough for the strong nuclear force to take over and trigger a reaction.

Applications in Nuclear Physics and Medicine

Artificial transmutation has a surprisingly wide range of uses:

• Medical radioisotopes: Technetium-99m is the most widely used isotope in medical imaging (bone scans, cardiac stress tests). Iodine-131 is used to treat thyroid cancer by delivering targeted radiation to thyroid tissue.
• Superheavy element synthesis: Elements like nihonium (113) and oganesson (118) don't exist in nature. They're created entirely through transmutation, expanding the periodic table and testing predictions about nuclear stability.
• Nuclear fuel production: Uranium-238 can be transmuted into fissile plutonium-239 through neutron capture, which is how breeder reactors generate new fuel.
• Nuclear waste management: Long-lived radioactive isotopes can potentially be transmuted into shorter-lived species, reducing the time waste remains hazardous. This is still a major engineering challenge.
• Fundamental research: Transmutation reactions reveal details about nuclear structure, decay processes (alpha, beta decay), and even how elements form inside stars (nucleosynthesis).

Particle Accelerators in Transmutation

Fundamental Principles

Particle accelerators use electromagnetic fields to push charged particles to high speeds and high kinetic energies. The basic physics governing this process is the Lorentz force:

$\mathbf{F} = q(\mathbf{E} + \mathbf{v} \times \mathbf{B})$

where $q$ is the particle's charge, $\mathbf{E}$ is the electric field, $\mathbf{v}$ is the particle's velocity, and $\mathbf{B}$ is the magnetic field.

The two field types play different roles:

• Electric fields do work on the particle, increasing its kinetic energy.
• Magnetic fields change the particle's direction without changing its speed (since the magnetic force is always perpendicular to the velocity). This is how beams are steered and bent into circular paths.

Key Components and Techniques

Several engineering systems work together inside an accelerator:

• Radio-frequency (RF) cavities generate oscillating electric fields timed to match the particle's arrival, giving it a "kick" of energy on each pass.
• Vacuum systems remove air from the beam pipe so particles don't scatter off gas molecules. This greatly increases the particle's mean free path.
• Quadrupole magnets act as focusing elements, squeezing the beam to keep it narrow and well-defined (similar to how a lens focuses light, but using magnetic fields).
• Cooling systems, often using liquid helium at around 4 K, keep superconducting magnets at the extremely low temperatures they need to operate without electrical resistance.

Role in Artificial Transmutation

Accelerators supply the high-energy projectiles needed to overcome the Coulomb barrier and trigger nuclear reactions. The specific transmutation products you get depend on:

• The type of projectile particle (protons, alpha particles, heavy ions)
• The kinetic energy of the beam
• The target material

Because accelerators offer precise control over beam energy and intensity, researchers can systematically study how nuclear reactions behave across a wide range of elements and energies. This level of control is what separates modern transmutation from early experiments using natural radioactive sources.

Types of Particle Accelerators

Linear and Circular Accelerators

Linear accelerators (linacs) arrange accelerating structures in a straight line. The particle passes through each RF cavity once, gaining energy at each stage. Linacs are often used for initial acceleration before injection into a circular machine, or for applications needing pulsed beams. Notable examples include SLAC at Stanford and LINAC4 at CERN.

Cyclotrons bend particles into a spiral path using a constant magnetic field. As the particle gains energy and moves faster, its orbit radius increases, tracing an outward spiral. Cyclotrons are compact enough to fit in hospitals, making them the workhorse for producing medical isotopes and delivering proton therapy. Major facilities include TRIUMF in Canada and PSI in Switzerland.

Synchrotrons use a fixed-radius circular ring but vary the magnetic field strength as particles accelerate, keeping them on the same orbit. This design scales to very high energies. The Large Hadron Collider (LHC) at CERN is a synchrotron with a 27 km circumference. The Advanced Photon Source at Argonne National Laboratory is another example, used as a high-brightness light source.

Specialized Accelerators

• Tandem Van de Graaff accelerators use a clever trick: a negative ion is accelerated toward a positive terminal, stripped of electrons at the terminal, and then accelerated again as a positive ion heading away from the terminal. This doubles the energy gain from a single voltage source. They're particularly useful for accelerating heavy ions (e.g., ATLAS at Argonne).
• Fixed-field alternating gradient (FFAG) accelerators combine the constant field of a cyclotron with the strong focusing of a synchrotron. They can cycle very rapidly, making them promising for proton therapy applications (e.g., EMMA at Daresbury Laboratory, UK).
• Storage rings don't accelerate particles further but maintain high-energy beams circulating for extended periods. This is essential for collider experiments where two beams need to intersect repeatedly. Examples include RHIC at Brookhaven National Laboratory and the now-decommissioned Tevatron at Fermilab.
• Spallation neutron sources slam high-energy protons into a heavy metal target (like mercury or tungsten), knocking out large numbers of neutrons. These neutrons are then used for materials science and neutron scattering experiments. Examples include SNS at Oak Ridge and ISIS at Rutherford Appleton Laboratory.

Products of Artificial Transmutation

Types of Transmutation Products

Transmutation can produce a wide variety of isotopes depending on the reaction:

• Stable isotopes like carbon-13 and oxygen-18, useful as tracers in research
• Short-lived radioisotopes like fluorine-18 (half-life of about 110 minutes), used in PET scans
• Long-lived radioactive species like plutonium-239 (half-life of about 24,100 years), relevant to nuclear energy

The yield and purity of the product depend on the target material, the projectile type and energy, and the reaction cross-section (a measure of how likely a particular nuclear reaction is to occur, expressed in units of area).

Two broad categories of reactions produce different kinds of products:

• Neutron-induced transmutations tend to produce heavier isotopes of the same or nearby elements. For example, uranium-238 captures a neutron and, after two beta decays, becomes plutonium-239.
• Charged-particle reactions can produce proton-rich isotopes. For example, bombarding oxygen-18 with protons yields fluorine-18: $^{18}_{8}\text{O} + ^{1}_{1}\text{H} \rightarrow ^{18}_{9}\text{F} + ^{1}_{0}\text{n}$

Applications and Implications

Superheavy element creation pushes the periodic table to its limits. Element 117 (tennessine), for instance, was synthesized by fusing calcium-48 projectiles with berkelium-249 targets. These experiments test theoretical predictions about the "island of stability," a region where certain superheavy nuclei might have unexpectedly long half-lives due to closed nuclear shells.

Nuclear waste transmutation could convert long-lived actinides into shorter-lived fission products, dramatically reducing the timescale of radioactive waste storage. The concept is sound, but scaling it to handle real waste volumes remains a significant engineering challenge.

Medical isotope production continues to advance. Lutetium-177 enables targeted radiotherapy for neuroendocrine tumors, while gallium-68 provides high-resolution PET imaging. These isotopes are produced in cyclotrons or reactors through carefully chosen transmutation reactions.

Studying transmutation products also generates valuable nuclear data: binding energies, decay modes, and nuclear structure information. Analyzing the decay chains of superheavy elements, for example, helps physicists understand nuclear shell structure and refine theoretical models of the nucleus.