CERN in Geneva is the world's largest facility for the study of fundamental particles. The equipment that usually serves science can sometimes be used for practical purposes too. That's for example the case for the protons emerging from the Proton Synchrotron Booster (PSB): they are used in the large particle accelerator LHC for scientific experiments. The protons can also be used to produce isotopes that are useful in radiation medicine. Such isotopes are produced in the recently opened facility CERN-MEDICIS.
Atoms consist of a positively charged atomic nucleus surrounded by a number of negatively charged electrons, so that atoms appear as neutral objects. The total charge of an atomic nucleus determines the corresponding chemical element in the periodic table. Atomic nuclei, in turn, consist of protons and neutrons. The number of protons in the atomic nucleus defines the chemical properties. The number of neutrons can vary, which leads to atoms that are identical in their chemical properties but differ in their mass. Isotopes are atomic nuclei in which the number of protons is exactly defined but the number of neutrons can be different.
Of the more than 3300 known isotopes, only a small number are stable. Most are unstable, so they transform into other atomic nuclei through radioactive decay. These radioactive isotopes – shortly: radioisotopes – have different applications. Some of them are used in radiation medicine. They can be used for example to diagnose and treat cancer. Different isotopes are suitable for different medical purposes. The production of radioisotopes is demanding. In December 2017 started its operations. The facility at the site in Meyrin near Geneva is dedicated to the task of producing suitable radioisotopes for medical applications.
Cancer diagnosis with Terbium isotope 155
The first and so far only radioisotope manufactured by CERN-MEDICIS is Terbium 155Tb. This isotope has recently been used by doctors to diagnose prostate cancer. "Before the isotopes in hospitals can be used for the benefit of patients, they have to come a long way. The isotopes have their origin in the research facilities that particle physicists at CERN have built for basic research, "says Stefano Marzari, a Swiss mechanical engineer who has received his training at the EIF (Engineering School of Fribourg) and has been involved in the development of CERN-MEDICIS from the outset.
There is actually a long way to go until a Terbium isotope is ready for patient use. At the very beginning there is a pressure vessel filled with hydrogen gaz. The nucleus of a hydrogen atom consists of exactly one proton and zero, one, two or more neutrons (zero neutrons is the version most often found in nature). These protons are the starting material needed to make 155Tb. The hydrogen nuclei are accelerated in the electric field of the linear accelerator at CERN, subsequently they are fed into the ring accelerator PSB where they are accelerated again, till they reach an energy of 1.4 Giga electron Volt (GeV).
From PSB via ISOLDE to CERN-MEDICIS
At this point, the protons are split into separate ways: one part of the particles accelerated in the PSB get into further ring accelerators and when they reach 5,000 times higher energy - around 7,000 GeV – they are used for pioneering particle physics experiments in the Large Hadron Collider. Another part of the PSB protons, comes to ISOLDE, short for: Isotope Separator On Line Device. uses protons to generate radioactive ion beams for scientific experiments in nuclear physics as well as materials science and life sciences. The ion beams are formed when the protons strike targets made of special materials.
Here the paths between particle physics research and medical application are definitely deviating: "Only 10% of the protons that reach ISOLDE are used by the experiment; the rest flies through ISOLDE unused. We are now using these previously lost protons at CERN-MEDICIS for medical purposes. So, we are working with a waste product", smiles Stefano Marzari. This means that the protons that pass unused through the ISOLDE targets are directed to a CERN-MEDICIS target and there used for isotope production.
Extraction of Terbium isotopes
In future, CERN-MEDICIS will use metals such as Tantalum, ceramics such as Uranium or Silicium carbides as targets. For the isotope produced so far, namely 155Tb, the target metal used is the transition metal Tantalum, which is used, for example, in implants and for the fabrication of bone nails. The target is a tube about 20 cm in length; inside are about 20 rolls of Tantalum foil, each about 1 cm wide. When protons hit the foil, 155Tb isotopes are generated. With this result the operators of CERN-MEDICIS are far from finishing their job: "The big challenge is to get the isotopes out of the target", says Stefano Marzari.
For extraction, the target containing Terbium isotopes is placed in a bunker twenty yards away. This room is protected with 40 cm thick steel walls, inside there is low air pressure. These precautions ensure that the target does not release radioactive particles into the environment in an uncontrolled manner. In the bunker, the Terbium isotopes are extracted from the target in four steps: first, the Tantalum tube, which contains a large number of other isotopes in addition to 155Tb, is heated to 2000 °C in a vacuum vessel. The 155Tb isotopes change into the gaseous state of aggregation. All isotopes, which outgas by heat, are first ionised, then accelerated in an electric field and finally deflected laterally by an electromagnet - the lighter isotopes stronger, the heavier ones weaker. In this way the different types of isotopes are separated depending on their mass. The 155Tb isotopes are finally collected on a 2 x 1 cm metal plate. In this form, the isotopes are then sent to the partner institutes or hospitals. There, they are released from the plate using a chemical process. Then they can be used for cancer diagnostics or treatment.
Isotopes for new applications
"We are very pleased to be able to use now the high-energy protons originally produced for scientific purposes at CERN for medical applications," says Thierry Stora, spokesperson and project coordinator for CERN-MEDICIS. The 155Tb isotopes are only the beginning. In the future, a wide range of isotopes will be produced here, creating the basis for new innovative applications in radiation medicine.
Author: Benedikt Vogel