Demonstration of inkjet-printed targets for activation cross-section measurements: deuteron-induced reactions on strontium up to 24 MeV

In the world of nuclear science, researchers often need to understand how atoms behave when they are struck by fast-moving particles. Imagine a target made of a specific element, like strontium, sitting in a beam of deuterons, which are a type of heavy hydrogen nucleus. When these particles collide, they can knock pieces off the strontium atoms or fuse with them, creating entirely new, unstable atoms called radioisotopes. Scientists care deeply about this process because these new atoms are not just curiosities; they are vital tools. Some are used to treat cancer by delivering radiation directly to tumors, while others act as tracers that allow doctors to see inside the human body using special cameras. To use these tools safely and effectively, scientists must know exactly how much of each new atom is created at different energy levels. This requires measuring something called a cross-section, which is essentially a way of quantifying the likelihood of a collision happening. However, making the targets needed for these experiments is notoriously difficult when the material is a metal like strontium, which reacts violently with air and moisture, making it impossible to roll into thin sheets like other metals.

A team of researchers at RIKEN in Japan has found a clever way around this obstacle, successfully measuring these collision probabilities for the first time using a novel target preparation method. Instead of trying to forge a thin metal sheet, they used a standard commercial inkjet printer to deposit tiny droplets of a strontium solution onto a thin aluminum foil. As the liquid dried, it left behind a uniform layer of strontium nitrate crystals, thin enough to let the particle beam pass through but dense enough to create the necessary reactions. The team fired a beam of deuterons with energies up to 24 million electron volts at these printed targets. By carefully measuring the gamma rays emitted by the resulting radioactive atoms, they were able to map out exactly how much yttrium was produced at each energy level. This work is significant because it proves that inkjet printing can be a reliable tool for creating high-quality targets for nuclear physics, opening the door to studying elements that were previously too difficult to handle.

The researchers focused on creating several different forms of yttrium, an element that is chemically similar to strontium but has different radioactive properties. Among the most important findings was the first-ever measurement of how much of a specific, short-lived form of yttrium, known as 86mY, is produced. They also measured the production of other forms, including 86gY, 87mY, 87gY, and 88Y, across a wide range of energies. To ensure their results were accurate, they compared their data against existing records and computer predictions. They found that their measurements generally matched previous studies, though they were slightly higher in the low-energy range where the reactions just begin. When they compared their results to the TENDL-2023 library, a major database used by scientists to predict nuclear behavior, they discovered that the computer models tended to overestimate the amount of yttrium produced, particularly at lower energies. This discrepancy is important because it tells database creators that their models need fine-tuning to better reflect reality.

The success of this experiment relies heavily on the quality of the targets. The researchers used a technique called autoradiography, which involves placing a special imaging plate over the target to see exactly where the radioactive material was deposited. This analysis showed that the inkjet-printed targets were remarkably uniform, with variations in thickness of less than three percent. This level of consistency is crucial because any unevenness in the target could distort the energy of the particle beam and skew the results. By confirming that the printed dots were evenly spread and that the beam energy remained stable as it passed through the stack of targets, the team could trust their numbers. They also used a monitoring reaction involving copper to double-check their beam settings, finding that their experimental setup was performing exactly as expected.

The implications of these measurements extend beyond just filling a gap in a data table. The data helps refine the understanding of how to produce specific medical isotopes. For instance, the energy range studied in this experiment corresponds to the optimal conditions for producing 88Y, a long-lived isotope useful for calibrating radiation detectors. For the other isotopes, while higher energies are often preferred for large-scale production, the low-energy data provided here serves as a critical benchmark. It allows scientists to verify that their theoretical models are correct, which is essential for predicting how much impurity might be created alongside the desired medical isotope. By demonstrating that inkjet printing can create targets for reactive metals like strontium, this study offers a new, practical path for future experiments, ensuring that the nuclear data needed for medical and scientific applications continues to become more precise and reliable.