TUDA

With TUDA (the TRIUMF U.K. Detector Array), the TRIUMF UK Detector Array, TRIUMF scientists and international collaborators are making critical insights into key stellar element formation pathways that can only be experimentally studied using accelerated rare isotopes. 

TUDA, and facilities such as DRAGON and EMMA, are core parts of TRIUMF’s astrophysics research program. TUDA’s precision astrophysics nuclear reaction measurements are used by researchers worldwide and integrated into the latest stellar computational models, providing astronomers with a clearer view of our cosmos’, and our, elemental story.  

TUDA’s specialty is the analysis of charged particle reactions. In these, two nuclei fuse but then break apart again into two different sized particles, a heavy and a much lighter one, both different from the initial two particles.  

In some key stellar charged particle reactions, the heavy product is lighter than the heaviest reactant. For example, in one key reaction in stars, a nucleus of fluorine-18 (¹⁸F) fuses with hydrogen, but spits-out an alpha particle (a helium nucleus) leaving the lighter oxygen-15 (¹⁵O). This produces a stellar nuclear element formation cycle akin to the game Snakes-and-Ladders. Heavier elements are created by consecutive fusion reactions, but then the elemental building process “slides” down to lighter nuclei via the charged particle reaction by emitting an alpha particle.    

For nuclear astrophysicists working to understand stellar nucleosynthesis, the key is to determine how probable particular charged particle reactions are at a given stellar temperature, and this can only be determined experimentally in laboratories, like TRIUMF, with access to accelerated rare isotopes. Similarly, the combination of TRIUMF rare isotopes and TUDA’s high detector sensitivity means that many of these charged particle measurements are the first of their kind.  

Precisely measuring charged particle reaction rates enables astronomers to peer directly into the hearts of stars and their explosions by using these real laboratory experimental measurements to interpret observational data and fine-tune stellar computational codes. 

TUDA is part of TRIUMF’s extensive national and international collaborations. The TUDA facility was developed and constructed by researchers at the University of Edinburgh, with infrastructure provided by TRIUMF. It’s used by researchers from the University of Edinburgh, University of York, Simon Fraser University, and TRIUMF.  

TUDA is a 2 metre long tube-shaped facility that can be mounted at the end of a beam line in either ISAC-I or –II. In overview, exotic-ion beams from ISAC-I or –II are fired at a target and the charged nuclear reaction products, or recoils, are detected downstream by an array of sophisticated silicon strip detectors.  

The Target

Simulating a stellar reaction begins with TUDA scientists selecting a beam of a key element in the stellar nucleosynthesis, for example the rare isotope ¹⁸F. A beam of ¹⁸F is accelerated, with exploding star-like energy, at a plastic target, for example polyethylene, a material which consists of only carbon and hydrogen. 

Most of the fluorine nuclei are deflected and scattered by the target nuclei, but some react with hydrogen nuclei to create ¹⁵O and an alpha particle. TUDA can also be used with a gas cell target, for example either hydrogen, helium or deuterium gas, with thin metal windows, in order to perform other types of charged particle reactions.  

The Detectors

TUDA’s versatility and precision comes from the ability to arrange up to four disk-shaped silicon array detectors almost anywhere within its 2-meter-long central vacuum chamber. This enables TUDA scientists to maximize detection efficiency for different experiments, which is critical given the very small probabilities for these reactions. 

These silicon detectors function similarly to a digital camera’s detector. They are segmented into pixels and thus able to detect location and energy, in this case not of visible light but of charged particles. Importantly, the detectors record both the heavy and lighter particles produced from a charged particle reaction in coincidence (at the same time), and linking these is the tell-tale evidence of the kind of reaction that took place. (In a charged particle reaction, because of the conservation of momentum, the products will exit the target in opposite lateral directions, for example, one to the left, the other to the right.) 

One type of detector is the diameter of a vinyl LP (about 26 cm)  and a second type of detector is the size of a music CD (about 14 cm), all only a third-of-a-millimetre (0.3 mm) thick. The detectors are placed as close as possible downstream from the target in order to cover as much particle deflection angle as possible, and thus detect almost every reaction that occurs. TUDA researchers continue an experiment until they’ve collected enough reaction data to precisely determine a reaction’s probability. 

The custom-built electronics for the detectors have excellent linear response, and low background noise.  As a result, the energy deposited in a pixel is precisely measured, and there’s little to no degradation of the signal or noisy data. This all means the experiment has the superior accuracy and sensitivity needed to make measurements that have not previously been possible of very weak charged particle reactions.