DRAGON

TRIUMF’s unique DRAGON (Detector of Recoils And Gammas of Nuclear Reactions) facility is giving astronomers a clearer view of our stardust origins by simulating the rapid nuclear reactions that take place in exploding stars. It is the only facility in the world capable of experimentally measuring many of these astrophysical reactions.  

DRAGON is a recoil mass spectrometer, a tool designed to simulate the nuclear reactions in stars and then identify the reaction products, the recoils.  

In the Big Bang, the only elements formed were hydrogen, helium and minute quantities of lithium and beryllium. All of the other naturally occurring elements of the Periodic Table, from carbon to gold and uranium were, and are being, created in stars.  

The big question is: how? Observational astronomers and astrophysics theorist work to figure-out the nuclear reaction processes in stars by studying stars’ elemental fingerprints. These are the specific ratios of different elements astronomers see produced by exploding stars, such as nova and supernova. However, in many cases, the specific underlying processes that produce the observed elemental fingerprints are still a mystery, unexplained by stellar models.  

This is why DRAGON’s experimental results are so important. The high temperature in an exploding star creates a zoo of very short-lived radioactive isotopes not formed in usual stellar burning. The presence of these rare isotopes creates tens of thousands of different possible nuclear reaction pathways, and which path occurs has a huge influence on the elements that will be produced. What’s key to determining these reaction pathways is the probability that one of these rare isotopes will react. 

DRAGON precisely measures the reaction rates of these short-lived radioactive isotopes, particularly a fusion reaction called radiative capture. For example, in 2013 images from the Hubble Space Telescope identified fluorine for the first time in the ejecta of a nova explosion. To enable astronomers to make sense of fluorine’s observed presence, DRAGON researchers measured the probability of one of the key fluorine production radiative capture pathways, the fusion of neon-19 (¹⁹Ne) with hydrogen, producing sodium-20 (²⁰Na) and a gamma ray.

DRAGON is designed to experimentally pinpoint the rapid nuclear processes and pathways that occur in exploding stars, primarily for elements created in nova explosions. Novas occur in stellar pair systems, often involving a red giant star (a bloated, dying Sun-like star) and a white dwarf, (the dense, hot carbon-rich cinder of such a star in close proximity). The white dwarf gravitationally draws material, primarily hydrogen, from its neighbour onto it surface until this nuclear fuel builds up to tipping point and explodes in a nuclear flare. 

TRIUMF is an associate member of the Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements (JINA-CEE), a world-leading repository for stellar nuclear reaction information. Thus, DRAGON’s precision astrophysics nuclear reaction data is used by researchers worldwide and integrated into the latest stellar computational codes and models, providing astronomers with new eyes to follow the elemental story of our cosmos’- and us.

Located in the centre of TRIUMF’s ISAC-I experimental hall, DRAGON is a 21-meter-long, semi-circular apparatus divided into three key parts: a “head”; “body”; and “tail”.  

The “head” is the target, the site of the star-like nuclear reactions. The long “body” is where fusion products separate from the unreacted beam, and the “tail” is the detector.  

DRAGON’s “Head”: The Gas Target and Gamma Ray Detector

This is where DRAGON’s radiative capture nuclear reactions take place. DRAGON’s “brain” is a triangular chamber that can be filled with hydrogen or helium gas, the main fusion fuel in stars. Short-lived radioactive isotope beams produced in ISAC-I, of the varieties and energies similar to those found in exploding stars, are fired at the gas target, resulting in the same nuclear fusion reactions that occur in stars. For example, when an atom from a beam of sodium-21 (²¹Na) isotopes slams into an atom of hydrogen gas, it fuses to produce magnesium-22 (²²Mg) , which emits gamma rays of specific energies.  

One of DRAGON’s distinctive features is that this gas target chamber has tiny entrance and exit holes, six and eight millimeters in diameter, for the ISAC beam and recoil products, to pass through. The holes are necessary because otherwise the recoil products would be contaminated with fusion products from the beam isotopes reacting with the window material.  

However, this means the gas chamber is also like trying to keep a balloon inflated when it has two holes in it, especially since the rest of DRAGON is at vacuum pressure, a million times lower pressure than in the gas target cell. Left alone, the target gas would leak-out in a fraction of a second. To solve this conundrum, DRAGON’s gas target is surrounded by a sophisticated and powerful pumping system that captures and recirculates the gas that spills out of the holes. 

The gas target is also almost completely surrounded by an array of 30 germanium gamma-ray detectors. These detectors record the energy signatures and timing of the gamma rays inherent to each radiative capture reaction.  

DRAGON’s “Body”: The Magnetic Dipole and Electric Dipole Separators

A central experimental challenge with DRAGON is that radiative capture reactions have very low probability and occur relatively infrequently. On average, there’s one fusion recoil for every 100 billion incoming isotope atoms. 

The recoils created in the gas chamber exit as a tiny fraction of the unreacted beam, which, like a river, continues into DRAGON’s “body”. Thus, DRAGON’s main task is beam suppression – to separate out the short-lived recoils from the unreacted beam particles.   

Like panning for gold, DRAGON uses a series of electromagnetic and physical filters to sift-out the recoil gems. This is accomplished in two main steps, first using a magnetic dipole and then an electric dipole as filters, and a series of slits to physically stop unwanted beam particles. 

Magnetic Dipole

The beam and recoils come out of the gas target in a range of charge states and this mixed overall beam is focused using magnetic quadrupoles and steered into a magnetic dipole chamber.  

This is DRAGON’s coarse filter, where the isotopes are separated by how much electrical charge they possess. This can be done because in a magnetic field, particles follow a path based on the ratio of their momentum-to-charge. The greater its charge, the more a particle will curve in a magnetic field. For example, two particles with the same momentum and both with four electrons missing, will follow the same magnetic path. In DRAGON, the researchers know the predicted charge state of their experimental recoil product and set the magnet to direct only this charge state through the magnetic dipole’s 2.5 cm wide exit aperture. As a result, all of the particles with a different charge state slam into the wall of the charge slit box and are filtered out.  

Electric dipole

The electric dipole is DRAGON’s fine filter separating particles of different masses. The particles exiting the magnetic dipole’s slit all have the same charge and momentum, but critically, the recoils are travelling slower. As with the magnetic dipole separation, DRAGON researchers set the electric dipole to bend and direct only the recoil atoms through a narrow mass exit slit. Thus, only atoms with the predicted mass, charge, and kinetic energy of the desired recoil atoms pass through the slit, and all the others are stopped in the slit box.  

The Tail: DRAGON’s Detectors

With the bulk of DRAGON’s separation work accomplished, the cleaned beam of recoil atoms is sent through another nearly identical set of magnetic and electric dipoles. However, this time the objective is to refocus the beam so that the particles are positioned as they were when they left the gas chamber. Thus, when they hit the detectors there’s a spatial correspondence between the detections in the tail, and the gamma rays detected in the head. 

DRAGON’s “tail” has three main detectors that can be used individually or in combination to identify the recoils.  The Double-Sided Silicon Strip Detector records the number of recoils, their energy, timing, and exactly where they hit the detector. A gas-filled ionization chamber detector and a micro-channel plate detector can also be used to record the number of recoils, their time of impact, but additionally can identify recoils by mass, via measuring their velocity, and atomic number.   

Correlated with the final detection of recoils, the timing and energy of the gamma ray detections provides crucial information to separate-out the real events recorded on the recoil detector from background noise, such as the detection of cosmic rays. 

DRAGON is a recoil separator for radiative capture reactions using something called ‘inverse kinematics’. This mouthful of a description makes sense when taken one step at a time, starting at the end. 

“Inverse kinematics” refers to the fact that DRAGON mimics stellar fusion reactions by firing short-lived radioactive isotopes at a target of hydrogen or helium gas. Historically, it was the other way around. Before the advent of sophisticated in-flight, rare isotope separators such as ISAC-I, the only way to create fusion reactions was to accelerate hydrogen or helium at a target. Thus, DRAGON reverses (or inverts) this kinematic (motion of objects) process. 

DRAGON specializes in measuring a key type of stellar nuclear reaction called radiative capture. This is a nuclear reaction in which an isotope (in DRAGON’s case usually a short-lived exotic one) fuses with a hydrogen or helium nucleus to form a heavier element, which subsequently emits a gamma ray. Thus, it’s a fusion capture reaction followed by a radiative component, the emission of the gamma ray. 

Finally, at its core, DRAGON is a recoil separator. The product of a nuclear fusion reaction is a recoil. In DRAGON, the radiative capture recoils are swamped in a river of the unreacted beam isotopes and DRAGON’s is uniquely able to separate these recoils from unreacted beam isotopes. 

For more, read TRIUMF scientist Chris Ruiz’s academic review article Recoil separators for radiative capture using radioactive ion beams.