IRIS

TRIUMF’s IRIS (ISAC Charged Particle Reaction Spectroscopy Station) experimental facility is giving physicists a unique view of the strong force and unusual transformations in nuclear structure when nuclei are pushed to the extreme. 

The research team, led by Saint Mary’s University physicist Rituparna Kanungo, is using ISAC-II produced rare, short-lived isotopes to view the inner workings of these exotic nuclei and mimic the nuclear reactions that occur in stellar explosions such as supernovae and neutron star mergers. 

Many stylized images of the atomic nucleus depict a random berry like clump of protons and neutrons. However, these nucleons are in fact precisely organized in an onion-like structure of nucleon energy layers, separated by large gaps at specific nucleon numbers. This organization is described by the nuclear shell model, and ultimately determined by the nature of the strong force which binds protons and neutrons. 

The shell model fully describes the long-lived, stable isotopes common on Earth, which usually have equal, or nearly equal, numbers of protons and neutrons. The carbon isotope carbon-12 (¹²C), for example, has six neutrons and six protons. 

However, nuclear physicists have discovered that in isotopes, those with extremely lopsided numbers of protons and neutrons, such as isotopes common in stellar processes, the known picture of nuclear shells changes. 

Using ISAC II’s ability to produce extreme, short-lived isotopes, IRIS is using them to induce star-like nuclear reactions that enable researchers to construct an unparalleled image of nuclear structure at the extreme, particularly the valence nucleons, those furthest from the nuclear core.  

IRIS is an example of an experimental facility conceived of, and led, by a Canadian university researcher, capitalizing on a collaboration with TRIUMF’s state-of-the-art experimental physics team, and unique radioactive beam capabilities. The majority of the IRIS components were fabricated at TRIUMF.  

 IRIS research involves collaborations with researchers from many Canadian and international institutions, including TRIUMF’s theory and experiments group, the University of Guelph, McMaster University, Simon Fraser University in Canada, the Research Center for Nuclear Physics and High Energy Accelerator Research Organization (KEK) in Japan, Lawrence Livermore National Laboratory, Lawrence Berkeley National Laboratory and the University of Tennessee in the United States, Technical University of Darmstadt in Germany, GANIL in France, and the University of Edinburgh in the United Kingdom.

Located in the ISAC-II hall, IRIS peers into the nucleus by bombarding a frozen hydrogen target with extreme isotopes and detecting the scattered products of the nuclear reaction, a process called reaction spectroscopy. IRIS is the first in the world to use frozen hydrogen targets in the low-energy reaction spectroscopy of rare isotopes.  

With IRIS, researchers use two different nuclear reactions to deduce nuclear structure: transfer reactions and scattering. In a transfer nuclear reaction, one or two protons or neutrons is transferred from a beam isotope to a target nucleus, or vice versa. Transfer reactions can be indirect probes of the neutron or proton capture reactions that play a central role in element formation in stars. In a scattering reaction, an incoming beam isotope, for example carbon-10 (¹⁰C) is analogous to a huge boulder colliding with a wall of pebbles, the target nuclei. This cause the “pebbles” to scatter to scatter widely, while the “boulder” if far less deflected. 

In both transfer reactions and scattering, the created products and the scatter pattern are together the equivalent of an energy fingerprint—a complex, highly distinctive indicator of the isotope’s nuclear structure, particularly the location of outer protons and neutrons. 

Extreme isotopes are short-lived and difficult to produce, and so IRIS specializes in an optimized infrastructure where the number of available pebbles – the solid hydrogen target nuclei – is much larger than commonly used targets. This enables IRIS to harvest detailed data from the most difficult to study rare isotopes with low-intensity beams and complements TRIUMF’s TIGRESS reaction studies of precision gamma spectroscopy. 

IRIS works in a staged process that moves from identifying and tagging individual beam isotopes, colliding them with a frozen hydrogen target, and using a trio of detectors to record the energy and pinpoint locations of the scattered particles.  

Optimizing the experimental design

Before running an experiment, IRIS researchers, including members of TRIUMF’s Theory Group, simulate and optimize an experiment and often also use ab initio, or first principle, theoretical calculations for estimating expected results. This involves running Monte Carlo, or event randomized, computer models of the experiment. The simulations guide the researchers in designing an experiment, for example, determining the target thickness to use with a particular rare isotope, or the distance at which to place detectors.  

Ionization Chamber –  Counting and cleaning isotopes

To start, IRIS isotopes pass through a low-pressure ionization chamber that tags and counts individual isotopes and sorts the beam isotopes.  For example, in an IRIS experiment studying the rare isotope¹⁰C with six protons and just four neutrons, some of the isotopes in the ISAC-II beam are boron-10 (¹⁰B), with five protons and five neutrons (the ISAC-II beam is sorted by mass, and thus isotopes of identical mass are often mixed, and must be experimentally separated). 

IRIS researchers don’t want their results muddied by recording the interactions of the¹⁰B with the target. So, the ionization chamber is used to identify and place an atomic charge tag on each and every isotope, or event. The atomic charge number of each element is different for example for carbon is six while that of boron in five. The 15-centimeter-long, and five-by-five-centimeter sided ionization chamber is filled with low-pressure isobutane gas (at about 1/40th atmospheric pressure). The ionized isotopes passing through are all positively charged and as a result, as each ion passes through the chamber it strips electrons from the isobutane molecules, creating a collection of electrical charges that’s detected in the chamber.  

The strength of these electrical charges, their voltage, depends on the magnitude of an individual ion’s positive charge. Thus, carbon with six protons creates a stronger electrical signal than boron with five protons. About 2000 isotopes-per-second zip through the ionization chamber during an IRIS experiment, and every one of these events is tagged and identified to guide the final analysis. This count enables the IRIS researchers to calculate the probability of a given reaction occurring, a key component in elucidating nuclear structure.

The frozen hydrogen target

To maximize the rate of nuclear reactions with rare isotopes, IRIS is pioneering the use of frozen hydrogen targets. In either a nuclear transfer or scattering reaction experiment with IRIS, the goal is to maximize the number of possible interactions, the reaction yield, between isotopes and target nuclei. To achieve this, researchers want as dense a target as possible, i.e. as many hydrogen nuclei as possible packed into the smallest area.  

Traditionally, scattering experiments have used hydrogen gas, or a polyethylene foil. In a gas, the hydrogen nuclei are spaced far apart, and in the polyethylene, being a molecule with hydrogen and carbon the number of hydrogen nuclei is smaller, the results are also complicated by the presence of carbon. 

To maximize and simplify the number and type of interactions, IRIS researchers developed a solid, frozen hydrogen target system. The tiny, 5 mm diameter, thin, frozen hydrogen target is created using a copper target cell lined with an ultra-thin (4.5 millionths-of-a-meter) silver foil that’s cryogenically cooled to 4° Kelvin. Hydrogen gas is sprayed through a diffuser onto the silver foil, instantly freezing to form a solid target of densely packed hydrogen nuclei, ranging in thickness from about 50 to 150-millionths-of-a-meter. The desired thickness of an IRIS target is achieved by controlling the volume of gas released. IRIS targets use either of hydrogen or its heavier isotope deuterium depending on the specific type of reaction to study.

Detectors

IRIS’ trio of detectors are designed to record the location, energy and time of all the reaction products from the interactions of rare isotopes with the hydrogen target. When an isotope hits the target, it produces either nuclear transfer or scattering reactions and the detectors are designed and positioned to optimize the detections of these different reactions. The detectors can be placed from 8 cm to 75 cm from the target. 

Behind, or downstream, from the target are two dartboard-like circular, segmented, silicon semiconductor detectors, each with a hole at the bulls-eye. In scattering reactions, the hydrogen target nuclei are much less massive than the incoming isotope and so are scattered more widely, just as a more massive bowling bowl scatters stationary, less massive, bowling pins.  

The first detector has a much larger circumference than the second and detects the lighter widely scattered hydrogen nuclei, and other light reaction products. Both downstream detector configurations are made up of two layers each of which is a separate detector. The first layer is a super-thin silicon wafer, just 100 µm thick. A particle passes through this layer, depositing some of its energy. The second layer is 12 mm thick cesium iodide scintillator detector which stops the particle.  

The correlation between the energy deposited in the detector’s first layer, and the final stopping layer, enable IRIS researchers to determine the particle’s mass and charge and thus its type, for example whether its hydrogen, deuterium or perhaps an alpha particle created in a nuclear reaction. The segmentation of the detector provides pinpoint information on the particle’s angle of scattering after reaction. 

In a reaction, the heavier mass isotopes are scattered far less and thus pass through the hole in the centre of the first detector and hit a second, smaller circumference segmented semiconductor detector combination (two layers of same semiconductor detectors) further downstream.  

Any beam isotopes that don’t interact with target nuclei continue on a straight path through the holes at the centre of the downstream detectors. At the very end of IRIS’ vacuum chamber is a strong radiation resistant scintillator detector that records the impact of these unreacted beam isotopes. This allows monitoring the transmission of the beam through the IRIS detector setup.  

IRIS is also equipped with a pair of upstream semiconductor detectors similar to those used for detecting downstream reaction particles. These upstream detectors are particularly important in transfer reaction experiments involving the emission of a proton from a deuterium target, a significant portion of which scatter backwards. 

Detecting the overall energy of each event, and the scattering angles of the heavy and light particles, enables IRIS researchers to deduce the detailed nuclear structure of the rare isotopes.