- 01 Overview
- 02 TIGRESS Science Highlights
- 03 How It Works
With TIGRESS (TRIUMF-ISAC Gamma Ray Suppressed Spectrometer), an in-beam gamma ray spectrometer, TRIUMF scientists are opening a new era of high-precision nuclear structure experiments with rare isotopes.
Much of physicists’ understanding of nuclear structure has come from gamma ray spectroscopy. TRIUMF’s unique gamma ray spectroscopy program with TIGRESS and GRIFFIN is extending this to rare, radioactive isotopes.
Using ISAC-II’s high-energy rare isotope beam, TIGRESS experiments involve smashing isotopes into a target to energize the nuclei and probe their structure by detecting emitted gamma rays.
TIGRESS is particularly powerful at enabling TRIUMF scientists to study how the number of neutrons and protons in a nucleus determines its shape. Stable nuclei are usually spherical, and their structure and related behaviour is dominated by a small number of protons and neutrons moving largely independent of one another. However, under the right circumstances, nuclei can behave as a single coherent system. With these atoms, the nucleus can distort from spherical into football, curling rock and even pear-shaped. In some cases, the addition of energy to a spherical nucleus can cause it to shift shape. In other cases, with special numbers of protons or neutrons in the nucleus, the most energetically-favoured shape is one of the distorted ones.
TRIUMF scientists have used TIGRESS to study how the addition of neutrons, or energy, changes the shape of a strontium nucleus. As an example, TIGRESS has used ISAC-II beams of strontium-95 to study the shape of its isotopic neighbors 94Sr and 96Sr. 95Sr is a spherical nucleus. Just adding a single neutron forms 96Sr, which turns out to also be spherical; however, adding both a neutron and energy can shift it to a highly deformed football shape. By contrast, all the states formed by removing a neutron from 94Sr, regardless of how much energy is added, seem to be spherical.
These experiments are providing TRIUMF scientists and partners with insights that are fueling understanding in how collective nuclear identity emerges from the basic interaction between protons and neutrons, and how this influences heavy element formation in stars.
TIGRESS operates with a suite of auxiliary detectors that are crucial to its ability to create unique views of nuclear structure. These auxiliary detectors include: SPICE, SHARC, BAMBINO, TIP, and DESCANT.
TIGRESS was built by a team of Canadian scientists primarily from TRIUMF, the University of Guelph, Université de Montréal, and University of Toronto. The TIGRESS team has grown to include researchers from Simon Fraser University, St. Mary’s University, and University of British Columbia, and international collaborators from the United States (Lawrence Livermore National Laboratory, Colorado School of Mines, University of Rochester), the UK (University of York, University of Surrey, University of Liverpool), and France (LPC Caen).
02 TIGRESS Science Highlights
How magic are the magic numbers?: Tracking single-particle levels in sodium-26How magic are the magic numbers?: Tracking single-particle levels in sodium-26: The standard shell model explains tightly bound nuclei in terms of magic numbers associated with large energy gaps between quantum mechanical single-particle levels. These magic numbers are well understood in nearly stable nuclei, but the magic numbers change in nuclei with a large neutron excess. As published in Physics Letters B (2014), a TIGRESS-SHARC experiment measured reactions of radioactive sodium-25 (25Na) beams on targets containing deuterium (one proton, one neutron). In particular, reactions where one neutron was taken out of the deuterium, measured the single-particle wave function composition of 26Na. The results showed that the neutron single-particle levels are already starting to be squeezed together by a complicated feature of the neutron-proton interaction, an effect which ultimately leads to the breakdown of the neutron magic number 20 in the so-called “island of inversion” around magnesium-32 (32Mg).
Testing modern nuclear structure theoriesTesting modern nuclear structure theories: As published in Physics Letters B (2018), this recent TIGRESS result measured the gamma-ray excitation transition rate in magnesium-22 (22Mg) and its mirror sodium-22 (22Ne). Mirror nuclei are pairs of nuclei where there are as many protons in one as neutrons in the other and vice versa. Based on “isospin symmetry”, the relative transition rates in mirror pairs ought to be well understood. However, in selected nuclei around atomic mass 21 to 24, there appears to be a much higher transition rate in the proton-rich pair compared to the neutron-rich mirror partner. This discrepancy cannot be explained within the phenomenological shell model, the standard model for nuclear structure, but for gamma-ray transition rates, it invokes an empirical “effective charge” parameter. The 22Mg experiment was two-fold: 1) measure the transition rate in 22Mg with high enough precision to determine if the anomalous proton-rich transition rate was an anomaly in selected nuclei or a trend across the whole mass region; and, 2) compare results to modern calculations that do not use effective charges but that calculate transition rates from first principles. The results clearly showed that, indeed, 22Mg’s transition rate is anomalously high compared to 22Ne, and furthermore, the first principles also were also unable to reproduce it.
Role of the continuum in Beryllium-11Role of the continuum in Beryllium-11: The exotic nucleus beryllium-11 (11Be) is a one-neutron halo nucleus, one in which the last proton or neutron appears to be in a large, extended orbit around an otherwise normal core. The extent to which 11Be truly behaves as a lone neutron in a large orbit around a 10Be core has been the subject of two important TIGRESS experiments, both of which employed scattering of 11Be off of heavy metal thin foil targets. The first, also using TRIUMF's Bambino, as measured the ratio of elastic scattering to Coulomb excitation, the process in which energy is transferred to a nucleus from the electric field of another, and (in most cases) re-emitted as a gamma ray. As reported in Physical Review Letters (2017), this high-precision measurement revealed the influence of coupling to the continuum, the role of virtual states corresponding to unbound, free neutrons interacting with 10Be. A second experiment with TIGRESS, published in Physics Letters B (2014), also identified breakup reactions, those in which enough energy is transferred to break 11Be into 10Be and a free neutron, a “continuum” state. The first measurement was accurate enough to validate a first-principles calculation of the gamma-decay rate, which revealed the influence of the coupling of virtual, continuum states to the overall structure of the bound states of 11Be. The second showed that the overall reaction dynamics depended not just on the continuum but also on highly excited states in the 10Be core itself.
Coexisting shapes in strontiumThe shape of an atomic nucleus is a result of a delicate interplay between macroscopic, liquid drop-like and microscopic shell structure effects. Nuclei with a closed shell configuration are spherical in their ground states, but away from magic numbers deformed ground states are observed. Small changes in the nucleon number can lead to rapid changes in deformation and states of different deformation can coexist at close excitation energies. In the first ISAC experiment with a post-accelerated heavy beam, recently published in Physics Letters B, the shape coexistence in 96Sr was studio with TIGRESS and SHARC by means of the d(95Sr,p) transfer reaction. These results suggest coexistence of three different configurations in 96Sr and strong shape mixing of two excited 0+ states.
03 How It Works
TIGRESS measures gamma ray spectroscopy in much the same way as GRIFFIN). However, TIGRESS performs in-beam, or in-flight, gamma ray spectroscopy with accelerated nuclei moving at approximately 10,000 to 40,000 kilometres per second, about three to 12 percent the speed of light. (GRIFFIN, by comparison, measures gamma rays that have been delivered at low energies and stopped.)
A TIGRESS experiment starts with a dedicated beam of selected rare isotopes in ISAC-II. The isotopes are fired at a paper-thin target foil in the reaction chamber in TIGRESS’s core, producing up to three different types of nuclear reactions.
The lowest energy reaction is a Coulomb excitation reaction. In these reactions, the beam energy is chosen so that the electric repulsion of the positive beam and target nuclei prevents them from touching. The beam energy usually gets transferred into kinetic energy of the beam and target. However, when the nuclei come close enough together, some of the beam energy is instead transferred into internal energy in the beam, the target, or both. In TIGRESS experiments this typically happens for 1 out of every 1000 scatters.
At higher energies, the electric repulsion is overcome, the nuclei can touch and the two other types of nuclear reactions occur.
In a fusion reaction, beam and target nuclei fuse to form a new nucleus with the mass and charge equal to the sum of the two participating nuclei. Much of the beam energy is transferred into internal energy of the new nucleus.
In a transfer reaction, a small number of protons or neutrons move from a beam nucleus to a target one, or vice versa.
Once internal energy is imparted to a nucleus, in order to return to the stable ground state, these excited nuclear states spontaneously emit gamma rays at very well-defined energies. The energy, rate, and direction of the emitted gamma rays reveals whether the constituent protons and neutrons are behaving relatively independently, or as a single system.
Surrounding TIGRESS’ reaction chamber is its gamma ray spectroscopy heart: an array of 16 high-purity germanium gamma ray detectors, each containing four high-purity germanium detectors cooled with liquid nitrogen. Each of the detectors consists of four germanium crystals, each of these with eight-separate outer electrical contacts for gathering the electrical signals produced when a gamma ray hits the germanium. The electrical segmentation enables TIGRESS scientists to better identify where in the detector the gamma ray struck.
This location information is crucial because the gamma rays are Doppler shifted. They were emitted by a nucleus moving at three to 12 percent the speed of light. Thus, as with the difference in the pitch of a train whistle depending on whether a train moving towards or away from you, the gamma radiation is also wavelength shifted. To correct for this, TIGRESS scientists use information from the gamma ray’s detection location, and the direction and speed of the isotope, to remove the Doppler effect and relate the energy measured in the laboratory from the moving nucleus, to its internal (de-)excitation energy.
Finally, TIGRESS contains a crucial system for filtering-out, or suppressing, partial gamma ray hits. TIGRESS’s detectors are surrounded by Compton suppression shields, in essence another layer of detectors called scintillator detectors. These exist because there are three ways that gamma rays interact with matter, including the germanium detector crystals. It’s only in one way, the photoelectric effect, in which all the gamma ray’s energy is transferred to the germanium in a single event.
In the other two modes, called Compton scattering and pair production, the gamma ray does the equivalent of shattering. It deposits some of its energy in the germanium, while some scatters out of the crystal. The Compton suppression shields record these scattered bits of energy, enabling TIGRESS scientists to remove (or suppress) these partial gamma ray energy events recorded in the germanium detectors. This suppression is equivalent to removing background noise, enabling TIGRESS scientists to achieve high sensitivity in their experiments, particularly with low energy gamma rays.
To learn more about TIGRESS, please see here.