TIGRESS

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).   

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.