- 01 Overview
- 02 GRIFFIN Science Highlights
- 03 How it Works
GRIFFIN (Gamma Ray Infrastructure For Fundamental Investigations of Nuclei) is the world’s most powerful tool for the decay spectroscopy of rare isotopes, and it is providing TRIUMF scientists an unparalleled view of the interplay of forces that create nuclear structure.
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. GRIFFIN and TIGRESS form a tag-team duo, each unique and complimenting the other’s capabilities in TRIUMF’s world-class use of gamma ray spectroscopy to help unravel the mysteries of nuclear structure.
GRIFFIN uses ISAC-I beams delivered to and stopped at its central focus, and measures the gamma rays emitted from radioactive nuclei after they decay. This decay spectroscopy is a powerful tool for studying nuclear structure by observing the gamma radiation emitted when radioactive nuclei decay.
GRIFFIN is enabling scientists to chart the unexplored nuclear frontier using rare, radioactive isotopes produced by ISAC-I. There are 288 stable isotopes, some with half-lives of billions of years. Yet, it’s estimated that there are about 7000 unstable, radioactive isotopes, with very brief half-lives, usually found only in exploding stars. Most of these rare isotopes have never been produced or studied on Earth. However, these rare isotopes are a gold mine for understanding the inner workings of the nucleus. For example, GRIFFIN provides highly precise, statistically rigorous information on aspects such as the branching ratio of nuclear gamma ray decay, the probability of a decay occurring along one of many particular paths.
Together, the gamma rays detected by GRIFFIN tell a detailed nuclear story, one that’s providing crucial data for TRIUMF scientists and others world-wide to test, clarify and extend theoretical models of nuclear structure. This has important implications for fields from beyond-Standard Model physics, new materials research, and the discovery of new exotic isotopes for nuclear medicine.
GRIFFIN is designed to operate with a suite of additional detectors, including DESCANT, each of which can detect additional kinds of particles, including beta particles, neutrons and internal conversion electrons, in order to produce a unique, overall view of nuclear structure.
02 GRIFFIN Science Highlights
Clarifying a key step in the origin of the elementsClarifying a key step in the origin of the elements: As reported in Physical Review C (2016) researchers using GRIFFIN have produced the highest-precision measurement ever of the half-life of cadmium-130 (130Cd), a rare isotope that's a cornerstone for understanding cosmic element formation. Astrophysical observations are providing mounting evidence that heavy elements are forged in the merger of neutron stars. However, to understand and accurately simulate this element formation process (r-process), it's necessary to experimentally characterize the key rare isotopes involved. This is especially the case for the half-lives of isotopes with masses of about 130 since theoretical models have been tuned to reproduce the half-life of 130Cd and then predict half-lives in the entire region. Thus, scientists used GRIFFIN to produce a 130Cd half-life measurement three-times more precise than the previously adopted world average, a result which will help astrophysicists more clearly see our stardust origins.
First measurement and ab initio calculation of rare excited nucleus' energy lossFirst measurement and ab initio calculation of rare excited nucleus' energy loss Using GRIFFIN, researchers confirmed the existence of a very rare case of energy loss from excited scandium-50 (50Sc) nuclei and the TRIUMF theory group produced the first ab initio calculation of this transition rate. Nuclei in hot excited states become more stable by emitting radiation in the form of gamma (γ) rays, high energy photons, that carry away both energy and angular momentum. Usually, the gamma rays carry away one or two units of angular momentum and do not change the parity. As reported in Physical Review C (2017), scientists used the GRIFFIN spectrometer to identify the angular momentum of states in 50Sc using γ-γ angular correlations. The existence was of a transition magnetic octupole was confirmed which carries away three units of angular momentum at once. This decay is so rare that the parent state lives for a full half a second instead of the typical one- trillionth of a second.
Half-life measurement provides clearer view of the weak forceHalf-life measurement provides clearer view of the weak force: Scientists using GRIFFIN achieved a half-life measurement of magnesium-22 (22Mg) three times more precise than the previously adopted world average. As reported in Physical Review C (2017), this high-precision measurement provides a clearer view of the dynamics of the weak force. Precision measurements of the ft values for superallowed Fermi β-decay transitions between isobaric analog states provide fundamental tests of the Standard Model's description of electroweak interaction. These transitions provide a stringent test of the conserved vector-current (CVC) hypothesis, and in combination with other values, they also provide the most precise determination of Vud, the most precisely determined element of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix. Researchers used a 4π proportional gas counter and the GRIFFIN spectrometer to make the 22Mg half-life measurement, resolving a discrepancy between the two previously published 22Mg half-life measurements.
03 How it Works
GRIFFIN is a spectrometer constructed to precisely and efficiently detect gamma rays emitted by the radioactive decay of exotic nuclei.
Gamma rays are a form of electromagnetic radiation like visible light but more energetic. And, like all light, gamma rays are photons, or individual packets of energy, and as such carry information about their source.
Spectroscopy is the use of the light spectrum, or different photon transitions between energy levels, to study atoms and nuclei. This is because electron and nucleon transitions emit light at particular wavelengths. With atoms, a spectroscope records the energy emitted by the transition of electrons from one orbital to another. The gamma ray spectroscopy of nuclei records the energy emitted by the transitions and transformations of neutrons and protons among orbitals in he nucleus. Thus, GRIFFIN’s detections are a telltale sign of the forces at play within the atomic nucleus. In the nucleus, protons and neutrons are arranged in orbitals, or energy levels, analogous to the quantum levels of electrons. Groups of orbitals, with similar energies, are organized into shells. An excited nuclear state decays to a more stable lower-energy state by emitting a gamma ray. However, this energy loss, or transition, can occur from the excited energy state to any lower state, or a cascade of them, with the emission of a gamma ray of different energy in each case.
In a GRIFFIN experiment, the first step is the arrival of an ISAC beam of mass-separated rare nuclei. These are fired into, and embedded in, a thin Mylar tape at the center of GRIFFIN’s detector array. (There’s even recycling in nuclear physics: the Mylar tape is repurposed computer tape.)
Since most of the radioactive nuclei quickly decay into radioactive daughter nuclei, it’s a moving tape collector. Like a conveyor belt, the tape is moved after an initial measurement, so that the used tape is contained behind a lead shield and gamma rays from daughter nuclei don’t interfere with the detection of gamma rays from the parent nuclei of interest.
GRIFFIN’s detector is a clover-leaf shaped array of 64 high-purity germanium crystals, arranged into 16 clover-shaped gamma ray detectors (four germanium crystals make the clover shape). Germanium, a semiconductor similar to silicon, is used in gamma ray detectors because its electrical characteristics make it ideal for turning gamma ray energy into a tiny electrical current. Each custom-made germanium crystal is 9 cm long and 6 cm in diameter, with the outer edges tapered at 22.5 degrees over the first 30 mm of length in order to provide for close packing in the overall detector array.
The germanium crystals are contained within a vacuum chamber and cooled with liquid nitrogen to -175 °C. The cooling reduces the thermal excitations of germanium valence electrons. Gamma ray interactions produce small electrical signals in the germanium, and this cooling is necessary to reduce the background electronic noise masking the signals of interest.
GRIFFIN includes a state-of-the-art digital data acquisition system. It’s able to achieve a sustained data throughput of 300 Mb per second, enabling the system to capture as many as 50,000 gamma ray interactions-per-second from each of the 64 germanium crystals.
By recording the gamma rays from millions of a specific isotope decay, GRIFFIN is able to create a spectrum, or an energy fingerprint, that enables scientists to distinguish details of nuclear structure and interactions. A key observable is the branching ratio, which is an experimentally derived measure of the probability that any one of several possible decay routes will occur, and as such reveals the inner workings of the nucleus.
With the addition of cerium-doped lanthanum bromide crystal gamma ray detectors, GRIFFIN is also able to measure and record, ultra-fast gamma decays from daughter nuclei, ones that occur in as little as ten trillionths of a second.