- 01 Overview
- 02 DSL Science Highlights
- 03 How It Works
With the DSL (Doppler Shift Lifetimes) facility, TRIUMF scientists are shining light on element formation in stars by measuring nuclear excited states that last for only femtoseconds, thousandths-of-a-trillionth-of-a-second.
A nuclear-excited state is a nucleus that contains excess energy and spontaneously releases it, for example as a gamma ray, to reach a more stable nuclear configuration.
How long a nuclear-excited state lasts, its lifetime, typically ranges from femto- to picoseconds, or trillionths-of-a-second. Nonetheless, infinitesimally small lifetime differences can make a big difference in the elements created by a star.
All else being equal, the shorter the lifetime of an excited state, the greater the probability it will be formed in a fusion reaction, creating a new and different element. This can have a dramatic long-time ripple effect on the elements formed by a star, just as a taking a left turn rather than a right turn at the beginning of a long trip can take you to a very different destination.
Excited state lifetimes are particularly important to a key stellar fusion process called resonant capture. This occurs when the quantum wave function describing the two reacting nuclei is very similar to the wave function of the product nucleus formed when they fuse together.
What’s key for DSL experiments is that the rate of resonant capture reactions can be deduced by studying the lifetime of excited states of the product nucleus, and these lifetimes can only be determined experimentally.
Thus, the DSL facility’s precise measurements are providing theoretical astrophysicists worldwide with critical empirical data for the finessing of astrophysical models and for interpreting observations. For example, DSL experiments are illuminating the nature of stellar explosions in binary star systems and the ages of the Milky Way’s oldest stars.
03 How It Works
The DSL facility is a versatile, portable facility that can be mounted at the end of a beamline in ISAC-II.
DSL gets its name from the fact that it determines nuclear-excited state lifetimes by measuring the Doppler shift of the gamma rays emitted as a nucleus decays to a lower energy state. (TRIUMF’s TIGRESS Integrated Plunger uses a similar process).
The DSL facility consists of a target chamber, a liquid nitrogen dewar (a specialized, insulated container for holding this cryogenic material), a customized, two-stage charged-particle detector, two gamma ray detectors, electronics, and a data acquisition system.
A DSL experiment begins by firing an ISAC-II beam of selected high-energy heavy ions at a target to induce nuclear reactions that produce the desired nuclear excited states.
The target is a thin metal foil, often gold, with helium ions implanted just a tenth-of-a-micrometer below the surface. After hydrogen, helium is the most common fuel in stars, and is also a key element in studying stellar fusion reactions in the lab.
To counter the heating effects of the ion beam on the foil, the target and surrounding structure are cooled by a liquid nitrogen system that refrigerates the target, preventing the evaporation of the helium from the foil.
The ISAC-II beam ions and the helium react via nuclear transfer reactions in which they exchange one, or more, protons and/or neutrons. This results in two product nuclei, a light ejectile and a heavy recoil nucleus in an excited state. For example, bombarding a helium-3 (3He) nucleus with an oxygen-16 (16O) beam produces a helium-4 (4He) ejectile and an excited oxygen-15 (15O) recoil. The heavy recoil is slowed and eventually stopped in the foil.
The lighter ejectile nuclei pass through the metal foil and are detected by the DSL facility’s two-stage charged-particle detector. The detector consists of a pair of particle detectors separated by a small distance so that the particles are detected by the back detector shortly after being detected in, and passing through, the front detector. Particles lose some of their energy in each detector. The ratio of energy lost in the front compared to the back detector is characteristic of the charge and mass of the particle.
The total energy lost in both detectors guides DSL scientists in determining which excited states in the heavy recoil nucleus were populated.
Collecting Gamma Rays
The lifetime of the excited state is determined by measuring the energies of the gamma rays it emits when the state decays, and correlating this with the known stopping time of the recoil in the foil (determined experimentally and based on theoretical models).
What’s key is that the detected energy of the gamma ray depends on the speed of the nucleus at the time it was emitted. The DSL facility typically employs two gamma ray detectors, one located just beyond the target along the beam path, the other to the side.
If a nucleus is moving toward a detector when it emits a gamma ray, the detected energy of this gamma ray will be Doppler-shifted and will appear to have a higher energy than if it were emitted when the nucleus was at rest. This is analogous to the situation in which if a train is moving towards you, the frequency of its whistle is Doppler-shifted and you hear a higher frequency than if the train were stationary. The shorter the lifetime of the excited state is, the faster the recoil will be moving when the gamma ray is emitted, and the larger the Doppler shift.
By collecting hundreds or thousands of gamma rays from the excited state of interest, DSL scientists obtain a distribution of gamma ray energies that can be used to pinpoint the lifetime of the state.