DSL

DSL is TRIUMF's Doppler Shift Lifetimes facility. Initiated by Gordon Ball and Barry Davids and implemented by Randy Churchman, Rituparna Kanungo, and Mythili Subramanian in 2005, DSL is is an experimental setup for the measurement of the lifetimes of excited states of nuclei.

The goal of nuclear astrophysics is to understand the creation of the chemical elements and the production of energy in stars and other astrophysical environments. Nuclear physicists contribute to this effort by measuring the rates of nuclear reactions. Ideally, this is done by directly measuring in the lab the probability that a particular reaction occurs at the energies found in the Big Bang, stars or stellar explosions. Unfortunately, in most cases this is impossible because the probabilities are so low that experimental backgrounds or time constraints prevent direct measurements. Another difficulty is that many of the atomic nuclei involved are radioactive and don't survive long enough for an experiment to be performed.

Even when a direct measurement is impossible, there are other ways to determine a nuclear reaction rate indirectly. Most reaction rates of astrophysical importance are dominated by a process known as resonant capture that yields an excited state of the compound nucleus formed in the reaction. For quantum mechanics afficianados, resonant capture occurs when the wave function describing the two reacting nuclei is very similar to the wave function of the excited state in the compound nucleus. The details aren't important here, but when resonant capture is the dominant process in a reaction, its rate can be deduced by studying the decays of excited states of the compound nucleus that is the end product of a reaction. One of the decay properties that is of primary importance is the decay rate, or equivalently the lifetime of the excited state of interest.

A measurement of the excited state's lifetime requires several steps. First, one must populate the excited state. At the DSL facility, this is accomplished through the use of transfer reactions, in which two reacting nuclei exchange one or more protons or neutrons, leaving the recoiling nucleus in an excited state. We induce these reactions by colliding a beam of nuclei with a thin metal target into which a layer of light nuclei has been implanted. Our first experiment used a gold target implanted with helium nuclei. The reaction occurs in the thin implanted layer, and the products of the reaction pass through that layer, continuing through the metal foil. The two products of the reaction have drastically different properties. The lighter of the two passes right through the metal foil, losing a small amount of energy, and is detected in a charged-particle detector. Measuring the remaining energy of the light product allows us to determine which excited state in the heavy recoiling nucleus has been excited. The heavy recoil nucleus on the other hand loses all of its energy in the metal foil, slowing down and eventually stops. The time it takes to stop depends on the energy and charge of the recoiling nucleus and some properties of the foil's atomic nuclei, such as their charge and mass. Many measurements of the stopping powers of different materials along with theoretical calculations have allowed quite precise determinations of the stopping times for nuclei with speeds around a few percent of the speed of light.

How can one measure the lifetime of the excited state formed in the reaction? These lifetimes typically range from femtoseconds to picoseconds. A femtosecond is 1/1000 of a picosecond, which is itself only a trillionth of a second. So we're talking about some pretty short times here! When the excited states we're interested in decay, they usually do so by emitting a gamma ray. The lifetime is determined by measuring the energies of the gamma rays that are emitted when the state decays. Here we take advantage of the Doppler effect: the shift in energy (and frequency) of radiation emitted by a moving source. Depending on the lifetime of the excited state, the recoiling nucleus will emit its gamma ray while still moving or after it has stopped in the foil. The energy of the gamma ray depends on the speed of the nucleus at the time it was emitted. One of our gamma ray detectors is located just beyond the target along the beam path. The energies of gamma rays emitted by recoil nuclei that are still moving forward are measured by this detector to have larger energies than gamma rays emitted by already stopped recoils because of the Doppler effect.

By collecting a number of gamma rays from the excited state of interest, we obtain a distribution of gamma ray energies that is characteristic of the lifetime of the state. The reason for this is that the decay of the excited state is a random process like the decay of radioactive nuclei. One can't predict when exactly a given nucleus will decay, but the distribution of decay times can be understood on the basis of simple statistics and characterized by a single number, the mean lifetime. Finally then, knowing the stopping characteristics of the recoil nucleus in the metal foil allows us to deduce the mean lifetime of an excited state from the energy distribution of the gamma rays it emits.

Our first experiment with DSL, performed in November 2005, was a measurement of the lifetime of the 4.03 MeV state in 19Ne. The experiment was a success, and the data have been analyzed and submitted for publication. A paper describing the measurement is available for download here. In September 2006 we studied the lifetimes of this and other states in 19Ne with the highest precision yet achieved. This experiment was led by my PhD student Mythili Subramanian. A preprint describing our results is available for download here. For more information, please contact Barry Davids by e-mail.