EMMA, the ISAC-II Recoil Mass Spectrometer

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EMMA (ElectroMagnetic Mass Analyser) is a recoil mass spectrometer for TRIUMF's ISAC-II facility. ISAC-II provides intense beams of radioactive ions with masses up to 150 atomic mass units to international scientists studying nuclear structure and nuclear astrophysics at Canada's national subatomic physics lab. The energies of these beams depend on the specific nuclei being accelerated, but typical top speeds range from 10-20% of the speed of light. At these energies, different types of nuclear reactions can be studied to learn about the structure of exotic nuclei and the nuclear reactions that produce the chemical elements in stars and stellar explosions. In order to learn about the properties of these nuclei and measure the rates of reactions that produce the chemical elements in stars, it is first necessary to identify the products of these reactions. An atomic nucleus can be uniquely identified by measuring its charge and its mass. The charge of a nucleus is simply the number of protons it contains, while its mass is the sum of the number of neutrons and protons that make it up. How can one measure the mass and charge of a nucleus? The trajectories of nuclei in magnetic fields are bent according to their momentum and charge, while in electric fields the trajectories depend on the energy and charge of the nucleus. By combining electric and magnetic fields in a particular way, one can cancel the energy and momentum dependence of the trajectories, and bend them according to their masses and charges alone. Hence a device that combines electric and magnetic fields can be used to separate an interesting nucleus produced in a nuclear reaction from all of the other nuclei produced in different nuclear reactions and from the beam used to initiate the reactions. Such a device is called a recoil mass spectrometer, because it analyzes the products of nuclear reactions, called recoils, dispersing them according to their mass and charge. Once complete, EMMA will be used in approximately 1/3 of the experiments at ISAC-II.