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Scientists have produced over 100 new neutron-rich isotopes for elements between vanadium and rubidium at the GSI laboratory in Darmstadt, Germany, using a novel technique reported by Monique Bernas of the Institut de Physique in Grenoble, France, Friday afternoon at the Joint APS/AAPT Meeting. Unlike conventional target-fission techniques, in which a target of metallic foil is hit by a beam of light particles, Bernas has developed a new method that relies on projectile fission.
The new isotopes were made by accelerating uranium-238 up to an energy of 750 MeV per nucleon and colliding them with beryllium and lead targets. The fragments are then separated using the high-performance spectrometer FRS at GSI-Darmstadt. "All these neutron-rich isotopes are produced daily in nuclear power reactors, but they occur so rarely that they could not have been observed before," said Bernas. "This is the first direct observation of every single type of isotope produced in fission."
Fission is the most efficient means of producing neutron-rich isotopes, and since its discovery in 1938, more than 400 new radioactive isotopes have been found and studied, separated mainly by radiochemical methods. When the first uranium beam was accelerated at relativistic energies at the Lawrence Berkeley Laboratory, the fission cross-sections were measured in a pioneering experiment using small silicon detectors. However, while elements could be identified, the isotopes were not separated. On-line mass separation techniques were established in 1967, but the process was still chemistry-dependent and inefficient for some elements with short-lived isotopes.
Finally, in-flight separation techniques based on large spectrometers combining electric and magnetic fields were developed to separate ionized fission fragments independent from their chemical properties. They are fast, with the ability to identify fragments in a millionth of a second. However, only one in a million of the fission products are transmitted through the magnet. According to Bernas, the efficiency of her projectile-fission method is more than four orders of magnitude greater than that of former in-flight methods, and the time required for separation and identification is shorter than any beta-decay half-lives.
In the present experiment, rare fragments of all elements produced in fission are unambiguously identified event by event by measuring energy loss after separation by the spectrometer and time-of-flight along a well-defined path in the magnets. This is because the fragments move at nearly the beam velocity, close to the uranium beam direction, and are totally ionized, making them easier to detect than in previous experiments, where they emerged at low velocities in any direction with different ionic states. Using this technique, a large number of isotopes can be simultaneously observed; with target fission, only light elements have been identified in flight.
The new isotopes do not last very long; their expected half lives range between 20 to 700 milliseconds. However, a number of them qualify as "r-process" nuclei. According to Bernas, in our universe, about half of the abundance of elements heavier than iron was produced by rapid neutron-capture reactions occurring on a short time scale, and sequential captures therefore lead to the production of extremely neutron-rich isotopes. Later, these isotopes beta-decay back to the stable atomic nuclei.
Thus, the projectile-fission method opens a wide field for nuclear structure investigations. For example, the spectroscopic analysis of the newly observed isotope of nickel-78 - a doubly magic nucleus with 28 protons and 50 neutrons - will provide a crucial test for nuclear structure models, and will be used to determine residual interactions in the shell model picture. "Fundamental characteristics need to be known for these nuclei in order to understand mass abundancy in the solar system and to constrain astrophysical models for supernova explosions," said Bernas of the effect.
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