Researchers produce heaviest silicon isotope yet

An NSCL team has produced the heaviest silicon isotope ever observed. In a recent experiment, silicon-44 ions traveling at approximately half the speed of light (330 million miles per hour) were separated out from many millions of billions of beam particles and reaction products and identified one-by-one as they traveled through the superconducting magnets of the Coupled Cyclotron Facility’s A1900 fragment separator.

The research has been accepted for publication in Physical Review C.

Chemical elements can exist in a range of masses, where each mass corresponds to a different number of neutrons in the nucleus. A fundamental question in nuclear science is how many neutrons can be added to or removed from a chemical element before it becomes unstable. Probing when a given nucleus comes apart due to internal strain is one way researchers test and extend nuclear theory.


Figure 1: The results of the identification of the reaction products reaching the end of the A1900 fragment separator included three silicon-44 ions (red circle). more

At present, the complete range of masses is only known up to oxygen, an element with 8 protons in its nucleus (atomic number = 8). Stable isotopes of silicon (atomic number = 14) have 14, 15 or 16 neutrons in the nucleus. However, earlier experiments have shown that a silicon nucleus can exist with as few as 8 or as or as many as 29 neutrons.

Using techniques developed at NSCL, an intense beam of calcium ions was directed at a tungsten target for about four hours. The heaviest reaction products were sifted out by the A1900 separator and then individually identified by a set of radiation detectors. These detectors were sensitive enough to record atomic numbers, time-of-flight through the separator, and positions and angles of the measured particles. Combining these data produced a unique identification of the numbers of neutrons and protons in each ion.

One key result is clearly visible in the distribution of ions shown in Figure 1. Namely, three silicon-44 ions (red circle) were observed in this study – a first-ever achievement. Previously, the heaviest observed silicon isotope was silicon-43, which was detected in a 2002 experiment at the RIKEN facility in Japan.

Each point in the graph corresponds to one particle detected in the experiment. The data included expected gaps for unstable combinations of neutrons and protons, such as fluorine-28, oxygen-25 and oxygen-26 (gray circles). The grouping of dots is due to the fact that the atomic number Z and mass number A (the number of protons and neutrons together) of every nucleus are integers. Because the ions emerged from the A1900 separator fully stripped of electrons, Q, the charge on each ion, was equal to the atomic number Z.

The data also reveal a subtle feature of the production rates of various silicon isotopes. Note that the detector also recorded lighter silicon isotopes, silicon-42 and -43 (green circles), with 28 and 29 neutrons, respectively.

Silicon-42 has the same number of neutrons as calcium-48, the isotope with 20 protons and 28 neutrons used as the ion source in the experiment. Calcium-48, more valuable than gold by weight, makes up just 0.19 percent the natural calcium found on Earth. Silicon-42 is produced when six protons in a given calcium-48 isotope are removed upon striking the stationary target.

In contrast, silicon-43 and -44 actually have more neutrons than the calcium isotope. So making these rare isotopes means not only removing six protons but also picking up one or two neutrons from the stationary target. The event is exceedingly rare, thus the relatively sparse data points in the silicon -43 and -44 circles in Fig 1.

The research was supported by the National Science Foundation and Michigan State University.

NSCL is a world-leading laboratory for rare isotope research and nuclear science education.

The paper is available on the arXiv preprint server at: http://arxiv.org/abs/0705.0349.

For more information on the A1900 fragment separator, see: www.nscl.msu.edu/tech/devices/a1900.

- Dave Morrissey (morrissey at nscl.msu.edu or 517-333-6321), April 16, 2007