ATOMS AND NUCLIDES
ATOMS AND NUCLIDES
Matter, as we understand it, is composed of a hierarchy of building blocks. Living creatures, as well as our inanimate surroundings, are made of molecules, which in turn are made of atoms, whose mass resides almost entirely in the nuclei. The nuclei include protons and neutrons, which ultimately consist of quarks and gluons. On the microscopic scale of the natural elements, the building blocks of matter are elegantly classified in three major schemes :
The question "when and how did it all start" is partly answered, to our present understanding, by the Big Bang model.
It is for example fascinating to realize that some of the light elements, say hydrogen, helium, lithium, which we observe today in Nature, were produced (or more exactly their nuclei were) about three minutes after the Big Bang. Around us on Earth, the light isotope of helium (3He) is largely "primordial" in this sense. The case of helium on Earth is actually interesting : its more abundant isotope (4He) is continuously created by natural radioactivity (alpha) and this "radiogenic" 4He is added on Earth to the primordial 4He.
This brings about related questions :
Our research, extending to several disciplines, is devoted to these questions.Literature and Links :
The stable nuclides which constitute matter as we know it, ourselves, our computer chips, the planets around us, have been synthesized in the stars of our Galaxy. Myriads of stars of our Galaxy may have contributed to produce the atoms we are made of. This is in principle possible since once a stable nuclide has been produced, it is not likely to disappear. Stable nuclides are insensitive to time.
On the other hand, unstable nuclei, which decay via the various radioactivity processes, constitute physical clocks : the very presence of an unstable nuclide indicates that it must have been produced, and no matter how, at most a few half-lives ago. In some cases, the relative number of such nuclides will act as a clock with a scale comparable to its half-life. Amazingly, these scales range from many million years to less than picoseconds.
Let us take two examples taken from two different disciplines.
Nuclei, beyond the mere interest in their physical structure and in the way they decay or interact, occupy a tremendous importance in our quest to understand the past. None of the other building blocks of matter plays this role.
RARE PARTICLES AND THEIR DETECTION
We describe here in general terms the experimental method we use to detect and "count" very rare atoms or particles, such as the cosmogenic nuclides we discussed above. Accelerator Mass Spectrometry (AMS), a sophisticated detection technique,identifies and counts rare atoms, such as radiocarbon and other cosmogenic nuclides, at an unprecedented level of sensitivity.
Our group has been a world leader in the development of the technique and new applications. Through these efforts and those of other laboratories, AMS is now accessing radionuclides with half-lives ranging up to million years as geophysical chronometers or environmental tracers.
The method links techniques of particle acceleration, mass spectrometry, nuclear identification and computer data analysis.
A series of sophisticated filters are applied, based on different physical and chemical principles, to discriminate a desired species from the background matrix : ion source chemistry, physical dispersion by magnetic and electrostatic elements, molecular dissociation, measurements of energy, stopping power and time-of-flight.
The main component of the experimental apparatus is a particle accelerator. Our experiments are performed at the 14UD Pelletron accelerator at the Weizmann Institute (Rehovot) and at the Argonne Tandem-Linac Accelerator (ATLAS) at Argonne (USA).
AREAS OF RESEARCH
Cosmogenic nuclides with half-lives much longer than that of radiocarbon (5,730 years) offer the fascinating prospect of revealing insight into the far past. Archaeological dating by radiocarbon is limited in practice to ages below 40,000 to 50,000 years BP (Before Present) but nuclides such as 10Be (1.6 million years), 26Al (760,000 years) and 41Ca (100,000 years) may, in principle, lead to information on older times.
We are presently studying the concentrations of 10Be and 26Al produced in flints by the continuous exposure to cosmic rays. Flint, a polycrystalline form of quartz, has an enormous importance as raw material for the fabrication of tools in the Stone Age. Flint minerals close to the surface and exposed to cosmic rays for long periods of time, build up 10Be and 26Al by spallation reactions of the energetic cosmic particles on its constitutents (Si and O). On the other hand, flint minerals and tools buried under soil (either before having been manufactured or later) have no significant buildup of cosmogenic nuclides and any such nuclides will in fact decay with their own long radioactive half-life. By measuring 10Be and 26Al concentrations, we expect to derive information on the history of the material.
A similar scenario may be applied to the very rare radionuclide 41Ca (100,000 years) which is produced on the Earth surface by the capture of a neutron (itself produced in the atmosphere by cosmic rays) by the most abundant isotope of calcium, 40Ca. Because of the large calcium content of bones (mainly as phosphate) and on the other side the archaeological importance of dating of bones, the possibility of a "radiocalcium" method is exceptionnally interesting. We are presently measuring the (extremely low) 41Ca content of Ca minerals, modern and ancient bones to assess the feasibility of the method.
This research is a collaboration between the Racah Institute and the Center of Archaeological Science at the Weizmann Institute.
The stable nuclides which form matter around us have been synthesized in stars and followed a long itinerary in interstellar space until their eventually merging into a nebula and condensing into the Solar System. The time of this formation is about 4.5 billion years ago, which we know from the decay of radioactive nuclides such as uranium and thorium isotopes. These long-lived nuclides, as of course the stable ones, are still around us. Shorter-lived nuclides, present in the Early-Solar matter, have decayed and are now extinct.
However, the Interstellar Medium (ISM), present in the Galaxy, resembles in many aspects the Proto-Solar nebula and is expected to contain "freshly" synthesized nuclides. It is now known that grains of ISM penetrate into the Solar System and such grains have even be detected in the higher atmosphere of the Earth. They may therefore reach the Earth and give us the amazing possibility of studying matter of an unknown kind.
We are presently pursuing a search for such material in Earth reservoirs such as deep-sea sediments and nodules where the possible accumulation of ISM accreted material may be detectable. The unmistakable signature for such material would be the presence of "short-lived" nuclides extinct in the present Solar System : candidates for such nuclides are 244Pu (81 Myrs), 247Cm.
In another kind of study, we are producing in the laboratory short-lived nuclides by nuclear reactions between accelerated particles and a target. This " laboratory nucleosynthesis", physically similar to that occurring in stars or supernovae, allows to measure directly the production yields of various nuclides and compare them to observations and astrophysical models : 7Be (57 days) produced in thermonuclear fusion reactions in the Sun, 44Ti (60 yrs) produced in core-collapse supernovae are cases presently investigated.
This research is a collaboration between the Racah Institute, Weizmann Institute, Argonne National Laboratory (USA) and University of Notre-Dame (USA).
Enviromental and Geophysical Research
Detection of naturally occurring and artificial nuclides is of wide-ranging interest in geophysical and environmental sciences. It is observed that the nuclide 129I for example, has increased in concentration in the environment by several orders of magnitude in the last forty years, due to nuclear power and nuclear reprocessing plants. The first measurements of environmental 129I were performed by our group at the Racah Institute, following the nuclear reactor accident of Chernobyl in 1986. Concentrations of 129I in marine seaweeds collected along the Mediterranean Israeli Coast and Red Sea coast act as a monitor of global pollution transport.
The rare 236U isotope of uranium is considered as an integrating neutron monitor over the last 100 million years in geological sites. In uranium minerals, it is naturally produced in the capture by 235U of a neutron. 236U is also abundantly produced in nuclear reactors and can be used in nuclear safeguard programs for the monitoring of nuclear activity.
This research is a collaboration between the Racah Institute and Soreq NRC (Yavne, Israel), sponsored by the International Atomic Energy Agency (Vienna).
The case of 81Kr is particularly appealing because its half-life allows in principle the dating of water (for example ground water aquifers) in the scale of hundred thousand years) by comparing the 81Kr content of the aquifer with that of contemporary water. The scheme has been applied for the first time to the Great Australian aquifer and dates of ground waters have been obtained.
In a different scale of time (hundreds of years), 39Ar is used to date ocean water samples from various origins. The age of water is an important input in the modeling of oceanic transport, a mechanism which influences heavily the global climate.
Experimentally, the cosmogenic noble gases pose different and difficult problems of detection, due to their very low abundances and to the fact they require very large accelerators of a different type than used in other AMS analysis. These experiments have been performed at the National Superconducting Cyclotron (USA) and at Argonne National Laboratory (USA).
This research is a collaboration between the Lamont-Doherty Earth Observatory (Columbia University, USA), the University of Vienna, Argonne National Laboratory, the National Superconductive Cyclotron Laboratory (USA), and the Racah Institute.
The ability to count atoms rather than the decay radiation of radioactive nuclides is triggering a major breakthrough in the use of tracers in biomedicine. Tracers consist in an amount of material added to a biological system in order to follow its biochemical evolution. Radioactive tracers are conventionnally used for the ease of detection of their decay radiation (e.g. gamma rays, positrons) in scanners and X-ray radiography.
The AMS atom counting method described above, offers several advantages : (i) since the detection capability is independent of the radioactive half-life, tracing with very long-lived nuclides becomes possible over unlimited periods of time; (ii) the sensitivity of the detection permits tracing the evolution of an element using a minute amount of tracer, not modifying the normal load of the element in a biological system; (iii) no collateral radiation risk exists due to the combination of minute amount of tracer and the long radioactive half-life.
We are presently involved in a pilot experiment where the feasibility of calcium tracing with the isotope 41Ca is studied. The rate of bone resorption and the net long-term calcium balance are important for understanding the mechanism of onset of osteoporosis and hormonal effects. We are following the evolution of a 125 nanogram dose of 41Ca injected to a patient by monitoring the 41Ca concentration in blood and urine over time.
This research is a collaboration between the Racah Institute, Triumf Laboratory (Vancouver, Canada) and University of British Columbia (Vancouver, Canada).
The use of illustrations and links taken from various sources is gratefully acknowledged.