Origins Program

Where did the universe come from? Do other habitable planets like Earth exist in the cosmos? Does life dwell elsewhere in the Solar System or beyond? People have speculated about these questions throughout history and have probably wondered about them, in some form, since early humans first looked up at the stars. Virtually every culture has origins myths. Now, for the first time, scientific answers appear within our grasp.

Under a sweeping program called "ORIGINS," the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF) and the Department of Energy (DOE) hope to combine their respective skills and expertise to learn how all objects -- from quarks to quasars, from galaxies to people -- are interrelated. With a new-generation of telescopes, accelerators, ground-based telescopes and other instruments -- some of them already planned and funded -- our nation's foremost research organizations will explore our universe, hoping to eventually answer the most profound question of all: are we alone?

Nova Cygni 1992 as seen from the Hubble Space Telescope about six months after the explosion. The ejecta have spectra strongly indicative of the rp-process of explosive nucleosynthesis.

We do know that galaxies are the universe's star nurseries: they recycle chemical elements bred in one generation of stars to form the next. We also know that without galaxies, without this regenerative process, human life could not exist; the very iron in our blood comes from recycled star ashes. But to understand the ORIGINS of galaxies and of the current-day universe, we must look back farther into the past, to objects more distant -- and younger -- than current telescopes can see. We also must gain a better understanding of the physical laws governing the universe.

To answer these fundamental questions, ORIGINS proposes the development of several experiments using telescopes, instruments and spacecraft. They include the ground-based MilliMeter Array, a radio telescope comprised of 40 closely-linked antennae, and the Next Generation Space Telescope, which would orbit high above Earth, perhaps beyond the Moon, to peer back even farther into space and time.

Satellites such as Microwave Anisotropy Probe, now being built, will map variations in the intensity of the cosmic microwave background, as will balloon-borne instruments and detectors at the South Pole. Laboratory experiments will attempt to detect particles that comprise the mysterious dark matter, and accelerators such as the Relativistic Heavy Ion Collider, the Tevatron, and the Large Hadron Collider will seek to recreate the conditions that existed just after the Big Bang. The full range of processes which create the elements in stars will be studied at the national Isotope Separator On Line (ISOL) Facility.

Studies with the Tevatron at DOE's Fermilab focus on the earliest moments after the Big Bang, when nature's basic particles and forces evolved and the large-scale structure of the universe was established. The Relativistic Heavy Ion Collider at Brookhaven will recreate the critical process that occurred about a millionth of a second later, when the expanding universe cooled sufficiently for the condensation of primordial matter into protons and neutrons, the basic constituents of all known matter.

Cauldrons of Nucleosynthesis This initial condensation, however, could not form chemical elements heavier than hydrogen and helium. The synthesis of the heavier elements -- the building blocks of life and of our civilization -- require the formation of stars. Deep inside these "cauldrons of nucleosynthesis," helium is converted to carbon, which is then "burned" to produce oxygen, and so forth. This helium-burning sequence terminates with iron, the most stable of all nuclei. Other elements, including some of the heavier isotopes, are then formed one-by-one by the slow capture of neutrons.

Knowing the rates of stellar nuclear reactions is crucial for understanding the nucleosynthesis chain for the creation of elements. The neutrinos that escape from our sun's core measure the overall rate of nuclear activity, but cannot specify the pathways by which nuclear reactions produce increasingly heavier elements. Scientists have conducted laboratory experiments to measure some of the nuclear reactions that would have produced the light elements during normal stellar burning. However, most heavy elements are produced in nova or supernova explosions, the cataclysmic death of stars that can no longer support their structure by stellar burning. These spectacular stellar events not only create elements from iron to uranium, but also disperse them into space for recycling into the next generation of stars and planets. These stellar explosions occur so fast that their production involves reactions of radioactive nuclear isotopes that no longer occur on Earth.

Isotopes Studies However, these reactions can be studied using new radioactive ion beam facilities. Such experiments require two-stages: first, radioactive isotopes are created in a nuclear reaction and then the pertinent radioactive product is selected, accelerated, and used to induce a reaction of astrophysics interest.

Early prototypes of such facilities are DOE's Radioactive Ion Beam Facility at Oak Ridge National Laboratory and the NSF-supported upgrade of the National Superconducting Cyclotron Laboratory at Michigan State University. A Canadian group at TRIUMF is working on a low-energy radioactive beam facility called ISAC (Isotope Separator/ACcelerator) which is expected to come on-line in 1999. A next-generation facility that would further enhance these investigations is the proposed national Isotope Separator On Line (ISOL) Facility, planned for a DOE national laboratory. Available early in the 21st century, this facility would play a key and vital role under the ORIGINS program.

The NSAC Long Range Plan for Nuclear Science strongly recommends the development of a cost-effective plan for the next generation ISOL-type facility. Pre-conceptual designs for this ISOL facility are being prepared by scientists at Argonne National Laboratory and at Oak Ridge National Laboratory.

The nucleus is a system of protons and neutrons governed by the strong interaction. The study of its properties is an exploration of the symmetries and degrees of freedom that define such a system. The ideas and methods employed have a large overlap with other areas of many-body physics both fundamental and applied. Results from nuclear structure research are important for our understanding of astrophysical processes and the nucleosynthesis that produced the chemical elements of the world in which we live.

Nuclear processes played, and continue to play, a key role in the evolution of the cosmos from the original "big bang" to the complex universe of galaxies, stars and planets we inhabit today. Nuclear reaction rates determined the abundances of the elements that were produced during the "big bang". Nuclear and gravitational forces are the main power sources in our universe, providing the prodigious energies we see from stars, supernovas and the cores of galaxies. The nuclear equation of state determines the ultimate fates of stars after their nuclear fuels are exhausted. And a complex network of nuclear reactions that occur in the central cores of stars is responsible for producing the elements necessary for life to form and prosper. Thus nuclear processes have governed the chemical evolution of the galaxy since its birth. Many of the nuclei involved in these processes are unstable and do not exist on earth.

Until recently, the study of the behavior of nucleons in the quantal nuclear many-body environment has been mostly confined to nuclei near stability and to some proton rich nuclei. The stable nuclei that surround us constitute less than 10% of all the nuclear systems that should exist. They are represented by the red squares in Figure I.2. They are the energetically most favored and tightly bound nuclei, and are located at the bottom of the valley of stability. By adding either protons or neutrons one moves outward towards the ridges of the valley of stability, finally reaching the drip lines where the binding of nucleons ends.

The advent of beams of short-lived, radioactive nuclei is providing new opportunities to create and study unstable nuclei far from stability. This opens an unexplored region of nuclei with the promise of new phenomena and new symmetries, possibly quite different from those in the stable regime. The effects on nuclear properties are expected to be largest near the drip lines, where the binding of nucleons (protons and neutrons) ends. If such modifications are confirmed, we will need to revisit our understanding of the atomic nucleus and of the diversity of quantal phenomena that it exhibits. These unstable nuclei are also critical for an understanding of astrophysical phenomena.

Stars like our sun are essentially very large fusion reactors where hydrogen nuclei are fused into helium. The reactions usually occur at energies much too low, and temperatures much too high, to be studied in the laboratory. Therefore nuclear physicists have developed techniques for measuring reaction rates down to the lowest possible energies, usually at smaller university-based accelerators, and then extrapolating these data down to stellar energies. In our sun, the most uncertain reaction rate is that for fusing hydrogen with Be-7 to form B-8. Several new techniques are being developed that should substantially reduce the uncertainty in this rate. A crucial test of our theories of stellar energy generation is provided by the neutrinosemitted in the fusion process. These neutrinos are the only particles that can pass directly from the center of the sun to the earth where they are detected as described below.

In later stages of stellar evolution, when the hydrogen fuel is exhausted, many stars fuse helium to form carbon, and then helium and carbon to form oxygen. The helium plus carbon fusion rate determines the relative amounts of carbon and oxygen in massive stars, which has a profound effect on the heavier nuclei produced when such stars explode as supernovas. Despite heroic efforts, the helium plus carbon fusion rate is still very poorly known. Improving our understanding of this process is one of the key challenges in nuclear astrophysics.

In binary stars, nova explosions can occur when material from one star falls onto the surface of its companion. When temperatures and densities become high enough, protons react with unstable nuclei before they can decay, in a process known as the hot CNO cycle. The rates of most important reactions in this sequence were recently determined from laboratory measurements (see Figure IV.1).

At still higher temperatures another proton fusion sequence is initiated, producing heavier elements that are detected in the debris of nova explosions (see Fig I.A). Studies of key reactions in this sequence will be one of the important thrusts of laboratory nuclear astrophysics for the next several years. Most of these reaction studies require the use of radioactive beam facilities.

Massive stars end their lives in spectacular explosions known as supernovas. The explosions begin with the collapse of the star's central iron core until enormous densities (about four times normal nuclear matter) are achieved. There follows a trampoline-like rebound that sends a shock wave propagating outward. The core, heated by its gravitational collapse, then cools by emitting neutrinos. Great progress has recently been made in understanding how the shock wave, the interaction of the neutrinos with matter outside the core, and the convection induced by this neutrino heating combine to create the supernova explosion.

Such supernovas are the major factories that enrich our galaxy with new elements, producing and ejecting the ashes of stellar burning (common elements like carbon, oxygen and neon) as well as many less abundant, heavier elements made in the explosion itself. One important goal of nuclear astrophysics is to combine laboratory measurements of nuclear properties and supernova theory to predict the observed abundances of heavy elements. Many of the heavy nuclei, including all the transuranic elements, are made by the rapid capture of neutrons produced in great numbers. Modelling this process requires a detailed knowledge of the properties of nuclei far from the valley of stability, many of which can be studied using radioactive beams.

Access to exotic nuclei provides other outstanding scientific benefits. Since neutrons, unlike the positively charged protons, do not carry an electric charge and consequently do not repel each other, many neutrons can be added to nuclei starting from the valley of stability. The possibilities for new discoveries are especially intriguing near the neutron-drip line. The outer regions of such nuclei become nearly pure low-density neutron matter. Recent mean-field calculations suggest that such a matter distribution would give rise to a shell-model potential different from the usual one, and the pattern of single-particle orbits may change, causing shell structure to alter. A predicted weakening of the spin-orbit potential could cause further alterations in these orbits. Several such effects are illustrated in Figure I.3.

In concert with these changes in the potential, residual interactions will be dramatically altered. Protons and neutrons will occupy very different orbitals. Strong pairing fields are expected due to coupling with the particle continuum, and the valence proton-neutron interaction may change as well. Collective phenomena, and their evolution with neutron number N and proton number Z, will be different and the basic theoretical approach to near-drip line nuclei may well have to be reformulated. Indeed, near the neutron-drip line shell structure may be washed out and magic numbers may essentially disappear. Consequently, the many-body symmetries associated with shell structure near the valley of stability may become qualitatively different.

Of course, if shell structure and residual interactions change, nuclear binding and the limits of nuclear existence will also be altered. Already, existing models do not agree even on which nuclei exist. As shown in Fig. I.4 difference theories vary in their predictions of the neutron drip line by 20-25 neutrons, that is, by more than the span in neutron number of known nuclei of a given element. Thus, the opportunities to study exotic nuclei such as these with radioactive beams has significant impact both in astrophysics and in our understanding of nuclear structure itself.

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