Science and Production of Exotic Nuclei

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1 Science and Production of Exotic Nuclei NAS - Board of Physics and Astronomy R. F. Casten Yale April 22, 2006

2 Themes and challenges of Modern Science Complexity out of simplicity How the world, with all its apparent complexity and diversity, can be constructed out of a few elementary building blocks and their interactions Simplicity out of complexity How the world of complex systems can display such remarkable regularity and simplicity Understanding the nature of the physical universe Manipulating nature for the benefit of mankind Nuclei: Two-fluid, many-body, strongly-interacting, quantal systems provide laboratories for frontier research in all four areas

3 Simplicity out of complexity. Astonishing simplicity in a complex many-body object counts J + 8 J + 6 J + 4 J + 2 J energy (kev)

4 Two views of nuclear structure Single-particle motion Single-particle excitations with residual interactions Bulk collective motion Macroscopic shape of nuclear matter Protons, neutrons fermions j = half-integer (orbital + intrinsic) Phonons bosons Pauli Principle: At most 2j + 1 particles in a given orbit

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6 The New Frontiers of Physics with Exotic Nuclei Terra incognita huge gene pool of nuclei Four Frontiers 1. Proton Rich Nuclei 2. Neutron Rich Nuclei 3. Heaviest Nuclei 4. Evolution of structure within these boundaries We can customize our system fabricate any nucleus (designer nuclei) controlling the number of constituent protons and neutrons to isolate and amplify specific physics or interactions

7 Scope of RIA Science The scientific questions that RIA can address are crucial for our understanding of the universe, and are a link to our ability to explain natural phenomena that range over distance scales spanning 42 orders of magnitude from the proton (10 15 m) to the whole of the universe (10 27 m). Just as nuclei themselves play essential roles in the cosmos, the conceptual techniques of nuclear science have close links with those of quantum many-body physics on the nanoscale and, hence, are important in understanding the quantum world. Moreover, nuclei are the interface between QCD and the fundamental forces and particles in nature on the one hand and the atomic and macroscopic world on the other.

8 Nuclear Structure and Nuclear Astrophysics What binds protons and neutrons into stable nuclei and rare isotopes? How does structure evolve with proton and neutron number What is the origin of simple patterns in complex nuclei? Where and how did the elements from iron to uranium originate? What causes stars to explode?

9 Microscopy, Concept of "mean field" V ij U i r = r i -r j r Ψ = Ψ nl, E = E nl H.O. E = ħω (2n+l) E (n,l) = E (n-1, l+2) E (2s) = E (1d) Clusters of levels shell structure Pauli Principle ( 2j+1 nucleons in orbit with angular momentum j) magic numbers, inert cores Concept of valence nucleons key to structure. Manybody few-body: each body counts. Addition of 2 neutrons in a nucleus with 150 can drastically alter structure

10 Simple Observables R 4/2 B( E 24 ; + 2+ ) B( E22 ; 0 ) E (kev) J π 1 B( E2; J J ) Ψ E2 Ψ 2J + 1 i f i f i 2

11 Classifying Structure -- The Symmetry Triangle Deformed Deformed Sph. Dynamical Symmetries, Phase/shape Transitions Benchmarks

12 Phase Transitions in Atomic Nuclei? order parameter critical point control parameter R 4/ Nd Sm Gd Dy N

13 Critical Point Symmetries First Order Phase Transition Phase Coexistence Energy surface changes with valence nucleon number E E β β β X(5) Bessel equation % 2 ξ % v ξ ξ = 0; 2 % z z % ξ β = ( ) 0. w Iachello v ( + 1) 9 1/2 L L = + 3 4

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15 P Competition between spherical-driving pairing interaction and deformation-driving p-n interaction = Np Nn p n ~ N + N pairing p n Loci of P~5 First X(5) Region P crit ~ 5 Challenge to microscopy: Why these symmetries? In which nuclei? Why in specific nuclei? New symmetries in exotic nuclei?

16 New Features in Weakly Bound Nuclei Spatially extended wave functions Halo Nuclei 11 Li V (r) r Normal nuclear density Density (log) p-n core n-skin Radius (fm) V (r) r Diffuse Normal potential New Magic Numbers, Altered Mean Field, Shell Structure New form of matter low density, diffuse, spatially extended, nearly pure neutron matter

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19 Change in Shell Structure? (Reduction of spin-orbit interaction) QUESTION: Are there major new shell gaps developing in the neutron-rich region, that could have major implications for structure and nucleosynthesis? Extrapolation of observed trend METHOD: Proton-adding reactions on Sn isotopes studied with a new solenoid spectrometer EXAMPLE: 138 Sn(α,t) 139 Sb 4 He target ~ 50μg/cm particles/s 12 MeV/u beam 5 mb/sr over at least 1 sr: ~300 cts/wk for each state

20 Pairing Correlations Questions one hopes to answer: Microscopic origin Dependence of the range of the force on the proton and neutron densities Dependence on the surface

21 Breakdown of BCS pairing? QUESTION: Does BCS pairing, which concentrates the L=0 strength in the ground state, break down in neutron-rich nuclei? METHOD: Neutron-pair transfer on Sn isotopes studied with a new solenoid spectrometer EXAMPLE: 138 Sn(t,p) 140 Sn Tritium target ~ 50μg/cm particles/s 20 MeV/u beam 0.5 mb/sr over at least 1 sr: ~30 cts/wk for each state In 138 Sn(t,p) will it be like this with continued BCS pair correlations as in other Sn isotopes? - or like this with disappearing of BCS correlations?

22 Transfer reactions with post-accelerated radioactive ion beams Two proton transfer reaction on Hg isotopes to probe the π(2p-2h) component N = 104 (midshell) N = Po 189Po 191Po Z = Pb 184 Hg 3 He( 184 Hg(T 1/2 =31 s),n) 186 Pb at 10 MeV/u σ 50 μbarn (cfr. 204 Hg( 3 He,n) 206 Pb: R.E. Anderson et al., PRC19 (1979) particles per second gas cell at 5 bar/ 1 cm: at/cm reactions/hour One neutron transfer reaction on e.g. Pb isotopes 190 Pb(d,p) 191 Pb 191 Po(d,p) 192 Po 185 Hg(d,p) 186 Hg prolate oblate odd Hg oblate Spectroscopic factors 0 + even Hg prolate

23 Classic Near-Barrier Coulomb Excitation with reaccelerated beams NEW PHYSICS Modification of nuclear structure due to neutron excess Impact of single particle states and gaps New collective modes, Influence of weak pairing Advantages Very sensitive to shapes and transitions (diagonal and off diagonal matrix elements) Flexible: Multi-step to study structural evolution Single-Step to study strength functions Clean...at sub-barrier energy it is the only mechanism Precise...an exact theory Efficient

24 Coulomb Excitation (with low intensity beams) 10 3 Take existing data set from beam Coulex of 138 Ce on 700 μg/cm 212 C with Gammasphere. Rescale 1pna for 14hrs to various scenarios: p.p.s for 5 days 10 4 p.ps for 5 days 10 3 p.p.s for 5 days 10 5 Even at 100 particles per second spectroscopy is possible at least for first excited state. 1pna

25 Z > 114, n-rich: Where does the periodic table end? Super-Heavy Elements Studies What is the shell structure at the highest Z s? (what is the right theoretical description) Predictions What are the properties of the heaviest elements? (stability, mass, decay modes,..) Rare isotopes may well be the only way to reach the island of super-heavy elements σ(fusion) ~ 1nb 1pb (who knows for n-rich beams?) σ(fusion) sharply peaked reaccelerated beams at precise energy Possible with intense n-rich beams 90,92 Kr, 90,92 Sr,.. (>10 11 /s) 1 atom/week

26 How does the physics of nuclei impact the physical universe? Frequency (Hz) protons What is the origin of elements heavier than iron? How do stars burn and explode? What is the nucleonic structure of neutron stars? X-ray burst Time (s) Nova T Pyxidis 4U stellar stellar burning burning neutrons rp rp process process Nuclear Input (experiment and theory) Masses and drip lines Nuclear reaction rates Weak decay rates Electron capture rates Neutrino interactions Equation of State Fission processes Crust Crust processes processes p process process process n-star KS RIA intensities (nuc/s) > s-process s-processprocess r process Mass 10known 2 Half-life 10-2 known nothing known Supernova E

27 Applications United States leadership in nuclear science is vital to the nation's well-being as well. RIA will have profound benefits to society; it will play an important role in the 21st Century's advances in modern technology, medicine, the environment, and national security. The pursuit of the scientific opportunities that drive RIA will enhance the training of the next generation of nuclear scientists. This field provides a superb venue to educate those who will seek to exploit nuclei for the benefit of humankind and the security of our nation.

28 RIA Discovery Potential, Spin-offs Comprehensive nuclear theory Reaching the limits of nuclear binding Discovery/study of exotic nuclear topologies Discovery of new structural symmetries Study of phases of nuclei and nuclear matter Crucial ingredients for astrophysics Tests of fundamental symmetries Unforeseen Discoveries Applications to medicine, national security, Training the next generation of scientists who know and can exploit the atomic nucleus

29 Exotic Nuclei and RIA It is the overall consensus of the international nuclear structure and nuclear astrophysics communities that the future of the study of atomic nuclei requires advanced facilities for access to nuclei far from the valley of stability. Discovery potential to produce a paradigm change that will transform nuclear structure and astrophysics like atomic physics was changed by the laser or condensed matter physics by the transistor. The aim is not to study all newly available species, but to use this expanded gene pool of exotic nuclei to select those that isolate or amplify specific physics.

30 Rare Isotope Science History of the concept: : A selection of highlights 1980 s Early experiments with exotic nuclei 1991 LRP Advanced radioactive beam facility cited as a possible future initiative 1996 LRP - ISL top priority for new construction upon completion of RHIC 2002 LRP - RIA as top priority for major new construction Seven NSAC reports reaffirm support for RIA 2003 DOE 20 Year Facilities Plan -- Strategic Plan Tied for Third

31 LRP 2002: RECOMMENDATION 2 The Rare Isotope Accelerator (RIA) is our highest priority for major new construction. RIA will be the world-leading facility for research in nuclear structure and nuclear astrophysics. The exciting new scientific opportunities offered by research with rare isotopes are compelling. RIA is required to exploit these opportunities and to ensure world leadership in these areas of nuclear science.

32 Facilities for the Future of Science A Twenty Year Outlook (2003)

33 RARE ISOTOPE SCIENCE ASSESSMENT Statement of Task The committee will define a scientific agenda for a U.S. domestic rare-isotope facility, taking into account current government plans. ***** The committee will carry out a thorough independent assessment of the importance to the nation of the science agenda for the Rare Isotope Accelerator. ****** In preparing its report, the committee will address the role that such a facility could play in the future of nuclear physics, considering the field broadly, but placing emphasis on its potential scientific impact on nuclear structure, nuclear astrophysics, fundamental symmetries, stockpile stewardship and other national security areas, and future availability of scientific and technical personnel. The need for such a facility will be addressed in the context of international efforts in this area. In particular, the committee will address the following questions: What science should be addressed by a rare isotope facility and what is its importance in the overall context of research in nuclear physics and physics in general? What are the capabilities of other facilities, existing and planned, domestic and abroad, to address the science agenda? What scientific role could be played by a domestic rare-isotope facility that is complementary to existing and planned facilities at home and elsewhere? What are the benefits to other fields of science and to society of establishing such a facility in the United States?

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40 Summary Exotic Nuclei Complexity Simplicity Links to nano-science, high energy physics, and the cosmos Paradigm-Changing Discovery Potential Comprehensive Understanding of Atomic Nuclei Applications

41 Backups

42 Approaches to Nuclear Structure Microscopic Approximate solutions to real nuclei Ab initio, No core, Monte Carlo Effective Interactions, Many degrees of freedom Density Functional Theory Numerically intensive. Revolutionary advances enhanced ability to predict wide variety of nuclei promise of a comprehensive theory. Macroscopic Exact solutions to ideal nuclei Many-body symmetries. Few degrees of freedom. Simple patterns, quantum numbers, selection rules, phases. Analytic, intuitive understanding -- WHAT symmetries WHERE? Challenge to microscopy Why THESE symmetries: In which nuclei: Why in THESE nuclei?

43 Z 82, N < π Z > 82, N > 126 ν Z > 82, N < 126