Astrophysics, Symmetries, and Applied Physics at Spallation Neutron Sources 9812382496, 9789812382498, 9789812776242

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ASAP 2002

Astrophysics, Symmetries, and Applied Physics at Spallation Neutron Sources Editors

Paul E. Koehler

Christopher R. Gould

Robert C. Haight

Timothy E. Valentine

Symmetry Experiments

Proposed ASAP Beam Line at SNS

Astrophysics, Symmetries, and Applied Physics at Spallation Neutron Sources

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ASAP 2002 Astrophysics, Symmetries, and Applied Physics at Spallation Neutron Sources Editors

Paul E. Koehler Oak Ridge National Laboratory, USA

Christopher R. Gould North Carolina State University, USA

Robert Haight Los Alamos National Laboratory, USA

Timothy E. Valentine Oak Ridge National Laboratory, USA

V f e World Scientific wll

New Jersey • London Singapore • Si • Hong Kong

Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: Suite 202, 1060 Main Street, River Edge, NJ 07661 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

ASTROPHYSICS, SYMMETRIES, AND APPLIED PHYSICS AT SPALLATION NEUTRON SOURCES Copyright © 2002 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

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ISBN 981-238-249-6

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FOREWORD AND ACKNOWLEDGMENTS Intense beams of epithermal neutrons from spallation sources are enabling the exploration of new vistas of research in nuclear astrophysics, the study of violation of fundamental symmetries, and applied nuclear physics. There has been, and continues to be an active and productive world-wide community engaged in these areas of research at existing facilities such as electron linacs (e.g. at the Oak Ridge Electron Linear Accelerator, ORELA) and van de Graaff laboratories (e.g. in Karlsruhe, Germany and at the Tokyo Institute of Technology in Japan) as well as new efforts at spallation sources (e.g. at the Los Alamos Neutron Science Center, LANSCE and the CERN n_TOF facility). Building on, the long and distinguished history of research at these facilities, the much higher flux available at spallation sources makes it possible to work with much smaller samples as well as to make measurements with much higher precision. These new capabilities of spallation sources are enabling the exploration of new, exciting areas of research such as: • The nucleosynthesis of the elements in dynamic stellar environments such as pulsing red giant stars and supernovae. • The study of fundamental symmetries such as the nature of parity violation in complex nuclei and the search for violations of time-reversal invariance. • The feasibility and efficiency of using accelerator-driven sub-critical assemblies to transmute dangerous, long-lived radioactive waste to more benign materials as well as several other topics in applied nuclear physics such as criticality safety and nuclear medicine. A unifying feature of all of these fields is the need for the highest intensity source of pulsed epithermal neutrons. The new Spallation Neutron Source (SNS) being built at Oak Ridge National Laboratory (ORNL) will be by far the highest flux pulsed source of epithermal neutrons in the world when it comes on line in 2006. Although the main thrust of the science program at the SNS will be materials science, the facility could provide outstanding opportunities for research in nuclear astrophysics, fundamental symmetries, and applied nuclear physics. To review the current status of these fields and to begin to assemble the scientific case and the community of researchers for future experiments at the SNS, a workshop on "Astrophysics, Symmetries, and Applied Physics" was held during March 11-13 at ORNL. Over 60 scientists, representing 11 different US and 4 different foreign universities as well as many national laboratories around the world participated in the workshop. We thank the speakers for their excellent presentations and everyone for participating in the discussions and making the workshop a success. The scientific organizing committee would like to thank Fred E. Bertrand, Jr. (ORNL Physics Division), John C. Nemeth (Oak Ridge Associated Universities, ORAU), and R. Gil Gilliland (ORNL) for financial support to make the workshop

v

vi possible. We would also like to thank Doro Wiarda (ORNL, Physics Division) for setting up the web site for the workshop. Finally, we would like to thank Ken Carter and staff of ORAU for providing technical and administrative support to the collaboration and to Carlene Stewart (ORAU) for her excellent help in organizing and running the workshop. Paul Koehler Chris Gould Bob Haight Tim Valentine

Schedule for the Workshop on Astrophysics, Symmetries, and Applied Physics at Spallation Neutron Sources

Sunday March 10

19:00-21:00

Reception at Pollard Auditorium in Oak Ridge

Monday March 11

Welcome (Chair: G. Young) 9:00-9:10 9:10-9:15 9:15-9:30

Welcome/G. Young/ORNL Welcome/R. Townsend/ORAU Welcome/J. Roberto/ORNL

Nuclear Astrophysics (Chair: G. Young) 9:30-10:15 10:15-10:45

Laboratory Experiments for Neutron Capture Nucleosynthesis! F. Kappeler, FZK Karlsruhe, Germany DANCE at LANSCE J. Ullmann, LANL

10:45-11:00

Break

11:00-11:30 11:30-12:00

The Astrophysics Program at the CERN n_TOF Facility A. Mengoni, ENEA Bologna, Italy Recent Astrophysics Results from ORELA and Possible Future Experiments at ORELA and SNS P. Koehler, ORNL

12:00-13:30

Lunch

Nuclear Astrophysics (continued) (Chair: Art Champagne) 13:30-13:50

13:50-14:10

Sensitivity of Isotope Yields to Reaction Rates in the Alpha-Rich Freezeout C. Jordan, Clemson University Neutron Reactions of Light Nuclei from Astrophysics and Nuclear Physics Interest

VIII

14:10-14:30 14:30-14:50

14:50-15:10

15:10-15:30

Y. Nagai, Osaka University, Japan "Offline" Radioactive Targets R. Rundberg, LANL Measurement ofthep(n, y)d Cross Section for Big Bang Nucleosynthesis at the Spallation Source LANSCE E. Esch, LANL Phonon Properties of Materials from Resonance Doppler Broadening J. E. Lynn, LANL Break

Applied Nuclear Physics (Chair: R. Haight) 15:30-16:15 16:15-16:45

Applied Physics at Spallation Neutron Sources P. Oblozinsky, BNL Applied Physics Measurements at the CERN n_TOF Facility E. Gonzalez-Romero, CIEMAT Madrid, Spain

Tuesday March 12

Applied Nuclear Physics (continued) (Chair: P. Oblozinsky) 9:00-9:30

Activities of the DOE Nuclear Criticality Safety Program (NCSP) at the Oak Ridge Electron Linear Accelerator (ORELA) T. Valentine, ORNL

9:30-9:50

Parameters for Nuclear Reaction Calculations -Needs for Improvement M. Herman, IAEA Vienna, Austria New Al(n, tf Measurements and Criticality Safety L. Leal, ORNL

9:50-10:10

10:10-10:30

Break

10:30-10:50

Time-of Flight Spectrometer GNEIS with Spallation Neutron Source A. Laptev, PNPI, Gatchina, Russia Measurements of Neutron Capture Cross Sections of Long-Lived Fission Products H. Harada, JNC, Japan

10:50-11:10

IX

11:10-11:30

11:30-11:50

11:50-13:30

Temperature Measurements in Dynamically-Loaded Systems Using Neutron Resonance Spectroscopy at LANSCE V. Yuan, LANL Radioactive Targets from RIA J. Blackmon, ORNL Lunch

Symmetries (Chair: C. Gould) 13:30-14:00 14:00 -14:30

14:30-14:50 14:50-15:10

15:10-15:30

Parity Violation in Neutron Resonances G. Mitchell, North Carolina State University New Experimental Capabilities for Parity Non-Conservation and Time Reversal InvarianceViolation S. Penttila, LANL Symmetry Tests at the Japanese Spallation Neutron Source Y. Masuda, KEK, Japan Violation of Fundamental Symmetries in Resonance Neutron Induced Fission W. Furman, JINR Dubna, Russia Physics of the Fission Process and Parity Violation in Neutron Induced Reactions V. Gudkov, University of South Carolina

15:30-15:50

Break

15:50-16:20

New Possibilities for Parity Violation Studies A. Hayes, LANL Time Reversal Tests and Sign Correlations in Heavy Nuclei C. Gould, North Carolina State University A Five-Fold Correlation Experiment to Measure Time Reversal Invariance Violation Using Neutron Resonances in Holmium P. Huffman, NIST

16:20-16:40 16:40-17:00

X

Wednesday March 13

SNS and Discussions (Chair: Paul Koehler) 9:00-9:30 9:30-10:15

SNS Letter of Intent and Instrument Development Team Processes T. Mason, ORNL SNS Technical Overview P. Ferguson, ORNL

10:15-10:30

Break

10:30-12:00

Discussion of SNS Beam Line Requirements, Possible Experimental Program, and LoI/IDT Issues Discussion leader: P. Koehler, ORNL

ASAP 2002 PARTICIPANT LIST Name

Affiliation

Email

Chuck Alexander

Oak Ridge National Laboratory

[email protected]

Dan Bardayan

Oak Ridge National Laboratory

[email protected]

Jon Batchelder

Oak Ridge Associated Universities

[email protected]

Jeff Blackmon

Oak Ridge National Laboratory

[email protected]

Carl Brune

Ohio University

[email protected]

Ken Carter

Oak Ridge Associated Universities

[email protected]

Art Champagne

University of North Carolina

[email protected]

Walter Furman

JINR, Dubna, Russia

[email protected] inr.ru

Kazuyoshi Furutaka

Japan Nuclear Cycle Development Inst.

[email protected]

T. Vincent Cianciolo

Oak Ridge National Laboratory

[email protected]

Aaron Couture

University of Notre Dame

[email protected]

Yaron Danon

Rensselaer Polytechnic Institute

[email protected]

Felix Difilippo

Oak Ridge National Laboratory

[email protected]

Ernst Esch

Los Alamos National Laboratory

[email protected]

Phillip D. Ferguson

Oak Ridge National Laboratory

[email protected]

Enrique Gonzalez

CIEMAT

[email protected]

Christopher Gould

North Carolina State University

[email protected]

Geoffrey Greene

Los Alamos National Laboratory

[email protected]

Colorado School of Mines

[email protected]

Uwe Greife Vladimir Gudkov Gueorgui Gueorguiev Robert Haight Hideo Harada Jack Harvey Anna Hayes Michal Herman Paul R. Huffman Masayuki Igashira Cal Jordan Franz Kaeppeler Guinyun Kim Paul Koehler

University of South Carolina

[email protected]

University of Florida

george@serverl .nuceng.ufi.edu

Los Alamos National Laboratory

[email protected]

Japan Nuclear Cycle Development Inst.

[email protected]

Oak Ridge National Laboratory

[email protected]

Los Alamos National Laboratory

[email protected]

International Atomic Energy Agency

herman@ndsalpha. iaea.org

Natl. Institute of Standards and Tech.

[email protected]

Tokyo Institute of Technology

[email protected]

Clemson University

[email protected]

FZK, Karlsruhe

[email protected]

Kyungpook National University

[email protected]

Oak Ridge National Laboratory

[email protected]

XII

Raymond Kozub

Tennessee Technological University

[email protected]

Alexandre Laptev

Petersburg Nuclear Physics Institute

[email protected]

Luiz Leal

Oak Ridge National Laboratory

[email protected]

Eric Lynn

Los Alamos National Laboratory

eric, [email protected]

Thorn Mason

Oak Ridge National Laboratory

[email protected]

Yasuhiro Masuda

High Energy Accelerator Research Org.

[email protected]

Alberto Mengoni

ENEA-Applied Physics Division

[email protected]

Gary Mitchell

North Carolina State University

[email protected]

Paul Mueller

Oak Ridge National Laboratory

[email protected]

Yasuki Nagai

Osaka University

[email protected]

John Neal

Oak Ridge National Laboratory

[email protected]

Pavel Oblozinsky

Brookhaven National Laboratory

[email protected]

Toshiro Osaki

Tokyo Institute of Technology

[email protected]

Peter Parker

Yale University

[email protected]

Seppo Penttila

Los Alamos National Laboratory

[email protected]

S. Raman

Oak Ridge National Laboratory

[email protected]

Wolfgang Rapp

University of Karlsruhe

[email protected]

John-Paul Renier

Oak Ridge National Laboratory

[email protected]

James B. Roberto

Oak Ridge National Laboratory

[email protected]

Bob Rundberg

Los Alamos National Laboratory

[email protected]

Royce Sayer

Oak Ridge National Laboratory

[email protected]

Kenneth Toth

Oak Ridge National Laboratory

[email protected]\

Ronald Townsend

Oak Ridge Associated Universities

to wnsenr@orau. gov

John Ullmann

Los Alamos National Laboratory

[email protected]

Timothy Valentine

Oak Ridge National Laboratory

[email protected]

Steve Wender

Los Alamos National Laboratory

[email protected]

Jerry Wilhelmy

Los Alamos National Laboratory

[email protected]

Glenn Young

Oak Ridge National Laboratory

[email protected]

Vincent Yuan

Los Alamos National Laboratory

[email protected]

xiii

CONTENTS Preface Schedule for the Workshop on ASAP Participant List Neutron Capture Nucleosynthesis: Astrophysical Processes and Laboratory Approaches F. Kappeler The Detector for Advanced Neutron Capture Experiments at LANSCE J.L. Ullmann, R.C. Haight, L. Hunt, E. Seabury, R.S. Rundberg, J.B. Wilhelmy, MM. Fowler, D.D. Strottman, F. Kaeppeler, R. Reifarth, M. Heil and E.P. Chanberlin Astrophysics Program at the CERN n_TOF Facility A. Mengoni

v vii xi

1

16

25

Recent Astrophysics Results from ORELA and Possible Future Experiments at ORELA and SNS P.E. Koehler

32

Sensitivity of Isotope Yields to Reaction Rates in the Alpha Rich Freezeout G.C. Jordan IV and B.S. Meyer

42

Neutron Reactions of Light Nuclei from Astrophysics & Nuclear Physics Interest Y. Nagai, T. Shima, A. Tomyo, H. Makii, K. Mishima, M. Segawa, M. Igashira and T. Ohsaki

52

XIV

Measurement of the n+p—>d+y Cross Section for Big Bang Nucleosynthesis with the Spallation Neutron Source at the Los Alamos Neutron Science Center E.-I. Eschm, J.M. O'Donnell, S.A. Wender, D. Bowman, G. Morgan and J. Matthews

58

Phonon Properties of Materials from Neutron Resonance Doppler Broadening Measurements J. Eric Lynn

65

Applied Nuclear Physics at Spallation Neutron Sources Pavel Oblozinsky

73

Applied Physics Measurements at the CERN n_TOF(1) Facility E. Gonzalez

83

Activities of the DOE Nuclear Criticality Safety Program (NCSP) at the Oak Ridge Electron Linear Accelerator (ORELA) Timothy E. Valentine, Luiz C. Leal and Klaus H. Guber Parameters for Nuclear Reaction Calculations - Needs for Improvements M. Herman Aluminum Data Measurements and Evaluation for Criticality Safety Applications L.C. Leal, K.H. Guber, R.R. Spencer, H. Derrien and R.Q. Wright Nuclear Physics Investigations at the Time-of-Flight Spectrometer GNEIS with Spallation Neutron Source O.A. Shcherbakov, A.B. Laptev andA.S. Vorobyev

97

107

115

123

XV

Measurement of Neutron Capture Cross Sections of Long-lived Fission Products H. Harada, S. Nakamura, K. Furutaka, T. Katoh, M.M.H. Miah, O. Shcherbakov, H. Yamana, T. Fujii and K. Kobayashi Temperature Measurements in Dynamically-loaded Systems Using Neutron Resonance Spectroscopy (NRS) atLANSCE V.W. Yuan

131

138

Radioactive Target Production at RIA J.C. Blackmon

146

Parity Violation inEpithermal Neutron Resonances G.E. Mitchell, J.D. Bowman, S.I. Penttila and E.I. Sharapov

155

New Experimental Capabilities for Parity Non-conservation and the Time Reversal Invariance Violation in Neutron Transmission S.I. Penttila T-Violating Three-fold Correlation in Neutron Transmission Y. Masuda

164

175

Violation of Fundamental Symmetries in Resonance Neutron Induced Fission A. Barabanov, W. Furman and A. Popov

184

Physics of the Fission Process and Parity Violation in Neutron Induced Reactions Vladimir Gudkov

194

XVI

Possibilities for Studies of Parity Violation at the SNS Using the Capture Gamma Reaction A. C. Hayes and Luca Zanini Time Reversal Tests with Epithermal Neutrons C.R. Gould An Experiment to Search for Parity-conserving Time Reversal Invariance Using Epithermal Neutrons from the Spallation Neutron Source P.R. Huffman Neutronic Characteristics of the Spallation Neutron Source P.D. Ferguson, E.B. lverson and F.X. Gallmeier Workshop Summary: Opportunities in Astrophysics, Symmetries, and Applied Physics at Spallation Neutron Sources Paul Koehler, Christoper Gould, Robert Haight and Timothy Valentine

202

209

217

225

233

Letter of Intent to the Spallation Neutron Source

234

Author Index

245

N E U T R O N C A P T U R E NUCLEOSYNTHESIS: ASTROPHYSICAL PROCESSES A N D LABORATORY APPROACHES F. K A P P E L E R Forschungszentrum

Karlsruhe,

Institut fur Kernphysik, Postfach Karlsruhe, Germany E-mail: [email protected]

3640,

D-76021

Neutron reactions are responsible for the formation of the elements heavier than iron. The corresponding scenarios relate to helium burning in Red Giant stars (s process) and to supernova explosions (r and p process). The status of the relevant neutron data for the various scenarios are briefly summarized, followed by an outline of the essential experimental techniques. The direct impact of laboratory results on the interpretation of the observed abundance patterns and their role as crucial tests for astrophysical models are illustrated by representative examples. The very high flux at spallation neutron sources provide a unique possibility for investigating numerous difficult and hitherto inaccessible cases, in particular cross sections of the important radioactive nuclei. In combination with advanced detector concepts these facilities provide a promising step towards a quantitative picture of galactic chemical evolution.

1

Introduction

A first clue for the origin of the chemical elements was obtained in the 1930ies by the analysis of carbonaceous chondrites, a class of primitive meteorites, which preserved the original composition of the protosolar nebula *. At about the same time, nuclear burning was identified as the stellar energy source 2 3 4 ' ' . However, it was not before 1952, when Merrill 5 discovered Tc lines in the spectra of red giant stars - an unstable element with isotopic half-lives much shorter than the stellar evolution time - that stellar nucleosynthesis was accepted as the origin of the chemical elements. The various aspects of this new field of Nuclear Astrophysics, i.e. the elemental composition of astronomical objects, the standard abundance distribution, the nucleosynthesis mechanisms, and the related nuclear physics, were eventually combined in the fundamental and seminal paper by Burbidge, Burbidge, Fowler, and Hoyle 6 . A comprehensive summary of the 40 years of progress in nucleosynthesis since B 2 FH was published recently by Wallerstein et al. 7 . Reviews on more specific topics can be found elsewhere 8>9,10,11,12,13,14,15

1

2

100 MASS NUMBER

150

Figure 1. The isotopic abundance distribution in the solar system (from Ref. 1 7 ).

2 2.1

The Observed Abundances The Solar System

Any nucleosynthesis model must be checked against observations. Originally, the composition of the solar system was considered a standard which can be reliably derived by spectroscopy of the photosphere and by meteorite analyses 16,17 p r o m this distribution (Fig. 1) the signatures of the dominant scenarios can be inferred, starting with the very large primordial H and He abundances from the Big Bang. The abundances of the rare elements Li, Be, and B, which are difficult to produce because of the stability gaps at A=5 and 8, but are easily burnt in stars, were mostly formed by spallation reactions induced by galactic cosmic rays. Stellar nucleosynthesis starts with the ashes of He burning, 12 C and 1 6 0 , which are partly converted to 14 N by the CNO cycle in later stellar generations. In subsequent stages of stellar evolution, the light elements up to the mass 40 to 50 region are produced by charged particle reactions during C, Ne, and O burning 7 . The corresponding yields show a strong preference for the most stable nuclei built from a-particles. This part of the distribution is strongly influenced by the Coulomb barrier, resulting in an exponential decrease with increasing atomic number Z. Ultimately, Si burning leads to such high temperatures and densities that nuclear statistical equilibrium is reached. Under these conditions matter is transformed into the most stable

3 nuclei around Fe, giving rise to the dominant maximum at A = 5 6 . Due to the increasing Coulomb barriers the abundances of all heavier nuclei up to the actinides are essentially shaped by neutron capture nucleosynthesis, leading to a fairly flat distribution characterized by the pronounced r and s maxima. These twin peaks are the signatures of the slow (s) and rapid (r) neutron capture processes discussed below. 2.2

Galactic

Evolution

While the solar abundance distribution is characteristic for most stars it represents just the average enrichment of the Galaxy 4.55 Gyr ago. The chemical evolution prior to this point has become an intense field of investigation. Spectral analysis of stellar atmospheres has become an ever refined source of information. W i t h the astonshing sensitivity of ground and satellite based telescopes extremely faint a n d / o r metal poor stars can be observed from the UV to the far IR, providing an almost complete element p a t t e r n of these objects 1 8 ' 1 9 . Likewise, chemically peculiar stars, which witness ongoing sprocess nucleosynthesis in their deep interiors, or the expanding supernova ejecta can be accessed in great detail as well. For more t h a n three decades, direct spectroscopic observations have been complemented by analyses of circumstellar dust grains from AGB stars or supernovae, which survived the homogenization in the protosolar cloud and are preserved as minute inclusions in meteorites n > 1 4 . T h e isotopic composition of these presolar grains clearly exhibit enrichments, which can be a t t r i b u t e d t o particular nucleosynthetic scenarios such as the s or r process. T h e wealth of new and exciting information on the chemical evolution of the Galaxy calls for an expanded and improved nuclear physics d a t a base, which is indispensable for the quantitative interpretation of these observations, and hence for understanding the history of the universe. 3

N e u t r o n Capture Scenarios

W h e n the concept of neutron capture nucleosynthesis was first formulated 6 the s and r processes were already identified as the mechanisms responsible for the sharp maxima in the abundance distribution. These mechanisms are illustrated in Fig. 2, which shows the respective reaction p a t h s in t h e chart of nuclides. T h e s process being characterized by relatively low neutron densities implies neutron capture times much longer t h a n typical /?-decay half-lives. Therefore, the s-process reaction p a t h follows t h e stability valley as indicated

Seed for s-Process

s-Process Reaction Path

s-Branchings ( M N i 79Se 8 5 Kr,...)

Figure 2. An illustration of the neutron capture processes responsible for the formation of the nuclei between iron and the actinides. The observed abundance distribution in the inset shows characteristic twin peaks, which refer to the points where the s- and r-reaction paths encounter magic neutron numbers. Note that a p process has to be invoked for producing the proton rich nuclei that are not reached by neutron capture reactions. (For details see discussion in text.)

by the solid line in Fig. 2. The s abundances are determined by the respective (n,7) cross sections averaged over the stellar neutron spectrum, such that isotopes with small cross sections are building up large abundances. This holds for nuclei with closed neutron shells giving rise to the sharp s-process maxima in the abundance distribution at A=88, 140, and 208. This represents an illustrative example for the intimate correlation between the relevant nuclear properties and the resulting abundances, a phenomenon that can be used for probing the physical conditions during nucleosynthesis. The r-process counterparts of these maxima are caused by the effect of neutron shell closure on the /3-decay half-lives. Since the r process occurs in regions of extremely high neutron density (presumably during stellar explosions in supernovae) neutron captures are much faster than /3-decays. Therefore, the r-process path is driven off the stability valley until nuclei with neutron separation energies of « 2 MeV are reached. At these points, (n,7) and (7,n) reactions are in equilibrium, and the reaction flow has to wait for /3-decay

5

to the next higher element. Accordingly, the r abundances are proportional to the half-lives of these waiting point nuclei. This means that r-abundance peaks accumulate also at magic neutron numbers, but at significantly lower A compared to the related s-process maxima, resulting in the typical twin peaks of the abundance distribution. While the observed abundances are dominated by the s and r components, which both account for approximately 50% of the abundances in the mass region A>60, the rare proton-rich nuclei can not be produced by neutron capture reactions. This minor part of the abundance distribution had to be ascribed to the p process that is assumed to occur in explosively burning outer shells of supernovae 20>12. Among these processes, the s process is best accessible to laboratory experiments as well as to stellar models and astronomical observations 9 . Attempts to describe the r and p processes are hampered by the large uncertainties in the nuclear physics data far from stability 8 ' 2 1 , but also - and perhaps more severely - by the problems related to a detailed modelling of the stellar explosion 20>22>23. Obviously most isotopes received abundance contributions from the s and r processes. But as indicated in Fig. 2 there are neutron-rich stable isotopes (marked r) that are not reached by the s process because of their short-lived neighbors. Consequently, this species is of pure r process origin. In turn, these r-only nuclei terminate the /?-decay chains from the r-process region, making their stable isobars an ensemble of s-only isotopes. The existence of these two subgroups is of vital importance for nucleosynthesis, since their abundances represent important tests for stellar models.

4

The Case of the s Process

The diret impact of neutron reactions for the processes sketched before is illustrated at the example of the s process. The main nuclear physics input for s-process studies are the (n,7) cross sections averaged over the thermal neutron spectra characteristic for the stellar sites of the s process, typically between T 8 ~ 1 and 3 (in units of 108 K). This information is required for all nuclei along the reaction path from Fe to Bi. In addition, the /?-decay rates for unstable isotopes, which act as branching points in the reaction chain, have to be evaluated 2 4 .

6

4-1

Laboratory Neutron Sources

Neutrons in the energy range between 0.3 and 300 keV required for such measurements are produced in several ways: (i) At low-energy particle accelerators, nuclear reactions, such as 7 Li(p,n) 7 Be offer the possibility of tailoring the neutron spectrum exactly to the energy range of interest. This has the advantage of low backgrounds, allowing for comparably short neutron flight paths to compensate limitations in the neutron source strength 9 ' 25 . (ii) Much higher intensities can be achieved at linear accelerators via (7,n) reactions by bombarding heavy metal targets with electron beams of typically 50 MeV. The resulting spectrum contains all energies from thermal to near the initial electron energy. Since the astrophysically relevant energy range corresponds only to a small window in the entire spectrum, background conditions are more complicated and measurements need to be carried out at larger neutron flight paths. In turn, the longer flight paths are advantageous for high resolution measurements which are important in the resonance region. Refs. 26 27 ' are recent examples of astrophysical measurements at such facilities. (iii) Spallation reactions induced by energetic particle beams provide the most prolific sources of fast neutrons. An advanced spallation source suited for neutron time-of-flight (TOF) work is the LANSCE facility at Los Alamos, allowing for measurements on very small samples as well as on radioactive targets 28 - 29 . While the situation at LANSCE is characterized by a comparably short flight of 20 m and a time resolution of 250 ns (similar to what is planned at the SNS in Oak Ridge) the new n_TOF facility at CERN represents a complementary approach aiming at higher resolution (185 m flight path, 7 ns pulse width) 30 ' 31 - 32 . 4-2

Measurement of Neutron Capture Rates

The experimental methods for measuring (11,7) cross sections fall into two groups, TOF techniques and activations. In principle TOF techniques can be applied to all stable nuclei and require a pulsed neutron source for determining the neutron energy via the flight time between neutron production target and capture sample. Capture events are identified by the prompt 7-ray cascade in the product nucleus. The best signature for the identification of neutron capture events is the total energy of the emitted 7-cascade. To use this feature for accurate (11,7) cross section measurements requires a detector that operates as a calorimeter with good energy resolution such as the Karlsruhe in BaF 2 detector 33 . In the 7-spectrum of a perfect calorimeter, all capture events would fall in a line at the neutron binding energy (typically between 5 and 10 MeV), well separated

7

from backgrounds, which are inevitable in neutron experiments. In this way, an efficiency for capture events of 96 to 98% can be obtained, allowing for cross section uncertainties of ±1 %. Similar calorimeters are presently under construction at Los Alamos and at CERN. Activation in a quasi-stellar neutron spectrum provides a completely different approach for the determination of stellar (n,7) rates, but is restricted to those cases, where neutron capture produces an unstable nucleus. This method has superior sensitivity, allowing to use sub-/ig samples, and is highly selective, which means that isotopically enriched samples are not required. Quasi-stellar neutron spectra can be produced via the 7 Li(p,n) 7 Be 34 ' 35 by bombarding thick metallic lithium targets with protons of 1912 keV, only 31 keV above the reaction threshold. The resulting neutrons exhibit a continuous energy distribution very similar to a Maxwell-Boltzmann distribution for kT = 25 keV. The possibility to use minute samples makes the activation technique an attractive tool for investigating unstable nuclei of relevance for s-process branchings 36 . For example, a measurement of the 155 Eu cross section (ti/2=4-96 yr) could be performed with a sample of only 88 ng corresponding to 3.4xl0 1 4 atoms. This aspect is essential for minimizing the sample activity and, hence the radiation hazard, to a manageable level 3T . 4-3

Theoretical Calculations

In spite of the experimental progress, cross section calculations remain indispensable for determining the (n,7) rates of unstable nuclei with high specific 7-activity as well as the (possible) differences between the laboratory values and the actual stellar cross sections, which can be affected by thermally populated nuclear states with low excitation energies. Theoretical reaction rates are particularly important for explosive scenarios, where nuclei far from stability are involved and where experimental data are completely missing 38,39 Another essential issue are weak interaction rates under astrophysical conditions, both for He burning 24 and explosive scenarios 8 . 5 5.1

s Process Models The Canonical s Process

This phenomenological model 9 ' 40 was suggested by the empirical assumptions that temperature and neutron density are constant during the s-process and that a certain fraction G of the observed 56 Fe abundance was irradiated by an exponential distribution of neutron exposures. Then, an analytical expression

8

can be derived to calculate for all involved isotopes the characteristic s-process quantity, i.e. the product of the stellar cross section and the respective s abundance. Apart from the two parameters G and TQ, which are adjusted to fit the abundances of the s-only nuclei, the stellar (n,7) cross sections (a) are the only input data required for determining the overall abundance distribution. This approach includes also the treatment of the particular sprocess branchings. Given the very schematic nature of the classical approach, it was surprising to see that it provides an excellent description of the s-process abundances. Fig. 3 shows the calculated (cr)Ns values compared to the corresponding empirical products of the s-only nuclei (symbols) in the mass region between A = 56 and 209. The error bars of the empirical points reflect the uncertainties of the abundances and of the respective cross sections. One finds that equilibrium in the neutron capture flow was reached between magic neutron numbers, where the (cr)A?s-curve is almost constant. The small cross sections of the neutron magic nuclei around A~88, 140, and 208 act as bottlenecks for the capture flow, resulting in the distinct steps of the crN-curve. 5.2

Stellar Models

In terms of stellar sites, the main component can be attributed to helium shell burning in low mass stars, where neutron production and concordant s-processing occur in two steps by the 1 3 C(a,n) 1 6 0 reaction at relatively low temperatures around T g ~ l and by the 22 Ne(a,n) 25 Mg reaction at Tg~3 (see Refs. 10 ' 41 for details). The weak component can be ascribed to core He burning in massive stars 42 . 6

s-Process Branchings

Branchings in the reaction chain of the s process occur at unstable nuclei with sufficiently long half-lives that neutron capture can compete with /?decay. The resulting abundance pattern provide direct clues with respect to stellar neutron density, temperature, and pressure and allow to characterize the He-burning zones, where the s process actually takes place. Fig. 4 shows the s-process branchings at 147 Nd and 1 4 7 ' 1 4 8 Pm, which are defined by the sonly nuclei 148 Sm and 150 Sm. Since 148 Sm is partly bypassed by the reaction flow, its (