Stars, Accelerators and Underground Laboratories Marialuisa Aliotta School of Physics University of Edinburgh Scottish Universities Physics Alliance nuclear astrophysics nuclear reactions in stars experimental approach underground studies why underground LUNA – pioneering project future facilities perspectives at Boulby John Adams Institute, Oxford, June 14th 2007

KEY to understand Universe at large (EXPERIMENTAL) NUCLEAR ASTROPHYSICS study energy generation processes in stars study nucleosynthesis of the elements Courtesy: M. Arnould MACRO-COSMOS intimately related to MICRO-COSMOS NUCLEAR PHYSICS KEY to understand Universe at large

In search of the building blocks of the Universe… What are we made of? Greek philosophers 4 building blocks earth water air fire 1896 Mendeleev 92 building blocks (chemical elements) questions: how, where and when have the elements been made?

abundance distribution Overview BIRTH gravitational contraction Interstellar gas Stars explosion ejection DEATH mixing of interstellar gas abundance distribution energy production stability against collapse synthesis of “metals” thermonuclear reactions

Stellar nucleosynthesis elements synthesised inside the stars nuclear processes well defined stages of stellar evolution courtesy: M. Wiescher, JINA lectures on Nuclear Astrophysics Burbidge, Burbidge, Fowler, Hoyle (B2FH, 1957) "for his theoretical and experimental studies of the nuclear reactions of importance in the formation of the chemical elements in the universe" 1983 Nobel Prize

during quiescent stages Nuclear processes in stars charged-particle induced reaction mainly neutron capture reaction during quiescent stages of stellar evolution mainly during explosive stages of stellar evolution involve mainly STABLE NUCLEI involve mainly UNSTABLE NUCLEI

stellar reactions on earth Experimental Approach Laboratory requirements and techniques

determines exponential drop in abundance curve! Reactions between charged particles In stellar plasmas: available kinetic energy from thermal motion at temperature T Average kinetic energy: kT ~ 8.6 x 10-8 T[K] keV T ~ 15x106 K (e.g. our Sun)  kT ~ 1 keV T ~ 1010 K (Big Bang)  kT ~ 2 MeV Ecoul ~ Z1Z2 (MeV) nuclear well Coulomb potential V r r0 during quiescent burnings: kT << Ec Ekin ~ kT (keV) reactions occur through TUNNEL EFFECT tunneling probability P  exp(-2)  (E) = exp(-2) S(E) reaction cross section: nuclear fingerprint determines exponential drop in abundance curve!

Relevant energy: the Gamow peak most effective energy region for thermonuclear reactions Gamow peak tunnelling through Coulomb barrier  exp(- ) Maxwell-Boltzmann distribution  exp(-E/kT) relative probability energy kT E0 E0 convolution between MB distribution and tunnel probability Gamow peak E0 = f(Z1, Z2, T) Example: T ~ 15x106 K (T6 = 15) reaction Coulomb barrier (MeV) E0 (keV) Reaction rate (Gamow peak area) p + p 0.55 5.9 7.0x10-6  + 12C 3.43 56 5.9x10-56 16O + 16O 14.07 237 2.5x10-237 STRONG sensitivity to Coulomb barrier H-burning discrete stages of nuclear burning: He-burning C/O-burning … nuclear fingerprint

Experimental approach Gamow peak: energy window where information on nuclear processes must be obtained BUT kT << E0 << Ecoul 10-18 barn <  < 10-9 barn major experimental difficulties Procedure: measure (E) over as wide a range as possible, then extrapolate down to E0! CROSS SECTION S-FACTOR  (E) = exp(-2) S(E) S(E) = E (E) exp(2) non resonant process interaction energy E extrapolation direct measurement S(E) LINEAR SCALE LOG SCALE  direct measurements E0 Ecoul Coulomb barrier (E) non-resonant resonance extrapolation needed ! low-energy tail of broad resonance -Er sub-threshold resonance DANGER OF EXTRAPOLATION ! Er

a possible solution to the extrapolation procedure! Going underground: a possible solution to the extrapolation procedure! reduce background (mainly from cosmic rays) improve signal-to-noise ratio for reaction of interest “Some people are so crazy that they actually venture into deep mines to observe the stars in the sky"                    Naturalis Historia – Pliny, 44 A.D.

The LUNA facility LUNA (Laboratory Underground for Nuclear Astrophysics) Gran Sasso - Italy Laboratori Nazionali del Gran Sasso (1400 m rock -> 106 shielding factor) Radiation LNGS/surface muons neutrons photons 10-6 10-3 10-1 LUNA 400kV The (present) LUNA Collaboration Italy (INFN Gran Sasso, Napoli, Genova, Padova, Milano, Torino, Legnaro) Germany (Ruhr-Universität Bochum) Hungary (Atomki Debrecen)

earth’s surface  0.5 cts/s LNGS underground  0.0002 cts/s HP Ge-Detector 3 MeV < Eg < 8 MeV earth’s surface  0.5 cts/s LNGS underground  0.0002 cts/s 4 BGO summing detector

LUNA (use the Moon to study the Sun) LUNA – Phase I: 50 kV accelerator (1992-2001) investigate reactions in solar pp chain R. Bonetti et al.: Phys. Rev. Lett. 82 (1999) 5205 3He(3He,2p)4He C. Casella et al.: Nucl. Phys. A706 (2002) 203-216 d(p,g)3He @ lowest energy:  ~ 20 fb  1 count/month @ lowest energy:  ~ 9 pb  50 counts/day

~ factor 2 lower than NACRE LUNA LUNA – Phase II: 400 kV accelerator (2002-2006) 14N(p,g)15O slowest reaction in CNO cycle C N O 13 15 12 14 6 7 8 lowest measured s = 0.24x10-12 barn astrophysical S-factor Lemut et al Phys Lett B634 (2006) 483 reaction rate ~ factor 2 lower than NACRE

14N(p,g)15O: bottleneck of CNO cycle major astrophysical implications: pp-chain CNO Imbriani et al A&A 420 (2004) 625 age of globular clusters increased by 1Gy increase in age of Universe (in better agreement with other determinations) solar neutrino flux from CNO reduced by factor 2 implications for Borexino detector delay in onset of CNO cycle for massive stars so far: only three reactions studied directly at Gamow peak !! new/upgraded underground facilities are very much needed !!

current LUNA capabilities LUNA is the only underground accelerator in the world for low-energy NA current LUNA capabilities (400 kV machine) limited to acceleration of H and He beams only direct kinematics studies are possible (beam-induced background on target impurities) reactions producing neutrons not allowed an upgrade of current facility seems unlikely because of severe space constraints at LNGS

Open Problems: carbon burning in advanced stages of stellar evolution neutron sources for s-process Ne, Na, Mg and Al nucleosynthesis in AGB stars isotopic composition of Novae ejecta …

An opportunity for the UK The Boulby Mine: An opportunity for the UK

Boulby Mine a working potash and salt mine Cleveland - North East England the deepest mine in Britain (850m to 1.3km deep) Middlesborough Whitby Staithes York courtesy: S. Paling

roadways & cavern excavated in Potash & Rock salt layer over 40 km of tunnel dug each year (now > 1000 km in total) Heliminer Map of excavations Mine Shafts Dark Matter Research Areas courtesy: S. Paling

Surface facilities The John Barton Building Surface facilities ~200m2: Admin & support facility: Office space, computing facilities, workshop, loading bay, showers, kitchen, laundry, conference room, remote monitoring of underground courtesy: S. Paling

CPL support & facilities Cleveland Potash Facilities Shaft and Tunnel Maintenance Ventilation Health and Safety Emergency rescue Surface & Underground transport Mechanical / Electrical workshops Clean rooms Chemical handling facilities Electrical Supply courtesy: S. Paling

Why is Boulby Special? Requirements for an underground lab... Low Backgrounds Deep (to shield from cosmic rays) Low background rock/lab (and/or adequate shielding) Plenty of Laboratory space Easy access for equipment Proximity of services Good infrastructure + facilities 1.1 km (2805 mwe) CR muons attenuated by ~106 Salt = low in Uranium /Thorium (~ 67 / 125 ppb)  Low neutron background ~1000 m2, Potential for expansion Via mine shaft (2.0m3 cage) + Transport underground 20min  Whitby, Saltburn 1hr  York, Leeds, Middleborough < 5hrs  London, Manchester etc. JIF Surface facility JIF Underground facilities Cleveland Potash Support courtesy: S. Paling

Advantage of salt mine: extremely low g background at Eg < 2-3 MeV

inverse kinematics studies The facility a complementary facility to LUNA ideal to study reactions otherwise not feasible high voltage machine + appropriate ion source for production & acceleration of heavy nuclei inverse kinematics studies

What facility? the accelerator beams Ion (charge state) Particles per second H+ 5.0x1014 He+ 5.0x1013 He2+ 1.0x1013 Xe17+ 2.9x1012 Kr15+ 4.1x1011 Ar11+ 5.6x1011 Ne6+ 1.0x1012 Fe11+ 5.7x1011 (estimated) Ni11+ 3 MV single-ended machine (e.g. NEC, Pelletron) ECR source (e.g. for high intensity (~500 mA) 12C beam at high charge states) Beam-lines + detection systems (gamma, neutron, charged particles) studies possible both in direct and inverse kinematics estimated cost for accelerator + ECR source: ~2.5 M€ = 1.7 M£

the Physics

12C+12C importance: evolution of massive stars Gamow region: 1 – 3 MeV min. measured E: 2.1 MeV (by g-ray spectroscopy) passive lead & concrete shielding Crab Nebula SN 1054 12C+12C 12C(12C,a)20Ne and 12C(12C,p)23Na channels Spillane et al PRL 98 (2007) 122501 a channel p channel Aguilera et al. PRC 73 (2006) 64601 further measurements currently in progress in Bochum (Germany) however: major improvements expected for measurements underground!

13C(,n)16O importance: s-process in AGB stars Gamow region: 130 - 250 keV min. measured E: 270 keV 13C(,n)16O M. Heil, PhD Thesis - Karlsruhe, 2002 Courtesy: F. Strieder contributions from sub-threshold states? mainly hampered by cosmic background  good case for underground investigation

22Ne(,n)25Mg importance: s-process in AGB stars Gamow region: 400 - 700 keV min. measured E: ~550 keV 22Ne(,n)25Mg Jaeger PRL 87 (2001) 202501 mainly hampered by cosmic background  good case for underground investigation reaction rate still uncertain by orders of magnitude uncertain nucleosynthesis predictions similar considerations apply also to 22Ne(a,g)25Mg reaction Karakas et al ApJ 643 (2006) 471

Other examples abundances of Ne, Na, Mg, Al, … in AGB stars and nova ejecta affected by many (p,g) and (p,a) reactions shaded areas indicate current estimate of reaction rate uncertainties Iliadis et al. ApJ S134 (2001) 151; S142 (2002) 105; Izzard et al A&A (2007) submitted !! new measurements underground are very much needed !!

The proposal Experimental Low-Energy Nuclear Astrophysics

Statement of Interest already submitted to Science and Technology Facilities Council Stage 1: Feasibility Study (duration: ~18 months, cost: ~ € 300k) neutron & gamma-ray background measurements  best location within mine infrastructure design (by engineering company) accelerator and beam tests (at Saclay) Stage 2: Construction, Commissioning, Exploitation (duration: ~24+12 months, cost: ~ €3-5M) construction of infrastructure commissioning of facility initial exploitation full proposal in preparation

ERC Start Grant submitted budget: €1.9M overall duration: 5 years aim: team of people for development of state-of-the-art equipment team: PI + 2 senior PDRAs tasks: background measurements requirements for infrastructure beam and accelerator tests (e.g. Saclay) oversee design and construction of infrastructure equipment (gamma-ray array, neutron detector, gas target,…) exploitation of equipment first at surface facilities and then underground Stage 2 submission deadline (if Stage 1 successful): September 17th 2007

Summary Experimental Nuclear Astrophysics: very lively research field quiescent evolution: generally well understood direct measurements at relevant astrophysical energies severely hampered by cosmic-background only few reactions measured so far new/upgraded underground facilities needed

future opportunities Boulby salt mine: an ideal location in the UK Middlesborough Whitby Staithes York depth ~ 1100 m background level ~ factor 10-30 lower than at GS no space constraints (no interference with other experiments) existing support and safety facilities opportunities for involvement at various level

if interested please get in touch: The End if interested please get in touch: m.aliotta@ed.ac.uk