“(Come in under the shadow of this red rock), And I will show you something different from either Your shadow at morni

1. Background Models for Muons and Neutrons Underground “(Come in under the shadow of this red rock),  And I will show you something different from either  Your shadow at morning striding behind you  Or your shadow at evening rising to meet you;  I will show you fear in a handful of dust.” Joseph A. Formaggio University of Washington LRT Workshop December 13th, 2004 --T.S. Eliot, The WasteLand

2. Based on recent article by C.J. Martoff and J.A. Formaggio, ARNPS, 54 361(2004) • By no means complete on all background sources for each experiment. Rather, cover major backgrounds for different experimental arch-types. • Focus on physical processes and Monte Carlo implementations. “(Come in under the shadow of this red rock),  And I will show you something different from either  Your shadow at morning striding behind you  Or your shadow at evening rising to meet you;  I will show you fear in a handful of dust.” --T.S. Eliot, The WasteLand

3. “A Conspiracy of Events” • Experiments • Next generation of sensitive experiments: • Dark Matter Experiments (CDMS, Picasso, Xenon, etc.) • Solar Neutrino Experiments (CLEAN, LENS, etc.) • (0nbb) Experiments (Majorana, EXO, CUORE, etc.) • Laboratories • New underground facilities being planned in the U.S. and Canada. • Simulations: • Backgrounds become a key issue in these and other next generation projects. • Trying to encompass wide range of energies and particle types within one system.

4. Sources of Background • Natural Radioactivity • (a,n) reactions from uranium and thorium decay chains. • Spontaneous fission. • Muon-Induced Activity • Muon capture. • Includes neutron production from neutron photo-production and subsequent secondary activity. • Isotope production (direct/secondary) m Capture m Spallation U/Th Chain m Spallation m Capture U/Th Chain U/Th Chain m Capture m Spallation

5. Uranium & Thorium Chains • For deep underground facilities, often the main source of background for experiments. • Contributes to both photon and neutron background in the detector. • Natural concentrations in surrounding environment, as well as detector materials.

6. M. J. Carson et al, Astrophys. J.607, 778 (2004). Neutrons from Radioactivity • Main sources from (a,n) reactions from Po decays. • Most abundant elements below neutron production threshold. • Typical production from reactions on 9Be, 13C, 17O, 25Mg and 43Ca. • Concentrations differ depending on rock type.

7. Spontaneous Fission • Neutron production can also take place through spontaneous fission: • Falls rapidly beyond 2 MeV • Sub-dominant process • Typically less than 30% of (a,n) neutron production. • Multiple neutrons produced.

8. Hagiwara K, et al. Phys. Rev.D66:010001(2002) Cosmic Ray Flux • Once below ~30 mwe, cosmic ray flux is dominated primarily by muons. • For muons that reach deep sites, the LVD parameterization works well to determine incoming rate and spectrum. • Well measured by existing underground experiments. • Uncertainties typically associated with rock density and composition (<Z2/A>). Vertical muon flux as function of depth.

9. Muon Capture • Source of neutron production, typically dominant at shallow depths. m- + A(Z, N)  nm + A(Z-1, N+1) • One or more neutrons typically produced, depending on target material. m-fraction Neutron multiplicity Stopping rate Capture rate

10. Muon Capture • Source of neutron production, typically dominant at shallow depths. m- + A(Z, N)  nm + A(Z-1, N+1) • One or more neutrons typically produced, depending on target material. • Here, Pc = Gc/(Gc+QGd), and X(1,2) = (170 s-1, 3.125) Suzuki T, et al. Phys. Rev. C 35: 2212 (1989)

11. Neutron Spectrum At 300 mwe Muon Spallation • Actually, a complex process, since a number of physics processes are at play: • Virtual photon exchange. • Electromagnetic interactions. • Secondary production from particle showers.

12. Virtual Photon Exchange • Two dominant theories: • Weizsacker & Williams formalism. • Treat virtual photon as a real photon exchange. • Bezrulov & Bugaev formalism: • Treat in the framework of a generalized vector meson dominance model • Includes nuclear shadowing effects. • In general, two methods differ by ~30%, depending on the energy and target type. • Both describe the reaction in terms of a virtual photon flux, coupled with a real photo-neutron cross-section.

13. Photo-neutron Production • Processes involved: • Giant Dipole Resonance (below 30 MeV) • Quasi-deuteron production. • Pion resonance • Hard scattering • Finally, one must consider re-interactions of primary neutrons produced at the vertex. IAEA Database Chadwick et al. R. Schmidt et al. Full Monte Carlo simulations necessary!

14. Neutron Production Data • Limited available data for neutron production underground. • Main measurements made in scintillator (LVD, Palo Verde, etc.). • Lead and other targets available through the Artemovsk Scientific Station. • Energy dependence appears to follow simple scaling law: Lead Scintillator Nn = 4.14 × 10-6Em0.75 n/(m g-1cm-2)

15. Target Dependence • Only limited number of underground target measurements made, mostly from the Artemovsk Scientific Station. • Also appear to have simple scaling dependence. • Target measurements also performed at the CERN SPS muon beam facility. • Limited since secondary reactions difficult to probe. • Monte Carlo estimates place this closer to A0.76. Nna A0.90+0.23

16. Neutron Energy Spectrum • Spectral comparisons between data and Monte Carlo done by Y.F. Wang et al and Kudryavtsev et al. • Global parameterizations seem to break down below 20 MeV for neutron energies. • Can be parameterized by simple dependence on muon energy. Y-F Wang, et al. Phys. Rev. D 64: 013012 (2001)

17. Isotope Production • Isotope production at the surface from hadronic showers. • Below surface from capture, (n,p) reactions, or direct spallation. • Muon spallation measured in CERN’s SPS muon beam facility. • Performed for a number of final states, including 11C, 7Be, 11Be, 10C, 8Li,6He, 8B, 9C, and 9Li+8He. Hagner T, et al. Astropart. P. 14: 33 (2000)

18. MUSIC SNO Photo-production Code GDR QD p-res. Pythia Hadron Propagation Simulation Techniques • Muon Propagation: • First order, Gaisser parameterization. • MUSIC & PrompMu deliver fast propagation through dense material. • FLUKA and GEANT4 • Neutron Production: • Comprehensive Monte Carlo often required. • SNO hybrid system • GEANT4 and FLUKA tested against existing data. LVD Flux

19. Outlook • Physics processes behind neutron production from natural radioactivity, muon capture, and muon spallation well understood. • Neutron energy spectrum varies as a function of source and depth. • Monte Carlo codes improving in incorporating decay chains and neutron spallation products. • Limited data still available for direct MC/data comparisons.