“(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.” Background Models for Muons and Neutrons Underground Joseph A. Formaggio University of Washington LRT Workshop December 13 th, T.S. Eliot, The WasteLand

“(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 Based on recent article by C.J. Martoff and J.A. Formaggio, ARNPS, (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.

“A Conspiracy of Events” Experiments Next generation of sensitive experiments: Dark Matter Experiments (CDMS, Picasso, Xenon, etc.) Solar Neutrino Experiments (CLEAN, LENS, etc.) (  ) 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.

Sources of Background Natural Radioactivity ( ,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)  Capture U/Th Chain  Spallation U/Th Chain  Spallation  Capture U/Th Chain  Spallation  Capture

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.

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

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

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 ( ). Vertical muon flux as function of depth. Hagiwara K, et al. Phys. Rev.D66:010001(2002)

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

Muon Capture Source of neutron production, typically dominant at shallow depths.  - + A(Z, N)   + A(Z-1, N+1) One or more neutrons typically produced, depending on target material. Here, P c =  c /(  c +Q  d ), and X (1,2) = (170 s -1, 3.125) Suzuki T, et al. Phys. Rev. C 35: 2212 (1989)

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. Neutron Spectrum At 300 mwe

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.

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. Full Monte Carlo simulations necessary! IAEA DatabaseChadwick et al.R. Schmidt et al.

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 N n = 4.14 × E  0.75 n/(  g -1 cm -2 )

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 A N n  A

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: (2001)

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 11 C, 7 Be, 11 Be, 10 C, 8 Li, 6 He, 8 B, 9 C, and 9 Li+ 8 He. Hagner T, et al. Astropart. P. 14: 33 (2000)

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 MUSIC SNO Photo-production Code GDRQD  -res. Pythia Hadron Propagation

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.