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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.”.

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slide1

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

slide2

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

a conspiracy of events
“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.
sources of background
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

uranium thorium chains
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

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.
spontaneous fission
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.
cosmic ray flux

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.

muon capture
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

muon capture10
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)

muon spallation

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.
virtual photon exchange
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
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!

neutron production data
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)

target dependence
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

neutron energy spectrum
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)

isotope production
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)

simulation techniques

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

outlook
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.
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