Understanding Backgrounds in Underground Environments

1. Neutrons, and Muons, and Photons (oh my!) Backgrounds in Underground Environments J. Formaggio University of Washington

2. Categories • Three main types of background to consider: • Natural radioactivity (U/Th/K) • Muons (see Jon’s talk) • Muon-induced backgrounds (mostly neutrons and isotopes) q13

3. Uranium Thorium Uranium/Thorium Chains • U/Th chain of most concern for underground experiments • Source of neutron activation via alpha decays. Most relevant are high energy alphas: • 216Po (6.8 MeV a) • 212Po (8.8 MeV a) • 218Po (6.0 MeV a) • 214Po (7.7 MeV a) • 210Po (5.3 MeV a). • However, not enough to overcome the energy threshold for (a,n) production in 16O, 28Si, 24Mg, and 40Ca. b a 232Th 238U

4. Geology Rock & Concrete Composition (%) at Modane • Uranium and thorium concentrations vary depending on type of rock and surrounding geology. • Typical values range from (1-6 ppm U and 3-30 ppm Th) • Rock composition also determines corresponding neutron activation. • Example of measurements made at Modane (4800 m w.e.) given here. Contamination of U, Th, and 40K

5. Neutrons from U/Th • Main activity comes from (a,n) activation of surrounding rock (thus dependent upon composition). • Presence of 238U also contributes to neutron production via spontaneous fission (232Th and 235U too small to consider). • Spontaneous fission issue since typical multiplicity is ~2 neutrons/fission. • Neutron energy spectrum (sp. fiss.):  En1/2 exp(-En/1.29) (En in MeV) Given in n/g/yr

6. Muon Capture • Though at 300 m w.e. most muons are at high enough energy that muon capture is not an issue, it must still be calculated. • Multiplicity from evaporative processes possible. • Process includes “direct” neutron production (hard) and “evaporative” (soft). *Note, fluxes not normalized to each other!

7. Muon-Induced Background • Currently somewhat difficult to estimate w/o accurate Monte Carlo simulations. • Limited data set exists for scintillator and lead targets. Agree well with FLUKA simulations. Nn = 4.14 x 10-6 Em0.74(/m/s/cm-2) • Based on our depth of 300 m w.e., should expect less than 0.1 neutrons/g/year. • However, energy spectrum is much harder than natural radioactivity.

8. Neutron Spectrum • Neutron spectrum (from muons) difficult to model, especially at low energies. • FLUKA parametrization: • Using muon spectrum lowers rate by ~15% and softens neutron spectrum.

9. 9Li / 8He Contamination • Correlated backgrounds from neutron activation, such as 9Li and 8He can pose serious background to the experiment. • Some codes (COSMO) set up to estimate activation from surface neutrons (see table). • Can modify code to adjust for neutron rate at the required depth.

10. Codes currently available: Muons: MUSIC MUon SImulation Code (MUSIC), for muon propagation through rock. Code available for use (currently making sure it works). Neutrons: FLUKA Optimal for neutron production from muons (and other processes) Code at hand, but not adapted to our studies Cosmogenics: COSMO Models cosmogenic activity Code available. Almost adopted to handle underground neutron flux. Neutrons: MNCP Neutron propagation at low energies (below 20 MeV) Used by SNO. U/Th chain: SOURCES Good for U/Th alpha chains No info at this time Practical Tools Available