Neutrons and Muons and Photons oh my Backgrounds

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

Categories n Three main types of background to consider: q 13 n Natural radioactivity (U/Th/K) n Muons (see Jon’s talk) n Muon-induced backgrounds (mostly neutrons and isotopes)

Uranium/Thorium Chains n n U/Th chain of most concern for underground experiments Source of neutron activation via alpha decays. Most relevant are high energy alphas: n n n (6. 8 Me. V a) 212 Po (8. 8 Me. V a) 232 Th (6. 0 Me. V a) 214 Po (7. 7 Me. V a) 210 Po (5. 3 Me. V a). 238 U 216 Po 218 Po However, not enough to overcome the energy threshold for (a, n) production in 16 O, 28 Si, 24 Mg, and 40 Ca. Uranium Thorium b a

Geology n n Uranium and thorium concentrations vary depending on type of rock and surrounding geology. Rock & Concrete Composition (%) at Modane 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 40 K

Neutrons from U/Th n n Main activity comes from (a, n) activation of surrounding rock (thus dependent upon composition). Presence of 238 U also contributes to neutron production via spontaneous fission (232 Th and 235 U too small to consider). Spontaneous fission issue since typical multiplicity is ~2 neutrons/fission. Neutron energy spectrum (sp. fiss. ): En 1/2 exp(-En/1. 29) (En in Me. V) Given in n/g/yr

Muon Capture n n n 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!

Muon-Induced Background n n 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 Em 0. 74 (/m/s/cm-2) n n 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.

Neutron Spectrum n n n 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 Li n n n / 8 He Contamination Correlated backgrounds from neutron activation, such as 9 Li and 8 He 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.

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