Boulby02 Sim

Neutron background at Boulby mine: 

Neutron background at Boulby mine V. A. Kudryavtsev, P. K. Lightfoot, J. E. McMillan, M. Robinson and N. J. C. Spooner University of Sheffield P. F. Smith, N. J. T. Smith and J. D. Lewin Rutherford Appleton Laboratory R. Lüscher and I. Liubarsky Imperial College of Science, Technology and Medicine, London IDM2002, York (UK), 2-6 September 2002

Neutron production: Outline: 

Neutron production: Outline Sources of neutron background. Neutrons from radioactivity in rock. Neutrons from muons Calculation of muon flux and spectrum Measurement of muon flux at Boulby Simulation of neutron production Comparison with experimental data Conclusions

Neutron background: Sources: 

Neutron background: Sources Two classes: Neutrons associated with local radioactivity (rock/salt + detector components): spontaneous fission and (a,n) reactions; Neutrons produced by cosmic-ray muons: muon-induced spallation reactions, neutrons produced in hadronic and electromagnetic cascades initiated by muons. The main background is due to the (a,n) reactions. It is important for experiments with sensitivity better than 10-5 pb to spin-independent WIMP-nucleon interactions. Low-energy neutrons from radioactivity can be suppressed by the shield made from hydrogen-rich material. Neutron fluxes from rock activity have been measured at several underground sites - Gran Sasso, Modane, Baksan.

Neutron background: Sources: 

Neutron background: Sources Neutron flux from muons is typically 0.1% or less of the neutron flux from the rock activity at large depth underground. Muon-induced neutrons at large depth can become a problem for experiments with sensitivity better than 10-7 -10-8 pb. It is more difficult, however, to suppress it because: Its spectrum is harder and neutrons can travel far from muon track and be collected in the detector from long distances; High-energy neutrons can produce nuclear recoils with energies well above the detector threshold; Any shielding, other than pure hydrogen, will be a target for muons to produce neutrons: heavy material (shielding against photons) will result in the enhanced production of neutrons. Neutrons from muons can be rejected by active muon veto in addition to the thick passive shielding.

Neutron background from rock activity: 

Neutron background from rock activity U and Th contamination in Boulby rock (halite - NaCl): <60 ppb U and <300 ppb Th; Large variation from sample to sample; On average ~30 ppb U and ~150 ppb Th. Simulations of neutron transport with MCNP - preliminary results: About 210-8 neutrons/g/s in rock; About 10-6 neutrons/cm2/s above 100 keV penetrating lead (15 cm) and copper (10 cm) castle; Attenuation of neutron flux in CH2

Muon spectrum and absolute flux: 

Muon spectrum and absolute flux Neutron production rate depends on the muon flux and energy spectrum. Muon flux provides absolute normalisation for neutron production rate - direct proportionality between muon flux and neutron flux. Estimate of the muon flux depends on the depth, rock density, surface relief and rock composition. The best check of the simulations - direct measurements, as has been done at Gran Sasso, Modane, Soudan, Kamioka… Boulby mine does not have large detector that can measure the flux of cosmic-ray muons, such as MACRO and LVD at Gran Sasso, Frejus at Modane, Soudan 2 at Soudan or Kamiokande and Superkamiokande at Kamioka. But there is an active Compton veto around the ZEPLIN I detector at Boulby. It is used now for the muon flux measurements. Neutron flux depends also on mean muon energy. Mean muon energy at Boulby underground lab is about 260 GeV.

Neutron flux from cosmic-ray muons: 

Neutron flux from cosmic-ray muons Simulations of cosmic-ray muons with the MUSIC code (Antonioli et al. Astropart. Phys., 7 (1997) 357; Kudryavtsev et al. Phys. Lett. B, 471 (1999) 251) and its by-product (MUSUN - MUon Simulation Underground - Kudryavtsev et al. Phys. Lett. B, 494 (2000) 175). Simulation of neutron production and transport with the FLUKA code (Fasso et al. Proc. MonteCarlo 2000 Conf., Lisbon, 2000, p. 159 and p. 995). Tests of the simulations - comparison with experimental data. Characteristics to investigate: Neutron production rate as a function of muon energy (depth); Neutron production rate as a function of atomic weight of material; Neutron energy spectrum; Neutron flux as a function of distance from muon track. Comparison with experimental data - mainly LVD at Gran Sasso (similar depth and mean muon energy). Full 3D Monte Carlo for Boulby - in progress.

Muon flux underground: 

Muon flux underground Flat surface above the laboratory; Effect of rock composition: blue squares (Boulby rock - 5-7% higher <Z> and <A> - normalised to the standard rock); Effect of parameterisation of muon energy spectrum at surface: black open circles -Gaisser's parameterisation (Gaisser. Cosmic Rays and Particle Physics, 1990) normalised to the LVD best fit (Aglietta et al. Phys. Rev. D, 58 (1998) 092005); Less than 10% difference at 3 km w.e. (2-3)% uncertainty in <Z> and <A> is acceptable for the muon and neutron flux calculations, if, and only if, the column density = depth  density is well known; Use of correct muon spectrum at surface is important: LVD best fit is the preferred option for large depth (> 2 km w.e.); An error in the column density (either depth or density) of 2% results in a 10% error in muon flux at 3 km w.e.; Best test - direct measurement of muon flux

Muon energy spectrum and mean energy: 

Muon energy spectrum and mean energy Mean energy as a function of depth: Red circles - standard rock and LVD best fit; Blue squares - Boulby rock and LVD best fit; Black open circles - standard rock and Gaisser's parameterisation Mean energy normalised to the red circles. Less than 5% differences for all depths. Mean muon energy can be calculated accurately if the depth and density is known. Less than 4% uncertainty in neutron flux.

Muon flux measurements at Boulby: 

Muon flux measurements at Boulby ZEPLIN I veto system: 1.2 tonnes of liquid scintillator viewed by 10 PMTs independent DAQ (4 PMTs) is used about 50 muons/day Distributions in: track length muon energy deposition number of detected photoelectrons

Muon flux measurements at Boulby: 

Muon flux measurements at Boulby Measured spectrum of muon energy depositions (preliminary) - in agreement with simulations

Neutron production as a function of muon energy: 

Neutron production as a function of muon energy Neutron production rate (in liquid scintillator) as a function of muon energy: red solid line - fit blue dashed line - fit from Wang et al. Phys. Rev. D, 64 (2001) 013012 Red squares - simulations with real muon spectrum for 0.6 km w.e. and 3 km w.e. Neutron production from real muon spectrum is smaller by (10-15)% than the one from muons, all having the energy equal to the mean energy of the spectrum.

Neutron production rate as a function of atomic weight of material: 

Neutron production rate as a function of atomic weight of material Red circles with solid line - results of simulations and fit; blue points - results of the CERN NA55 experiment with thin target - only muon-induced spallations - Chazal et al. Nucl. Instrum. & Meth A., 490 (2002) 334; hep-ex/0102028 red squares - simulations with muon-induced spallations only (experimental data are for certain scattering angles and are normalised arbitrarily to the simulations)

Neutron production in various processes: 

Neutron production in various processes Neutron production in various processes: muon spallation hadronic cascades e.m. cascades Scintillator: 5% 75% 20% <A>=10.4, Em=280 GeV Lead: 3% 55% 42% A=207.2, Em=280 GeV Scintillator: 27% 38% 35% <A>=10.4, Em=10 GeV Neutron production in electromagnetic cascades is more important for heavy targets due to Z2/A - dependence of cross-sections of radiative processes.

Neutron spectrum: 

Neutron spectrum Red open circles - simulations for scintillator (real muon spectrum) Solid line - fit for 280 GeV muons from Wang et al. Phys. Rev. D, 64 (2001) 013012 normalised to our simulations Blue open circles - simulations for NaCl (real muon spectrum) Parameterisation from Wang et al. describes simulations at E>50 MeV (muon spectrum instead of fixed energy?) Softer neutron spectrum for heavier targets

Neutron energy spectrum: 

Neutron energy spectrum Comparison with the LVD data (Aglietta et al., hep-ex/9905047) - data corrected for the proton quenching factor. Blue open circles with error bars - corrected LVD data normalised to the simulations; Red points - simulations with FLUKA.

Spectrum of neutrons coming from the rock: 

Spectrum of neutrons coming from the rock Rock (halite) volume -202020 m3; Cavern (laboratory hall) volume - 665 m3; Neutron production rate in salt; (NaCl) at Boulby underground lab is 7.5610-4 neutrons/muon/(g/cm2); No difference between m+ and m- - negligible contribution from stopping muons (negative muon capture).

Lateral distribution of neutrons: 

Lateral distribution of neutrons Comparison of FLUKA results (red points) with the LVD data (blue open circles with error bars). LVD data are normalised to the simulations.

Suppression of neutron-induced rate: 

Suppression of neutron-induced rate Expected recoil rate in the DRIFT-type detector as a function of recoil energy for several values of thickness of CH-shielding - preliminary 60 cm of shielding suppress almost completely neutron flux from activity in salt (more than 5 orders of magnitude), while muon-induced neutrons with harder spectrum are less suppressed Rate (events/kg/day/keV)

Conclusions and outcomes: 

Conclusions and outcomes 20-30 cm of H-rich material (CH2) are enough to protect dark matter detectors with sensitivities down to 10-7 -10-8 pb from neutrons from rock activity. Required thickness increases roughly by 10 cm for every order of magnitude in sensitivity improvement. MUSIC (MUSUN) and FLUKA provide reasonably good description of muon spectra and neutron production underground. Best test - direct measurements of muon and neutron fluxes. Muon flux at Boulby - measurements with liquid scintillator (ZEPLIN I veto). Neutron flux at Boulby - DRIFT I (DRIFT II), veto systems (ZEPLIN I, II). Active muon veto with 90% efficiency will be enough for detectors with sensitivities down to 10-8 pb.