Muon-induced neutron background at Boulby mine Vitaly A. Kudryavtsev University of Sheffield UKDMC meeting, ICSTM, London, 27 June 2002

Neutron background Two classes: –Neutrons associated with local radioactivity: spontaneous fission and ( ,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 ( ,n) reactions. It is important for experiments with sensitivity better than pb to spin-independent WIMP-nucleon interactions. Muon-induced neutrons can become a problem for experiments with sensitivity better than pb. Low-energy neutrons from radioactivity can be suppressed by the shield made from hydrogen-rich material. Neutrons from muons can be rejected only by active muon veto. Neutron fluxes have been measured at several underground sites, which were considered to be the world-class underground laboratories (Gran Sasso, Modane). What's about Boulby?

Neutrons from radioactivity Neutrons from radioactivity are simulated with MCNP (see the talk by John McMillan). There was also simulation performed by David Lewin and Peter Smith (it would be very useful to have a note summarising all results). However, MCNP does not provide absolute neutron flux, but only neutron flux suppression in various materials as a function of neutron energy and material thickness. So, the result from MCNP will tell us what thickness is needed to suppress the neutron flux by a certain factor. Absolute normalisation should come from other experiments or simulations. Hence, the uncertainty in the absolute value and neutron spectrum are quite high. Direct measurements of neutron flux and possibly spectrum would be very useful.

Neutrons produced by muons This is not the first simulation of muon- induced neutrons - Peter Smith has also performed simulations with his code. More detailed and sophisticated codes (FLUKA, GEANT etc.) are available now and can be used for this purpose. GEANT3 is not reliable for muon simulation (muon inelastic scattering is underestimated by a factor of 10). GEANT4 should be fine but it has not been extensively tested yet. FLUKA is proven to give reliable results.

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. Geological survey of the Boulby mine (shaft) has been used to evaluate the rock parameters. Integrated over solid angle muon flux at Boulby has been calculated as 2.74  cm -2 s -1. At present the flux is known with 20% uncertainty. It will be measured with (3-5)% accuracy with ZEPLIN I scintillator veto. Neutron flux depends also on mean muon energy. Mean muon energy at Boulby underground lab is 264 GeV. The uncertainty is about 3%, which results in 2% uncertainty in the neutron flux.

Neutron flux from cosmic-ray muons Starting with understanding the code and simple tests, which will prove that the code works properly. Things 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 (not done yet).

Neutron production: outcomes Neutron production rate (in liquid scintillator) as a function of muon energy:  Neutron production rate as a function of atomic weight of material: Neutron production in: hadronic cascades e.m. cascades Scintillator 80% 20% =10.4 Lead 58% 42% A=207.2

Neutron production: outcomes Neutron production rate in salt (NaCl) at Boulby underground lab is 7.56  neutrons/muon/(g/cm 2 ), which is about half of Peter's calculations. No difference between    and   . Muons sampled according to the energy spectrum produce (10-15)% less neutrons than muons, all having mean energy from this spectrum. Neutron energy spectrum can be approximated by equation from Wang et al. Phys. Rev. D, v. 64 (2001) , but only above 50 MeV.

Neutron production: outcomes Reasonably good agreement with measured spectrum (LVD) if LVD data are corrected for proton quenching factor. (In fact measured energy is not equal to neutron energy). Lateral distribution of neutrons as a function of distance from muon track is in very good agreement with LVD data. Moving towards full 3D Monte Carlo. Looking forward measuring muon and neutron fluxes at Boulby.