Backgrounds in direct detection experiments05/11/2014

• Program:

– Sources of background

∗ External background (Gamma and neutrons)∗ Internal Backgrounds (for crystals and for liquids separately)

– Background reduction strategies: underground operation and shielding

– Analysis methods to reduce backgrounds

– Statistical significance of a signal

• Literature:

– G. Heusser, Low-radioactivity background techniques, Annu. Rev. Nucl. Part. Sci. 45(1995) 543

– D.-M. Mei and A. Hime, Muon-induced Background Study for Underground Laboratories,Phys. Rev. D 73 (2006) 053004, arXiv:astro-ph/0512125

– G. J. Feldman and R. D. Cousins, A Unified Approach to the Classical Statistical Analysis ofSmall Signals, Phys. Rev. D57 (1998) 3873 , arXiv:physics/9711021

– W. R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer-Verlag, 1987second revision 1994

• Material for the lecture:

Figure 1: Left: Background spectra of a Ge detector without shield (top), with 15 cm lead shield(middle), and with shield and at 500 m.w.e. (bottom). Right: flux of cosmic ray secondaries andtertiary-produced neutrons in a typical Pb shield as function of shielding depth. Figure from G.Heusser, Annu. Rev. Nucl. Part. Sci. 45 (1995) 543.

Figure 2: Total muon flux measured for the various underground sites as function of the equivalentvertical depth relative to flat overburden. Figure from D.-M. Mei and A. Hime, Phys. Rev. D 73(2006) 053004, arXiv:astro-ph/0512125.

Figure 3: Examples of detector shielding, of the DAMA detector (left) and of the XENON100detector (right).

Figure 4: (Left)Event distribution in the XENON100 detector, the dashed line represent the fiducialvolume used for analysis. Figure from XENON100, Phys. Rev. Lett. 109 (2012) 181301. (Right)Variable containing the charge over light ratio as function of the nuclear recoil energy. Calibrationdata using gamma sources is shown in blue while nuclear recoils from a neutron source are shownin red. Figure from the XENON collaboration.

Figure 5: Pulse shape of electrons and alpha particles in liquid argon. Figure from the ArDMcollaboration.

Figure 6: Poisson distribution for various values of the mean µ. Figure from W. R. Leo, Techniquesfor Nuclear and Particle Physics Experiments.