MANTENIMIENTO CUATRIMESTRAL DE LA UNIDAD COR-3

Solid Scintillator Detectors

Solid scintillator detection experiments use high purity inorganic scintillator crystals as their target. Photomultiplier tubes (PMTs) are used to detect scintillation from energy de- position in the crystal. The crystals are grown at sizes on the order of cm3, and larger tar- gets are constructed using arrays of scintillator crystals. Multiple targets are built this way to increase an experiment’s combined target mass. One detector of this type is DAMA, a NaI detector based at the LGNS underground laboratory, which consisted of nine NaI targets of mass 9.7 kg, and took data over seven annual cycles. The experiment was then upgraded to DAMA/LIBRA, consisting of 25 NaI targets of mass 9.7 kg. DAMA/LIBRA collected 1.33 tonne-years of data over 14 annual cycles in combination with previous DAMA data. An oscillation in event rate was observed at 9.2σ C.L. for recoil energies of 2-4 keVr, with a period of 0.996±0.002 years and an amplitude of 1.9±0.2% of the

average rate [64]. Analysis of the uncertainty and modulation due to crystal temperature, WIMP flux, ambient pressure surrounding the crystals, radon background and electronics noise yielded that none of the effects investigated were large enough to be consistent with no rate modulation [65, 66]. From scattering with sodium, an SI cross section discovery region at 2 × 10−40cm2 is implied for WIMP masses (10-15) GeV. From scattering with iodine, an SI cross section at 2 × 10−41 cm2 is implied for WIMP masses (6-20) GeV. These signal regions are excluded by other experiments as shown in Figures 1.11 and 1.12.

Liquid Noble Gas Detectors

Particle scattering in liquid noble gases produces detectable signals in the form of scin- tillation light and electrons from ionisation. Liquid nobles are transparent to their own scintillation light wavelength for the path lengths at present detector sizes, and scintilla- tion is detected using PMTs. Scintillation in liquid nobles is discussed in Section 1.5. Ionisation electrons are detected by drifting them towards a gaseous region in a detector using a uniform electric field. In the gaseous region they produce light through scintilla-

1.4. DETECTION STRATEGIES CHAPTER 1. INTRODUCTION

tion and electroluminescence, which is detected by PMTs.

Single phase detectors use noble gases in the liquid phase and can collect only scintil- lation light. The detected scintillation timing profile is different for electronic and nuclear recoils as the energy loss per unit length dE/dx of the incident particle is larger in neu- trons and WIMPs than electrons and gammas. The exploitation of the timing profile to discriminate between electronic and nuclear recoils is known as Pulse Shape Discrimina- tion (PSD), which is described in more detail in Section 1.5.

XMASS is an example of a liquid xenon single phase detector based in the Gran Sasso National Laboratory, Italy, which consists of an 835 kg spherical target region sur- rounded by 642 PMTs. XMASS demonstrated a rejection of electron recoil events with energies between 4.8 keVee and 7.2 keVee by a factor of 7.7±1.1(stat)+1.2_−0.6(sys)×10−2,

whilst retaining 50% nuclear recoil acceptance [67]. For electronic recoil energies be- tween 9.6 and 12 keVee the rejection factor at 50% nuclear recoil acceptance improves

to 7.7±2.8(stat)+2.5_−2.8(sys)×10−3. Using 0.818 tonne-years of data and a nuclear recoil en- ergy threshold of 4.8 keVr, XMASS-1 set a 90 C.L. upper limit on the SI WIMP-nucleon

scattering cross section of 3.2 × 10−41cm2at a WIMP mass of 140 GeV [68].

DEAP-3600 is an example of tonne-scale liquid argon single phase detector with a 3263 kg target mass (originally 3600 kg) before fiducialisation, based in SNOLAB in Sudbury, Ontario, in Canada. With its first result DEAP-3600 has set the most stringent 90% C.L. upper limit on the SI WIMP-nucleon scattering cross section using argon, at 1.2 × 10−44 cm2for a 100 GeV WIMP mass, having seen no candidate events [3]. The limit, shown in solid blue in Figure 1.11, is set using a 9.87 tonne-day exposure, electronic recoil rejection leakage probability at < 1.2 × 10−7 events and a 64-132 keVr energy region of interest. Over three years of data taking with a 1000 kg fiducial mass and 48 keVr nuclear recoil energy threshold it is projected to reach a sensitivity to SI WIMP-

nucleon scattering cross section of 10−46cm2for a 100 GeV WIMP mass [2]. The DEAP- 3600 projected 90% C.L. upper limit for 3 tonne-years of data taking is shown in dashed blue in Figure 1.12. The DEAP-3600 detector is described in Chapter 2.

of data from ionisation. Scintillation and ionisation signals are known as S1 and S2 re- spectively. Discrimination between electronic and nuclear recoils in dual phase detectors is performed using the ratio S1:S2. Multiple dual phase detectors have been constructed using the time projection chamber (TPC) format. A TPC consists of cylindrical target mass in the liquid phase with a layer of gas above it, a uniform electric field parallel to the cylindrical axis, and a layer of PMTs above and below the target mass. Electron time of flight relative to the initial scintillation time is used to locate the position of an event along the cylindrical axis, and PMTs are used to reconstruct position perpendicular to the axis.

LUX is an example of a xenon TPC constructed in the Sanford Underground Research Facility in the USA, with a 250 kg target mass. LUX set a 90% C.L. upper limit on the SI WIMP-nucleon scattering cross section limit at 2.2 × 10−46 cm2 for a 50 GeV WIMP mass [69, 70]. LUX used a 332 live day exposure with 100 kg fiducial volume, and a nuclear recoil energy threshold of 3keV was set using the energy at which PSD has ≥50% nuclear recoil acceptance. Electronic recoil leakage into the WIMP PSD cut was observed to occur with an average 0.2% probability [69]. The combination of this with a previous 92 live day exposure allows LUX to set a 90% C.L. upper limit at 1.1 × 10−46 cm2 for a 50 GeV WIMP mass [71]. XENON-1T is another TPC constructed in the Laboratori Nazionali del Gran Sasso in Italy. At time of writing, XENON1T has set the world-leading 90% C.L. upper limit on the SI WIMP-nucleon scattering cross section, at 7.7×10−47cm2for a 35 GeV WIMP. XENON1T used 34.2 live day exposure using a 1042±12 kg fiducial mass, in a 5-40 keV nuclear recoil energy region of interest. Thereafter the XENON-1T experiment is projected to reach a 90% C.L. upper limit on the SI WIMP-nucleon scattering cross section of 1.6 × 10−47 cm2 at a 50GeV WIMP mass with a 2 tonne-year exposure, 1 tonne fiducial mass, 0.5% electronic recoil leakage at 50% nuclear recoil acceptance and a 4-50 keV nuclear recoil energy region of interest [72]. This is then projected to be exceeded by the LZ collaboration which follows LUX, whose SI sensitivity is projected to reach below 3 × 10−48cm2at a 40 GeV WIMP mass, using a 5.6 tonne fiducial volume, the PSD rejection power observed in the 332 live day

1.4. DETECTION STRATEGIES CHAPTER 1. INTRODUCTION

LUX result, and a 6 keVrnuclear recoil energy threshold [69]. The LUX and XENON-1T

observed limits are shown in Figure 1.11, and the LZ projected limit is shown in Figure 1.12.