Radioactive Source Calibration

The radioactive sources are deployed periodically over the course of the commissioning of the detector and during stable running, in order to calibrate the energy reconstruction, pulse timing and position reconstruction in the detector. Two sources are used to this end: an AmBe neutron source, which produces WIMP-like nuclear recoils; and a22Na gamma source, which produces electronic recoils.

A 22Na gamma source is used to produce electronic recoils in argon for calibration of the energy resolution of the detector, which at time of writing is estimated to undergo gamma decay at a rate of 333 kBq. The source emits a 1.27 MeV photon, which is of sufficiently high energy to be detected in the argon after scattering in intermediate materials. The positron from the22Na β+ decay annihilates with a nearby electron in the source to produce two 511 keV photons in opposite directions, which are detected and used for tagging of a decay event.

The gamma source is placed at the centre of a cylindrical stainless steel canister, which houses on either side a tagging system composed of two 8.5 mm thick scintillator crystals, made of Cerium-doped Lutetium Yttrium Orthosilicate (LYSO), and behind each a com- pact Hamamatsu R9880U PMT. Two cable housings either side of the canister enclose the PMT cables and a steel umbilical attached to the canister which ensures that any force applied on the cabling is exerted on the canister, not the PMT or its connection.

A 74 MBq AmBe neutron source is used to produce nuclear recoils. The241Am under- goes alpha decay. The alpha undergoes alpha capture in a9Be nucleus, which stimulates

2.8. CONCLUSION CHAPTER 2. THE DEAP-3600 DETECTOR

neutron production and creates an excited12C. The excited12C de-excites to produce a 4.4 MeV photon. The neutron source is placed at the centre of a cylindrical steel canister. On either side of the source is a 51 mm thick NaI scintillator crystal, in front of a 38 mm ETL 9102 PMT. The PMT detects scintillation in NaI from the 4.4 MeV photon which is used to tag a neutron decay.

The sources are deployed into the calibration tubes shown in the drawing in Figure 2.27. The neutron source is deployed into three vertical tubes, Cal A, B and E, which are at closest to the steel shell at the equator. The gamma source is deployed into Cal F, which is mounted on rails attached to the steel shell and which crosses over at the neck.

Figure 2.27: a) A drawing showing the calibration tubes. Three vertical tubes, Cal A, B and E allow sources to be deployed close to the equator, and Cal F allows sources to be deployed at points around the detector, and close to the neck. b) A photograph showing the two gamma calibration racks on the left and the neutron calibration rack on the right.

2.8

Conclusion

The design of the DEAP-3600 detector was motivated by background reduction, which is achieved through material selection, preparation and handling during construction and commissioning. The content in this chapter provides an overview of the detector as a

whole and the backgrounds the detector is designed to mitigate. The remainder of back- ground mitigation is performed in analysis, and the success of that undertaking is de- pendent on a detailed simulation of the detector. In the case of the alpha backgrounds, position reconstruction is of paramount importance, and accurate position reconstruction relies heavily on detector simulation. That simulation is described in the next chapter.

Chapter 3

Simulation, Analysis and Optics

This chapter discusses the simulation of the DEAP-3600 detector. The simulation and analysis software is described. The PMT electronics and DAQ simulation is described, which produces data which mimics real detector data. The analysis variables from event reconstruction which are used in later analysis are also described. The detector optical model is then discussed, which the reconstruction model relies on to model the effect that the variation of the position of a point-like scintillation event has on the response of the PMT charge readout. The effect of variation of the optical model on those parameters which affect the variation of PMT charge with position is discussed, using simulation and data from the AARF calibration source and uniform39Ar background.

3.1

Simulation

In this section, the detector simulation is discussed. The Reactor Analysis Tool, or RAT, software package has been adapted for use in simulation and analysis in DEAP-3600. RAT was originally designed by Stanley Seibert for spherical liquid scintillator experi- ments instrumented with PMTs. A generic open-source version is available at Ref. [139]. The software package provides a framework which enables Monte Carlo simulation of a detector and data analysis to take place in the same software and in the same instance. The software also provides a command-line interface which can be scripted.

text JSON-like table structure as a macro file to interface with GEANT4 geometry classes. Material and optical properties are encoded in the same way. Any geometric, optical or material property of the detector can be changed by the user in a single line in a RAT script as necessary. Particle and particle-material interactions are then handled in GEANT4. The simulated detector is shown compared to the detector as constructed in Figure 3.1. The top images compare the AV and light guides without PMTs and filler blocks installed. The middle images compare the AV with PMTs, copper shorts and filler blocks installed. The bottom image compares the steel shell in simulation with the AV enclosed, without calibration tubes, to the detector before the water tank was filled with water.