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Liquid Xenon Gamma Screening. Luiz de Viveiros Brown University. Simulation Settings.

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Simulation Settings

- A simulation is set up to analyze the level of radiation in a Liquid Xenon detector caused by contaminants in Photomultiplier Tubes. The objective is to determine the efficiency of Xenon in screening gammas from the PMTs and to determine whether there are sufficiently large regions in the detector with background radiation below acceptable levels.
- The target background is 6 x 10-3 counts/keV/kg/day.
- Multiple scatter events can be filtered out by identifying the events with a large deposition spread, measured by calculating the energy weighted standard deviation of interactions in an event, and it is given by:

- The depth of a event is defined as the energy scaled average position of all the interactions in that event, given by:

- The target is a cylinder of Liquid Xenon, 30cm high and 30cm in diameter (65 kg). The PMT is represented by an isotropic point source of gamma-rays. The source is located 5cm above the liquid surface. All simulations are run with 1 million emitted photons.

The simulation is run with a source that simulates the emission lines of the physical PMTs. For this simulated PMT, we used the Hammamatsu R8778 tube. The radiation is due to 3 contaminants: Potassium (13 mBq), Uranium (5 mBq) and Thorium (13 mBq).

2D Hitogram – Energy vs. Depth

Potassium / Uranium / Thorium Source – 31 mBq Total Activity

On the rate scale, 0.00128 is the smallest detected rate and 0.006 is the target background rate

Potassium / Uranium / Thorium – 31 mBq Total Activity

Energy Histogram

Depth Histogram

Background Analysis – 1MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

2D Histogram – Energy vs. Depth

On the rate scale, 0.00048 is the smallest detected rate and 0.006 is the target background rate

Background Analysis – 1MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

Depth: 10 cm – 20 cm

Depth: 20 cm – 30 cm

Radial Histograms

Background Analysis – 1MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

Copper Cryostat

1cm thick

2D Histogram – Energy vs. Depth

On the rate scale, 0.00048 is the smallest detected rate and 0.006 is the target background rate

Background Analysis – 1MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

Effects of Copper Cryostat

With Cryostat

Without Cryostat

Radial Histograms

Depth: 10 cm – 20 cm

Middle of Detector

Background Analysis – 1MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

Effects of Copper Cryostat

With Cryostat

Without Cryostat

Radial Histograms

Depth: 20 cm – 30 cm

Bottom of Detector

Background Analysis – 1MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

Effects of Copper Cryostat

With Cryostat

Without Cryostat

Depth Histograms

Background Analysis – 2.5MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

2D Histogram – Energy vs. Depth

On the rate scale, 0.00019 is the smallest detected rate and 0.006 is the target background rate

Background Rates for Sources of Different Energies

Each source emitted 1 Million Photons,

And has rated activity of

20 Low-Activity PMTs, 6 mBq each

Number of Detected Photons

for Sources of Different Energies

Each source emitted 1 Million Photons

1MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

Detectors of Different Depths

20cm, 30cm and 40cm

2D Histogram – Energy vs. Depth

1MeV Gamma Source

20 Low-Activity PMTs, 6 mBq each

Detectors of Different Depths

20cm, 30cm and 40cm

Depth Histograms

Conclusions

- The top 10 cm of the detector present high background rates and can be eliminated to bring the overall background below the target rate.
- The outer layers of the detector have higher background due to the backscattering of particles off the cryostat. Two courses of action can reduce this background – increasing the radius of the detector and/or filtering out the events in the region closer to the cryostat.
- The thickness of the detector does not significantly affect the background rate at all depths in the detector. The advantage of adding more Liquid Xenon to the bottom of the detector is the increase in volume of the fiducial region with very low background.

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