Características y ventajas propias de la figura

There are known inaccuracies with the PC geometry in MaGe; they are listed below. Most of the inaccuracies are a result of missing components and in some of these cases the contribution that these missing components are expected to make to the PC background can be estimated from similar components in the PC and Demonstrator background model. The inaccuracies with the PC background model are listed below by order of importance.

1. GasketThe Viton gasket is not in the PC geometry and therefore is not included in the PC background model. The PC cryostat is vacuum-sealed with two Vi- ton gaskets rather than with the cleaner parylene film that is being used in the Demonstrator; therefore their contribution to the PC background is higher than what is expected for the Demonstrator.

The gaskets are expected to contribute 56 cts/ROI/ton/yr (as detailed in Sec- tion A.3.1). Of all the inaccuracies listed, the gaskets are expected to contribute the most to the PC backgrounds.

2. Passive Shielding and Cavern The passive shielding and cavern are not in- cluded in the PC background model; this includes the outer copper shield, lead shield, radon purge box, liquid nitrogen, muon veto panels, concrete walls, floors and cavern. Furthermore there is SS hardware in the outer copper shield that is not included in the PC background model.

The outer copper shield is expected to contribute 6.88 cts/ROI/ton/yr (as de- tailed in Section A.3.2).

The contribution from the lead shielding is assumed to be the same as for the Demonstratorbackground: 0.627 cts/ROI/ton/yr. This rate is found from the Demonstrator background model, where both the inner and outer Cu shields

are installed. However since the inner Cu shield is not installed in the PC the rate at which the lead shield contributes to the PC background will be slightly higher.

3. CablesThe signal and HV cables are not included in the PC background model. The PC cable geometry in M_aG_e is outdated; so much so that certain portions of cable conflict with other components in the PC geometry. Therefore several portions of the cable geometry have been removed from the PC geometry, and no simulations have been done with what is remaining. The signal cables in the PC are known to be higher in radioactivity than the cables in the Demonstrator and therefore their contribution to the PC background is higher than what is expected for the D_emonstrator.

The cables are expected to contribute 1.4 cts/ROI/ton/yr (as detailed in Sec- tion A.3.3).

4. Components On Top of the ColdplateThe signal connectors sitting on top of the coldplate are not included in the PC background model. Additionally, there are two temperature sensor assemblies above the coldplate that are not included in the PC background model.

The contributions from the connectors are assumed to be the same as for the D_emonstrator background: 0.299 cts/ROI/ton/yr. However this count rate is found using outdated connectors; several modifications have been made to the components that sit on top of the coldplate and work is ongoing to include these updates into the Demonstratorbackground model. One main difference is that the collective mass of the (current) components is higher than that used in the Demonstrator background model. Furthermore, for the PC, two temperature sensors are above the coldplate; the temperature sensors are known to be high

in radioactivity (see Chapter 6) and therefore the expected contribution of 0.299 cts/ROI/ton/yr is most likely an underestimate.

5. Calibration System The calibration system is not included in the PC back- ground model. At the time of this work the geometry of the calibration system was still being implemented into the PC and D_emonstratorgeometries. There- fore it is not included in the PC geometry, even though it is present in the actual experimental setup. The contribution from the calibration system is assumed to be the same as for the Demonstrator background: 1.3·10−3 cts/ROI/ton/yr. 6. Coldplate During the commissioning of the PC the geometry of the coldplate

was changed. This is not reflected in the PC geometry in MaGe. As a result, the shielding of detectors from what sits about the coldplate is different in the M_aG_egeometry than from the actual module.

7. Temperature Sensor AssembliesThe geometry and location of the tempera- ture sensor assemblies is approximated based on pictures taken in the lab while the PC was being commissioned. Because the components are small and located very close to the detectors, a small change in the geometry or location could cause a drastic change in how the detectors are shielded from the assembly. Fur- thermore, the PC background model does not include the temperature sensors themselves nor the masses of Kapton tape that are located at roughly the same position as the sensors. It is assumed that any activity in the sensor itself (and/or the Kapton tape) can be distributed among the other components that clamp the sensor to the string (i.e. the solder, sensor clamp and screw).

8. Cable Guides The cable guides are not included in the PC background model. Given that the cable guides are attached to the hollow hex rods it is assumed that any activity in the cable guides can be distributed among the hollow hex

rods and other nearby string parts.

9. Tie Rod Split Nuts for S1 and S3 The tie rod split nuts for strings 1 and 3 are not included in the PC background model. Their total mass is 13.56 g. It is assumed that any activity in the nuts can be distributed among the tie rods and other nearby string parts.

10. Thermal Shield Supports and Wedges The thermal shield support and wedges are not included in the PC background model. Their total mass is roughly 112.5 g. It is assumed that any activity in the supports and wedges can be dis- tributed among the thermal shield.

11. HV RingsIn the current geometry, there is only one geometry for the HV ring and it is used for all the detector units. This is not accurate; the HV rings for S1D3 and S1D4 are incorrect in the current PC geometry.

12. Material Composition of Silicon Bronze The cryostat clamping nuts are made of silicon bronze. In the PC geometry their composition is considered to be 97% copper and 3% silicon. However there are known impurities in similar materials; most notably, some sources of silicon bronze are reported as containing up to 0.5% lead [SiB].

13. Unknown if OFHC Cu or UGEFCuAs detailed in Sections 5.4 and 6.3, com- ponents are grouped during simulations and when comparing the PC background model to data. Components are only grouped together if they are expected to have the same activity; therefore the components made of OFHC Cu are grouped separately from those made of UGEFCu. The material and history of all parts in the Demonstrator can be found in the Majorana Parts Tracking Database (PTDB) [Abg15]. The PTDB was still under development during the building

and installation of the PC and therefore the material of several copper parts in the PC is unknown. It is assumed that these parts are made of OFHC Cu. There- fore some parts may be incorrectly considered to be OFHC Cu when they are in fact made from UGEFCu. This does not affect the simulations; the isotopic com- position and density of the two materials are the same in the M_aG_e materials database, so for simulation purposes they are essentially the same. However this does affect how the components are grouped and therefore how the simulations are compared to data (Chapter 6).

5.4 Component Grouping in the Prototype Cryostat Background Model