determine to what effect possible radioactivity in that part would have on the detectors of the PC. Each part inside of the passive shielding is simulated for possible 238U and
232Th activity. Additionally, parts made out of OFHC Cu and SS are simulated for
possible60Co activity and the masses of solder in the temperature sensor assemblies are simulated for possible 210Pb activity. The inner cavity volume of the PC is filled with
nitrogen gas in the MaGe geometry and is only simulated for possible222Rn activity.
When simulating a single part for a single nuclide, the location of the primary vertex of the radioactive nuclide is randomly placed within the volume of the part and the nuclide is allowed to decay. Regardless of whether a particle from the decay deposits energy in a detector, this constitutes a single event. For a typical part in the PC, thousands to hundreds-of-thousands of events must be simulated for each part and for each nuclide to gather enough statistics for the detector’s resulting energy spectrum. However rather than simulate each individual part of the PC, parts that are made of the same material, and are thus expected to have the same levels of radioactivity, are grouped together and simulated as one. For example, there are 24 cryostat clamping bolts in the PC geometry. In the PC these bolts are all made of SS and therefore
should have roughly the same 238U, 232Th and 60Co activities. Furthermore each bolt
is roughly the same radial distance from each detector and therefore all of the bolts should have a similar effect on the detector’s energy spectrum and count rate. So rather than simulate each individual bolt on its own, the 24 bolts are grouped together and simulated as a whole. The feature to simulate a group of components was added to the MaGeframework as part of the work to create the PC background model. Therefore, the details on how the radioactive nuclides are distributed throughout the group of components is further explained in this work and can be found in Chapter A.4.
The groups used for the PC background model simulations can be found in Ta- ble 5.1. Table 5.1 also lists which radioactive nuclides are simulated for each group: “U” indicating 238U, “Th” indicating 232Th, “Co” indicating 60Co, “Rn” indicating
222Rn and “Pb” indicating210Pb. The masses reported in Table 5.1 are the total mass
of the entire group as calculated by MaGe. As seen in Table 5.1, the copper parts for a string are split into two groups: one for the UGEFCu components of the string and one for the OFHC Cu components of the string. (Detailed lists of which parts in the strings are made of UGEFCu and OFHC Cu can be found in Tables C.6– C.11.) The same is generally true for the copper cryostat components, however the Cryostat Top Lid and Cryostat Bottom Lid are dealt with separately from the other copper components. This is due to the fact that the lids were fabricated via metal spinning and there is no known assay on the process; hence they are put into their own groups.
Table 5.1: The groups used for the PC background model simulations. The “Material” column refers to the actual material of the part(s) in the PC. The “Part(s)” column lists the parts in each group; the part names are the same as those found in Tables C.1–C.4. The “Total Mass” column is the total mass of the entire group. (For the masses of the individual parts see Tables C.1–C.4.)
Group Nuclides Material Part(s) Total Mass [kg]
Simulated
1 Rn Nitrogen Gas Inner Cavity Volume 0.292
2 U/Th/Co OFHC Cu Thermosyphon Mount Plate 5.05
Thermosyphon Tube
Thermosyphon Hoop Adapter Thermosyphon Cold Plate Adapter Thermosyphon Bolts (×6)
3 U/Th/Co OFHC Cu Cryostat Hoop 32.1
Cross Arm Tube
Cryostat Clamping Rails (×16) Thermal Shield Annulus
ColdPlate
4 U/Th/Co OFHC Cu Cryostat Top Lid† 7.01
5 U/Th/Co OFHC Cu Cryostat Bottom Lid† 21.2
6 U/Th/Co OFHC Cu String 1 0.516
7 U/Th/Co OFHC Cu String 2 0.299
8 U/Th/Co OFHC Cu String 3 0.753
9 U/Th UGEFCu Thermal Shield Can 3.23
10 U/Th UGEFCu String 1 0.285
11 U/Th UGEFCu String 2 0.033
12 U/Th UGEFCu String 3 0.172
13 U/Th/Co SS Cryostat Clamping Bolts (×24) 0.259
14 U/Th/Co SS Thermal Shield Screws (×14) 2.21·10−2
15 U/Th/Co SS Temperature Sensor Screw for S1D1 2.99·10−4
16 U/Th/Co SS Temperature Sensor Screw for S1D4 2.99·10−4
17 U/Th/Co SS Temperature Sensor Screw for S2D1 2.99·10−4
18 U/Th/Co SS Temperature Sensor Screw for S3D1 2.99·10−4 19 U/Th/Co SS Temperature Sensor Screw for S3D5 2.99·10−4 20 U/Th Si-Bronze Cryostat Clamping Nuts (×24) 6.77·10−2
21 U/Th NXT-85 String 1 HV Nuts (×12) 8.60·10−3
22 U/Th NXT-85 String 2 HV Nuts (×3) 2.16·10−3
23 U/Th NXT-85 String 3 HV Nuts (×15) 10.8·10−3
24 U/Th NXT-85 String 1 Crystal Insulators (×12) 14.9·10−3 25 U/Th NXT-85 String 2 Crystal Insulators (×3) 2.74·10−3 26 U/Th NXT-85 String 3 Crystal Insulators (×15) 13.7·10−3
27 U/Th NXT-85 String 1 Center Bushings (×4) 5.92·10−4
28 U/Th NXT-85 String 2 Center Bushings (×1) 1.48·10−4
29 U/Th NXT-85 String 3 Center Bushings (×5) 7.38·10−4
30 U/Th PEEK Temperature Sensor Clamp for S1D1 5.13·10−4 31 U/Th PEEK Temperature Sensor Clamp for S1D4 5.13·10−4 32 U/Th PEEK Temperature Sensor Clamp for S2D1 5.13·10−4
33 U/Th PEEK Temperature Sensor Clamp for S3D1 5.13·10−4
34 U/Th PEEK Temperature Sensor Clamp for S3D5 5.13·10−4
35 U/Th Silica with String 1 LMFE Substrate (×4) 4.22·10−4
Gold Traces String 1 LMFE Traces (×4)
36 U/Th Silica with String 2 LMFE Substrate (×1) 1.06·10−4 Gold Traces String 2 LMFE Traces (×1)
37 U/Th Silica with String 3 LMFE Substrate (×5) 5.28·10−4
Gold Traces String 3 LMFE Traces (×5)
38 Rn/Th/Pb Solder Temperature Sensor Solder for S1D1 0.201·10−4 39 Rn/Th/Pb Solder Temperature Sensor Solder for S1D4 0.201·10−4 40 Rn/Th/Pb Solder Temperature Sensor Solder for S2D1 0.201·10−4
41 Rn/Th/Pb Solder Temperature Sensor Solder for S3D1 0.201·10−4
42 Rn/Th/Pb Solder Temperature Sensor Solder for S3D5 0.201·10−4