Before the assembly and operation of the Demonstrator, a single test cryostat was built. This cryostat, referred to as the PC, was built to test the clean assembly procedures and data acquistion (DAQ) that are to be used for the Demonstrator. The PC contained three strings with a total of ten natural germanium detectors. Of the ten detectors, eight were modified-Broad Energy Germanium (BEGe) detectors from CANBERRA; these are the same type of natural detectors used in the Demonstrator.
The other two detectors were larger-mass ORTECrdetectors. ORTECr has fabricated
the enriched detectors for the Demonstrator and initially fabricated two detectors similar in size to the enriched detectors but made from natural germanium; these were the two ORTECr detectors in the PC.
The work in this thesis focuses primarily on the PC and therefore a naming con- vention is used to discuss the individual detectors and strings. The strings are referred to as Strings 1, 2 and 3. String 1 (S1) holds four detectors: two BEGes and the two ORTECr detectors. In Fig. 1.9 it is the string pictured in the background to the left.
String 2 (S2) holds one BEGe detector and is in the background to the right in Fig. 1.9. String 3 (S3) holds five BEGe detectors and is in the foreground in Fig. 1.9. The de- tectors in the strings are numbered in increasing value as one moves away from the coldplate, with SxD1 being the detector closest to the coldplate. As an example, in Fig. 1.9, S3 is in the foreground and its top detector is S3D1 while its bottom detector is S3D5. Seven of the ten detectors are used in the analysis presented here. The status and mass of each detector of the PC can be found in Table 1.3.
Table 1.3: The masses of the PC detectors.
Detector Mass [g] Status
S1D1 631 NOT included; unstable gain S1D2 633 Included in analysis
S1D3 904 Included in analysis S1D4 1013.5 Included in analysis
S2D1 644 NOT included; unstable gain S3D1 622 Included in analysis
S3D2 646 Included in analysis
S3D3 630 NOT included; no HV connection S3D4 631 Included in analysis
S3D5 627 Included in analysis
Figure 1.9: The three strings of the PC. In the foreground is String 3 which holds five detectors. In the background to the left is String 1 which holds four detectors. In the background to the right is String 2 which holds one detector. The detectors in the strings are numbered in increasing value as one moves away from the coldplate, with SxD1 being the detector closest to the coldplate.
the PC is designed to mimic the Demonstrator as much as reasonably possible. However there are several differences between the PC and the Demonstrator. While achieving the lowest possible background is the goal of the Demonstrator this was not necessarily true of the PC. Therefore some modifications were made that sacrificed the ultra-low background for cost and scheduling purposes. For example, while the PC uses the same low-mass detector and string designs as the Demonstrator, many of the copper parts are made from OFHC Cu rather than the cleaner UGEFCu that are being used in the Demonstrator. As another example, the PC is located in the compact shield at the 48500 level at SURF that is to be used for the Demonstrator, however the shielding was not entirely complete during the time that the PC was being operated. The following is a complete list of the important differences between the PC and the Demonstrator.
1. Temperature Sensor AssembliesFor testing purposes, five temperature sen- sors were installed in the PC (and are not installed in the Demonstrator). The temperature sensors were soldered to their cabling. A clamp made of Polyether Ether Ketone (PEEK) and a stainless steel screw were used to clamp the sen- sor to the string to monitor temperature stability and cooling. The temperature sensors, solder, cabling and SS screws were not assayed. The material PEEK – which is what the clamps were made of – has been assayed and is known to have a relatively high amount of natural radioactivity compared to the preferred polymer, NXT-85, that is being used in the Demonstrator.
2. OFHC CuSeveral parts in the PC were made of OFHC Cu, while their Demonstrator counterparts are made of UGEFCu. Also, the time that the OFHC Cu parts spent
above ground was not tightly controlled and therefore the cosmogenically-induced radioactivity (e.g. 60Co) in the OFHC Cu is expected to be higher.
3. SS Several parts in the PC were made of SS, while their Demonstratorcoun- terparts are made of UGEFCu. These SS parts include some of the cryostat clamping hardware and some of the outer copper shield fasteners.
4. Silicon BronzeSome parts of the PC cryostat clamping hardware were made of silicon bronze, while their Demonstrator counterparts are made of UGEFCu. 5. Metal Spinning The top and bottom cryostat lids of the PC were fabricated via metal spinning. The top and bottom cryostat lids of the Demonstrator were not fabricated this way as there is no known assay on the procedure. 6. Radon Purge The radon purge system was not in its final state and there-
fore higher levels of 222Rn were expected in the inner cavity volume during the
operation of the PC (than for the Demonstrator).
7. Active and Passive Shielding The inner copper shield was not installed in the PC. The poly shield and muon veto were only partially installed. Additional shielding is required where the cross arm tube penetrates the passive shielding and was not installed in the PC. Additionally there was SS hardware in the outer copper shield of the PC.
8. Gasket The PC cryostat was vacuum-sealed with a Viton gasket rather than with a cleaner parylene film that is being used in the Demonstrator.
9. Cables The signal cables in the PC were known to be higher in radioactivity than the cables in the Demonstrator.
10. Thermosyphon Supports The thermosyphon supports were made of PEEK, while their Demonstrator counterparts are made of a cleaner polymer.
11. Detector CosmogenicsUnlike the detectors of the Demonstrator, the time that the detectors of the PC spent above ground was not tightly controlled.
Therefore the cosmogenically-induced radioactivity in the detectors was expected to be higher than for the detectors of the Demonstrator.
Much of the work presented in this thesis attempts to understand the prototype module backgrounds and exactly how the differences between the PC and Demonstrator modules contribute to the backgrounds of the PC. As previously mentioned, all of the materials used in the Demonstratorare assayed and extensive MC simulations have been performed to predict the backgrounds that the materials are expected to con- tribute to the energy spectrum of each detector. A background model for the PC was created using the same tools used to create the Demonstrator background model. By creating a background model of the PC and comparing it to data, the background model of the Demonstrator can be verified and possible future issues can be iden- tified. Creating an accurate background model also requires understanding the energy resolution of each detector. Chapters 3 and 4 look at thoroughly characterizing the energy response function for each detector of the PC. Chapter 5 describes the PC background model created for this work. Chapter 6 compares the PC background model to data. This chapter also discusses the implications for the background model for the Demonstrator and ongoing and future work. The conclusions of this work and the current status of the PC and Demonstrator are discussed in Chapter 7.
CHAPTER 2: DATA ACQUISITION, PROCESSING AND SELECTION