The $^{16,17}$N Source Chamber Geometry

The major features listed below have been included in the Monte Carlo simulation, and are illustrated in Figure 12.2. Nine detector elements are defined (the detector element media and numerical priority is presented following the region description):

• CAN - outer container (stainless steel; priority = 540)
• RING - annulus which divides the interior of CAN (stainless steel; 541)
• OPT - optical coupling (acrylic; 550)
• GAP - lower chamber gap between CAN and SCINT (air; 543)
• SCINT - plastic scintillator (dark acrylic; 544)
• GAS - volume within scintillator (air; 545)
• INSTR - upper chamber volume (air; 543)
• PMT_E - exterior of PMT (aluminum; 544)
• PMT_I - interior of PMT (vacuum; 549)

Figure 12.2: The geometry regions of the $^{16}$N source chamber.

The detector elements are embedded in the following manner. The outer container (CAN) has only the annulus (RING) as an interior region. There are three elements embedded within RING: the optical coupling (OPT), the lower chamber volume (GAP) and the instrumentation volume (INSTR). OPT is also embedded within INSTR and GAP to facilitate the handling of the common boundaries. The plastic scintillator (SCINT) is embedded within GAP, and the gas volume (GAS) is embedded within SCINT. Likewise, the exterior of the PMT (PMT_E) is embedded within INSTR and the interior of the PMT (PMT_I) within PMT_E.

The simulation of the source chamber is not complete to the level of including all the nuts and bolts. In fact, while all of the major features are present, there have been simplifications. The actual geometry of the region between the upper and lower chambers is quite complex. For example, the optical coupling is treated as a single, solid cylinder of acrylic, rather than the scheme featuring optical pads and acrylic. Indeed, mechanical fasteners, gas line couplings and the high-voltage converter have all been omitted. These components were judged to have minor influence on the attenuation of $\gamma$-rays escaping the lower chamber. In addition, simulations did not attempt to include the presence of the manipulator, nor any hardware necessary to couple the source chamber to the manipulator, unless specifically noted. The effect of such components on the $^{16}$N Nhit spectrum will be studied in future simulations as the design of this hardware is finalized.

The source chamber geometry is intended to make use of all detector elements. If a simpler model of the source chamber is desired, one must work from the outside-in, beginning with CAN. Carefully refer to the description of the relationship between embedded regions when choosing which to disable or enable. The effect of a specific region may also be investigated by setting the media to D$_2$O or to air, as one desires. The source may be operated in the following regions:

D$_2$O

H$_2$O

ACRC_IVL (chimney inner region, D$_2$O volume)

PAN_ZONE (PMT panel zone)

HEX (PMT empty hex cells)

ABS_SKIRT (between PMT panels))

The $^{16,17}$N source chamber geometry is compiled as part of the standard SNOMAN executable program, however, it is not a default feature in the geometry. A command file (calibration_16n.cmd) is read to enable the source chamber in the geometry. Additionally, the default detector element media and the source location may be modified within this command file. A script has been added to the standard set of SNOMAN tools (run_16n.scr) to provide for position specification on the command line at the time of program execution. The script will translate but not rotate the source, it will call calibration_16n.cmd and may be edited to include other command files.