Automatic Petri Dish Sample Changer For Compton Suppressed HP-Ge Detection System

Photo of Elan Herrera, Chris Hoffman, James Tonthat, Sam Woelke Students: Elan Herrera, Chris Hoffman, James Tonthat, Sam Woelke

Sponsor: UT Austin Nuclear Engineering Teaching Laboratory

Date: Spring 2013

Requirements:
The new sample changer system is required to be simple, accurate and reliable. The changer is required to queue a minimum of 20 Petri dishes (standard 60mm O.D.) while maintaining a sample transient time of under 1 minute. The positioning accuracy is limited to within 5 mm of concentric alignment with the detector aperture. The sample must sit at a height of 10 mm or less from the detector film with a maximum of 2 mm thickness for the supporting sample holder. The budget is $30,000, including prototyping, the sample changer system and the steel support structure.

Problem:
The Nuclear Engineering Teaching Laboratory (NETL) requested the design of a new, automatic sample changer for a high-purity germanium detection system. The sample changer will handle Petri dishes containing naturally occurring radioactive material (NORM) found in geological samples. These samples come from oil and natural gas drilling sites, and they are used to verify the radiation levels at the work site are below the regulated limits.

Solution:
The team designed the automatic sample changer system to meet all of the specifications while ensuring the detector will remain safe through the extent of operation and possible accident scenarios. The system is based on a series of interlocking subassemblies, which then attach to the underside of the steel structure. The sample magazine is a gravity-fed system designed for 20 samples, but is expandible. The transport track utilizes a Velmex XSlide linear screw drive to move the sample into position with a balance of speed and accuracy. The u-channel bridge design ensures the sample travels straight and will not exit the track on its own. The transport claw is a thin, notched, low-density polyethylene part that acts as a fail-safe mechanism in case of detector collision. Likewise, the Velmex controller has inputs from adjustable limit switches, and a magnetic reed home switch, to ensure the system operates as intended. While the transport track mounts to the detector shroud for loading support, the shroud also offers protection for the detector arm while in the raised, detection position. The total budget is roughly $6,000 for materials and manufacturing. The team will be working in conjunction with NETL over Summer 2013 to fabricate, assemble and test the system and its controller.

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