Implementation of a Portable HPGe for Field Contamination Assay

Area of Science:

• Nuclear instrumentation
• Radiation detection and measurement
• Environmental monitoring

Background:

• Portable high purity germanium (HPGe) detectors are crucial for field radiation measurements.
• Accurate modeling of HPGe detectors is essential for reliable contamination assays, especially in challenging environments.
• Existing methods may struggle with large-area or low-count radioactive contamination assessments.

Purpose of the Study:

• To develop and validate a Monte Carlo N-Particle (MCNP) model of a portable HPGe detector.
• To enable operational assay of radioactive contamination over large areal distributions.
• To assess the detector model's performance for low-count events and its detection limits.

Main Methods:

• Detector model construction using MCNP based on x-ray images and check source responses.
• Detector parameter calibration using large count spectra.
• Validation of the model against known responses and assessment of error distributions.

Main Results:

• A validated MCNP model of the HPGe detector was successfully created.
• Rigorous reproducibility was achieved for high activity measurements.
• The model allows for contamination assay in large areas, like the Waste Isolation Pilot Plant (WIPP) exhaust shaft, with results within a factor of 2 accuracy.
• Detection limits were found to be well below levels that would permit legal environmental release.

Conclusions:

• The MCNP detector model provides a reliable tool for operational assay of widespread radioactive contamination.
• The model demonstrates accuracy and sensitivity suitable for environmental monitoring and regulatory compliance.
• This approach enhances the capability to measure contamination in previously inaccessible or difficult-to-assess large areas.