Answer to Question #532 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

The following question was answered by an expert in the appropriate field:

How do I do effective shielding so that a 10 mm NaI detector can be used to pinpoint a 2-4 mm radioactive area from a distance of 25 cm?

A method for pinpointing a small spot from such a distance using a thin NaI detector coupled to a standard (nonimaging) photo-multiplier tube is the use of a collimator. The collimator will essentially be a small hole passing through a shield in front of the detector. Much like a camera (only without lens effects), the "field of view" for the detector will be limited solely by the diameter and length of the input collimator. The geometry can either be calculated or can be modeled by drawing a scale drawing of the configuration of interest. The trade-offs that must be balanced are shield thickness versus detection efficiency—since a thinner shield will have a wider focal angle and will therefore require a smaller hole to in order to keep the "image" area small at 25 cm. As an auxiliary issue, using such a system will likely be difficult unless some type of location system is used. A simple method would be to attach a small laser diode to the detector and align the focal image points.

Bobby Coleman
Oak Ridge National Laboratory

Answer posted on December 29, 2000. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.