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Radiation Alert Ranger – Detector Efficiency Statement for Cs-137 and All Built-In Isotopes

S.E. International, Inc. (SEI) is the original equipment manufacturer (OEM) of the Radiation Alert Ranger, a hand-held GM-based radiation detection instrument. This document provides the factory-programmed detector efficiency values for the Ranger, explains the measurement geometry under which those values were determined, and describes the procedure a customer may use to determine or verify efficiency independently using a known source and the Radiation Alert Wipe Test Plate.

1. Instrument and Detector Description

The Radiation Alert Ranger uses an internal halogen-quenched, uncompensated GM tube with a thin mica end window. The mica window has an areal density of 1.4 to 2.0 mg/cm2 and an effective window diameter of 45 mm (1.77 inches). The instrument is calibrated and its exposure rate modes are optimized for Cs-137. The energy sensitivity for Cs-137 is 3,340 CPM/mR/hr. Instrument accuracy is typically +/-15% from the factory and +/-10% with NIST source calibration.

2. Measurement Geometry

All factory-programmed efficiency values listed in this document were determined using the Radiation Alert Wipe Test Plate (WTP) geometry. The Wipe Test Plate positions the sample planchet at a fixed, reproducible distance of 1 centimeter from the end window of the GM detector. This geometry must be replicated when using the built-in efficiencies to calculate activity. Using a different source-to-detector distance or geometry will produce inaccurate activity results.

Efficiencies are geometry specific. Results obtained without the Wipe Test Plate in position, or with the source positioned other than in the planchet depression, are not covered by these values and will require independent efficiency determination.

3. Factory-Programmed Efficiency Values

The following efficiencies are programmed into the Ranger at the factory and are accessible via the instrument menu (Menu > Pick Efficiency) or through the Observer USB Software Calibration Panel. Values are expressed in counts per disintegration (c/d) and as a percentage.

4. Cs-137 Efficiency – Additional Context

Cs-137 decays primarily by beta emission (Ebeta-max = 512 keV, 94.6% abundance) with an accompanying 662 keV gamma photon (85.1% yield via Ba-137m). The factory efficiency value of 0.2078 c/d (20.78%) for the Ranger reflects the combined detector response to both the beta and gamma components at the wipe test plate geometry (source-to-window distance of 1 cm). The large pancake GM window of the Ranger (45 mm effective diameter) contributes to this relatively high efficiency compared to smaller window detectors.

Accuracy of the Cs-137 efficiency is typically +/-15% from the factory. For applications requiring tighter accuracy, SEI offers NIST source calibration, which reduces the accuracy specification to +/-10%. Contact SEI to arrange NIST calibration.

5. Procedure for Customer-Determined Efficiency Using the Wipe Test Plate

Customers who require a site-specific or independently verified efficiency value may determine it using the procedure below. This is appropriate when using a source geometry or activity level that differs from factory conditions, or when regulatory requirements specify a site-determined efficiency.

Required Materials:

Radiation Alert Ranger with Wipe Test Plate attached, a calibration source of known activity deposited on a planchet (activity certified and decay-corrected to the date of measurement), and a means to record count data.

Example: If R(net) = 2,078 CPM and A = 10,000 DPM, then E = 2,078 / 10,000 = 0.2078 c/d (20.78%). Both the count rate and activity must use the same time base (CPM and DPM, or CPS and Bq).