This section contains the following information:

Topic

Portable Survey Meters

Laboratory Counters

Instrument Characteristics

Portable Survey Meters

Performing radiation level and contamination surveys is an important aspect of the overall radiation safety program. These are generally performed using portable survey meters.

- Surveys for Radiation Levels

Radiation level surveys can be performed using two types of detectors:

Ion Chambers; and

Energy Compensated Geiger-Mueller (GM) Detectors

The meter reading for these surveys must be in units of mR/hr.

- Surveys for Contamination

Contamination surveys can be performed using two types of detectors:

Sodium Iodide Detectors; and

Thin end-window or pancake GM Detectors

The meter reading for these surveys is typically in units of counts per minute (CPM). Some instruments with built in computers convert the CPM data to disintegrations per minute (DPM) if the detection efficiency is known.

Sodium-Iodide detectors are used to detect gamma emitting radioactive materials. Thin end-window NaI detectors have been developed to detect low energy gamma's emitted from iodine-125. Larger NaI detectors (up to 1 inch thick) and energy compensated GM detectors are used to monitor for higher energy emitters such as chromium-51, iodine-131, or zinc-65.

Thin end-window and pancake G-M detectors are typically used for detecting beta emitting radioactive materials such as carbon-14, sulfur-35, and phosphorus-32.

Very specialized detectors are needed to detect low energy beta emitters such as hydrogen-3 or nickel-65, or to detect alpha emitters such as radium-226.

- Instrument Calibration

Radiation level and contamination surveys must be performed with a properly calibrated instrument. Calibrations must be performed annually or after repair. The instrument does not need to be recalibrated when batteries are changed.

The detection efficiency for the radioisotope being monitored must also be known since the Nuclear Regulatory Commission requires survey results in units of activity or DPM.

Another general point to follow in selection of survey meters is as follows:

Meters having a readout in CPM are acceptable for contamination surveys.

Meters having a readout in mR/hr are acceptable for radiation level surveys.

It is useful to determine the instrument's minimum detectable activity using the guidance in Section 6.2. Some instruments are unable to monitor low levels of contamination with sufficient accuracy to separate the reading from the normal fluctuations in background.

Laboratory Counters

- Liquid Scintillation Counters

Liquid scintillation counters should be calibrated annually using established standards of known activity. The counting efficiency for commonly used radioisotopes should be known to calculate activity from counting data.

The minimum detectable activity of the counter for various isotopes and counting times should also be calculated.

Liquid scintillation counters often contain an external standard reference that contains cesium-137 or radium-226. These sealed sources are the responsibility of the instrument's manufacturer when the instrument is surplused. Either the Permit Holder or the LRPO should list these sources on their inventory.

- Gamma Counters

Gamma counters should be calibrated annually using established standards of known activity. The counting efficiency for commonly used radioisotopes should be known to calculate activity from counting data.

The minimum detectable activity of the counter for various isotopes and counting times should also be calculated.

The calibration standards should be listed either on the Permit Holder's or the LRPO's inventory.

The Technical Bulletin "Determining Lower Limit of Detection (LLD) and Minimum Detectable Activity (MDA) for Radiation Measurements" is included in Section 6.2.

Instrument Characteristics

The following table lists the general types of radiation emitted by radioisotopes in use at USDA facilities and the recommended survey instrument and laboratory counter to be used.

Typical detection efficiencies for commonly used survey probes are also listed.

Table 4.5 - 1: Selecting appropriate instruments
 

Characteristic Radiation	Portable Survey Instrument	Laboratory Instrument
Low Energy Beta H3, Ni63	None Recommended	Liquid Scintillation Counter.
Medium Energy Beta C14, S35, Ca45	Ratemeter with end window or pancake GM detector.	Liquid Scintillation Counter.
High Energy Beta P32	Ratemeter with end window or pancake GM detector.	Liquid Scintillation Counter.
Low Energy Gamma or X-ray I125	Ratemeter with thin NaI(Tl) scintillation probe.	Gamma Counter with semiconductor or scintillation detector. Liquid Scintillation Counter.
Medium - High Energy Gamma Cr51, I131, Cs137, Co60	Ratemeter with thick NaI(Tl) scintillation probe or Energy Compensated GM.	Gamma Counter with semiconductor or scintillation detector.

Typical detector efficiencies and sensitivities for portable survey meters.
 

Detector	Typical Efficiency		Detector	Sensitivity
End-Window GM	5% for C14 5% for S35 30% for P32		Energy Compensated GM	1,200 CPM per mR/hr
Pancake GM	10% for C14 10% for S35 50% for P32		Gamma Scintillator	175,000 CPM per mR/hr
Low-Energy Gamma Scintillator	35% for I125