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Office of Homeland Security & Emergency Coordination
Radiation Safety Division (RSD)
United States Department of Agriculture
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Instrumentation

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Purpose

Performing radiation level and contamination surveys is an important aspect of the overall radiation safety program.

Radiation detecting instruments are necessary for the following purposes:

  • package surveys;
  • contamination surveys;
  • dose rate measurements;
  • other specialized air sampling, bioassay, or effluent release measurements.

 

Portable Survey Meters

Radiation level surveys are generally performed using portable survey meters. Survey meters are portable radiation detection and measurement instruments used to check personnel, equipment, and facilities for radioactive contamination, or check external or ambient ionizing radiation fields to evaluate the direct exposure hazard. They are synonymous with scintillation counters and Geiger counters. In USDA, such instruments are usually GM detectors and sodium iodide detectors.

 

Geiger–Müller (GM) Detector

A Geiger–Müller detector is a tube, consisting of electrodes and filled with a low-pressure inert gas and a halogen gas, that measures activity. The walls of the tube form a cathode, while the anode is a wire passing up the center of the tube with a potential difference of several hundred volts, but no current flowing. When ionizing radiation passes through the tube, gas molecules are ionized. The tube's electrodes accelerate the positive ions towards the cathode and the electrons towards the anode. The ion pairs further ionize the gas, creating an avalanche of charged particles. This results in a short, intense pulse of current which passes (or cascades) from the negative electrode to the positive electrode and is measured or counted. These current pulses are processed in a count-rate meter that produces a read-out on a scale, and an audible sound. GM measurements do not relate easily to dose-rate. But GM detectors can be calibrated for dose-rate, when working with specific, known isotopes.

Thin end-window and pancake GM detectors are typically used for detecting beta emitting radioactive materials such as carbon (C-14), sulfur (S-35), and phosphorus (P-32).

 

Sodium Iodide (NaI) Scintillation Detector

A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, or organic liquid (see liquid scintillation counting) that fluoresces when struck by ionizing radiation. A photomultiplier tube measures the light from the crystal. The photomultiplier tube is attached to an electronic amplifier and other electronic equipment to count and quantify the amplitude of the signals produced by the photomultiplier. When a charged particle strikes the scintillator, a flash of light is produced. The association of a scintillator and photomultiplier with the counter circuits forms the basis of the scintillation counter apparatus. A common and useful scintillating material is sodium iodide, with traces of thallium.

Sodium-Iodide (NaI) 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 (I-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 (Cr-51), iodine (I-131), or zinc (Zn-65).

 

Which Survey Meter to Use

With the exception of tritium (H-3), Permit Holders must have a portable survey meter available to monitor for fixed contamination and for radiation levels. This consists of a count rate meter and a detector. Commonly used radioisotopes and appropriate detectors are listed below: 

Use a Geiger-Muller (GM) detector for:

  • Carbon (C-14),
  • Sulfur (S-35),
  • Phosphorus (P-32), and
  • Phosphorus (P-33),

Use a Sodium Iodide (NaI) detector for:

  • Iodine (I-125),
  • Cesium (Cs-137),
  • Chromium (Cr-51),
  • Zinc (Zn-65),

Portable survey instruments must be calibrated yearly by commercial firms or other qualified individuals. Detection efficiencies for radioisotopes used in the laboratory should also be determined. Portable survey instruments should be tested before each use with a small check source to verify proper instrument operation. Instruments shall be recalibrated when failing to respond to a check source or when serviced.

 

Units

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.

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.

Radiation level surveys can be done with a Geiger-Mueller (GM) or scintillation detector that has been calibrated for specific isotope energies. The meter reading for these surveys must be in units of milli-rad/hr.

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

Meters having a read-out in CPM are acceptable for contamination surveys.

Meters having a read-out in milli-rad/hr are acceptable for radiation level surveys.

 

Laboratory Counting Equipment

Contamination wipes are generally counted in large laboratory counters. Such counters are used to evaluate lab survey wipes or leak test wipes. In USDA, such instruments are usually liquid scintillation counters and gamma counters. The calibration should consist of an efficiency determination.

 

Liquid Scintillation Counters

A liquid scintillation counter is a standard laboratory equipment for measuring radiation from beta-emitting nuclides.

Samples are dissolved or suspended in a "cocktail" containing an aromatic solvent (historically benzene or toluene, but more recently less hazardous solvents have come into favor) and small amounts of other additives known as fluors, scintillants, or scintillators. Beta particles emitted from the sample transfer energy to the solvent molecules, which in turn transfer their energy to the fluors; the excited fluor molecules dissipate the energy by emitting light. In this way, each beta emission results in a pulse of light. The samples are placed in small glass or plastic vials that are loaded into the liquid scintillation counter. The counter has a photomultiplier tube that counts the pulses of light as each vial is counted.

Liquid scintillation counters often contain an external standard reference that contains cesium (Cs-137) or radium (Ra-226). These sealed sources are the responsibility of the instrument's manufacturer when the instrument is surplused.

 

Gamma Counters

A Gamma Counter is standard laboratory equipment that measures gamma radiation emitted by a radionuclide. Up to 300 samples are placed in sealed vials or test tubes, and move along a serpentine track on a horizontal plane. One at a time, they move down inside a shielded detector, set to measure specific energy windows characteristic of the particular isotope. Due to the heavy shielding of the detector, a gamma counter often is not portable. In principle, the gamma counter is like a scintillation detector except that a scintillation crystal such as sodium iodide (NaI) surrounds the sample. The gamma rays interact with the crystal, are absorbed, and produce light.

 

Which Laboratory Counting Equipment to Use?

The following table lists the radioisotopes in use at USDA facilities and the recommended laboratory counter to be used.

Use a Liquid Scintillation Counter for:

  • tritium (H-3),
  • nickel (Ni-63),
  • carbon (C-14),
  • sulfur (S-35),
  • calcium (Ca-45),
  • phosphorus (P32).

Use a Gamma Counter or a Liquid Scintillation Counter for :

  • low energy gamma,
  • Iodine (I-125).

Use a Gamma Counter for:

  • Chromium (Cr-51),
  • Iodine (I-131),
  • Cesium (Cs-137),
  • Cobalt (Co-60)

 

Instrument Calibration

Portable survey instruments shall be calibrated yearly by commercial firms or other qualified individuals. Detection efficiencies for radioisotopes used in the laboratory should also be determined. Portable survey instruments should be tested before each use with a small check source to verify proper instrument operation. Instruments shall be recalibrated when failing to respond to a check source or when serviced. For some instruments, it may be useful to determine the “minimum detectable activity” at the time of calibration. Some instruments are unable to monitor low levels of contamination with sufficient accuracy to separate the reading from the normal fluctuations in background.

Laboratory counting equipment shall be calibrated annually by counting standards of known activity. This can be done in the lab. The calibration should consist of an efficiency determination and a determination of the Minimum Detectable Activity for commonly used isotopes. Additionally, calibration, repair, and maintenance can be arranged through the manufacturer or vendor. Factory representatives will typically calibrate and maintain these counters on an annual basis.