What are Radioisotope Identification Devices (RIIDs)?
Radio Isotope Identification Devices (RIIDs) are tools designed to identify radioactive materials by measuring the energy of emitted gamma rays. Law enforcement, customs and other personnel are being equipped with RIIDs to prevent illegal movement of radioactive material. When radiation sources are detected by screening devices such as radiation gate monitors, the RIID is used to determine if the radioactive source constitutes a high-level threat. Radiation first responders, firefighters, and other responders also use the RIID to assess the situation during radiation emergencies.
How do RIIDs work?
Most radioisotopes emit gamma rays with specific energies. The gamma ray emitted by a radioactive source hits a detector inside the RIID and is converted into a signal indicating the energy of the incident gamma ray. The number of gamma rays at each energy is counted and plotted against the energies in an energy spectrum showing characteristic energy peaks.
The identification is based on matching the peaks in the spectrum with known peaks and the gamma emitter peak ratio. This matching is done using proprietary isotope recognition software, which is a key component of these devices. The two main features of RIID are sensitivity and power resolution.
– Resolution is a measure of how close two energy peaks can be and still be distinguishable; The lower the percentage resolution, the better the detector’s ability to distinguish two or more closer peaks.
– Sensitivity is a measure of how effectively incident gamma rays are detected; this determines the counting time required to obtain the spectrum.
These features are a function of the size and material type of the RIID detector.
Probe types:
The first is an efficient, robust and relatively inexpensive Sodium Iodide (NaI) detection system. However, its recognizability is limited.
A more sophisticated detector is the high-purity, solid-state germanium (HPGe) detector system which exhibits considerable discriminating power and can distinguish hundreds of different gamma-ray energies in a single spectrum. The HPGe system has about 50 times better resolution than the NaI system. Resolution is the ability to distinguish one nucleotide from another of similar energy.
Feature
The handheld RIID is battery powered, has built-in software for spectrum analysis, and is capable of identifying the most common radioisotopes encountered by emergency responders. Radioisotopes are divided into four groups:
– Special nuclear material (SNM), plutonium, highly enriched uranium (HEU) and neptunium, which can be used in nuclear weapons;
– Medical isotopes (used in radiotherapy and medical imaging);
– Industrial isotopes (used in weld testing equipment, civil engineering equipment, food irradiators), and
Natural radioactive substances (NORMs), commercial products such as ceramics and fertilizers containing radioactive elements such as potassium, uranium, thorium and radium.
Some RIIDs also contain neutron detectors, which can enhance the detection of neutron-emitting SNM isotopes. The RIID shall be designed in accordance with ANSI Standard N42.34 (2006) or ASTM Standard C1237-99 which defines a minimum acceptable level of sensitivity to neutrons and gamma radiation.
Many RIIDs are equipped with radiation dose-ratio meters, which can be set to alarm to alert the user that they are approaching significant sources of radiation.
However, this device still has limitations. Radioactive sources can be shielded so that gamma or neutron radiation falls below the RIID detection limit or the spectrum is greatly distorted. For example, about 1 inch of lead will reduce emissions from plutonium by about 1000, and 0.25 inch of lead will reduce emissions from HEU by the same factor. The power calibration of some RIIDs may be affected by temperature changes such as movement between indoors and outdoors, which may lead to false determinations. Some devices contain a built-in test source for recalibration. Isotope recognition software currently used in RIID may misidentify the radioactive source even when the data collected using the RIID has been properly calibrated.
