Instrumentation for radioactivity
Instrumentation for radioactivity is of many types, due to different applications (e.g., analysis vs. safety), needs for portability, and the intensity and types of expected radiation. Instruments need to measure: [1]
Different types are needed, variously, for:
- Health risk to individuals
- Analysis of nuclear materials that emit alpha particles
- Tritium survey
Detector technology
Instruments operate by one of two general principles: ionization or excitation.[2]
Ionization
Portable ionization chamber
One of these instruments is a small, air-filled container in which a quartz fiber is suspended, with a microscope that allows the shadow of the fiber to be read against a graduated scale. When the instrument is initialized, the fiber is charged to 200 volts, causing it to read a cumulative radiation exposure of zero. As the device is struck by ionizing radiation, the ions created in the air cause the fiber to discharge.
Some are direct, or self-reading, while others are indirect, or nonself-reading. There is also a variety of pocket ionization chambers that read at different rates (0.01-200 mR and 1-500 R). Pocket ionization chambers, primarily measure whole body gamma exposure (with some x-radiation). According to the Federal Emergency Management Agency, the advantages of the devices are:[3]
- cumulative exposure can be read at any
time or location without ancillary equipment, by the user
- The chambers are reusable by simple electrical recharging
- Long shelf-life with little to no
maintenance requirements; sealed at the time of manufacture and are relatively insensitive to environmental conditions.
- Measure gamma exposure accurately.
Major disadvantages include:
- "The exposure readings on the devices may be
sensitive to a significant mechanical shock (for example, if dropped more than a few feet to a concrete surface).
- The initial cost of a pocket dosimeter is high."
- They do not measure neutron, beta, X-ray, or alpha accumulation
Excitation
The basic field survey instrument that can detect alpha particles is an scintillometer, such as the AN/PDR-77, which comes with a set of probes variously intended for alpha, beta/gamma, and low-energy X-ray radiation. The X-ray probe allows detection of plutonium and americium contamination. "Knowing the original assay and the age of the weapon, the ratio of plutonium to americium may be computed accurately and so the total plutonium contamination may be determined. [4]
Field applications
Health
Yet another set of instruments are used to measure health risks to individuals. These include portable ionization chambers, film badges, and thermoluminescent personal dosimeters.
Tritium survey
Different instruments, such as the AN/PDR-73 or AN/PDR-74, are used for tritium survey.
Alpha survey
Analysis
There are limits to what can be determined with portable equipment. For more complex analysis, either a transportable laboratory needs to be brought to the site, or, if safety permits, to have representative samples taken to a laboratory. Analysis of radioactive trace elements, for example, can help identify the source of fuel for a given contamination incident. Some of the less portable,, but powerful instrumentation includes gamma spectroscopy, [[low background alpha and beta counting and liquid scintillation counters for extremely low energy beta emitters such as tritium.
The DoD directive makes the distinction clear that detection is harder than measurement, and the latter is necessary for MASINT:
Nuclear radiation is not easy to detect. Radiation detection is always a multistep, highly indirect process. For example, in a scintillation detector, incident radiation excites a fluorescent material that de-excites by emitting photons of light. ... the quantitative relationship between the amount of radiation actually emitted and the reading on the meter is a complex function of many factors. Since those factors may only be controlled well within a laboratory. Such a laboratory either must be moved to the field, or samples brought to a fixed laboratory.[4]
Detectors based on semiconductors, such as germanium, have better intrinsic energy resolution than scintillators, and are preferred where feasible for gamma-ray spectrometry. Neutron detection is improved by using hydrogen-rich scintillation counters, such those using a liquid rather than a crystal scintillation source.
References
- ↑ Office of the Assistant to the Secretary of Defense for Nuclear and Chemical and Biological Defense Programs (February 22, 2005). Nuclear Weapon Accident Response Procedures (NARP).
- ↑ Measuring Radioactivity, Integrated Environmental Management, Inc.
- ↑ Emergency Management Agency, Radiological Emergency Response Independent Study, Federal Emergency Management Agency, IS 301, pp. 4-7 to 4-8
- ↑ 4.0 4.1 United States Department of Defense, DoD 3150.8-M, "Nuclear Weapon Accident Response Procedures (NARP)"