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:

• Alpha particles
• Beta particles
• Neutrons
• X-rays
• Gamma rays

Detectors are most likely to respond to the most energetic gamma rays, although some detectors are more selective, such as a zinc sulfide scintillation counter that responds only to alphas. A key, however, to measuring specific radiation types is shielding, such as subtracting, from the total radiation, that which is measured through a shield that only admits gammas, giving beta and alpha.

Different types are needed, variously, for:

• Health risk to individuals
• Analysis of nuclear materials
• Tritium survey
• Mineral prospecting

Detector technology

Instruments operate by one of four general principles:

• Ionization detectors: Direct effects of radiation on the conductivity of electrically active material, such as Geiger-Mueller counters, portable ionization chambers, proportional counters
• Excitation detectors: Indirect effects of ionizing radiation produce visible light that is measured: thermoluminescent dosimeters, scintillometers
• chemical: photographic film badges
• Other: activation foil, Cerenkov detector, bioassays, mass spectrometry

Ionization detectors

In ionization detectors, the incident radiation creates ion pairs in the detector. Ionization detectors are common in measuring radioactivity, but smoke detectors also make use of ionization.

The substance to be ionized can be either gas (most common) or solid (semi-conductors). Gas filled chambers can be operated as either ion chambers, proportional counters, or Geiger- Mueller (GM) tubes. A typical solid ionization detector is a germanium-lithium (GeLi) detector used in a multichannel analyzer Ionization detectors may be characterized as intended for personal, survey, or laboratory use, and if they use gaseous or solid detectors.

Portable ionization chamber

A gas-filled personal exposure instrument, the portable ionization detector 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).

Advantages With direct-reading instruments, 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 storage conditions. Measure gamma exposure accurately.

Disadvantages "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," although it is reusable They do not measure neutron, beta, alpha or some X-ray accumulation

Geiger-Mueller

Geiger-Mueller counters are ionization detectors built around a gas-filled metal tube, with a wire charged to approximately +1000 volts down the center of the tube. When charged particles, such as ions or electrons from outside the tube, or X-rays or gamma rays that hit the outside of the tube and cause an electron to be emitted from the inside of the tube, the particles are attracted to the anode in the center. As particles traverse, the moving particles collide with more atoms in the gas, until the resulting "avalanche" produces sufficient electron flow to generate a measurable current pulse.

Counter circuitry counts all pulses alike, with no differentiation by their energy or type other than they have enough energy to penetrate the tube. In practical counters, various shields or windows go around the tube to screen out less energetic particles; multiple tubes with different shielding can allow differentiation.

Advantages They offer immediate reading of the current rate With filters, they can give some analysis

Disadvantages The instrument is much larger than pocket units It requires a high-voltage supply, usually heavy batteries Tubes can be fragile

Smoke detector

Not all smoke detectors use ionization, but that is the most common method. In an ionization-based detector, an ₂₄₁americium source emits alpha particles that ionize air molecules in a chamber, giving a known current flow. The ₂₄₁Am is"embedded in a gold foil matrix...made by rolling gold and americium oxide ingots together to form a foil approximately one micrometer thick. This thin gold-americium foil is then sandwiched between a thicker (~0.25 millimeter) silver backing and a 2 micron thick palladium laminate. This is thick enough to completely retain the radioactive material, but thin enough to allow the alpha particles to pass."

If smoke and combustion products enter the chamber, they reduce ionization and thus the conductivity of the air inside. The drop in current flow triggers the alarm.

Tritium survey

TRIUMF has built a tritium-in-air monitor that illustrates the problems of shielding and differential radiation measurement. "The measurement of tritium in the air presents a problem because the average energy of the beta particles emitted by tritium is so low (5.7 keV) that it is not possible to make a detector with walls thin enough to be penetrated by the tritium betas. Instead the air to be sampled is drawn through an ionisation chamber by a pump so that all of the beta particle energy is used to create ion pairs in the chamber.

The instrument is compensated for use in gamma fields by having a second chamber in the monitor which is sealed and measures only the gamma field. The difference between the readings of the two chambers is a measure of the tritium concentration. "

In the U.S. military, for applications such as surveys after nuclear accidents and for nuclear MASINT, while the T-446 and T-290A are tritium detectors, the AN-/PDR-74 is a measurement instrument containing a "IM-246 light weight tritium air monitor to detect airborne radioactive gases. Also, the instrument is calibrated directly in terms of tritium activity but may also be used to detect other radiogases or to monitor gamma radiation if appropriate calibration factors are applied to the meter reading. The instrument is battery operated (D cells) and has an audible alarm when radioactivity exceeds a preset level."

Excitation

"In excitation detectors, the incident radiation excites the atoms of the detector material. The atoms give off the excess energy in the form of visible light." In some instruments, not usually considered radiation survey, the emitted light can be formed into images, as in computed tomography.

Thermoluminescent dosimeters

Thermoluminescent dosimeter (TLD) badges, like film dosimeters, are issued to radiation workers and emergency responders who have the potential to be exposed to ionizing radiation. Thermoluminescent crystals inside the device are analogous to the film in film badges, except that radiation exposures cause them to glow, and the emitted light can be measured and exposure calculated from its intensity. TLDs are especially good for measuring low-level radiation exposure, in the 1-105 rem range.

Advantages TLDs can be used with radiation fields of widely varying energy and intensity. They can store information for long periods of time without fading. TLDs are reusable, and can be used for many applications because of their small size. TLDs are less energy dependent than film dosimeters or pocket ion chambers.

Disadvantages They cannot be analyzed immediately. Environmental factors such as humidity and heat may affect the results.

Scintillometer

Like a TLD, the crystal in a scintillometer emits light when irradiated. Unlike a TLD, it emits only a burst when struck, and the intensity of the burst is low, requiring that the light be detected and recorded by electronics, such as a photomultiplier. Scintillometers are most often used for detecting and measuring, but their "big brothers" in nuclear medicine, such as a ingle photon emission computed tomography (SPECT scanner), can create images.

A basic field survey scintillometer, such as a military AN/PDR-77, 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.

Detector types in use include:

• Zinc sulfide (ZnS): detect alphas in the presence of other types of radiation by energy discrimination. A thin coating of zinc sulfide (a phosphor) is placed behind an alpha-transparent thin entrance window; alpha particles go through the window and produce measurable light flashes; higher-energy particles interact too quickly to produce the flash. A nonquantitative alpha detector using ZnS is called a spinthariscope, and is used in introductory education.
• Sodium iodide (NaI) for "low levels of gamma/x-ray radiation. These detectors typically read out in units of cpm, but with proper calibration and within proper energy limits, they may be used as a microroentgen ("micro-R") meter to measure low exposure rates. A sodium iodide detector is a solid chunk of material with an outer casing. The thickness of the casing prohibits detection of alpha and beta radiation". Large NaI scintillation crystals are the heart of the Anger camera that forms images in various nuclear medicine scanners.
• Plastic scintillators are organic chemicals that, depending upon the material and way in which it is packaged, plastic scintillators can be used for alpha, beta, gamma or neutron detection.

Chemical

Film badges

Film dosimeters, or film badges, do not provide direct response, but are inexpensive and widely used for safety monitoring of personnel routinely exposed to radiation. The badge consists of several layers:

• Front, with identification information and a window for exposure
• Selective filters to screen out radiation not of interest
• One or more radiosensitive films, which will be developed in photographic chemicals
• Possible filters to prevent backscatter into the film
• Mechanical closure and attachment to clothing

After a timed period of exposure, the film is extracted and developed; the density of the developed film is proportional to the intensity of the radiation received. Some film-filter combinations may have different sensitivities in different parts of the film.

Advantages Film badges can record wide ranges of exposure over time: between 10 mrem to 1500 mrem for gamma and x-radiation, and 50 mrem to 1000 rem for beta radiation. They can differentiate between different types of radiation by energy level They are small, lightweight and inexpensive, although there is a continuing cost for film and developing

Disadvantages The response of the film to radiation is energy dependent; at energies less than 300 keV, the response tends to increase. The films cannot be read immediately. Environmental conditions such as heat and humidity will affect the film’s response to radiation. Film badges may be left or lost at the site of the radiation accident

Bioassays

Bioassays, in the context of measuring human radioactive exposures, are analytical technologies that detect the type and amount of radionuclides in a urine sample to determine the amount of internal radionuclide contamination that a person has received during a radiological or nuclear incident.

Mass spectrometry

When used to measure human exposures, mass spectrometry technologies detect the actual number of radionuclide atoms instead of the alpha, beta, or gamma emissions.

Field applications

Health

Yet another set of instruments are used to measure health risks to individuals. According to the Federal Emergency Management Agency, there are three portable types, each with advantages and disadvantages: portable ionization chambers, film badges, and thermoluminescent personal dosimeters. Portable detectors share the problems that they may become contaminated and artificially raise the exposure reading. Small units can be easily lost.

Individual dose measurements for emergency responders is the responsibility of the Safety Officer in the Incident Command System.

Detection of Nuclear Weapons and Materials

Given the danger of smuggled nuclear weapons, as well as the special nuclear materials (SNM) to build them, intense attention is being paid to detecting them. The problem divides roughly into systems practical for fixed or minimally mobile installation at transportation ports of entries, and for field searches for weapons development. A representative set of technologies includes:

1. A new scintillator material to improve detector performance and lower cost. This project was terminated in January 2010.
2. GADRAS, an application using multiple algorithms to determine the materials in a container by analyzing gamma-ray spectra. If materials are the “eyes and ears” of detectors, algorithms are the “brains.”
3. A project to simulate large numbers of experiments to improve detection system performance.
4. Two Cargo Advanced Automated Radiography Systems (CAARS) to detect high-density material based on the principle that it becomes less transparent to photons of higher energy, unlike other material.
5. third CAARS to detect material with high atomic number (Z, number of protons in an atom’s nucleus) based on the principle that Z affects how material scatters photons. This project was terminated in March 2009.
6. A system to generate a 3-D image of the contents of a container based on the principle that Z and density strongly affect the degree to which muons (a subatomic particle) scatter.
7. Nuclear resonance fluorescence imaging to identify materials based on the spectrum of gamma rays a nucleus emits when struck by photons of a specific energy.
8. The Photonuclear Inspection and Threat Assessment System to detect SNM up to 1 km away, unlike other systems that operate at very close range. It would beam high energy photons at distant targets to stimulate fission in SNM, producing characteristic signatures that may be detected.

Laboratory 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 spectrometry, low background alpha and beta counting and liquid scintillation counter 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.

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.