 | The examples and perspective in this article may not represent a worldwide view of the subject.(December 2010) |
Radiation Portal Monitors (RPMs) are passive radiation detection devices used for the screening of individuals, vehicles, cargo or other vectors for detection of illicit sources such as at borders or secure facilities. Fear of terrorist attacks with radiological weapons spurred RPM deployment for cargo scanning since 9/11, particularly in the United States.
RPMs were originally developed for screening individuals and vehicles at secure facilities such as weapons laboratories. [1] They were deployed at scrap metal facilities to detect radiation sources mixed among scrap that could contaminate a facility and result in a costly clean up. [ citation needed ] As part of the effort to thwart nuclear smuggling after the breakup of the Soviet Union, RPMs were deployed around that territory, and later around many other European and Asian countries, by the US Department of Energy (DOE) National Nuclear Security Administration (NNSA) Second Line of Defense Program (SLD) [2] starting in the late 1990s. After the attack of 9/11, the US Customs and Border Protection (CBP) started the Radiation Portal Monitor Program (RPMP) to deploy RPMs around all US borders (land, sea and air). [3]
Radiation Portal Monitor (RPM) was designed to detect traces of radiation emitted from an object passing through a RPM. Gamma radiation is detected, and in some cases complemented by neutron detection when sensitivity for nuclear material is desired. [4]
First generation RPMs often rely on PVT scintillators for gamma counting. They provide limited information on energy of detected photons, and as a result, they were criticized for their inability to distinguish gamma rays originating from nuclear sources from gamma rays originating from a large variety of benign cargo types that naturally emit radioactivity, including cat litter, granite, porcelain, stoneware, bananas etc. [5] Those Naturally Occurring Radioactive Materials, called NORMs account for 99% of nuisance alarms. [6] It is worth noting that bananas have erroneously been reported as the source of radiation alarms; they are not. Most produce contains potassium-40, but packing density of fruits and vegetables is too low to produce a significant signal. PVT does have the ability to provide some energy discrimination, which can be exploited to limit nuisance alarms from NORM. [7]
In attempt to reduce the high nuisance alarm rates of first generation RPMs, the Advanced Spectroscopic Portal (ASP) program was called into life. Some of the portal monitors evaluated for this purposes are based on NaI(Tl) scintillating crystals. These devices, having better energy resolution than PVT, were supposed to reduce nuisance alarm rates by distinguishing threats from benign sources on the basis of the detected gamma radiation spectra. ASPs based on NaI(Tl) had a cost several times that of first generation RPMs. To date, NaI(Tl) based ASPs have not been able to demonstrate significantly better performance than PVT based RPMs. [8]
The ASP program was canceled in 2011 [9] after continued problems, including a high rate of false positives and difficulty maintaining stable operation. [10]
In the scope of the ASP program, high purity germanium (HPGe) based portal monitors were evaluated. HPGe, having significantly better energy resolution than NaI(Tl), allows rather precise measurement of the isotopes contributing to gamma ray spectra. However, due to very high costs and major constraints such as cryo-cooling requirements, US government support for HPGe based portal monitors was dropped.
RPMs geared for interception of nuclear threats usually incorporate a neutron detection technology. The vast majority of all neutron detectors deployed in RPMs to date relies on He-3 tubes surrounded by neutron moderators. Since the end of 2009, however, the global He-3 supply crisis [11] has made this technology unavailable. The search for alternative neutron detection technologies has yielded satisfactory results. [12]
The latest technology being deployed at ports [13] uses pressurized natural helium to directly detect fast neutrons, without the need for bulky neutron moderators. Utilizing recoil nuclei following neutron scatter events, natural helium glows (scintillates), allowing photomultipliers (e.g. SiPMs) to produce an electrical signal. [14] Introducing moderators and lithium-6 to capture thermalized neutrons further increases the detection capabilities of natural helium, at the expense of losing the initial information of the neutrons (such as energy) and reducing sensitivity to shielded neutron-emitting materials.
RPMs are deployed with the aim to intercept radiological threats as well as to deter malicious groups from deploying such threats.
Radiological dispersal devices (RDDs) are weapons of mass disruption rather than weapons of mass destruction. "Dirty bombs" are examples of RDDs. As the name suggests, an RDD aims at dispersing radioactive material over an area, causing high cleanup costs, psychological, and economic damage. Nevertheless, direct human losses caused by RDDs are low and not attributed to the radiological aspect. RDDs are easily fabricated and components readily obtainable. RDDs are comparatively easy to detect with RPMs due to their high level of radioactivity. RDDs emit gamma radiation as well as sometimes, depending on what isotopes are used, neutrons.
Improvised nuclear devices (INDs) and nuclear weapons are weapons of mass destruction. They are difficult to acquire, manufacture, refurbish, and handle. While INDs can be constructed to emit only low amounts of radiation making them difficult to detect with RPMs, all INDs emit some amounts of gamma and neutron radiation.
Gamma radiation as well as neutron radiation can cause RPMs to trigger an alarm procedure. Alarms caused by statistical fluctuations of detection rates are referred to as false alarms. Alarms caused by benign radioactive sources are referred to as nuisance alarms. Causes of nuisance alarms can be broken up into several large categories:
This article relates primarily to RPMs deployed for screening trucks at ports of entry. Over 1400 RPMs are deployed at US borders and a similar number at foreign locations for the purpose of interdicting illicit radiological and nuclear material. The US deployments cover all land border vehicles, all seaport containerized cargo, and all mail and express courier facilities. Efforts are also being made to deploy similar measures to other cross border vectors including:
RPMs are also deployed at civilian and military nuclear facilities to prevent theft of radiological materials. Steel mills often use RPMs to screen incoming scrap metal to avoid radioactive sources illegally disposed in this way. Garbage incineration plants often monitor incoming material to avoid contamination.
Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources.
A Geiger counter is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental physics and the nuclear industry.
Neutron activation analysis (NAA) is the nuclear process used for determining the concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The method is based on neutron activation and therefore requires a source of neutrons. The sample is bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well known. Using this information, it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it. A particular advantage of this technique is that it does not destroy the sample, and thus has been used for analysis of works of art and historical artifacts. NAA can also be used to determine the activity of a radioactive sample.
A beta particle, also called beta ray or beta radiation, is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β− decay and β+ decay, which produce electrons and positrons respectively.
A dirty bomb or radiological dispersal device is a speculative radiological weapon that combines radioactive material with conventional explosives. The purpose of the weapon is to contaminate the area around the dispersal agent/conventional explosion with radioactive material, serving primarily as an area denial device against civilians. It is, however, not to be confused with a nuclear explosion, such as a fission bomb, which by releasing nuclear energy produces blast effects far in excess of what is achievable by the use of conventional explosives.
A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses.
Radiological warfare is any form of warfare involving deliberate radiation poisoning or contamination of an area with radiological sources.
Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Exposure can be from a source of radiation external to the human body or due to internal irradiation caused by the ingestion of radioactive contamination.
A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate. Sometimes, the excited state is metastable, so the relaxation back down from the excited state to lower states is delayed. The process then corresponds to one of two phenomena: delayed fluorescence or phosphorescence. The correspondence depends on the type of transition and hence the wavelength of the emitted optical photon.
Radioactive contamination, also called radiological pollution, is the deposition of, or presence of radioactive substances on surfaces or within solids, liquids, or gases, where their presence is unintended or undesirable.
Scintigraphy, also known as a gamma scan, is a diagnostic test in nuclear medicine, where radioisotopes attached to drugs that travel to a specific organ or tissue (radiopharmaceuticals) are taken internally and the emitted gamma radiation is captured by external detectors to form two-dimensional images in a similar process to the capture of x-ray images. In contrast, SPECT and positron emission tomography (PET) form 3-dimensional images and are therefore classified as separate techniques from scintigraphy, although they also use gamma cameras to detect internal radiation. Scintigraphy is unlike a diagnostic X-ray where external radiation is passed through the body to form an image.
Neutron detection is the effective detection of neutrons entering a well-positioned detector. There are two key aspects to effective neutron detection: hardware and software. Detection hardware refers to the kind of neutron detector used and to the electronics used in the detection setup. Further, the hardware setup also defines key experimental parameters, such as source-detector distance, solid angle and detector shielding. Detection software consists of analysis tools that perform tasks such as graphical analysis to measure the number and energies of neutrons striking the detector.
In health physics, whole-body counting refers to the measurement of radioactivity within the human body. The technique is primarily applicable to radioactive material that emits gamma rays. Alpha particle decays can also be detected indirectly by their coincident gamma radiation. In certain circumstances, beta emitters can be measured, but with degraded sensitivity. The instrument used is normally referred to as a whole body counter.
Cargo scanning or non-intrusive inspection (NII) refers to non-destructive methods of inspecting and identifying goods in transportation systems. It is often used for scanning of intermodal freight shipping containers. In the US it is spearheaded by the Department of Homeland Security and its Container Security Initiative (CSI) trying to achieve one hundred percent cargo scanning by 2012 as required by the US Congress and recommended by the 9/11 Commission. In the US the main purpose of scanning is to detect special nuclear materials (SNMs), with the added bonus of detecting other types of suspicious cargo. In other countries the emphasis is on manifest verification, tariff collection and the identification of contraband. In February 2009, approximately 80% of US incoming containers were scanned. To bring that number to 100% researchers are evaluating numerous technologies, described in the following sections.
Nuclear MASINT is one of the six major subdisciplines generally accepted to make up Measurement and Signature Intelligence (MASINT), which covers measurement and characterization of information derived from nuclear radiation and other physical phenomena associated with nuclear weapons, reactors, processes, materials, devices, and facilities. Nuclear monitoring can be done remotely or during onsite inspections of nuclear facilities. Data exploitation results in characterization of nuclear weapons, reactors, and materials. A number of systems detect and monitor the world for nuclear explosions, as well as nuclear materials production.
Survey meters in radiation protection are hand-held ionising radiation measurement instruments used to check such as personnel, equipment and the environment for radioactive contamination and ambient radiation. The hand-held survey meter is probably the most familiar radiation measuring device owing to its wide and visible use.
Radiation monitoring involves the measurement of radiation dose or radionuclide contamination for reasons related to the assessment or control of exposure to radiation or radioactive substances, and the interpretation of the results.
Leonidas D. Marinelli was the American radiological physicist who founded the field of Human Radiobiology. He is best known for
A radionuclide identification device is a small, lightweight, portable gamma-ray spectrometer used for the detection and identification of radioactive substances. It is available from many companies in various forms to provide hand-held gamma-ray radionuclide identification. Since these instruments are easily carried, they are suitable for first-line responders in key applications of Homeland Security, Environmental Monitoring and Radiological Mapping. These devices have also found their usefulness in medical and industrial applications as well as a number of unique applications such as geological surveys. In the past two decades RIIDs have addressed the growing demand for fast, accurate isotope identification. These light-weight instruments require room temperature detectors so they can be easily carried and perform meaningful measurements in various environments and locations.
A radioactive source is a known quantity of a radionuclide which emits ionizing radiation; typically one or more of the radiation types gamma rays, alpha particles, beta particles, and neutron radiation.
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