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Radiation Portal Monitors (RPMs) are passive radiation detection devices used for the screening of vehicles or other cross-border vectors. Fear of terrorist attacks with radiological weapons spurred RPM deployment for cargo scanning since 9/11, particularly in the US.

Detected radiation
RPMs are designed to detect ionizing radiation sufficiently penetrating to escape from a container. In most cases gamma radiation is detected, in some cases complemented by neutron detection when sensitivity for nuclear material is desired.

PVT (gamma ray detection)
First generation RPMs often rely on PVT scintillators for gamma counting. They provide very little information on energy of detected photons, and as a result, they were criticized for their inability to distinguish gammas originating from nuclear sources from gammas originating from a large variety of benign cargo types that naturally emit radioactivity, including bananas, cat litter, granite, porcelain, stoneware, etc. Those Naturally Occurring Radioactive Materials, called NORMs account for 99% of nuisance alarms.

NaI(Tl) (gamma ray detection)
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

HPGe (gamma ray detection)
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.

3He (thermal neutron detection)
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, the global He-3 supply crisis however has made this technology unavailable. The search for alternative technologies has not yielded satisfactory results.

Radiological threats
RPMs are deployed with the aim to intercept radiological threats as well as to deter malicious groups from deploying such threats.

Radiological dispersal devices
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 can 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
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.

Alarms
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. Common causes of nuisance alarms include naturally occurring radioactive materials (NORM), technically enhanced NORM (TENORM), and medical isotopes.

Deployment
This article relates primarily to RPMs deployed for screening trucks at ports of entry. Increasingly efforts are being made to deploy similar measures to all cross border vectors including
 * Pedestrian radiation portal monitoring
 * Air freight radiation portal monitoring
 * Crane based radiation portal monitoring
 * Air luggage radiation portal monitoring
 * Postal package radiation portal monitoring
 * Railway radiation portal monitoring

RPMs are also deployed at civillian 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.