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Introduction

Nuclear disarmament is heightening the fear that weapons-grade nuclear material from growing stockpiles could fall into the hands of radical states or the terrorists they sponsor. Economic disarray coupled with a huge reserve of nuclear materials is adding to the world community's anxiety. With the upcoming election for the extension of the Non-Proliferation Treaty (NPT) in the Spring of 1995, the question of safeguarding civil plutonium is also gaining concern worldwide. Safeguarding nuclear material is the responsibility of all countries with nuclear production capabilities. In the absence of long-term disposal solutions for plutonium and enriched uranium (many countries consider these materials national treasures to be saved for future use in breeder reactors and other power-generation applications) technology is available to help to minimize the insider threat of unauthorized removal at the source - the production facility.

Automatic portal monitors for pedestrians and vehicles exiting a nuclear facility have been jointly developed by Canberra Industries and the U.S. Department of Energy's Los Alamos National Laboratory to detect the characteristic radiation emitted by weapons-grade or reactor-grade nuclear material. Once radioactive material is detected, portable, hand-held gamma-ray spectrometers provide proof positive that the suspect material is Special Nuclear Material (SNM) - plutonium, uranium enriched >20%, ²³⁵U and ²³³U.

A Worldwide Concern

Under the strategic-disarmament treaties, the United States and Russia will take 100 metric tons of plutonium out of warheads over the next 10 years while the nuclear power fuel cycle will produce an additional 110 tons. Recently, U.S. and Russian leaders indicated a desire to speed up the disarmament schedule, which could increase the rate of growth of SNM stockpiles. In 1994, in response to a rash of attempted nuclear material smuggling incidents in Europe, the U.S. and European nations agreed to raise $100 million to improve nuclear security in Russia.

Huge stockpiles of plutonium and enriched uranium exist throughout the world within countries with developed nuclear power and weapons capabilities. The United States, Japan, United Kingdom, France, Germany, Israel and several others must protect their SNM from diversion throughout the fuel and weapons cycles.

Controlling Theft of SNM

An effective approach to stopping unauthorized removal of SNM from facilities is to apprehend people or vehicles thought to be carrying SNM before they leave the site. The strategy for detecting the presence of SNM relies on sensing emitted radiation. SNM emits both gamma rays and neutrons, which provide characteristic identifying signatures when measured by sensitive counting systems. Portal monitors jointly developed by Canberra and Los Alamos National Laboratory [1] have been used in the United States at the exits of material access areas and facilities producing or storing fissile material for over 15 years as a means of diversion control. These same portals can be used at border control points to detect weapons material that has made it out of the production facility.

The portals collect and analyze radiation data and sound an alarm to notify authorities of possible diversion of SNM. The portals are not designed to identify the material causing the alarm, just to provide highly sensitive screening of persons and vehicles, with a low false alarm rate. The response to an alarm is site-specific. As an example, the first response to an alarm could be detention of the person causing the alarm and verification that the detected radioactivity is caused by weapons material, not a medical radioisotope or uncontrolled radioactive source such as ⁶⁰Co. Canberra's U-Pu InSpector portable gamma spectrometer with unique isotopics software quickly and accurately confirms the presence of SNM by measuring the isotopic content of the sample. A detailed description of the U-Pu InSpector is provided in a separate application note.

Types of Monitors

Plutonium emits gamma-rays and energetic neutrons from spontaneous fission of the even isotopes. Highly enriched uranium (HEU) emits gamma rays, but very low levels of neutrons. Therefore, two measurement techniques are employed in SNM portal monitors - neutron and gamma. The choice depends on the type and amount of SNM to be detected.

Gamma-ray monitor  - Shielded, large-area plastic scintillators detect gamma rays emitted by HEU and/or plutonium. The scintillators are shielded with lead on three sides to reduce the background and improve the sensitivities of the monitors. There are two reasons plastic scintillation detectors are preferred for detecting shielded plutonium over NaI(Tl) detectors. They are efficient at detecting fast neutrons and prompt-fission gamma rays transmitted through the shielding, and they are large enough to intercept much more of this radiation than equal-cost NaI(Tl) detectors.

Neutron monitor - ³He proportional counters inside a hollow, high-density polyethylene enclosure detect thermalized neutrons from spontaneous fissions in small quantities (fractions of a gram) of shielded plutonium. There are two benefits of neutron monitoring: even though neutron backgrounds are usually low, measurements can be made in the presence of a high gamma background and neutrons emitted by SNM readily penetrate containers that might be used to try to hide SNM sources from detection to give a signature for the presence of SNM in proportion to its mass.

At uranium facilities, the need to monitor for HEU limits the choice to a gamma-ray monitor. At plutonium facilities either gamma-ray or neutron monitors may be used, depending on the desired detection limit. If both HEU and plutonium are present at one location, monitoring requirements for both materials will have to be considered.

Relying on a single measurement technique is not always the best method for diversion control because each technique can be "fooled", eluding detection. Combination neutron and gamma portals allow detection of SNM even if it is shielded with a high-Z material such as lead. This combination is very effective for both vehicle and pedestrian monitors. Gamma pedestrian portals with metal detectors also detect shielded SNM by providing an indication of suspicious metal containers that could be used by pedestrians to shield SNM. If not adequately shielded, the gamma portion will also detect the SNM.

General Description

Pedestrian and Vehicle SNM Monitors employ much of the same radiation measurement, acquisition and analysis technology. How they are configured is of course quite different. To detect SNM in both vehicles and pedestrians, the monitor senses a radiation intensity increase by comparing its monitoring measurements with an alarm threshold derived from previous unoccupied background measurements. Detection sensitivity and nuisance-alarm rates are directly related. Systems must be carefully designed, implemented and evaluated to assure the desired level of performance is consistently achieved.

Factors affecting portal monitor performances are isotopic content, chemical composition, size of the SNM particles, ambient background; electronic noise; distance between the detectors; type, number or size of radiation detectors; and passage speed (for pass-through mode).

• Isotopic content. Reactor-grade plutonium has a lower detection limit than weapons-grade plutonium, both with gamma and neutron portals. Also HEU is much easier to detect in a gamma portal than Low Enriched Uranium (LEU) or ^natU because of the larger gamma emission rate from ²³⁵U.
• Chemical composition of the SNM. PuF₄ emits two orders of magnitude more neutrons than plutonium metal or PuO₂. Therefore, its detection limit will be lower using a neutron portal, or a combination neutron and gamma portal.
• Particle size. Gammas emitted from the center of large lumps of uranium or plutonium will or attenuated and lost to detection. The high energy spontaneous fission neutrons emitted by plutonium do not suffer self-attenuation in lumps. This is another reason to combine gamma and neutron detection in SNM portals.
• Shielding. High density absorbers strongly attenuate the gamma intensity, but not the fission neutrons, supporting the use of gamma and neutron techniques for detecting plutonium.
• Background. There is a square root relationship between the background level and the detection limit. Detection limits obtained with neutron portals at 2100 m are typically two to three times larger than at sea level for similar operating parameters because the background is larger. Neutron portals are an effective method for detecting plutonium in areas with large gamma backgrounds, or gamma-ray backgrounds that vary and cause nuisance alarms.
• Surface area of the detectors and the distance between the detectors. These parameters affect the geometric efficiency or response of the system.
• Count time or travel speed. As discussed below in more detail, the algorithms in Canberra's SNM portals make them less susceptible to variations in the speed than other portals. There is still a maximum speed that provides a counting time sufficient to meet the required minimal detectable activity (MDA).

Even though they can be used for detecting any gamma or neutron radiation, Canberra's portals were designed specifically to detect SNM with the maximum sensitivity for a given configuration and a minimum false alarm rate. The requirements for monitors that are designed to detect possible diversion of SNM are different from monitors that are designed to detect other radioactive sources or contamination on persons and vehicles. The latter are designed to detect possible surface contamination or unshielded sources such as ⁶⁰Co, while the SNM monitors are designed to detect material where the subject will probably do anything to avoid detection. This has its repercussions on the detection techniques used, the algorithms and the philosophy employed in both types of radiation monitors.

To meet the sensitivity and false alarm rates required for detection of SNM, Canberra's portal designs include features not used in most standard gamma portals designed to detect contamination. They include:

• Enhanced modular design with automated operation. Facilities rely on continuous operation of SNM portals during working hours. If portals are not operational due to failure of components, guards must hand monitor all pedestrians and vehicles exiting the site. This is time consuming, resulting in long delays. The portal controller, low-noise photomultiplier tubes (PMTs), high-quality plastic scintillators, and lead shields give a low false alarm rate to make the portals more reliable. Therefore, routine maintenance is minimal. If maintenance is required, the modular design of the electronics and detector assemblies allow easy, field maintenance, thereby, minimizing the downtime of the portals. The automated operation of these intelligent radiation detection systems lowers operator training requirements.
• Low-noise PMT with tube base assembly. The PMT assembly is an integral unit that is attached to the plastic scintillator with a mechanical holding fixture. If the PMT fails, this configuration is easier to repair than if the PMT is glued to the plastic scintillator. Without this feature, phototube failure requires replacement of the entire detector assembly or cutting off the PMT, polishing the plastic and regluing another PMT to the plastic.
• Portal controller with summed input from all detectors for maximum sensitivity and minimum false alarm rate. The controller is designed to implement specific self-diagnostic tests upon power-up and during monitoring to verify state-of-health. The sophistication of the program in the microprocessor-based controller allows authorized personnel to change operating parameters in the field to turn a walk-through monitor into a wait-in monitor, or vice versa.
• A variance analyzer that operates continuously to analyze variations in the background. If variations are larger than statistically expected, a variance alarm is sounded and the deviation is printed. The alarm will also sound when the background exceeds an upper limit, indicating that the background is too large to meet the required MDA, or that the system electronics is noisy; or when the background drops below a lower limit, indicating electronics failure. This feature prevents diversion of SNM by intentionally altering the background, and is an important design specification for SNM portals.
• An RS-232 communication port that allows authorized personnel to set or change all calculation and threshold parameters which are permanently stored in nonvolatile RAM. These parameters can be changed by a standard ASCII terminal or from a computer with a serial port. The RS-232 communication port makes the unit more versatile because changes can easily be made in the field as required instead of at the factory, reducing down time.
• The use of the Sequential Probability Ratio Test (SPRT) [2] instead of moving average or fixed ratemeter alarm points. Once the portal is occupied, the portal controller examines very small count intervals. Each time interval is analyzed using the SPRT and compared to two thresholds: a background and a background plus transient. The SPRT not only shortens the count time for stationary counts but detects transient increases in the radiation intensity. Therefore, the portals still perform well if the passage times are faster or slower than normal. The SPRT also reduces the number of nuisance alarms compared to systems with fixed ratemeter alarm points.
• SCA and an LLD instead of LLD only. This allows user to set lower and upper discriminators to minimize counts from cosmic rays and reduce false alarm rates.
• The doors to the gamma portals are made of a low-Z material instead of steel to minimize gamma attenuation and allow detection of low energy gamma rays from plutonium and uranium. All doors have locks to prevent access to electronics that could affect the integrity of the measurement.

Vehicle Portal Monitoring

The configuration of vehicle portal monitors varies depending on the required detection limit. There are four standard configurations of vehicle monitors: JPM-11A Gamma Vehicle Monitoring Station [3], JPM-12A Gamma Vehicle Monitor, JPM-31A Neutron Vehicle Monitor and JPM-32A Neutron/Gamma Vehicle Monitor. The JPM-11A is operated while the vehicle is stationary while the JPM-12A, JPM-31A and JPM-32A are drive-through monitors. A summary of SNM vehicle monitors is shown in Table 1.

Table 1.

The JPM-11A Gamma Portal Monitoring Station resembles a carport with detectors located above and below the stopped vehicle and arranged to view the entire vehicle. The large number of detectors, therefore, large surface area, in combination with a wait-in mode of operation makes the JPM-11A capable to detecting subgram quantities of low-burnup and reactor-grade plutonium or tens of grams of HEU.

Four plastic scintillators and preamplifiers are located overhead and pairs of plastic scintillators and preamplifiers are located in six covered trenches below the vehicle. The four overhead detectors operate as a single data channel and pairs of trenches operate as three independent data channels. Measurements are separately analyzed for a total of four independent monitors. An indication from only one data channel that radiation is present produces an alarm. However, all data channels must agree that no radiation is present before the vehicle is allowed to leave.

The JPM-12A Gamma Vehicle Monitor is designed for drive-through monitoring where larger quantities of SNM may be encountered, for example, fabricated items containing ten grams or larger quantity of low-burnup plutonium, or kilogram quantities of HEU, spent fuel, ²³⁸Pu, ²³³U or high-burnup plutonium. It usually is positioned on opposite sides of a lane of traffic near a vehicle trap to force the vehicle to slow down, which improves sensitivity.

The JPM-31A Neutron Vehicle Monitor is a drive-through monitor that can detect tens of grams of shielded weapons-grade or reactor-grade plutonium, which cannot be detected by gamma-based detectors due to attenuation of the gamma rays in the shielding. It is also used where the gamma-ray background varies and may contribute to nuisance alarms, or the gamma-ray background is too large to meet the required MDA.

The JPM-32A Neutron/Gamma Vehicle Portal Monitor is a combination system that includes both neutron and gamma detectors, similar to those used in the JPM-12A and JPM-31A discussed earlier. The combination of neutron and gamma detectors provides all of the advantages described on page 2.

Pedestrian Portal Monitoring

SNM pedestrian portals monitor individuals leaving a material access area. Similar to vehicle monitors, Canberra's pedestrian portal monitors are available in several detector configurations that can be operated in either a walk-through or wait-in mode, depending on the desired detection levels. Monitoring occurs as the pedestrian walks between two pillars housing the detectors. There are four standard configurations of pedestrian monitors: JPM-21A Gamma Pedestrian Portal Monitor, JPM-22A Gamma Pedestrian Portal Monitor with Metal Detector, JPM-41A Neutron Pedestrian Portal Monitor and JPM-42A Neutron/Gamma Vehicle Portal Monitor. A summary of SNM pedestrian monitors is shown in Table 2.

Table 2.

The JPM-21A has four plastic scintillator detectors. In a walk-through mode, the JPM-21A can detect subgram quantities of weapons-grade or reactor-grade plutonium and gram quantities of HEU.

When the gamma detectors are integrated with a commercially-available metal detector, the JPM-22A can detect shielded SNM by providing an indication of suspicious metal containers that could be used by pedestrians to shield SNM. The portal controller automatically sets the sensitivity levels in the metal detector based on the direction of traffic. This feature allows different detection levels for metal going into the plant than that going out. In addition to detecting possible SNM shielding, this feature allows workers to carry metal tools, etc. into a facility but prevents removal of potentially contaminated metal without authorization.

The JPM-41A Neutron Pedestrian Portal detects gram masses of shielded SNM that cannot be detected by gamma-ray based monitors. It is also used where the gamma-ray background varies. The portal configuration is identical to the JPM-31A Neutron Vehicle Monitor with fewer ³He detectors.

The JPM-42A is based on a Los Alamos National Laboratory design. It is an integrated neutron and gamma-ray portal monitor for detecting gram masses of shielded plutonium. Lower detection limits are possible for unshielded plutonium and HEU using the gamma detectors. The JPM-42A also takes up less space than a separate JPM-21A and JPM-41A.

Summary

SNM pedestrian and vehicle monitors have been successfully used for over 15 years as a diversion tool to reduce the insider threat of theft of SNM from production facilities. These portals which are designed for maximum sensitivity, minimum false alarm rates, easy and reliable operation can also be used worldwide as a tool to detect SNM at border access points. More detailed information on the detection limits for different types of SNM materials are available on request.

References

1. American Society for Testing and Materials, Standards C993-92, C1112-93, C1236-93.
2. P.E. Fehlau, K.L. Coop, and K.V. Nixon, "Sequential Probability Ratio Controllers for Safeguards Radiation Monitors", Proceedings of the 6th ESARDA Symposium on Safeguards and Nuclear Material Management, Venice, Italy, May 14-18, 1984.
3. P.E. Fehlau, "Vehicle SNM Monitors: Results of an Evaluation", Nuclear Materials Management, Proceedings of the 24th Annual Meeting of the Institute of Nuclear Materials Management, Vail, Colorado, July 10-13, 1983.