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reportRadiological and Nuclear Detection
9.4.3 Short description

International smuggling of weapons grade nuclear material presents a significant security challenge. Between 1993 and 2004, the International Atomic Energy Agency has reported 18 incidents that related to smuggling of weapons grade nuclear materials. The proliferation of nuclear weapons can take place through the borders at ports, airports, road passengers and through the postal system. A tactical or improvised nuclear weapon would be small enough to be transported in modular containers. It was reported that 25 kilograms of highly enriched uranium or
4 kilograms of plutonium-239 would be adequate for a nuclear explosive device. A significant challenge is presented by dirty bombs, which would present a significant challenge to health of civilians. Americum-241, californium-252, cesium-137, cobalt-60, iridium-192, and strontium-90 are radioactive species that could possibly be used for a dirty bomb. The radioactive isotopes californium-252 and americium-241, are used in the oil industry and smoke detectors respectively, therefore are easy to obtain for a dirty bomb [72]. Weapon grade plutonium and highly enriched uranium can also be used for radiological dispersal devices. Radiological dispersal devices can thus contain a variety of radioactive species that emit gamma rays, neutrons and/or bremsstrahlung radiation.

Radiation monitoring is largely done by detecting gamma rays emitted by radioactive materials. The gamma rays cover a spectrum of energies. Gamma rays passing through matter deposit a part of their energy resulting in electrons that can be detected. One method of detection of photons in a detector is a process called scintillation. Plastic scintillators, pure crystal scintillators, and solid state devices are used for detecting gamma radiation. The drawback of plastic scintillators is the low energy absorption due to the low density of the material. As a result the instruments using such detectors cannot identify the radioactive material accurately. Pure crystal scintillators and solid state devices are relatively better at absorbing all the energy of gamma rays
due to their higher density and atomic number. Radioactive isotopes such as strontium-90 emit beta rays, which when shielded produce bremsstrahlung radiation which can be detected by the
same methods as gamma rays. Weapon grade plutonium and highly enriched uranium are relatively less radioactive with respect to gamma ray emission in comparison to other isotopes, therefore making them more difficult to detect. Neutrons are also emitted by weapon grade
plutonium, often making detection easier due to the low natural background from cosmic radiation. The neutron detectors function by detection of protons released from nuclei struck by
neutrons, of fission daughters or by measuring gamma rays, electrons and other charged particles. Uranium isotopes emit alpha particles and gamma rays, and not as many neutrons [72].

A number of strategic tools are used in the detection of radiation such as radiation portal monitoring equipment, personal radiation detectors, hand held detectors and x-rays systems for imaging of shielding. Radiation detection systems can be passive or active. Passive systems for detection of radiation include radiation portal monitoring equipment, mobile systems, hand held, backpack and belt monitoring systems, all of which have been deployed. Mobile x-ray and fixed
systems have been used for penetration of cargo containers for suspected cargo. Plutonium and a few other radioactive materials emit neutron and tus neutrons are of particular interest in
detection applications at border crossing [73].

Radiation portal monitoring equipment has been deployed for border crossing and port application in detection of illicit nuclear material. Detectors of gamma rays based on polyvinyltoluene (PVT) and thallium doped crystalline sodium iodide have been demonstrated and
deployed. For passive screening of gamma rays, the energy range of interest for detection was between 20 keV to 3 MeV [74]. A comparison of radiation portal monitoring equipment for border
security was done using gamma ray and neutron detectors. A comparison of polyvinyltoluene and thallium doped crystalline sodium iodide for vehicle based radiation portal monitoring has also
been evaluated in the literature. The spectral capability of NaI(Tl) is superior to PVT for isotopic identification, though the cost of NaI(Tl) has been reported to be much higher than PVT. A range
of environmental and operational factors determine the suitability of a detector in different operating scenarios. Each detector type offers some advantages for various operating conditions
of portal monitoring systems [75].

Energy based alarm algorithms with enhanced sensitivity over gross counting have been implemented for radiation portal monitoring equipment. The energy information obtained from
plastic scintillators can be used to distinguish between naturally occurring radioactive material and special nuclear material. The energy based algorithm was considered to be a much desired
improvement in detection over gross count algorithms. One of the main limitations of radioaction portal monitoring systems is the presence of naturally occurring radioactive isotopes that can
present a significant operational challenge [73,76].

Nuclear weapons detection in transportation cargo has been demonstrated with a range of techniques, including both passive and active detection. The photo-fission of neutron emission
induced by gamma rays forms the basis of one active detection approach. The technique and its effectiveness has been demonstrated for radioactive material in simulated shipping containers
and air-cargo [77]. Detection of nuclear weapons in cargo has been demonstrated using a pulsed
beam of neutrons, that produce fission events and detection of their fissionable material is done from the beta delayed neutron emission or beta delayed high-energy gamma radiation. This is
another of the several active interrogation detection methods, and has been demonstrated for simulated shipping cargo [78].

Nuclear Resonance fluorescence, another potential active interrogation technique, has been demonstrated in the detection of isotopes of uranium in a laboratory. The basis of the method is a
unique signal that is relevant to each nuclei. The technique combined with effective algorithms has been demonstrated in the laboratory as a possible method that may be applicable to detection of material in sea containers, truck containers, trucks and other vehicles [79].

Monitoring of radioactive xenon in air has been used to detect nuclear weapons explosions as part of the worldwide network of the Comprehensive Test Ban Treaty verification effort. A
prototype single phoswich detector has been used to detect beta particles and gamma rays from radioxenon isotopes [80]. High resolution inductively coupled plasma mass spectrometry and
accelerator mass spectrometry have been demonstrated in detecting ultra low level of uranium isotopes in marine environments. The uranium isotope signature provides valuable information on
origin of uranium. The method has useful applications in monitoring radioactivity in depleted uranium environments and undeclared nuclear activity or movement of nuclear material [81].

Detection of radioisotopes using a distributed sensor network, as opposed to central fixed systems, has been proposed and developed. These distributed sensor network, coupled with a monitoring portal has been demonstrated. The sensor array consisted of sodium iodide
scintillators that were connected to a platform for processing of gamma counts. The performance of the array was reported to be higher than that of a single detector, though that is a controversial
claim. The advantage of this proof of concept is that it may be inexpensive, further research is aimed at increasing sensitivity and developing an integrated platform for chemical and biological
threats [71].

The use of various detectors for radioactive species has been mentioned earlier. A system for simultaneous detection of radiation species such as x-rays, gamma rays, neutrons and minimum
ionising particles has been observed in the literature. The sensitivity of the scintillators in the research was achieved using nano-sized particles, dopants and extruded plastic material. Three
different type of detectors have been described, which identify specie of radiation. Nano-sized particles of lithium have been used in neutron detectors. The wavelength shifting fibre absorbs
scintillator light at a wavelength and re-emitting it at a higher wavelength to better match the photodetector used [82].

The use of nanoscale materials for detection of radiation is expected to overcome single crystal based detectors limitations such as size and cooling requirements to very low temperatures. Nanophosphor has been mentioned as a candidate material for scintillators and detectors.
Cerium doped lanthanum halides (less than 10nm in diameter) have also been mentioned as suitable candidates for scintillators nanocomposites. Due to their brightness and short decay
lifetime they are very effective in gamma ray detection. Scaling up of the synthesis of cerium lanthanum fluoride to kilogram quantities remains a further research challenge that remains to be
addressed [83].

Enhanced optical properties of nanocomposites made of existing scintillator materials have been reported in the literature. The nanocomposites offer enhanced light output, decreased costs and
scalability have been demonstrated at the proof of principle stage. Cerium doped lanthanum fluoride has been synthesized nanoparticles having a size of 25 -100nm, have shown a three
times increase in light intensity as compared to bulk material used for scintillation. Further research in the area was identified as synthesis of nanophosphors as scintillators and their
fabrication as nanocomposites. Measurement of absolute light yield and linearity of the nanocomposite were mentioned as challenges for characterization [84].

Solid state semiconductor detectors offer advantages over gas filled detectors and scintillator detectors due to excellent energy resolution and higher efficiency. High purity germanium detectors are the gold standard for gamma ray detection but require cryogenic temperatures. A number of semiconductors have been suggested for application such as cadmium zinc telluride (CZT), cadmium telluride, gallium arsenide, indium phosphide, mercury iodide and thallium bromide. CZT offer advantages due to its wide band gap, high resistivity and commercial
availability. The higher resistivity is a desirable characteristic as it decreases noise level thus improving the resolution of detection. A synthesis process for producing nanowire arrays of CZT
has been mentioned in the literature for detecting gamma ray radiation. In the process CZT was electrodeposited on a titanium dioxide nanotubular template. Stacks of CZTs with very high
resistivity were fabricated and connected. It was experimentally demonstrated that the flow in the current increased when exposed to a radiation source. The potential of nanowires being used as
a radiation detector at room temperature was at a much lower bias applied in relation to bulk material detectors. Very high sensitivity to radiation was experimentally demonstrated [85].

Other recent methods have been mentioned in the literature such as the Neutron Imaging Camera for detection of weapon grade plutonium at borders. The camera is based on three dimensional image tracker developed initially for applications in gamma ray astrophysics. The
working principle is based on measuring the energy and position in three dimensions of the charged particles moving through the camera medium. The application was successful demonstrated to identify radiation at stand-off distances and in the presence of other background emissions [86]. An Electronic Neutron Dosimeter has been mentioned in the literature for detecting radiation. It uses scintillators on a pair of photomultiplier tube minimizing the power
consumption and increasing operational times. The dosimeter has been prototyped with results exceeding electronic neutron dosimeter standards [87].

Microcantilevers were reported to detect an alpha particle as it impinges on an electrically insulated metallic surface by undergoing a deflection of a few nanometres. The particle is
detected by a shift in the resonance frequency due to electrostatic forces. A single alpha particle can be detected using this method, however other conventional methods have been shown to have higher sensitivity as compared to these detectors [88].

One of the main challenges of detecting these threat radioactive materials is the shielding using lead and other dense materials for gamma rays, and hydrogenous materials for neutrons. An additional challenge is interference from medical isotopes and other slightly radioactive, but relatively innocuous materials such as smoke detectors, fertilizers, television sets, abrasives and glazed ceramics. The approach of using X-ray scanners has been used in the United States, for any shielding that may be used to hide radiological weapons. Other methods such as active interrogations using gamma ray and neutrons have also been reported. As mentioned above, these induce fission in uranium and plutonium, resulting in gamma rays and neutrons that are
detected [72].

 

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Tags: radiation portal monitoring, sensor array, spectrometric method, nuclear resonance fluroescence, nanomaterials based detector

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