Recently a secret group of fewer than 10 people in undercover congressional operation was easily able to buy the raw ingredients for a dirty bomb in US. This has set off alarms among some lawmakers and officials in Washington about risks that terrorists inside the United States could undertake a “dirty bomb” attack and the harmful effects of radiation from such an event. Apart from terrorists, there is also threat of accidents involving nuclear material. Security agencies are increasingly seeking out better, smaller and less expensive detection devices to detect and prevent this type of terrorist attack.
Radiation can be readily detected with equipment carried by many emergency responders, such as Geiger counters, which provide a measure of radiation dose rate. Other types of instruments are used to identify the radioactive element(s) present. But they only work within a range of a few metres. Radiation portal monitors are used to detect the invisible gamma and neutron radiation and warn security officials of unauthorized movements of nuclear materials at borders and checkpoints.
One of thrust areas of DARPA is to counter CBRNE threats by developing and testing networked, mobile and cost-effective nuclear- and radiological-weapons detectors that can easily be deployed to provide real-time surveillance over city-scale areas. DARPA under their SIGMA program had designed a pocket-sized radiation detector to help foil terrorists attempting to detonate concealed dirty bombs or full-blown nuclear weapons in or around U.S. cities and crucial government and industrial infrastructure. Direct measurement of gamma and neutron emission remains one of the few definitive to detect and identify special nuclear materials and radiological sources, DARPA officials say.
When radioactive elements decay, they produce energetic emissions (alpha particles, beta particles, or gamma rays) that can cause chemical changes in tissues. Gamma and X-rays can travel long distances in air and can pass through the body exposing internal organs; it is also a concern if gamma emitting material is ingested or inhaled.
Beta radiation can travel a few yards in the air and in sufficient quantities might cause skin damage; beta-emitting material is an internal hazard if ingested or inhaled. Alpha radiation travels only an inch or two in the air and cannot even penetrate skin; alpha-emitting material is a hazard if it is ingested or inhaled.
Alpha Particles are the cores (nucleus) of helium atoms. Even though they are easily stopped, they can damage the atoms of whatever they strike. Beta Particles – Beta particles are electrons flying along without the rest of the atom. It takes a little more to stop a beta particle, but thin metal – even tinfoil will stop most of them. Gamma Radiation – Gamma rays are made of the same particles as non-ionizing radiation, but at much, much higher energies. Gamma Rays come from the decay of radioactive elements here on Earth. Gamma Rays are very hard to stop, and a Gamma source even quite distant from the body can be very dangerous.
The average person in the United States receives a “background” dose of about one-third of a rem* per year— about 80% from natural sources including earth materials and cosmic radiation, and the remaining 20% from man-made radiation sources, such as medical x-rays. * A rem is a measure of radiation dose, based on the amount of energy absorbed in a mass of tissue. Dose can also be measured in Sieverts (1 Sievert=100 rem).
A variety of detectors can be used in radiation monitoring systems, but the most common types fall into one of two basic designations: crystalline-based materials and gas-filled chambers. Both configurations are based on the movement of free electrons moving through a medium and the accumulation and control of ions through electrical methods. It is important to note, however, that while some of the same detection principles apply to neutron radiation, the detection and measuring of neutron generally requires specialized equipment and a different set of analysis factors.
The available radiation detecting technologies is of three type
- Semiconductor detectors, eg. Si, Ge, GaAs, CdTe;
- Gaseous detectors, eg. propotional counters, Gas Electron Multiplier (GEM) detectors;
- Scintillators, e.g. metamaterials (SMM).
As the requirements for greater accuracy, efficiency, or sensitivity increases, so does the complexity of the detector and its operation. Reliable quality assurance (optical and electrical) prior to installation is essential to guarantee reliable performance of
detectors, interconnections and electronics.
Some of the types of commonly used detectors are:
The most common detector is the Geiger-Mueller counter, commonly called the Geiger counter. It uses a gas-filled tube with a central wire at high voltage to collect the ionization produced by incident radiation. It can detect alpha, beta, and gamma radiation although it cannot distinguish between them. Because of this and other limitations, it is best used for demonstrations or for radiation environments where only a rough estimate of the amount of radioactivity is needed.
Scintillators are usually solids (although liquids or gases can be used) that give off light when radiation interacts with them. The light is converted to electrical pulses that are processed by electronics and computers. Examples are sodium iodide (NaI) and bismuth germanate (BGO). These materials are used for radiation monitoring, in research, and in medical imaging equipment.
Si photodiodes have no internal gain and often rely on low noise properties of externally connected amplifiers for better S/N. These lightweight and compact photodetectors are suitable for compact scintillation-based radiation detectors without an identification function. Photomultiplier tubes (PMT) can vary in size ranging from 8 mm to more than 100 mm in diameter. Their high internal gain of up to 10⁶ and extremely low dark count make them the best detectors for all types and sizes of scintillation-based radiation detection systems.
An MPPC consists of multiple avalanche photodiode pixels operating in Geiger mode. These lightweight and compact photodetectors have high internal gain of up to 10⁶, making them the best alternative for your scintillation-based radiation detection system.
Solid state X-ray and gamma-ray detectors
Silicon and germanium detectors, cooled to temperatures slightly above that of liquid nitrogen (77 K), are used for precise measurements of X-ray and gamma-ray energies and intensities. Silicon detectors are good for X-rays up to about 20 keV in energy.
Germanium detectors can be used to measure energy over the range of >10 keV to a few MeV. Such detectors have applications in environmental radiation and trace element measurements. Germanium gamma ray detectors play the central role in nuclear high-spin physics, where gamma rays are used to measure the rotation of nuclei. Large gamma-ray detection systems, such as Gammasphere and Eurogam are made of these detectors.
Neutrons are much harder to detect because they are not charged. They are detected by nuclear interactions that produce secondary charged particles. For example, boron trifluoride (BF3) counters make use of the 10B(n,a)7Li reaction to detect neutrons.
Often one uses a moderator, such as paraffin, to slow the neutrons and thus increase the detection efficiency. These detectors are used to monitor the neutron fluxes in the vicinity of a reactor or accelerator. Liquid scintillators can measure both neutrons and gamma rays. By carefully measuring the shape of the electronic signal, scientists can and distinguish between these two types of particles.
US Navy new radiation detector using scarce Helium-3
Even the smallest atomic “dirty bombs” emit some neutron radiation, but the Helium-3 used in the thermal neutron detectors is in short supply. The only source of Helium-3 is decayed Tritium from a nuclear reactor, and only one reactor in the U.S. is producing it. Each year, thousands of liters of Helium-3 gas is made available for government research, national security activities, and medical diagnostic procedures. But demand as high as 70,000 liters per year can outpace government supply, due to a substantial increase in the use of neutron detectors for homeland and national security applications, according to the Department of Energy.
But in April 2019, the U.S. Navy was assigned U.S. Patent 10,247,848 for a new technology titled “Helium-3 Gas Proportional Counter,” invented by Drs. Brian Justus and Alan Huston of the Naval Research Laboratory, and Brian’s brother, Dr. Alan Justus of the DOE’s Los Alamos National Laboratory.
The scientists discovered that rectangular gas tubes have more surface area for detection and are more efficient than cylindrical gas tubes in mating with the neutron detector’s polyethylene moderator. But the more important discovery was that instead of pressurizing the gas tubes with lots of Helium-3, dense Xenon gas (or Krypton ) could be added, at lower pressures, which means more detectors can be built using smaller amounts of Helium-3.“The engineering innovations taught in this invention will achieve an overall four-fold reduction in the consumption of Helium-3 and still achieve improved detector efficiency,” the patent states, with an estimated cost savings of $20,000 to $30,000 per detector.
NASA studies space applications for GaN crystals
Two NASA teams are examining the use of gallium nitride, a crystal-type semiconductor compound first discovered in the 1980s, and currently used in consumer electronics such as laser diodes in DVD readers, to enhance space exploration. Among its many attributes, gallium nitride—GaN, for short—demonstrates less electrical resistance and thus loses only a small proportion of power as heat. The material can handle 10 times the electrical current of silicon, enabling smaller, faster, and more efficient devices. In addition, it’s tolerant to a wide range of temperatures, resistant to radiation, and as it turns out, adept at detecting energetic particles.
Gallium-nitride transistors or semiconductors became available commercially in 2010, but they have not yet found their way into space scientists’ instruments, despite their potential to reduce an instrument’s size, weight, and power consumption. There’s a reason for that, said Lauenstein.
Even though gallium-nitride is predicted to be resistant to many types of radiation damage encountered in space, neither NASA nor the U.S. military has established standards characterizing the performance of these transistor-enabled devices when exposed to the extreme radiation in space. With the funding, Lauenstein and MacDonald are teaming with the Los Alamos National Laboratory in New Mexico, a parts manufacturer, and the NASA Electronic Parts and Packaging to establish criteria assuring a GaNs-type device could withstand the effects of potentially harmful particles produced by galactic cosmic rays and other sources.
When struck by galactic cosmic rays or other energetic particles, electronic equipment can experience catastrophic or transient single-event upsets. “We have standards for silicon,” Lauenstein said. “We don’t know if the methods for silicon transistors would apply to gallium-nitride transistors. With silicon, we can assess the threshold for failure.”
For de Nolfo and Hunter, gallium-nitride offers a potential solution for building a detector and imaging neutrons, which are short-lived and typically expire after about 15 minutes. Neutrons can be generated by energetic events in the Sun as well as cosmic ray interactions with Earth’s upper atmosphere. The neutrons generated by cosmic rays in the atmosphere can add to Earth’s radiation belt—a swatch of radiation surrounding Earth that among other things can interfere with onboard satellite electronics—when they decay. Researchers have discovered GaN can form the basis of a highly sensitive neutron detector. “The gallium-nitride crystal could be game-changing for us,” de Nolfo said.
Under their concept, Hunter and de Nolfo would position a gallium-nitride crystal inside an instrument. As neutrons entered the crystal, they scatter off gallium and nitrogen atoms and, in the process, excite other atoms, which then produce a flash of light revealing the position of the neutron that initiated the reaction. Silicon photomultipliers attached to the crystal convert the flash of light into an electrical pulse to be analyzed by the sensor electronics.
“Gallium-nitride is reasonably well understood in the photo-electronics industry, but I think we’re pushing the envelope a little on this application,” Hunter said, adding that the beauty of the concept is that it would contain no moving parts, use little power, and operate in a vacuum. If it works, the instrument would benefit different space science disciplines and the military in detecting nuclear material, he added.
Ceramic insulator in Mobile phone could reveal your radiation exposure after a nuclear disaster
A ceramic insulator found in many devices, such as cell phones and fitness trackers, gives off a glow under high heat that reveals its past nuclear radiation exposure, researchers report in the February 2019 Radiation Measurements. That insight may allow experts to gauge someone’s radiation dose in a matter of hours, whereas typical blood tests can take weeks. “Everybody panics when it comes to radiation,” says study coauthor Robert Hayes, a nuclear engineer at North Carolina State University in Raleigh. Quickly estimating people’s risk of radiation-related sickness after a nuclear disaster could help triage emergency medical treatment.
When nuclear radiation floods the ceramic in electronic components called surface mount resistors, the radiation rearranges the distribution of electrons in defects in the ceramic’s crystalline structure. If heated to hundreds of degrees Celsius, the ceramic glows, and the wavelengths of light that make up that luminescence reveal the material’s electron distribution. From there, researchers can determine the dose of radiation that caused the material’s electron reshuffling.
Hayes and NC State colleague Ryan O’Mara tested their technique by blasting surface mount resistors with 0.005, 0.015, 0.03, 0.06, 0.125, 0.25 or 0.5 grays of radiation. (One gray represents one joule of radiation per kilogram of target material.) For the lower levels of radiation, the researchers could generally estimate doses within about 0.01 grays using the technique; at 0.5-gray exposure, the uncertainty was 0.05 grays.
This test is sensitive enough to judge whether someone likely needs immediate treatment for radiation poisoning, which may result from one to a few grays of radiation, Hayes says. It could also indicate whether someone has an increased risk of cancer — which could be induced by about 0.2 grays.But the machine used to measure the ceramic’s luminescent glow costs about $150,000, so people in areas affected by nuclear disasters would have to send their personal electronics to specialized facilities for testing.