Friday, 28 September 2012

Know your security detectors, including the hidden ones

The technology that goes into detecting people armed with weapons and drugs in  modern Airports are getting more sophisticated. Airport security uses a host of tools from the handheld wands, the step through metal detectors or the Backscatter X-ray machines not to mention possible hidden ones. Metal detectors use  three technologies Very low frequency (VLF), Pulse induction (PI) and Beat-frequency oscillation (BFO).
If the Pulse induction metal detector is over a metal object, the pulse creates an opposite magnetic field in the object. When the pulse's magnetic field collapses, causing the reflected pulse, the magnetic field of the object makes it take longer for the reflected pulse to completely disappear. This process works something like echoes: If you yell in a room with only a few hard surfaces, you probably hear only a very brief echo. This type of metal detector is normally associated with step through detectors.
security sensitive areas have began using a device that scans the whole body for contraband. The scan produces an imagine of the skin, and will show objects like guns, drugs, other weapons under clothing. This procedure is often used an alternative to whole body strip searches. The device uses a low energy x-ray or gamma source to produce the image. X-rays come in many flavors from hard to soft with hard being the most penetrating. All x-rays are ionizing radiation this means the x-ray photons break DNA, chemical bonds by dislodging electrons and generate destructive free radicals. Backscatter x-ray uses the Compton Effect to reflect or scatter x-rays back to detectors.
Backscatter x-ray systems use a complex mechanical assembly to sweep an intense pencil beam of x-rays head to toe. The x-rays leave a trail of damaged DNA and cells from head to toe. Most x-ray are absorbed by the body tissue a small fraction is back-scattered or reflected like a window back to special detectors. The twelve year old fuzzy public images below reveal a great deal. If you inspect the lower leg and arms of the man shown you will see "bone shadows". This is a result of the x-rays penetrating the skin and being absorbed by the underlying bones, the same shadow is seen on the top of the head showing the skull. Fewer x-rays are being "back scattered" from the bone making this region dark in the image. Clearly the x-rays penetrate the skin and reach bones and deeper radio sensitive tissues such as ovary, testicles. thyroid, breast and eyes.

To put the radiation dose received into perspective:
Naturally occurring ionizing radiation is all around us. We are continuously exposed to this background radiation during ordinary living. In 42 minutes of ordinary living, a person receives more radiation from naturally occurring sources than from screening with any general-use x-ray security system.
The national radiation safety standard (see below) sets a dose per screening limit for the general-use category. To meet the requirements of the general-use category a full-body x-ray security system must deliver less than the dose a person receives during four minutes of airline flight. TSA has set their dose limit to ensure a person receives less radiation from one scan with a TSA general-use x-ray security system than from two minutes of airline flight. A person would have to be screened more than a thousand times in one year to exceed the annual radiation dose limit for people screening that has been set by expert radiation safety organizations.

Emerging THz technological applications,THz waves are found between microwaves and infrared on the electromagnetic spectrum. This type of radiation was chosen for security devices because it can penetrate matter such as clothing, wood, paper and other porous material that’s non-conducting. The terahertz spectrum is so good for scanning; infrared cameras can see through some obstacles. The effect is even more pronounced with the lower wavelength terahertz radiation.
This type of radiation seems less threatening because it doesn’t penetrate deeply into the body and is believed to be harmless to both people and animals. THz waves may have applications beyond security devices. Research has been done to determine the feasibility of using the radiation to detect tumors underneath the skin and for analyzing the chemical properties of various materials and compounds. The potential marketplace for THz driven technological applications may generate many billions of dollars in revenue.
The past several years the possible health risks from cumulative exposure to THz waves was mostly dismissed. Experts pointed to THz photons and explained that they are not strong enough to ionize atoms or molecules; nor are they able to break the chains of chemical bonds. They assert—and it is true—that while higher energy photons like ultraviolet rays and X-rays are harmful, the lower energy ones like terahertz waves are basically harmless. While that is true, there are other biophysics at work. Some studies have shown that THZ can cause great genetic harm, while other similar studies have shown no such evidence of deleterious affects.
A company called Genia Photonics has developed a programmable picosecond laser that is capable of spotting trace amounts of a variety of substances. Genia claims that the system can detect explosives, chemical agents, and hazardous biological substances at up to 50 meters. This is why the US Department of Homeland Security is so keen on getting it into airports.
This device relies on classic spectroscopy; just a very advanced form of it. A spectrometer is simply a device that uses radiated energy to characterize a material. In the case of Genia’s scanner, it is using far-infrared radiation in the terahertz band. It’s important to understand that this is not an imaging device, but a tool for reading absorbance spectra (in this case it is a very sensitive electronic nose). Some of the radiation emitted will be absorbed by everything (and everyone) it is directed at. Because different compounds absorb that energy in different ways, the profile of energy returned to the scanner can tell you what it “saw.”
This kind of picosecond laser reads the environment in real-time. That gunpowder residue on your hand from hunting the other day, cannabis smoke particles in your hair, or even a bit of (explosive-boosting) nitrate fertilizer stuck to your shoe could trigger this scanner. Perhaps this invasive technology might replace the pat down option or even the x-ray and the terahertz body scanners. In future this technology could be a key device to detect improvised bomb devices or a suspicious criminals for gun powder or drug traces. Perhaps another gadget for law enforcement in the not so distant future...

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