Engineers from the Lawrence Livermore National Laboratory (LLNL) have designed a phone which contains a compact radiation detector, making easy for police forces to check for radioactive material hidden in large cities. In "Cellphone sniffs out dirty bombs," New Scientist writes that the phone measures continuously the level of radiation around it and transmits it to a central computer via an always-on Internet connection. The phone will also send time and location information gathered from its GPS unit. When these phones are deployed around the U.S., they will form a radiation monitoring network dubbed the RadNet. Such phones should be available for about $1,000 in a few months first to military personnel or police officers, then to U.S. Postal service personnel or delivery service workers. It should take more time before you can buy one yourself at a Wal-Mart store. Read more...
Here are the opening paragraphs from the New Scientist article.
A smart phone that can detect radiation may soon be helping the police to find the raw materials for radioactive “dirty bombs” before they are deployed. The phones will glean data as the officers carrying them go about their daily business, and the information will be used to draw up maps of radiation that will expose illicit stores of nuclear material.
The detector is the brainchild of engineers at the Lawrence Livermore National Laboratory (LLNL) in California, US, who developed it in response to the rise in illicit trafficking of radioactive materials (see graphic). Customs officers at ports and airports already wear pagers that detect radiation. But any radioactive material not picked up by border controls can be hidden in towns and cities, with little chance that it will be found.
And here is the solution proposed by the LLNL engineers.
Now LLNL engineers funded by the US Department of Homeland Security have devised a way to tackle the problem. They have turned a multi-function internet cellphone into a wireless sensor that will feed data into a new type of radiation monitoring network that they are calling a RadNet.
The phone transmits radiation readings continuously over an always-on internet connection to a central computer. A GPS receiver in the phone labels the data with a time and location, allowing it to be used to build up a radiation map of a particular area.
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Here is a picture of a prototype of such a cell phone with its compact radiation detector and its GPS unit (Credit: LLNL). |
How much will cost such a device?
The challenge for the LLNL engineers was to devise a radiation sensor cheap enough to make the project viable. "It’s relatively straightforward to make a $10,000 radiation detection device that works well," says project leader Bill Craig at LLNL’s Radiation Detection Center (RDC). "But the target price of these units is $1000. That’s the phone, the whole thing."
This paper from the RDC, "Compact Radiation Detector and Global-Positioning-System Unit in a Cell Phone," gives additional details about these phones (the above illustration comes from this paper).
New-generation gamma-ray sensors and a global-positioning-system (GPS) module are built into a cell phone. The device uses pixelated cadmium zinc telluride (CdZnTe, often abbreviated CZT) detectors coupled with an ultra-low-power readout with moderate energy resolution. The device requires no cooling and is battery-powered (24 to 48 hours on a single charge).
Because the poorer-quality sections of the CZT crystal are left unconnected, we can use less-expensive, commercial-grade detector materials. The material is literally "sliced and diced" from the ingot, with no material selection or individual detector testing required. This approach dramatically lowers the cost of each detector, which costs less than $350 now and will be less than $100 when the device is mass-produced.
Let's return to New Scientist for more details about the detectors.
Craig’s team cut costs by compromising on the quality of the cadmium zinc telluride (CZT) semiconductor crystals that lie at the heart of the detectors. When a gamma-ray photon strikes the CZT it knocks a number of electrons out of position, producing a cascade of electron-hole pairs. A voltage applied across the crystal turns these into a current whose strength depends on the energy of the incident photons. This in turn allows the radionuclide that generated the gamma rays -- caesium-137 or cobalt-60, for example -- to be identified.
The reason why these detectors are relatively inexpensive is thta the team used cheap crystals.
They divide the crystal into 64 separate sensing "pixels" in which each pixel acts as a detector on its own. They then simply discard the output from the 10% to 15% of pixels that do not work because of defects. The team has found that by using four of these crystals in each phone, they can achieve reasonable sensitivity.
Let's move back to the RDC paper to check for future uses of these phones.
The device reports its health when reporting its data; system maintenance consists of swapping out a malfunctioning unit in much the same way pager or cell phone companies do. The only maintenance requirement is charging the battery periodically. Each device can be used as a programmable radiation alarm, personal dosimeter, search instrument, and analysis tool.
A central processing system monitors the data from the network of devices and provides additional sensitivity by tracking below-threshold alerts, correlating measurements from different detectors passing the same location at different times, and iteratively adjusting system thresholds to account for transient events.
Such devices are ideally suited for military personnel, Transportation Security Administration screeners, U.S. Customs and Border Protection agents, U.S. Postal Service personnel, public safety personnel, delivery service workers, and even nuclear search teams.
For more information about nuclear security, you can visit the web sites of the National Nuclear Security Administration (NNSA) and its Office of Nonproliferation Research and Engineering.
Sources: Jenny Hogan, New Scientist, December 9, 2004; and various web sites
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