The receiver sensitivity could be approximated by knowing the kind of
treatment the cat got (diagnostics, or outright treatment, which
determines the isotope used and therefore its gamma spectral signature and
therefore the attenuation level by the vehicle), its weight (and therefore
the approximate dose of the isotope), and the distance of the detector
from the vehicle at the maximum proximity (the article states 80 ft).
The radiological treatment/diagnostics isotopes that are injected into the
body are usually highly radioactive and short-halftime, in molecules
designed to be preferentially accumulated in the cancer tissue. The
radiation level emitted by the cat is therefore expected to be pretty
high.
I assume to achieve the required long-distance sensitivity they are using
large scintillator crystals.
Such detectors, with suitable crystals, are capable of discriminating
between differnet isotopes. Each isotope emits radiation with
characteristic energy (it is commonly stated in kiloelectronvolts or
megaelectronvolts - keV or MeV, but it can be recalculated to wavelength
or frequency, to make it more familiar to the people here who are used to
another part of the EM spectrum). The suitable scintillators have a
repeatable behavior where the high-energy photon is braked in the
scintillator's structure and the energy it hands over is reradiated as
photons of a relatively narrow range of wavelengths, specific for the
scintillator; the number of photons emitted for each MeV of the gamma
photon absorbed is characteristic for the scintillator used). Therefore
each absorbed gamma photon produces a flash of light, with the flash
intensity proportional to the gamma photon energy. The flash is then
amplified, typically by a photomultiplier tube. The attached electronics
counts the flashes and measures their intensity, ignoring the known false
alarms (e.g. potassium-40, a characteristic pest in banana shipments) and
sometimes using a built-in library of energies to identify the isotopes
present. (The signal processing is a little more complicated, due to the
not exactly simple behavior of interaction of ionizing radiation with
matter, but it is nothing new for many decades already.) Thallium-doped
sodium iodide or caesium iodide is typically used for gamma spectrometers.
I assume more modern types use organic scintillators (polymers with some
polycyclic stuff added), but I am not certain what is the contemporary
state of technology.
Alternative detectors are based on large blocks of a semiconductor
(typically germanium). While being more sensitive, they require cooling
and are more suitable for laboratories and space probes than for field
deployment.
More detailed description of the technology is here:
http://en.wikipedia.org/wiki/Gamma_spectroscopy
On Tue, 25 Mar 2008, TSCM/SO Group wrote:
> Talk about receiver sensitivity......
>
>
> Nuclear detectors in Washington State detect radioactive cat Published 25
> March 2008
>
> DHS has radiation monitors along Interstate 5 to make sure no radioactive
> material is being smuggled into the country; the monitors are so sensitive
> that they detected a sick cat in car driving at 70 miles per hour (the cat
> was taken home after cancer radiation treatment at the vet)
>
> In a meeting last week of 200 San Juan Islanders, deputy chief Joe Giuliano
> of the Border Patrol explained to agency's policy to conduct citizenship
> checks on domestic ferry runs.
>
> The Seattle Times's Danny Westneat writes that near the end of his
> presentation, while trying to convince the audience that the point is to
> root out terrorists, Giuliano revealed that DHS has been monitoring
> Interstate 5 for nuclear dirty bombs.
>
> They do it with radiation detectors so sensitive it led to the following
> incident.
>
> "Vehicle goes by at 70 miles per hour," Giuliano told the crowd. "Agent is
> in the median, a good 80 feet away from the traffic. Signal went off and
> identified an isotope [in the passing car]."
>
> The agent raced after the car, pulling it over not far from the monitoring
> spot (near the Bow-Edison exit, 18 miles south of Bellingham).
>
> The agent questioned the driver, then did a cursory search of the car,
> Giuliano said. "Turned out to be a cat with cancer that had undergone a
> radiological treatment three days earlier," Giuliano said.
>
> He added: "That's the type of technology we have that's going on in the
> background. You don't see it. If I hadn't told you about it, you'd never
> know it was there."
>
> Mitch Davis
> TSCM/Special Operations Group
> 20 Music Square West,Suite 208
> Nashville,TN 37203 USA
> 615 251 0441
> Fax 615 523 0300
> mit..._at_tscmusa.com
> www.tscmusa.com
> "maintaining a higher degree of excellence"
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Received on Sat Mar 02 2024 - 00:57:21 CST