# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 6 MKIDs for Security Applications  
A further area for which the use of MKIDs is being investigated is security applications. Currently, there are groups based in Cardiff, Glasgow, Rome, Helsinki, and Groningen that are developing passive GHz and THz imaging cameras based on MKIDs, for security applications such as airport screening, the detection of hidden objects such as landmines, and the screening of trucks and vans. MKIDs are a promising technology for these applications because they allow large arrays of detectors due to their multiplexabiliy, with time responses of \(\sim\) 10\({}^{-4}\) s, while detecting frequencies from about 300 to 1000 GHz (1.0-0.3 mm), at which certain materials which are opaque at optical wavelengths become transparent. While active THz imaging is already used in airports around the world for security screening, MKIDs would, due to their improved sensitivity, allow for passive THz imaging with fast time responses, allowing for subjects to be scanned while simply walking past the camera, greatly reducing the time needed for each traveler to be screened [148; 149; 150; 151; 152]. [PERSON] et al. in Cardiff University have developed a passive 350 GHz (860 \(\upmu\)m) field-scanning LEKID camera [148], operating at a \"quasi-video\" frame rate of 2 Hz, and noise equivalent temperature of \(\sim\)0.1 K per frame based on a linear array of 152 LEKIDs. The array is fabricated from an aluminum film deposited onto a high purity silicon wafer. This camera successfully demonstrated sufficient spatial resolution, sensitivity and scanning speed to detect concealed objects in a person's clothing or luggage. The array is housed in a cryostat with a 25 cm diameter window, into which incoming light is coupled using lenses and a flat beam forming mirror. At any one moment, a thin, horizontal strip of the object is scanned with the mirror, while oscillating the mirror allows the full object to be covered. This array achieved a pixel yield of 85%, with the unusable pixels being attributed to non-uniformity in the aluminum film thickness, causing overlapping resonators. This LEKID array was used to detect objects hidden inside a subject's coat, like a wallet, an air pistol, and some coins. [PERSON] et al. [151] in Helsinki have developed a dual-band imaging system, CONSORTIS, based on 8208 kinetic inductance bolometers (KIBs) in an array. This is also being developed to be used as a passive, \"walk by\" scanner for security screening purposes. KIBs are equivalent to TKIDs thermal detectors, consisting of an absorber and inductive thermometer on a thermally confined membrane. This thermometer is based on the temperature dependence of the kinetic inductance effect, and is coupled to an external capacitor, creating a resonance. The KIBs are then read out in the same manner as TKIDs. One third of the 8208 pixels are designed to operate at a frequency band centered at 250 GHz (1.2 mm), while the remaining two thirds of the pixels are sensitive around 500 GHz (600 \(\upmu\)m). This system has demonstrated detection of an aluminum bar as well as plastic and paper objects underneath a person's jacket. They report that they demonstrated the basic operation of this system at detecting concealed objects, and that work is ongoing to scale up this technology to allow all the detectors in the array to be read out simultaneously. It is reported that an issue with proper thermalization of the full focal plane has been resolved, and that work is ongoing to fully parallelize the readout. [PERSON] et al. [150] in Rome have designed and fabricated high \(T_{c}\) MKIDs, based on YBa\({}_{2}\)Cu\({}_{3}\)O\({}_{7}\) (YBCO), reaching a critical temperature of up to 89 K, for use in security applications. They aim to use high \(T_{c}\) MKIDs to allow the use of cheaper, more reliable cryogenic systems, needing to only reach temperatures of 20 to 40 K, as opposed to typical mK temperatures.