KNN classification of metallic targets using the magnetic polarizability tensor

Walk-through metal detectors are used at check points for preventing personnel and passengers from carrying threatening metallic objects, such as knives and guns, into a secure area. These systems are capable of detecting small metallic items, such as handcuff keys and blades, but are unable to distinguish accurately between threatening objects and innocuous items. This paper studies the extent to which a K-nearest-neighbour classifier can distinguish various kinds of metallic objects, such as knives, shoe shanks, belts and containers. The classifier uses features extracted from the magnetic polarizability tensor, which represents the electromagnetic properties of the object. The tests include distinguishing threatening objects from innocuous ones, classifying a set of objects into 13 classes, and distinguishing between several similar objects within an object class. A walk-through metal detection system is used as source for the test data, which consist of 835 scans and 67 objects. The results presented show a typical success rate of over 95% for recognizing threats, and over 85% for correct classification. In addition, we have shown that the system is capable of distinguishing between similar objects reliably. Overall, the method shows promise for the field of security screening and suggests the need for further research. © 2014 IOP Publishing Ltd

‘Snitches get stitches’: US homicide detectives' ethics and morals in action

This paper draws upon evidence from a short but intensive period of ethnographic fieldwork with a specialist homicide squad in a large US city. A range of homicides were observed during the study, and the discussion that follows describes a number of cases in depth and the difficulties the detectives experience in obtaining evidence from witnesses who may be frightened or unwilling to help them. The way they regard these problems and lack of cooperation, and the techniques they use to obtain information or confessions from suspects are explored. To analyse these problems, the paper reflects upon the ‘ruses’ detectives were observed to use in their attempts to obtain confessions and the way they rationalise these methods in terms of their personal and professional ethics