One of the more frightening scenarios that civil defence teams worry about is the possibility that a bomb contaminated with radioactive material would be detonated in a heavily populated area.

Various research teams have considered this problem and come to similar conclusions–that the actual threat to human health from such a device is low. Some even claim that terror groups must have come to a similar conclusion which is why we’ve not been on the receiving end of such an attack. The panic such a device would cause is another questions.

Today Theodore Liolios from a private institution called the  Hellenic Arms Control Center in Thessaloniki in Greece, goes through the figures.

He says the most likely material to be used in such an attack is Cesium-137, widely used throughout the developed and developing world as a source for medical therapies.

The unstated implication is that it would be relatively easy to get hold of this stuff from a poorly guarded hospital. Exactly this happened in Goiania in Brazil when an abandoned hospital was broken into and its supply of cesium-137 distributed around the surrounding neighbourhoods. The incident left 200 people contaminated. Four of them died.

But a dirty bomb would not be nearly as lethal. The trouble with them (from a terrorist’s point of view, at least) is that distributing radioactive material over a large area dramatically reduces the exposure that people receive. Particularly when most could be warned to stay indoors or be evacuated (unlike the Goiania incident in which most people were unaware they were contaminated).

Liolios calculates that anybody within a 300 metre range of a dirty bomb would increase their lifetime risk of cancer mortality by about 1.5 per cent. And then only if they were unable to take shelter or leave the area. That’s about 280 people given the kind of densities you expect in metropolitan areas.

And he goes on to say that it is reasonable to assume that a cesium-137 dirty bomb would not increase the cancer mortality risk for the entire city by a statistically significant amount.

But the terror such a device might cause is another question. Liolios reckons that current radiation safety standards would mean the evacuation of some 78 square kilometres around ground zero. That would affect some 78,000 people, cost $7.8m per day to evacuate and some $78m to decontaminate.

That seems a tad conservative but however it is calculated, it may turn out to be chickenfeed compared to the chaos caused by panic, which may well result in more deaths than the bomb itself could claim. How to calculate the effect of that?

Ref: arxiv.org/abs/0902.3789: The Effects of Using Cesium-137 Teletherapy Sources as a Radiological Weapon

Plot the number of people killed in terrorists attacks around the world since 1968 against the frequency with which such attacks occur and you’ll get a power law distribution, that’s a fancy way of saying a straight line when both axis have logarithmic scales.

The question, of course, is why? Why not a normal distribution, in which there would be many orders of magnitude fewer extreme events?

Aaron Clauset and Frederik Wiegel have built a model that might explain why. The model makes five simple assumptions about the way terrorist groups grow and fall apart and how often they carry out major attacks. And here’s the strange thing: this model almost exactly reproduces the distribution of terrorists attacks we see in the real world.

These assumptions are things like: terrorist groups grow by accretion (absorbing other groups) and fall apart by disintegrating into individuals. They must also be able to recruit from a more or less unlimited supply of willing terrorists within the population.

Being able to reproduce the observed distribution of attacks with such a simple set of rules is an impressive feat. But it also suggests some strategies that might prevent such attacks or drastically reduce them in number . One obvious strategy is to reduce the number of recruits within a population, perhaps by reducing real and perceived inequalities across societies.

Easier said than done, of course. But analyses like these should help to put the thinking behind such ideas on a logical footing.

Ref: arxiv.org/abs/0902.0724: A Generalized Fission-Fusion Model for the Frequency of Severe Terrorist Attacks

How much force does it take to stab somebody to death? Strangely enough, forensic scientists do not know.

A number of groups have attempted to measure the forces necessary to penetrate skin but the results are difficult to apply to murder cases because of the sheer range of factors at work. The type and sharpness of the knife; the angle and speed at which it strikes; the strength of skin which varies with the age of the victim and the area of body involved; these are just a few of parameters that need to be taken into account.

So when giving evidence, forensic scientists have to resort to relative assessments of force.

“A mild level of force would typically be associated with penetration of skin and soft tissue whereas moderate force would be required to penetrate cartilage or rib bone. Severe force, on the other hand, would be typical of a knife impacting dense bone such as spine and sustaining visible damage to the blade,” says Michael Gilchrist at University College Dublin and pals who are hoping to change this state of affairs.

They’ve developed a machine that measures the force required to penetrate skin–either the animal kind or an artificial human skin made of polyurethane, foam and soap.

The surprise they’ve found is that the same knives from the same manufacturer can differ wildly in sharpness. And the force required for these knives to penetrate the skin can differ by more than 100 per cent.

That could have a significant bearing in some murder cases. And that’s important because in many European countries such as the UK, stabbing is the most common form of homicide.

Gilchrist and co say their work could even help tease apart what has happened in that most common of defences: “he ran onto the knife, your honour”.

The key thing here is the speed and angle of penetration. The angle can be measured easily enough but the speed is another matter altogether. Gilchrist and co say future work may throw some light on this.

Ref: arxiv.org/abs/0811.3955: Mechanics of Stabbing: Biaxial Measurement of Knife Stab Penetration of Skin Simulant