Building a Death ray

After reading and hearing some truly interesting views about a thwarted terrorist effort to build an X-ray based weapon I thought it would be interesting to examine this idea and maybe try to assess the feasibility of such weapons. Firstly I will confess I have somewhat of a bizarre interest in beam weapons or directed energy research1 as it is often known. When I was a student I liked to read the crazy conspiracy sites with their illogical notions of existing but covered up death rays. I also read up on the very real and surprisingly extensive cold-war era research into beam weapon technology. If you are interested in reading up on this and other interesting weapons of that era (orbital anti-ICBM atomic bomb powered X-ray lasers being my favourite) you could do a lot worse than starting with these two awesome books.

Images of Books titled "Beam weapons" and "Weapons of Tomorrow"
So to start I will say that beam weapons are most certainly possible and an advanced modern army or state would have little trouble building them. Some limited prototypes actually exist. However they are not in widespread use for a very good reason namely that they are wildly impractical weapons. The conspiracy style death beam device which is generally an easily hidden machine that can fire a beam causing immediate death accurately at a target hundreds of miles away simply does not exist. Oh and that is not to be read as "I am unaware of" I mean "does not exist". I can be so sure of this statement due to various laws of physics and limits of engineering which prevent such devices being produced. If you follow the vast majority of the cold-war projects to their end the reasons given to stop funding them provide a fairly comprehensive list of these issues. Rumours persist that these projects merely went underground but in my humble opinion these are merely the ravings of the paranoia. I am happy to be proven wrong if anyone has hard evidence otherwise on this latter point. 

However let us step away from conspiracies and focus on this specific terrorist incident. Details on this case are very limited however we can piece together what they were attempting by examining publicly released details. These are some of the crucial details:

  • The device would be portable.
  • The device would use X-rays.
  • The victims would be affected days later.
  • The damage would be delivered whilst the victims slept.
  • The victims would die.
  • The device would be remote activated.

X-rays are merely a form of radiation. It is not so hard to calculate how much radiation we need for a decent weapon. The effects of high amounts of radiation are well known even though they do vary considerably from case to case. Absorbed radiation is called absorbed dose and is measured in units of grays (Gy) which are defined as the mean energy delivered (in joules) per kilogram. To make it likely otherwise healthy people die you would have to supply at very least 5 Gy. 

Next we will assume, somewhat generously, that the victims sleep for ten hours. You could argue the radiation might be delivered over multiple days but then the proximity required (I will show this later on) for the attack means the terrorists would likely be caught midway through the attack. Plus there are some media sources claiming the terrorists claimed the victims would be killed in a night. So to get the dose needed in this time period works out at a victim averaged intensity of 0.5 Gy/hr. This is a lotbut initially seems achievable given that some medical treatments work out at a higher rate. Although of course in medical irradiation3 the total dose is very rarely delivered in one go and sensitive organs are deliberately spared. 


So how could terrorists produce X-ray radiation? Taking into account the condition that the radiation was to be activated remotely4 and setting vastly expensive/complex equipment and under-powered sources to the side there is actually only two devices they could use. These are an X-ray tube like this

image of X-ray tube adapted from http://en.wikipedia.org/wiki/File:Fixed_anode_x-ray_tube.jpg
or a linear accelerator (linac) like this.

Image of a medical linac
I am of course assuming the terrorists are going to repurpose some existing source. You might argue that maybe they would construct an entirely bespoke radiation source for the weapon. This is unlikely as it would take months or years of research and construction to even match the commonly available devices and really why would they bother?  

So which of the two available devices do you go with?
 Well starting from a purely physics perspective you would probably prefer the linac. The X-rays these produce can be very high energy and thus very penetrating going up to a beam that will easily pass an external wall or several inches of lead. Secondly linac technology whilst inefficient is scalable to very high powers. However, linacs are highly complex and they are immensely expensive (say $2-300,000 pounds for a decent 2nd hand one) and generally require trained engineers to set them up and maintain them. However for now let us assume the terrorist have acquired an evil engineer who has the right skills to buy and set-up a linac. Linacs can appear quite small like the following elegant radiotherapy system:

Image of the Cyberknife (a small linac mounted on a car assembly robot arm)
But even this linac is far from portable and most of the bulky backroom bits (not shown in the image) are not designed to be moved about at all. If stripped down to the most crucial components it would still take something like 10 kW as a minimal to run. So if we wanted to run it portably we would need a decent generator. The generator would be of a size that you could probably wheel about but certainly not carry. The noise from this generator is not exactly going to help any attack go undetected. In fact to have any chance of this attack not being screamingly obvious you will need some way to hide the EMF, heat, noise and inevitable leaky radiation. Hiding all that is going to require heavy shielding materials but is possible.

So whilst a portable linac is achievable the complexity and inefficiency means an intense weaponizable portable linac is pretty close to impossible. Portable relatively low intensity linacs (used for high energy imaging) are possible and are even commercially available. You can judge for yourself from the following image of one whether you would notice it parked outside your house or not.

Image of a hefty truck armed with a linac for X-ray cargo inspection.
In spite of the initial promise of good penetration and high intensity I think we can assume linacs are out. Mostly for stealth and practical reasons but also as one of the released details from the investigation was that the terrorists planned their X-ray machine to be powered by a makeshift 2000 watt battery. The exact efficiency of a linac in terms of converting electrical energy to X-rays is strongly dependent on design and working parameters but when running continously for hours above 0.025% is very unlikely. Hence this battery is simply too small to power a linac capable of making a radiation beam intense enough to act as a weapon5.

So on to the other device. X-ray tubes are much simpler basically operating by directing a high voltage accelerated electron beam on to a metal target in vacuum. See here if you want a much more detailed explanation. All X-ray tubes run at about 1% efficiency in terms of converting their input electrical energy to X-rays. This conversion efficiency cannot really be increased as the efficiency is due to the inherent likelihood of atomic processes which the device relies upon to make the X-rays. So given this set efficency the terrorists' 2000 watt battery will generate ~20 watt of X-rays. If their target was an 80 kg man then as 1 watt = 1 joule per second this means we are delivering 20/80 or 0.25 Gy a second, right? A massive 900 Gy/hr! Potential terrorists rejoice. Well not quite.

Firstly the metal target will block and absorb about half of the X-rays and the rest will be emitted isotropically (in all directions) from a point on the target's surface. As practical X-ray lens or mirrors do not exist you cannot focus the X-ray radiation. Hence you can think of the X-rays as being like the heat from a very small campfire, as you move away from the source the intensity drops very quickly. Without going into the maths it works out a man 3 m (10 ft) away would only get about 1-0.5% of the emitted rays so the potentially lethal 900 Gy/hr beam has now dropped to the less scary 9-4.5 Gy/hr. It gets worse for the terrorists.

The penetrating power of the X-ray beam produced by an X-ray tube is dependent on the high voltage applied to the tube. This voltage is limited by the tube design. Go too high and you get electrical sparking and wreck your tube. Generally medical X-ray imaging and treatment is done at 20,000-250,000 V (20-250 kV). Considering these voltages are set to go only partly through a human and we want to through an external wall I think we can assume we want at least the top end of this voltage range. High voltage above 250 kV is very awkward6 to work with and thus high power high voltage supplies above this energy are actually quite rare. So let us assume the terrorists also know this and so have choosen 250 kV as their operational energy. So just how penetrating is a 250 kV beam? Well I took this X-ray image of two flat stones about 5 mm thick at that tube energy:

X-ray image of two stones showing the shadow is about 50% of the main beam
We learn from this image that even a thin stone cuts the strength of the radiation beam in half. The standard brick as used in UK houses is twelve times as thick and reduces the beam power to about 0.05%7 of the original strength.  So, even ignoring distance fall off, if our source was pressed up against the wall and our victim was pressed against the other side we would still be limited to a dose rate of less than 0.5 Gy/hr. It gets worse again for our hypothetical terrorist weapon designer.

X-ray radiation that is produced from a tube comes in a wide spectrum of energies. As you go through any material the lowest energy (weakest) rays are absorbed first. This means the more you go through material the weaker but more penetrating the remaining radiation beam gets. As the beam gets more penetrating more and more of it will pass through the target and naturally this means less and less dose is deposited. Basically you cannot win as a weak beam is easily blocked and a penetrating beam goes through your target. So coupling all these degrading factors together.

A 2000 W battery powering an X-ray tube gives:    
20 W of X-rays
Half of these X-rays are directly absorbed in the target:    
10 W
1% of the emitted rays hit a normal sized man 3 m away:            
0.1 W
A brick wall reduces the original beam to 0.05% of its intensity:      
0.000,05 W
40% of this beam is absorbed in the victim:                
0.000,02 W
who weight 80 Kgs:                                              
0.000,000,25 Gy
for 10 hours ( 36,000 seconds):                              
0.009 Gy or 9 mGy

This is almost an ideal case and yet it only results in a negligible dose. In fact the dose is so low that if you exposed a million people to this dose you would not be able to detect any ill effects even if you followed all of the exposed for their whole lifes. 

So what this post has demonstrated is that the scheme cooked up by this "evil genius" as some articles would have you believe is actually a load of bollocks. His claims that he could made 'Hiroshima on a light switch' or that 'everything with respiration would be dead by the morning' are either some sort of weird terrorist ranting or evidence of a utter lack of basic knowledge about radiation. Even if he further optimized his scheme the best he could achieve is to increase his victims risk of cancer by a very small amount (he would be far more successful by selling them very cheap cigarettes).  

There are ways to adapt the ideas that would make them a bit more effective. However for somewhat obvious reasons I do not want to go into these. In a nutshell portable X-ray weapons are perfect for science fiction stories but happily in the real world they are next to useless.


Footnotes

  1. I always thought this was an odd name as what is a gun if not a directed energy weapon.
  2. The highest confirmed radiation rates in-between reactors units at Fukushima was 400 mSv/hr which would actually result in far less than 0.5 Gy/hr. Of course not one of the radiation workers received doses anywhere near any of the high values I am discussing here.
  3. Patients are placed directly under the gantry of a very highly powered and most certainly not portable linac. The dose delivered is carefully monitored and modulated. It is about as far from a weapon as you could possibly imagine. 
  4. I am using this and the fact that no source mentioned them to discount radioisotopes.
  5. I could have started here and skipped the linac discussion but hey I just wanted to talk about linacs. 
  6. This is due to the increasingly large distances required to avoid electrical break down.
  7. It's not quite the value you might expect due to beam hardening and air gaps.