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Chocolate bombs

Again I feel the need to comment on the science of chocolate and explosives again. I feel that after the comments made in http://answers.yahoo.com/question/index?qid=20100602221408AA9hKfm that I should enlighten the public a little more regarding chocolate and things which go bang.  

I would like to talk a little about bomb detection and imaging systems. I only have access to the public domain literature and I will not be giving out a “how to” guide on getting a bomb / gun past security staff. This is in the same way as I will not give you details of how to cook street drugs.  

One modern and interesting thing is the (n,p) reaction of nitrogen-14 with a high energy neutron. When this occurs a gamma ray with about 10 MeV of energy is formed. The equation for the reaction is shown below.  

14N + n –> 14C + p + gamma 

When you look at such an equation it is important to do two things. Firstly make sure that the equation is balanced in terms of charge, the nucleons (bits of the nucleus {protons, neutrons}), momentum and also balanced in terms of energy. In this reaction the neutron is a very fast neutron (at least 10 MeV) and a lot of energy is taken away by the gamma ray. It is important to note that energy and mass can be interconverted, but when this happens then both particles and antiparticles are made.  The energy / mass conversion can be understood by the famous equation E = mc2.

 

  

A good example of a series of nuclear reactions which might occur inside your head, if you go for a PET scan are. 

18F –> 18O + antielectron + neutrino  

  

  

The neutrino will fly off and is almost never stopped or detected on earth while the antielectron then takes part in a second reaction. 

  

   

antielectron + electron –> gamma photon + gamma photon.  

   

Because the momentum is balanced the two photons fly off in opposite directions. I think that the momentum of the electrons (including the antielectron) has little effect on the directions of the gamma rays. The momentum of a photon can be described by the equation.  

   

P = plank’s constant (h) / wavelength  

   

As wavelength = speed of light (c) / frequency (f)  

   

and energy = hf  

   

Then I am sure that you can work out a relationship for getting the momentum of a photon from the energy. I know I can do it, but you can go some of the work yourself. It is saturday morning and I need to take tree branches to the waste disposal site so I have too little time to write every last equation !  

   

Now you might ask what have nuclear reactions got to do with bombs.  

   

A bomb typically contains a nitrogen rich substance (explosive), this nitrogen can be detected if neutron source is used to provoke the n,p reaction in the nitrogen present in the explosive. If a suitable energy dispersive gamma detector is present then it can be used to make an estimate of how much nitrogen is present inside an object. The idea is that a bomb will be more nitrogen rich than a non-bomb. The only problem is that it is not perfect there are some objects which could give a false positive result. One such item would be a poly of nitrile rubber gloves.  

   

Nitrile rubber is a polymer of acrylonitrile, which makes it very nitrogen rich. Maybe you should find out what acrylonitrile is and work out the nitrogen content of a nitrile glove.  

   

Chocolate is very low in nitrogen so I would not expect a bar of chocolate to give a false positive result in this test. I must go now and do some garden cleaning.  

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