Irradiator

I don't think that I have ever posted an images of what our detector looks like, or the setup.  Since I had to get detailed drawings for MCNPX/Geant4 simulations, I decided to make a post about our irradiator.  On the left is an image of what our irradiator looks like.  

There are three main compartments of the irradiator.  One is the source, the other is the lead tube, and the other is the cadmium tube.  In the picture to the left the lead tube is  one the left, and the cadmium tube is on the right (it has electrical tape around it since cadmium is harmful if you ingest it).

A view from the top of the irradiator with the top shield missing is show on the left. The right most section is where the source goes.  Going to the left there is a 2.125" block of polystyrene to help moderate (slow down) the neutrons before they go to the lead and cadmium tube.  In the last section on the left we store other sources that need to be moderated.

The most important aspect of the irradiator is the Cf-252 source.  Not only is this source expensive (There is only one facility in the US capable of making Cf-252 sources, the HIFR at ORNL), but it is the reason why I will probably never have kids (not really, see 1).  Martin calculated the source at 1.3 million neutrons/s; I am in the processes of repeating those measurements.  In general Cf-252 emits neutron of energies around 1 MeV. (This is actually quite small.  When the neutron undergoes a capture reaction in Li, it emits 4.7 MeV).    The source is about an inch long and encased in a lead pig (2).

The purpose of the lead and cadmium tubes is to be able to isolate the neutron response.  Neutrons interactions are governed by a probability distribution, and if they do interact in other parts of the irradiator they create secondary particles which could (and do) cause photons to be adsorbed by the Photo Multiplier Tube (PMT).  Cadmium has a very high neutron adsorbition cross section, so we assume (I am working on validating how accurate this assumption is with MCNPX) that any response from the cadmium tube is from this extra radiation.  We can then subtract the cadmium spectrum from the lead spectrum to get the net neutron response (3).

1. Father is of the opinion that I should wear a lead cup.  This would make things worse.  The neutron would react with the lead producing high energy electrons.  These in turn would produce bremsstrahlung (photons) which would not be adsorbed in the lead, and would end up in my junk.  If one was really worried about adsorbing neutrons they would use a hydrogen dense compound (polystyrene, water) because the hydrogen can adsorb all of the energy of a single neutron collision.  Hydrogen stops much easier than neutrons because they have have +1 charge (the electron is usually stripped off). 
2.  If you have something to adsorb the bremsstrahlung (such as polystyrene) then lead is great for stopping neutrons.  If you don't you are generally better off with no shielding.
3. There isn't a particular reason why the other tube is covered with lead.  In short it was done to humor Neel.  Neel (post doc who make the polymer detector) thought that a lead tube would help eliminate the low energy counts which destroy his detectors efficiency, but if you have a highly conductive metal (such as lead) and a current (such as a cascade effect of electrons in the PMT) than the electrons create a magnetic field in the tube, which in turn enhances the electron cascade. When Neel first suggested this I mentioned this problem to him, but he pushed for it anyway.  Once implemented I was correct, and the lead tube actually made it worse. Physics!!  (We still use the lead tube because Neel and I don't get along very well . . .)

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