Radiation and Long-Term
Space Flight

Bonner Ball Neutron Detector

Here is a rundown on how the Bonner Ball neutron detector works:

A small stainless steel shell contains helium 3 (He3) at 6 atmospheres pressure (the pressure is high enough to contain a lot of atoms but not high enough to burst the shell). It is covered with a thick layer of hydrogen-rich material such as polyethylene.

 

Inside the shell is a thin wire which is connected to an external electronics package. The output signal is measured by the current in the wire when it is biased to approximately +1000 volts. Here are details on how a fast neutron eventually becomes a current in the wire:

 1) Fast neutrons are thermalized by polyethylene. I.e., protons in polyethylene with mass almost identical to the neutron quickly distribute (via elastic collisions) the high neutron energy to zillions of protons, producing a slow or "thermal" neutron.

2) The thermal neutrons undergo nuclear reactions with the He3 inside the Bonner Ball:

n + He3 -> p + H3 (tritium) + (764 keV)
In words, the above reaction symbols mean that a neutron impacts and has a nuclear reaction with an atom of He3. The arrow points to the reaction products which are a proton, a positively ionized atom of tritium and 764 keV of kinetic energy to be shared by the two particles. We know that the He3 must have a charge of -1 because the proton has a charge of +1 and charge is conserved (the total charge is zero on the left hand side)

Due to their 1:3 mass ratio, the p and H3 must share the energy in a 3:1 ratio. Thus the proton gets 573 KeV and the tritium nucleus gets 191 KeV. Both these guys are way too hot to be attracted to things biased to only a few thousand volts (they will only be moderately deflected by 100,000 volts). So how are these products to be detected?

3) Answer: ionization of the background gas! There is a high probability for ionization of the He3 gas by these energetic particles produced in the nuclear reaction.   Considering only the fast proton for now (the fast tritium can produce electrons too):

p + He3 -> p + e- + He3+.
Again, in words, a fast proton strikes a slow He3 atom in the gas giving a fast proton (which has lost a small amount of energy in the collision), a low energy electron and the resulting He3 atom minus this electron (a positive ion). In other words, the proton knocks loose one of the two electrons flying around the He3 nucleus.  This reaction does not involve any changes to the nuclear particles; this is an atomic or chemical reaction.  The proton can produce many free electrons before it finally loses all of its kinetic energy.

The freed electron can be easily detected by collection to a low voltage biased wire (i.e., very low energy negative electrons will be sucked in very nicely by a wire charged to positive 1000 volts).  The collected electrons constitute a current which is measured by an external amplifier.  The current is proportional to the rate which neutrons strike the Bonner Ball.

Note that the designers really get their money's worth out of the He3 gas since it is used in the original nuclear reaction and in the final atomic reaction to produce a charge signal. Pretty clever, huh?. Apparently this design originates from a paper by J. W. Leake (J. W. Leake, "Nuclear Instruments and Methods", Vol. 63, page 329, 1968).