Lithium (p,n) Neutron Generators

[Wilfried Heil]

A number of high powered neutron sources employ lithium as the target material. When the lithium is used in the form of a liquid metal, it can absorb and transport the beam power without structural damage. It is also quite efficient for neutron generation.

The reaction has a surprising peculiarity:

Li-7 + p (p,n) -> Be-7 + n - 1.6 MeV
Be-7 + e- (EC) -> Li-7 + 0.86 MeV (gamma)

The threshold of the reaction is 1.6 MeV and it produces one neutron. The Be-7 then captures an electron, with a halflife of 53 days, and becomes Li-7 again.

In effect, the Li-7 acts as a catalyzer which fuses protons and electrons to neutrons. However, it needs energy to do this.

There is a low energy side reaction which has no threshold and consumes some of the lithium: Li-7 + p -> Be-8 -> 2 alpha + 17.3 MeV. The latter is a true aneutronic fusion reaction, followed by fission of the Be-8.

[Richard Hester]

Back in high school, I was thinking of doing fusion with the Li7-p reaction. Cockroft and Walton investigated this one with their newly invented multiplier stack long ago. The threshold energy is pretty high, though.

[Wilfried Heil]

The good part of this threshold fusion reaction is that most of the excess proton energy will be carried away by the neutron. By adjusting the proton accelleration voltage, the neutron energy can be tuned within the range of 30..600 keV. These neutrons are not emitted isotropically, but are kinetically focused in the forward direction, which greatly increases performance.

The Cockroft-Walton multiplier is still used in almost all of these accellerators.

A medical application for such epithermal neutrons is "Boron Neutron Capture Therapy" (BNCT). As promising as it sounds, I think it is best to never need it.

[Carl Willis]

This reaction is attractive for accelerator-based neutron sources because it is endothermic and has a high cross-section. The endothermicity gives rise to a strongly forward-directed angular distribution for the neutrons via kinematic constraint. At a proton energy of 2.5 MeV, one can expect a maximum neutron yield of about 1E+12 n / s / mA.

My dissertation work is likely to focus on the design of BNCT treatment facilities using this reaction. Wilfried's right though, you don't ever want to be recommended for such a treatment--that would imply that you have a virtually incurable malignant brain tumor.

Attached is a piece of "eye candy", a cut along the axis of a target and flux-tailoring scheme for BNCT that was designed here at OSU and uses the 7Li(p,n) reaction. This is a mesh tally output from the radiation transport modeling program MCNPX, with logarithmic color scale indicating neutron flux magnitude. In the middle of the red region is the circular planar target. Below that is a conical moderator and finally a recess that is the "treatment port" where the victim's skull is placed. There are materials surrounding the moderator to reflect neutrons and delimit the beam where necessary. Finally, as mentioned, the Be-7 product is radioactive. It produces some gamma rays of ~480 keV that are very important in shielding design and in figuring out how to protect and handle spent targets (for BNCT machines, hundreds of curies of Be-7 can accumulate).

-Carl

[Javier Lopez]

If you are interested in collimate or focusing the X rays you can see at XMM X ray telescope that uses lots of concentric tubes covered by gold.

[Wilfried Heil]

What is the total dose administered to a patient in BNCT therapy?

It would consist of the neutron dose, the (intended) dose from Boron dissociation and the prompt gammas from capture reactions. The boron concentration in the body isn´t known, but could be deduced by measuring the gamma field. With 10^13 mostly thermalized neutrons/s in total and irradiation times of 30 minutes, I would expect this to add up to doses which I do not consider survivable.