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Lithium disintegration as a source of heat
John T. Lenaghan
12th May 2022
The reaction 7Li ( p , α ) 4 He (Q = 17.35 MeV) is available as a source of heat. Given a proton
source and accelerator of beam current 150 mA at 1 MeV 1 if every proton were to cause a
disintegration would yield approximately 2.6 MW. One disintegration per 2 000 protons is assumed
to be available yielding 1.3 kW.
Ajzenberg-Selove and Lauritsen (1974) list 7Li ( p , α ) 4 He as 21(e) and Qm = 17.348 with a
reference to 8Be for the reaction details listed as reaction 21 there. The cross section is stated to
be 3.7 ±0.4 mb with proton energy of 561 keV. The research quoted states a quadratic fit for the data
that might be extrapolated. Such an extrapolation should be checked against research that has not
been reported here.
Henderson (1932) reports that the target used was only 0.002 inches thick and material lithium
fluoride (LiF). The resulting rate of disintegrations is 40 per 109 protons with energy 1 MeV.
The expectation is that with a careful management of the beam shape and the ‘thickness’ of the
target the low rates of disintegrations reported in the earliest papers in the research should be
improved firstly by a factor of 250,000 by that increase in ‘thickness’ then a change of target
material from LiF and lithium hydroxide (LiOH) to lithium hydride (LiH) that would reduce the
shielding electrons from twelve to about three giving another four fold increase in the rate of
disintegrations per proton count. LiH has two advantages over the pure metal as a target. Firstly, the
melting point of 692 degrees Celsius is more easily used thermodynamically to transfer energy
compared to 180.5 degrees Celsius. Secondly, the excess of hydrogen atoms from the unreacted
beam of protons would evolve as gas instead of reacting with the target. It is also noted that
increasing the thickness of the target, that is it’s overall volume, the alpha particles carrying the
energy are stopped inside the target releasing that energy to the heat of the target material. The
helium that results is just evolved as a gas. In cloud chamber studies the alpha particles left tracks of
approximately 8.5 cm. The target material being seven hundred times more dense than the cloud
chamber the alpha particles would be stopped 125 µm from their source. No radiation escapes.
It is noted that if alpha particles can only travel 125 µm in the target the geometry of beam and
target would have to be rather special to let the protons cause two million times more
disintegrations than with a thin flat wafer of a target perpendicular to a narrow beam. And yet more
special to let the heat transfer through the material of the target without ablation. It is believed that
‘thickness’ may be better stated as ‘surface area.’
It is expected that many ‘state of the art’ design improvements would yield ever more
disintegrations per beam proton count. And that there would also be available improvements in the
designs of proton sources and accelerators to have a beam current of about 1 ampere. At these
currents and nearly 100% protons reacting there would be released over 10 MW of heat energy that
would need carefully designed retorts for the target material so that the heat may be transferred to a
cold reservoir by a heat engine generating electricity.
1] Wiesner, C. & Chau, Loi & Dinter, H. & Droba, M. & Heilmann, Manuel & Joshi, Ninad &
Mäder, D. & Metz, A. & Meusel, O. & Mueller, Igor & Noll, D. & Podlech, Holger & Ratzinger,
Ulrich & Reichau, Hermine & Reifarth, R. & Schempp, Alwin & Schmidt, S. & Schweizer, W. &
Volk, K. & Wagner, Christopher. (2010).
Proton Driver Linac for the Frankfurt Neutron Source.
AIP Conference Proceedings. (15–19 December 2009).
pages 487-492.
doi: 10.1063/1.3480247
Ajzenberg-Selove and Lauritsen (1974)
Ajzenberg-Selove, F. and Lauritsen, T.
Energy levels of light nuclei A = 5—10,
doi: 10.1016/0375-9474(74)90780-5
Nucl. Phys. A
volume 227,
pages 1-243
Henderson (1932)
Henderson, M. C.,
The Disintegration of Lithium by Protons of High Energy,
doi: 10.1103/PhysRev.43.98
Phys. Rev.,
volume 43,
issue 2,
pages 98-102,