Re: Alpha-Be neutron source construction

["Dale E. Boyce" <dale@radpro.uchicago.edu>]

Jeff,

The Be(a,n) reaction is exothermic by 5.65 MeV.  The tunneling
probability for the alha is extremely energy dependent however.
The 5.3 MeV Po210 alpha yields 80/1e6 while the 5.155 MeV Pu239
yields only 60/1e6.  I would certainly expect some neutrons
from Po210 in close contact with Be, but any mass of material
between the source and the Be will degrade the energy and lower
the yield.  The tunneling probability varies as:

exp(-8 Pi Z z e^2/hv(arccos(T/B)^0.5 -(T/B)^0.5(1-T/B)^0.5))

where Z ans z are the proton numbers of the alpha and Be
h is Planck's constant, T is the c.m. kinetic energy of the
a be collision, and B is the coulomb barrier height.
Oh, and v is the velocity of the alpha I believe and not frequency.
Ref. "Nuclear Chemistry" Friedlander, Kennedy, Miller, and Macias
ISBN 0 471 86255-X

You will also see a decrease in yield due to geometry.  Besides
the obvious factor of two you lose automatically, the energy degradation 
due to source thickness, source covering material, intervening air,
and any film on the Be will be exagerated for alpha's that are not emitted 
perpendicular to the source.  It may be that there is sufficient
intervening material that you effectively have a very small solid
angle into which alphas may be emitted that will result in a
probability of the (a,n) reaction occuring.

Dale
dale@radpro.uchicago.edu