Answer to Question #2095 Submitted to "Ask the Experts"

Q

What is the best (in terms of high flux) portable source of thermal neutrons?

A

First, any practical portable neutron source will not provide you with thermal neutrons. You will have initially fast to intermediate energies, with a spectrum of neutron energies into the million electron volt (MeV) range. To obtain thermal energies (i.e., in the eV range), you will have to moderate these neutrons through collision interactions with hydrogenous material—such as water, heavy water, paraffin, high-density polyethylene, etc.

Commercially available sources of neutrons include ²⁵²Cf that normally undergoes an alpha decay, but has about 3% of its decays through spontaneous fission. The neutron emission rate for ²⁵²Cf is about 2.3 X 10⁶ neutrons per second per microgram. Alternately, sources using the (alpha in - neutron out) nuclear reaction with beryllium (Be) are common. Modern sources typically use ²⁴¹Am, but other alpha emitters have been used in the past, such as ²²⁶Ra and ²³⁹Pu, i.e., often called AmBe, RaBe, and PuBe sources. The RaBe and PuBe sources have 1.7 X 10⁶ and 1 X 10⁶ neutrons per second per curie yields respectively. Herman Cember's book "Introduction to Health Physics" (published by McGraw Hill) has a good discussion of this subject.

Lastly, depending on your needs, project budget, and source availability, any of these may satisfy your needs. In fact, the Conference on Radiation Control Program Directors offers a list of "sources in need of a good home," versus the owner/licensee continuing to store no-longer-needed sources or pay for radioactive waste disposal. You might contact them for the most recent list.


David J. Allard, CHP