Answer to Question #8536 Submitted to "Ask the Experts"

Category: Radiation Basics — Neutrons

The following question was answered by an expert in the appropriate field:

Q

For photons, over a very wide range of energy and for almost all elements, it is possible to find data for the attenuation coefficients and for the absorption coefficients. However, is it possible to find data for the attenuation coefficients and the absorption coefficients for neutrons? If the answer is yes, please let me know where.

A

Many sources provide neutron cross-section information. The data are usually presented as atomic cross sections for specific isotopes of selected elements, with units of cm2. Photon attenuation coefficients and absorption coefficients are most often presented as macroscopic cross sections, usually the mass attenuation or mass absorption coefficient, with common dimensions of cm2 g-1, or the volume cross section, with common units of cm-1 (i.e., cm2 cm-3), also referred to as the linear attenuation coefficient. If you are given the neutron atomic cross section for a specific isotope, you can obtain the mass macroscopic cross section for a given material by multiplying the atomic cross section for the isotope by the isotope atom density, in atoms per gram, of the material.

As an example, consider the absorption of thermal neutrons by 10B to produce the well-known (n,a) reaction. Assume that the material being irradiated is pure borax, Na2B4O7, which contains boron of natural composition. Natural boron contains 19.9 atom percent 10B and 80.1 atom percent 11B, and the density of anhydrous borax is 2.37 g cm-3. The molecular weight of borax is 201.2, and the atomic weight of natural boron is 10.81. The thermal neutron (n,a) cross section of 10B is 3,840 barns. Making use of Avogadro's number, we can calculate the mass macroscopic absorption cross section (for the (n,a) reaction) of the borax as: 

Sm = (6.022 x 1023 molecules borax/201.2 g borax)(4 atoms B/molecule borax)

        (0.199 atoms 10B/atom B)(3,840 x 10-24 cm2/atom 10B) = 9.15 cm2 g-1.

If we preferred the volume macroscopic cross section, we would simply multiply the above value by the mass density of the borax.

A convenient source of neutron cross section information is the Brookhaven National Laboratory's National Nuclear Data Center. The "Library" box above the periodic table should say "ENDF/B-VII.0 (USA, 2006)," and the "Sublibrary" box should say "Neutron reactions." Simply click on the element of interest in the periodic table, then select the isotope of interest from the mass numbers that will be displayed to the right of the table. A pop-up window will appear, and you can select to display the data for the reaction of interest; the "Plot" option for display is very convenient. If you select it you will see the cross-section data plotted as a function of neutron energy, and when you position the cursor on the curve you will see the exact energy (in eV) and the cross section at that energy (in barns).

If you are a member of the Health Physics Society, you may access the Members Only section of the HPS site and activate the HP Toolbox, which is the last item shown in the list of available materials. Once you open this you will see the major topic areas listed, and you may click on "Neutrons" to obtain other cross-section information.

I hope you find what you need. Good luck.

George Chabot, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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