Each atom has a known value for its neutron scattering cross section, with that for hydrogen being the largest. Although in X-ray scattering the interaction depends on the atomic number of an atom, for neutrons the interaction is much more complex and hard to predict. The thermal neutron cross section of a molecule represents the probability that neutrons of a given energy interact with the material, and it is dependent not only on the atoms that are present, but also on their molecular structure, dynamics and on the temperature.
The knowledge of the scattering cross section of amino acids is important when investigating the interaction of neutrons on biological systems during medical procedures such as Boron Neutron Capture Therapy, or those caused by secondary emissions during proton therapy.
Although the use of neutron scattering techniques for biological investigations is widely used at ISIS and other facilities, the precise measurement of the thermal neutron cross section of amino acids was yet to be performed.
As part of the CNR-STFC partnership and within the ISIS@MACH Research Infrastructure, a group of researchers has used the VESUVIO beamline at ISIS to measure the cross sections for all twenty amino acids. By using two different detectors on the instrument at the same time, the group were able to accurately measure the neutron cross sections across a range of energies from fractions of a meV to tens of keV. By investigating all the amino acids across a large range, their results, published in Journal of Physics: Condensed Matter, form a comprehensive reference for those who need it.
In addition to collecting data on the amino acids, the group were also able to devise a method of predicting the cross section depending on the chemical formula of the molecule. They used their Average Functional Group Approximation (AFGA) technique to predict the interaction of other organic materials, then taking measurements to confirm their predictions were correct.
As amino acids are the basic building blocks for all proteins, the researchers' results will provide crucial information for medical applications where neutrons interact with tissue. In addition to therapeutic applications, their data will also be of use to researchers who use neutrons to investigate complex biological systems such as proteins, muscles or other human tissues.
The full paper can be found at DOI: 10.1088/1361-648X/abfc13