6 Li may be used in fusion power systems, as noted above, and is the starting material for producing tritium by neutron absorption:

|Li+ &n->-?T + $He

In some types of thermonuclear power systems it is desirable to use a blanket of lithium enriched in 6 Li to increase the volumetric rate of neutron capture to produce tritium.

7 Li hydroxide is now used in some water-cooled reactors to inhibit corrosion by control of hydrogen ion concentration. Because the thermal-neutron absorption cross sections of the lithium isotopes are 6 Li, 940 b, and 7Li, 0.037 b, it is necessary to use 7 Li containing less than

0. 01 percent 6 Li. 7Li metal, which melts at 180°C, was proposed as coolant for an aircraft — propulsion reactor, because of its low vapor pressure at high temperature and low neutron — absorption cross section.

1.2 10B

The thermal-neutron absorption cross section of natural boron, which contains 19.61 percent 10B, is 759 b, whereas that of separated I0B is 3837 b. Thus, enriched 10B is useful in applications where the highest volumetric rate of neutron absorption is wanted. Examples are compact shielding for thermal neutrons and control rods for fast reactors.

Neutron-capture therapy is an experimental technique for selective destruction of cancerous tissue surrounded by healthy tissue. In this technique a compound of 10 В that is selectively absorbed by the cancer is injected into the bloodstream, followed by irradiation of the cancerous tissue by a beam of neutrons. Energetic alpha particles, produced by the reaction

*?B + Jn^-fHe + ^y

where the neutron beam reacts with the boron compound in the cancer, destroy the cancer while leaving the neighboring healthy tissue, containing less boron, less affected.

1.3 13C

Carbon, hydrogen, oxygen, and nitrogen are the elements that occur in greatest abundance in living systems. Tracer experiments using either radioactive isotopes or separated natural isotopes are of great importance in understanding biochemical reactions. Although with carbon there is the possibility of using the short-lived radioisotope "C or the very long-lived 14 C, for many experiments it is preferable to avoid radioactivity and use separated stable 13 C. Another important use of 13 C is in nuclear magnetic resonance experiments on the structure of carbon compounds. By synthesizing a compound with a 13 C atom in a known location, it is possible to draw conclusions about the configuration of the molecule, because 13 C has a nuclear magnetic moment and 12 C has none.

1.4 15 N

1SN can be used in very much the same way as 13C, as a tracer for nitrogen compounds and in nuclear magnetic resonance experiments. The fact that the longest-lived nitrogen radioisotope, 13 N, has a half-life of only 10 min gives 15 N added significance.

An additional possible use suggested for 15 N is in U15N fuel material for a fast reactor. 15 N has a lower absorption and inelastic scattering cross sections for fast neutrons than the more abundant 14 N. Its use avoids 14 C production from the reaction 14 N + ln -* 14 C + 1H.

1.5 Oxygen Isotopes

Because the longest-lived oxygen radioisotope, 15 0, has a half-life of only 124 s, the separated isotopes 17 0 and 18 О are valuable in tracer experiments. The nuclear magnetic moment of 17 0 gives it application in determining molecular structure by nuclear magnetic resonance measurements.