Question

Briefly describe how radioisotopes are produced from stable isotopes using a nuclear reactor and a cyclotron and give at least one example of a radioisotope used in nuclear medicine that is produced in each manner along with the nuclear reaction equation for production of each.

Answer 1

Production of Radioisotopes

Stable elements tend to become radioactive when the neutron-proton ratio (N/Z ratio) in the nucleus is changed or altered. Radioisotope production process makes use of altering N/Z ratio of the element by bombarding suitable target material with neutrons or charged particles.

Neutron bombardment (using a nuclear reactor) in general yields “neutron rich” isotopes which usually decay by β^-(negatron) emission and often accompanied by gamma (γ) radiation.

In contrast, charged particle bombardment (in a cyclotron) produces “neutron deficient” isotopes and they decay either by β⁺(positron) or Electron Capture (EC) mode.

Neutron bombardment of selected targets in reactors is by far the most common route for radioisotope production. Charged particles of sufficient energy yield a product which is not isotopic with the target element and results in high specific activity product.

Radioisotopes from a nuclear reactor

1. Some (n, γ) reactions produce a short lived radioisotope which decays to another radioisotope having a longer half-life. For example, (n, γ) reaction with tellurium yields a product ¹³¹I. The product isotope is easily separable from the target and leads to high specific activity.

¹³⁰ Te(n, γ)¹³¹Te --- β decay----> ¹³¹I (used in nuclear medicine for the dignosis of thyroid related diseases)

Radioisotopes from a cyclotron

Positron emitting isotopes such as ¹¹C, ¹³N, ¹⁵O and ¹⁸F are short lived (t_1/2 in the range of minutes). Their production in cyclotron is followed by rapid radiochemical processing to yield labelled molecules.

¹⁸O(p, n)¹⁸F (used in Positron Emission Tomography with FDG (fluorodeoxyglucose). These are produced in medical cyclotrons which are compact as comapred to conventional cyclotrons.