Radioanalytical chemistry

Alpha decay is characterized by the emission of an alpha particle, a ⁴He nucleus. The mode of this decay causes the parent nucleus to decrease by two protons and two neutrons. This type of decay follows the relation:

${\displaystyle {}_{Z}^{A}\!X\to {}_{Z-2}^{A-4}\!Y+{}_{2}^{4}\alpha }$[3]

Beta decay is characterized by the emission of a neutrino and a negatron which is equivalent to an electron. This process occurs when a nucleus has an excess of neutrons with respect to protons, as compared to the stable isobar. This type of transition converts a neutron into a proton; similarly, a positron is released when a proton is converted into a neutron. These decays follows the relation:

${\displaystyle {}_{Z}^{A}\!X\to {}_{Z+1}^{A}\!Y+{\bar {\nu }}+\beta ^{-}}$
${\displaystyle {}_{Z}^{A}\!X\to {}_{Z-1}^{A}\!Y+\nu +\beta ^{+}}$[4]

Gamma ray emission follows the previously discussed modes of decay when the decay leaves a daughter nucleus in an excited state. This nucleus is capable of further de-excitation to a lower energy state by the release of a photon. This decay follows the relation:

${\displaystyle {}^{A}\!X^{*}\to {}^{A}\!Y+\gamma }$[5]

Schematic of an ionization detector

Gaseous ionization detectors collect and record the electrons freed from gaseous atoms and molecules by the interaction of radiation released by the source. A voltage potential is applied between two electrodes within a sealed system. Since the gaseous atoms are ionized after they interact with radiation they are attracted to the anode which produces a signal. It is important to vary the applied voltage such that the response falls within a critical proportional range.

Schematic of a solid-state detector

The operating principle of Semiconductor detectors is similar to gas ionization detectors: except that instead of ionization of gas atoms, free electrons and holes are produced which create a signal at the electrodes. The advantage of solid state detectors is the greater resolution of the resultant energy spectrum. Usually NaI(Tl) detectors are used; for more precise applications Ge(Li) and Si(Li) detectors have been developed. For extra sensitive measurements high-pure germanium detectors are used under a liquid nitrogen environment.[6]

Scintillation detectors uses a photo luminescent source (such as ZnS) which interacts with radiation. When a radioactive particle decays and strikes the photo luminescent material a photon is released. This photon is multiplied in a photomultiplier tube which converts light into an electrical signal. This signal is then processed and converted into a channel. By comparing the number of counts to the energy level (typically in keV or MeV) the type of decay can be determined.

Samples with very low concentrations are difficult to measure accurately due to the radioactive atoms unexpectedly depositing on surfaces. Sample loss at trace levels may be due to adhesion to container walls and filter surface sites by ionic or electrostatic adsorption, as well as metal foils and glass slides. Sample loss is an ever present concern, especially at the beginning of the analysis path where sequential steps may compound these losses.

Various solutions are known to circumvent these losses which include adding an inactive carrier or adding a tracer. Research has also shown that pretreatment of glassware and plastic surfaces can reduce radionuclide sorption by saturating the sites.[7]

Since small amounts of radionuclides are typically being analyzed, the mechanics of manipulating tiny quantities is challenging. This problem is classically addressed by the use of carrier ions. Thus, carrier addition involves the addition of a known mass of stable ion to radionuclide-containing sample solution. The carrier is of the identical element but is non-radioactive. The carrier and the radionuclide of interest have identical chemical properties. Typically the amount of carrier added is conventionally selected for the ease of weighing such that the accuracy of the resultant weight is within 1%. For alpha particles, special techniques must be applied to obtain the required thin sample sources. The use of carries was heavily used by Marie Curie and was employed in the first demonstration of nuclear fission.[8]

Isotope dilution is the reverse of tracer addition. It involves the addition of a known (small) amount of radionuclide to the sample that contains a known stable element. This additive is the "tracer." It is added at the start of the analysis procedure. After the final measurements are recorded, sample loss can be determined quantitatively. This procedure avoids the need for any quantitative recovery, greatly simplifying the analytical process.