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Application of Nuclear Radiation in Agriculture

Introduction

In recent years, the application of nuclear radiation in the agricultural sector has gained beneficial importance in optimising crop yields in terms of managing irrigation. Omnia applies this technology as a diagnostic tool in providing a comprehensive service to farmers. Since radioactivity is quite often regarded as a no-go option, a brief discussion is presented to set producers’ minds at rest concerning the possible pros and cons involved. Similar to electricity, artificial radiation such as in X-ray equipment, can be switched off to remove the risk involved. Natural radioactivity, on the other hand, cannot be switched off, but it can be prevented from propagating into its surroundings, making it safe to handle and use as is necessary. Radioactive substances may be either in their gaseous, liquid, powder or solid states and each demand separate handling procedures according to their nuclear properties. Radioactive solids are relatively easier to handle than their two counterparts.

The history of radioactivity

The discovery of radioactivity took place over several years and was preceded in 1895 by the discovery of X-rays by Conrad Roentgen. Roentgen discovered X-rays through his work using fluorescing materials surrounded by photographic plates for which he was awarded the Nobel Prize in physics. Not knowing what kind of rays he was dealing with, he named it X-rays, meaning ‘unknown’. The popular TV series of X-Files, immediately comes to mind. Learning about Roentgen’s work, Henri Becquerel, together with Pierre and Marie Curie – two of his students – further explored this discovery. In 1896 up to 1911 radioactivity was subsequently discovered by Becquerel. Marie Curie further discovered Polonium and Radium, for which she also was awarded a Nobel Prize, becoming the first women to receive such a prestigious award. Unfortunately, Marie Curie died at a young age due to the still unknown hazardous effects of over-exposure caused by radioactive radiation. Building on his predecessor’s work, Ernest Rutherford applied this form of radiation to study the structure of atoms and in 1919 he used alpha particles to convert one type of atom into another. This would be the first “splitting of an atom”, opening up the potential to the peaceful and also unfortunate destructive ability of nuclear energy.

Although the term radiation or radioactivity understandably brings fear into most people’s minds, properly harnessed and managed applications of the same entity are now part of our lives in many different ways. In fact, it is important to note that, together with our bodies, our environment and cosmos is constantly bombarded by radioactive energy from outer space, but at very low doses. This may include ingestion of minute quantities of various contaminated food sources originating from our surroundings which is part of our existence. These (safe) levels of radiation are called background radiation and are believed to be harmless and even beneficial to our survival.  

To get back to physics, the term radioactivity, as it is now understood, describes the spontaneous emission of (nuclear) energy in different forms, i.e. either or both as particles or rays from the nucleus of an unstable atom (either natural or artificial) to revert to the stable condition. Radiation is essentially energy in motion.

So what makes radioactivity harmful?

Radiation in general may be divided into two distinct types, viz. ionizing and non-ionizing. In the former, the energy of the radiation is sufficiently high to strip electrons from an atom with which it collides, thereby changing its chemical properties and consequently damaging living cells, such as DNA, making it vulnerable to cancer. In the latter, the energy is insufficient to trigger any chemical changes in cellular composition. Figure 1 illustrates the relative inherent penetrating power of the different types of activity through commonly used shielding materials. A thorough knowledge of nuclear physics is paramount in choosing a specific radioactive substance for a specific application and taking all risks into consideration in designing a safe system.  

Radioactivity from ionizing particle emission is measured with a Geiger counter. The Geiger counter has a low pressure tube filled with neon, helium or argon gas charged with high voltage. When an ionizing particle passes through the tube, a readout is given. Readouts are given either in radiation activity, or dose, or absorbed dose. Older models use old non-Si Units, where activity, dose and absorbed dose is given in Curie, Rem and Rad respectively. Roentgen is also sometimes used as an old unit for exposure. Today, the new Si Units are used, also in South Africa, and Becquerel, Sieverts and Gray are used to express activity, dose and absorbed dose.

Figure 1. Illustration of the behavior of different radioactive particles in terms of distance travelled and penetration.
Figure 1. Illustration of the behavior of different radioactive particles in terms of distance travelled and penetration. https://www.mirion.com/...radiation.../types-of-ionizing-radiation

 

Radiometric equipment used by Omnia

Figure 2. Troxler or CPN 503 DR neutron moisture probe as used by Omnia. These are the gold standard in measuring soil moisture and have been around since the 1950s.
Troxler or CPN 503 DR neutron moisture probe as used by Omnia. These are the gold standard in measuring soil moisture and have been around since the 1950s.

Neutron moisture probes make use of Americium 241 and Beryllium 9, which are radioactive. Americium has a half-life of 432 years and is an alpha emitter and Beryllium a neutron emitter. As stated earlier, neutrons can travel for hundreds of meters and are only stopped by cement or water. Therefore, calibration of neutron moisture or density gauges must be done in the absence of cement barriers or large water bodies. The Americium and Beryllium is present in pellet form in a sealed chamber inside the probe that is lowered into the soil for making measurements. Important to note is that radioactive particles cannot be switched on or off and is constantly radiating harmful particles. Therefore in the CPN 503 moisture instrument, the probe or sealed nuclide is stored inside the instrument in a lockable silicon-based paraffin wax to limit exposure. Neutron moisture gauges measure soil moisture indirectly. The high speed neutrons emitted by the probe collide with hydrogen atoms in the soil and water. Their energy is then lost and low energy or slow neutrons are created. Some of the slow neutrons are then reflected back to the source tube and counted by the neutron detector. These raw values are then corrected to volumetric soil water content via a calibration equation. It is worth mentioning that neutron moisture gauges are more accurate at depths of over 15 cm, since neutrons escape from the soil into the surrounding air at shallow depths and will not be detected.

Regulatory requirements

The regulatory requirements regarding the ownership of and use of neutron soil moisture probes are controlled by the Department of Health (DoH) under the South African Hazardous Substances Act, 1973 (Act 15 of 1973). The body responsible for administering this legislation is the Directorate: Radiation Control, Department of Health. Under this act, all entities owning, using and transporting sealed radioactive sources must have authority to do so. Omnia Fertilizer’s Authority has recently being updated under this act and Omnia owns three registered neutron moisture probes under Authority File 3043 with the DoH. Under this authority, a trained Radiation Protection Officer (RPO) is responsible for the safe code of practice of instruments used internally. Under the Omnia authority, a complete set of internal rules was drawn up and submitted as part of the Authority Annexure. These rules deal with issues such as safe storage, emergency plans, safe handling and transport, dose limits and regulatory requirements. Due to the nature of the instruments, leak tests must be performed annually on each instrument. Leak tests, also called swab or smear tests, are done to ensure that no radioactive powder is leaking from the sealed source. Once these are in place, the DoH issues an annual return, whereby the owner of these devices needs to declare compliance in all responsible aspects mentioned. All relevant application forms, safety codes and acts are available from the following Dropbox : http://tinyurl.com/pne5nyv

Responsible officer, internal rules for Omnia, safe conduct and leak tests are also available on request from Willem Jonker, RPO for Omnia Authority File 3043.

Conclusions

This brief overview of principles involved in the use of equipment based on low-level radioactivity in ground water measurements, serves to inform and enlighten persons for whom the subject is outside of their scope. Omnia’s custodianship regarding its responsibility to comply with national regulatory matters is demonstrated.

My sincere thanks to W. Jonker (Sr), formerly from the Nuclear Energy Corporation South Africa (NECSA) and David Allen, RPO from Allen Associates for their contributions.