Application of isotopes in industry and technology

Nuclear radiations are real entities of the physical world, which can be used for the benefit of man to improve his quality of life. In fact, the nuclear radiations emitted by radioactive atoms, given the ease with which they can be detected, allow these atoms to be used as radioactive tracers of the chemical elements to which they belong, which leads to their use in visualizing the paths that follow the elements in the physical, chemical and biological systems in nature.

Thus, radioactive tracers make it possible to unravel the mechanisms of operation or transformation of the material world, saving the patient laboratory task that would have to be carried out through thousands and thousands of analyzes to obtain similar knowledge. For this reason, it is not an exaggeration to state that, in the last fifty years, the body of knowledge about the physical world has more than doubled, with the help of radioactive tracers, which are the great paradigm of scientific research on material systems.

But the applications of radioactive atoms are not limited exclusively to this extension of the perceptive capacity with the help of a detector; radioactive atoms, hermetically confined, become radiation emitting sources, whose interaction with matter provides signals to measure properties of surrounding objects. Radiations can pass through opaque objects, weakening in proportion to the matter they find in their path, or they can be reflected, giving information about the density of the medium where they bounce; or they can excite the emission of other radiations, characteristics of the elements present.

These interaction phenomena are the basis of a multitude of devices used in the automatic control of manufacturing processes of sheet products (paper, plastic, metal sheets, etc.), of metallic coatings on plastic substrates or on other metals (zinc-plated, chrome-plated, silver, etc.), of level switches in liquid tanks, etc. By means of these devices, nuclear radiation not only saves man from the routine tasks of delayed control of industrial processes, but also allows real-time control to be carried out with all the advantages that this entails for productivity, saving raw materials, or The quality of products.

In short, nuclear radiation -alpha, beta, neutron and gamma photon particles- offer a wide repertoire of interactive possibilities with matter, from which multiple applications are derived, either helping to detect imperceptible phenomena, or measuring by transmission, reflection or fluorescence the material properties seen by the radioactive source.

Why do atomic clocks allow us to date geological events?

In the past, events such as the formation of rocks, volcanic eruptions, climatic variations, glaciations, etc. took place. To reconstruct the chronology of this geological past it is necessary to have a clock that indicates the elapsed time; which presupposes, in turn, that indelible signs of the passage of time are imprinted on the materials that the past has bequeathed -be they fossils, rocks or crystalline minerals-.

For this need, atomic clocks come in handy, as they are the only natural systems capable of providing an absolute reading of elapsed time; and this for a very simple reason, because the radionuclides, which are the basis of this precision watchmaking, have an invariant temporal property, namely, that the probability of disintegration of their atoms is a constant; which refers to that decreasing exponential law with time for the relative number of remaining radioactive atoms (father radionuclide), and increasing for that of the stable atoms that are formed in the disintegration (daughter nuclide).

For a watch to be useful it must keep time correctly, that is, it must be set on time. This is the operation that nature carried out in due time, when the igneous rocks were consolidated, the volcanic lava cooled, or the sediments were stratified; at that moment the clocks were reset. Well, the father radionuclide was freed from the son generated until then, and it was only from this moment that the son began to accumulate within the crystalline lattice of the mineral; accumulation that ends when the quantity of remaining radionuclide and daughter formed is determined by destructive analysis.

With these data, the problem is determined as long as a previous condition is met: that the mineral has behaved as a sealed system and has not lost an appreciable amount of matter, neither from the parent nor from the child. Take, for example, the case of potassium-40, which disintegrates over a period of 1,300 million years, to give the noble gas argon-40 (stable); there is no doubt that at the moment of the formation of a potassium mineral (be it feldspar, granite, etc.), the clock was reset, because argon is a volatile noble gas that escapes, and only from then it could accumulate in the crystalline network of the mineral, in such a way that the number of its atoms, at the moment of analysis, serves as a quantitative signal of the elapsed time.

Of course, the driving radionuclide of a clock must have sufficient "winding" to measure the time it is intended to measure; and, for this reason, it is resorted to using clocks that have a half-life according to the distance of the event; Thus, with carbon-14 (5,730 years) only events from the end of the Quaternary can be dated, but with other radionuclides, such as aluminum-26 (0.7 million years, for short, 0.7 Ma), iodine-129 ( 17 Ma), rubidium-87 (50,000 Ma), any event in Earth's geological evolution can be dated.

Now then, in order for atomic clocks to be used, it is necessary that some natural force has created them without the help of man; This force was the supernova explosion that shaped the solar system, an explosion that gave rise to the formation of the
early radionuclides, such as rubidium-87, potassium-40, and the isotopes of uranium and thorium, which still survive. To this class of clocks we should add another, based on cosmogenic radionuclides, formed in the continuous bombardment of the Earth by cosmic radiation, which causes constant levels of radioactivity of carbon-14, and other radionuclides of relatively short periods, in living beings, in sediments, etc. When the living being dies or the sediment is hidden, the activity of the radionuclides it contains begins to decrease, taking its measure over time.

In summary, atomic clocks have allowed man to build that science of nature that is geochronology, when he has learned to read the temporal records existing in material objects.

While the number N (relative) of the "father" atom decreases exponentially, that of the "son" grows in a complementary way with time, ( T=half-life).

Can a small radioactive source replace a chemical analysis laboratory?

Yes, under certain circumstances; for example, in the automatic control of impurities in the raw materials contributed to an industrial process, or in the elemental analysis of the strata crossed by a borehole. The foundation of these analytical applications is based on the existence of specific interactions of gamma and neutron radiation with the atoms of the elements that make up the material medium.

And what, one may ask, are these specific interactions? They are those that take place with atomic particles whose energy levels are characteristic of each element; such is the case of the deep electrons of the atomic crust, where X-rays are generated; or the nucleons (neutrons and protons) that form the atomic nucleus, where nuclear reactions take place, which generate gamma photons or other particles.

Well, the deep electrons are accessed through interactions of gamma radiation -photoelectric and Compton effects-, which remove electrons and create cascades of fluorescent X-rays, by filling in the cortical gaps produced. As for nuclear reactions, the radiations that easily enter the atomic nucleus are neutrons, which cause the instantaneous emission of gamma radiation or other particles.

In both cases, to carry out these analytical applications, a radiation source is required - gamma in the first case, and neutrons, in the second - and a detector of the resulting radiation. Among the analytical applications of the sources, it is worth highlighting the control of sulfur (impurity) in crude oil circulating through pipelines, or that of ash in coal and lignite on conveyor belts. Regarding the analytical applications of neutron sources, its dominant field is the analysis of the elemental composition of materials located in inaccessible places, such as hydrocarbon prospecting boreholes (up to 7,000 meters deep), coal (1,000 meters) or metal products in general (more superficial).

Through these analytical techniques it is possible to evaluate the resources of a mining basin and plan their exploitation; For example, in the carboniferous case it is possible to specify the impurity content of the coal veins, their thickness and depth, their calorific value, etc. Naturally, many other advanced technologies, of an electronic and computer nature, concur in these applications, without which it would not be possible to analyze the composition of subsoil strata, located several kilometers deep from the surface of the earth's crust.

Is industrial radiography based on the same principles as medical rediography?

The radiographic examination of the human body is well known to all, because it is the most widely applied physical technique in medicine, and we have all had x-rays of the chest, stomach, etc. Regarding industrial radiography, what is intended is to verify, through X-rays or gamma radiation, the quality of the components of the technological systems; As in the case of medical x-rays, it is non-destructive testing, so that if the x-ray image is satisfactory, the component can be considered as good, without having suffered any reduction in its physical integrity.

The basic principles on which medical radiography and industrial radiography are based are, of course, the same, since X and gamma radiation do not distinguish at all whether it is living matter belonging to an organism or inert matter belonging to a metallic component. of a system. Who is different is the specialist who requests and interprets the radiographic image, who is sometimes an expert in human anatomy, and other times, a technical specialist in metallic constructions; but both make use of the same fundamental principle, the difference in absorption suffered by any beam of radiation on its path from the source to the point considered on the photographic plate, depending on the elemental composition and the amount of matter interposed.

One difference, however, is worth mentioning: while the patient goes to the X-ray room for the medical examination, it is not always possible to take the component of the technological system to the industrial radiography laboratory, due to its immobility; and then the use of scintigraphic sources, which are easily transportable to the site of the project (oil pipeline, bridge, dam, thermal or nuclear power plant, etc.), becomes exceptionally important to verify in situ the quality of its construction. The mobility of the sources and their adaptability to the most diverse circumstances are their most appreciated qualities in modern technological applications.

Can nuclear radiation be used in the restoration of artistic objects?

Nuclear radiation (especially gamma radiation) have two characteristic properties: on the one hand, they are ionizing and form free radicals, which allows them to be used as polymerization catalysts when they act on monomers containing double bonds (such as ethylenic, vinyl , etc.); on the other hand, ionizing radiations have, at high doses, biocidal effects; that is, they inhibit biological reproduction and, as a consequence, produce cell death, from which their use as sterilizing agents is derived.

In a work of art in a state of obvious deterioration (whether it is a wooden statue, a scroll, etc.), the first thing to do is sterilize it to eradicate xylophagous insects, eliminate fungi, etc.; and, secondly, it is necessary to consolidate it so that the environment (humidity, polluting chemical compounds in the atmosphere, etc.) does not continue to deteriorate it.

Well then, the aforementioned properties allow gamma radiation to be used to perform both operations at the same time, sterilization and consolidation; for which all that is required is to have impregnated the work of art, after cleaning it, with a monomeric solution that, due to the effect of gamma radiation, will be transformed in situ into a polymeric substance, which will give it consistency and protect it. of the possible harmful environmental action.

Can artistic or historical forgeries be discovered using nuclear techniques?

The term of forgery in art or history covers a very broad casuistry: author, time, place, style, etc. Only two aspects will be considered here: those related to the attribution of authorship and to historical dating.

The use of nuclear techniques is based on two unique properties of radiation, which allow:

• Carry out non-destructive analysis (or with minimal sampling) to discover the "fingerprints" of the works; These "footprints" are formed by the micro-constituent elements that accompany the raw material with which the work of art was made: marble or bronze in sculpture, clay in ceramics, silicate in glass, pigments in numismatics, etc., and which vary according to authors and times.
• Dating the time of historical or archaeological objects, through the radioactive decrease produced by time in certain radionuclides present in the material substrate of the object; such is the case of carbon-14, present in its day in living plants and animals in equilibrium with the natural radioactive level of carbon, which will later give rise to less radioactive fossil remains; carbon-14, without a doubt, is the most important radionuclide (5,730 years of half-life) for dating objects related to the history of man; there are other radionuclides of more specific application, such as lead-210 (with a 20-year period), which usually accompanies the white lead used in paints, or tritium (12 years old), which enters the water cycle, allow vintages of wines etc.

Nuclear techniques have made it possible to clarify the authenticity of works of art and to establish the chronology of the evolution of human cultures on an objective basis.

Does the reader know that a good part of the products for medical use are sterilized by means of nuclear radiation?

The ionizing radiations emitted by radionuclides have the property of inhibiting cell reproduction and, with this, cause the death of microorganisms, insects and, in general, any living being, if the applied radiation dose is sufficient. This biocidal property of radiation has many practical applications but, among all of them, it stands out for its importance for human health, the sterilization of products frequently used in clinics and in surgery, where a high degree of asepsis is required; such is the case of products such as gloves, syringes, gauze, probes, cannulas, pipettes, containers, etc., and, in general, how many products are "throw away".

The great advantage of this technique lies in the penetrating power of gamma radiation, such as that emitted by cobalt-60, which can sterilize products at relatively low doses (25 kGy) once packaged and ready for shipment. supply, which avoids any possibility of recontamination due to manipulations prior to use.

From the economic point of view, it is also important that the products can be manufactured using normal environments, instead of sterile environments (much more expensive), knowing that subsequent dowsing will allow for greater degrees of asepsis than those required by health regulations.

The aforementioned advantages have meant that dowry sterilization has reached full industrial development in the most advanced countries, using cobalt-60 irradiators (and, sometimes, cesium-137) of several million curies, which allow to treat annually about 3 million m3 of products ready for supply. With this, dowsing has displaced the classic procedure of fumigation with ethylene oxide, which has already been banned in many countries (USA, Japan, Australia, and now in the EU), because it has been discovered that it gives rise to carcinogenic waste, which can affect patients and healthcare personnel.

Is it true that most plastic materials are obtained using nuclear rations?

Plastics, so widely used today, are materials made of organic polymers, to which some secondary component is added to give them body (load additives) or to provide them with suitable properties (coloring, flexibility, incombustibility, etc.); but the essential base is, as has been said, organic polymers. And, it is worth asking, what are these substances? They are simply organic compounds of high molecular weight, whose structure is formed by the repetition of small units, to which the root mer alludes, from the Greek meros, which means part; the successive union of these parts is known as polymerization, and gives rise to linear chains with thousands of units (polymer).

In nature there are many polymeric substances -cellulose, cotton, wool, proteins and DNA itself- whose importance is not necessary to highlight. When man discovered the structure of these substances, he was able to manufacture them synthetically, even designing the properties he wanted to obtain. From this point of view, the polymers obtained through the use of nuclear radiation (gamma, above all) represent the most perfect degree of achievement of plastic materials to cover the special uses demanded by modern technology.

The role of radiation has to do with the polymerization reaction to the extent that it creates free radicals, which drive not only the linear growth of polymer chains but also the formation of interchain bonds (crosslinking), which give the polymers special properties. Examples of materials obtained by irradiation are the following:

• Natural rubber, obtained by crosslinking latex without adding vulcanizers (sulfur, zinc oxide, etc.), which give toxic residues; the products are soft to the touch and are used in the form of surgical gloves, catheters, etc.
• Insulating plastics (electrical cables), which by cross-linking acquire greater thermal and electrical resistance, essential for IT and low-voltage applications.
• Graft copolymers, which insert a polymer onto any material substrate (paper, wood, metal or other plastic); Its applications range from the resinification of wood, for highly resistant uses, to the coating of Teflon kitchen utensils, or the provision of prostheses and biocompatible materials..
• Finally, among many other applications, the new galenic forms of medicines (enzymes, antibodies, etc.), confining them in plastic membranes from which they diffuse slowly, thus providing a continuous and regulated bioavailability of the drug. As can be seen, nuclear radiation has multiple applications, and there are many materials and utensils that are manufactured taking advantage of its positive properties.

Could nuclear radiation contribute to the disappearance of world hunger?

Nuclear radiation, especially gamma radiation, has a far-reaching application for humanity, which is the preservation of food by irradiation, in which the sterilizing property of radiation is taken advantage of (destruction of microorganisms) and, also, to the enzymatic retardation of fruit maturation, inhibition of seed germination, etc.

Irradiation is one more procedure in food conditioning, which adds to the long repertoire of those already in existence - cooking, freezing, refrigeration, dehydration, vacuum packaging, fermentation, salting, smoking, adding chemical preservatives, etc.- , each of which has its own scope of application, although nothing prevents a combination of them from being used, as in our case the irradiation of frozen, dried or vacuum-packed foods, etc., or the application of procedures normal culinary to irradiated foods; on the other hand, the health authorities are prohibiting the use of chemical preservatives (methyl bromide, dibromoethylene, etc.), whose gap is being filled (or may be in the near future) by gamma photon irradiation of cobalt-60.

Food irradiation currently has two main streams of development; reducing food losses after harvesting, and improving the sanitary quality of food in general.

Regarding the reduction of losses, the following cases can be cited:

• The irradiation of fresh fruit, to eliminate insects (fruit flies, above all), which cause real havoc in more than a hundred varieties of fruit during storage, while delaying the ripening process, prolonging life useful commercial.
• The destruction of larvae in cereals, legumes and seeds, which devour, in their weevil phase, large quantities of the stored reserves.
• The inhibition of sprouting in bulbs and tubers (potato, onion, garlic, etc.), which stops the spontaneous germination process of these products.

In Third World countries it is estimated that between thirty and fifty percent of the food collected is lost.

The other side tends towards compliance with increasingly strict microbiological specifications in food, which often carries unacceptable amounts of pathogenic germs (salmonella, trichina, campylobacter, etc.); Also, irradiation is applied to the sanitization of spices (above all, for the manufacture of sausages), and in the preparation of special diets for patients with low immunological defenses. In terms of food hygiene, there is still a long way to go, even in the most developed countries, where every year one in two people suffer from an infectious episode transmitted by food.

The preservation of food by irradiation contains great potential to remedy the problem of hunger in the world, but the level of technological and cultural development of the Third World prevents, today, from benefiting from this technique.

Is radioactivity induced in food preservation by irradiation?

In the food sector, some national authorities, some industrialists and some consumers associate food irradiation with the induction of radioactivity in them, with the loss of nutritional value, and with possible long-term risks caused by continued consumption. of these products; this, without counting on the strong opposition of certain pseudo-environmental groups, who believe that any technique related to the nuclear is intrinsically rejectable.

Therefore, food irradiation has, from the outset, a very unfavorable environment for its industrial deployment, an essential step to improve the quality of food in general and the problem of hunger in the world, in particular. This singular situation has motivated, for years, the development of extensive research plans on irradiated foods, coordinated by the United Nations Organizations -FAO, WHO, IAEA and the Codex Alimentarius Commission- by virtue of their double mission, both to promote development as to remedy the most urgent problems in the most disadvantaged populations.

The problem of the induction of radioactivity has a remote scientific foundation, which adds to an attitude of radical distrust towards scientists and official institutions. Indeed, there is no doubt that irradiating food, or any other material, with radiation endowed with sufficient energy will induce nuclear reactions, and that these will produce radioactive atoms. For this reason, they are specified in the good practice procedures, authorized by the Codex Alimentarius Commission, that:

• Gamma radiation, used in food irradiation, must have energy of less than 5 MeV, which guarantees the absence of nuclear reactions in the component elements of the food..
• Accelerated electrons, which are used as an alternative, should have energy less than 10 MeV, because such electrons cannot induce - indirectly, via braking gamma radiation - nuclear reactions to a significant extent.

Therefore, nuclear sciences and regulatory institutions have provided the necessary technological safeguards to prevent the induction of radioactivity; Furthermore, it is enough for the gamma radiation used to come from isotopic sources (such as cobalt-60), so that the expected conditions are automatically fulfilled, because there are no radionuclides that emit gamma photons above about 3 MeV.

Currently, fifty countries (the most developed) have authorized the irradiation of food for public consumption, an essential condition for there to be international trade in them. Also, in part, this slowness of commercial penetration is due to the extremely conservative nature of the food industry, which does not risk investment until all regulatory pitfalls have been ironed out and the public has been properly informed, so that it is predisposed for its acceptance. .

Why are radioactive isotopes so useful in nature science research?

Nature is constituted, in its simplest version, by the representative atoms of the chemical elements that appear in the Periodic Table. But, as is well known, each element can be formed by several classes of isotopic atoms, that is, by atoms that, having the same atomic number, differ in their mass numbers. With this, it turns out that the representative atom of an element is a fictitious atom, which represents a mixture of isotopes; Usually this mixture is that of the stable isotopes (and long-lived radioactive ones, if any) with which the element occurs in nature, but this does not exclude that the same element can occur with a great variety of different isotopic compositions, especially everything, after man learned to isotopically enrich chemical elements and to transmute them through nuclear reactions, creating radioisotopes that did not previously exist in nature.

When you want to have a new representation, more consistent with the existence of the isotopes of the elements, it is necessary to forget about the simplicity of the Periodic Table and resort to the so-called Nucleidic Table. In fact, some 2,000 different kinds of atoms (nuclides) are now known; only about 300 of these nuclides are stable, and with them nature has shaped the isotopic composition of natural elements; the remaining 1,700 are radioactive (radionuclides), and have been created by man through nuclear research and technology. These radionuclides are obviously radioactive isotopes of the known elements, and it can be said that there is no element for which several of these isotopes are not known.

The existence of radioactive isotopes is of great empirical importance, and has given rise to the fact that every chemical element can be presented in two versions; one, the "stable", formed exclusively by stable isotopes (worth the redundancy); and another, that of radioelement, in which at least one of its isotopes is radioactive. Of course, every radioelement is ephemeral and becomes its "stable" form over time, but while this is happening, the radioelement is, by chemical identity, a tracer to the corresponding stable element. Naturally, in those cases of elements that do not have any stable isotopes, such as radium, uranium, thorium, plutonium, etc., they are themselves permanently radioelements, which spontaneously trace their paths in nature.

The trace of the chemical elements, in conclusion, through their respective radioelements is a fact of great importance, because it allows us to visualize, with the help of a detector, the paths that the elements follow in the physical, chemical and biological systems in which they intervene. . The use of radioactive isotopes (for several decades) has therefore had a paradigmatic character for scientific research into nature, insofar as it has made it possible to clarify most of the evolutionary or transformation mechanisms of systems. materials.

What are the main applications of radioactive tracers?

Any radioactive isotope can be used as a radioactive tracer of the chemical element to which it belongs. The only required condition is that the radioactive isotope is part of the same chemical entity as the element in question; This forces, in many cases, to carry out specific chemical operations, which are known as marking. Nowadays, there are commercial catalogs of marked compounds such as, for example, benzene with tritium substituting for hydrogen, or with carbon-14 substituting for stable carbon; Obviously, this labeled benzene behaves in the same way as normal benzene, and is used as its radioactive tracer in many organic chemistry research problems.

Here are some examples of the use of radioactive tracers in different disciplines:

• Agriculture: Nutrient-soil-plant relationships can be studied, with special reference to trace elements, fertilizers, intecticides, etc.
• Biology: Very small concentrations of enzymes, hormones, drugs, poisons, etc. can be determined by means of the radioimmunoassay technique (RIA), which makes use of the specificity of antigen-antibody reactions.
• Chronology: Geological and historical events can be dated by studying the radionuclides that act as atomic clocks.
• Pharmacology: The metabolism of drugs can be studied, before authorizing their public use, and of the metabolites and secondary reactions to which they give rise.
• Hydrology: River and turbine feed flows, or leaks in swamps, sediment dynamics, etc. can be measured.
• Medicine: The desease can be diagnosed through the use of radiophamaceuticals that visualize the functional status of specific organs: brain, thyroid, heart, lung, skeleton, etc., or by locating abscesses and metastases.
• Mining: The natural radioactivity or uranium, thorium and potassium can be measured in prospecting boreholes, which provides information regarding the minerals associated with these radioelements.

As can be seen, the use of radioactive tracers provides valuable information in all domains of natural sciences.

What is activation analysis?

It is a technique for the identification and quantification of the constituent elements of a substance, which is based on the measurement of the radionuclides that are formed (activation) when irradiating a representative sample of it.

This technique can be practiced using various elementary particles (photons, protons, neutrons, alpha particles, etc.), but the modality that has acquired the most importance is the one that uses neutrons (neutron activation), since these particles are very abundant in the vicinity from the core of a reactor or easy to obtain by means of neutron sources, such as those of americium-beryllium, etc. On the other hand, neutrons are constituent particles of the atomic nucleus, which enter it with great ease, giving rise to radioisotopes that emit beta radiation and, sometimes, gamma radiation, which is what is commonly used to measure nuclei. formed radionuclides.

The fundamental characteristic of activation analysis is its great sensitivity, at least for certain elements, such as sodium, magnesium, chlorine, potassium, manganese, cobalt, uranium, etc., which can be determined even in very low concentrations, below one part per million (1 ppm), which would be difficult or impossible using other techniques.

Next in importance, as a valuable feature, is the fact that activation analysis can be used as a non-destructive test, preserving the object analyzed in its physical integrity, with the only exception that a very small number of its stable atoms have been transformed. in radioactive; but this does not matter, because the induced radioactivity decays, in general, very quickly and the object recovers in a short time its original stable condition.

Among the specific applications of analysis by activation, it is worth mentioning the quantification of impurities in technological materials (quality control) and of micro-constituent elements in valuable objects (artistic, historical, etc.), to identify their origin or time, and in meteorites and extraterrestrial rocks, in search of cosmochemical connections.

What are isotope generators of electricity?

They are devices that contain a radionuclide, hermetically confined in a metallic capsule, whose radiations are fully absorbed by its walls. Therefore, the capsule is equivalent to a small heat source, since this is the form in which the radiation energy is finally manifested. A circuit formed by thermocouples (between a hot and a cold spot, Peltier effect) is coupled to this heat source to generate an electric current, like that of a galvanic battery, but with a much longer duration, if the radionuclide is of period length.

The radionuclides used are always alpha emitters, because this radiation stops in the first microns of the capsule walls (usually stainless steel). Plutonium-238, with a half-life of 88 years, and curium-244, with an 18-year half-life, which can provide electrical powers of the order of watts per gram of confined radioactive material, for several years, are preferably used.

The applications of isotopic generators, which, as can be seen, provide very small powers, are reserved for very special uses, such as:

• Pacemakers, implanted subcutaneously to regulate the heart rate, requiring very small powers, in the order of microwatts; They are being displaced in modern times by long-life lithium batteries (10 years).
• Batteries for remote use, low power (watts), to power observation devices and signal transmission in inaccessible places, terrestrial or marine (without possible maintenance).
• Space navigation batteries, with power in the order of kilowatts, to feed the instrumentation of terrestrial satellites and planetary probes; In this case, the batteries are referenced with the English acronym SNAP-X for Space Nuclear Auxiliary Systems, followed by X (an integer), which if it is odd indicates that the energy comes from an isotopic generator, and if it is even, from an small nuclear reactor With SNAPs, the solar system has been explored -Apollo, Pioneer, Voyager missions, etc.- and, in some cases, small observatories have been set up on nearby planets, which transmit information to Earth.

When was the Earth formed?

From a provincial point of view of the universe, such as that of an inhabitant of a planet in the solar system, cosmic time is divided into two large sections: the pre-solar stage, which corresponds to the time elapsed since that fantastic original explosion, the Big Bang, by which an expanding universe of galaxies was created; and the solar stage, in which a small part of the matter of our galaxy -the Milky Way- separated and materialized in the Sun, the planets and the meteorites.

The solar stage is considered, in turn, to have two parts: one, the transition interval between the initial nebular state and the formation of the chemical compounds that make up the planets and meteorites, which is assigned a duration of about 100 Ma; and another, from the end of the transition interval to the present day, which is the period of time that constitutes the age of the Earth.

There is still a very important event to refer to known as the "Last Minute" of nucleosynthesis, in which chemical elements of high atomic number were formed for the last time, which would later become part of the matter of our solar system; this "Last Minute" of the creation of elements took place just before the start of the transition interval, and was promoted, almost certainly, by a supernova explosion -the last stage of stellar nucleosynthesis- such as those now observed by astronomers in other galaxies in the universe. Well, in this "Last Minute" the atomic clocks were started, with which it has been possible to measure the transition interval and the age of the Solar System, considered common for all its components and, therefore, equivalent to the age of the Earth, which has been determined to be about 4,550 Ma.

How has the age of the Earth been determined?

The age of the Earth has been measured using the atomic clocks contained in the most primitive materials of the Solar System to which access has been had, such as:

• The oldest terrestrial rocks.
• Moon rocks brought by Americans and Soviets.
• The meteorites that the Earth intercepts in its wandering around the Sun.

There have been two kinds of watches used: some, with "little winding", which stopped while the transition interval elapsed, and others, with "long winding", which have been running to this day. The prototypes of these watches are the following:

• The iodine-129 clock, which, driven by this radionuclide, decays with a period of 17 Ma to xenon-129 (stable), and which makes it possible to measure time lapses of the order of one hundred million years. Readings from this clock can be made on meteorites which, being very small celestial bodies, cooled immediately after their formation; thus it has been observed by the accumulated xenon-129, that all the meteorites were formed during the transition interval, some, like the Allede meteorite, at the very beginning of the interval, and others, like the Guareña meteorite (to cite only Spanish names ) about 100 Ma later (at the latest). In the interim of the transition, it is thought that the planets were also formed, by gravitational accretion of small asteroids; but this has been known, in part, with the competition of "long winding" watches, the prototype of which is cited below.
• The rubidium-87 clock, which, driven by this radionuclide, decays, with a period of about 50,000 Ma, to strontium-87 (stable), which accumulates in all rubidium-containing minerals; the data provided by various meteorites confirm the linearity of this temporal accumulation of strontium-87, which allows the results to be extrapolated to time zero, from the beginning of the transition interval. This has been ratified using another clock of the same class, the uranium-238 clock, which decays with a period of 4,507 Ma, to give lead-206 (stable).

In summary, using different atomic clocks, it has been possible to determine that the age of the Earth (and the Solar System as a whole) is 4,550 Ma, and that its formation required a transition interval, between the galactic nebula and the concretion of the planets, about 100 Ma.

What role do radiation detectors play?

It is well known that man has no perceptive capacity for alpha, beta, gamma, neutron radiation, etc.; therefore, for man it is as if nuclear radiation did not exist. Radiation detectors are devices created to fill this sensory deficiency, by transforming the interactions of radiation with matter into signals perceptible by man, or by instruments (counters) that are charged with keeping track of them. .

What interactions are used for this purpose? Molecular ionizations and excitations, which are the most elementary forms of radiation-matter interaction; for this, the sensitive material of the detector is carefully selected, according to the nature of the radiation-problem, and the way of measuring the electrical charges produced in the ionization after the excitation. These three requirements -material, radiation and measurement- give rise to a large number of possible detectors, of which the most used are those belonging to the following classes:

• Gaseous ionization detectors, which, under the action of an electric field, collect the charges formed in a gas, giving rise to a current (ionization chamber) or to discrete pulses (proportional and Geiger-Müller counters); these detectors are useful in the metrology of all radiations.
• Scintillation detectors that, equipped with a photomultiplier, see the flashes of light emitted by phosphorescent substances when radiation passes; there are scintillation crystals, suitable for gamma radiation metrology, and scintillation liquids, for alpha and beta radiation.
• Solid-state detectors, which, equipped with crystals of semiconductor elements -diamond, silicon or germanium-, become conductive at low temperatures due to the effect of radiation, giving rise to impulses classifiable by size, using multichannel analysers; These detectors are the foundation of high-resolution gamma spectrometry, which makes it possible to analyze complex mixtures of radionuclides, without the need for prior radiochemical separations.

In this brief description of the main types of detectors, it will have been possible to appreciate the prosthetic function that detectors have for man, giving instrumental coverage to his sensory deficiency in the field of nuclear radiation. With the help of the detectors, all the knowledge of the subatomic world that is now possessed has been built..

Do you know that nuclear radiation is used to improve agricultural crops?

Currently human nutrition is based on the cultivation of a few plant species, which have been the result of some ten thousand years of agricultural practices, aimed at selecting the most appropriate varieties to meet man's food needs.

Since the beginning of the century, it has been known that the variability of species is the consequence of gene mutations that occur spontaneously in plants; that is, small variations in one of the many genes -of the order of one hundred thousand- that define the characters of a plant species. These spontaneous mutations have, however, a very limited field of application, because their frequency of appearance is very low, since they are due to radiation from the natural radioactive background or mutagenic chemical compounds existing in the environment. Added to this is the fact that mutations are random in nature and randomly modify the characters of plants, both improving and worsening them. What traditional agricultural practices did was, ultimately, patiently select the variants that appeared and that presented positive aspect modifications, that is, with greater resistance to climatic conditions, pathogenic germs, pests, etc., or with a higher content of trophic substances (proteins, fats, sugars, etc.).

Now that a wide repertoire of radiation sources is available, the efficient use of mutations invites us to artificially induce them in the most promising species, in order to shorten the slow natural evolutionary process, going from millennia of customary agriculture to mere decades; because, although it is easy to increase the rate of mutations, it is necessary to go through the expression phase of the mutations, which are the crops of the resulting plants, on which the selection of the advantageous varieties must be carried out, which entails some years of experimentation.

Fortunately, today there are many research centers dedicated to the genetic selection of seeds, and between the years 1970 and 1990 more than a thousand crops have been introduced, especially in the cereal sector, which now cover large agricultural areas in the countries with the greatest demographic problems (China, India, Japan, etc.).

The genetic selection of agricultural crops is the true "green revolution" that humanity needs, of which we are still in its beginnings. In vitro plant cultivation (rapid clonal reproduction), recently developed, and biotechnology (directed gene transfer between different species), which has achieved its first successes, are the main pillars on which future agri-food development will rest.

Did you know that nuclear radiation is used to eradicate agricultural pests?

Despite the prolonged use of powerful insecticides over decades, on the order of 20 percent of agricultural crops are still lost, destroyed by insect pests. If we add to this the fact that flies and mosquitoes are disease transmitters, it is easy to conclude that insects are responsible for a good part of the food deficiencies and the quality of life of the human species. Fortunately, in recent years an insect sterilization technique has been successfully applied to control the most devastating pests.

The technique itself is very simple: insects are massively produced in factories, which are sexually sterilized with doses of the order of 100 Gy of cobalt-60 gamma radiation; The treated insects are released in a programmed way into nature, where they mate, without consequences, with the native insects, with which the pest population decreases to the point where it can be eradicated.

Here are some examples of the application of this technique:

• The screwworm fly lays eggs in the wounds of warm-blooded animals, where its larvae develop parasitically, penetrating the tissues, causing great suffering and even death in the animal. The plague has already been eradicated in North America, where a factory to control the plague in the Caribbean region continues in full production. The fight against this fly has also begun in the Maghreb countries, which suffered accidental contamination in 1988.
• The tsetse fly, which is the propagation vector of the parasite that causes sleeping sickness (trypanosomiasis), affects an area 20 times greater in tropical Africa than Spain, where livestock farming is totally ruined; The eradication problem is very complex, not only because of its size, but also because the tsetse denomination includes at least 30 subspecies, which require the development of as many varieties of sterile insects, and a coordinated program involving 36 different States.
• The fruit fly (or Mediterranean fly), which is one of the most harmful pests for fruit crops worldwide; It seems to be native to Southeast Africa, but it has spread to the Mediterranean basin and from there to other continents. The fight against sterile insects, combined with insecticides, has already started in several countries with good results, but the technique is being perfected in the sense of eliminating the females in the breeding process in the factories, because the sterile females continue to have the instinct to lay their eggs in the pulp of the fruit, thus opening pathways of infection for other pathogenic germs and, because they uselessly distract the sexual attention of the sterile males.
• The lepidopteran caterpillar, which defoliates large areas of trees, especially in the US; In this case, a variant called F1 or inherited sterility is being tested, consisting of irradiation at a lower dose in the butterfly phase, which, although it is sufficient to sterilize females, is only 30 to 60 percent effective. of cases in males. After the release, the females that mate with native males do not give rise to offspring, and the males that mate with native females give rise to reduced offspring, whose individuals are also completely sterile; with which the reproductive chain is definitively interrupted.

As seen with the examples reviewed, nuclear radiation has beneficial applications for the eradication of pests, making unnecessary the use of insecticides, which are producing dangerous chemical contamination of the biosphere.