THE RADIOISOTOPE PART 1

2500 years ago Democritus said sweetness and bitterness warmth cold and color are only appearances in truth nothing exists but atoms and the void ever since the beginning of history lonely men have searched for a pot of gold under a distant rainbow now the prospector has become in a sense of scientists now the object of the search is not only gold silver and copper but also uranium a radioactive metal which signifies its present by the reading on a Geiger counter scientists foresee that in future years uranium may supply power to heat our homes drive our cities an engineer's dream a future reality today blueprints for atomic power will require years to develop and apply but atomic energy has still another aspect the nuclear reactor incubator of plutonium precursor of the atomic power plant of the future is also the source of radioactive materials known as Radio isotopes available now as peacetime weapons in the continuing struggle for health material satisfactions and scientific progress the radioisotope is probably the most important new means of perception and investigation since the invention of the microscope to use this new means of perception the scientist has been forced to acquire many novel techniques absorb a variety of concepts unfamiliar to him the radioisotope becomes useful to the scientists only when these concepts have taken their places in an orderly structure of method and underlying theory the first concepts the scientist faces are those which have to do with the nature of matter and the dynamics of its smaller components now it is known that the atom long considered the basic unit of matter is itself a void in which planetary particles called electrons revolve in great orbits around a central nucleus the nucleus also has a structure is composed of protons and neutrons each proton has a weight of 1.00 758 atomic mass units the neutron is slightly heavier in the usual notation the mass is shown in round numbers one for both proton and neutron the proton carries one unit of positive charge indicated in subscript the neutron has no charge the electron has one unit of negative charge its mass is insignificant compared to that of a proton or neutron again charge is shown in subscript mass in superscript in describing an atom the same notation is followed the atomic number of the element in subscript is equal to the number of electrical charges carried by the protons in the nucleus the mass number in superscript is equal to the sum of protons and neutrons for instance the designation for helium is to helium for normally the positive charge of the two protons in the nucleus is balanced by the negative charges of an equal number of orbital electrons atoms can be classified according to the number of protons in the nucleus or the atomic number and according to the number of neutrons in terms of proton number and neutron number it is possible to define any atom normal hydrogen atoms for instance have one proton and no neutrons helium with two protons usually has two neutrons lithium most often has three protons and four neutrons and so forth all the other atoms are identified according to the same system the number of protons or the atomic number identifies the element all atoms of the same element have the same number of protons however in most elements there are found atoms of different Neutron numbers these different forms of the same elements are isotopes atoms of the same element alight chemically differing only in the number of neutrons the sum of protons and neutrons give the mass number for instance an atom of helium which has two protons and two neutrons as a mass of for a lithium atom with three protons three neutrons has six mass units and so on different isotopes of the same element have different mass numbers since they have a different number of neutrons again to complete the description of each atom the element is accompanied by its atomic number in subscript most elements have two or more isotopes however a few have only one for instance cobalt element number 27 has only one natural isotope all cobalt atoms found in nature have the same atomic weight 59 and therefore the same number of neutrons however most elements have more than one natural form for instance zinc number 30 on the chart has five natural isotopes in zinc as in most elements the number of neutrons is greater than the number of protons the number of neutrons can be found by subtracting the proton number from the mass number higher on the chart a few additional elements have only one isotope though most have several at higher atomic numbers the ratio of neutrons to protons becomes greater and greater in the natural isotopes of mercury for instance the lightest atoms have 116 neutrons 36 more neutrons than protons the heaviest have 124 neutrons 44 more neutrons and protons to illustrate neutron proton ratio the isotopes can be plotted on a graph of atomic number against neutron number in this graph a straight diagonal line would intersect all points at which proton number equals neutron number however if the isotopes found in nature are plotted on such a graph it is found that they depart from the diagonal in other words they have a preponderance of neutrons / protons a preponderance which tends to increase at higher atomic numbers with a few exceptions these positions represent atoms with stable neutron proton ratios capable of existing indefinitely without any observable change the empty spaces represent neutron proton ratios which would not be stable it is reasonable to assume that if an atom existed in an empty space on the chart it would show a tendency to change itself to an atom having a more stable neutron proton ratio for instance and at'em occupying a position above the natural isotopes would have a disproportionately high ratio of protons to neutrons and would tend toward the position of one of the naturally stable isotopes below it similarly and Adam having a disproportionate excess of neutrons / protons would tend to take a position of one of the naturally stable isotopes above it this illustrates one of the primary principles of nuclear stability the tendency of all atoms to reach an equilibrium in terms of a stable neutron proton ratio up to atomic number 83 the natural isotopes are stable with a few exceptions however above atomic number 83 all the isotopes are unstable they all have a disproportionate number of neutrons either too many or too few without exception these isotopes are radioactive and return to stability through a process known as radioactive decay for example uranium the heaviest element found in nature and significant amounts has three natural isotopes each uranium isotope tends to return to stability through a series of transformations ending in a stable form of lead an example of this process is the way in which uranium-238 changes to lead 206 the transformation from instability to stability is a complex process which takes place in a series of separate steps the first step occurs when the uranium atom throws off a nuclear particle of for mass units composed of two protons and two neutrons this single unit is known as an alpha particle with this change the uranium 238 atom becomes thorium 234 which has two less units of charge on the nucleus and four less units of mass the nucleus of the thorium atom emits an electron which possesses a negative unit of charge and negligible mass this electron thrown off by the nucleus of the thorium atom is known as a beta particle and is usually represented by the Greek letter beta with this emission of a negatively charged beta particle for iam 234 in turn changes to proto actinium 234 an atom with 91 charges instead of 90 on the nucleus proto actinium 234 throws off a beta particle increasing atomic number by 1 thus changing to uranium 234 uranium 234next throws off an alpha particle to become thorium 234 missions further alpha particles are given off resulting in radium-226 radon 222 polonium 218 and finally led to 14 and unstable isotope two beta particles are emitted successively producing bismuth 214 and polonium 214 polonium 214 gives off another alpha particle to produce led to 10 still unstable again one beta particle is emitted in each of two changes to form polonium-210 with the emission of a final alpha particle polonium-210 goes to lead 206 a stable isotope thus in radioactive decay a complex series of nuclear transformations has taken place from an unstable neutron proton ratio to a stable state in every step of radioactive decay the transformation is from parent to daughter and parent to daughter atoms of the parent element decaying to produce atoms of a daughter element the same terminology applies whether or not the transformation is to a higher or lower atomic number there is no way of foretelling when any particular unstable nucleus will decay from one step to the next however in any sample of a radioactive isotope having millions of billions of atoms some of the atoms are always in the process of decay the number of atoms decaying in a unit time in a sample of radioactive isotope is called the activity of that sample activity or rate of decay is generally stated in terms of military's or MC for short a given sample of radioactive material is said to have an activity of one military when 37 million decay or disintegrate per second the military is the commonly accepted measure of radioactivity decay from a parent isotope to a daughter isotope can therefore be illustrated in terms of the number of unstable atoms and the time on this graph the number of unstable parent atoms that exist at any given instant can be represented by a point since the parent substance is decaying fewer unstable parent atoms will be present at any later time in the language of simple calculus the number of parent atoms has decreased by Delta M during a time interval delta T the slope of the line between the two points corresponds to the rate of decay the rate of decay Delta n over delta T is proportional to M the number of parent atoms present or equal to minus lambda M the minus sign indicates that n decreases with time lambda is called the decay constant and is characteristic of the particular parent radioisotope if decay is plotted as a continuous series of short time intervals an exponential formula is derived which is the basic standard formula for radioactive decay in this formula n is the number of radioactive parent atoms presence at a given time n sub 0 is the number of radioactive parent atoms at the beginning of the first time interval e is the base of the natural logarithms lambda of course is the decay constant of the parent substance since the number is proportional to the activity the symbol n can be replaced by a throughout the formula accordingly the same curve can equally well represent the change of activity rather than number versus time from this equation for radioactive decay activity at any moment can be determined in terms of the initial activity in any decay process there is a point in time at which activity is equal to one half of that present initially this time during which activity has decreased by half is known as the half-life in successive half-lives activity continues to decrease by successive halves until finally only a small fraction of the original activity remains hence the decay rate for a specific radioisotope is usually given in terms of half-life it is often convenient to substitute half-life for the decay constant lambda the equivalent of lambda is point 693 / half-life this formula is often used in finding activity at any time T when half-life is known plotted in terms of the logarithm of activity against time exponential decay is shown as a straight line figures for half-life are generally available in isotope literature each radioisotope has a specific half-life the half-life may vary greatly from a fraction of a second to thousands or billions of years for instance one isotope of cadmium has a half-life of two point three days while carbon-14 decays to its daughter nitrogen after a half-life of roughly fifty seven hundred years or take the case of uranium u-238 has a half-life of four and a half billion years giving a straight line on a graph calibrated in terms of days or weeks in contrast to the long half-life of u-238 thorium 234 the daughter of u-238 has a half-life of twenty 4.1 days thus while all steps of decay are unpredictable in a single atom the statistical rate of decay in a large number of atoms can be predicted and measured with great accuracy examining the various steps in the decay series beginning with u-238 it appears that there are at least two ways in which unstable atoms can change to a more stable form by throwing off an alpha particle and losing four units of mass and two electrical charges or by throwing off a beta particle an increasing atomic number by one in the process of decay of u-238 to stable lead each of the two types of transformation occurs several times in eight steps of decay eight alpha particles are given off six beta particles are emitted one would assume that these fourteen particles would account for the mass difference between uranium 238 and led 206 let's check this assumption the exact weight of an alpha particle is known from accurate determinations the eight alpha particles thrown off way 32.0 to two units of mass the mass of beta particles is also known with great accuracy adding the weights it is found that 32.0 25 units of total mass have been given off as discrete particles during the decay process but if the weight of lead 206 is subtracted from that of uranium 238 we find that the total mass lost in the decay is somewhat greater than that represented by the alpha and beta particles thrown off in other words point zero four five units of mass are unaccounted for representing a minut fraction of a gram according to Einstein's formula energy and mass are equivalent substituting in solving the formula it is found that this mass represents a really significant energy in the transition from uranium to lead this energy has been released in addition to the alpha and beta particles emitted most of this energy released during uranium decay has been applied in giving the ejected particles their high speed in other words as kinetic energy additional energy is also released as a form of electromagnetic energy known as gamma radiation to summarize the decay of uranium 238 resulting in stable led is accompanied by the release of alpha particles beta particles and gamma radiation this is generally true a change from instability to stability is accompanied by energy released as discrete particles or electromagnetic radiations or both it is the energy released by the decay of uranium in uranium bearing or which makes it possible to detect it with the Geiger counter even though mixed in minut proportions with a variety of other substances the decay series beginning with uranium 238 is only one of several a similar decay scheme begins with u-235 another with thorium to 30 an otherwise elements near it on the chart have very long half-lives which have permitted them to survive from an early age of greater radioactivity they are feted by the inevitable process of decay to return in time to a stable state lower on the scale of elements are found a few unstable survivors naturally radioactive isotopes of long half-life such as rhenium 187 lutetium 176 10 124 rubidium 87 and potassium 40 all of these return to stability in a single step potassium-40 offers a good illustration of the three possible modes of nuclear transformation as one possibility potassium-40 may give off a beta particle increasing its nuclear charge by one and changing to calcium 40 in this case the beta particle is accompanied by excess energy as a gamma ray or potassium 40 might emit a positive electron reducing atomic number to become argon-40 the positive electron is known as a positron as the third possibility the potassium nucleus may absorb an electron becoming argon-40 without emitting any discrete particle in all three cases there is a change in atomic number without any significant change in Mass the sum of neutrons plus protons remains the same obviously each change must involve a shift in the neutron proton ratio in the nucleus for example when potassium-40 changes to calcium 41 of the neutrons in its nucleus has become a proton and an electron in other words a neutron of zero charge and one mass unit has become a proton of positive charge in mass one and an electron of one negative charge and very slight mass mass and charge units are conserved in the transition when potassium-40 emits a positron and changes to argon-40 a proton in the nucleus has changed to form a neutron and a positron this type of nuclear change is called positron decay in the third possibility one of the protons in the nucleus may join with an electron to form a neutron this transformation would seem contrary to logic since it is generally assumed that there are no free electrons in the nucleus such a phenomenon can best be explained by referring to the nucleus and its surrounding electrons it sometimes occurs that an electron from the inner or K shell is captured by the nucleus and is immediately absorbed by a proton the opposite electrical charges neutralize one another and the proton becomes a neutron this transformation is known as k electron capture when it occurs it causes a series of changes in the outer electron shells first the empty space and the K orbit is filled by an electron from an outer orbit the empty space in the second orbit is filled by an electron from an orbit further out and so forth in this cascade of electrons from outer to inner orbits x-rays are released which represent the difference in potential energy between different electron shells of the daughter Adam the K x-ray which corresponds to the energy released in capture of the K electron represents most of the measurable energy released thus when potassium-40 captures a k electron and changes to argon-40 the transformation is accompanied by the emission of x-radiation while a single atom of potassium-40 can decay in only one of three ways with a large number of potassium-40 atoms present some will decay in each way energy is released as positrons beta particles and gamma rays and in k capture as x-rays in potassium-40 decay positron emission is difficult to substantiate occurring rarely if at all in k capture the x-rays emitted are of low energy and difficult to measure in other radioisotopes any of these radiations may be given off in different proportions and relative energies decay and emission of energy as radiation occurs in all radioisotopes however uranium-235 and one or two other radioisotopes have a second and unique characteristic if it were possible to observe a single atom of uranium-235 and to penetrate through the electron shells to the nucleus itself it would appear to be in a constant state of agitation in all probability it would throw off an alpha particle as the first step in the decay process however in a gram of u-235 in each second at least one atom behaves quite differently it becomes more and more unstable until it splits into two fragments each a lighter atom of lower atomic number in addition two or more neutrons are left over from the reaction this is the reaction of spontaneous fission in a single fishin taking place in a minut fraction of a second considerable energy is released about four times as much as in all steps of a long decay process of u-238 to stable lead part of the energy is kinetic energy giving velocity to the neutrons and the two fragments of uranium the remainder of the energy is given off as gamma radiation and bigger radiation in uranium fishin one two three neutrons are emitted if enough uranium atoms are present one of the neutrons may strike and be absorbed by another nucleus of u-235 with the added mass this nucleus in turn fishin's releasing still other neutrons and the process continues this is a chain reaction such as that which takes place in an uncontrolled manner in the atomic bomb however if you're any emission occurs in uranium mixed with some light substance such as graphite the neutrons are slowed down by collision with the carbon atoms because of its slowing action on the neutrons the graphite or other light substance is known as a moderator the neutrons because of their reduced velocity are much less likely to escape from the uranium and are much more likely to penetrate to the nuclei of the uranium atoms if the amount of uranium and moderator is too small a large proportion of the neutrons will escape and the reaction will die out if the structure of moderator and uranium is larger than a certain critical size the reaction will sustain itself