Download - CHAPTER 1 Nuclear Radiation - Review · CHAPTER 1 Nuclear Radiation - Review Some Nuclear Units Nuclear energies are very high compared to atomic processes, and need larger units

CHAPTER 1 Nuclear Radiation - Review

Some Nuclear Units

Nuclear energies are very high compared to atomic processes, and need larger units

However, the sizes are quite small and need smaller units:

Nuclear masses are measured in terms of atomic mass units with the carbon-12 nucleus defined as having a mass of exactly 12 amu. It is also common practice to quote the rest mass energy as if it were the mass. The conversion to amu is:

Engineering Aspects of Food Irradiation 1

Introduction

2

Relativistic Energy

The famous Einstein relationship for energy

includes both the kinetic energy and rest mass energy for a particle. The kinetic energy of a high speed particle can be calculated from

The relativistic energy of a particle can also be expressed in terms of its momentum in the expression

The relativistic energy expression is the tool used to calculate binding energies of nuclei and the energy yields of nuclear fission and fusion.

Kinetic Energy

Kinetic energy is energy of motion. The kinetic energy of an object is the energy it possesses because of its motion. The kinetic energy* of a point mass m is given by

Engineering Aspects of Food Irradiation

Introduction

Kinetic energy is an expression of the fact that a moving object can do work on anything it hits; it quantifies the amount of work the object could do as a result of its motion. The total mechanical energy of an object is the sum of its kinetic energy and potential energy.

For an object of finite size, this kinetic energy is called the translational kinetic energy of the mass to distinguish it from any rotational kinetic energy it might pos-sess - the total kinetic energy of a mass can be expressed as the sum of the transla-tional kinetic energy of its center of mass plus the kinetic energy of rotation about its center of mass.

*This assumes that the speed is much less than the speed of light. If the speed is comparable with c then the relativistic kinetic energy expression must be used.

Rest Mass Energy

The Einstein equation includes both the kinetic energy of a particle and the energy it has as a result of its mass. If the particle is at rest, then the energy is expressed as

which is sometimes called its rest mass energy.

Conservation of Energy

The relativistic energy expression is a statement about the energy an object contains as a result of its mass and is not to be construed as an exception to the principle of conservation of energy. Energy can exist in many forms, and mass energy can be considered to be one of those forms.


Introduction

4

Pair Production

Every known particle has an antiparticle; if they encounter one another, they will annihilate with the production of two gamma-rays. The quantum energies of the gamma rays is equal to the sum of the mass energies of the two particles (including their kinetic energies). It is also possible for a photon to give up its quantum energy to the formation of a particle-antiparticle pair in its interaction with matter.

The rest mass energy of an electron is 0.511 MeV, so the threshold for electron-positron pair production is 1.02 MeV. For X-ray and gamma-ray energies well above 1 MeV, this pair production becomes one of the most important kinds of interactions with matter. At even higher energies, many types of particle-antiparti-cle pairs are produced.

Relativistic Kinetic Energy

The relativistic energy expression includes both rest mass energy and the kinetic energy of motion. The kinetic energy is then given by

This is essentially defining the kinetic energy of a particle as the excess of the parti-cle energy over its rest mass energy. For low velocities this expression approaches the non-relativistic kinetic energy expression.

Kinetic Energy for v/c<<1

The relativistic kinetic energy expression can be written as


Introduction

and the square root expression then expanded by use of the binomial theorem

given

substituting gives

Relative scale model of an atom and the solar system

Do you perceive a gold ring to contain a larger fraction of solid matter than the solar system?


Introduction

6

On this scale, the nearest star would be a little over 10,000 miles away.

Data for Scale Model of Atom

Nuclear Size and Density

Various types of scattering experiments suggest that nuclei are roughly spherical and appear to have essentially the same density. The data are summarized in the expression called the Fermi model:


Introduction

where r is the radius of the nucleus of mass number A. The assumption of constant density leads to a nuclear density.

The most definitive information about nuclear sizes comes from electron scattering.

Nuclear Density and the Strong Force

The fact that the nuclear density seems to be independent of the details of neutron number or proton number implies that the force between the particles is essentially the same whether they are protons or neutrons. This correlates with other evidence that the strong force is the same between any pair of nucleons.


Introduction

8

Nuclear Forces

Within the incredibly small nuclear size, the two strongest forces in nature are pit-ted against each other. When the balance is broken, the resultant radioactivity yields particles of enormous energy.

The electron in a hydrogen atom is attracted to the proton nucleus with a force so strong that gravity and all other forces are negligible by comparison. But two pro-tons touching each other would feel a repulsive force over 100 million times stron-ger! So how can such protons stay in such close proximity? This may give you some feeling for the enormity of the nuclear strong force which holds the nuclei together.

The Electromagnetic Force


Introduction

One of the four fundamental forces, the electromagnetic force manifests itself through the forces between charges (Coulomb's Law) and the magnetic force, both of which are summarized in the Lorentz force law. Fundamentally, both magnetic and electric forces are manifestations of an exchange force involving the exchange of photons. The electromagnetic force is a force of infinite range which obeys the inverse square law, and is of the same form as the gravity force.

The electromagnetic force holds atoms and molecules together. In fact, the forces of electric attraction and repulsion of electric charges are so dominant over the other three fundamental forces that they can be considered to be negligible as determiners of atomic and molecular structure. Even magnetic effects are usually apparent only at high resolutions, and as small corrections.

Nuclear Particles

Nuclei are made up of protons and neutrons bound together by the strong force. Both protons and neutrons are referred to as nucleons. The number of protons is called the atomic number and determines the chemical element. Nuclei of a given element (same atomic number) may have different numbers of neutrons and are then said to be different isotopes of the element.

Proton

Along with neutrons, protons make up the nucleus, held together by the strong force. The proton is a baryon and is considered to be composed of two up quarks and one down quark.


Introduction

10

It has long been considered to be a stable particle, but recent developments of grand

unification models have suggested that it might decay with a half-life of about 1031 years. Experiments are underway to see if such decays can be detected. Decay of the proton would violate the conservation of baryon number, and in doing so would be the only known process in nature which does so.

When we say that a proton is made up of two up quarks and a down, we mean that its net appearance or net set of quantum numbers match that picture. The nature of quark confinement suggests that the quarks are surrounded by a cloud of gluons, and within the tiny volume of the proton other quark-antiquark pairs can be pro-duced and then annihilated without changing the net external appearance of the pro-ton.

Neutron

Along with protons, neutrons make up the nucleus, held together by the strong force. The neutron is a baryon and is considered to be composed of two down quarks and one up quark.

A free neutron will decay with a half-life of about 10.3 minutes but it is stable if combined into a nucleus. The decay of the neutron involves the weak interaction as indicated in the Feynman diagram to the right. This fact is important in models of the early universe. The neutron is about 0.2% more massive than a proton, which translates to an energy difference of 1.29 MeV.


Introduction

The decay of the neutron is associated with a quark transformation in which a down quark is converted to an up by the weak interaction.

The Strong Force

A force which can hold a nucleus together against the enormous forces of repulsion of the protons is strong indeed. However, it is not an inverse square force like the electromagnetic force and it has a very short range. Yukawa modeled the strong force as an exchange force in which the exchange particles are pions and other heavier particles. The range of a particle exchange force is limited by the uncer-tainty principle. It is the strongest of the four fundamental forces.


Introduction

12

Nuclear Notation

Standard nuclear notation shows the chemical symbol, the mass number and the atomic number of the isotope.

Example: the isotopes of carbon. The element is determined by the atomic number 6. Carbon-12 is the common isotope, with carbon-13 as another stable isotope which makes up about 1%. Carbon 14 is radioactive and the basis for carbon dating.

Since the protons and neutrons which make up the nucleus are themselves consid-ered to be made up of quarks, and the quarks are considered to be held together by the color force, the strong force may be considered to be a residual color force. In the standard model, therefore, the basic exchange particle is the gluon which medi-ates the forces between quarks. Since the individual gluons and quarks are con-tained within the proton or neutron, the masses attributed to them cannot be used in the range relationship to predict the range of the force. When something is viewed as emerging from a proton or neutron, then it must be at least a quark-antiquark


Introduction

pair, so it is then plausible that the pion as the lightest meson should serve as a pre-dictor of the maximum range of the strong force.

Isotopes

The different isotopes of a given element have the same atomic number but differ-ent mass numbers since they have different numbers of neutrons. The chemical properties of the different isotopes of an element are identical, but they will often have great differences in nuclear stability. For stable isotopes light elements, the number of neutrons will be almost equal to the number of protons, but a growing neutron excess is characteristic of stable heavy elements. The element tin (Sn) has the most stable isotopes with 10, the average being about 2.6 stable isotopes per element.

Information about the isotopes of each element and their abundance can be found by going to the periodic table and choosing an element. Then take the link to nuclear data.

Radioactivity

Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Because the nucleus experiences the intense conflict between the two strongest forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation. The most common types of radiation are called alpha, beta, and gamma radiation, but there are several other varieties of radioactive decay.

Radioactive decay rates are normally stated in terms of their half-lives, and the half-life of a given nuclear species is related to its radiation risk. The different types of radioactivity lead to different decay paths which transmute the nuclei into other


Introduction

14

chemical elements. Examining the amounts of the decay products makes possible radioactive dating.

Radiation from nuclear sources is distributed equally in all directions, obeying the inverse square law.

Alpha Radioactivity

Composed of two protons and two neutrons, the alpha particle is a nucleus of the element helium. Because of its very large mass (more than 7000 times the mass of the beta particle) and its charge, it has a very short range. It is not suitable for radia-tion therapy since its range is less than a tenth of a millimeter inside the body. Its main radiation hazard comes when it is ingested into the body; it has great destruc-tive power within its short range. In contact with fast-growing membranes and liv-ing cells, it is positioned for maximum damage.


Introduction

Alpha particle emission is modeled as a barrier penetration process. The alpha par-ticle is the nucleus of the helium atom and is the nucleus of highest stability.

Alpha Barrier Penetration

The energy of emitted alpha particles was a mystery to early investigators because it was evident that they did not have enough energy, according to classical physics, to escape the nucleus. Once an approximate size of the nucleus was obtained by Rutherford scattering, one could calculate the height of the Coulomb barrier at the radius of the nucleus. It was evident that this energy was several times higher than the observed alpha particle energies. There was also an incredible range of half lives for the alpha particle which could not be explained by anything in classical physics.

The resolution of this dilemma came with the realization that there was a finite probability that the alpha particle could penetrate the wall by quantum mechanical tunneling. Using tunneling, Gamow was able to calculate a dependence for the half-life as a function of alpha particle energy which was in agreement with experimen-tal observations.


Introduction

16

Alpha Binding Energy

The nuclear binding energy of the alpha particle is extremely high, 28.3 MeV. It is an exceptionally stable collection of nucleons, and those heavier nuclei which can be viewed as collections of alpha particles (carbon-12, oxygen-16, etc.) are also exceptionally stable. This contrasts with a binding energy of only 8 MeV for helium-3, which forms an intermediate step in the proton-proton fusion cycle.

Alpha, Beta, and Gamma

Historically, the products of radioactivity were called alpha, beta, and gamma when it was found that they could be analyzed into three distinct species by either a mag-netic field or an electric field.

Penetration of Matter

Though the most massive and most energetic of radioactive emissions, the alpha particle is the shortest in range because of its strong interaction with matter. The


Introduction

electromagnetic gamma ray is extremely penetrating, even penetrating considerable thicknesses of concrete. The electron of beta radioactivity strongly interacts with matter and has a short range.

Radioactive Half-Life

The radioactive half-life for a given radioisotope is the time for half the radioactive nuclei in any sample to undergo radioactive decay. After two half-lives, there will be one fourth the original sample, after three half-lives one eight the original sam-ple, and so forth.

Graph of Radioactive Decay

The radioactive half-life gives a pattern of reduction to half in any successive half-life period.


Introduction

18

Radioactive Decay Paths

Radioactivity involves the emission of particles from the nuclei. In the case of gamma emission, the nucleus remaining will be of the same chemical element, but for alpha, beta, and other radioactive processes, the nucleus will be transmuted into the nucleus of another chemical element. Each decay path will have a characteristic half-life, but some radioisotopes have more than one competing decay path.


Introduction

Radioactive Dating

Because the radioactive half-life of a given radioisotope is not affected by tempera-ture, physical or chemical state, or any other influence of the environment outside the nucleus save direct particle interactions with the nucleus, then radioactive sam-ples continue to decay at a predictable rate. If determinations or reasonable esti-mates of the original composition of a radioactive sample can be made, then the amounts of the radioisotopes present can provide a measurement of the time elapsed.

One such method is called carbon dating, which is limited to the dating of organic (once living) materials. The longer-lived radioisotopes in minerals provide evi-dence of long time scales in geological processes. While original compositions can-not be determined with certainty, various combination measurements provide self-consistent values for the times of formations of certain geologic deposits. These clocks-in-the-rocks methods provide data for modeling the formation of the Earth and solar system.

Beta Radioactivity

Beta particles are just electrons from the nucleus, the term “beta particle” being an historical term used in the early description of radioactivity. The high energy elec-trons have greater range of penetration than alpha particles, but still much less than gamma rays. The radiation hazard from betas is greatest if they are ingested.

Beta emission is accompanied by the emission of an electron antineutrino which shares the momentum and energy of the decay.


Introduction

20

The emission of the electron's antiparticle, the positron, is also called beta decay.

Electron and Antineutrino

Early studies of beta decay revealed a continuous energy spectrum up to a maxi-mum, unlike the predictable energy of alpha particles. Another anomaly was the fact that the nuclear recoil was not in the direction opposite the momentum of the electron. The emission of another particle was a probable explanation of this behav-ior, but searches found no evidence of either mass or charge. Pauli in 1930 pro-posed a particle called a neutrino which could carry away the missing energy and momentum. With no charge and no mass, it was hard to detect, and not until 1953 was experimental detection of the neutrino achieved. For symmetry reasons, the particle emitted along with the electron from nuclei is called an antineutrino. The emission of a positron is accompanied by a neutrino.

Positron and Neutrino

The emission of a positron or an electron is referred to as beta decay. The positron is accompanied by a neutrino, a massless(?) and churchless particle. Positrons are emitted with the same kind of energy spectrum as electrons in negative beta decay because of the emission of the neutrino


Introduction

Beta Energy Spectrum

In the process of beta decay, either an electron or a positron is emitted. Because either a neutrino or an antineutrino is emitted as well, there is a spectrum of ener-gies for the electron or positron, depending upon what fraction of the reaction energy Q is carried by the massive particle.

Energy and Momentum Spectra for Beta Decay

Beta emission has a characteristic energy spectrum. It is accompanied by the emis-sion of an electron antineutrino which shares the momentum and energy of the decay.


Introduction

22

This experimental energy spectrum is from G. J. Neary, Proc. Phys. Soc. (London), A175, 71 (1940).

The emission of the electron's antiparticle, the positron, is also called beta decay.

Gamma Radioactivity

Gamma radioactivity is composed of electromagnetic rays. It is distinguished from x-rays only by the fact that it comes from the nucleus. Most gamma rays are some-what higher in energy than x-rays and therefore are very penetrating. It is the most useful type of radiation for medical purposes, but at the same time it is the most dangerous because of its ability to penetrate large thicknesses of material.


Introduction

Other Radioactive Processes

While the most common types of radioactive decay are by alpha, beta, and gamma radiation, several other varieties of radioactivity occur:

Electron capture: A parent nucleus may capture one of its own electrons and emit a neutrino. This is exhibited in the potassium-argon decay.

Positron or positive beta decay: Positron emission is called beta decay because the characteristics of electron or positron decay are similar. They both show a charac-teristic energy spectrum because of the emission of a neutrino or antineutrino.

Internal conversion is the use of electromagnetic energy from the nucleus to expel an orbital electron from the atom.

Electron Capture

Electron capture is one form of radioactivity. A parent nucleus may capture one of its orbital electrons and emit a neutrino. This is a process which competes with positron emission and has the same effect on the atomic number. Most commonly,


Introduction

24

it is a K-shell electron which is captured, and this is referred to as K-capture. A typ-ical example is

In the middle range of the periodic table, those isotopes which are lighter than the most stable isotopes tend to decay by electron capture, and those heavier decay by negative beta decay. An example of this pattern is seen with silver isotopes, with two stable isotopes plus one of lower mass which decays by electron capture and one of heavier mass which decays by beta emission.

Internal Conversion

Internal conversion is another electromagnetic process which can occur in the nucleus and which competes with gamma emission. Sometimes the multipole elec-tric fields of the nucleus interact with orbital electrons with enough energy to eject them from the atom. This process is not the same as emitting a gamma ray which knocks an electron out of the atom. It is also not the same as beta decay, since the emitted electron was previously one of the orbital electrons, whereas the electron in beta decay is produced by the decay of a neutron.

An example used by Krane is that of 203Hg, which decays to 203Tl by beta emis-sion, leaving the 203Tl in an electromagnetically excited state. It can proceed to the ground state by emitting a 279.190 keV gamma ray, or by internal conversion. In this case the internal conversion is more probable. Since the internal conversion process can interact with any of the orbital electrons, the result is a spectrum of internal conversion electrons which will be seen as superimposed upon the electron energy spectrum of the beta emission. The energy yield of this electromagnetic transition can be taken as 279.190 keV, so the ejected electrons will have that energy minus their binding energy in the 203Tl daughter atom.

Electron emissions from the Hg-203 to Tl-203 decay, measured by A. H. Wapstra, et al., Physica 20, 169 (1954).


Introduction

At higher resolution, the internal conversion electrons from the L, M and N shells can be resolves. Z. Sujkowski, Ark. Fys. 20, 243 (1961).

At even higher resolution, the three L shells can be resolved. From C. J. Herrlander and R. L. Graham, Nucl. Phys. 58, 544 (1964).


Introduction

26

The resolution of the electron detection is good enough that such internal conver-sion electron spectra can be used to study the binding energies of the electrons in heavy atoms. In this case, the measured electron energies can be subtracted from the transition energy as indicated by the gamma emission, 279.190 keV.

Binding energies for T-203l

K: 85.529 keV

LI: 15.347 keV

LII: 14.698 keV

LIII: 12.657 keV

M: 3.704 keV

In addition to information from the internal conversion electrons about the binding energies of the electrons in the daughter atom, the relative intensities of these inter-nal conversion electron peaks can give information about the electric multipole character of the nucleus.

Discovery of Radioactivity

Radioactivity was discovered by A. H. Becquerel in 1896. The radiation was classi-fied by E. Rutherford as alpha, beta, and gamma rays according to their ability to penetrate matter and ionize air.


Introduction

Nuclear Reactions

Many kinds of nuclear reactions occur in response to the absorption of particles such as neutrons or protons. Other types of reactions may involve the absorption of gamma rays or the scattering of gamma rays. Of particular note is the resonant absorption of gamma rays in the Mossbauer effect. Specific nuclear reactions can be written down in a manner similar to chemical reaction equations. If a target nucleus X is bombarded by a particle a and results in a nucleus Y with emitted par-ticle b, this is commonly written in one of two ways.

We can characterize the energetics of the reaction with a reaction energy Q, defined as the energy released in the reaction. The Q is positive if the total mass of the prod-ucts is less than that of the projectile and target, indicating that the total nuclear binding energy has increased. The probability of a given type of nuclear reaction taking place is often stated as a “cross section”.

Some Nuclear Reactions

* The nuclear reaction in the atmosphere which produces carbon-14 for radiocar-bon dating.


Introduction

28

Data from C. W. Li, W. Whaling, W. A. Fowler, and C. C. Lauritson, Physical Review 83:512 (1951)

Nuclear Cross Section

To characterize the probability that a certain nuclear reaction will take place, it is customary to define an effective size of the nucleus for that reaction, called a cross section. The cross section is defined by

The cross section has the units of area and is on the order of the square of the nuclear radius. A commonly used unit is the barn:

A standard old story was that in the early days of the field, a particular cross section turned out to be much bigger than expected. An experimenter exclaimed “Why, that's as big as a barn!” and a unit name was born.

Scattering Cross Section

The concept of cross section, as its name suggests, is that of effective area for colli-sion. The cross section of a spherical target is

In aiming a beam of particles at a target which is much smaller than the beam, as in the Rutherford scattering experiment, the cross section takes on a statistical nature.


Introduction

Rutherford worked out the scattering cross section for alpha particles of kinetic energy KE scattering off a single nucleus with atomic number Z. The cross section for scattering at a greater angle than some chosen angle is

More detail is needed for the prediction of the number of scattering events to expect above that angle. When the details about the beam and target are included, the frac-tional scattering rate is given by

The process for dealing with multiple targets is illustrated below


Introduction

30

Scattering Cross Section

The concept of cross section, as its name suggests, is that of effective area for colli-sion. The cross section of a spherical target is

The units of cross section are then area units, but for nuclear scattering the effective area is on the order of the cross sectional area of a nucleus. For a gold nucleus of mass number A=197, the radius determined from the nuclear radius relationship is about 7 fermis.

The most common unit for cross section for nuclear scattering interactions is the barn. The cross section in barns for alpha scattering above a selected angle is a stan-dard part of the analysis of Rutherford scattering. In the case of 6 MeV alpha parti-cles scattered from a gold foil, for example, you don't know the impact parameter for any given alpha particle, so the calculation of the scattered fraction takes on a statistical character. For alpha particles bombarding a gold foil of thickness 1 micrometer, less than 1 in 100,000 would meet the conditions for scattering at over 140°. The cross section for scattering at smaller angles will be larger, because the alpha doesn't have to come as close to be scattered through the smaller angle. For example, the cross section for scattering through 90° or above is 11.3 barns.


Introduction

For neutron scattering, there is no coulomb repulsion, so there is no appreciable scattering unless the impact parameter is less than the effective radius for the strong interaction. Therefore for a given target geometry the cross sections are smaller, typically measured in millibarns.

The Value of the Cross-Section Concept

If you are firing projectiles which you cannot see at targets which are much smaller than atoms, how can you make sense out of what you detect on the other side of the target? This kind of scenario highlights the value of the concept of cross-section.

If you had a uniform beam of projectiles and encountered a target which intercepted 1% of the area of the beam, you would expect 1% of the projectiles to be scattered. That basic concept can be extended to the calculation of the fraction of alpha parti-cles scattered from a gold foil.

Suppose you had a beam of incident alpha particles with rate Ri = 10,000 /s on a gold foil of thickness 10-6 meters and the effective area (cross-section) for each gold nucleus was 100 barns. The fraction of those 10,000 particles scattered would be the fraction of the beam area which is covered by one of the scattering gold nuclei. The fraction of the beam covered can be calculated as follows


Introduction

32

Multiplying the number of targets per square meter times the cross-section for each target gives you the fraction of the area which is covered by target and therefore the fraction of the beam which would be scattered:

For an incident rate of 10,000/s, an average of 5.88 particles/s would be expected to be scattered.

To put this example more directly in the context of Rutherford scattering, you can use the cross-section calculation to verify that for 6 MeV alpha particles incident upon this gold film, the cross-section would be 100 barns for scattering above about


Introduction

37 degrees. Another context would be the calculation of scattering within an angu-lar window. For example, suppose you start at 20 degrees, where the calculated cross-section is 363 barns for scattering above that angle. The scattering cross-sec-tion for scattering at 23.4 degrees is about 263 barns, so the cross-section for scat-tering between 20 and 23.4 degrees is about 100 barns. Using finite detectors and windows like this, the Rutherford team was able to compare the number of scat-tered alpha particles at different scattering angles and confirm that they followed the calculated pattern for coulomb scattering. Then at higher energies, when they detected a departure from the pattern, they could judge that they had interacted with a different kind of force (the strong force) and could then imply a radius for the nucleus.

Radiation Risk

Because the energies of the particles emitted during radioactive processes are extremely high, nearly all such particles fall in the class of ionizing radiation.

Ionizing Radiation

The practical threshold for radiation risk is that of ionization of tissue. Since the ionization energy of a hydrogen atom is 13.6 eV, the level around 10 eV is an approximate threshold. Since the energies associated with nuclear radiation are


Introduction

34

many orders of magnitude above this threshold, in the MeV range, then all nuclear radiation is ionizing radiation. Likewise, x-rays are ionizing radiation, as is the upper end of the ultraviolet range

All nuclear radiation must be considered to be ionizing radiation! In addition, the upper end of the electromagnetic spectrum is ionizing radiation.

Activity of Radioactive Source

The curie (Ci) is the old standard unit for measuring the activity of a given radioac-tive sample. It is equivalent to the activity of 1 gram of radium. It is formally defined by:


Introduction

Intensity of Radiation

The roentgen (R) is a measure of radiation intensity of x-rays or gamma rays. It is formally defined as the radiation intensity required to produce and ionization charge of 0.000258 coulombs per kilogram of air. It is one of the standard units for radiation dosimetry, but is not applicable to alpha, beta, or other particle emission and does not accurately predict the tissue effects of gamma rays of extremely high energies. The roentgen has mainly been used for calibration of x-ray machines.

Absorbed Dose of Radiation

The rad is a unit of absorbed radiation dose in terms of the energy actually depos-ited in the tissue. The rad is defined as an absorbed dose of 0.01 joules of energy per kilogram of tissue. The more recent SI unit is the gray, which is defined as 1joule of deposited energy per kilogram of tissue. To assess the risk of radiation, the absorbed dose is multiplied by the relative biological effectiveness of the radia-tion to get the biological dose equivalent in rems or sieverts.


Introduction

36

Biologically Effective Dose

The biologically effective dose in rems is the radiation dose in rads multiplied by a “quality factor” which is an assessment of the effectiveness of that particular type and energy of radiation. For alpha particles the relative biological effectiveness (rbe) may be as high as 20, so that one rad is equivalent to 20 rems. However, for x-rays and gamma rays, the rbe is taken as one so that the rad and rem are equivalent for those radiation sources. The sievert is equal to 100 rems.


Introduction

Radiation Detection

Nuclear radiation and x-rays are ionizing radiation and they can be detected from the ionizing events they produce.

Ionization Counters

Radiation detection can be accomplished by stretching a wire inside a gas-filled cylinder and raising the wire to a high positive voltage. The total charge produced by the passage of an ionizing particle through the active volume can be collected and measured. Different names are used for the devices based on the amount of voltage applied to the center electrode and the consequent nature of the ionizing events. If the voltage is high enough for the primary electron-ion pair to reach the electrodes but not high enough for secondary ionization, the device is called and ionization chamber. The collected charge is proportional to the number of ionizing events, and such devices are typically used as radiation dosimeters. At a higher voltage, the number of ionizations associated with a particle detection rises steeply because of secondary ionizations, and the device is often called a proportional counter. A single event can cause a voltage pulse proportional to the energy loss of the primary particle. At a still higher voltage, an avalanche pulse is produced by a single event in the devices called Geiger counters.

Scintillation Counters

Radiation detection can be accomplished by the use of a scintillator: a substance which emits light when struck by an ionizing particle. The scintillation detectors used in the Geiger-Marsden experiment were simple phosphor screens which emit-ted a flash of light when struck by an alpha particle. Modern scintillation counters may use single crystals of NaI doped with thallium. Electrons from the ionizing event are trapped into an excited state of the thallium activation center and emit a photon when they decay to the ground state. Photomultiplier tubes are used to intensify the signal from the scintillations. The decay times are on the order of 200 ns and the magnitude of the output pulse from the photomultiplier is proportional to the energy loss of the primary particle.

Organic scintillators such as a mixture of polystyrene and tetraphenyl butadine. They have the advantage of faster decay time (about 1 ns) and can be molded into experimentally useful configurations.


Introduction

38

Particle Track Devices

Radiation detection can take the form of devices which visualize the track of the ionizing particle. Cloud chambers can show the track of a passing particle which can be photographed. D. A. Glaser's invention of the bubble chamber in 1952 largely replaced the cloud chamber. Placed in an intense magnetic field, the curva-ture of the tracks of the primary particles and their products give information about their charge and momentum.

Spark chambers can also visualize the tracks of particles and has the advantage that the paths can be recorded electronically.

Interaction of Radiation with matter

You may click on any of the types of radiation for more detail about its particular type of interaction with matter. The different parts of the electromagnetic spectrum have very different effects upon interaction with matter. Starting with low fre-quency radio waves, the human body is quite transparent. (You can listen to your portable radio inside your home since the waves pass freely through the walls of your house and even through the person beside you!) As you move upward through microwaves and infrared to visible light, you absorb more and more strongly. In the lower ultraviolet range, all the uv from the sun is absorbed in a thin outer layer of your skin. As you move further up into the x-ray region of the spectrum, you become transparent again, because most of the mechanisms for absorption are gone. You then absorb only a small fraction of the radiation, but that absorption involves the more violent ionization events. Each portion of the electromagnetic spectrum has quantum energies appropriate for the excitation of certain types of physical processes. The energy levels for all physical processes at the atomic and molecular levels are quantized, and if there are no available quantized energy levels with spacings which match the quantum energy of the incident radiation, then the material will be transparent to that radiation, and it will pass through


Introduction

Microwave Interactions

The quantum energy of microwave photons is in the range 0.00001 to 0.001 eV which is in the range of energies separating the quantum states of molecular rota-tion and torsion. The interaction of microwaves with matter other than metallic conductors will be to rotate molecules and produce heat as result of that molecular motion. Conductors will strongly absorb microwaves and any lower frequencies because they will cause electric currents which will heat the material. Most matter, including the human body, is largely transparent to microwaves. High intensity microwaves, as in a microwave oven where they pass back and forth through the food millions of times, will heat the material by producing molecular rotations and torsions. Since the quantum energies are a million times lower than those of x-rays, they cannot produce ionization and the characteristic types of radiation damage associated with ionizing radiation.


Introduction

40

Infrared Interactions

The quantum energy of infrared photons is in the range 0.001 to 1.7 eV which is in the range of energies separating the quantum states of molecular vibrations. Infra-red is absorbed more strongly than microwaves, but less strongly than visible light. The result of infrared absorption is heating of the tissue since it increases molecular vibrational activity. Infrared radiation does penetrate the skin further than visible light and can thus be used for photographic imaging of subcutaneous blood vessels.

Visible Light Interactions

The primary mechanism for the absorption of visible light photons is the elevation of electrons to higher energy levels. There are many available states, so visible light is absorbed strongly. With a strong light source, red light can be transmitted through the hand or a fold of skin, showing that the red end of the spectrum is not absorbed as strongly as the violet end.

While exposure to visible light causes heating, it does not cause ionization with its risks. You may be heated by the sun through a car windshield, but you will not be sunburned - that is an effect of the higher frequency uv part of sunlight which is blocked by the glass of the windshield.


Introduction

Ultraviolet Interactions

The near ultraviolet is absorbed very strongly in the surface layer of the skin by electron transitions. As you go to higher energies, the ionization energies for many molecules are reached and the more dangerous photoionization processes take place. Sunburn is primarily an effect of uv, and ionization produces the risk of skin cancer.

The ozone layer in the upper atmosphere is important for human health because it absorbs most of the harmful ultraviolet radiation from the sun before it reaches the surface. The higher frequencies in the ultraviolet are ionizing radiation and can pro-duce harmful physiological effects ranging from sunburn to skin cancer.


Introduction

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X-rays

Since the quantum energies of x-ray photons are much too high to be absorbed in electron transitions between states for most atoms, they can interact with an elec-tron only by knocking it completely out of the atom. That is, all x-rays are classified as ionizing radiation. This can occur by giving all of the energy to an electron (pho-toionization) or by giving part of the energy to the photon and the remainder to a lower energy photon (Compton scattering). At sufficiently high energies, the x-ray photon can create an electron positron pair.


Download - CHAPTER 1 Nuclear Radiation - Review · CHAPTER 1 Nuclear Radiation - Review Some Nuclear Units Nuclear energies are very high compared to atomic processes, and need larger units

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