Outline. Radiation Interactions. Spurs, Blobs and Short Tracks. Introduction. Radiation Interactions 1

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1 Outline Radiation Interactions Introduction Interaction of Heavy Charged Particles Interaction of Fast Electrons Interaction of Gamma Rays Interactions of Neutrons Radiation Exposure & Dose Sources of Radiation Exposure 2 Spurs, Blobs and Short Tracks Energy deposition occurs in discrete events at the nanometer level. Introduction These events are categorized as spurs, blobs, short tracks and branch tracks. The amount of energy deposited determines whether an event will cause a spur or a larger event. 4 Radiation Interactions 1

2 Spurs, Blobs and Short Tracks Remember: Secondary electrons that are emitted with high energy after a scattering event are called δ rays. They will create their own track. Spurs, Blobs and Short Tracks Blobs Spurs ev Primary <100 ev < 5000 ev >5000 ev Short Tracks -rays Branch Tracks (from Mozunder and Magee 1966) A 1 MeV electron will deposit about 65 % of its energy in isolated spurs, 15 % in blobs and 20 % in short tracks. 5 6 Entity Energy Deposition Events for Low LET Radiation Energy Deposited (ev) Size (nm) Number of Water Molecules Per Event Energy (%) Events (%) Spur <100 2 (diam.) 1100 ~80 95 Blob <500 7 (diam.) 6000 ~20 5 Short Track DNA 2 (diam.) Nucleosome disc 5.7 Thickn. 5.5 Radius Diameter of the area in which energy is deposited is similar to the diameter of DNA and nucleosomes. Spurs, Blobs and Short Tracks In a typical cell volume, a dose of 1 Gy will produce ~75,000 spurs and ~4,000 blobs. The energy deposited in these events is sufficient enough to cause several ionizations. Example: An energy of ~32 ev is needed to produce an ion pair in water. This means a 100 ev spur could contain 3 ion pairs. 7 8 Radiation Interactions 2

3 Charged vs. Uncharged Radiation The radiation produced by the radiation sources previously discussed can be grouped into: Charged Particulate Radiation (Directly Ionizing) Heavy charged Particles (Range ~ 10 5 m) Fast Electrons (Range ~ 10 3 m) Uncharged Radiation (Indirectly Ionizing) Neutrons (Range ~ 10 1 m) X Rays and Gamma Rays (Range ~ 10 1 m) Interactions Charged particulate radiations interact continuously with the electrons in any medium through which they pass. This interaction is due to the Coulomb force. The interactions result in excitations and ionizations. Uncharged radiations are not affected by coulomb forces. Instead they undergo a direct, catastrophic interaction. It radically alters the properties of the radiation Interactions This interaction results in the complete or partial transfer of energy to a secondary particle. This secondary particle is charged and causes ionization. Interaction of a photon produces an electron. Interaction of a neutron produces a heavy charged particle. Uncharged radiation will remain undetected if this catastrophic interaction does not occur within the detector volume. Interaction of Heavy Charged Particles 11 Radiation Interactions 3

4 Nature of the Interaction Nature of the Interaction Heavy charged particle interact primarily through Coulomb forces. These forces are due to the positive charge of the particle and the negative charge of the orbital electrons. Direct interaction with a nucleus is possible but occurs only very rarely. Such direct encounters are not significant for the response of a radiation detector. The charged particle interacts with many electrons simultaneously when it enters the medium. In each interaction energy is transferred from the particle to an electron Nature of the Interaction Stopping Power As the result of the interaction ion pairs are produced. Each pair consists of an electron and a positively charged atom. Typically these ion pairs recombine very quickly to form neutral atoms. They can however be separated and detected by introducing an electrical field. This is the basis for several types of radiation detectors. The linear stopping power S is defined as the differential energy loss per unit path length. de S dx (in MeV cm 1 ) Radiation Interactions 4

5 Stopping Power The classical expression for the stopping power is given by the Bethe formula: S 4 de 4 π e z 2 dx m v where 2 2 m0 v NZ ln I v ln 1 c v, z = velocity, charge of the primary particle NZ = Number of absorber electrons per cm 3 I = Mean ionization and absorbing potential v c Stopping Power Several important conclusions can be drawn from this equation: The stopping power varies with 1/v 2. The stopping power increases as the velocity decreases. The stopping power varies with z 2. Particles with the greatest charge will experience the highest energy loss Stopping Power Stopping Power For different materials the energy loss depends primarily on the product NZ. High atomic number, high density materials will have the greatest stopping power. 19 Variation of the specific energy loss in air versus energy of the charged particle shown. Source: Knoll, G. F., Radiation Detection and Measurement, 4 th Edition, John Wiley (2010) 20 Radiation Interactions 5

6 Particle Range The range of heavy charged particles in matter is well defined. For small absorber thicknesses the only effect is an energy loss. Because the particle tracks through the absorber are quite straight, the total number of particles that reach the detector remains the same. No attenuation takes place until the thickness of the absorber approaches the length of the shortest track. Particle Range Increasing the thickness further stops more and more particles until the number drops rapidly to zero Particle Range Particle Range The mean range is defined as the absorber thickness that reduces the particle count to exactly half of its original value. This definition is most commonly used in numerical range tables. The extrapolated range is obtained by extrapolating the linear portion at the end to zero. Historically this type of experiment was used to determine the energy of alpha particles indirectly. An alpha particle transmission experiment. I is the detected number of alpha particles through an absorber thickness t, whereas I 0 is the number detected without the absorber. The mean range R m and the extrapolated range R e are indicated. 23 Source: Knoll, G. F., Radiation Detection and Measurement, 4 th Edition, John Wiley (2010) 24 Radiation Interactions 6

7 Interaction of Fast Electrons Interaction of Fast Electrons Electrons lose their energy at a lower rate compared to heavy charged particles. The mass of the electron is the same as that of the orbital electrons with which it is interacting. A much larger fraction of its energy can be transferred in a single encounter. As a result the electron can experience large deviations from its path. Electrons do not take a straight path through the absorbing medium. 26 Specific Energy Loss Electrons can lose energy through collisions as well as through radiative processes, in particular Bremsstrahlung. The total linear stopping power for electrons is therefore composed of two components: de dx de dx c de dx r Absorption of Monoenergetic Electrons The energy losses through collision can be described by a modified Bethe formula. 27 Transmission curve for monoenergetic electrons. R e is the extrapolated range. Source: Knoll, G. F., Radiation Detection and Measurement, 4 th Edition, John Wiley (2010) 28 Radiation Interactions 7

8 Absorption of Monoenergetic Electrons Even small absorber thicknesses lead to electron loss due to scattering. Therefore a plot of the number of detected electrons versus absorber thickness begins to drop immediately. It gradually approaches zero for large absorber thicknesses. The electrons that penetrate the greatest absorber thickness will be the ones whose initial direction has been changed the least. Range of Electrons in Matter The concept of range is less definite for electrons. This is because their total path length is much greater than their distance of penetration. The electron range can be obtained from a transmission plot by extrapolation of the linear portion of the curve to zero. The specific energy loss of electrons is much lower than that of heavy charged particles, so their path length is hundreds of times greater Absorption of Beta Particles Absorption of Beta Particles The transmission curve for beta particles differs from that for monoenergetic electrons because of their continuous energy distribution. The initial slope is much steeper due to the rapid absorption of low energy beta particles. For most beta spectra the curve has a near exponential shape. This is only an empirical approximation and does not have a fundamental base. 31 Transmission curves for beta particles from 185 W (endpoint energy of 0.43 MeV). Source: Knoll, G. F., Radiation Detection and Measurement, 4 th Edition, John Wiley (2010) 32 Radiation Interactions 8

9 Backscattering The fact that electrons often undergo large angle deflection can lead to backscattering. This means an electron can be deflected enough to scatter out of a medium which it entered. Backscattering is most pronounced for electrons with low incident energy and absorbers with high Z. Backscattering can influence the efficiency of beta emitter measurements because the electrons are scattered by the source backing. Positron Interactions The major mechanism of energy loss for positrons is based on coulomb interactions. The impulse and energy transfer for particles of similar mass is the same, no matter whether the interaction involves a repulsive or attractive force. Therefore the tracks of positrons in an absorber are similar to those of electrons. The main difference however is the annihilation radiation emitted by positrons Interaction of Gamma Rays Interaction of Gamma Rays When a photon interacts with matter, its energy is either partially or completely transferred into electron energy. This means the photons disappears entirely or is scattered through a significant angle. This behavior is very different from charged particles, which slow down gradually through many interactions with absorber atoms. 36 Radiation Interactions 9

10 Interaction of Gamma Rays A large number of possible interaction mechanisms are known for gamma rays in matter. Only three major types play a role in radiation measurements: Photoelectric absorption Compton scattering Pair production Photoelectric Absorption In the photoelectric absorption process a photon transfers all of its energy to a bound electron. The photon disappears entirely and the electron is ejected as a photoelectron. This interaction is not possible with a free electron due to momentum conservation. The photoelectron appears with an energy: E hν E e b Photoelectric Absorption Photoelectric Scattering Process As a result of the photoelectron emission a vacancy in one of the bound shells is created. This vacancy is quickly filled by an electron from a higher shell. As a result one or more characteristic X rays may emitted. These X rays are generally reabsorbed close to the original site, but their migration and escape can influence the response of detectors. In some cases an Auger electron is emitted instead of the X ray. E=hν E=ΔB.E. 2. E=hν B.E Radiation Interactions 10

11 Photoelectric Absorption This process is the predominant interaction for gamma ray energies of less than a few hundred kev. It has a higher probability in high Z materials. The probability for photoelectric absorption scales as: Z τ cons E γ Compton Scattering Compton scattering is the predominant interaction mechanism for gamma rays energies typical of radioisotope sources. Scattering takes place between the incident gamma ray and a loosely bound or free electron in the absorbing material. The incoming photon transfers a portion of its energy to the electron. The photon itself is deflected at an angle Θ and the electron is emitted as a recoil electron Compton Scattering Process Compton Scattering E=hν E=hν hν B.E. The energy transferred depends on the scattering angle of the photon. The detailed derivation of this relationship can be found in the textbook. The scattering probability depends on the number of electrons available as scattering targets. It increases linearly with Z. Compton scattering is the most important energy loss mechanism for energies from a few hundred kev up to a few MeV. E=hν Radiation Interactions 11

12 Pair Production Pair production is possible if the energy of the gamma ray exceeds twice the rest mass of an electron (1.02 MeV). The gamma ray disappears and is replaced by an electron positron pair. The interaction must take place in the coulomb field of a nucleus. All photon energy in excess of 1.02 MeV is converted into kinetic energy shared between the electron and the positron. Pair Production The positron will subsequently slow down in the medium and annihilate with another electron, releasing two 511 kev photons in the process. The pair production probability remains very low until the gamma ray energy approaches several MeV. The magnitude of the probability varies approximately with Z 2 of the absorber, but no simple expression exits for this relation Interaction Type vs. Z and Energy Interaction of Neutrons The relative importance of the three major types of gamma ray interaction. The lines show the values of Z and hν for which the two neighboring effects are just equal. Source: Knoll, G. F., Radiation Detection and Measurement, 4 th Edition, John Wiley (2010) 47 Radiation Interactions 12

13 General Properties Neutrons carry no charge and therefore can not interact by means of the coulomb force. If the neutron interacts at all, it interacts with the nucleus of the absorbing medium. As a result the neutron either disappears and is replaced by a secondary radiation, or its energy and direction is significantly changed. The secondary radiation is almost always a heavy charged particle. The probability of the various types of interaction depends on the neutron energy. Slow Neutron Interactions The significant interactions for slow neutrons are elastic scattering and neutron induced nuclear reactions. The kinetic energy of slow neutrons is very small, therefore only little energy is transferred in the scattering. Consequently, this interaction can not be used to detect slow neutrons. The most important interactions are neutron induced reaction. These allow for the detection of the secondary radiation emitted Fast Neutron Interactions The probability of the most useful neutron induced reactions drops off with increasing neutron energy. Scattering however becomes more important, because more energy can be transferred, resulting in recoil nuclei being emitted. At sufficiently high energies inelastic scattering can take place. In this case the recoil nucleus becomes excited and deexcites by emission of a gamma ray. Neutron Cross Sections For fixed energy neutrons the probability per unit path length is constant for each interaction type. This probability is generally expressed in terms of the cross section σ per nucleus. The cross section has the units of an area and is traditionally measured in barn (10 24 cm 2 ). Multiplying the cross section by the number of nuclei N per unit volume gives the macroscopic cross section Σ. N σ Σ Radiation Interactions 13

14 Neutron Cross Sections Σ can be interpreted as the interaction probability per unit path length (length 1 ). Each type of interaction has a specific cross section. The total cross section can be obtained by adding up the individual cross sections. Σ tot Σ scatter Σ rad.capture... This quantity has the same significance for neutrons as the linear attenuation coefficient for gamma rays. Neutron Cross Sections The results of a narrow beam attenuation will follow: where I I 0 e Σtot t t = Distance the neutron travels The neutron reaction rate density reactions per volume per time is defined as ΣΦ. Φ is called the neutron flux and has the unit length 2 time Gamma Ray Exposure Radiation Exposure & Dose The concept of gamma ray exposure was introduced early in the history of radioisotope research. It is defined only for sources of X and gamma rays. The exposure is defined as the charge dq due to ionization from secondary electrons in a volume element of air with mass dm. The exposure value X is given by dq/dm. The SI unit of gamma ray exposure is C kg Radiation Interactions 14

15 Gamma Ray Exposure The historical unit has been the roentgen (R). It is defined as the amount of exposure that produces 1 electrostatic unit of charge per 1 cm 3. 1 R = 2.58 x 10 4 C/kg. Gamma Ray Exposure The gamma ray exposure at a known distance d from a spherical sources with an activity α can be calculated as: α X Γ δ d 2 Γ δ is given in units of (R cm 2 )/(hr mci) Absorbed Dose Two different materials will in general absorb different amounts if subjected to the same gamma ray exposure. Many important phenomena scale with the amount of energy absorbed per unit mass. Therefore a unit is needed that measures this quantity. The energy absorbed from any type of radiation per unit mass of absorber is defined as the absorbed dose. Absorbed Dose The SI unit of absorbed dose is the Gray (Gy). It is defined as 1 J kg 1. The historical unit of absorbed dose has been the rad. It is defined as 100 ergs / g. 100 rad equal 1 Gy. An absorbed dose in air of 33.8 Gy corresponds to a gamma ray exposure of 1 C kg Radiation Interactions 15

16 Dose Equivalent The absorption of equal amounts of energy per mass does not guarantee the same biological effect for different types of radiation. The extend of biological damage can vary by as much as an order of magnitude depending on whether the energy is deposited by heavy charged particles or by electrons. The severity and permanence of biological effects is directly related to the local rate of energy deposition along the particle track. Dose Equivalent This quantity is know as the linear energy transfer, L. Radiations with a large L value tend to result in greater biological damage than those with lower values for L. To quantify the biological effects better, the concept of dose equivalent has been introduced. A unit of dose equivalent is defined as the amount of any type of radiation that results in the same biological effect as one unit of dose delivered in the form of low LET radiation Dose Equivalent The dose equivalent H is the product of the absorbed dose D and the quality factor Q. Dose Equivalent The quality factor increases with linear energy transfer L. H D Q Source: Knoll, G. F., Radiation Detection and Measurement, 4 th Edition, John Wiley (2010) Radiation Interactions 16

17 Dose Equivalent All fast electron radiations of interest have a quality factor of 1 because L is sufficiently low. The same is true for X and gamma rays. The quality factor is much higher for charged particles and neutrons. Units of Dose Equivalent The unit of H depends on the corresponding unit of the absorbed dose D. If D is expressed in Gy, then H is defined in units of Sievert (Sv). If D is expressed in rad, then H is defined in units of rem. 1 Sv = 100 rem Categories of Exposure Sources Sources of Radiation Exposure The sources of radiation exposure to the general population can be divided in five broad categories: 1. Exposure from ubiquitous background radiation, including radon in homes. 2. Exposure to patients from medical procedures. 3. Exposure from consumer products or activities involving radiation sources. 4. Exposure from industrial, security, medical, educational and research radiation sources. 5. Exposure of workers that results from their occupations. 68 Radiation Interactions 17

18 Presentation of Results The exposure is presented as annual values for: 1. Collective effective dose (S) (person sievert) This is the cumulative dose to a population of individuals exposed to a given radiation source or group of sources. 2. Effective dose per individual in the U.S. population (E US ) (millisievert) Obtained by dividing S by the total number of individuals in the U.S. population whether exposed to the specific source or not. Presentation of Results 3. Average effective dose to an individual in a group exposed to a specific source (E Exp ) (millisievert). This excludes individuals that are not subject to exposure from the specific source of radiation Sources of Radiation Exposure (1982) Sources of Radiation Exposure (1982) Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 93 (1987) 71 Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 93 (1987) 72 Radiation Interactions 18

19 Effective Dose per Individual in the U.S. Population (1982) Effective Dose per Individual in the U.S. Population (1982) Source S (person Sv) E US (msv) E Exp (msv) Ubiquitous background 690, Radon & Thoron 460, Space 65, Internal 90, Terrestrial 65, Medical 123, Source S (person Sv) E US (msv) E Exp (msv) Consumer 29, Miscellaneous Nuclear Fuel Cycle Occupational 2, Total 835, Diagnostic X rays 91, Nuclear Medicine 32, Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 93 (1987) 73 Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 93 (1987) 74 Sources of Radiation Exposure (2006) Sources of Radiation Exposure (2006) Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 160 (2009) 75 Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 160 (2009) 76 Radiation Interactions 19

20 Sources of Radiation Exposure (2006) Effective Dose per Individual in the U.S. Population (2006) Source S (person Sv) E US (msv) E Exp (msv) Ubiquitous background 933, Radon & Thoron 684, Space 99, Internal 87, Terrestrial 63, Medical 899, CT 440, Nuclear Medicine 231, Interventional Fluoroscopy 128, Radiography & Fluoroscopy 100, Consumer 39, Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 160 (2009) 77 Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 160 (2009) 78 Effective Dose per Individual in the U.S. Population (2006) Source S (person Sv) E US (msv) E Exp (msv) Industrial 1, Occupational 1, Medical Aviation Commercial Nuclear Power Industry & Commerce Education & Research Government, DOE,Military Total 1,870, Source: National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States, NCRP Report 160 (2009) 79 Radiation Interactions 20