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  • Glossary

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    1. Simulation of radiation processing and related terms

    Benchmarking - comparing model predictions to independent measurements or calculations under similar conditions using established criteria of uncertainty.

    Build - up factor - the ratio of the total dose, particle fluence, exposure or other quantity due to primary and secondary (scattered) radiation, at a target (or field point) location to the dose due to primary radiation at that location. The concept of build - up applies to the transport of photons.

    Deterministic method - a method using mathematical equations (transport equations) to directly calculate the radiation field over all space as a function of radiation source and boundary conditions.

    Discussion - The point kernel and discrete ordinate methods are examples of deterministic methods.

    Discrete ordinates - a deterministic method for approximate numerical solution of the transport equation in which the direction of motion is divided into a finite number of discrete ordinate angles.

    Discussion - In the discrete ordinates approximation, the transport equation becomes a set of coupled equations, one for each discrete ordinate. Particle behaviors along paths intermediate to described paths are approximated by a weighted average (numerical quadrature) of adjacent paths. The method is useful for both electron and photon beam sources when appropriate assumptions can be made.

    Empirical model - a method derived from fitting an approximating function to experimental data or Monte Carlo calculation result.

    Discussion - Empirical models are generally developed by fitting equations (for example, polynomial) to experimental data or simulation output derived from another mathematical method.

    Histories - a particle history is the record of all simulated interactions along its track as used in stochastic or Monte Carlo simulations.

    Discussion - A history begins with the starting position, energy and direction of a particle, follows all its interactions, and terminates in one of several outcomes such as absorption, escape from the boundary of the problem, or reaching a cut - off limit (such as a cut - off energy). A particle history is the systematic generation of a random, simulated particle track that is obtained according to the known physical interactions of either electrons or photons with the material being traversed.

    Mathematical method - a method of solution of an electron and/or photon transport problem using algebraic relations and mathematical operations to represent the system and its dynamics.

    Mathematical model - a mathematical description of a physical problem based on physical laws and/or empirical correlation.

    Monte Carlo method - a simulation method used for calculating absorbed dose, energy spectra, charge, fluence and fluence rate in a volume of interest using a statistical summary of the radiation interactions. A Monte Carlo calculation consists of running a large number of particle histories (simulations) until some acceptable statistical uncertainty in the desired calculated quantity (such as dose) has been reached.

    Discussion - This calculation method is suitable for problems involving either electrons or photons or both.
    This technique produces a probabilistic approximation to the solution of a problem by using statistical sampling techniques.

    Point kernel method - a deterministic method for calculating dose based on integrating the contributions from point sources.

    Discussion - The point kernel method is typically used for photon transport applications. The radiation source is modeled as a large set of point sources. The absorbed dose, dose equivalent or exposure is estimated at a dose point by integrating the contribution from each of the point sources. A multiplicative value (the semi - empirical build - up factor) is used to account for the contribution from scattered (indirect) radiation from regions not in the direct path between the source point and field point.

    Radiation field - a function describing the particle density and the distributions of energy, direction and particle type at any point.

    Radiation transport theory - an analytical description of the propagation of a radiation field according to the physical laws governing the interactions of the radiation.

    Discussion - In its most general form, transport theory is a special branch of statistical mechanics, which deals with the interaction of the radiation field with matter.

    Semi - empirical model - an empirical model in which the fitting parameters are constrained so that the model satisfies one or more physical laws or rules.

    Discussion - The satisfaction of such physical rules may enable the model to be applicable over a wide range of energies and materials.

    Stochastic methods - methods using mathematical equations containing random variables to describe or summarize the physical processes in the system being studied. A random variable is a variable whose value is a function of a statistical distribution of random values. The Monte Carlo method is the only stochastic method discussed in this guide.

    Transport equation - an integrodifferential equation describing the motion of particles or radiation through a medium. This equation contains various terms corresponding to sources of particles, particle streaming and particle scattering in and out of an infinitesimal volume of phase space.

    Uncertainty - a parameter associated with the result of a measurement, that characterizes the spread of values that could reasonably be attributed to the measurand or derived quantity.

    Validation - accumulation of documented experimental evidence, used to demonstrate that the mathematical method is a reliable prediction technique.

    Discussion - Validation compares a code or theory with results of an appropriate experiment.

    Verification - confirmation by examination of evidence that the mathematical method has been properly and successfully applied to the problem.

    Discussion - It is important to know the type of radiation sources, geometries, energies, etc. for which a code has been validated. The calculated results will also depend on quantities at the user´s disposal such as cutoff energy (for Monte Carlo) or mesh size (for discrete ordinate methods).
    Verification demonstrates that theory was implemented in the way intended, and that the simulation was performed in accordance with its requirements and specifications.

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    2. Electron beam processing and related terms

    Average beam current - time - averaged current produced by an electron beam generator.

    Beam current - a measure of the number of electrons exiting an accelerator each second.
    Beam power - the power or energy flux of a radiation beam. Usually given in watts (?J/s).

    Broad beam - a stream of incident radiation where the lateral dimensions are relatively large with respect to the size of the absorber or target (as contrasted to a narrow - beam).

    Beam width - dimension of the irradiation zone perpendicular to the beam length and direction of the electron beam specified at a specific distance from where the beam exits the accelerator.

    Discussion - For a radiation processing facility with a conveyor system, the beam width is usually perpendicular to the flow of motion of the conveyor. Beam width is the distance between the points along the dose profile which are at a defined level from the maximum dose region in the profile. Various techniques may be employed to produce an electron beam width adequate to cover the processing zone, for example, use of electromagnetic scanning of pencil beam (in which case beam width is also referred to as scan width), defocussing elements, and scattering foils.

    Electron accelerator - a device for imparting large amounts of kinetic energy to electrons.

    Electron beam - continuous or pulsed stream of high energy electrons.

    Electron energy - kinetic energy of the electrons in the electron beam.

    Electron energy spectrum - particle fluence distribution of electrons as a function of energy.

    Electron equilibrium - A condition that exists in an incremental volume within a material under irradiation if the kinetic energies and the number of electrons entering that volume are equal to those leaving the volume.

    Electron range - penetration distance along the beam axis of electrons within homogeneous material.

    Half - entrance depth(R50e) - depth in homogeneous material at which the absorbed dose has decreased 50 % of the absorbed dose at the surface of the material.

    Half - value depth (R50) - depth in homogeneous material at which the absorbed dose has decreased 50 % of its maximum value.

    Electronvolt (eV) - a unit of energy. One electronvolt is the kinetic energy acquired by an electron in passing through a potential difference of one volt in vacuum. It is approximately equivalent to 1 eV = 1.60219 x 10 - 19 J.

    Entrance dose - absorbed dose in a product at the entrance surface (or extrapolated to this surface), i.e. where the radiation beam enters the product.

    Irradiation facility - an engineering plant, housing the radiation source and all the ancillary equipment required for carrying out a radiation process.

    Irradiation geometry - the spatial description of the relative positions of the process load and the radiation source during the radiation treatment (comprises source - to - product distance, size, spacing, shape and position of scattering or shielding materials, etc.).

    Irradiation unit - a volume of product with a specified loading configuration processed as a single entity; this term is not relevant to bulk - flow processing.

    Narrow beam - In beam attenuation measurements, a radiation beam in which only the unscattered and small angle forward scattered radiation reach the detector.

    Optimum thickness (Ropt) - depth in homogeneous material at which the absorbed dose equals the absorbed dose at the surface where the electron beam enters.

    Practical range (Rp) - distance from the surface of homogeneous material where the electron beam enters to the point where the tangent at the steepest point (the inflection point) on the almost straight descending portion of the depth dose distribution curve meets the depth axis.

    Pulse beam current - for a pulsed accelerator, the beam current averaged over the top ripples (aberrations) of the pulse current waveform; this is equal to Iavg/wf, where Iavg is the average beam current, w is the pulse width, and f is the pulse rate.

    Pulse rate - for a pulsed accelerator, the pulse current repetition frequency in hertz, or pulses per second; this is also referred to as the repetition (rep) rate.

    Pulse width - for a pulsed accelerator, the time interval between the half peak beam current amplitude points on the leading and falling edges of the pulse beam current waveform

    Radiation source - an apparatus or radioactive substance in a suitable support that constitutes the origin of the ionizing radiation (e.g. an electron accelerator or 60Co source rods in a frame).

    Reference material - homogeneous material of known radiation absorption and scattering properties used to establish characteristics of the irradiation process, such as scan uniformity, depth - dose distribution, throughput rate, and reproducibility.

    Simulated product - a mass of material with attenuation and scattering properties similar to those of a particular material or combination of materials; this material is sometimes referred to as dummy product or phantom.

    Scanned beam - an electron beam which is swept back and forth with a varying magnetic field.

    Discussion - this is most commonly done along one dimension (beam width), although two dimensional scanning (beam width and length) may be used with high - current electron beams to avoid overheating the beam exit window of the accelerator.

    Scattering - the change in direction of a particle or photon due to a collision or interaction with another particle, atom or system.

    Secondary radiation - radiation resulting from the interaction of primary radiation with any medium.

    Stopping power - this could refer to either the linear stopping power, S, or the mass stopping power, S/p. Its value depends on the material and the energy of the charged particle. The mass stopping power of a material, for charged particles (such as electrons), is the quotient of dE by ρ dl, where dE is the energy lost by a charged particle in traversing a distance dl in the material of density ρ, thus, S/p = dE/p dl in SI units of J*m2/kg.
    The linear stopping power is then defined as S = dE/dl in SI units if J/m.

    Throughput - in radiation processing, this is the amount of product emerging from the irradiator per unit time (kg/h) multiplied by the absorbed dose (J/kg). This should be equal to the power of the radiation source multiplied by the energy utilization efficiency.

    Utilization efficiency - the fraction of radiation energy emitted by the radiation source that is absorbed by the total product during the irradiation cycle.

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    3. X - ray processing and related terms

    Bremsstrahlung - broad spectrum electromagnetic radiation emitted when an energetic electron (or a charged particle) is influenced by a strong magnetic or electric field, such as that in the vicinity of an atomic nucleus (the German word literally means 'braking radiation'). In practice, it is produced when an electron beam strikes any material (converter). The bremsstrahlung spectrum depends on the electron energy, and the converter material and its thickness, and has its maximum energy corresponding to the maximum energy of the incident electrons.

    Discussion - When a beta particle (electron) passes close to a nucleus, the strong attractive coulomb force causes the beta particle to deviate sharply from its original path. The change in direction is due to radial acceleration, and in accordance with classical theory the beta particle loses energy by electromagnetic radiation at a rate proportional to the square of the acceleration. This means that the bremsstrahlung photons have a continuous energy distribution that ranges downward from a theoretical maximum equal to the kinetic energy of the beta particle. Practically, bremsstrahlung is produced when an electron beam strikes any material (converter). The bremsstrahlung spectrum depends on the electron energy, converter material, and its thickness.

    Conversion efficiency - also referred to as conversion ratio. In the production of bremsstrahlung (X rays) by the slowing down of electrons or other charged particles, it is the ratio of the energy flux of the resulting photons emitted in the forward direction to the energy flux of the incident electrons.

    X - ray converter - a device for generating X rays (bremsstrahlung) from an electron beam, consisting of a target, means for cooling the target, and a supporting structure.

    X - ray target - that component of the X - ray converter that is struck by the electron beam. It is usually made of metal with a high atomic number, high melting temperature, and high thermal conductivity.

    X rays - the common name for the short wavelength electromagnetic radiation emitted by high - energy electrons when they are accelerated, decelerated or deflected by strong electric and magnetic fields. The term includes both bremsstrahlung from nuclear collisions and the characteristic monoenergetic radiation emitted when atomic electrons make transitions to more tightly bound states.

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    4. Gamma ray processing and related terms

    Becquerel, (Bq) - the name for the SI unit of activity, equal to one transformation per second. It supersedes the traditional special unit curie (Ci), 1 Bq = 2.7 x 10 - 11 Ci (approximately, for exact value see curie)

    Batch - defined quantity of bulk, intermediate, or finished product that is intended or purported to be uniform in character and quality, and which has been produced during a defined cycle of manufacture.
    Batch - type irradiator - irradiator in which the irradiation containers are introduced or removed while the radioactive source is in the storage room

    Buildup - in the passage of radiation through a medium, the increase with depth of energy deposition due to the forward moving secondary radiation. It leads to a maximum in the depth - dose curve. For example, for 60Co y radiation, this maximum occurs at a depth of about 0.5 cm in water.

    Container - Carrier, tote, cart, tray or other container in which product is loaded to traverse the irradiation field. In some instances, this may be the actual product package.

    Continuous - type irradiator - irradiator that can be loaded and unloaded with product while the source is in the processing mode.

    Gamma ray - short wavelength electromagnetic radiation (photons) emitted from radioactive substances in the process of nuclear transition.

    Half - life - the time required for a radioactive isotope to decay to half its original curie content.

    Photon - a particle that has no mass or charge and is the basic unit of electromagnetic energy.

    Source activity - quantity of the radionuclide 60Co or 137Cs measured in becquerels of curies (1 curie =3.7 x 1010 becquerels, where 1 becquerel = 1 disintegration per second).

    Activity, A - the amount of radionuclide in a particular energy state at a given time defined as A = dN/dt, where dN is the expectation value of the number of spontaneous nuclear transformations from the given energy state in the time interval dt. The SI derived unit of activity is the reciprocal second (s - 1), termed the becquerel (Bq). The traditional special unit was the curie (Ci), use of which is being phased out.

    Curie (Ci) - the special unit of activity, which is superseded by the becquerel (Bq).
    The curie is defined as: 1 Ci =3.7 x 1010 Bq (exactly).

    Dwell time - the time interval during which a process load is at rest at an irradiation position in a shuffle - dwell irradiator.

    Half - life (radioactive), T 1/2 - for a radionuclide, the time required for the (radio)activity to decrease, by a radioactive decay process, by half. In gamma radiation processing, it is used to determine the reduction of the source strength (activity) of a y ray source with time.

    Irradiation Cycle - The entire sequence of events experienced by a process load from the instant it enters the irradiation chamber (beginning of radiation treatment) to the instant it leaves the irradiation chamber (completion of radiation treatment).

    Irradiation geometry - the spatial description of the relative positions of the process load and the radiation source during the radiation treatment (comprises source - to - product distance, size, spacing, shape and position of scattering or shielding materials, etc.).

    Irradiator - assembly that permits safe and reliable radiation processing, including the source of the radiation, conveyor and source mechanisms, safety devices, and shield.

    Isotopes - nuclides having the same atomic number, Z (i.e. the same chemical element) but having different mass number, A.

    Plaque source - an arrangement of radionuclide sources in the planar configuration.

    Product overlap - with radionuclide irradiators, the extension of the product beyond 1 the cross - sectional dimensions of the source, as the product passes the plaque source. This technique is usually employed to improve the energy absorption efficiency in multipass, single position or multiposition, one or two direction irradiation facilities.

    Production run - series of process loads consisting of materials, or products having similar radiation absorption characteristics, that are irradiated sequentially to a specified range of absorbed dose. This refers to both continuous flow and shuffle - dwell processes. Also see process load.

    Radiation source - an apparatus or radioactive substance in a suitable support that constitutes the origin of the ionizing radiation (e.g. an electron accelerator or 60Co source rods in a frame).

    Shuffle - dwell irradiator - an irradiator in which a process load moves I discontinuously past the irradiation source, alternately being moved (indexed) to a new irradiation position and then remaining at rest for a specified period at that position.

    Source activity augmentation - also referred to as source activity enhancement. Introduction of additional radionuclide source material or higher activity source material in specific places in a large area plaque source to improve dose uniformity in the process load being treated. Typically such augmentation is required at the edges of a source plaque near which the dose would otherwise tend to diminish.

    Source overlap - for a radionuclide irradiator, it is the extension of the radiation source beyond the cross - sectional dimensions of a process load as it passes the plaque source or a collimated beam. For an accelerator source, it is the sweep of the beam, especially for a scanned electron beam, beyond the edges of a process load.

    Source, strength - with reference to a γ-ray source, it is the activity level of the radioactive material expressed in becquerels (or curies).

    Transit dose - for a stationary radiation source, this is the dose received by the product during its movement into and out of the radiation field. For a stationary product, it is the dose received by the product while the movable source moves into or out of its irradiation position.

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    5. Dose, Dosimetry and related terms

    Absorbed dose, D - the mean energy, dE imparted by ionizing radiation to matter in a volume element divided by the mass, dm, of matter in the volume element D = dE/dm.
    The SI derived unit of absorbed dose is the gray (Gy), where 1 gray is equivalent to absorption of 1 joule per kilogram (= 100 rads).

    Absorbed dose rate, D=dD/dt - the change in absorbed dose, dD, divided by the time interval, dt,during which absorption occurs. The SI unit is the gray per second (Gy/s); the traditional special unit was the rad per second (rad/s), use of which is being phased out.

    Absorbed - dose mapping - Measurement of absorbed dose within a process load using dosimeters placed at specified locations to produce a one, two or three - dimensional distribution of absorbed dose, thus rendering a map of absorbed - dose values.

    Calibration - comparison of a measurement system or device of unknown accuracy to a measurement system or device of known accuracy (traceable to national standards) to detect, correlate, report, or eliminate by adjustment, any variation from the required performance limits of the unverified measurement, system or device.

    Dose uniformity ratio - ratio of the maximum to the minimum absorbed dose within the irradiation unit; it is a measure of the degree of uniformity of the absorbed dose; the concept is also referred to as the max/min dose ratio.

    Dosimeter - device or system having a reproducible, measurable response to radiation, which can be used to measure the absorbed dose in given material.

    Dosimetry system - system used for determining absorbed dose, consisting of dosimeters, and measuring instrumentation and procedures for the system's use.

    Ionizing radiation - Any type of radiation consisting of charged (directly ionizing) particles or uncharged (indirectly ionizing) particles, or both, that as a result of physical interaction, creates ions by either primary or secondary processes. For example, the charged particles could be positrons or electrons, protons, or other heavy ions and the uncharged particles could be X rays, y rays or neutrons.

    Gray (Gy) - the SI derived unit of absorbed dose of ionizing radiation, being equal to one joule of energy absorbed per kilogram of matter undergoing irradiation. Its relationship to the now obsolete special unit, the rad, is 1 Gy = 100 rad.

    National standard - standard recognized by an official national decision as basis for fixing the value, in a country, of all other standards of the quantity concerned.

    Primary standard - dosimeter: dosimeter, of the highest metrological quality, established and maintained as an absorbed dose standard by a national or international standards organization.

    Routine dosimeter - dosimeter calibrated against a primary reference or transfer standard dosimeter and used for routine dosimetry measurements.

    Traceability - the property of a result of a measurement whereby it can be related to appropriate standards, generally international and national standards, through an unbroken chain of comparisons.

    Transfer standard dosimeter - dosimeter, often a reference standard 'dosimeter, intended for transport between different locations for use ; as an intermediary to compare absorbed dose measurements.

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    6. Radiation sterilization and related terms

    Bioburden: population of viable microorganism on raw material, a component, a finished product, and/or a package just prior to sterilization.

    Bioburden estimate: value established for the number of microorganisms comprising the bioburden by applying to a viable count or presterilization counts a factor compensating for the recovery efficiency.

    Colony - forming unit (cfu): visible growth of microorganisms arising from a single cell or multiple cells.

    D10 - value (decimal reduction value): dose (expressed in kGy) required to achieve inactivation of 90% of a population of the test organisms under stated exposure conditions where it is assumed that the death of microorganisms follows first order kinetics.

    False negative: test results interpreted as no growth, either where growth was present but not detected, or where viable microorganisms failed to grow.

    False positive: result of a test of sterility where turbidity is interpreted as growth arising from the sample tested, when the growth resulted from extraneous microbial contamination or the turbidity arose from an interaction between the sample and the test medium.

    Fractional positive: quotient with the number of positive sterility tests in the numerator and the number of samples in the denominator.

    Incremental dose: dose within a series applied to a number of product units or portions thereof and used in dose setting methods to establish or confirm the sterilization dose.

    Microbiological quantity: product bioburden and microbial resistance to radiation.

    Microorganism: anyone - celled unit of sporangia and non - sporangia bacteria, viruses, fungi, and protozoa.

    Process development - documented program of studies, which is performed in order to define the sterilization process based upon the product/packaging/loading pattern and/or equipment limitations.

    Process parameter - specified value for a process variable.

    Product qualification - obtaining and documenting evidence that the health care product will be acceptable for its intended use after exposure to radiation.

    Reference load: specified sterilization load made up to represent the most difficult combination of products to be sterilized.

    Sample item portion (SIP): defined portion of a health care product unit that is tested.

    Standard distribution of resistance (SDR): a hypothetical distribution (based on measurements of the radiation resistances of selected microbial isolates) comprising a series of increasing D10 values and associated probabilities of occurrence.

    Sterility assurance level (SAL): probability of a viable microorganism being present on a product unit after sterilization. SAL is normally expressed as 10 - n.

    Sterility test: test performed to determine if viable microorganisms are present.

    Sterilization: validated process used to render a product free of all forms of viable microorganisms.

    Note: In a sterilization process, the nature of microbial death is described by an exponential function. Therefore, the presence of viable microorganisms on any individual item can be expressed in terms of probability. While this probability may be reduced to a very low number, it can never be reduced to zero. The probability can be expressed as a sterility assurance level (SAL).

    Sterilization dose audit: action taken to detect whether or not a change in sterilization dose is needed.

    Sterilization load: goods that are to be or have been sterilized simultaneously in the same sterilization chamber. Note: the sterilization load may include more than one manufacturing batch or lot.

    Sterilization process: all treatments that are required to accomplish sterilization to include preconditioning (if used), the sterilization cycle, and aeration.

    Survivor curve: graphical representation of inactivation against exposure to stated conditions.

    Test of sterility: test performed to establish the presence or absence of viable microorganisms on product units (or portions thereof) when subjected to defined culture conditions.

    VD. maximal acceptable verification dose for a given bioburden land verification sample size.

    Verification dose (D** kGy): a dose of radiation estimated to produce an SAL of 10 - 2 for a product unit or portion thereof, and used in dose setting methods to establish or confirm the sterilization dose.

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