Air kerma rate constants for gamma emitters used most often in practice

It is often required to estimate the dose rate at a distance from radionuclides that are sources of X rays and gamma rays. Such calculations may be required for planning radiation protection measures in the vicinity of radioactive sources or patients containing radionuclides, calibrations of radiation instruments or for estimating the absorbed dose rate to patients receiving brachytherapy. The factor relating activity and air kerma rate is called air kerma rate constant--gamma(delta). In this paper, the results of recalculation of this quantity for unfiltered point sources of radionuclides in practice used most often are given. The calculations included corrections for internal conversion of X rays and gamma rays and detailed accounting of the generation of the K and L series X rays from internal conversion and electron capture. Particular air kerma rate constants were calculated for each discrete line in the photon spectrum of radionuclide with a yield per decay event >0.01% and the energy >20 keV. Since the energy structure of the photon spectra and accessible discrete numerical values of the mass energy-transfer coefficient for air are not the same, the cubic spline interpolation was used to obtained the coefficient, where the photon spectrum data are available. In the calculation, the latest gamma ray spectral data for all radionuclides and latest data for the mass energy-transfer coefficient for air are used. Air kerma rate constants for the following 35 radionuclides are calculated: 11C, 13N, 15O, 18F, 24Na, 42K, 43K, 51Cr, 52Fe, 59Fe, 57Co, 58Co, 60Co, 67Ga, 68Ga, 75Se, 99Mo, 99mTc, 111In, 113mIn, 123I, 125I, 131I, 127Xe, 133Xe, 137Cs, 152Eu, 154Eu, 170Tm, 182Ta, 192Ir, 197Hg, 198Au, 201Tl and 241Am.     

Calculation of absorbed dose around a facility for disposing of low activity natural radioactive waste (C3-dump)

A C3-dump is a facility for disposing of low activity natural radioactive waste containing the uranium series 238U, the thorium series 232Th and 40K. Only the external radiation owing to gamma rays, X-rays and annihilation photons is considered in this study. For two situations--the semi-infinite slab and the tourist geometry--the conversion coefficients from specific activity to air kerma rate at 1 m above the relevant level are calculated. In the first situation the waste material is in contact with the air but in the tourist geometry it is covered with a 1.35 m thick layer. For the calculations, the Monte Carlo radiation transport code MCNP is used. The yield and photon energy for each radionuclide are according to the database of Oak Ridge National Laboratory. For the tourist situation, the depth-dose distribution through the covering layer is calculated and extrapolated to determine the exit dose.     

Estimation of the peak entrance surface air kerma for patients undergoing computed tomography-guided procedures

The purpose of this work was to develop a method for estimating the patient peak entrance surface air kerma from measurements using a pencil ionisation chamber on dosimetry phantoms exposed in a computed tomography (CT) scanner. The method described is especially relevant for CT fluoroscopy and CT perfusion procedures where the peak entrance surface air kerma is the risk-related quantity of primary concern. Pencil ionisation chamber measurements include scattered radiation, which is outside the primary radiation field, and that must be subtracted in order to derive the peak entrance surface air kerma. A Monte Carlo computer model has therefore been used to calculate correction factors, which may be applied to measurements of the CT dose index obtained using a pencil ionisation chamber in order to estimate the peak entrance surface air kerma. The calculations were made for beam widths of 5, 7, 10 and 20 mm, for seven positions of the phantom, and for the geometry of a GE HiSpeed CT/i scanner. The program was validated by comparing measurements and calculations of CTDI for various vertical positions of the phantom and by directly estimating the peak ESAK using the program. Both validations showed agreement within statistical uncertainties (standard deviation of 2.3% or less). For the GE machine, the correction factors vary by approximately 10% with slice width for a fixed phantom position, being largest for the 20 mm beam width, and at that beam width range from 0.87 when the phantom surface is at the isocentre to 1.23 when it is displaced vertically by 24 cm.     

Calculation of conversion coefficients for clinical photon spectra using the MCNP code

In this work, the MCNP4B code has been employed to calculate conversion coefficients from air kerma to the ambient dose equivalent, H*(10)/Ka, for monoenergetic photon energies from 10 keV to 50 MeV, assuming the kerma approximation. Also estimated are the H*(10)/Ka for photon beams produced by linear accelerators, such as Clinac-4 and Clinac-2500, after transmission through primary barriers of radiotherapy treatment rooms. The results for the conversion coefficients for monoenergetic photon energies, with statistical uncertainty <2%, are compared with those in ICRP publication 74 and good agreements were obtained. The conversion coefficients calculated for real clinic spectra transmitted through walls of concrete of 1, 1.5 and 2 m thick, are in the range of 1.06-1.12 Sv Gy(-1).     

Determination of conversion factors from air kerma to operational dose equivalent quantities for low-energy X-ray spectra

The conversion coefficients from air kerma to ICRU operational dose equivalent quantities for STUK's realisation of the X-radiation qualities N-15 to N-60 of the ISO narrow (N) spectrum series were determined by utilising X-ray spectrum measurements. The pulse-height spectra were measured using a planar high-purity germanium spectrometer and unfolded to fluence spectra using Monte Carlo generated data of the spectrometer response. To verify the measuring and unfolding method, the first and second half-value layers and the air kerma rate were calculated from the fluence spectra and compared with the values measured using an ionisation chamber. For each radiation quality, the spectrum was characterised by the parameters given in ISO 4037-1. The conversion coefficients from the air kerma to the ICRU operational quantities H(p)(10), H(p)(0.07), H'(0.07) and H(*)(10) were calculated using monoenergetic conversion coefficients at zero angle of incidence. The results are discussed with respect to ISO 4037-4, and compared with published results for low-energy X-ray spectra.     

Personal dose equivalent conversion coefficients for photons to 1 GeV

The personal dose equivalent, H(p)(d), is the quantity recommended by the International Commission on Radiation Units and Measurements (ICRU) to be used as an approximation of the protection quantity effective dose when performing personal dosemeter calibrations. The personal dose equivalent can be defined for any location and depth within the body. Typically, the location of interest is the trunk, where personal dosemeters are usually worn, and in this instance a suitable approximation is a 30 × 30 × 15 cm(3) slab-type phantom. For this condition, the personal dose equivalent is denoted as H(p,slab)(d) and the depths, d, are taken to be 0.007 cm for non-penetrating and 1 cm for penetrating radiation. In operational radiation protection a third depth, 0.3 cm, is used to approximate the dose to the lens of the eye. A number of conversion coefficients for photons are available for incident energies up to several megaelectronvolts, however, data to higher energies are limited. In this work, conversion coefficients up to 1 GeV have been calculated for H(p,slab)(10) and H(p,slab)(3) both by using the kerma approximation and tracking secondary charged particles. For H(p)(0.07), the conversion coefficients were calculated, but only to 10 MeV due to computational limitations. Additionally, conversions from air kerma to H(p,slab)(d) have been determined and are reported. The conversion coefficients were determined for discrete incident energies, but analytical fits of the coefficients over the energy range are provided. Since the inclusion of air can influence the production of secondary charged particles incident on the face of the phantom, conversion coefficients have been determined both in vacuo and with the source and slab immersed within a sphere in air. The conversion coefficients for the personal dose equivalent are compared with the appropriate protection quantity, calculated according to the recommendations of the latest International Commission on Radiological Protection (ICRP) guidance.     

Monte Carlo determination of the conversion coefficients Hp(3)/Ka in a right cylinder phantom with 'PENELOPE' code. Comparison with 'MCNP' simulations

This work has been performed within the frame of the European Union ORAMED project (Optimisation of RAdiation protection for MEDical staff). The main goal of the project is to improve standards of protection for medical staff for procedures resulting in potentially high exposures and to develop methodologies for better assessing and for reducing, exposures to medical staff. The Work Package WP2 is involved in the development of practical eye-lens dosimetry in interventional radiology. This study is complementary of the part of the ENEA report concerning the calculations with the MCNP-4C code of the conversion factors related to the operational quantity H(p)(3). In this study, a set of energy- and angular-dependent conversion coefficients (H(p)(3)/K(a)), in the newly proposed square cylindrical phantom made of ICRU tissue, have been calculated with the Monte-Carlo code PENELOPE and MCNP5. The H(p)(3) values have been determined in terms of absorbed dose, according to the definition of this quantity, and also with the kerma approximation as formerly reported in ICRU reports. At a low-photon energy (up to 1 MeV), the two results obtained with the two methods are consistent. Nevertheless, large differences are showed at a higher energy. This is mainly due to the lack of electronic equilibrium, especially for small angle incidences. The values of the conversion coefficients obtained with the MCNP-4C code published by ENEA quite agree with the kerma approximation calculations obtained with PENELOPE. We also performed the same calculations with the code MCNP5 with two types of tallies: F6 for kerma approximation and *F8 for estimating the absorbed dose that is, as known, due to secondary electrons. PENELOPE and MCNP5 results agree for the kerma approximation and for the absorbed dose calculation of H(p)(3) and prove that, for photon energies larger than 1 MeV, the transport of the secondary electrons has to be taken into account.     

Review of reconstruction of radiation incident air kerma by measurement of absorbed dose in tooth enamel with EPR

Electron paramagnetic resonance dosimetry with tooth enamel has been proved to be a reliable method to determine retrospectively exposures from photon fields with minimal detectable doses of 100 mGy or lower, which is lower than achievable with cytogenetic dose reconstruction methods. For risk assessment or validating dosimetry systems for specific radiation incidents, the relevant dose from the incident has to be calculated from the total absorbed dose in enamel by subtracting additional dose contributions from the radionuclide content in teeth, natural external background radiation and medical exposures. For calculating organ doses or evaluating dosimetry systems the absorbed dose in enamel from a radiation incident has to be converted to air kerma using dose conversion factors depending on the photon energy spectrum and geometry of the exposure scenario. This paper outlines the approach to assess individual dose contributions to absorbed dose in enamel and calculate individual air kerma of a radiation incident from the absorbed dose in tooth enamel.     

Corrections to air kerma and exposure measured with free air ionisation chambers for charge of photoelectrons, Compton electrons and Auger electrons

The signal charge from a free air ionisation chamber for the measurement of air kerma and exposure consists of not only the charge of ion pairs produced by secondary electrons (i.e. photoelectrons, Compton electrons and Auger electrons), but also the charge of the secondary electrons and single and multiple charged ions formed by the release of the secondary electrons. In the present work, correction factors for air kerma and exposure for the charge of the secondary electrons and ions were calculated for photons with energies in the range from 1 to 400 keV. The effects of an increase in the W value of air for low-energy electrons were also taken into consideration. It was found that the correction factors for air kerma and exposure have a maximum value near a photon energy of 30 keV; in the lower energy region, the correction factor for exposure monotonically decreases with a decrease in photon energy except for a small dip due to K-edge absorption by argon atoms in air. The values of the correction factors were found to be 0.9951 and 0.9892, respectively, for a spectrum with a mean energy of 7.5 keV, the reference X-ray spectrum with the lowest mean energy in ISO 4037-1. The air kerma correction is smaller than that for exposure, because for air kerma the signal due to the charge of secondary electrons and ions is partly compensated by the decrease in the number of ion pairs produced by the secondary electrons due to the increase of the W value of air for lower energy electrons.     

Calibration of a 7.6 cm x 7.6 cm (3 inch x 3 inch) sodium iodide gamma ray spectrometer for air kerma rate

An experimental procedure is described for converting a gamma ray spectral measurement from a 7.6 cm x 7.6 cm (3 inch x 3 inch) sodium iodide (NaI) detector to air kerma rate. The calibration procedure involves measuring the energy deposited in the detector using 10 radioactive sources of known activity covering an energy range from 60 keV to 1,836 keV. For each of the 10 sources, gamma ray spectra were measured with the source at different angles to the detector axis. The total energy deposited in the detector for the ten sources was confirmed by Monte Carlo calculations. The spectra measured at different angles were combined to produce a spectrum that would represent a homogeneous semi-infinite source of radiation. The resultant spectrum was then subdivided into 10 energy regions. Based on the known air kerma rates due to the sources, a calibration coefficient was calculated for each of the 10 energy regions. These calibration coefficients could then be used to convert the energy deposited in the 10 regions of an unknown spectrum to air kerma rate. The calibration procedure was confirmed by comparing the results from the detector with those from calibrated collimated beams of 137Cs and 60Co. A comparison of measurements using a calibrated pressurised ionisation chamber with those from a similar Nal spectrometer in Finland provided additional confirmation of the calibration procedure.     

The scope for measuring the effective dose of external irradiation and the use of operational quantities

The effective dose of external radiation may be measured as a physical quantity by an indirect method based on direct measurements of air kerma for photons, neutron fluence, or electron fluence, where calculated conversion factors for given irradiation conditions are used. The same method is used for measuring operational quantities. __________ Translated from Izmeritel’naya Tekhnika, No. 5, pp. 47–52, May, 2008.     

~(137)Cs γ射线空气比释动能基准石墨空腔电离室的研制

根据Bragg-Gray理论研制了圆柱形石墨空腔电离室,用于~(137)Cs空气比释动能基准的建立。通过理论计算灵敏体积内部电场分布,优化了电离室壁和收集极之间接地保护的结构设计,在此基础上,对研制的电离室的饱和曲线、本底电流以及稳定性等电学性能进行了测试,其结果表明完全达到设计要求。对电离室的各项修正因子进行了理论计算和实验测量,实现了~(137)Cs空气比释动能的量值复现,其合成标准不确定度为0.25%。     

Gamma shielding factor for typical houses in Brazil

The housing features in a country depend much on its climate. Dwellings in warm countries are much lighter constructions than in cold ones, which will reflect on the amount of shielding against radiation they provide. In addition to that, wealth is another factor that influences the building's finishing. Great effort has been taken to determine parameters to more accurately estimate dose to a population in case of a radioactive or nuclear accident. Nevertheless, most available data are concerned with typical housing in cold climate countries. This study aims to determine shielding factors for typical building materials used in the southeast of Brazil, a warm area, due to radioactive material deposited on the surrounding field, walls and ceiling of the external surfaces. The shielding factors determination was performed by simulation with the MCNP5 Monte Carlo computer code. The air kerma indoors for the 300, 662 and 3000 keV photon energies have been determined for three different housing patterns, ranging from the very simple to a very complex structure. The shielding factor, defined as the ratio of the air kerma indoor to the air kerma in open field, for the most simple house type and 300 keV photon energy was found to be twice of the best finished one for the same energy.     

A system for rapid large-area monitoring of gamma dose rates in the environment based on MCP-N (LiF:Mg,Cu,P) TL detectors

One lesson learned from the Chernobyl accident was that the spatial distribution of far-field contamination was strongly non-uniform due to local variation of atmospheric conditions, such as wind direction, rain etc. An environmental monitoring system using highly sensitive thermoluminescent LiF:Mg,Cu,P (MCP-N) detectors has been completed and field-tested. The system consists of 3000 MCP-N detectors in 1000 TLD cards (three TLDs per card), two Mikrolab automatic TL readers, heating ovens, and specially developed software which includes a database for rapid evaluation of results. The main dosimetric parameters of MCP-N dosemeters, such as thermally-induced fading, light sensitivity, minimum detectable dose, self-dose, zero-dose, energy response up to 6-7 MeV, influence of annealing and readout conditions on detector stability, have been tested. About 100 locations over an area of about 15,000 km2 in the south of Poland were selected for measurements lasting from 4 days to 3 months. The kerma rates measured over a 4 day screening period agree well with kerma rates determined over a 75 day monitoring period. Results from short- and long-term exposure periods agree well with those performed using MTS-N (LiF:Mg,Ti) over southern Poland in 1985, before the Chernobyl accident. Thus, using the system based on MCP-N detectors, one is able simultaneously to monitor environmental radiation kerma rates at a large number of locations over periods of four days or less. Provided natural background kerma rates at selected monitoring points are available prior to the accident, the system can be applied to assess kerma rates rapidly in the environment, following a nuclear accident.     

Optimised geometry to calculate dose rate conversion coefficient for external exposure to photons

A single-parameter geometry to describe soil is achieved for Monte Carlo calculation of absorbed dose rate in air for photon emitters from natural radionuclides. This optimised geometry based on physical assumptions consists of the soil part whose emitted radiation has a given minimum probability to reach the detector. This geometry was implemented in Geant4 toolkit and a significant reduction in computation time was achieved. Simulation tests have shown that for soil represented by a cylinder of 40 m radius and 1 m deep, >98% of the calculated dose rate conversion coefficients in air at 1 m above the ground is generated by only 6% of the soil volume in the case of uniform distribution of radioactivity, and >99.2% of the calculated dose rate for an exponential distribution. When the soil is represented by the entire optimised geometry, 99% of the conversion coefficients values are reached for a soil depth of 1 m and 100% for that of approximately 2 m.     

Dosimetry of a nearly monoenergetic 6 to 7 MeV photon source by NaI(Tl) scintillation spectrometry

The dosimetry of a nearly-monoenergetic 6–7 MeV photon source developed at the National Bureau of Standards (NBS) for radiation protection instrument calibration has been carried out by NaI(Tl) scintillation spectrometry. This approach uses calculated 3 in. × 3 in. NaI(Tl) detector-response functions that have been shown to be reasonably accurate up to 20 MeV. A least-squares fit of the appropriate response functions to a selected region of the pulse-height distribution determines the primary 6–7 MeV photon fluence. The uncertainty in the fluence determination is based on the χ² of the fit, the statistics of the data, and the uncertainty in the response functions. The air kerma delivery due to the primary photons at a reference point in the photon field was calculated from the primary photon fluence. The uncertainty in the determination of air kerma delivery for primary photons was less than 5% (1 std. dev.). The primary high-energy photon contribution to the detector response was subtracted from the data and the remaining distribution due to lower-energy photons was evaluated by spectrum unfolding analyses. The spectrum-unfolded results indicate that a contribution of approximately 12% of the total air kerma was mostly from a continuous distribution of photons extending up to 4.5 MeV.     

The response of lif thermoluminescence dosemeters to photon beams in the energy range from 30 kV x rays to 60Co gamma rays

The energy response of standard (TLD-100) and high-sensitivity (TLD-100H) LiF thermoluminescence dosemeters (TLDs) has been studied for photon beams with mean energies from about 25 keV to 1100 keV. Canadian primary standards for air kerma were used to establish the air kerma rates for each of the photon beams. TLDs were mounted in a PMMA holder and the air kerma response was measured as a function of energy. The EGSnrc Monte Carlo code was used to model the TLD holder and calculate the absorbed dose to the TLD chip per unit air kerma for each beam. The measured and calculated results were combined to obtain the intrinsic dose response of the TLD chip. Broadly, our results are consistent with existing data, which show a marked difference in the energy dependence of the two materials. However, the precision of our measurements (standard uncertainty of about 0.6%) has permitted the identification of features that have not been noted before. In particular, the energy dependence of the two materials is quite different in the important energy region delimited by 137Cs and 60Co gamma rays.     

Simulation of medical electron linac bremsstrahlung beam transport in typical shielding materials

The Monte Carlo N-particle code MCNP version 4C3 was used to investigate the backscattering and transmission of high-energy photons in concrete, iron and lead at deep penetration. A typical bremsstrahlung beam from a 24 MV linac was used, and the transmission up to 15 mean-free paths was studied. Broad beam slab geometry was used. Estimates of the transmission in terms of absorbed dose to tissue ratio and air kerma ratio were performed for the primary and secondary components of the transmitted beam in the three materials. The tissue dose and air kerma buildup factors were calculated and fitted to Berger's equation. Finally, the differential dose albedo values for common reflected angles were determined.     

Calculated neutron air kerma strength conversion factors for a generically encapsulated Cf-252 brachytherapy source

The ²⁵²Cf neutron air kerma strength conversion factor (S_KN/m_Cf) is a parameter needed to convert the radionuclide mass (μg) provided by Oak Ridge National Laboratory into neutron air kerma strength required by modern clinical brachytherapy dosimetry formalisms indicated by Task Group No. 43 of the American Association of Physicists in Medicine (AAPM). The impact of currently used or proposed encapsulating materials for ²⁵²Cf brachytherapy sources (Pt/Ir-10%, 316L stainless steel, nitinol, and Zircaloy-2) on S_KN/m_Cf was calculated and results were fit to linear equations. Only for substantial encapsulation thicknesses, did S_KN/m_Cf decrease, while the impact of source encapsulation composition is increasingly negligible as Z increases. These findings are explained on the basis of the non-relativistic kinematics governing the majority of ²⁵²Cf neutron interactions. Neutron kerma and energy spectra results calculated herein using MCNP were compared with results of Colvett et al. and Rivard et al.     

Air kerma to Hp(3) conversion coefficients for a new cylinder phantom for photon reference radiation qualities

The International Organization for Standardization (ISO) has issued a standard series on photon reference radiation qualities (ISO 4037). In this series, no conversion coefficients are contained for the quantity personal dose equivalent at a 3 mm depth, H(p)(3). In the past, for this quantity, a slab phantom was recommended as a calibration phantom; however, a cylinder phantom much better approximates the shape of a human head than a slab phantom. Therefore, in this work, the conversion coefficients from air kerma to H(p)(3) for the cylinder phantom are supplied for X- and gamma radiation qualities defined in ISO 4037.