Ratio Of Absorption Coefficients Research Articles

Response of LiF-TLD micro-rods around 125I radioactive seed

The EGSnrc Monte Carlo system has been used to calculate the energy response of LiF-TLDs of different sizes around 125I permanent brachytherapy sources. The source model includes the effects of an encapsulation, self-absorption within the source and in the welded ends of the encapsulation. The LiF-TLD material has cylindrical geometry (micro-rod) with diameters ranging from 1 mm to 5 mm and a length of 6 mm. The energy response factor (relative to 60Co γ-rays) for a LiF-TLD calibrated in 60Co γ-rays and then irradiated by an 125I permanent brachytherapy source varies between 1.32 ± 0.2% (1 SD) and 1.406 ± 0.2% (1 SD) for 5 mm and 1 mm diameter micro-rods, respectively. The energy response factor depends on the radius and the polar angle (r, θ) of the measurement point. For a LiF-TLD of diameter 1 mm calibrated at 1 cm on the transverse axis (r = 1.0 cm, θ = 90°) of the 125I source in water, the energy response factor decreases by a maximum of 3.5% within the 6 cm × 6 cm × 6 cm calculation region. For the 5 mm diameter LiF-TLD, the energy response factor decreases by a maximum of 5% in the same region. An examination of the photon energy spectra showed that the photon spectrum does not change significantly in water within the 3D calculation region (6 cm × 6 cm × 6 cm). The mass energy absorption coefficient ratio of water to LiF-TLDs does not vary by more than 0.5% in this calculation grid. The results, however, show that there is a change in the photon spectrum with distance from the source and with polar angle for LiF-TLDs. This difference in the energy spectrum gives rise to a difference in the mass energy absorption coefficient ratio of water to LiF (calculated by taking into account the difference in photon fluence in water and LiF) and that calculated assuming that the photon spectrum in water and in the LiF-TLD is identical.

Calculation of UV attenuation and colored dissolved organic matter absorption spectra from measurements of ocean color

The absorption of ultraviolet and visible radiation by colored or chromophoric dissolved organic matter (CDOM) drives much of marine photochemistry. It also affects the penetration of ultraviolet radiation (UV) into the water column and can confound remote estimates of chlorophyll concentration. Measurements of ocean color from satellites can be used to predict UV attenuation and CDOM absorption spectra from relationships between visible reflectance, UV attenuation, and absorption by CDOM. Samples were taken from the Bering Sea and from the Mid‐Atlantic Bight, and water types ranged from turbid, inshore waters to the Gulf Stream. We determined the following relationships between in situ visible radiance reflectance, Lu/Ed (λ) (sr−1), and diffuse attenuation of UV, Kd(λ) (m−1): Kd(323nm) = 0.781[Lu/Ed(412)/Lu/Ed(555)]−1.07; Kd(338nm) = 0.604[Lu/Ed(412)/Lu/Ed(555)]−1.12; Kd(380 nm) = 0.302[Lu/Ed(412)/Lu/Ed(555)]−1.24. Consistent with published observations, these empirical relationships predict that the spectral slope coefficient of CDOM absorption increases as diffuse attenuation of UV decreases. Excluding samples from turbid bays, the ratio of the CDOM absorption coefficient to Kd is 0.90 at 323 nm, 0.86 at 338 nm, and 0.97 at 380 nm. We applied these relationships to SeaWiFS images of normalized water‐leaving radiance to calculate the CDOM absorption and UV attenuation in the Mid‐Atlantic Bight in May, July, and August 1998. The images showed a decrease in UV attenuation from May to August of approximately 50%. We also produced images of the areal distribution of the spectral slope coefficient of CDOM absorption in the Georgia Bight. The spectral slope coefficient increased offshore and changed with season.

Secondary extinction and diffraction behaviors in cylindrical crystals.

The X-ray and neutron diffraction properties in absorbing cylindrical crystals are systematically explored within the framework of transfer equations and the kinematic diffraction approximation. The calculated power ratio distribution, the integrated reflection power ratio and the secondary-extinction factor y( micro ) are expressed as functions of the Bragg angle theta(B), the reduced radius sigma(0)rho = tau(0) and the ratio of absorption coefficient to diffraction cross section micro /sigma(0) = xi(0). Numerical solutions were obtained for all theta(B) (0-90 degrees ) and samples with tau(0) from 0 to 30, and xi(0) from 0 to 25. The relationship between the power ratio distribution curves, the integrated reflection power ratio and the diffraction geometry of cylindrical crystals is obtained for the first time and analyzed in detail. A dip was found in the curve of the extinction factor y( micro ) against tau(0) for given theta(B) and xi(0), and the position of this minimum shifts toward smaller tau(0) with increasing xi(0) or theta(B). A large decrease of y( micro ) with decreasing theta(B) at low angle appears when micro rho > 3.5 and 25 > xi(0) > 0.2. The rate of change of y( micro ) in this region increases with tau(0). All of this will be important for the refinement of diffraction data. The influence of different kinds of mosaic distributions on the integrated reflection power ratio and the extinction factor was also studied. The transmission coefficients A(*) were calculated using two different methods, and an inaccuracy of these numbers in Vol. II of International Tables for X-ray Crystallography (1972) in the range theta(B) < or = 15 degrees and micro rho > or = 15 was found by comparison.

An investigation of entrance surface dose calculations for diagnostic radiology using Monte Carlo simulations and radiotherapy dosimetry formalisms

Our aim in this work was to investigate the methodology used in the determination of the entrance surface dose (ESD) in diagnostic radiology. In kV x-rays for low-energy photons (tube potential up to 160 kV, HVL: 1–8 mm Al), the ESD is based on the use of the ratio of mass-energy absorption coefficients and backscatter factors. A full simulation of the photon and electron transport in a kilovoltage x-ray unit, using the Monte Carlo code BEAM/EGS4, was performed to obtain an accurate beam phase space for use in dose calculation. The modelled phase space was experimentally validated for the beam qualities (measured HVL: 3.3 mm Al–2.2 mm Cu) and showed good agreement between calculated and measured HVLs, air kerma and relative dose distributions. We have computed the conversion factors from air kerma to water or soft tissue absorbed dose at the surface of a phantom for beam qualities (HVL: 3.3–8.35 mm Al). The same model was also used to calculate the ESD in water and in soft tissue for the low-energy photon range considered. The results show that the numerical differences between the air kerma and the water kerma based backscatter factors are insignificant. The same conclusion was reached for the (μen/ρ) ratios, for soft tissue to air, evaluated using either the primary photon spectra or the spectra at the surface of a phantom. Furthermore, the good agreement obtained for the computation of the conversion factors with a full BEAM/EGS4 model confirms the previous studies which are based on different sources for the spectral distribution and different beam geometries (pencil beam or point source assumptions). On the other hand, the ESD in water or soft tissue is well described either with the Bair or the Bw formalism. Conversion factors from air kerma to ESD in these media are proposed in this work for several beam qualities in diagnostic radiology.

The G11.11-0.12 Infrared-dark Cloud: Anomalous Dust and a Nonmagnetic Isothermal Model

The G11.11-0.12 infrared-dark cloud has a filamentary appearance, both in absorption against the diffuse 8 μm Galactic background and in emission from cold dust at 850 μm. A detailed comparison of the dust properties at these two wavelengths reveals that standard models for the diffuse interstellar dust in the Galaxy are not consistent with the observations. The ratio of absorption coefficients within the cloud is κ8/κ850 ≤ 1010, which is well below that expected for the diffuse interstellar medium where κ8/κ850 ~ 1700. This may be due to the formation of ice mantles on the dust and grain coagulation, both of which are expected within dense regions of molecular clouds. The 850 μm emission probes the underlying radial structure of the filament. The profile is well represented by a marginally resolved central region and a steeply falling envelope, with Σ(r) ∝ r-α, where α ≥ 3, indicating that G11.11-0.12 is the first observed filament with a profile similar to that of a nonmagnetic isothermal cylinder.

Clinical implementation of AAPM TG61 protocol for kilovoltage x-ray beam dosimetry.

Historically, there have been a variety of dosimetry protocols used for kilovoltage x-ray therapy beams with a set of conversion factors and correction factors taken from different references. Corresponding to the continued installation and use of kilovoltage machines, the American Association of Physicists in Medicine (AAPM) presented a unified protocol developed by Task Group 61 (TG61). TG61 determines the absorbed dose to water with an ionization chamber calibrated in air in terms of air kerma (Nk). TG61 presents both an in-air method and an in-phantom method. In this work we only examine the TG61 in-air method. Our traditional dosimetry procedure, which is based upon NCRP Report 69 and on material found in standard medical physics texts, has been compared to the TG61. A variety of kilovoltage beam energies were examined with a set of various field sizes and source to surface distances. TG61 published updated data for the mass absorption coefficient ratios, backscatter factors, and the average energy per ion pair factor. The following conclusions have been reached: (1) Our traditional procedures and the TG61 protocol for in-air measurements are equivalent. (2) The conversion and correction factors used in TG61 are different by up to 4.5% compared to the old factors that we have used. (3) The application of the TG61 factors can result in up to 5% differences in the determination of the absorbed dose.

Isotope effects in the vibrational spectrum of the SF6 molecule

The IR absorption spectra of solutions of SF6 in liquid argon are studied at a temperature of 93 K in the concentration interval 10−5–10−7 mole fractions. A sample with a natural abundance of isotopes and a monoisotope 34SF6 sample are studied. The frequencies, half-widths, and relative intensities of bands in the vibrational spectrum of all isotopomers of the molecule are determined. For the 34SF6 molecule, the ratio of integral absorption coefficients of fundamental bands A(ν4)/A(ν3)=0.07(6) is larger than 32SF6:A(ν4)/A(ν3)=0.66(4) for the 32SF6 molecule, which corresponds to the same signs of P′3 and P′4. The change in the intensity of the ν2+ν6 and ν5+ν6 bands upon isotopic substitution is explained by the change in the resonance contributions of due to the isotope shift of the ν3 band.

Excitation cross section of erbium in semiconductor matrices under optical pumping

Based on a detailed consideration of excitation mechanisms of erbium in crystalline and amorphous matrix we present an analysis of the physical meaning of the Auger excitation cross section of Er{sup 3+} ions in semiconductor. It is demonstrated that large values of Auger excitation cross sections under optical pumping in semiconductor matrices are due to large values of band-to-band absorption coefficient exceeding by several orders of magnitude the absorption coefficient of erbium in dielectric SiO{sub 2} and Al{sub 2}O{sub 3} matrix. The Auger excitation cross section of Er{sup 3+} in semiconductor matrices is roughly given by the ratio of the matrix absorption coefficient to concentration of Er{sup 3+} ions. While the analysis of the excitation cross section is carried out for Er-doped crystalline and amorphous silicon, the results are expected to be applicable to the other rare-earth doped semiconductors. Based on low-temperature experimental results for crystalline silicon we get the Auger excitation coefficient of c{sub A}{sup cryst}{approx}7 x 10{sup -10} cm{sup 3}s{sup -1} and the effective excitation cross section {sigma}{sub eff}{sup cryst}=(2-8) x 10{sup -12} cm{sup 2}. For amorphous silicon at 100 K we obtain {sigma}{sub eff}{sup amorph}{approx}1.4 x 10{sup -14} cm{sup 2}.

Dose measurements in inhomogeneous bone/tissue and lung/tissue phantomsfor angiography using synchrotron radiation

For angiography using synchrotron radiation we measured the absorbed dosedistribution in inhomogeneous phantoms with thin LiF:Mg, Cu, P, LiF:Mg, Tithermoluminescent dosimeters (TLDs) in tissue and lung substitutes, and withMg2SiO4:Tb TLDs in bone substitute for 33.32 keV monoenergeticphotons from synchrotron radiation. The energy responses of the TLDs weremeasured in air for 10-40 keV monoenergetic photons. The values at 30 keVbecame smaller by 30% for LiF:Mg, Cu, P and larger by 22% for Mg2SiO4:Tb than the ratio of the mass energy absorption coefficients of theTLDs to that of air. These values were used to modify the calculated responseof the TLDs in each phantom material. The absorbed dose distribution obtainedwas compared with that calculated using the Monte Carlo transport code EGS4expanded to a low-energy region, and their agreement was confirmed takinglinear polarization into account. In the bone substitute the dose increased bya factor of 3.9, while behind the bone the dose decreased drastically becauseof photon attenuation. In the lung substitute a slight dose difference fromthat in soft tissue was observed because of its different density. TheLiF:Mg, Cu, P TLDs exhibited a better energy response, higher sensitivity andwider linear regions than did the other tissue-equivalent TLDs in thelow-energy region.

The Effect of Interfacial Viscosity on the Kinetics of Formation of Silver Nanoparticles Using Water-in-Oil Microemulsions as Nanoreactors

AbstractSilver nanoparticles were prepared by the method of mixing of two microemulsions having similar chemical composition but different reactants in their respective aqueous core. One microemulsion contains silver nitrate in the aqueous core and other contains sodium borohydride. The silver nanoparticles formed were characterized using UV-Visible absorption spectra and TEM micrographs. Effect of chain length of solvent and addition of Arlacel-20 to the AOT/Heptane, AOT/Decane and AOT/Dodecane microemulsion on particle size and absorption spectra of silver nanoparticles was studied. TEM photographs showed agglomerated and bigger particles in case of pure AOT/dodecane whereas addition of Arlacel-20 showed dispersed and smaller particles. Reaction kinetics was observed for silver nanoparticles using UV-visible spectrophotometer. Silver nanoparticles prepared using pure AOT surfactant showed plasmon band (416 nm) immediately after preparation whereas no absorption band of silver nanoparticles was observed for mixed surfactant microemulsion of AOT and Arlacel 20 for few hours indicating the reaction kinetics is slowed down upon addition of Arlacel-20. Growth rate of silver nanoparticles was plotted by monitoring absorption coefficient ratio (ε416/ε475) as a function of time. AOT/heptane system showed slower growth rate as compared to AOT/decane and AOT/dodecane and also larger particle size. Presence of Arlacel-20 significantly decreases the growth rate in all three alkanes and this observation can be explained using the concept of rigidity of surfactant film at the oil/water interface. It is proposed that higher the interfacial viscosity, slower is the coalescence rate of nanodroplets in the microemulsion system, and hence slower the growth rate of particles and smaller is the final particle size.

FTIR spectroscopic characterization of structural changes in polyamide-6 fibers during annealing and drawing

First, we report the development of Fourier transform infrared (FTIR) spectroscopic methods to determine the α/γ-crystalline phase ratio of polyamide-6 fibers and, in combination with density measurements, the total crystallinity. Using density determinations of the crystallinity of pure α and pure γ samples, we found the absorption coefficient ratio for the 930 (α) and 973 cm−1 (γ) bands to be 4.4, from which we could obtain the α/γ ratio for any polyamide-6 sample. The application of this FTIR method to the quantitative analysis of phase changes during thermal treatment and the drawing of polyamide-6 was then made. We confirmed that crystallization during thermal treatments involved increases in both phases and did not involve crystal-to-crystal transformation, whereas drawing involved both crystallization of the amorphous phase in the α form and γ→α transformation. Finally, we revisited the band assignments for the amorphous phase of polyamide-6 and found that the band at 1170 cm−1 was not an amorphous band but, because its absorbance was independent of crystallinity, could be used as an internal reference band. The band at 1124 cm−1 was reliably attributed to the amorphous phase. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 536–547, 2001

In‐phantom response of LiF TLD‐100 for dosimetry of HDR source

An experiment was carried out to reevaluate the response of LiF TLD-100 rods (1 mmx1 mmx6 mm) at different depths in a water substituting phantom to provide an answer to a prevailing controversy about the over-response of LiF to the softened photon spectra of 192Ir HDR source at depths in phantom due to its photon energy dependence. Claims of some authors that LiF TLDs over-responds by 8.5% at 10 cm depth in phantom, necessitating depth-dependent correction factors even for an 192Ir source and of some others for no over-response were evaluated. The over-response of LiF TLD-100 rods, against a calibrated ion chamber having a photon energy-independent response within 2%, was found to be not exceeding 2.5% at a depth of 10 cm in the phantom as compared to a depth at 1 cm, for a precision of the order of +/- 1% (1sigma) in the TLD measurements. By using ISO equivalent photon beams, photon energy dependence of the dosimeters was evaluated and for LiF TLD-100 rods it was found to be in close agreement (within 3%) with the ratios of mass energy absorption coefficients of LiF and water in the range of effective photon energy from 26 keV to 1.25 MeV. Parameters that could contribute to the discrepancy in the reported values of experimental results have been discussed.

Determination of absorbed dose to water in reference conditions for radiotherapy kilovoltage x-rays between 10 and 300 kV: a comparison of the data in the IAEA, IPEMB, DIN and NCS dosimetry protocols

A comparison of four of the most commonly used dosimetry protocols for the determination of absorbed dose to water in therapeutic kilovoltage x-rays using an ionization chamber (IAEA TRS-277, IPEMB, DIN and NCS) has been carried out. Owing to the different energy ranges and HVLs recommended by each protocol, backscatter factors, water-to-air mass energy absorption coefficient ratios and perturbation correction factors have been recast to a common quality range that all protocols satisfy individually to make a comparison possible. The results of the comparison show that in the sometimes reduced quality range originally included by the different protocols, determinations of absorbed dose to water at all beam qualities agree to within ±1.0% with that obtained using the second edition of the IAEA TRS-277 code of practice (1997). The extrapolation of data to a common beam quality range practically preserves the agreement for all the protocols except for that issued by the NCS at the extremes of the range, where differences of up to 1.8% and 1.4% have been found for low and medium energies respectively. In all cases the DIN protocol yields very good agreement with TRS-277.

Photon fluence perturbation correction factors for solid state detectors irradiated in kilovoltage photon beams

Dose perturbation correction factors, (p), for LiF, CaF2 and Li2B4O7 solid state detectors have been determined using the EGS4 Monte Carlo code. Each detector was simulated in the form of a disc of diameter 3.61 mm and thickness 1 mm irradiated in a clinical kilovoltage photon beam at a depth of 1 cm in a water phantom. The perturbation correction factor (p) is defined as the deviation of the absorbed dose ratio from the average mass energy absorption coefficient ratio of water to the detector material, (en/)med,det, which is evaluated assuming that the photon fluence spectrum in the medium and in the detector material are identical. We define another mass energy absorption coefficient ratio, (en/)med,det, which is evaluated using the actual photon fluence spectrum in the medium and detector for LiF and CaF2 rather than assuming they are identical. (en/)med,det predicts the average absorbed dose ratio of the medium to the detector material within 0.3%. When the difference in atomic number between the cavity and the phantom material is large then their photon fluence spectra will differ substantially resulting in a difference between (en/)med,det and (en/)med,det. The value of (p) calculated using (en/)med,det is up to 27% greater than unity for a cavity of CaF2 in 50 kV x-rays. When the atomic number of the medium and detector are similar, their photon fluence spectra are similar, and the difference between (en/)med,det and (en/)med,det is small. For instance their difference for LiF is less than 2%. The average mass energy absorption coefficient ratio, (en()/)w,LiF, evaluated using the mean or representative energy, , is up to 8% different from (en/)w,LiF. For calcium fluoride the difference between (en/)w,CaF2 and (en()/)w,CaF2 is up to 42% in the energy range studied.

Determination of X Ray Effective Energy and Absorbed Dose Using CaF2:Mn and LiF:Mg,Ti Thermoluminescence Dosemeters

The results of the X ray effective energy determination using the energy dependence of the response ratio of two thermoluminescence dosemeters are presented. CaF 2 :Mn dosemeters with a large, and LiF:Mg,Ti with a much lower energy dependence, were used. The calibration factors were obtained by irradiating the dosemeters with 137 Cs gamma rays, and their energy dependences were measured with X ray beams with known effective energies and also with a 241 Am gamma ray source. The measured values, i.e. the energy calibration curves, were compared with the calculation using the ratios of mass energy absorption coefficients and pertaining uncertainties were discussed. The results show that, in addition to the determination of the X ray effective energy, this method enables the determination of absorbed dose in the specified material to compensate for the systematic error due to the energy dependence inherent in each type of detector used.

Dichroism of a Polyimide Chain for Ultraviolet Light

We have determined the absorption coefficient ratio a‖/a⊥ of a poly [4, 4′ -oxydiphe-nylene-pyromellitimide] (PMDA-ODA) chain in the wavelength range from 240 nm to 380 nm, where a‖ and a⊥ are the absorption coefficients of the polyimide chain for ultraviolet (UV) light polarized parallel and perpendicular to the chain direction, respectively. To accomplish that, we have measured the polarized UV absorption spectra of a rubbed polyimide film whose molecular orientation was previously determined by polarized infrared absorption spectroscopy. The a‖/a⊥ ratio becomes maximum in the wavelength region from 290 nm to 330 nm : a‖/a⊥ ≃ 5. The absorption coefficient peaks at ∼290 nm. These results sugest that linearly polarized UV light at 290 nm is most suitable for causing anisotropic decomposition of the polyimide chain. At 266 nm the a‖/a⊥ ratio was compared with the ratio of the photoinduced decomposition rates of the polyimide chain oriented parallel and perpendicular to the polarization direction of UV light. The anisotropy of the photoinduced decomposition rates of the polyimide chain is much smaller than that of the UV absorption coefficients.

The response of photosynthetic absorption coefficients to irradiance in culture and in tidally mixed estuarine waters

The accuracy of models for primary production and light propagation depends on correct assignment of absorption to photosynthetic pigments. The phytoplankton absorption coefficient is comprised of two components: photosynthetic and photoprotective absorption coefficients. A method based on the fluorescent excitation of chlorophyll a is used to quantify the photosynthetic absorption coefficient for phytoplankton grown in culture and sampled from Puget Sound, Washington. The difference spectrum between total phytoplankton and photosynthetic absorption should be equivalent to photoprotective absorption. For cultures, the difference spectra exhibit peaks near 460 and 490 nm and broad‐band absorption between 400 and 450 nm. However, for field samples an additional pronounced peak is observed around 440 nm, similar in shape to the chlorophyll a Soret peak. If the 440‐nm peak were associated with photosystem I chlorophyll a, the photosynthetic absorption coefficient will be underestimated by &lt;15% for these samples. Variability in both coefficients is predictable as a function of irradiance. The photosynthetic coefficient varies inversely with growth irradiance, and the photoprotective coefficient varies directly with irradiance. This direct relationship with irradiance accounts for much of the variability in the spectral shape of the total phytoplankton absorption coefficient. The ratio of the photosynthetic absorption coefficient to the total phytoplankton absorption coefficient increases as a function of decreasing irradiance for cultures and for field samples collected from stratified regions of the water column. This ratio is a photoadaptive parameter that can serve to integrate physiological response to irradiance and has the potential to provide estimates of mixed layer dynamics.

Mass-energy absorption coefficient and backscatter factor ratios for kilovoltage x-ray beams

For low-energy (up to 150 kV) x-rays, the ratio of mass-energy absorption coefficients for water to air, , and the backscatter factor B are used in the conversion of air kerma, measured free-in-air, to water kerma on the surface of a water phantom. For clinical radiotherapy, similar conversion factors are needed for the determination of the absorbed dose to biological tissues on (or near) the surface of a human body. We have computed the ratios and B factor ratios for different biological tissues including muscle, soft tissue, lung, skin and bone relative to water. The ratios were obtained by integrating the respective mass-energy absorption coefficients over the in-air primary photon spectra. We have also calculated the ratios at different depths in a water phantom in order to convert the measured in-phantom water kerma to the absorbed dose to various biological tissues. The EGS4/DOSIMETER Monte Carlo code system has been used for the simulation of the energy fluence at different depths in a water phantom irradiated by a kilovoltage x-ray beam of variable beam quality (HVL: 0.1 mm Al-5 mm Cu), field size and source-surface distance (SSD). The same code was also used in the calculation of the B factor ratios, soft tissue to water and bone to water. The results show that the B factor for bone differs from the B factor for water by up to 20% for a 100 kV beam (HVL: 2.65 mm Al) with a 100 field. On the other hand, the difference in the B factor between water and soft tissue is insignificant (well within 1% generally). This means that the B factors for water may be directly used to convert the `in-air' water kerma to surface kerma for human soft tissues.

Quantum close-coupling calculations of collisional redistribution of light for Ba(1P ← 1S) perturbed by helium

Cross-sections for optical collisions of the barium atom with He are calculated for a range of detunings around the atomic resonance. Results are obtained from fully quantum mechanical coupled-channels calculations including the ground and two excited molecular 1σ and 1II states. Close-coupling calculations are carried out using the theoretical potential curves for Ba—He, obtained by means of a pseudopotential SCF-CI technique (Czuchaj, E., Rebentrost, F., Stoll, H., and Preuss, H., 1995, Chem. Phys., 196, 37). For accurate comparison with the experiment (Ni, S. Y., Goetz, W., Meijer, H. A. J., and Andersen, N., 1996, Z. Phys. D, 38, 303), the calculated absorption profile and polarizations have been thermally averaged over the collision energy. The theoretical absorption coefficient and linear polarization ratio agree quite well with the experimental data.

An alternative beam quality index for medium-energy x-ray dosimetry

To determine absorbed dose to water it is essential that the radiation quality of the x-ray beam is known accurately. The unique method of defining the beam quality is to acquire a detailed knowledge of the photon spectrum at the point of measurement in water, but this is impractical. The aim of this work was to investigate a quality index that uniquely defines the ratio of mass-energy absorption coefficients of water to air at 2 cm deep in water . Three parameters, HVL, mean energy at 2 cm deep in water and the ratio of the doses at 2 and 5 cm deep in water were investigated as functions of . The quality index that best defines is the ratio of doses at 2 and 5 cm depth in water.