PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

More documents

Recommendations

Info

I. Cibulskaite et al. / Medical Physics in the Baltic States 7 (2009) 72 - 75 DOSnrc (EGSnrc) [2] in each region of interest. Estimated relative statistical uncertainty of calculated doses was 0.8% in the case when 10 8 photon histories were taken for the simulation, and 0.2% - for 10 9 photon histories. The spectra of the X-ray beam, entered to the code system for interaction modelling, were generated for different X-ray tube potentials from the range of (25 - 32) kV when the tilting angle of the Mo anode was 16 0 . Generated X-rays were attenuated by 1 mm Be window of the X-ray tube, 0.03 mm thick Mo filter and 3 mm thick polymethylmethacrilic compression paddle of mammography unit. Additional filtration of the photons with the energies below 10 keV and above 20 keV was undertaken due to the fact, that Mo is characterized by K lines at the energies: 17.38 keV (Kα2), 17.49 keV (Kα1), 19.61 keV (Kβ1) and 19.96 keV (Kβ2) [3]. X-ray photon spectra were calculated using IPEM Spectrum Processor [4] and average photon energies for the applied X-ray tubes potentials were estimated taking into account the special geometry of the irradiation. 3. Results Adapted for specific geometry EGSnrc code was used for X-ray photon interaction with physical object modelling. The main characteristic that was chosen was radiation dose, that reflects information about energy transfer, absorption and scatter in every segment of the complex object irradiated by X-ray photons. The result of modelling are total and scatter doses in selected objects. It was considered that the total dose represents the energy, transferred to the target and the scatter dose is defined as a dose, which can be traced back to the photons that have been scattered in Compton and Rayleigh effects and to the photons that were created trough relaxation processes after Compton and photoelectric effect. Modelling of low energy (25-32) keV X-ray induced processes was performed for the constructions consisting of Si wafer (active volume) protected by different coating materials which are commonly used in radiation detector constructions. Some characteristics of the investigated coatings are provided in Table 1. X-ray interaction effects in the detector construction are dependent on X-ray attenuation and scattering processes in the coating material, which are influenced at least by the coating structure (polycrystalline, amorphous, polymer-like), coating density and thickness, and the construction of the layered structure (with or without chemical bonding between detector and coating material). A thin air gap existing between the coating and detector material was inserted into modelling geometry in the case of free standing polymeric foils. Nanothick interlayer structure between bulk material and coating was considered for the modelling of the structures 73 having direct deposited or grown coating layers on the top of the active volume. The thickness of Si target in investigated constructions varied within the range of 0.1 mm – 3.0 mm, the thickness of protective coatings was from the interval of 100 nm - 1.0 mm. Table 1. Coating materials Coating type Coating material Zeff Density, g/cm 3 Free Polyethylene 5.68 0.93 standing H – 14 %, C – 85 % polymeric foils Mylar, Polyethylenetereftalate H – 4 %, C – 62 %, O – 33 % 6.71 1.38 Rubber H – 11%, C – 88% 5.75 0.92 Carbon fibre 6.10 1.75 Direct deposited / grown layers SiO2 DLC film synthesized from C2H2: H – (24 % 11.5 diverse 2.32 1.60 2.12 - to 38%) SiOx-containing DLC film, synthesized from HMDSO: main components, O and Si (17% - 24%) diverse 1.87 - 2.20 Comparing the modelling results, radiation induced scatter dose of X-ray photons was higher in all coated Si specimens of different thickness, as compared to uncoated ones, while the total dose was less, due to the photon energy absorption in coating material. Increasing for scatter dose and decreasing for the total dose tendency was observed with the increasing density of coating material within the whole energy range of mammography examinations, when the thickness of coating was kept constant. Some variations of calculated scatter doses in Si targets of different thickness coated by different coating materials of the same thickness, representing results of the investigation are shown in Fig. 1. Lines in this figure indicate the highest and lowest scatter doses corresponding to the coating material, which were evaluated within the group of coated samples, having the same Si target thickness. A broadening of the range between the highest and lowest scatter dose within the group indicates the influence of coating material (increased density) on scatter dose in coated Si when the thickness of the active Si volume is decreased. The highest scatter dose was estimated for Si-DLC film containing SiOx combination, which represented the highest density (2.2 kg/m 3 ) of all investigated coatings. Scatter doses calculated for Si coated by nanothick DLC film were included in Fig 1 for the comparison of the results.
Scatter dose ( Gy /mA s) 1.55E-12 1.45E-12 1.35E-12 1.25E-12 1.15E-12 1.05E-12 9.50E-13 8.50E-13 I. Cibulskaite et al. / Medical Physics in the Baltic States 7 (2009) 72 - 75 y = 2E-14x + 8E-13 y = 2E-14x + 3E-13 0.1 mm Si 0.5 mm Si 7.50E-13 y = 2E-14x + 2E-13 6.50E-13 1.5 mm 5.50E-13 y = 2E-14x + 2E-14 4.50E-13 3.50E-13 y = 2E-14x - 1E-14 24 25 26 27 28 29 30 31 32 33 X-ray tube voltage (kV) Si without coating 1 mm mylar coating 200 nm DLC coating 1 mm rubber coating 1 mm DLC coating 1 mm polyethylene coating Fig. 1. Simulated scatter doses in Si samples of different thickness coated by different coatings of the same thickness. Detector thicknesses are indicated. Three possible cases were investigated for the estimation of the influence of the coating thickness on the X-ray attenuation processes in irradiated detector constructions: 1) coating thickness h is of the same order as the thickness of the active layer d (h ≈ d); 2) h/d ≤ 0.1; and 3) h
Page 1 and 2:
2009 PROCEEDINGS OF THE 7 th INTERN
Page 3 and 4:
Executive editor: D.Adlienė CONFER
Page 5 and 6:
11.15-11.30 Daiva Leščiute, Valda
Page 7 and 8:
MEDICAL PHYSICS IN THE BALTIC STATE
Page 9 and 10:
S. Mattsson / Medical Physics in th
Page 11 and 12:
S. Mattsson / Medical Physics in th
Page 13 and 14:
L. Bumbure et al. / Medical Physics
Page 15 and 16:
Average brightness Brightness range
Page 17 and 18:
Page 19 and 20:
D. Lesciute et al. / Medical Physic
Page 21 and 22:
Page 23 and 24: D. Kaskelyte et al. / Medical Physi
Page 25 and 26: MEDICAL PHYSICS IN THE BALTIC STATE
Page 27 and 28: V. Poderys et al. / Medical Physics
Page 29 and 30: E D A B C V. Poderys et al. / Medic
Page 33 and 34: A. Kulbickas / Medical Physics in t
Page 37 and 38: M. Franckevicius / Medical Physics
Page 39 and 40: M. Franckevicius / Medical Physics
Page 41 and 42: J. Puiso et al. / Medical Physics i
Page 43 and 44: J. Puiso et al. / Medical Physics i
Page 45 and 46: T. Landberg et al. / Medical Physic
Page 55 and 56: S. Plaude et al. / Medical Physics
Page 59 and 60: Table 1. Optimization of number of
Page 63 and 64: C.-M. Nilsson / Medical Physics in
Page 67 and 68: S. Bernhardsson et al. / Medical Ph
Page 69 and 70: S. Bernhardsson et al. / Medical Ph
Page 71 and 72: 2.3. Biological test. Y. Dekhtyar e
Page 73: MEDICAL PHYSICS IN THE BALTIC STATE
Page 79 and 80: N. Kazuchits et al. / Medical Physi
Page 81 and 82: V. Minialga et al. / Medical Physic
Page 83 and 84: Temperature, o C 36 35 34 33 32 31
Page 85 and 86: O. Klepalov et al. / Medical Physic
Page 89 and 90: G. Adlys / Medical Physics in the B
Page 91 and 92: G. Adlys / Medical Physics in the B
Page 93 and 94: D. Mikšys / Medical Physics in the
Page 95 and 96: L. Krynke et al.. / Medical Physics
Page 99 and 100: M. Pučetaitė et al. / Medical Phy
Page 105 and 106: D. Bernatavičius et al. / Medical
Page 109 and 110: N. Vaiciunaite / Medical Physics in
Page 111 and 112: D. Adlienė et al. / Medical Physic
Page 113 and 114: MP Competences Generic Competences
Page 117 and 118: S. Tabakov et al. / Medical Physics
Page 119 and 120: J. Laurikaitienė et al. / Medical
Page 121 and 122: R, % 100,0 99,5 99,0 98,5 98,0 97,5
Page 123 and 124: S. Mockevičienė et al. / Medical
Page 125 and 126:
show all

PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

Create successful ePaper yourself

Delete template?

Save as template?